The Deep Motor Branch of the Ulnar Nerve and Its Role in Hand Spasticity: A Narrative Review for Surgical Applications

The deep motor branch of the ulnar nerve (DMBUN) plays a critical role in intrinsic hand function and deformities related to spasticity. Traditionally described as a single motor trunk, recent anatomical studies demonstrate multiple branching patterns and gender-based differences that challenge classical descriptions. These findings have important implications for selective surgical procedures, particularly hyperselective neurectomy (HSN), which has emerged as a promising treatment for hand spasticity. Evidence from cadaveric mapping, case series, and clinical outcome reports suggests that HSN targeting the DMBUN can achieve durable tone reduction and functional improvement with minimal complications. Conversely, the DMBUN is also at risk of iatrogenic injury during carpal tunnel release, underscoring the importance of respecting anatomical landmarks. Although current outcomes are encouraging, the literature is limited by the use of small cohorts and the absence of randomized controlled trials. Future studies should prioritize patient-specific anatomical mapping and long-term prospective data. Integrating anatomical precision with selective surgical strategies offers the best opportunity to improve outcomes for patients with hand spasticity. KEYWORDS: Ulnar nerve; Deep motor branch; Hyperselective neurectomy; Hand spasticity; Anatomical variation; Carpal tunnel surgery.