Ulnar nerve motor

1. High-Yield Summary

Ulnar nerve motor evaluation demands rigorous clinical examination to assess neuromuscular integrity across extrinsic forearm flexors and intrinsic hand musculature. The diagnostic evaluation establishes the presence, severity, and anatomical level of motor denervation across the following foundational principles:

• Dual Anatomical Entrapment Sites: The ulnar nerve is susceptible to mechanical compression proximally at the elbow within the cubital tunnel (retroepicondylar groove, Osborne ligament, flexor carpi ulnaris aponeurosis) and distally at the wrist within Guyon canal (pisohamate hiatus). Proximal entrapment compromises extrinsic flexors (FCU, FDP 4/5) and intrinsic hand muscles, whereas distal Zone 2 compression selectively paralyzes the deep motor branch while sparing forearm flexors and cutaneous sensation.
• Pathognomonic Physical Signs: Advanced motor denervation manifests through classic physical maneuvers: Froment sign (compensatory thumb interphalangeal flexion via the median-innervated flexor pollicis longus during key pinch), Jeanne sign (concomitant thumb MCP hyperextension), Wartenberg sign (persistent abduction of the fifth digit from unopposed extensor digiti minimi), and Egawa test (inability to abduct and adduct the middle finger with the hand resting flat).
• The Ulnar Paradox: High ulnar nerve lesions at or proximal to the elbow produce deceptively milder digital clawing than low lesions at the wrist. Denervation of the flexor digitorum profundus to digits 4 and 5 eliminates active DIP flexion, softening the visual claw posture despite a more profound total neuromuscular deficit.
• Biomechanical Reconstruction Guidance (Bouvier Maneuver): The Bouvier test assesses the functional competence of the extrinsic extensor digitorum communis (EDC) apparatus. A positive Bouvier test confirms that preventing MCP hyperextension restores active IP extension, validating isolated MCP stabilization (e.g., Zancolli lasso tenodesis); a negative test indicates central slip attenuation or fixed contracture requiring tendon transfer routed directly into the lateral bands.
• Diagnostic Benchmarks & Anomalous Confounders: While individual motor tests demonstrate high specificity (88% to 98%), sensitivity varies with the degree of axonal injury. Anomalous neural communications, including Martin-Gruber anastomosis (15% to 20% prevalence) and Riche-Cannieu anastomosis, can preserve intrinsic muscle function despite severe proximal ulnar nerve pathology, requiring meticulous electrodiagnostic correlation.

2. Clinical Indications & Target Pathology

Comprehensive physical examination of the ulnar nerve motor branch is indicated across a broad spectrum of acute traumatic, compressive, and degenerative upper extremity disorders:

• Functional Grip and Pinch Deficiency: Patients presenting with progressive loss of grip strength (>50% deficit) or key pinch power (up to 70% to 80% loss), subjective clumsiness, dropping handheld objects (water bottles, tools), or difficulty turning door keys and opening jars.
• Visible Intrinsic Muscular Atrophy: Hollowed first dorsal web space depression, guttering between the metacarpal shafts on the dorsum of the hand, flattening of the hypothenar eminence, or prominence of the flexor tendons within the palm.
• Digital Postural Abnormalities: Resting hyperextension of the metacarpophalangeal (MCP) joints with flexion of the proximal and distal interphalangeal (PIP and DIP) joints of the ring and small fingers (ulnar claw hand), or persistent resting abduction of the small finger catching on pants pockets (Wartenberg sign).
• Suspected Cubital Tunnel Syndrome: Chronic compression, traction, or subluxation at the retroepicondylar groove or Osborne ligament, exacerbated by sustained elbow flexion during sleep, telecommunication, or occupational positioning.
• Suspected Guyon Canal Syndrome (Pisohamate Hiatus Entrapment): Isolated motor deficits or mixed sensorimotor symptoms originating from Zone 1 (proximal canal), Zone 2 (deep motor branch around the hook of the hamate), or Zone 3 (superficial sensory branch), frequently linked to carpal ganglion cysts, hook of hamate nonunions, or chronic handlebar compression (“cyclist palsy”).
• Differential Diagnosis of Hand Weakness: Distinguishing focal ulnar neuropathy from C8-T1 cervical radiculopathy, neurogenic thoracic outlet syndrome (TOS / Gilliatt-Sumner hand), motor neuron disease (amyotrophic lateral sclerosis), and peripheral polyneuropathies.

