Medial and Anterior Knee Anatomy

Layer 1: Deep Fascia

Layer 1 (see Fig. 1-1 ) consists of the deep fascia that extends proximally to invest the quadriceps, posteriorly to invest the two heads of the gastrocnemius and cover the popliteal fossa, and distally to involve the sartorius muscle and sartorial fascia. Anteriorly, layer 1 blends with the anterior part of layer 2 approximately 2 cm anterior to the SMCL. Inferiorly, the deep fascia continues as the investing fascia of the sartorius and attaches to the periosteum of the tibia. Layers 1 and 2 are always distinct at the level of the SMCL unless extensive scarring has occurred. The gracilis and semitendinosus tendons are discrete structures that lie between layers 1 and 2 and are easily separated from these two layers. However, according to Warren and Marshall, these tendons will occasionally blend with the fibers in layer 1 anteriorly before they insert onto the tibia. As depicted in Figure 1-4 , dissections and clinical experience of the authors concur in that there is a blending of layer 1 with a confluence of the semitendinosus and gracilis tendons at their common insertion onto the tibia; however, they are easily found as discrete structures more posteriorly. Thus, we recommend that when attempting to harvest the semitendinosus and gracilis tendons for an anterior cruciate ligament reconstruction, these tendons initially be identified 2 to 3 cm posterior and medial to the anterior tibial spine. This will allow for easier visualization of the tendons, which can then be traced to their insertions on the anterior tibia to allow for maximal tendon length at the time of harvest.

Layer 2: Superficial Medial Collateral Ligament and Posterior Oblique Ligament

The SMCL is a well-defined structure that spans the medial joint line from the femur to tibia. According to LaPrade and coworkers, the SMCL does not attach directly to the medial epicondyle of the femur, but is centered in a depression 4.8 mm posterior and 3.2 mm proximal to the medial epicondyle center. Other studies have described the MCL attaching directly to the medial epicondyle of the femur. The confusion lies in the confluence of fibers that reside in the area of the medial epicondyle that make it difficult to identify the precise attachment site of the SMCL. As shown in Figure 1-5 the authors agree with LaPrade and coworkers that the main fibers of the SMCL attach to an area just posterior and proximal to the medial epicondyle; but the origin of the SMCL is rather broad and, therefore, there are also superficial fibrous strands attaching anterior on the medial epicondyle and posterior in a depression on the medial femoral condyle.

The posterior fibers of the SMCL overlying the medial joint line, both above and below the joint, change orientation from vertical to a more oblique pattern that forms a triangular structure with its apex posterior, eventually blending with the fibers of the posterior oblique ligament (POL) ( Fig. 1-6 ). LaPrade and coworkers described two anatomic attachment sites of the SMCL on the tibia. The first is located proximally at the medial joint line and consists mainly of soft tissue connections over the anterior arm of the semimembranosus. The second attachment site is further distal on the tibia, attaching directly to bone an average of 61.2 mm from the medial joint line. In the authors' experience, there is a consistent attachment of the proximal portion of the SMCL to the soft tissues surrounding the anterior arm of the semimembranosus, but a discrete attachment to bone is found only distally (see Fig. 1-6 ).

The gracilis and semitendinosus lie between layers 1 and 2 at the knee joint. The sartorius drapes across the anterior thigh and into the medial aspect of the knee invested in the sartorial fascia in layer 1. The insertion of the sartorius, as described by Warren and Marshall, consists of a network of fascial fibers connecting to bone on the medial side of the tibia, but does not appear to have a distinct tendon of insertion such as the underlying gracilis and semitendinosus. However, LaPrade and coworkers located the gracilis and semitendinosus tendons on the deep surface of the superficial fascial layer, with each of the three tendons attaching in a linear orientation at the lateral edge of the pes anserine bursa.

