Ulnar nerve injuries, whether traumatic or iatrogenic, lead to profound motor and sensory impairments, including reduced grip strength, fine motor dysfunction, and deformities like claw hand. The ulnar nerve, originating predominantly from the C8-T1 roots, traverses anatomically significant regions like the cubital tunnel and Guyon’s canal, where it innervates muscles critical for hand function and provides sensory input to the ulnar hand and fingers. Functional recovery is constrained by the slow axonal regeneration rate (1 mm/day) and delayed presentation, necessitating alternative treatments such as nerve and tendon transfers. Diagnostic evaluation, including electrodiagnostic testing, imaging, along with clinical assessment is essential for treatment planning. Commonly performed transfers include the anterior interosseous nerve (AIN)-to-ulnar nerve transfers for proximal injuries and the opponens pollicis to deep ulnar motor branch for distal injuries. The development of novel nerve transfer and improved surgical techniques have demonstrated promising success for improving motor reinnervation and sensory restoration. These techniques have transformed the prognosis for ulnar nerve injuries, offering functional recovery and improved quality of life.
Background
Ulnar nerve injuries, whether traumatic or iatrogenic, are severe conditions with profound and often permanent implications for sensory and motor function. These injuries commonly result in impaired grip strength, reduced fine motor dexterity, diminished pinch strength, and clawing of the hand due to unopposed extrinsic muscle pull. Axonal regeneration after nerve injury is widely regarded to progress at a rate of approximately 1 mm per day (equivalent to 1 inch per month). However, even with meticulous microsurgical repair, functional recovery is frequently suboptimal, particularly in proximal injuries, due to the extensive distance axons must regenerate to reinnervate distal target muscles. , Early intervention offers the greatest potential for improved outcomes by minimizing denervation atrophy and preserving motor endplates. However, delayed presentation—often due to the insidious nature of certain injuries or delayed diagnosis—may preclude direct repair, necessitating alternative strategies. ,
When direct nerve repair is not feasible, advanced surgical approaches such as nerve grafting, tendon transfers, or nerve transfers are considered. Nerve transfer techniques can be categorized into 3 principal types: end-to-end, reverse end-to-side, and end-to-side transfers ( Fig. 1 ).
End-to-End Transfers (ETE): ETE transfers are optimal for proximal injuries with minimal or no potential for spontaneous recovery. This method prioritizes functional restoration by entirely redirecting donor axons to reinnervate the distal segment of the injured nerve. By completely severing and redirecting the donor nerve, ETE achieves robust axonal input to distal targets, albeit at the cost of donor nerve function and sectioning of the recipient nerve disrupts any chance for healing .
Reverse End-to-Side Transfers (RETS): RETS are particularly advantageous for distal or incomplete injuries, where some degree of spontaneous recovery is anticipated. Unlike ETE, RETS preserves the continuity of the recipient nerve while supplementing it with donor axons. This dual input enhances the regenerative potential and functional outcomes without compromising the recipient nerve’s existing structure. ,
End-to-Side Transfers (ETS): ETS are primarily utilized for sensory recovery, targeting reinnervation of the sensory territories within the ulnar nerve dermatome. By coapting the injured nerve to the side of the donor nerve without severing either, ETS facilitates axonal sprouting and sensory restoration while maintaining the structural integrity of the donor nerve.
Each transfer technique requires careful consideration of the injury’s severity, chronicity, and anatomical constraints to optimize outcomes. Additionally, adjunctive strategies such as postoperative rehabilitation are critical in maximizing functional recovery.
Indications
Ulnar nerve injuries with documented axonal damage on electrodiagnostic testing, coupled with clinical findings such as muscle atrophy or weakness, are the primary indications for nerve transfer procedures. In acute injuries, patients treated within 1 year of the initial insult have the highest potential for functional recovery. This is attributed to the preservation of viable motor endplates (MEPs), which undergo irreversible degeneration beyond this timeframe, rendering reinnervation efforts less effective. For chronic injuries or cases of compressive neuropathy, the identification of fibrillations on electromyography (EMG) serves as a critical marker for intact MEPs. Presence of these fibrillations indicates the potential for successful reinnervation, which is an essential prerequisite for nerve transfer surgery.
Anatomy
Course of the Ulnar Nerve: Brachium to Elbow
The ulnar nerve originates predominantly from the C8-T1 ventral rami, with occasional contributions from the C7 nerve root. These roots coalesce to form the lower (inferior) trunk of the brachial plexus, which transitions through divisions to form the medial cord. The medial cord gives rise to a significant portion of the median nerve with its distal continuation forming the ulnar nerve.
