ANATOMY AND BIOMECHANICS OF THE KNEE

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ANATOMY AND BIOMECHANICS OF THE KNEE JOHN P. GOLDBLATT, MD, and JOHN C. RICHMOND, MD

Successful treatment of the injured knee depends on a fundamental understanding of the anatomy and biomechanical function of the structures that comprise the knee. The major structures of the knee are presented anatomically, followed by a brief review of the biomechanics of each. Normal function, as well as the expected result of injury is presented. This review is intended to assist in clinical diagnosis, as well as treatment planning for the injured knee. KEY WORDS: knee, anatomy, biomechanics

© 2003 Elsevier Inc. All rights reserved.

A thorough knowledge of the complex anatomy and biomechanical function of the structures of the knee is essential to make accurate clinical diagnoses and decisions regarding the treatment of the multiple-ligament-injured knee. The following review is intended to provide the essential information required for an understanding of the anatomy and biomechanics of the normal knee, and the consequence of injury. Various techniques are utilized by researchers attempting to evaluate the biomechanics of the knee as a whole, as well as the function of individual structures. Mathematical modeling, experimental testing of knee specimens, anatomic dissection studies, and strain or force measurements in individual structures are a few of the possible methods. The two most commonly utilized experimental approaches have been termed the "flexibility method" and the "stiffness method." The flexibility method utilizes selective ligament sectioning, followed by observation of the response of the specimen to an applied load or moment. The process first measures the laxity in the intact knee in response to an applied force. A specific ligament, or combination of ligaments, is then transected, and the resulting increase in translation or rotation is measured. This increase reflects the loss of the sectioned structure, as well as the change in the interaction among the remaining structures. The stiffness method attempts to define the contribution of individual structures to overall restraint in response to predetermined displacements. Initially, the intact knee is translated, or rotated, a precise amount, and the restraining force that must be overcome to produce this displacement is measured. Next, a structure is sectioned, and the remaining force required to reproduce the original displacement is determined. The change in total restraining force represents the percentage contribution of the sec-

From the Department of Orthopaedics, Tufts-New England Medical Center, Boston, MA. Address reprint requests to John P. Goldblatt, MD, Tufts-New England Medical Center, Department of Orthopaedics, 750 Washington Street, Boston, MA 02111. © 2003 Elsevier Inc. All rights reserved. 1060-1872/03/1103-0002530.00/0 doi:10.1053/otsm .2003.35911

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tioned structure to the overall stability of the knee in the direction tested. Certainly, each method provides useful information, and each has inherent limitations. The information obtained from studies utilizing each technique must be integrated. Taken together, one can define the expected results of deficiency. The following summary, therefore, utilizes information from many types of studies in an effort to compile the consensus regarding the anatomy and function of the structures of the knee.

BONE ANATOMY The knee is a modified hinge joint that must allow flexion and rotation, yet provide complete stability and control under a great range of loading conditions. The knee consists of 2 joints: the femorotibial joint and the patellofemoral joint. The bony architecture of the femur, tibia, and patella contribute to the stability of the knee joint, along with static and dynamic restraints of the ligaments, capsule, and musculature crossing the joint. 1 The architecture of the bones dictates, to a certain extent, the allowed motion of the joint. FEMOROTIBIAL

JOINT

The femorotibial joint is the largest joint in the body, and is comprised of 2 condyloid articulations. The medial and lateral femoral condyles articulate with the corresponding tibial plateaus. Intervening medial and lateral menisci serve to enhance the conformity of the joint, as well as to assist the rotation of the knee. Simplistically, the femoral condyles are cam-shaped in lateral profile. The medial condyle has a larger radius of curvature than the lateral, and extends distal to the lateral on the anteroposterior (AP) projection. The lateral condyle extends anterior to the medial on the lateral projection, and can be identified by its terminal sulcus and groove for the popliteus insertion. 2 The proximal tibia is separated by the intercondylar eminence into an oval, concave medial plateau, and a circular, convex lateral plateau. The medial and lateral compartments are asymmetrical, particularly anteriorly.3

Operative Techniques in Sports Medicine, Vol 11, No 3 (July), 2003: pp 172-186

The lateral condyle of the femur is smaller than the medial, both in the AP and proximodistal directions. This contributes to the valgus and AP alignment of the knee. These shapes allow the medial femur to rotate on the tibia through 3 axes, and the medial femur to translate, to a limited extent, in the AP direction. Laterally, the femur can freely translate in the AP direction, but can rotate around a transverse axis only near extension, s The 3-degree lateral inclination of the tibial plateau in relation to the joint line, and 9 ° posterior slope, creates an overall valgus and posterior-inferior alignment of between 10° to 12 ° in most knees.l,4 PATELLOFEMORAL JOINT

The patellofemoral articulation is a sellar joint between the patella and femoral trochlea. This joint is important to knee stability primarily through its role in the extensor mechanism. The patella increases the mechanical advantage of the extensor muscles by transmitting the extensor force across the knee at a greater distance from the axis of rotation. This increased moment arm reduces the quadriceps force required to extend the knee by 15% to 30%. The contribution of the patella to increasing the moment arm of the quadriceps varies over the range of motion. At full flexion, the lever arm of the quadriceps is increased approximately 10%, and this increases to 30% by 45 ° from full extension, and then once again decreases as the knee passes to terminal extension. 1 The stability of the patella in the trochlear groove is a combination of bony, ligamentous, and muscular restrain,s. The patella responds to a set of 3 forces: the pull of the quadriceps, hamstrings, and a net compressive force on the patellofemoral surfaces. In addition, several softtissue constraints contribute to the tracking of the patella within the trochlear groove. The constraints include the medial patellofemoral ligament, medial patellomeniscal ligament, medial patellotibial ligament, medial retinaculum, and lateral retinaculum. These ligaments are discussed in detail in later sections.

ANTERIOR CRUCIATE LIGAMENT (ACL)

contour of the posterior curvature of the condyle, 4 mm anterior to the articular margin. The long axis of the femoral attacl~nent is tilted obliquely, slightly forward from the vertical. 6 The origin of the ACL is 16 to 24 mm in largest diameter, s-7 11 mm in lesser diameter, 7 and is well posterior in the intercondylar notch. 5,6 The tibial attachment of the ACL is to a wide depressed area in front of, and lateral to, the medial tibial spine. The fiber insertion to the tibia is oval, 7 and occupies approximately one third of the sagittal width of the tibial plateau. ~ The overall footprint includes insertions to the base of the tibial spine, and a well-defined slip to the anterior horn of the lateral meniscus. Also variably present are additional attachments posteriorly that blend with the attachment of the posterior horn of the lateral meniscus, and to bone in front of the posterior horn of the medial meniscus. The average distance from the anterior border of the tibia1 articular surface to the anterior attachment of the ACL is 15 mm. 6 The reported average AP length of the ACL attachment ranges from 17 mm 7 to 30 ram, 6 and the width is 11 mm (Fig 1). 7 The tibial attachment is nearly twice the bulk width of the ACL, and has a distinct anterior toe at this attachment that adapts to the contour of the intercondylar roof in full extension. 8,9 From the femoral attachment, the ACL passes anteriorly, distally, and medially to the tibia. The ACL is inclined an average 25 ° from the tibial plateau, when viewed laterally. 1° The average length of the ACL is reported from 26 m m to 38 ram, and the average width is 11 ram. 4,6 The length of the ACL changes less than 2.5 mm through the arc of motion. 4 In the AP projection, the center of the tibia1 ACL attachment is just lateral to the exact center of the tibia. In the lateral projection, the center of the tibial attachment is at the junction of the anterior 40% and posterior 60% of the AP length of the tibial plateau, and spans 30% of this width. On the AP view, the entire ACL femoral insertion is lateral to the midline. The femoral attachment occupies the posterosuperior region of the intercondylar notch on the lateral view in extension, and extends posterior to Blumensaat's line (the roof of the intercondylar notch). The superior margin is at the approximate level of Blu-

