New insights into the development of popliteal cysts.

Background: This prospective study was designed to investigate the aetiology of popliteal (Baker's) cysts.

Methods: Some 426 legs in 312 individuals were investigated with ultrasonography for the presence of a popliteal cyst. Participants included healthy volunteers, patients with a painful knee, and a group of patients with signs and symptoms of deep vein thrombosis (DVT). The location, thickness and content of all cysts were recorded. In addition, the popliteal fossa was dissected in four cadaver knees.

Results: The prevalence of popliteal cyst increased with age and was significantly higher in those aged over 50 years (P < 0.001). Compared with controls (incidence 3.0 per cent), cysts were more common in patients with DVT symptoms (9.5 per cent; P = 0.141) and those with painful knees (19.8 per cent; P < 0.001). All cysts were located in the posteromedial aspect of the popliteal fossa; the majority extended between the deep fascia and the medial head of the gastrocnemius muscle. At this level an area without external support of the posterior capsule was identified between the two expansions of the semimembranosus muscle from the medial side (the oblique popliteal ligament and the expansion over the popliteus muscle) and the posterior cruciate ligament from the lateral side. This area was also located in the knee dissections. No association was found between popliteal cyst and acute DVT.

Conclusion: Popliteal cysts are relatively common in patients over 50 years old with a painful knee or with signs and symptoms of DVT. They form in the posteromedial popliteal fossa because the synovial capsule does not provide anatomical support in this region.