Baker's cyst in rheumatoid arthritis: an ultrasonographic study with a high resolution technique.

OBJECTIVE: To determine the prevalence of popliteal cyst (Baker's cyst) in rheumatoid arthritis (RA), through the use of a very sensitive and non-invasive method, high resolution ultrasonography. The present is the first such report in the literature. METHODS: Ninety-nine unselected consecutive patients with RA, after undergoing routine clinical and laboratory evaluation, had knee radiographs and ultrasound examinations of both knees, the popliteal fossae and calves, using an Ultramark 9ATL apparatus with a 3 MHz curved array and 10 MHz linear array heads and color doppler ability. RESULTS: A Baker's cyst was detected in 47 patients (47.5%) and in a total of 67 out of the 198 knees (33.8%). Four of the 67 cysts were ruptured. Only 29 of the 67 cysts (43.3%) had been diagnosed clinically. A statistically significant correlation was found between the presence of a Baker's cyst and clinical and radiologic involvement of the knee by rheumatoid arthritis (p < 0.025, and p < 0.05 respectively). There was a highly significant correlation between the presence of a cyst and ultrasonographically demonstrated joint effusion (p < 0.001). CONCLUSION: Baker's cyst is very common in RA but it may escape clinical detection. High resolution ultrasound scanning of the area is a simple, highly sensitive and non-invasive technique able to overcome this problem. Therefore, it should be more widely employed by clinicians in the diagnosis of popliteal cysts, which may sometimes be accompanied by significant morbidity.