3. Functional Anatomy & Biomechanical Mechanism

The ulnar nerve originates as the terminal continuation of the medial cord of the brachial plexus, carrying motor axons predominantly derived from the C8 and T1 ventral rami (with frequent C7 contributions via the lateral cord). Understanding its precise course and fascicular anatomy is critical for accurate motor localization:

• Brachial Course & Arcade of Struthers: Descending along the medial aspect of the brachial artery, the nerve pierces the medial intermuscular septum approximately 8 cm proximal to the medial epicondyle to enter the posterior compartment. Here, the nerve may be constrained beneath the arcade of Struthers, a musculoaponeurotic band extending from the medial head of the triceps to the medial intermuscular septum.
• Cubital Tunnel Architecture: The nerve enters the fibro-osseous cubital tunnel posterior to the medial epicondyle. The anterior border is formed by the posterior epicondylar groove, the lateral border by the olecranon, the floor by the posterior and transverse bands of the medial collateral ligament (MCL), and the roof by the arcuate ligament of Osborne. Distally, the nerve passes beneath the confluent aponeurosis connecting the humeral and ulnar heads of the flexor carpi ulnaris (FCU).
• Extrinsic Forearm Muscular Branches: In the proximal forearm, the ulnar nerve supplies direct motor branches to the FCU (humeral and ulnar heads) and the medial half of the flexor digitorum profundus (FDP), which powers DIP flexion of digits 4 and 5. The internervous plane of surgical dissection in the forearm lies between the FCU (ulnar nerve) and the flexor digitorum superficialis (median nerve).
• Guyon Canal & Anatomic Zones: Measuring 4 to 4.5 cm in length, Guyon canal is bounded by the volar carpal ligament and palmaris brevis superficially, the transverse carpal ligament and pisohamate ligament deeply, the pisiform medially, and the hook of the hamate laterally. Entrapment is classified into three zones: Zone 1 (proximal, mixed trunk), Zone 2 (distal deep motor branch surrounding the hamulus), and Zone 3 (distal superficial sensory branch).
• Terminal Deep Motor Branch Distribution: Exiting Guyon canal, the deep motor branch pierces the opponens digiti minimi, passes around the hook of the hamate, and travels across the deep palm within the subfascial plane. It supplies all hypothenar muscles (abductor digiti minimi [ADM], flexor digiti minimi brevis [FDMB], opponens digiti minimi [ODM]), all dorsal interossei (DAB 1 to 4: dorsal abducts), all palmar interossei (PAD 1 to 3: palmar adducts), the third and fourth lumbricals (digits 4 and 5), the adductor pollicis (transverse and oblique heads), and the deep head of the flexor pollicis brevis (FPB).

The biomechanics of intrinsic hand function and the resulting deformities are governed by precise vector mechanics:

• Biomechanics of the Intrinsic-Minus Hand (Duchenne Sign): Normal interossei and lumbricals pass volar to the transverse metacarpal ligament and the MCP joint axes, inserting into the extensor expansions and lateral bands dorsal to the IP joint axes. They act as dynamic MCP flexors and IP extensors. Loss of intrinsic motor innervation eliminates the primary MCP flexion vector. Unopposed pull from the extrinsic extensor digitorum communis (EDC) pulls the MCP joints into hyperextension. As the MCP hyperextends, EDC excursion is exhausted, loosening tension across the extensor hood. Concurrently, intact extrinsic flexors (FDP and FDS) exert unchecked flexion forces across the PIP and DIP joints, culminating in classic clawing.
• Biomechanics of the Ulnar Paradox: In a low ulnar nerve lesion (at the wrist), the intrinsic muscles are paralyzed, but the extrinsic FDP 4/5 units remain fully innervated. The intact FDP exerts maximal flexion moments across the denervated PIP and DIP joints, creating pronounced, severe clawing. In a high ulnar nerve lesion (at the elbow), the FDP to digits 4 and 5 is denervated; without active deep extrinsic flexion force, the DIP joints remain passive and relatively extended, softening the visual deformity despite greater total neurological impairment.

