The articular cavity of the knee joint is the largest joint space of the body. The cavity includes a space between and around the tibial and femoral condyles but also extend upwards behind the patella to include the patello-femoral articulation and further into the supra-patellar bursa which lies between the tendon of the quadriceps femoris muscle and the femur.
The synovial membrane lines the articular capsule and reflects onto the bone as far as the edges of the articular cartilage. It also lines the supra-patellar bursa and may also line any other bursas that communicate with the knee joint. The synovial membrane also covers the cruciate ligament except where the PCL is attached to the back of the capsule. The anterior cruciate ligament is therefore an intra-articular structure which is within the synovial cavity of the knee joint whereas the posterior cruciate ligament is an intra-articular structure which is outside the synovial cavity of the knee joint.
The infra-patellar fat pad which lies below the patella represents an anterior section of the median septum of tissues with the cruciate ligaments separating the two tibio-femoral articulations into medial and lateral compartments. From the synovial surface of the infra-patellar fat pad, a vertical fold frequently passes towards the cruciate ligament and attaches to the intercondylar fossa of the femur, anterior to the ACL and lateral to the PCL. This is called a ligamentum mucosum.
The numerous folds and recesses within the knee are potential sites for collection of wear debris, loose bodies and bacterial contamination. All these recesses need to be assessed arthroscopically in these situations.
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