Spontaneous Flexor Digitorum Profundus Tendon Rupture of the Little Finger in Patients with Rheumatoid Arthritis: A Report of Two Cases with Different Mechanisms

Abstract

Subcutaneous flexor tendon rupture in patients with rheumatoid arthritis is rare compared with extensor tendon rupture, with a reported incidence of approximately one-tenth that of extensor tendons. The pathophysiology of flexor tendon rupture is not fully understood. We report two cases of spontaneous flexor digitorum profundus tendon rupture of the little finger in patients with rheumatoid arthritis despite low disease activity and the absence of advanced joint destruction. Both patients presented with an inability to actively flex their little fingers without any history of significant trauma. Based on magnetic resonance imaging and intraoperative findings, in Case 1, direct invasion by synovitis was identified as the cause. However, in Case 2, a small bone spur on the hook of hamate was responsible for the rupture. In both cases, the suspected causative synovium or bone spur was excised, and tendon transfer surgery to the flexor digitorum profundus of the ring finger was performed. The postoperative outcomes were favourable, with improved total active motion and no re-ruptures observed during follow-up. These cases highlight that flexor tendon rupture can occur even in patients with rheumatoid arthritis with low disease activity and without advanced joint destruction. Careful evaluation of both the local inflammatory activity and mechanical factors is essential to identify the cause of rupture and prevent re-rupture.