The tibiofemoral and patellofemoral joints share a common capsule that has a complicated shape. The capsule is attached above to the femur, below to the tibia, and anteriorly to the patella (figure 6.13). To accommodate the extremes of full flexion and full extension of the tibiofemoral joint, the anterior part of the capsule is folded upward during extension and the posterior part of the capsule is folded downward during flexion. The upward fold, only present in extension, is referred to as the suprapatellar bursa (figure 6.13). The suprapatellar bursa intervenes between the quadriceps tendon and the anterior aspect of the femur just proximal to the patellar surface.
The downward fold, only present during flexion, is referred to as the gastrocnemius bursa. The gastrocnemius bursa intervenes between the posterior aspect of the tibial condyles and the gastrocnemius muscle. In general, a bursa is a flattened sac of synovial membrane containing synovial fluid. Bursas minimize friction between structures that slide across each other during normal movement (see chapter 7).
During flexion of the tibiofemoral joint, the suprapatellar bursa becomes progressively smaller and the gastrocnemius bursa becomes progressively larger as synovial fluid is redistributed from the suprapatellar bursa to the gastrocnemius bursa. The movement of synovial fluid is reversed during extension of the tibiofemoral joint; that is, the gastrocnemius bursa becomes progressively smaller and the suprapatellar bursa becomes progressively larger.
The suprapatellar bursa is separated from the femur by a pad of fat called the suprapatellar fat pad. Lying in the space bounded by the upper two thirds of the posterior aspect of the patellar ligament, the anterior intercondylar area of the tibia, and the anterior inferior aspect of the articular surface of the femoral condyles is a pad of fat called the infrapatellar fat pad. It is roughly triangular in sagittal cross section and is suspended superiorly from a fibro-adipose band called the infrapatellar fold. The infrapatellar fold is attached anteriorly to the inferior pole of the patella and posteriorly to the anterior border of the intercondylar notch (figure 6.13). The infrapatellar fat pad is attached anteriorly to the posterior aspect of the patellar ligament and extends at both sides of the patellar ligament. The infrapatellar fat pad also extends narrow branches halfway up each side of the patella; the branches are referred to as alar folds (alar = winglike). The infrapatellar fat pad cushions the patellar ligament and lower part of the patella during movements of the knee.