4. Step-by-Step Examination Technique

Patient Positioning

The patient is seated comfortably opposite the examiner with both upper extremities exposed from the mid-arm to the fingertips. The forearm rests supported on an examination table in neutral rotation and 90° of elbow flexion, allowing simultaneous side-by-side comparison of muscle bulk, resting digital posture, and web space fullness.

Examiner Hand Placement & Setup

The examiner uses one hand to firmly stabilize the proximal skeletal segment (preventing compensatory forearm, wrist, or adjacent finger movement) while the manipulating hand applies isolated, directional resistance against the tested digit. The examiner palpates the target muscle belly directly during contraction to distinguish true active motor recruitment from passive tenodesis or adjacent tendon substitution.

Provocative Maneuver Execution

A systematic ulnar nerve motor examination incorporates standardized provocative maneuvers, intrinsic muscle isolation, and extrinsic tendon testing:

• Froment Test (Adductor Pollicis Compensation): The patient grasps a standard sheet of paper between the volar pulp of the thumb and the radial aspect of the index finger middle phalanx (key pinch posture). The examiner firmly attempts to pull the paper away while instructing the patient to resist with maximal effort. The examiner directly observes the thumb interphalangeal (IP) joint for compensatory flexion.
• Jeanne Test (Thumb MCP Instability): Performed simultaneously during the Froment key pinch maneuver. The examiner observes the thumb metacarpophalangeal (MCP) joint for dynamic hyperextension (>20° to 30°) as the patient pinches forcefully against resistance.
• Wartenberg Sign (Fifth Digit Abduction Posture): The patient rests the pronated hand on the examination table with all fingers fully extended. The patient is instructed to actively adduct all digits together flush against one another. The examiner observes whether the small finger fails to adduct against the ring finger or drifts into persistent, involuntary abduction.
• Egawa Test (Middle Finger Lateral Deviation): The patient places the palm completely flat on a rigid, hard table surface with digits extended. The patient is asked to actively abduct the middle finger toward the radial side (testing the second dorsal interosseous) and then adduct/deviate it toward the ulnar side (testing the third dorsal interosseous), keeping the palm and adjacent digits pinned flat to the table.
• Pitres-Testut Sign (Pitres Sign of Small Finger Adduction): With the hand resting palm down on a flat surface and all fingers spread widely in extension, the patient is instructed to scrape or sweep the small finger along the table surface toward the ring finger without lifting the digit or flexing the interphalangeal joints.
• Cross-Finger Test (Interosseous Coordination): The patient is asked to extend the index and middle fingers and actively cross the index finger over the dorsal aspect of the middle finger without using the thumb or external assistance.
• Bouvier Maneuver (Extensor Apparatus Competence): In a patient exhibiting clawing of the ring and small fingers, the examiner manually blocks and stabilizes the MCP joints in 30° to 45° of flexion, eliminating hyperextension. The patient is then asked to actively fully extend the PIP and DIP joints while MCP flexion is maintained.
• First Dorsal Interosseous (FDI) Manual Muscle Testing: With the hand resting palm down on the table, the patient actively abducts the extended index finger radially. The examiner places a finger on the radial border of the index proximal phalanx, resisting abduction while palpating the contractile bulk of the first dorsal web space.
• Abductor Digiti Minimi (ADM) Manual Muscle Testing: With the hand supinated or flat, the patient actively abducts the small finger away from the ring finger. The examiner resists abduction by applying force against the ulnar aspect of the small finger proximal phalanx while palpating the hypothenar muscle belly.
• Flexor Carpi Ulnaris (FCU) Manual Muscle Testing: The patient actively flexes and ulnarly deviates the wrist against examiner resistance applied across the hypothenar carpus. The examiner palpates the taut FCU tendon along the volar-ulnar wrist just proximal to the pisiform.
• Flexor Digitorum Profundus (FDP 4/5) Manual Muscle Testing: The examiner firmly immobilizes the MCP and PIP joints of digits 4 and 5 in complete extension, isolating the distal interphalangeal (DIP) joints. The patient is instructed to actively flex the DIP joint against resistance applied to the distal fingertip pulp.

