Giant subpatellar synovial osteochondromatosis complicated by osteoarthritis of the knee

Rationale: Synovial osteochondromatosis (SOC) is a rare proliferative disorder of the synovium that most commonly affects the knee and is characterized by the formation of multiple cartilaginous nodules or intra-articular loose bodies. Subpatellar SOC is extremely rare because only a small amount of synovial tissue is present in the deep infrapatellar bursa. Prolonged mechanical irritation caused by subpatellar SOC may contribute to progressive cartilage damage and osteoarthritis. We report a patient with giant subpatellar SOC accompanied by severe knee osteoarthritis who underwent mass excision combined with total knee arthroplasty, a treatment approach that has rarely been described. Patient concerns: A previously healthy 71-year-old woman was admitted to our institution in March 2025 with a 2-year history of progressive pain and swelling in the left knee. Her symptoms had worsened during the preceding month, resulting in marked restriction of left-knee movement and substantial impairment of daily activities. Diagnoses: The patient was diagnosed with giant subpatellar SOC accompanied by severe osteoarthritis of the knee. Interventions: Examination revealed severe destruction of the left-knee articular cartilage, and the associated pain substantially impaired the patient’s daily activities. Because conservative treatment had failed to relieve her symptoms, she underwent excision of the giant subpatellar lesion combined with left total knee arthroplasty. Outcomes: Histopathological examination of the resected mass confirmed the diagnosis of giant subpatellar SOC. After surgery, the patient experienced substantial relief of left-knee pain and was able to walk with the assistance of a walker within 48 hours. At the latest follow-up, she reported no pain or gait impairment, and no clinical or radiological evidence of recurrence was observed at the 3-month follow-up. Lessons: SOC may arise in the subpatellar region despite the limited amount of synovial tissue and may occasionally develop into a large mass. Although this presentation is rare, SOC should be considered in the differential diagnosis of a calcified subpatellar mass to reduce the risk of misdiagnosis. Complete lesion excision combined with total knee arthroplasty may be an appropriate treatment option for patients with giant subpatellar SOC and end-stage knee osteoarthritis.