ANATOMY

The complex structure of the ACL reflects its important contribution to knee-joint function. Originally referred to as a crucial ligament because of the cruciate, or crossed, arrangement of the anterior and posterior ligaments within the knee, the irony of the ACL being crucial to the well-being of the knee joint has more recently been demonstrated, s An elegant anatomical dissection study by Girgis 6 of 20 cadavers and 24 fresh knees helped elucidate the complex anatomy of the ACL. The details of the footprints on the femur and tibia, as well as fiber bundle orientation are clearly described. The ACL femoral attachment is to the posterior part of the medial surface of the lateral femoral condyle, and is primarily oriented in line with the longitudinal axis of the femur with the knee in extension. 4,6 The footprint is in the shape of a segment of a circle. The anterior border is nearly straight. The posterior margin is convex, and follows the ANATOMY AND BIOMECHANICS OF THE KNEE

Fig 1. Tibial plateau showing the menisci and their relationship to the ACL and PCL attachments. Reprinted with per= mission. TM 173

mensaat's line. 9 These radiographic landmarks are of obvious use to the surgeon in evaluating tunnel placement during reconstruction. Girgis 6 identified a functionally distinct fascicular arrangement of the ACL, and divided these into anteromedial and posterolateral bands, named for their tibial origins. Subsequent authors confirm the fascicular arrangement of the ACL, 4,~1 and have included a possible intermediate band. 1~ The anteromedial fibers form the shortest band of the ACL and are tense in flexion.6,12 The remaining bulk of fibers originate from the distal femoral attachment, and insert posterolaterally on the tibia. This posterolateral band is taut in extension and lax in flexion (Fig 2). 4-6,I1,12 The orientation of the ACL becomes nearly horizontal with flexion, and the anteromedial band becomes taut almost immediately after flexion begins. 6 The more horizontal orientation of the ACL with flexion enables the ligament to function as a primary restraint to anterior tibial translation. This functional description of the ACL is somewhat of an oversimplification. The ACL is actually comprised of a continuum of fascicles, each of a different length. Therefore, a different portion of the ligament is taut and functional throughout the range of motion. 7,13 Minimum ACL strain occurs between 30 ° to 35 ° under normal passive motion. The anteromedial fiber strain relaxes from full extension to 30 ° to 35 °, and then tightens with further flexion to 120 °. The strain in the posterolateral fibers decreases immediately from full extension, with marked laxity between 15 ° to 70 ° flexion. The posterolateral fibers of the ACL, for angles greater than 15°, do not control anterior displacement of the tibia on the femur. When simulated quadriceps contraction is added, the anteromedial ACL strain significantly increases in the range of 0 ° to 45 ° of flexion. Quadriceps activity does not strain the ACL when the knee is flexed beyond 60 °. The clinical implication of this is 2-fold. First, in attempting to clinically evaluate the function of the ACL, the Lachman examination is most sensitive because it is performed at the position of the least amount of inherent strain in the ligament. Second, this suggests that rehabilitation of an injured or reconstructed ligament should be performed at angles greater than 60 ° until healed. 62

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a'-

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Fig 2. Changes in the shape and tension of the ACL in extension and flexion, in extension, lengthening of the posterolateral band B-B'. In flexion, lengthening of the anteromedial band A-A'. Reprinted with permissionA

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Because the ACL is arranged anatomically and functionally into distinct bands, it is evident that the ligament, as a whole, is not isometric. 14,15 During passive flexion of the knee, the anteromedial portion of the ACL lengthens while the posterolateral portion shortens, and the intermediate fibers do not change in length. 15 Odensten 7 measured the distance between the central points of the insertion areas on the tibia and lateral femoral condyle, and found that this distance was isometric over the entire range of motion. Recognition of this is important when trying to reconstruct the ligament. Many authors attempt to seek the position of femoral and tibial attachments whose separation distances remain nearly constant, or isometric, as the knee is flexed. Hefzy 14 showed that altering the femoral attachment had a much greater effect than the tibial attachment on ACL graft length. No femoral attachments were completely isometric. The axis of least variability in graft length was proximal-distal oriented, located near the native ACL femoral insertion. Fibers anterior to this line lengthen with flexion, and posterior to this line, slacken with flexion. Therefore, AP orientation has the greatest effect on fiber length. The widest part of the most isometric region is located proximally, along the roof of the intercondylar notch. 14 The overall configuration of the ligament is flat in extension. When flexed, the ACL twists on itself approximately 90°. 6,7,13 Samuelson ~2 demonstrated in an anatomical dissection study that the normal rotation of the ACL was due to the orientation of the 2 major bundles. A lateral twist developed in the ACL and became more pronounced as knee flexion increased. This inherent twist caused the tibia to rotate internally with respect to the femur approximately 55 ° . To recreate this inherent internal rotation with an ACL graft, the graft required 90 ° of lateral rotation on insertion. The anterior cruciate ligament is covered by a fold of synovial membrane that resembles a mesentery. 13,16 This synovial fold originates from the posterior intercondylar area, and completely envelops both the ACL and posterior cruciate ligament (PCL). 5 Thus, although the ligament is intra-articular, it is actually extrasynovial. The synovial envelope is richly endowed with vessels that originate from the middle geniculate artery, as well as a few smaller terminal branches of the medial and lateral inferior geniculate artery. 5,x3These vessels arborize to form a plexus that ensheaths the length of the ligament, and penetrates the ligament transversely to anastomose with the network of endoligamentous vessels. The blood supply to the ligament is largely of soft-tissue origin, and the ligamentosseous junctions contribute little to the vascular supply of the ligament. 5,16 The ACL is innervated by branches of the tibial nerve. Nerve fibers penetrate the joint capsule posteriorly, and travel with the periligamentous vessels surrounding the ligament. Smaller nerve fibers and end organs have also been identified within the substance of the ligament itself. Schutte 17 found that 3 morphological types of mechanoreceptors and free nerve endings were present within the substance of the ACL. Two of the slow-adapting Ruffini type subserve speed and acceleration. One rapidly adapting Pacinian corpuscle type signals motion. Free nerve GOLDBLATT AND RICHMOND

endings responsible for pain were also identified within the ligament, although relatively small in total number. This may explain w h y isolated injury to the ACL often results in little initial pain. The nerve supply is postulated to serve a vasomotor function, as well as a possible proprioceptive or sensory function to sense speed, acceleration, position, and direction of motion.

BIOMECHANICS The ACL is a keystone to controlled, fluid, and stable flexion and rotation of the normal knee. The ACL is a primary restraint to anterior translation of the tibia on the femur, and a secondary restraint to internal rotation, varus, valgus, and hyperextension.4,6,1°,11,13,1s,I8-22 The ACL does not resist posterior d r a w e r . 1°,2°,21,23,24 Sectioning the ACL produces a significant increase in anterior knee instability. The greatest amount of anterior translation after isolated ACL sectioning occurs between 15° and 450.20,23,25 During the clinical examination, the most effective position to conduct an anterior instability test is at 30 ° of flexion.2° The ACL reaches ultimate stress at approximately 15% strain, and gross failure is expected to occur when strain exceeds 15% to 30%, or displacement of about 1 cm. 15,1s Levy24 subjected knees to a 100 N anterior force. Intact knees demonstrated, on average, 3.4 m m anterior translation at full extension, and maximum, 4.7 m m at 30 ° flexion. After isolated sectioning of the ACL, maximum anterior displacement at 30 ° was 18.1 mm. In a similar study, Fukubayashi2° found that isolated sectioning of the ACL produced a greater than 2-fold increase in anterior displacement of the tibia, compared with the intact knee, when loaded in an anterior direction. As the flexion angle increased, the displacement decreased; however, sectioning did result in increased laxity at all angles. Later sectioning of the PCL did not alter translation in the anterior direction. Utilizing the stiffness method, Butler 1° ranked the ligamentous restraints to anterior-posterior motion in the human knee when displacement was fixed at 5 mm. The ACL provided 85% to 87% of the restraining force to anterior translation at 30 ° and 90 ° of knee flexion, when rotation was eliminated. Takeda is utilized a 5 ° of freedom kinematic linkage system, which allowed rotation, to investigate the contribution of the ACL to resistance against anterior drawer. The ACL restraint dropped to 74% to 83% of the total, indicating that constrained motion altered the normal function of the structures tested. ACL deficiency results in the disintegration of the normal rolling-gliding movement of the femur on the tibia. Rolling predominates in the initial 30 ° of flexion, and this shifts the contact point of the femur and tibia posteriorly. With ACL deficiency, the tibia moves into an anteriorly subluxated position, and then relocates with further flexion (pivot--shift phenomenon). This reduction is guided by the secondary restraints to anterior translation, notably the iliotibial band. The abnormal movement is suggested as a cause of potential injury to the menisci, as well as chondral surfaces. 26 ANATOMY AND BIOMECHANICS OF THE KNEE

