Extensor Pollicis Longus Tendon Rupture With Concomitant Electrodiagnostic and MRI Findings of Posterior Interosseous Nerve Palsy: A Report of 2 Cases and Review of the Literature.

CASES: Two patients with spontaneous loss of thumb interphalangeal joint extension were referred after history, electrodiagnostic, and/or magnetic resonance imaging (MRI) findings were felt to demonstrate a posterior interosseous nerve (PIN) palsy. Subsequent physical examination and additional directed studies suggested an alternative explanation: rupture of the extensor pollicis longus (EPL) tendon, subsequently confirmed at surgery. An extensor indicis proprius to EPL transfer restored thumb function.

CONCLUSION: Electrodiagnostic and MRI changes consistent with diffuse PIN palsy may occur in the setting of EPL rupture. An accurate diagnosis of EPL dysfunction requires careful physical examination and confirmatory imaging. Further study of this previously unreported phenomenon is required.