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chapter 1: normal anatomy

Soft Tissue Stabilizers

The patella is the central pole, the crossroad for converging retinacular elements: ligament, muscle, tendon, and capsule. (Fig. 1.12) of the extensive synovial expansion. The patellofemoral articulation cannot tolerate a tightly closed capsular cuff which explains why the capsule, often poorly defined, does not play its habitual stabilizing role. By contrast, the stabilizer system, comprising ligaments and tendons, is remarkably effective. The patella is solidly anchored to the knee, both in the transverse and in the longitudinal directions, by a cruciform soft tissue system, in which it is possible to distinguish both active and passive elements.

Passive Stabilizers

Inferiorly, the patellar tendon limits the proximal ascent of the patella from the tibia. This flattened tendon is 3 cm broad at its insertion into the apex of the patella and 2.5 cm wide at its insertion into the tibial tubercle. It is 5 to 6 cm long and 7 mm thick. Its orientation is roughly in the long axis of the lower extremity, but often slightly oblique laterally from proximal to distal, which adds to the tendency toward lateral displacement of the patella. Components of the peripatellar retinaculum interdigitate with the patellar tendon medially and laterally.

The lateral peripatellar retinaculum (26) is comprised of two major components, the superficial oblique retinaculum and the deep transverse retinaculum. The superficial oblique retinaculum is rather thin and runs superficially from the iliotibial band to the patella, as shown in Figures 1.13 and 1.14, A. On reflection of the superficial oblique retinaculum, there is a much denser deep transverse retinaculum (Fig. 1.14, B), which is comprised of three major components. The epicondylopatellar band portion, also known as the lateral patellofemoral ligament, was originally described by Kaplan (27). It provides superolateral static support for the patella, and its width correlates with patellar shape (28). The midportion of the deep transverse retinaculum courses directly from the iliotibial band to the patella and has a dense, fibrous attachment to the patella. This is the primary support structure for the lateral patella. There is also another band, the patellotibial band, which provides some distal inferior support for the lateral patella. This band has also been called the lateral meniscopatellar ligament, but is probably more appropriately described by its insertion on the tibia as the patellotibial band. Upon flexion of the knee, these lateral retinacular bands are drawn posteriorly along with the iliotibial band placing a lateral displacement force on the patella that would cause progressive tilting of the patella except that the medial stabilizers are also tightening and provide a counteracting force. When the medial stabilizers have been stretched, however, the static lateral stabilizers will create lateral tilt and displacement of the patella, which may lead to subluxation, excessive tilt, dislocation, or excessive lateral pressure syndrome. Balance between the medial and lateral static stabilizers, therefore, is very important in maintaining appropriate alignment of the extensor mechanism within the femoral trochlea. There is stronger retinacular support laterally than medially.

Medially, capsular condensations form a tough fibrous layer that inserts into the superior two‑thirds of the posterior part of the medial border of the patella. This medial patellofemoral ligament links the patella to the medial femoral epicondyle and passively limits lateral patellar excursion. Inferiorly, the medial meniscopatellar ligament inserts into the inferior third of the medial border of the patella, connecting the patella to the anterior part of the medial meniscus. This ligament is buried in the margins of the fat pad but can be palpated when the patella is put under load.

Above the patella is the central quadriceps tendon expansion of the quadriceps muscle (29). This thick (> 9 mm) band of rectus femoris and vastus intermedius tendons can become inflamed or overused like any other tendon. It can be a source of anterior knee pain. This region also provides plentiful tendon graft for cruciate ligament reconstruction. There is much more fibrocartilage at the central quadriceps tendon insertion than at the patellar tendon insertion into the tibia (30), possibly related to the greater absolute size of the central quadriceps tendon compared with the patellar tendon (Fig. 1.15).

The patellar tendon, the central quadriceps tendon, and the medial and lateral retinacula are the passive elements of soft tissue stabilization. They, along with the bony confines of the trochlea and the active stabilizers, define the limits of patellar excursion. Strictly speaking, however, both the medial and lateral retinacula are partially affected by active stabilizers to some extent. Most of the lateral retinaculum, for instance, originates in the iliotibial band that provides both active and passive stabilization of the patella.

Active Stabilizers

The four main muscular elements of the quadriceps fuse distally into the quadriceps tendon, which can still be identified as three separate layers at their insertion into the patella (Fig. 1.16). Superficially, the rectus femoris inserts into the anterior portion of the top of the patella, as well as the superior third of the anterior surface. The most superficial fibers continue over the anterior surface of the patella to operate a continuous bridge of tough fibrous tissue ending in the patellar tendon. These represent a direct tibial insertion of the quadriceps. In the midportion, the vasti medialis and lateralis unite in the midline to form a solid aponeurosis that inserts into the base of the patella just posterior to the insertion of the rectus femoris. The vasti insertions also continue medially and laterally into their respective borders of the patella. Medially, the muscle fibers and fibrous insertion descend more distally than on the lateral side. Koskinen and Kujala, however, have noted more proximal insertion of the vastus medialis in patients with patellar dislocation (31). The vastus intermedius inserts via a broad, thin tendon into the base of the patella, posterior to the other vasti, but anterior to the capsule. Medially and laterally, these insertions reinforce the respective patellofemoral ligaments.

There is some evidence (32) that the rectus femoris may initiate extension of the knee, but contributions from the vastus medialis and lateralis become important only during the latter part of knee extension. This work has suggested independent functioning of different quadriceps components and has supported rehabilitation efforts that emphasize selective strengthening of the vastus medialis to increase dynamic medial support of the patella.

Although the presence of a vastus medialis obliquus has been well accepted by most orthopedic surgeons, little attention was paid to the vastus lateralis insertion into the patella until description of the vastus lateralis obliquus in 1987 by Hallisey et al (33). In this study, it was noted that there is an anatomically distinct group of vastus lateralis fibers separated from the main belly of the vastus lateralis by a thin layer of fat. This muscle group interdigitates with the lateral intermuscular septum before inserting into the patella. Vastus lateralis obliquus fibers are important because they provide a direct lateral pull on the extensor mechanism by virtue of interdigitation with the lateral intermuscular septum. In addition, this relatively small muscle may be released at the time of lateral retinacular release without detaching the main vastus lateralis tendon. Lateral dynamic support of the patella, then, is very important in understanding patellar balance in the trochlea. Figure 1.17, A and B illustrates the vastus lateralis obliquus photographically and diagrammatically.


        

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