The posterior fibers of the ACL are the principal restraint to hyperextension, and are expected to fail first in injuries occurring in extension¢ 8 Isolated ACL sectioning allows an increase in hyperextension by 250.6,19 Because the pattern of tibial rotation also changes drastically after sectioning of the ACL, it is apparent that this ligament plays a vital role in the natural rotation of the tibia during AP motion. After ACL sectioning, the resulting secondary internal rotation that accompanies anterior translation is eliminated. The ACL constitutes a primary mechanism to control and produce internal rotation during AP motion. 2°

POSTERIOR CRUCIATE LIGAMENT (PCL) ANATOMY

The same study of 44 knees by Girgis 6 that provided much of the information regarding the ACL serves as an excellent resource, with elaboration by others, for the anatomy of the PCL. Once again, the details of the footprints on the femur and tibia, as well as fiber bundle orientation are clearly described. The femoral attachment of the PCL is to the lateral surface of the medial condyle. The attachment is in the form of a segment of a circle, and horizontal in its general direction, with the knee extended. The upper boundary is horizontal, and the lower boundary convex, and parallel to the lower articular margin of the condyle. The distal margin of the PCL is proximal to the articular surface by 3 mm. 6 Fibers attach to the femur in an anterior to posterior direcfion.27 The tibial attachment of the PCL is into a depression between the 2 plateaus, approximately 1 cm distal to the articular surface of the tibia, and extends distally several millimeters onto the posterior surface of the tibia. Fibers attach to the tibia in a medial to lateral direction. The PCL attaches with several additional slips, including a slip to blend with the posterior horn of the lateral meniscus. When the meniscofemoral ligaments from the lateral meniscus are absent, this slip from the PCL is quite prominent. The width of the tibial attachment averages 13 ram, and is noted to depend on the width of the intercondylar notch.6, 27 The average length of the PCL is 38 ram, and the average width is 13 mm. 6 The ligament is enclosed within synovium and is, therefore, extra-articular in an anatomic sense, aT,2s The synovium is reflected from the posterior capsule, and covers the medial, lateral, and anterior aspects of the PCL. Distally, the posterior portion of the PCL blends with the posterior capsule and periosteum, as The vascular supply to the PCL is from the middle genicular artery, which arises from the popliteal artery behind the popliteal surface of the femur. It runs anteriorly and penetrates the posterior capsule of the knee joint in the intercondylar notch. The artery supplies blood to both cruciates, synovial membrane, posterior capsule, and the epiphyses of the tibia and femur. The synovial tissue around the PCL is also a major blood source for the ligament. The base of the PCL is supplied by capsular vessels arising from the popliteal and inferior genicular arteries.13,27 175

I

Fig 3. Changes in the shape and tension of the PCL. In extension, lengthening of the posteromedial band A-A'. In flexion, lengthening of the anterolateral band B-B'. C-C' is the ligament of Humphrey attached to the lateral meniscus. Reprinted with permission. 6

The overall position of the ligament in the joint is located near the longitudinal axis of rotation, just medial to the center of the knee. It is directed vertically in the frontal plane, and angles forward 30 ° to 56 ° in the sagittal plane, depending on the degree of knee flexion. The PCL assumes a more vertical orientation in extension, and a more horizontal position in flexion.a7,28 The PCL is narrowest in the midsubstance, and fans out superiorly to its widest dimension, an average 32 mm, and to a lesser extent inferiorly.6 The medial fibers from the tibia insert posteriorly on the femur, and the lateral fibers from the tibia insert anteriorly on the femur. 27 Functionally, the PCL appears to be arranged in 2 inseparable bands, named for their insertion positions. 6,27 The anterolateral band is more robust, and arises from the convex portion of the femoral attachment. The anterolateral band is lax in extension, and becomes taut as flexion increases past 300.6,11,27 The posteromedial band is thinner, runs a more oblique course, and attaches more distally on the tibia. 6,27The posteromedial fibers are taut in extension and lax in flexion (Fig 3). 11"27 The bands are not entirely separable, and represent a simplification of the architecture. 23 No part of the PCL is totally isometric during range of motion. The bulk of the fibers of the PCL lengthen with flexion from 0 ° to 90 °. The femoral attachment of the fibers is the primary determinant of a given fiber's isometry during flexion and extension, particularly the proximal and distal (versus anterior and posterior) location. Therefore, the function of fibers of the PCL is determined primarily by the femoral attachment location. Grood 29 performed a study using an instrumented spatial linkage for measuring the length of discrete bundles within the ligament, and found that the most isometric location for graft placement was at the base of the bulletshaped region whose base is against the roof of the intercondylar notch. These results help define attachments that do not cause the substitute to become excessively tense or slack when the knee is flexed. Error in AP placement on the femoral side may be accepted; however, this is not true for proximal-distal orientation. The PCL substitute should be placed along the proximal attachment of the PCL.

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In chronically deficient knees with no identifiable footprint, the average distance to the native femoral origin is 11 m m from the junction of the notch with the trochlear groove. The region that results in 2 mm length variance through the range of motion extends approximately 1 cm from the roof in a posterior and slightly distal direction. The PCL length pattern is relatively insensitive to either proximal-distal or medial-lateral placement on the tibia. Therefore, a widely exposed tibial fossa is not as necessary during fixation on the tibial side. 29 The ACL and PCL tibial insertions are oriented perpendicular to each other in the longest dimension. The ligaments are separate in the sagittal plane in extension, and wind around each other in flexion.6 BIOMECHANICS

The PCL is a primary restraint to posterior translation of the tibia on the femur, and a secondary restraint to varusvalgus and external rotation.6,10,13,20,25,27,30 The PCL is the only isolated ligament to provide primary restraint to posterior translation at all angles of flexion.25 The PCL does not resist anterior d r a w e r . 1°,2°,23,24 Utilizing the stiffness method, Butler 10 ranked the ligamentous restraints to anterior-posterior motion in the human knee when displacement was fixed at 5 mm. The PCL provided 90% to 95% of the total restraining force to posterior translation at 30 ° and 90 ° of knee flexion. No other structure contributed more than 2% to the total restraint. Therefore, abnormal posterior tibial translation cannot occur without injury to the PCL. Grood s° utilized the flexibility method in a study that allowed 6° of freedom to determine the effect of sectioning the PCL and posterolateral structures (lateral collateral ligament, popliteus, and arcuate complex) on knee motion from 0 ° to 90 °. Isolated sectioning of the PCL resulted in an increase in posterior translation, and this increased as the knee was flexed, to a maximum at 90 °. These results reflect the increasing slackness in the remaining secondary restraints to posterior translation as the knee flexes. Conversely, if the knee demonstrated a similar increase in posterior translation at both 30 ° and 90 °, this suggested that the medial and lateral extra-articular ligaments may have lost some of their functional capacity. In a similar sectioning-type study, Fukubayashi2° demonstrated that isolated sectioning of the PCL produced an almost 3-fold increase in the amount of posterior displacement versus the intact knee, without affecting anterior displacement. Before sectioning, a 100 N posteriorly directed force resulted in 6-mm translation. Translation increased to over 15 m m with sectioning of the PCL. In PCL-deficient knees, the greatest posterior translation with a posteriorly directed force occurred at 75 ° flexion. Later sectioning of the ACL did not alter the translation in the posterior direction. During the clinical examination, the most effective position to conduct a posterior drawer test is at 75 ° knee flexion. After sectioning the PCL, the resulting secondary external rotation disappears. Therefore, it is apparent that this ligament plays a vital role in the natural rotation of the tibia during AP motion. The PCL causes coupled external rotation during posterior translation. In other words, the GOLDBLATT AND RICHMOND

PCL constitutes a primary mechanism controlling and producing external rotation during posterior translation. 2° The central location of the PCL makes it the center for rotational instability patterns of the knee. 28,3~ The PCL functions as a secondary restraint to external rotation and varus-valgus rotation. Isolated sectioning of the PCL yields no effect on varus-valgus or external rotation at any angle as long as the LCL and deep ligament complex are intact. 25 External rotation increases only 8° with the knee flexed and isolated PCL sectioning, and internal rotation increases only 3 °. Rotation is not affected in the extended position, and extension is unaffected as well. 6 Neither isolated nor combined sectioning of the PCL or posterolateral structures increase anterior translation.6,25,30,32

