Objective: To evaluate the prevalence and describe the findings of complicated 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 series of 382 (38%) consecutive patients with knee MR studies performed. Cysts were classified as simple (smooth walls, homogeneous content, signal intensity similar to that of the synovial fluid and with no changes in the surrounding tissues) or complicated (changes of wall, changes in the intensity of the content signal, presence of free bodies or changes in the surrounding tissues). Complications were detected in ten (6.9%) cysts, which were confirmed 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 and complicated cysts (chi 2, p = 0.09), although a statistically significant relationship was indeed detected between the presence of complications with 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 free bodies, and in one case pigmented villonodular synovitis with intracystic foci. Only two out of the five patients with cyst rupture had thrombophlebitis symptoms, the remaining of ruptures being asymptomatic. In three out of the four patients with free intracystic bodies they were considered secondary to degenerative osteochondral lesions, the other case having synovial chondromatosis.
Conclusions: Baker cysts are common and usually are not associated with complications. In our series, a prevalence of 6.8% of complicated cysts was observed, either by their contents or wall changes. The most common complication is rupture (50% of complicated cysts) which can be asymptomatic in up to 80% of cases, followed by free intracystic bodies (40% of complications) usually of degenerative origin. The complication is statistically associated with meniscal ruptures and aged patients.