In our experience, the sartorial fascia has a broad insertion onto the anteromedial border of the tibia and, with sharp dissection at its insertion, the underlying distinct tendons of the gracilis and semitendinosus are easily visualized ( see Fig. 1-4 ). At the level of the joint, layers 1 and 2 are easily separated from one another over the SMCL. However, farther anteriorly, layer 1 blends with the anterior part of layer 2 along a vertical line 1 to 2 cm anterior to the SMCL.

Also within layer 2 is the MPFL, which courses from the medial femoral condyle to its attachment onto the medial border of the patella. This is a flat, fan-shaped structure that is larger at its patellar attachment than its femoral origin, with a length averaging 58.3 mm (47.2-70.0 mm). Controversy exists regarding where the MPFL attaches at the medial femoral condyle. Mochizuki and coworkers described the MPFL as a fan-shaped structure with proximal fibers extending to the medial margin of the vastus intermedius, and distal fibers interdigitating with the medial retinaculum without a distinct attachment to the vastus medialis. LaPrade and coworkers noted that the MPFL attaches primarily to soft tissues between the attachments of the adductor magnus tendon and the SMCL, with an attachment to bone 10.6 mm proximal and 8.8 mm posterior to the medial epicondyle. Steensen and associates, from a dissection of 11 knees, believe the MPFL attaches along the entire length of the anterior aspect of the medial epicondyle. Smirk and Morris describe a variable origination of the MPFL on the femur. In dissections of 25 cadavers, the MPFL attached solely to the posterior aspect of the medial epicondyle, approximately 1 cm distal to the adductor tubercle in 44% of specimens. The adductor tubercle was included in the origin in 4%, the adductor magnus tendon in 12%, the area posterior to the adductor magnus tendon in 20%, and a combination of these in 4%. In 16% of the specimens, the MPFL attached anterior to the medial epicondyle. Fulkerson and Edgar described a distinct attachment of the MPFL to the medial quadriceps tendon and named this structure the medial quadriceps tendon–femoral ligament. Kang and associates described the femoral attachment point for MPFL fibers as two “relatively concentrated fiber bundles.” The authors identified an “inferior straight” bundle (what is commonly referred to as the MPFL ) that was the main static soft tissue restraint. The superior-oblique bundle was attached and associated with the vastus medialis obliquus (VMO) and was identified as a dynamic soft tissue restraint. The authors acknowledged that the two bundles were not entirely separable.

In our experience, the MPFL attaches in a depression posterior to the medial epicondyle and blends with the insertion of the SMCL ( Fig. 1-7 ). The anterior attachment of the MPFL consists of both attachments to the undersurface of the VMO and the proximal medial border of the patella. The work of Steensen and associates demonstrated that the VMO does not overlap the MPFL, with the exception in 3 of 11 knees in which only 5% of the width of the MPFL was deep to the VMO. However, LaPrade and coworkers reported that the distal border of the VMO attaches along the majority of the proximal edge of the MPFL before inserting onto the superomedial border of the patella. The midpoint of the MPFL attachment is located 41% of the length from the proximal tip of the patella along the total patellar length. Our experience is that the MPFL attaches to the proximal third of the patella, with the majority of the ligament connected to the distal portion of the VMO with fibrous bands (see Fig. 1-7 ).

The adductor magnus and medial gastrocnemius tendons also contribute to the medial anatomy of the knee; both attach on the medial femoral condyle. Similar to the SMCL attachment, the confluence of fibers over the medial femoral condyle makes it difficult to precisely identify the exact location of each attachment ( Fig. 1-8 ). The adductor magnus tendon is a well-defined structure attaching just superior and posterior to the medial epicondyle near the adductor tubercle. LaPrade and coworkers reported the adductor magnus does not attach directly to the adductor tubercle, but rather to a depression located an average of 3.0 mm posterior and 2.7 mm proximal to the adductor tubercle. The adductor magnus also has fascial attachments to the capsular portion of the POL and medial head of the gastrocnemius.