THE MENISCI AND MENISCOFEMORAL LIGAMENTS ANATOMY The medial and lateral compartments of the knee each has an intervening meniscus located between the femur and tibia. Grossly, the menisci are peripherally thick and convex,, and centrally taper to a thin free margin. The meniscal surfaces conform to the femoral and tibial contours. The medial meniscus is semicircular and approximately 3.5 cm in length. The posterior horn is wider than the anterior horn (Fig 1). The anterior horn has a variable attachment to the tibial plateau in the area of the intercondylar fossa in front of the ACL. Often, this attachment is to the anterior surface of the tibial plateau. The posterior fibers of the anterior horn merge with the transverse interrneniscal ligament, which connects the anterior horns of the 2 menisci. 33 The intermeniscal ligament, located approximately 8 m m anterior to the A C t , serves as the primary attachment site for the anterior horn of the medial meniscus in approximately one quarter of cases. 34 The posterior horn of the medial meniscus is firmly attached to the posterior intercondylar fossa of the tibia, anterior and medial to the PCL tibial attachment site. The periphery of the medial meniscus is attached to the capsule throughout its length, and the tibial portion of this attachment is called the coronary ligament. At its midpoint, the meniscus is firmly attached to the femur and tibia through a condensation of the joint capsule known as the deep medial collateral ligament. 33 The lateral meniscus is almost circular in gross morphology, and covers a larger portion of the tibial plateau than the medial meniscus. The lateral meniscus is approximately the same width from front to back (Fig 1). The bony attachments of the lateral meniscus are just anterior and posterior to the ACL onto the tibia. There is a loose peripheral attachment of the lateral meniscus to the joint capsule that allows greater translation of the lateral meniscus when compared with the medial meniscus. 33 The area of the lateral meniscus with no coronary ligament attachment, anterior to the popliteus tendon, is called the b a r e a r e a . 35,36

Two meniscofemoral ligaments attach the lateral meniscus to the medial femoral condyle. The anterior meniscofemoral ligament (ligament of Humphrey) is less than ANATOMY AND BIOMECHANICS OF THE KNEE

one third the diameter of the PCL, and arises from the posterior horn of the lateral meniscus. It runs anterior to the PCL to insert with the anterior fibers of the PCL on the femur, at the distal edge of the PCL. The posterior meniscofemoral ligament (ligament of Wrisberg) is as large as half the diameter of the PCL, and also arises from the posterior horn of the lateral meniscus. It passes obliquely behind the PCL, and inserts on the medial femoral condyle, along with the posterior PCL fibers. 27,37The posterior ligament has a more variable attachment to the meniscus. It can originate solely from the tibia, or posterior capsule, in which case it attaches indirectly to the lateral meniscus via posterior capsular and popliteus attachments. The meniscofemoral ligaments are variably present, and both may be present in the same knee. Heller 37 found a meniscofemoral ligament in 71% of 140 knees. Of all knees studied, the anterior ligament was present in 36%, the posterior in 35%, and both in 6%. In contrast, Girgis 6 never observed the 2 ligaments together in a dissection of 44 knees. In 30%, the 2 ligaments were not found. Instead, a prominent slip from the posterior cruciate ligament (PCL) was inserted into the posterior horn of the lateral meniscus. In the remaining 70%, Wrisberg's ligament was more commonly found, but when present, Humphrey's ligament was more robust. Each meniscofemoral ligament is identified by distinct femoral attachments readily distinguishable from those of the PCL. Their presence may be confused secondary to the variable tibial-sided attachments.

BIOMECHANICS With knee flexion form 0° to 120°, the menisci move posteriorly. In the midcondylar, parasagittal plane, the medial meniscus moves approximately 5.1 mm, and the lateral meniscus moves approximately 11.2 ram. Rotation of the knee also affects meniscal motion. Posterior motion of the medial meniscus is guided by the deep medial collateral ligament (MCL) and semimembranosus, whereas anterior translation is caused by the push of the anterior femoral condyle.1 The medial meniscus lacks the controlled mobility of the lateral meniscus.37 The posterior oblique fibers of the deep MCL limit motion in rotation and, therefore, the medial meniscus is at increased risk of tear. 33The lateral meniscus is stabilized, and motion guided, by the popliteus tendon, popliteomeniscal ligaments, popliteofibular ligament, meniscofemoral ligaments, and lateral capsule.37~39 The differential motion between the anterior and posterior horns of each meniscus allows the meniscus to mainlain conformity to the bony surfaces during flexion, as well as to avoid any block to motion as the femorotibial contact point moves with mofion. 13,26 In addition, meniscal motion allows continued load distribution during changes of position of the joint, during which the radius of curvature of the femoral condyles changes. 1,13,26,33 Although the menisci deepen the plateaus only slightly, this deepening provides for a more congruent and constrained surface with the femoral condyles. 1,3s The medial meniscus provides greater restraint to anterior translation than does the lateral meniscus, by acting as a buttress. This can be demonstrated by evaluation of the meniscus-deft-

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cient knee. A biomechanical study by Levy 24 of humancadaver knees compared intact knees to knees subjected to isolated medial meniscectomy, isolated ACL sectioning, or combined ACL sectioning and medial meniscectomy. Compared with intact knees, isolated medial meniscectomy did not significantly alter anterior-posterior displacement, nor coupled internal rotation. ACL-deficient knees demonstrate increased anterior translation when subjected to an anteriorly directed force, and this translation increased significantly with combined meniscectomy at all angles of flexion. The maximum anterior displacement occurred at 60 °, and the greatest percentage increase (58%) occurred at 90 °. The results confirm the role of the ACL as a primary restraint to anterior translation, and demonstrate that the medial meniscus acts as a secondary stabilizer to resist anterior translation. With sufficient anterior translation (in the ACL-deficient knee), the posterior horn of the medial meniscus is wedged between the tibial plateau and the femoral condyle, and is the mechanism suggested for the resistance provided by the meniscus. Thus, sacrifice of the medial meniscus after injury to the ACL may further compromise anterior stability. Meniscectomy alone, or combined with ACL sectioning, does not significantly affect posterior translation under 100 N posteriorly directed load. In contrast, the soft-tissue attachments of the lateral meniscus do not affix the lateral meniscus as firmly to the tibia. Levy 23 performed a similar study of the effect of lateral meniscectomy in the unloaded knee. Changes in motion in knees subjected to isolated lateral meniscectomy, ACL sectioning, and combined lateral meniscectomy and ACL sectioning were quantified. As expected, isolated ACL sectioning resulted in increased anterior translation when subjected to a 100 N anteriorly directed force at all angles of flexion. Isolated lateral meniscectomy did not significantly affect primary anterior or posterior translation. Combined lateral meniscectomy and ACL sectioning did not increase anterior translation significantly over ACL sectioning alone. This implied that the greater mobility of the lateral meniscus prevented it from contributing as efficiently as a posterior wedge to resist anterior translation of the tibia on the femur. This test was performed in unloaded knees, and should be extrapolated to predict the effect under loaded conditions with caution. Shoemaker 21 evaluated the role of the meniscus in anterior-posterior stability under loaded conditions in the ACL-deficient knee. This study examined changes in AP laxity due to progressive meniscectomy in the loaded knee at 20 ° of flexion, under 320 N and 925 N compressive force. After ACL sectioning, the resistance provided by the menisci to an anteriorly directed force increased with increasing axial load (57% at 925 N axial load). The higher the applied joint load, the greater the meniscal compression, and thus the greater the resistance to anterior force. The contribution from the lateral meniscus was minimal. The loaded menisci helped resist anterior translation in ACLdeficient knees; however, they were easily overcome at relatively low anteriorly directed forces of 200 N. In addition to their role in joint stability, the menisci serve additional functional roles, including load bearing