5. Positive Test Criteria & Interpretation

Each provocative maneuver and muscle isolation test provides specific diagnostic criteria reflecting underlying biomechanical compensation or focal motor paralysis:

• Froment Sign Interpretation: Positive when the patient exhibits acute hyperflexion of the thumb interphalangeal (IP) joint (>30°) to maintain grip on the paper. This reflects profound weakness or paralysis of the ulnar-innervated adductor pollicis, forcing compensatory recruitment of the median-innervated flexor pollicis longus (FPL).
• Jeanne Sign Interpretation: Positive when the thumb MCP joint dynamically hyperextends (>20° to 30°) during pinch. This demonstrates structural instability and loss of the stabilizing flexion vector normally provided by the adductor pollicis and deep head of the flexor pollicis brevis (FPB), unmasking capsular attenuation under lateral stress.
• Wartenberg Sign Interpretation: Positive when the small finger remains abducted away from the ring finger or cannot be adducted flush in full extension. This denotes paralysis of the third palmar interosseous muscle (PAD), leaving the small finger subject to the unopposed abducting vector of the radial-innervated extensor digiti minimi (EDM) and intact abductor digiti minimi.
• Egawa Test Interpretation: Positive when the patient cannot actively sweep the middle finger radially (2nd dorsal interosseous deficit) or ulnarly (3rd dorsal interosseous deficit) while the hand is held flat on the table, confirming intrinsic motor denervation.
• Pitres-Testut Sign Interpretation: Positive when the patient is unable to adduct the extended small finger toward the ring finger along the table surface, confirming selective paralysis of the 3rd palmar interosseous.
• Cross-Finger Test Interpretation: Positive when the patient fails to cross the index finger over the middle finger, establishing severe motor weakness of the first dorsal interosseous and second palmar interosseous.
• Positive Bouvier Test Interpretation: When manual blocking of MCP hyperextension enables the patient to actively achieve complete PIP and DIP joint extension. Clinical Interpretation: The extrinsic extensor digitorum communis (EDC) and central slip apparatus are intact; the claw deformity is purely dynamic due to loss of the intrinsic MCP flexion vector. Surgical Implication: The patient is an ideal candidate for an isolated MCP stabilization procedure (e.g., Zancolli lasso tenodesis or capsulodesis).
• Negative Bouvier Test Interpretation: When manual blocking of MCP hyperextension fails to restore active PIP/DIP extension. Clinical Interpretation: Extensor central slip attenuation, secondary sagittal band rupture, or a structural, fixed PIP joint flexion contracture (>30°) has developed. Surgical Implication: An isolated MCP flexion block will fail; the patient requires formal joint contracture release or reconstructive tendon transfers routed directly into the lateral bands (e.g., Stiles-Bunnell or Brand transfer).

Individual muscle strength is systematically quantified using the Medical Research Council (MRC) scale:

Quantitative dynamometry provides objective statistical baselines:

• Jamar Grip Dynamometry: Tested across all 5 handle positions (producing a normal bell-shaped curve). Ulnar motor neuropathy characteristically causes a >50% reduction in total grip strength compared to the contralateral dominant hand, with marked flattening of the force curve at wider handle spans due to loss of intrinsic MCP flexion power.
• Key / Lateral Pinch Dynamometry: Evaluated using a calibrated pinch gauge. Because the adductor pollicis provides up to 80% of key pinch force, motor denervation typically produces a profound 70% to 80% reduction in lateral pinch capacity relative to normal baseline.

