Prevalence and characteristics of complications of Baker cysts by MRI

Objective. To evaluate the prevalence and describe the findings of complica ted Baker cysts observed in a large series of knee MR studies. Materials and methods. A total of 145 Baker cysts were detected out of a se ries of 382 (38%) consecutive patients with knee MR studies performed. Cyst s were classified as simple (smooth walls, homogeneous content, signal inte nsity similar to that of the synovial fluid and with no changes in the surr ounding tissues) or complicated (changes of wall, changes in the intensity of the content signal, presence of free bodies or changes in the surroundin g tissues). Complications were detected in ten (6.9%) cysts, which were con firmed by means of direct observation (arthroscopy or surgery, n = 9 cases) or by means of clinical evidence with follow-up (n = 1 case). Results. No differences were detected in sex distribution between simple an d complicated cysts (chi (2), P = 0.09). although a statistically significa nt relationship was indeed detected between the presence of complications w ith older age (P = 0.003) and presence of meniscal lesions (P = 0.019). In five patients a rupture of the Baker cyst was found, in four intracystic fr ee bodies, and in one case pigmented villonodular synovitis with intracysti c foci. Only two out of the five patients with cyst rupture had thrombophle bitis symptoms, the remaining of ruptures being asymptomatic. In three out of the four patients with free intracystic bodies they were considered seco ndary to degenerative osteochondral lesions, the other case having synovial chondromatosis. Conclusions. Baker cysts are common and usually are not associated with com plications. In our series, a prevalence of 6.8% of complicated cysts was ob served, either by their contents or wall changes. The most common complicat ion is rupture (50% of complicated cysts) which can be asymptomatic in up t o 80% of cases, followed by free intracystic bodies (40% of complications) usually of degenerative origin. The complication is statistically associate d with meniscal ruptures and aged patients.