MR features of a giant cyamella in a patient with osteoarthritis: diagnosis and discussion
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The cyamella is infrequently encountered in humans with only a handful of reports in the literature describing its appearance on radiographs, CT, and MRI [1, 2]. It is occasionally referred to as the popliteal fabella or the fabella distalis. It is typically located within the tendon or, occasionally, the myotendinous junction of the popliteus muscle, which runs from the lateral femoral epicondyle, posteriorly and distally to the origin of the muscle on the proximal third of the posterior face of the tibial diaphysis. Most commonly, on radiographs, it is seen in the lateral femoral groove and is usually a small, ossified structure.
Uncommon in humans, it is frequently encountered in many other primates as well as other animals including dogs and kangaroos [3, 4]. It is postulated to assist joint function by modifying pressure on the tendon, diminishing friction, and redirecting force along the tendon. In the human, it appears to no longer play a critical role in joint function. There is little in the way of any description of pathology associated with the cyamella in humans, although Mishra and Jurist have reported one case of a painful dislocated cyamella associated with recreational basketball .
Akansel et al. reported an additional case of a cyamella, which was imaged with both MR and CT . Their case demonstrates an ossicle with a low signal border on T1- and T2-weighted spin echo imaging and T2*-weighted gradient imaging. CT images demonstrated marrow fat within the ossicle.
In both our case and that of Akansel, the cyamella appeared larger than most of those shown by Keats and Anderson , and they are more posteriorly and inferiorly located. The examples shown presumably represent cyamella located within the tendon whilst ours appears to be in the musculotendinous junction.
The fabella located at the medial side of the knee is a more commonly encountered sesamoid at the posterior aspect of the knee located within the gastrocnemius tendon . Its incidence is reported in the range of 12 to 16% of individuals. The fabella develops as a cartilaginous nodule within the gastrocnemius tendon, which only becomes visible at approximately age 12 when ossification occurs. To our knowledge, it is unclear whether some patients have a non-ossified fabella, which may not be radiologically identified on standard radiography. Pritchett reported that patients with osteoarthritis appear to have a higher incidence of fabella on their radiographs, in the range of 35%. He postulates that fabella may predispose the development of osteoarthritis. Patients with osteoarthritis often demonstrate a large fabella with an anterior facet that articulates with the femoral condyle. Whether the fabella predisposes to osteoarthritis, or whether the fabella becomes hypertrophic due to osteoarthritis in the knee is uncertain. It is interesting to note that in both our case and that of Akansel, we encountered markedly enlarged cyamella in patients with osteoarthritis. It is unclear whether the presence of osteoarthritis induced hypertrophy of the cyamella due to altered biomechanics.
Another differential diagnosis that needs to be considered in both our case and that of Akansel is the presence of a possible loose body. These bodies may occasionally sequester at the margins of the joint capsule or through herniated portions of the joint capsule and may, in some instances, contain marrow—particularly those that represent broken, large, mature osteophytes. Other less likely possibilities would be myositis ossificans or an osteochondroma.
- 5.Keats TE, Anderson MW. Atlas of normal roentgen variants that may simulate disease. 8th ed. Philadelphia: Mosby/Elsevier; 2007.Google Scholar