178

and shock absorption. The menisci transmit large loads across the joint, and their contact areas change with different degrees of knee flexion and rotation. Up to 50% of compressive load is transmitted through the menisci in extension, and 85% at 90 ° of flexion. Removal of a portion of the meniscus results in decreased contact area between the femur and tibia. Medial meniscectomy decreases the contact area by up to 70%. Resection of as little as 15% to 34% of the meniscus results in increased contact pressure by more than 350%. The resulting increased peak stresses and pressure concentrations lead to progressive degenerative changes closely resembling naturally occurring histologic, biochemical, and biomechanical changes of osteoarthritis. The degree of change is proportional to the extent of meniscectomy. 33,4°

MEDIAL STRUCTURES OF THE KNEE The supporting structures of the medial side of the knee can be divided into 3 discrete layers. 1,41 The most superficial layer, layer I, is the deep or crural fascia. This fascia is the first plane encountered deep to the subcutaneous tissue, and extends from the patella to the midline of the popliteal fossa. Anteriorly, this layer blends with layer II in a vertical line, 1 to 2 cm anterior to the anterior edge of the superficial MCLY The medial patellotibial ligament is an oblique condensation of the medial retinaculum of layer I. This ligament inserts 1.5 cm inferior to the joint line, on the anteromedial border of the tibia, and coalesces with the fibers of the medial patellofemoral ligament of layer II, at the medial border of the patella. The medial patellomeniscal ligament is deep to the patellotibial ligament, and runs from the inferior two thirds of the patella along the medial border of the infrapatellar fat pad to insert on the anterior portion of the medial meniscus. 42 Posteriorly, layer I overlies the 2 heads of the gastrocnemius and serves to support the neurovascular structures of the popliteal fossa. The sartorius inserts into the crural fascia and does not have a distinct tendon of insertion, as do the underlying gracilis and semitendinosus, which run between layer I and II. Anteriorly and distally, layer I joins the periosteum of the tibia, where the fibers of the sartorius insert on the tibia, and posteriorly it becomes the deep fascia of the leg. Layer II contains the superficial MCL, and is clearly defined by the parallel anterior fibers of this ligament (Fig 4). From the region of the femoral insertion of the anterior fibers, a transverse band runs forward in the plane of layer II from the adductor tubercle toward the patella, forming the medial patellofemoral ligament. 41 The patellofemoral ligament runs deep to the vastus medialis to insert on the superomedial patella, and sends a slip to the undersurface of the vastus medialis obliquus and vastus intermedius. 43 The posterior fibers of the superficial MCL are oblique in orientation, and more posteriorly merge with fibers of layer III at the posteromedial corner of the knee. These posterior oblique fibers blend with the capsule posteriorly to form a pouch. The pouch is augmented by contributions from the semimembranosus sheath, as well as variably from the semimembranosus tendon, to form the oblique popliteal ligament (Fig 4). GOLDBLATT AND RICHMOND

MEDIAL COLLATERAL LIGAMENT (MCL) Anatomy The MCL is composed of a superficial portion and a deep portion (Fig 4). 11 The superficial MCL originates on the medial epicondyle, and runs downward as a broad triangular band approximately 11 cm to its tibial insertion, deep to the gracilis and semitendinosus tendons. 1,13,41 The superficial MCL can be further subdivided into anterior and posterior portions. The anterior margin lies free except at its attachment sites to the tibia and femur, and is separated from the medial meniscus and deep capsular ligament by a bursa, where as the posterior margin passes obliquely backwards to an insertion in the medial meniscus. The anterior portion is taut in extension, and progressively tightens over the entire range of motion, whereas the posterior portion slackens with flexion. 41,44The inferior medial geniculate vessels and nerve intervene between the ligament and bone as the insertion of the collateral extends distal to the knee joint. 13 The deep portion of the MCL also can be divided into 2 subdivisions, the meniscofemoral and meniscotibial ligaments, defined by their respective insertions. 44 Fig 4o Structures of the medial side of the knee. Distinct insertions of semimembranosus tendon (1,2) and tendon sheath (3,4,5). The insert demonstrates sites of attachment of the superficial and deep MCL. "C" indicates the oblique popliteal ligament. Reprinted with permission. 41

The semimembranosus inserts to bone by a direct insertion at the posteromedial corner of the tibia, just below the joint line (Figs 4 and 7). Additional insertions of the semimembranosus include an anterior insertion around the medial side of the tibia just below the joint line, deep to layer II and distal to layer [II, as well as variable contributions to the posteromedial capsule and oblique popliteal ligament. The semimembranosus sheath contributes fibrous extensions into the posteromedial and posterior capsule. These include a direct extension upward over the medial condyle of the femur, a second extension across to the lateral condyle, forming the oblique popliteal ligament, and a third to blend with the oblique posterior fibers of the superficial MCL (Fig 4). Layer III is the true joint capsule. The lines of attachment of the capsule follow the joint margins, except anteriorly, where it extends cephalad to form the suprapatellar pouch. The anterior capsule is thin and redundant to accommodate the range of motion of the knee. Beneath the superficial MCL, the capsule thickens to form the vertically oriented, short fibers of the deep MCL (Fig 4). The meniscofemoral portion of the deep ligament extends from the femur to the mid-portion of the peripheral margin of the meniscus. The meniscotibial portion of the ligament anchors the meniscus, and is readily separated from the overlying superficial MCL. The remainder of the capsule is thin, and bulges as it extends from the femur to the meniscus. The meniscus is attached distally, along its margin, to the tibia by the coronary ligament, which is slack but very short, so that it does not allow excessive meniscal motion.~, 4~

ANATOMYAND BIOMECHANICSOF THE KNEE

Biomechanics

The MCL is an important restraint to valgus rotation and a check against external rotation and straight medial and lateral translation of the tibia. Warren 44 demonstrated that, regardless the order of ligament sectioning, the superficial portion of the MCL contributed greatest to stability. Sectioning the superficial portion of the ligament, while leaving the remainder intact, resulted in joint space opening under valgus load, over the entire range of motion. In addition, external rotation doubled in extension and progressed to a 3-fold increase by 90 °. Sectioning the deep ligament and posterior capsule produced almost no change in the behavior of the specimen under stress if the superficial fibers were intact. Utilizing the stiffness method, Grood 45 determined the contribution of the MCL to valgus stability. The superficial portion of the MCL provided the majority of the restraint to valgus rotation, from 57% at 5° of flexion to 78% by 25 ° flexion. Laxity was greatly reduced with the knee in full extension. The percentage contribution of the ligament in extension was reduced even further as joint-space widening increased. This was due to increasing tension in other posteromedial structures, mainly the posteromedial capsule. The role of the MCL increases with increasing flexion, as the posterior capsular structures become slack. The increase in valgus laxity after sectioning the MCL is greater in a more flexed position, and largest at 30 °, with up to 5.5 mm of joint-space opening. This measurement points out that a complete injury to the MCL may occur with even subtle laxity, and a large increase in joint laxity likely involves additional structuresol~, 44,45

MEDIAL PATELLOFEMORAL LIGAMENT Anatomy More recently, the medial patellofemoral ligament is recognized as a major restraint to lateral displacement of the

179

1 - first layer II. second layer III- third layer

prepate!tsr bursa (I) ~atella ;ulum (111 I tract (l)

ra! meniscus joint capsule {lff} popltteus tendon " (entering jo!r~ through hiatus) lateral ¢otlat~l ligament (Ill) in sup, lamina ~rcua~ ligament (Ill) in deep ~am!na ~ater~l inferior gentcutate a.

