Lateral synovial fold impingement with lateral femoral condylar osteophyte formation after unicompartmental knee arthroplasty: a case report.

Introduction and importance: Unicompartmental knee arthroplasty (UKA) for isolated compartmental osteoarthritis is associated with favorable long-term outcomes. However, persistent or new-onset postoperative pain unrelated to implant failure remains a diagnostic challenge. Lateral synovial fold impingement is a rare cause of knee pain but is often under-recognized following UKA. Presentation of case: We report a case of a 75-year-old male who underwent medial UKA for osteoarthritis. Although the initial postoperative course was uneventful, he developed lateral knee pain and mechanical symptoms 1-year after surgery. Magnetic resonance imaging revealed joint effusion and linear structure suggestive of a lateral synovial fold. Diagnostic intra-articular lidocaine injection resulted in immediate pain relief, indicating an intra-articular pain source. Arthroscopic evaluation confirmed a hypertrophic lateral synovial fold impinging on the patellofemoral joint, accompanied by osteophyte formation on the lateral femoral condyle. Arthroscopic resection of the synovial fold and osteophyte removal led to complete symptom resolution. Clinical Discussion: Although similar cases have been reported, this case highlights the importance of a stepwise diagnostic approach combining imaging findings and diagnostic intra-articular injection to identify non-implant-related causes of postoperative pain. Conclusion: Lateral synovial fold impingement should be considered in the differential diagnosis of delayed-onset lateral knee pain following medial UKA in patients with preserved implant integrity. Arthroscopic synovial resection with osteophyte removal is a minimally invasive and effective treatment option that can resolve symptoms without compromising the prosthesis.