In the brachium, the ulnar nerve runs medial to the axillary artery and lateral to the axillary vein, coursing between the brachialis and triceps muscles. It follows the superior ulnar collateral artery as it pierces the intermuscular septum, traversing the Arcade of Struthers—a fascial thickening located approximately 8 mm proximal to the medial epicondyle. The nerve then passes posteriorly to the supracondylar ridge, where the inferior ulnar collateral artery anastomoses with the superior ulnar collateral artery. Articular branches to the elbow joint may emerge around 7 mm proximal to the medial epicondyle.
Course of the Ulnar Nerve: Elbow to Forearm, Including Cubital Tunnel
Upon crossing the medial epicondyle, the ulnar nerve enters the cubital tunnel, a fibro-osseous passage bordered by Osborne’s ligament (roof), spanning the gap between the humeral and ulnar heads of the flexor carpi ulnaris (FCU) muscle, and the posterior band of the medial collateral ligament (floor). Elbow flexion significantly increases pressure within the tunnel, with studies reporting up to a twofold pressure rise during full flexion.
The ulnar nerve provides motor innervation to the FCU via multiple branches (∼3 on average) originating ∼25 mm distal to the medial epicondyle. The ulnar portion of the flexor digitorum profundus (FDP) receives its motor branch ∼3 cm distal to the medial epicondyle, with ∼20% of specimens exhibiting multiple branches. The nerve travels through a triangular region formed by the FCU (medial), flexor digitorum superficialis (FDS) (anterolateral), and FDP (posterolateral), becoming progressively superficial as it courses between the FDS and FDP muscle bellies. The dorsal cutaneous branch emerges ∼6-9 cm proximal to the wrist, supplying sensation to the dorsum of the small and ring fingers. ,
Course of the Ulnar Nerve: Wrist, Guyon’s Canal, and Hand
At the wrist, the ulnar nerve travels superficial to the transverse carpal ligament, passing through Guyon’s canal (∼4 cm in length). The canal’s boundaries include the roof (distal continuation of the antebrachial fascia), floor (transverse carpal ligament), ulnar border (pisiform, FCU, and abductor digiti minimi (ADM)), and radial border (hook of the hamate).
Guyon’s canal is divided into 3 zones. Zone 1, proximal to the bifurcation, contains the undivided ulnar nerve. Zone 2 contains the deep motor branch, which innervates the ADM, flexor digiti minimi brevis (FDMB), opponens digitis minimi (ODM), adductor pollicis, interossei, and lumbricals 3/4. Zone 3 contains the superficial branch, responsible for sensory innervation to the small and ulnar half of the ring fingers, as well as motor innervation to the palmaris brevis. ,
Examination and Diagnostic Evaluation
Forearm Muscles
The ulnar nerve provides motor innervation to the FCU and the FDP. The FCU is assessed by testing wrist flexion against resistance, with contraction producing a characteristic ulnar deviation due to its anatomical positioning relative to other wrist flexors. As the primary flexor contributing to ulnar deviation, evaluation of the FCU is critical in assessing proximal ulnar nerve integrity. The FDP is tested by assessing flexion at the distal interphalangeal (DIP) joints of the small and ring fingers. Notably, the FDP functions through a single muscle belly, resulting in simultaneous activation of its tendons during movement. Motor innervation of the ulnar-most portion of the FDP, traditionally attributed solely to the ulnar nerve, exhibits variability: in approximately 56% of individuals, this region is exclusively ulnar-innervated, while in others, it may receive dual innervation from both the median and ulnar nerves, complicating diagnostic evaluation.
Hand Muscles
The ulnar nerve innervates 15 intrinsic muscles of the hand, which include the interossei, lumbricals, hypothenar muscles, and additional muscles such as the adductor pollicis and the deep head of the flexor pollicis brevis. The interossei muscles are divided into 4 dorsal interossei, which are responsible for finger abduction, and 3 palmar interossei, which facilitate finger adduction. The functional integrity of these muscles is assessed by testing resistance to lateral finger movement. The third and fourth lumbricals, also innervated by the ulnar nerve, contribute to flexion at the metacarpophalangeal (MCP) joints and extension at the proximal and distal interphalangeal (PIP and DIP) joints. The hypothenar muscle group consists of the ADM, which abducts the small finger; the FDMB, which flexes the small finger; and the ODM, which facilitates opposition of the small finger. The ulnar nerve further innervates the adductor pollicis, a critical muscle for thumb adduction, as well as the deep head of the flexor pollicis brevis. Both of these muscles are challenging to isolate clinically but play essential roles in hand function. Additionally, the palmaris brevis, a superficial muscle innervated by the ulnar nerve, is occasionally evaluated in cases of suspected ulnar neuropathy, although it contributes minimally to overall hand mechanics.