Fig 5. Axial section of the anatomy of the layers of the lateral knee. I: first layer; I1: second layer; II1: third layer. Reprinted with permission. 36

fabellof~buiar ligament (Ill} biceps t e n o n (I} common peroneal m ligament Of Wrisberg oblique' poplitea!

fibular bead popliteus

ligament

distal knee-extensor mechanism. For this reason, it deserves separate mention. As described above, the hourglass-shaped ligament runs transversely in layer II from attachments to the adductor tubercle, as well as femoral epicondyle, and anterior bo~der of the superficial MCL. 41-4~ The proximal fibers of the ligament proceed anteriorly toward the vastus medialis obliquus, fanning out proximally to insert on the undersurface of the vastus medialis obliquus and the aponeurotic fibers of the vastus intermedius. The distal fibers insert anteriorly on the superomedial patella, extending inferiorly from the medial process. 42,43 The width of the medial patellofemoral ligament averages 1.3 cm. 43

Biomechanics Conlan 43 utilized the stiffness method to evaluate the medial soft-tissue restraints of the extensor mechanism in 25 fresh-frozen knee specimens performed in extension. The major stabilizer preventing lateral displacement of the patella was the medial patellofemoral ligament, followed, in decreasing order, by the medial patellomeniscal ligament, the medial retinaculum, and the medial patellotibial ligament. The contribution to restraint from the medial patellofemoral ligament was 23% to 80%, with an average 53% of the total restraint. Of note, the patellofemoral ligament was variable in size, and the restraint provided corresponded to the bulk of the ligament. In a similar study performed at 20 ° of flexion, Desio 42 found the contribution of the medial patellofemoral ligament to be 60%. In combination, the restraint of the patellofemoral and patellomeniscal ligaments accounted for approximately 75% of the medial restraining force at extension, and 20 ° flexion.42,43

LATERAL STRUCTURES OF THE KNEE The lateral compartment of the knee is divided into 3 sections. 46 The anterior one third of the lateral compartment includes the capsular ligament, which extends posteriorly from the lateral border of the patella to the ill180

otibial band. This region is reinforced by the lateral extension of the quadriceps tendon (retinaculum). These join to form a single layer at their attachment to the tibia. The middle one third is composed of the more superficial iliotibial (IT) band and a d ~ p e r capsuia~ Iigamen~. This section extends posteriorly to the lateral collateral ligament (LCL). The middle third capsular ligament attaches proximally to the lateral epicondyle of the femur, and distally to the tibial joint margin. The remaining posterior one third is termed the posterolateral corner. This region contributes significantly to the stability of the lateral knee through the intricate arrangement of many structures. 46 It is precisely this complexity that has resulted in so much study and controversy. Seebacher36 divides the lateral knee into 3 layers (Fig 5). Layer I is the superficial layer comprised of the iliotibial tract with its anterior expansion, and the superficial portion of the biceps with its posterior expansion. The peroneal nerve lies on the deep side of layer I, just posterior to the biceps tendon (Fig 6). Layer II is formed anteriorly by the retinaculum of the quadriceps, which extends along the anterolateral border of the patella. Posteriorly, the layer is incomplete, and is represented by 2 patellofemoral ligaments. The proximal ligament joins the terminal fibers of the lateral intermuscular septum. The distal ligament ends posteriorly on the fabella, or at the insertions of the posterolateral capsular reinforcements, and the lateral head of the gastrocnemius on the femoral condyle. Also part of layer II, the patellomeniscal ligament travels obliquely from the patella to the margin of the lateral meniscus, and terminates at Gerdy's tubercle (Fig 6 ) . 36 Finally, layer III, the deep layer, includes the lateral joint capsule, and its supporting ligaments (Fig 5). The coronary ligament is formed by the capsular attachment to the meniscus. Just posterior to the IT band, the underlying capsule divides into 2 laminae. The more superficial, the original capsule, encompasses the LCL and ends posteriorly at the fabellofibular ligament. B6 When a fabella is present, the fabellofibular ligament is found coursing parGOLDBLATT AND RICHMOND

3ralis ntem'~,~Sr;uiar ;uiar septum

~riOt 9enicut~te a, z and lateral head of 'ocltem~u$

biccp ,ion~

suprapatella~ r pouch

*sho

Fig 5. Structures of the lateral side of the knee. The figure on the left shows the major structures of Layer 1. On the right, Layer I is reflected from the lateral margin of the patella showing Layer 2. Reprinted with permission. 36

palella

i|i

pa|oliat reli~'~ac~ ulum with at~achr~er~t$ to: .acr:r,,ssory vsstt~ -!at. intermuscular sep~urn 4al~ila 4tio4ibia! trawl 4aL meniscus -|aL tuberr;le of tibia

\ fat pad ~t ca p.~ule

allet to the LCL from the fabella to the fibula to insert posterior to the insertion of the biceps tendon. If also present, the short lateral ligament runs adjacent to the lateral limb of the arcuate ligament from the femoral condylar origin of the lateral head of the gastrocnemius to the fibula.35 The deeper lamina runs along the edge of the lateral meniscus, forming the coronary ligament and the hiatus for the popliteus tendon, and terminates posteriorly at the Y-shaped arcuate ligament. The arcuate ligament spans the junction between the popliteus muscle and its tendon from the fibula to the femur. The popliteus tendon passes through the hiatus in the coronary ligament to attach to the femur, anterior and distal to the attachment of the LCL. The inferior lateral genicular artery runs in the space between the 2 laminae. B6 The final component of layer III is the popliteofibular ligament, which is found deep to the lateral limb of the arcuate ligament. The popliteofibular ligament arises from the posterior part of the fibula, posterior to the biceps insertion, and joins the popliteus at the musculotendinous junction. The popliteus muscle-tendon unit, therefore, is a Y-shaped structure with a muscle origin from the posterior part of the tibia, a ligamentous origin from the fibula, and a united insertion on the femur. 35 ILIOTIBIAL (IT) B A N D Anatomy

The IT tract is formed proximally at the level of the greater trochanter by the coalescence of fascial investments of the tensor fascia lata, gluteus maximus, and gluteus medius. It then attaches to the linea aspera of the femur through the lateral intermuscular septum. At the knee, it separates into 2 functional components: the iliotibial tract, and iliopatellar band. 47 The IT tract is divided into layers. The anterior portion of the superficial IT tract attaches to Gerdy's tubercle. A deep layer begins 5 cm proximal to the lateral epicondyle, ANATOMY AND BIOMECHANICS OF THE KNEE

separate from the superficial, and continues in the coronal plane to the lateral intermuscular septum of the distal femur. The final layer of the IT tract, the capsuloosseous layer, begins proximally as the lateral investing fascia of the lateral gastrocnemius tendon and the medial investing fascia of the short head of the biceps. Distally, the capsuloosseous, deep, and superficial layers of the tract merge to insert on the lateral tibial tuberosity, just posterior and proximal to Gerdy's tubercle. 4s The iliopatellar component of the IT tract connects the anterior aspect of the IT tract to the patella. The iliopatellar band is layered in a similar fashion to the IT tract. At the insertion into the patella, these layers are indistinct. However, on the femoral side, the layers are separate, and the capsuloosseous layer is seen to arise from a bony connection to the supracondylar process. This layer serves as the femoropatellar ligament, and travels parallel to the oblique fibers of the vastus lateralis. The iliopatellar band functions to resist a medially directed force to the patella, and is dynamically influenced by the vastus lateralis. 47 Biomechanics

The IT tract is an important stabilizer of the lateral compartment, and is instrumental in preventing varus opening of the knee. 49 In full extension, the IT tract may act as an extensor as well as static stabilizer. 13,26 As the knee flexes, the IT band tightens and moves posteriorly. The biceps fascial communication aids in maintaining tension over the range of motion. The IT tract, therefore, exerts a posteriorly directed and external rotation force on the lateral tibia. The tract is tightest at 10° to 30 ° of flexion and, therefore, may be most vulnerable to injury at this position. 38,5° Beyond 40 °, the tract becomes a flexor of the krtee. 26 During extension, the IT tract moves anteriorly, and is thus spared in most cases of varus stress and posterolateral injury,ss

181

LATERAL COLLATERAL LIGAMENT (LCL) Anatomy The LCL arises in a fan-like fashion in a fovea immediately posterior to the lateral epicondyle at an average 3.7 m m posterior to the apex of the epicondylar ridge. It is located between the superior fovea for the lateral gastrocnemius and the more distal popliteus. 2,48 The LCL is superficial to the tendon of the popliteus. 5~ The average length of the ligament is reported from 59.2 to 71 ~,2,52,53 and has a minimum diameter at its midpoint, where it is elliptical in shape. The average AP diameter is 3.4 mm, and the average medial to lateral dimension is 2.3 mm. The fibular attachment is into a superior and laterally facing V-shaped plateau on the head of the fibula. 2 The biceps tendon forms a semicircle around the distal rim of this plateau and overlies the LCL attachment, separated from the LCL by a bursa. 2,53 The lateral fibers of the LCL then continue distally, medial to the anterior arm of the biceps, to blend with the superficial fascia of the lateral compartment of the leg. 48 The LCL reinforces the posterolateral one third of the capsule. 38 Along its proximal course, the posterior aspect of the LCL is directly connected to the lateral aponeurotic expansion of the short head of the biceps. 48 The LCL differs from the MCL in that it is separated from the lateral meniscus. 54 The angle the LCL makes with the long axis of the femur changes with flexion. In full extension, the LCL is directed posteriorly, from proximal to distal. As flexion proceeds, this changes to an even greater anteriorly directed angle, due to the posterior translation and internal rotation of the tibia. 2 The LCL passes through vertical at 70 ° flexion, and at this point may provide less restraint to posterolateral rotation of the fibular head. 52

