History:

A 57 y/o male found a left popliteal mass noted for 6 months. There is a history of previous trauma over left knee with pain.

Questions:

1. What are the findings?
2. What is the differential diagnosis?
3. What is the diagnosis?

Answers: 

1. MRI of the left knee with/without contrast:
   l A huge cystic lesion with thickened wall is noted at the medial posterior aspect of the popliteal fossa, between the semimembranesus and medial head of the gastrocnemius, with constrict connection into the joint space. Its capsule is well enhanced. Some septation is noted.
   l Complex signal intensity within posterior horn of the medial meniscus suggesting meniscal tear.
   l Hyperintense patch on T2-weighted images over the anterior medial tibial plateau consistent with bone bruise..
   l Thickening of MCL with bright signal on T2-weighted images suggesting sprain injury.
2. Baker's cyst, Ganglion cyst, Aneurysm of popliteal artery, Varicose vein, Hematoma, Tumors of soft tissue or bone
3. Baker's cyst

Discussion

Synovial popliteal cyst is caused by an outpouching of synovial lining into a bursa. The most common location for its occurrence is in the semimembranesus-gastrocnemius bursa. The neck of the cyst frequently can be seen on MR imaging (about 35-55%), confirming communication with the joint space. The point of communication may become constricted or obliterated, and a clear communication with the joint may not be demonstrated. In the presence of communication, the incidence of a bursa presenting as a symptomatic mass effect (so-called Baker's cyst) is about 5%. The communication with the joint space and the typical location make the diagnosis of the Baker's cyst in this case. Popliteal cysts can be readily detected with ultrasound or MR imaging, showing a smooth, thin-walled collection of fluid that may contain septations, synovial tissues, or loose bodies. Inflammatory condition such as rheumatoid arthritis, and osteoarthritis are common predisposing factors. Ligamentous and meniscal tear, chronic effusions, and increased age are other associations. The associated posteromedial meniscal tear and medial collateral ligament sprain in this case indicate that the Baker's cyst is traumatic related. The primary differentiated diagnosis in the unruptured popliteal cyst is ganglion cyst, which are usually related to the cruciate ligament and more often the PCL. Rupture can result from sudden increased pressure in the cyst. So-called dissection with fluid extension into the soft tissue or along the fascial planes of the lower leg is due to persistent pressure increase in the bursa or cyst caused by check valve mechanism or chronic arthritis. In these two conditions, the differentiated list includes the cellulitis, fasciitis and myositis.

Reference

1. D. L. Janzen, C. G. Peterfy, J. R. Forbes, P. F. Tirman, H. K. Genannt. Cystic lesions around the knee joint: MR imaging findings. AJR 1994; 163: 155-161.