The medial gastrocnemius tendon inserts in a confluence of fibers in an area between the adductor magnus insertion and the insertion of the SMCL ( Fig. 1-9 A ). LaPrade and coworkers described a gastrocnemius tubercle on the medial femoral condyle in this region; however, these authors state that the tendon does not attach to the tubercle, but to a depression just proximal and posterior to the tubercle. In addition, fascial expansions from the lateral aspect of the medial gastrocnemius tendon form a confluence of fibers with the distal extent of the adductor magnus tendon in addition to the capsular arm of the POL (see Fig. 1-9 A ).

Layers 2 and 3 blend together in the posteromedial corner of the knee along with additional fibers that extend from the semimembranosus tendon and sheath that form the posteromedial capsule (see Fig. 1-9 ). LaPrade and coworkers used the term posterior oblique ligament (POL) for this same structure and described each of the three fascial attachments similar to Hughston and colleagues' original description. The superficial arm of the POL runs parallel to both the more anterior SMCL and the more posterior distal expansion of the semimembranosus. Proximally, the superficial arm blends with the central arm; distally, it blends with the distal expansion of the semimembranosus as it attaches to the tibia.

The central arm is the largest and thickest portion of the POL, running posterior to both the superficial arm of the POL and SMCL. It courses from the distal portion of the semimembranosus and is a fascial reinforcement of the meniscofemoral and meniscotibial portions of the posteromedial capsule. LaPrade and coworkers noted that this structure has a thick attachment to the medial meniscus. As the central arm courses along the posteromedial aspect of the joint, it merges with the posterior fibers of the SMCL and can be differentiated from the SMCL by the different directions of the individual fibers. The distal attachment of the central arm is primarily to the posteromedial portion of the medial meniscus, the meniscotibial portion of the capsule, and the posteromedial tibia.

The capsular portion of the POL is thinner than the other portions of this structure and fans out in the space between the central arm and the distal portions of the semimembranosus tendon. The capsular portion blends posteriorly with the posteromedial capsule of the knee and the medial aspect of the oblique popliteal ligament (OPL). It attaches proximally to the fibrous bands of the medial gastrocnemius tendon and fascial expansions of the adductor magnus tendon, with no osseous attachment identified.

The superficial portion of the POL is rather thin and appears to represent a confluence of fibers from the SMCL and the semimembranosus more distally. The capsular portion appears to represent a confluence of fibers from the semimembranosus, adductor magnus, and medial gastrocnemius (see Fig. 1-9 ). The central arm appears more robust, having contributions from the semimembranosus and medial gastrocnemius.

Controversy remains on whether three separate distinct anatomic structures make up the POL. Other authors have not found three distinct structures and note that with tibial rotation, different portions of the posteromedial capsule appear under tension but are not anatomically separate structures.

Semimembranosus.

Controversy exists with respect to the exact number of attachments of the semimembranosus tendon at the knee joint. However, it appears that three major attachments have been consistently identified. The common semimembranosus tendon bifurcates into a direct and anterior arm just distal to the joint line. LaPrade and coworkers described the direct arm attaching to an osseous prominence called the tuberculum tendinis , approximately 11 mm distal to the joint line on the posteromedial aspect of the tibia. These authors also note a minor attachment of the direct arm that extends to the medial coronary ligament along the posterior horn of the medial meniscus (see Fig. 1-6 ). A thinning of the capsule or capsular defect may be identified just distal to the femoral attachment of the medial head of the gastrocnemius and proximal to the direct arm of the semimembranosus. This is often the site of the formation of a Baker cyst.

Warren and Marshall believed the semimembranosus tendon sheath and not the tendon itself extends distally over the popliteus muscle and inserts directly into the posteromedial aspect of the tibia, with some fibers coalescing with SMCL fibers inserting in the same region. These authors contend that these fibers do not have functional significance, because no change was found in position or tension of the MCL when those fibers were transected. LaPrade and associates separated the distal tibial expansion into a medial and lateral division. Both divisions originating on the coronary ligament of the posterior horn of the medial meniscus are located on either side of the direct arm of the semimembranosus. The divisions then course distally to cover the posterior aspect of the popliteus muscle and insert onto the posteromedial aspect of the tibia, forming an inverted triangle in appearance. These authors noted the medial division attaches just posterior to the SMCL, whose fibers coalesce with the superficial arm of the POL (as previously noted by Hughston and colleagues ) rather than the MCL.