Biomechanics Because the LCL is located posterior to the axis of flexionextension rotation, and the radius of curvature of the lateral condyle decreases during flexion, it is tightest in extension and progressively relaxes with flexion beyond 300.2,45,49,52,54Additionally, posterior translation of the femorotibial contact point and coupled internal rotation of the tibia with flexion contribute to the change in tension in the LCL.2 The LCL appears to remain taut from 0° to 30 ° and, therefore, is most important in resisting varus instability over this range. However, the dynamic effect of the aponeurotic layers of the long and short heads of the biceps femoris provide continuous tension in the LCL.49,5° This may explain how the LCL continues to be a primary restraint to varus at all angles. Appropriate selection of attachment sites during reconstruction of the LCL may not simply be isometric, because the LCL slackens with flexion and, therefore, normally allows tibial external rotation. 2,45,54 Otherwise, a completely isometric graft may compromise this rotation. Tensioning a graft in no more than 30 ° of flexion, close to neutral rotation, is recommended. 2 The LCL is a primary restraint to varus at all positions of f l e x i o n , 25"38'45"49"52"55 and a secondary restraint to external rotation and posterior translation. 38,55 The LCL resists approximately 55% of applied varus load at full extension. 11

182

Ligament integrity can by tested with varus at 0 ° of flexi0n52; however, Veltri 56 showed that injury to the LCL was best demonstrated at 30 ° of flexion. The magnitude of increased varus rotation with isolated injury to the LCL is small (2° to 4°), and may be difficult to detect clinically. Sectioning of the popliteofibular ligament with an intact LCL results in no significant increase in varus rotation from 0° to 30 °, because the LCL is a primary restraint to varus. Yet an increase in varus is seen from 60 ° to 90 ° due to the slackening of the LCL with flexion. Combined sectioning of the LCL and popliteofibular ligament yields a very significant increase in varus rotation at all angles. 55 External rotation increases with isolated sectioning of the LCL at all angles of flexion except 60 °. Isolated sectioning of the LCL results in no change in primary internal rotation, posterior translation, or coupled anterior-posterior translation of the tibia on the femur. 25

POSTEROLATERAL CORNER Anatomy The complexity of the posterolateral corner stems largely from the variable presence of many of the component structures. The structures in this region provide both dynamic and static stability to the knee. The dynamic structures include the iliotibial band, the lateral head of the gastrocnemius, biceps femoris, and popliteus. These components are invariably present. Static structures are much more variable in presentation, and include the lateral collateral ligament, fabellofibular ligament, short lateral ligament, popliteofibular ligament, arcuate ligament, posterolateral capsule, posterior horn of the lateral meniscus, and lateral coronary ligament. The contribution to overall stability of several of these structures is debated. The controversy is due in part to the confusing terminology used to describe individual components. The short lateral ligament, for example, is described with no fewer than 8 names. 57 Many authors note variability of the structures of the posterolateral corner. In the dissections performed by Seebacher, 36 3 anatomic variations were described: 1) the arcuate ligament alone reinforced the capsule, 13%; 2) the fabellofibular ligament alone reinforced the capsule, 20%; and 3) both ligaments reinforced the capsule, 67%. The variable presence of these structures was related to the presence or absence of the fabella. If an osseous fabella was present, then the fabellofibular ligament was robust; if the fabella was absent, then the arcuate ligament was robust; and if the fabella was cartilaginous, then both ligaments were present and diminished in substance. The oblique popliteal ligament is invariably present as a wide band that runs diagonally from the distal tibial insertion of the semimembranosus toward the more proximal femoral origin of the lateral gastrocnemius over the lateral femoral condyle. It is formed from the oblique popliteal expansion of the semimembranosus muscle and the capsular arm of the posterior oblique ligament. 48 The lateral border of the oblique popliteal ligament constitutes the medial arch of the arcuate ligament over the popliteus muscle (Fig 7). 57

GOLDBLATTAND RICHMOND

Fig 7. Posterior knee: 1) popliteus muscle; 2) semimembranosus expansion; 3) posterior-superior popliteomeniscal fascicle; 4) arcuate ligament; and 5) popliteofibular ligament (curved arrow). Reprinted with permission. 39

The arcuate ligament is a complex arrangement of fibers oriented in various directions, and appears to be a consolidation of several anatomic structures. 48 The ligament is comprised of a lateral and medial limb, resulting in a Y-shaped configuration overlying the popliteus muscle at its musculotendinous junction (Fig 7). 35"38'48,51,57 The medial limb, as mentioned above, arises from the posterior part of the capsule at the distal part of the femur, proximal to the joint line, and courses medially to cross the midpoint of the joint at the level of the tibial insertion of the PCL. It terminates into the oblique popliteal ligamenL 35,38,39,48,51The lateral limb is not always distinct, and appears less prominent in older specimens.4S, 57 The lateral limb arises from the posterior part of the capsule, at the level of the superior edge of the posterior horn of the lateral meniscus, to span the junction between the lateral femoral condyle and the posterior fibular styloid. 35,36,38,48It courses laterally over the popliteus muscle and tendon, deep to the lateral inferior geniculate vessels, to insert on the posterior part of the fibula, posterior to the fabellofibular ligament.g5,4s, 5s The lateral limb frequently blends with the posterior capsule to such an extent that it cannot be dissected from the capsule or from the fascia covering the popliteus. 57 The variable presence of the short lateral ligament and the fabellofibular ligament results in significant controversy in the literature. In an effort to clarify the anatomy of these 2 ligaments, Kaplan 57 performed an extensive comparative anatomic dissection of human and nonhuman specimens, in which he defined the fabellofibular ligament as a distinct structure, homologous to the short lateral ligament. The fabellofibular ligament differed from the short lateral ligament in size and relationship to the LCL. ANATOMY AND BIOMECHANICS OF THE KNEE

The short lateral ligament is attached proximally to the posterior aspect of the supracondylar process of the femur, where its fibers blend with the lateral gastrocnemius tendon, 48 and distally to the fibular styloid and medial border of the fibular head. It blends with the capsule deep to the popliteus as a thickening of the capsule. 57 When a fabella is present, a strong ligament is found originating from the fabella, almost equal in size to the LCL and running almost parallel to it, to the fibular head. It inserts posterior to the insertion of the biceps tendon. The LCL inserts anterior to the biceps at the fibular head. The distance between the fabellar origin of this ligament and the femoral origin of the LCL is approximately 2 cm. Of note, the inferior lateral genicular vessels run over the capsule and lateral arch of the arcuate ligament, and pass deep to the LCL. In the presence of the fabellofibular ligament, the vessels run deep to this ligament as well, thus indicating the independence of the fabellofibular ligament from the capsule. The fabellofibular ligament is present whenever a fabella is found, and attenuated or absent when the fabella is absent. A fabella is present in approximately 8% to 16% of knees. In the absence of the fabellofibular ligament, the short lateral ligament represents a homologue of the fabellofibular ligament. 57 The mid-third LCL is a thickening of the lateral capsule of the knee. 48,49The ligament is thought to be semiequivalent to the deep MCL. This thickening extends from the capsular attachments just anterior to the popliteus tendon insertion on the femur to the lateral gastrocnemius attachment. It then extends distally to its tibial attachment, from slightly posterior to Gerdy's tubercle to the popliteus hiatus. The ligament is divided into 2 components. A meniscofemoral component extends from the femur to the meniscus, and a meniscotibial component extends from the meniscus to the tibia. ° The remaining structures of the posterolateral corner (LCL, popliteus complex, IT band) receive individual description in separate sections. Biomechanics Taken as a whole, the structures crossing the posterolateral corner of the knee provide resistance to tibial external rotation, varus rotation, and posterior tibial translation. 25,28,38,49,50,52,55 Various sectioning studies provide insight into the interaction among the ligaments of this region. Combined sectioning of the posterolateral structures, with an intact PCL, result in maximum increased external rotation, varus rotation, and posterior translation at 30 °. At low flexion angles, the bulk of the PCL is lax. When the PCL is sectioned along with these structures, posterior translation, varus, and external rotation further increase at all angles. 5° Intact knees demonstrate increasing external rotation with flexion to 90 ° when subjected to an external rotation moment. On isolated sectioning of the popliteofibular ligament, coupled external rotation increases at all angles, to a maximum of 30 °, at which point posterior translation is also maximum. Combined sectioning of the popliteofibular ligament and LCL yields continued increases in coupled external rotation, at a peak of 30°. 25,50 The LCL is

