Isolated Injury to the Motor Branch of Ulnar Nerve Following Penetrating Trauma: A Case Report

Isolated injury to the deep motor branch of the ulnar nerve at Guyon's canal is an uncommon condition, particularly following penetrating trauma, and may be overlooked because sensory function is often preserved. We report the case of a 39-year-old right-handed male dental assistant who presented with progressive weakness of the left hand 3 months after sustaining a puncture injury from a sharp dental instrument. Clinical examination demonstrated weakness and wasting of the intrinsic hand muscles with intact sensation. Nerve conduction studies and electromyography localized the lesion to the deep motor branch of the ulnar nerve. Surgical exploration identified a neuroma within Guyon's canal, which was excised, followed by tension-free epineural repair under microscopy. The patient was managed with postoperative splinting and hand therapy and showed gradual functional recovery. Electromyography demonstrated evidence of reinnervation at 6 months, and first dorsal interosseous muscle strength improved from M0 preoperatively to M4 at the 18-month follow-up, with resolution of Froment's and Wartenberg's signs and restoration of intrinsic hand function. This case emphasizes that isolated motor branch injury should be considered in patients with persistent intrinsic hand weakness after penetrating trauma around Guyon's canal, even in the absence of sensory deficits. Careful clinical assessment, appropriate electrodiagnostic evaluation, and surgical exploration can result in favorable functional recovery.