In our experience, as shown in Figure 1-10 , the semimembranosus tendon sheath and not the tendon itself comprises the distal tibial expansion, which includes a medial and lateral division with a central raphae separating the two. The anterior arm of the semimembranosus courses deep to the SMCL and attaches directly to bone just distal to the medial joint capsule on the tibia ( Fig. 1-11 ). There are fibrous connections between the SMCL and the anterior arm of the semimembranosus, but only the anterior arm of the semimembranosus has an osseous attachment in this region. Because both the direct and anterior arms of the semimembranosus anchor directly to bone and attach distal to the tibial margin of the medial joint capsule, they are not considered part of either layer 2 or layer 3 as described by Warren and Marshall.

The third major attachment of the semimembranosus is the OPL. Warren and Marshall described the semimembranosus tendon sheath forming fiber tracts that make up the OPL, although they admit some collagen fibers may come from the tendon itself. LaPrade and associates described a lateral expansion off the common semimembranosus tendon, just proximal to its bifurcation into the direct and anterior arms, that coalesces to form a portion of the OPL, in addition to the capsular arm of the POL. As shown in Figure 1-12 , it is difficult to appreciate distinct structures comprising the origin of the OPL because of the significant confluence of fibers in the region. However, there are fibers originating from both the semimembranosus tendon and its sheath that contribute to its origin.

The OPL is described as a broad fascial band that courses laterally and proximally across the posterior capsule. LaPrade and associates noted two distinct lateral attachments of the OPL (proximal and distal). The proximal attachment is broad, extending to the fabella, the posterolateral capsule, and the plantaris (see Fig. 1-12 ). It does not attach directly to the lateral femoral condyle. The distal attachment is on the posterolateral aspect of the tibia, just distal to the posterior root of the lateral meniscus, but not directly attaching to the lateral meniscus as described by Kim and coworkers. It is theorized that this may serve a functional role limiting hyperextension, but this has not been demonstrated in any biomechanic study to date.

LaPrade and associates also described a proximal capsular arm of the semimembranosus as a thin aponeurosis that traverses medially to laterally along the superior border of the OPL. As it courses laterally, it blends with the posterolateral capsule and inserts on the distal lateral femur just proximal to the capsular insertion while at the same time extending fibers to the short head of the biceps femoris tendon (see Fig. 1-12 B and C ).

Layer 3: Deep Medial Collateral Ligament and Knee Capsule

The capsule of the knee joint is thin anteriorly and envelops the fat pad. In this area, the capsule is easily separated from the overlying superficial retinaculum until it reaches the margin of the patella, where it is difficult to separate the capsule from the overlying superficial structures. Under the SMCL lies a vertical thickening of the knee capsule known as the distal medial collateral ligament (DMCL). The DMCL crosses the joint from the distal femur to the medial meniscus and inserts into the proximal tibia at sites adjacent to the articular surfaces of the femur and tibia. These separate divisions of the DMCL are named the meniscofemoral and meniscotibial ligaments. Warren and Marshall noted the meniscofemoral portion of the DMCL had a discrete attachment onto the distal femur at its articular margin. Similarly, the meniscotibial portion of the DMCL, also known as the coronary ligament , is easily separated from the overlying SMCL in layer 2 before attaching to the tibia at its articular margin ( Fig. 1-13 ).

The deepest structure on the medial side is the capsule of the knee, which envelops the entire joint and extends proximally up to the suprapatellar pouch and distally to the attachment site of the meniscotibial ligament on the tibia-articular cartilage border.