Prevalence of Baker's cysts in painful primary osteoarthritis of the knee: a musculoskeletal ultasound study.

Objectives: To investigate the prevalence of the Baker's cyst in patients with primary painful knee osteoarthritis (OA).
Methods: Prospective clinical and musculoskeletal ultrasound (MUS) findings in 150 patients with primary painful knee OA diagnosed by ACR criteria were studied. The relationship between the Baker's cyst and the variables joint effusion, synovitis, osteophytes and radiological grades of Kellgren and Lawrence were analysed.
Results: Baker's cysts were demonstrated by MUS in 54 (21.2%) of 254 knees or 40 (26.7%) of 150 patients. Only 18 (33.9%) of 54 cysts had been diagnosed clinically. One of the 54 cysts ruptured. Joint effusion, synovitis and osteophytes were detected in 59.8%, 30.3% and 48.8% of knees respectively. There was a highly significant correlation between the presence of Baker's cyst with knee effusion and synovitis (p<0.001 and p<0.001, respectively). There was also a significant correlation of the presence of Baker's cyst with osteophytes and the radiographic grade (p<0.01 and p<0.05, respectively).
Conclusion: The results indicate that Baker's cysts are common in knee OA, and that they may be missed clinically. Therefore, MUS should be more widely employed by clinicians in the diagnosis of Baker's cysts, which may sometimes be accompanied by significant morbidity