Articular cavity of the knee

The articular cavity of the knee contains several synovial recesses, which are important for the distribution of synovial fluid and the accommodation of joint movements. The primary recesses of the knee include:

1. Suprapatellar Recess: Located above the patella, this recess is an extension of the knee joint cavity and is often the site where effusion is most easily detected, especially in a 30° flexed position.[1]

2. Medial Parapatellar Recess: Situated on the medial side of the patella, this recess can be evaluated for fluid accumulation and is significant in the context of medial knee pathologies.[1]

3. Lateral Parapatellar Recess: Found on the lateral side of the patella, this recess is similarly assessed for effusion and lateral knee conditions.[1]

4. Posterior Cruciate Ligament (PCL) Recess: This recess is located behind the PCL and communicates with the medial femorotibial compartment. It is often distended during knee flexion.[2]It is a synovial recess that extends distally along the popliteus tendon. The SPR is characterized by its length, which averages approximately 22.64 ± 11.38 mm from the upper end of the lateral tibial condyle to the lower end of the depression.[1]

The SPR is closely associated with the popliteus muscle and tendon, and its morphology can be classified into three types based on its relationship with the popliteus muscle. Types 2 and 3, where the SPR extends to the proximal tibiofibular joint, may contribute to knee joint instability.[1] Additionally, the presence of the fabellofibular ligament can significantly increase the length of the SPR.[1]

Communication between the SPR and the proximal tibiofibular joint has been observed in approximately 27.5% of anatomical specimens, typically due to a defect in the posterior ligament of the fibular head.[2] This anatomical feature is important for understanding potential pathways for joint fluid distribution and implications for knee joint stability and injury.

5. Subpopliteal Recess: Extending distally along the popliteus tendon, this recess is located in the posterolateral region of the knee and can be involved in conditions affecting the popliteus muscle and surrounding structures.[3]

6. Infrapatellar (Hoffa's) Fat Pad Recesses: These include the infrahoffatic and suprahoffatic recesses, which are located along the posterior margin of the infrapatellar fat pad and are important in distinguishing between normal anatomical variations and pathological conditions.[4]

7. Posterior femoral recesses (Lateral and medial retrocondylar recesses) As the membrane extends posteriorly from both sides of the femur, recesses are formed between the posterior portion of both femoral condyles and the deep surface of the lateral and medial heads of gastrocnemius. In the midline, the posterior capsular recess may be identified behind the PCL as an extension from the medial femorotibial compartment 

8. Lateral and medial femorotibial recesses :

• The medial femorotibial recess is situated on the medial side of the knee, adjacent to the medial femoral condyle and the medial tibial plateau. It is often involved in the communication with the posterior cruciate ligament recess, as described in the study by de Abreu et al.. This recess can be visualized on MR imaging, particularly when contrast material is injected into the joint, highlighting its communication with the medial compartment of the knee

• The lateral femorotibial recess is located on the lateral side of the knee, adjacent to the lateral femoral condyle and the lateral tibial plateau. This recess is less commonly discussed in the literature but is similarly involved in the knee's synovial membrane dynamics.

9. Perimeniscal recesses: Along the medial and lateral aspects of the capsule, the synovial lining extends inferiorly from the articular margins of the femur as far as the meniscal attachments. The peripheral surfaces of the menisci themselves are not covered by the synovial membrane