Tumors of the synovium are rare and can involve the lining of the joints, tendons, or bursa. LA is a very rare benign primary intra-articular tumor of the synovium. It is known also as synovial lipomatosis, and is characterized by replacement of the subsynovial tissue by mature adipocytes, giving rise to prominent intra-articular villous proliferation of the synovium. Less than 60 cases have been published in the English language medical literature as shown by the database of medical search engines of Pub Med and OVID.1
The majority are case reports and the largest series of cases included six cases.3
I believe that this is the first case ever reported in Saudi Arabia.
The exact etiology of LA is not known; the majority of cases arise de novo. Other reports described association with previous trauma, chronic rheumatoid arthritis, underlying osteoarthritis, psoriasis, or a Baker cyst.2,3
Most cases present as joint pain, intermittent swelling, and joint effusion.2
The knee is the most commonly affected joint especially the suprapatellar pouch region. LA has occurred in other anatomical sites including the hip, elbow, shoulder, and wrist. The lesion was rarely reported to involve the synovial surface of the ankle joint.4
The majority of cases presented with monoarticular involvement; however, only a few reports described polyarticular involvement by LA.3,5,6
Most occurred in patients in their 5th to 7th decades of life;3
however, our case interestingly presented at the young age of 24 years.
Plain radiographs are usually non-specific for LA, but may show degenerative articular changes if there is a secondary osteoarthritis. Ultrasound can demonstrate joint effusion, and may also show villous synovial projections. In most cases, MRI is the diagnostic modality of choice, as it can demonstrate high signal intensity of fat in LA on both T-1 and T-2 weighted images. In our case, the initial MRI reported only mild effusion with no documentation of lesions. After the histopathologic establishment of the diagnosis of LA, the MRI report was revised by the radiologist in the light of the pathologic diagnosis and close discussion with the orthopedic surgeon. The lesions were determined to be localized. An additional axial T1-weighted sequence, including suprapatellar pouch, is therefore recommended when such rare soft tissue lesions are suspected. Although such lesions are extremely rare, in cases of persistent knee joint effusion, LA should be entertained in the differential diagnosis. The recommended management for LA is open synovectomy; however, arthroscopic synovectomy can be done with a favorable outcome.
This report confirms the significance of arthroscopy and the histopathologic examination for the confirmation of such a rare intra-articular entity. When such lesions are suspected, additional axial T1-weighted MRI sequences should be made to include the suprapatellar pouch region of the knee joint.