Special Tests and Signs
Several clinical signs are indicative of ulnar nerve dysfunction. Froment’s sign, characterized by hyperflexion of the thumb’s interphalangeal (IP) joint during key pinch, reflects compensatory recruitment of the flexor pollicis longus (FPL) in response to adductor pollicis weakness. Jeanne’s sign is identified by hyperextension of the thumb’s MCP joint during key pinch, further indicating adductor pollicis dysfunction. Wartenberg’s sign is manifested as persistent abduction of the small finger, caused by unopposed action of the extensor digiti minimi (EDM) (innervated by the posterior interosseous nerve) and weakened function (adduction) of the third palmar interosseous muscle. Clawing of the hand, another hallmark of ulnar nerve injury, is characterized by hyperextension at the MCP joints and flexion at the PIP and DIP joints ( Fig. 2 ). This deformity results from unopposed action of the extensor digitorum communis (EDC) and tension in the FDP. In cases of proximal ulnar nerve injury, clawing may be attenuated due to reduced tension in the FDP tendons, highlighting the anatomical interplay between roximal and distal nerve integrity.
Diagnostics
Electrodiagnostic studies (EDS), including nerve conduction studies (NCS) and EMG, are essential for evaluating ulnar nerve function. NCS involves stimulating the nerve proximally while recording responses distally, to measure conduction velocity (latency) and signal amplitude. Reduced conduction velocity indicates demyelination, whereas diminished amplitude reflects axonal loss.
EMG complements NCS by assessing muscle activity both at rest and during voluntary contraction. Fasciculations observed at rest indicate denervation, while polyphasic potentials during contraction suggest ongoing reinnervation. These findings provide critical insights into the extent of nerve damage and potential for recovery.
Advanced imaging modalities, such as high-resolution magnetic resonance imaging (MRI) and ultrasonography (US), have become increasingly valuable in the diagnostic evaluation of ulnar nerve injuries. MRI can identify structural abnormalities such as nerve swelling or neuroma formation, while US enables dynamic assessment of the nerve. Both modalities are useful for surgical planning, especially in cases requiring localization of proximal and distal nerve stumps.
Fascicular Topography
In the brachium, motor fascicles to the FCU are located anterolaterally, while those to the FDP are posterolateral. Sensory fascicles to the dorsal cutaneous nerve occupy a central and posterior position, while mixed fascicles for the intrinsic hand muscles are situated medially. In the proximal forearm, the radial and volar regions contain sensory fascicles, while the central region contains smaller motor fascicles. The ulnar region houses the dorsal cutaneous fascicles. After the dorsal cutaneous branch emerges, sensory fascicles shift radially, while motor fascicles remain ulnar in the distal forearm ( Fig. 3 ).
Axon Count
Several studies have explored axon counts and fascicular distribution within the ulnar nerve. Gesslbauer et al. demonstrated that sensory axons outnumber motor axons by a ratio of 9:1. At the mid-forearm, the total axon count was 30,915 ± 2,869, with motor axons comprising 3.88% ± 0.74%. At the wrist, the total axon count was 26,610 ± 3,133, with motor axons accounting for 4.66% ± 1.08%. Approximately 1,700 axons innervate the hand, of which 1,200 originate from the ulnar nerve and 500 from the median nerve.
Tereshenko et al. reported an overall axon count of 26,900 ± 5,000 in the ulnar nerve proximal to the dorsal cutaneous branch, with 1,300 ± 300 motor fibers. The dorsal cutaneous branch contains ∼9,300 axons, while the superficial branch has 21,400 ± 3,400 axons, including 51 ± 6.4 motor fibers. The deep branch comprises 8,900 ± 1,400 axons, with 1,200 ± 280 motor fibers. The ratio of motor-to-sensory axons increases from the proximal ulnar nerve (1:20) to the deep branch (1:7). This gradual increase highlights the functional specificity of distal motor branches, such as those to the intrinsic hand muscles.
Decision-Making in Ulnar Nerve Injury Management
General Indications and Considerations
End-to-end nerve transfers are preferred in cases of high-grade axonotmesis or neurotmesis, particularly when the likelihood of spontaneous motor recovery is low. This is often due to the significant distance required for motor reinnervation or the presence of an extensive nerve gap. Using a motor nerve branch as a donor minimizes the motor-sensory mismatch commonly encountered with mixed nerve grafts, as motor axons exhibit a natural preference for regenerating through motor pathways, while sensory axons tend to regenerate through sensory pathways.