183

slack with flexion; therefore, the popliteofibular ligament is dominant versus external rotation with knee flexion and, therefore, will fail first with external rotation. 52 As knee flexion progresses, the femoral origin of the popliteus moves upward from the tibial plateau, and the popliteofibular ligament complex maintains a more efficient tension and orientation for resisting tibial external rotation and posterior displacement than the LCL at all knee flexion angles. 52 The LCL is a secondary restraint to coupled external rotation. 25,5° Sectioning the posterolateral structures has no significant effect on the anterior limit of translation when an anteriorly directed force is applied, but it does change the posterior limit. This increase in posterior translation is significant between 0 ° and 45 °, and is accompanied by an increase in external rotation of the tibia. 3° Gollehon 25 demonstrated that isolated sectioning of the deep ligament complex (arcuate ligament, popliteus tendon, fabellofibular ligament, and posterolateral capsule) or the LCL yielded a slight increase in primary varus. Combined sectioning of the posterolateral complex and LCL produced a significant increase in varus and was clinically detectable, especially at 30 ° flexion. The fabellofibular ligament is under greatest tension when the knee is in full extension, but it relaxes with flexion. This ligament, therefore, may have greatest function in extension. 59

POPLITEUS AND POPLITEOFIBULAR LIGAMENT

Anatomy The popliteus muscle and popliteofibular ligament receive much discussion regarding an accurate description of their anatomy. Covey 5° uses the more functional term "popliteus complex" to describe these structures. The complex consists of both a dynamic component (popliteus musculotendinous unit) and a static component (popliteofibular ligament, popliteotibial fascicle, and popliteomeniscal fascicle) .50 The popliteus muscle originates from the posteromedial surface of the proximal 10 to 12 cm of the tibial metaphysis (Fig 8). 60 The direct expansion of the semimembranosus blends into the popliteus muscle fascia. The medial part of the popliteus muscle inserts into the posterior horn of the lateral meniscus and capsule via the superior popliteomeniscal fascicle. The lateral part of the muscle joins the arcuate ligament superficially, as well as deep connecting fibers from the fibula, to form the musculotendinous junction. 39 The tendon then passes through a hiatus in the coronary ligament, reinforces the posterior one third of the lateral capsule, and then crosses under the LCL to insert on the lateral femoral condyle. The insertion is crescent shaped, near the articular cartilage border, 3 to 5 mm above the superior aspect of the lateral meniscus, distal and anterior to the LCL. 38,39,59In extension, the insertion of the tendon into the femoral condyle creates a sinusoidal cartilage indentation at the border of the lateral femoral condyle called the sulcus statarius of Furst. In flexion, the popliteus tendon gradually slides into the sulcus. 39 The popliteus muscle is invariably described with little controversy; however, the popliteofibular ligament is only 184

Fig 8. Popliteus complex. The asterisk represents the popliteofibular ligament. Reprinted with permission, ss

more recently receiving significant appreciation. The popliteofibular ligament is a stout ligamentous structure, with an average length of 42.6 mm. The average cross-sectional area of the popliteofibular ligament measured 6.9 mm 2, compared with 7.2 mm 2 for the LCL and 13.7 mm 2 for the popliteus tendon. It descends from the musculotendinous junction of the popliteus to the posterosuperior prominence of the fibular head, adjacent to the insertion of the LCL, partially covered by the LCL, and deep to the lateral limb of the arcuate ligament (Fig 8). The fiber orientation of the popliteofibular ligament is oblique in its proximal third, where it fuses with the fibers of the popliteus tendon. In its distal two thirds, the fibers are oriented more vertically, similar in orientation to the LCL. 38,52,55,58,61 The ligament is comprised of 2 fascicles. The anterior fascicle originates from the tibia and anterior aspect of the fibular head, adjacent to the tibiofibular joint capsule. The posterior fascicle originates from the posterior aspect of the fibular head, adjacent to the posterior tibiofibular joint capsule. The 2 fascicles join in an inverted-Y pattern before insertion into the popliteus tendon and the inferior popliteomeniscal fascicle. This description is equivalent to the description by Seebacher 36 of the innermost lamina of the arcuate ligament (spanning the junction between the popliteus muscle and its tendon from the fibula to the femur). 39,61 The ligament acts as a pulley fixing the popliteus tendon during contraction, 51 and because it undergoes no significant length change during knee flexion, it becomes dominant with knee flexion secondary to the slackening of the LCL. 52 The biceps insertion is anterior to the popliteofibular ligament on the fibula, 38,52,55 which is important when considering the biceps to reconstruct the ligament, s5 The combined anatomic dissection and videoarthroscopic evaluation conducted by Staubli 39 of 175 knees taken through a range of motion and subjected to rotational moments, established the relationship of 2 addiGOLDBLATT AND RICHMOND

tional supporting structures arising from the popliteus tendon: the inferior and superior popliteomeniscal fascicles. The anteriorly located inferior fascicle blended into the middle segment of the lateral meniscus to form the floor of the popliteal hiatus. The posteriorly located superior fascicle blended into the posterior horn of the lateral meniscus to create the roof of the popliteal hiatus. The medial excursion of the lateral meniscus with varus was controlled by the popliteomeniscal fascicles and the popliteofibular ligament, and the lateral excursion of the meniscus with valgus was limited by the popliteus tendon.

Biomechanics The popliteus complex acts as a dynamic internal rotator of the tibia and as a static restraint to posterior tibial translation, varus rotation, and primary and combined external rotation of the tibia on the femur. 28,38,49,50,52,55 The popliteus appears to function both statically and dynamically to prevent external rotation, rather than merely acting as an active internal rotator of the tibia. It is the only major structure positioned at an oblique angle in the posterolateral corner of the knee, and thus is well suited to prevent tibial external rotation during flexion from 20 ° to 130 °, as well as varus from 0 ° to 90o.50 When compared with other components of the deep-ligament complex, sectioning of the popliteus results in significant increases in external rotation at 90 ° of flexion.25 The tendon appears to be composed of 2 fiber bundles. The anterior fibers become tense in flexion, and the posterior fibers tense in extension. This phenomenon is exaggerated by the addition of external rotation. 59 The popliteus is actively responsible for the screw home mechanism (tibial external rotation) during terminal extension. 55 In addition, the popliteus initiates flexion by unscrewing the locked, extended knee, and retracts the lateral meniscus to prevent its impaction by the femur. The popliteus is the only muscle that has a rotation effect on the j o i n t . 13,37,54 Once unlocked, the hamstrings can function to flex the knee. ls A major contribution to the static function of the complex is provided by the popliteofibular ligament. Several investigators have demonstrated that the popliteofibular ligament is a primary restraint to posterior translation, varus rotation, and primary and coupled external rotation. 39,52,55,56 Because the popliteofibular ligament is dominant, and well aligned to resist tibial external rotation at all angles of flexion, increased external rotation with testing at any angle indicates damage to the popliteofibular ligament. 52 Its role in preventing posterior translation is greatest at 30 ° of flexion, secondary to decreased effectiveness of the PCL in this position, ss

CONCLUSIONS The information presented represents a brief outline of the anatomy of the structures of the knee. The separate discussion of the biomechanics of these structures is important for the surgeon who is attempting to recreate the missing function in the injured knee. Accurate clinical evaluation is based on the assessment of changes from normal motion. Successful reconstruction requires precise ANATOMY AND BIOMECHANICS OF THE KNEE

anatomical graft placement, as well as tensioning in the appropriate degree of knee flexion and rotation. This review is intended to assist with diagnosis and surgical intervention.

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