Arthroscopic evaluation of anatomical variations of the oblique popliteal ligament and their association with popliteal cyst formation.

Introduction: The posteromedial compartment of the knee houses several important anatomical structures, including the oblique popliteal ligament (OPL), an accessory insertion tendon of the semimembranosus muscle. Popliteal cysts develop from the synovial bursa located between the medial gastrocnemius and the semimembranosus, typically secondary to intra-articular pathologies causing effusion. This study aimed to describe the normal anatomy of the postero-medial capsule of the knee and its anatomical variations, particularly in the presence of popliteal cysts.

Materials and methods: This was a prospective descriptive study conducted over four consecutive months, including 96 knees from 96 patients who underwent arthroscopy for various intra-articular pathologies. The anatomy of the oblique popliteal ligament was categorized into four types: integrated OPL (Type 1), prominent "cord-like" OPL (Type 2A), normally prominent OPL (Type 2B), and bulging OPL (Type 3). The presence of popliteal cysts was evaluated preoperatively using magnetic resonance imaging (MRI).

Results: Type 1 OPL was observed in 13 knees (13.5%), Type 2A in 22 (22.9%), Type 2B in 40 (41.7%), and Type 3 in 21 (21.9%). MRI revealed popliteal cysts in 20 knees (20.8%), of which 8 (8.3%) were symptomatic. No popliteal cysts were identified in Type 1 knees. Cysts were present in 2 of 22 Type 2A knees (9.1%), 6 of 40 Type 2B knees (15%), and 12 of 21 Type 3 knees (57.1%). A statistically significant relationship was found between the anatomical type of the postero-medial capsule and the presence of a popliteal cyst (p < 0.001).

Conclusion: Arthroscopic anatomy of the postero-medial capsule appears altered in the presence of a popliteal cyst. This alteration is characterized by a bulging OPL within the joint, creating a communication orifice at its superior margin leading to the cyst. Posteromedial synovectomies aimed at disrupting the unidirectional flow mechanism into the cyst should target a triangular area defined by the medial gastrocnemius, the semimembranosus, and the oblique popliteal ligament.

Level of evidence: IV; observational study.