chapter 1: normal anatomy
Synovium of the Patellofemoral Joint
The synovial limits of the patellofemoral joint are essentially the synovium of the anterior portion of the knee and consist of the suprapatellar pouch, the middle portion or peripatellar part, including the lateral recesses and the inferior portion or the infrapatellar fat pad.
Although this pouch can remain isolated and independent of the synovial cavity, there is generally a wide communication with the knee joint proper (17). The proximal extent is variable, but averages 4 to 5 cm from the proximal articular border of the femur. The synovium covers the anterior surfaces of the femur from which it is separated by the prefemoral fat pad. Anteriorly, the synovial pouch is covered by the extensor apparatus. The synovium is densely adherent to the central cartilaginous insertion of the quadriceps femoris distally, but separated from the medial and lateral vasti by a small quantity of fat. The superior reaches of the sac receive the insertion of aberrant fibers of the vasti, referred to as the tensor synovialis. These anatomical facts become important at the time of synovectomy. The distal aspect of the pouch usually communicates widely with the synovial cavity of the knee joint proper. This communication is marked by vestigial fibrous remnants of the embryonic diaphragm that separate the two cavities and more or less involute in the course of the development of the knee. This fibrous ring, the plica synovialis suprapatellaris, is particularly noticeable at the sides and, as it approaches the lateral aspects of the patella, approximately 1.5 cm above the base.
Here the synovium blends imperceptibly superiorly with the suprapatellar pouch and laterally and medially into the respective recesses. A small synovial fold or fringe less than 1 cm broad surrounds the patella. This is generally less evident at the level of the quadriceps femoris insertion into the patella proximally, but is a definite structure laterally, medially, and inferiorly. This middle synovial region represents the true synovium of the patellofemoral joint. Medially and laterally where the synovium is reflected from the respective condylar coverings to the undersurface of the quadriceps expansion, a synovial fold can be palpated to appreciate synovial thickness in disease. This area of synovium is not only the most exposed but also the most accessible. Both the lateral and medial recesses decrease in height from anterior to posterior to become continuous with the suprameniscal synovium. Medially, there is a plica synovialis mediopatellaris (18), a synovial pleat starting from the medial alar fold and running toward the medial part of the plica synovialis suprapatellaris (Fig. 1.11). This may become apparent during arthroscopy and can, in certain cases, produce symptoms similar to those in a patient with patellar articular cartilage degeneration. Hughston and Andrews (19) described plical inflammation in association with chondromalacia and Patel (20, 21) later described the arthroscopic anatomy of plicae. He noted three plicae: the infrapatellar plica (or ligamentum mucosum), the medial patellar plica (or shelf), which most often catches under the medial patella, and the suprapatellar plica. Jackson et al (22) and Dandy (23) emphasized the clinical importance of the medial patellar plica (shelf). Broom pointed out that plica irritation is frequently associated with abnormal patella mechanics. In fact, one must consider the possibility that an abnormally prominent medial infrapatella plica could result from chronic patella malalignment developmentally, with excess mesenchymal filling of a medial potential space-later to become a hypertrophic plica (24).
The infrapatellar fat pad is covered by a true synovial layer and, in turn, covers the extra articular portion of the posterior patellar surface. The fat pad extends superiorly to merge with the peripatellar fold on both sides of the patella. The superior extent of the fat pad is often past the midpoint of the articular surface of the patella. Posteriorly, it extends into the ligamentum mucosum, which inserts into the anterior border of the intercondylar notch. This bell shaped ligament, narrow at its femoral attachment, broad as it flows into the fat pad, is delineated medially and laterally by alar folds that merge into the fat pad (Fig. 1.7). Inferiorly, the ligamentum mucosum may form a thin vellum, separating the medial and lateral tibio femoral compartments. In full extension, with the patellar tendon under tension, the fat pad bulges anteriorly on each side of the tendon, giving the false impression of separate medial and lateral pads. This impression is accentuated in patella alta and genu recurvatum, both conditions reducing the space for the fat pad between the extensor apparatus and the femoral condyles. The fat pad may hypertrophy under conditions of recurrent injury and may be the source of symptoms (25).
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