General Contraindications
Nerve transfers are contraindicated when terminal denervation atrophy has occurred in the target muscle, as this condition prevents functional recovery. The absence of viable donor nerves, such as in cases involving multiple nerve injuries, also precludes nerve transfer. Additionally, donor nerves may need to be preserved for alternative procedures, such as tendon transfers, which adds complexity to the decision-making process. Thorough evaluation of the risks and benefits is essential to guide the selection of the most appropriate intervention.
Selecting the Type of Nerve Transfer
End-to-End Ulnar Nerve Transfer
Expert consensus supports the use of end-to-end ulnar nerve transfer in severe cases of ulnar neuropathy where there is a complete lack of motor response detected from the ulnar nerve. This approach is particularly beneficial in addressing motor deficits when intrinsic muscle function is completely lost.
Reverse End-to-Side Ulnar Nerve Transfer
The reverse end-to-side ulnar nerve transfer is indicated for less severe axonotmesis (Sunderland grades II-III) or cases with partial intrinsic function preserved via Martin-Gruber anastomoses. It is also appropriate for Sunderland grade IV or V injuries (neurotmesis) located between the elbow and up to 10 cm proximal to the wrist, provided the proximal injury has been addressed. This technique is further employed in cases of severe cubital tunnel syndrome with intrinsic muscle loss, low brachial plexopathy, or Parsonage-Turner syndrome.
Surgical Technique for Nerve Transfers
Proximity to Potential Donor Nerves: Motor Transfers
High ulnar nerve injuries above the elbow require reanimation to restore intrinsic hand function and protective sensation. One of the most common donor nerves is the anterior interosseous nerve (AIN), which can be transferred to the ulnar nerve in an end-to-end or reverse end-to-side manner. The AIN branches from the median nerve and innervates the radial half of the FDP, the FPL, and pronator quadratus. Its anatomical proximity to the ulnar nerve at the mid-forearm, after the branches of the FDP and FPL make it ideal for transfer. Distal ulnar nerve injuries (mid-forearm to wrist) may benefit from targeted transfers such as the median-to-ulnar nerve (Bertelli transfer). This involves transferring a branch of the median nerve innervating the opponens pollicis to the ulnar-innervated adductor pollicis, effectively restoring pinch strength and opposition.
Nerve Transfer: AIN to Ulnar Motor Transfer
The patient is positioned supine with the arm abducted and supinated on an arm table or board. A tourniquet is applied as proximally as possible to ensure optimal exposure, with careful sterile placement. It is important to avoid using a regional block, as local anesthesia may compromise the effectiveness of the nerve stimulator. The time of tourniquet use must also be considered when stimulating donor nerves, as prolonged use can affect signal transduction.
A Brunner-type incision is made along the ulnar aspect of the hook of the hamate, crossing the wrist crease to facilitate access, particularly when decompressing Guyon’s canal. An alternative approach involves a linear incision just radial to the FCU. The ulnar nerve and artery are located beneath the volar carpal ligament within Guyon’s canal, with the deep motor branch of the ulnar nerve traced from just distal to the pisohamate ligament.
Proximally, in the forearm, the ulnar nerve lies deep to the FCU tendon. During dissection, the dorsal cutaneous branch, emerging 6-9 cm proximal to the ulnar styloid, is identified. Potential compression sites, such as the hypothenar fascia, must be addressed to achieve thorough decompression. Internal neurolysis is performed from distal to proximal to isolate both sensory and motor fascicles of the nerve. The motor fibers of the ulnar nerve are traced to the proximal ulnar border of the pronator quadratus.
The distal AIN is exposed within the same incision and traced to its arborization in the pronator quadratus muscle ( Fig. 4 ). The AIN is transected as distally as possible to maximize graft length while preserving axon branches. A side-to-end repair is performed by creating an epineural window in the motor fascicle of the ulnar nerve. The AIN is then sutured to the epineurial window using microsurgical technique ( Fig. 4 ) In an end-to-end repair, a similar technique is followed, though the ulnar motor branch must also be transected proximally to facilitate a tension-free repair ( Fig. 5 ).
Related posts:
Evaluation and Grading of Peripheral Nerve Injuries
Complete Brachial Plexus Injuries: Review of Surgical Techniques and Functional Outcomes
Nerve Transfers for Radial Nerve Palsy: History, Outcomes, Alternatives, and Surgical Techniques
Nerve Transfers for Median Nerve Injury
Nerve Transfers for Brachial Plexus and Upper Extremity Nerve Injuries
Upper Extremity Targeted Muscle Reinnervation
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