Working Diagnosis:
Primary synovial chondromatosis of the left knee

Treatment:
Following surgical consultation, the patient underwent arthroscopic removal of loose bodies with partial synovectomy. Pathology report of the loose bodies showed fragments of benign cartilage with extensive fibrinoid degeneration, consistent with synovial chondromatosis. [Pphoto 3]

Outcome:
At two weeks post-op, she had mild knee pain, stiffness and swelling, but was overall recovering well. After this visit, she began physical therapy to improve left knee strength and range of motion. At one-month post-op, she continued advancing in physical therapy and had resumed light activity to begin progression to return to sport.
At three months post-op, she had resumed full activity and did not have a return
of left knee swelling. At this visit, persistence of right knee effusion was discussed, and an MRI of the right knee was ordered which showed non-specific synovitis. She was ultimately referred to a pediatric rheumatologist, and at time of case submission, had not received a formal diagnosis to account for the synovitis in her right knee.

Author's Comments:
Synovial chondromatosis (SC) is a benign metaplastic condition in which pedunculated cartilaginous bodies develop in the synovium, eventually detaching as they grow to become intra-articular loose bodies. The incidence is estimated to be 1 in 100,000 adults, and pediatric cases are exceedingly rare. It may be idiopathic or secondary to trauma or arthritis. Symptoms are non-specific such as pain, decreased range of motion, and swelling. As SC progresses, intra-articular densities with calcified borders may be seen on x-ray, and MRI is useful to confirm the diagnosis. Management involves surgical removal of loose bodies with or without synovectomy, although there is a chance of recurrence.

Editor's Comments:
Synovial chondromatosis (SC) is a rare condition, particularly in the pediatric population. Symptoms such as reduced range of motion and pain are non-specific, which can lead to a delayed diagnosis. In this case, the identification of a joint effusion alongside atypical symptoms in the absence of trauma warranted a broad differential and further imaging. Bloodwork can be useful to aid in the diagnosis and rule out inflammatory, infectious and systemic conditions. Although rare in the pediatric population, malignancies such as interosseous low-grade chondrosarcoma or synovial sarcoma should be considered. MRI can help differentiate benign versus malignant lesions. If left untreated, SC may progress and lead to secondary degenerative joint changes, underscoring the importance of early recognition and intervention.

References:
1. Habusta SF, Mabrouk A, Tuck JA. Synovial Chondromatosis. [Updated 2023 Apr 22]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470463/

2. Yothakol, N., Charuvanij, S., Siriwanarangsun, P., Lertwanich, P., Muangsomboon, S., & Sukharomana, M. (2022). Synovial osteochondromatosis mimicking juvenile idiopathic arthritis in an adolescent: a case-based review. Clinical rheumatology, 41(8), 2571-2580. https://doi.org/10.1007/s10067-022-06224-w

3. Systematic Arthroscopic Treatment of Synovial Chondromatosis of the Knee. Wengle, Lawrence J. et al.Arthroscopy Techniques, Volume 10, Issue 10, e2265 - e2270

4. Synovial Chondromatosis. (2024, July 13). Orthobullets. Retrieved May 18, 2025, from https://www.orthobullets.com/pathology/8051/synovial-chondromatosis

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