Age / Sex : 24 / M

Chief complaint : Knee pain, 4 months ago

What is your impression?

Two weeks later, you can see the final diagonosis with a brief discussion of this case (Please submit only one answer).

Courtesy : Sung Gyu Moon, Konkuk University Hospital

Discussion

Answer:  

Synovial chondromatosis (Synovial chondroid metaplasia)

Findings:

CR: soft tissue density in Kager’s fatpad with minute calcific foci

MR: well defined nodular mass before ACL, slightly heterogenous intermediate low signal intensity on T2WI, identical to articular cartilage signal intensity on FS PDI, intermediate signal intensity on T1WI, no blooming artifact on GE sequence (T2*)

Arthroscopy: numerous tiny cartilaginous materials, which were loosely attached in the synovium

Differential Diagnosis:

PVNS

Discussion:

Primary synovial chondromatosis is a benign proliferative neoplastic synovial process that can involve joints, bursae, and tendon sheaths. It affects males 2 to 4 times more frequently than females, most commonly in their third to fifth decades of life. Patients frequently present with pain, swelling, and/or stiffness.

As in PVNS, the knee is the most common site of synovial chondromatosis, involvement is rarely polyarticular, and it may not only be intra-articular but also extend into a popliteal cyst, ganglia, and other extra-articular soft tissues. Chronic erosions may be seen, although they are less frequent than in joints with less capacious capsules such as the hip. The cartilage masses calcify and ossify in the vast majority (70%–95%) of cases, and thus are visible on radiographs in most patients. Whether calcified/ossified or nonmineralized synovial chondromatosis is present, the joint spaces are usually normal and periarticular osteopenia is usually absent.

On MR imaging the nonmineralized, radiographically occult form of the disease is seen as lobular, mass-like areas of cartilage signal. These masses manifest as low or, more frequently, intermediate signal on T1-weighted sequences (depending on the degree of cartilage hydration) and high signal on fluid-sensitive sequences. On fluid-sensitive sequences, the masses are slightly lower in signal than joint fluid. The foci may be slightly heterogeneous in signal and may contain punctate areas of low signal on all pulse sequences, reflecting early mineralization. Larger low-signal areas are present on all pulse sequences when frank calcification of the masses is present on radiographs. The low signal is often more well-defined and rounded in comparison with the more amorphous and elongated areas of low signal seen in PVNS. When the masses are ossified, the bodies will be often numerous and similar in size, displaying cortical low signal and internal fatty marrow. The calcifications will be more prominent (ie, blooming) on gradient-echo sequences.

Malignant transformation of synovial chondromatosis to synovial chondrosarcoma is very rare. Unfortunately, there are usually no distinguishing features between the two on MR imaging. As in PVNS, rapid growth and/or rapidly deteriorating clinical symptoms can be suggestive of malignant transformation. Frank marrow invasion is atypical for benign disease, and thus should raise suspicion for malignant transformation.

References:

Flemming DJ, Hash TW 2nd, Bernard SA, Brian PS. MR imaging assessment of arthritis of the knee. Magn Reson Imaging Clin N Am. 2014 Nov;22(4):703-24