6. Diagnostic Accuracy & Statistical Benchmarks

The diagnostic accuracy of physical examination maneuvers for ulnar nerve motor evaluation has been established through prospective electrodiagnostic and operative validation trials:

7. Confounders, Diagnostic Traps & False Results

Accurate motor diagnosis requires systematic differentiation between true intrinsic denervation, anatomical neural variants, and biomechanical compensation:

• Martin-Gruber Anastomosis (MGA): Present in 15% to 20% of the normal population. Consists of a median-to-ulnar motor axon crossover in the proximal forearm, originating from the anterior interosseous nerve (AIN) or main median trunk and joining the ulnar nerve. In a complete high ulnar nerve laceration or severe cubital tunnel syndrome, motor axons bypassing the elbow via the median nerve re-enter the ulnar nerve distally, resulting in paradoxical preservation of the first dorsal interosseous and hypothenar muscles (false-negative examination). Electrodiagnostic stimulation of the median nerve at the elbow elicits an intrinsic hand CMAP, resolving the diagnostic anomaly.
• Riche-Cannieu Anastomosis (RCA): An anomalous communication in the palm between the recurrent motor branch of the median nerve and the deep motor branch of the ulnar nerve. RCA can provide median motor innervation to the adductor pollicis, flexor pollicis brevis, or first dorsal interosseous. In patients with complete ulnar nerve injury, RCA may preserve key pinch or index abduction, mimicking an intact ulnar nerve.
• Thumb Carpometacarpal (CMC) Osteoarthritis (Basal Joint Arthrosis): Painful thumb CMC arthritis causes severe weakness of lateral pinch and avoidance posturing that closely mimics a positive Froment or Jeanne sign (false-positive). Clinicians must perform the thumb axial grind test and palpate the adductor pollicis directly during isometric contraction to verify active motor tone.
• Flexor Pollicis Longus (FPL) Substitution Tricks: Patients with chronic intrinsic loss develop unconscious habituation patterns, firing the FPL to lock the thumb against the index finger during key pinch. The examiner must manually stabilize the thumb MCP joint in extension or 15° of flexion to isolate the adductor pollicis vector and eliminate false pinch stability.
• Wrist Extension Tenodesis Effects: Patients may actively extend the wrist to exploit passive finger flexion via flexor tenodesis, simulating intrinsic grip strength. All intrinsic motor testing must be conducted with the wrist maintained strictly in neutral (0° flexion/extension).
• Joint Hypermobility & Pseudo-Signs: Generalized ligamentous laxity (Beighton score ≥5) allows physiological thumb IP hyperflexion or MCP hyperextension during key pinch in the absence of denervation (Pseudo-Froment and Pseudo-Jeanne signs). Similarly, fifth-finger extensor tendon subluxation or sagittal band rupture can produce an abducted small finger posture mimicking Wartenberg sign (Pseudo-Wartenberg sign). Always test and compare against the contralateral asymptomatic extremity.

8. Comparative Utility & Clinical Decision Algorithm

Physical examination is the initial foundation of clinical decision-making, differentiating anatomical levels and directing surgical escalation:

Systematic Physical Examination Sequence

• Step 1 (Visual Inspection): Inspect for resting digital posture (ulnar claw hand), abduction of the small finger (Wartenberg sign), first web space hollowing, and guttering between metacarpals.
• Step 2 (Palpation & Provocative Testing): Palpate the retroepicondylar groove for nerve subluxation or tenderness; perform 60-second pressure provocation test and Tinel percussion across cubital tunnel and Guyon canal.
• Step 3 (Extrinsic Motor Testing): Quantify FCU and FDP 4/5 strength to localize high versus low lesion level.
• Step 4 (Intrinsic Motor Testing): Assess FDI, ADM, and AP strength; perform Egawa test, Pitres sign, and Cross-finger test.
• Step 5 (Pathognomonic Motor Signs): Execute Froment and Jeanne tests during resisted key pinch.
• Step 6 (Biomechanical Assessment): Perform the Bouvier maneuver in clawed digits to evaluate extensor mechanism competence.
• Step 7 (Quantitative Dynamometry): Measure Jamar grip strength and pinch gauge key pinch force.

High vs. Low Lesion Differentiation (The Ulnar Paradox)

• High Lesion (Cubital Tunnel / Elbow): FCU is weak; FDP 4/5 is paralyzed (absent DIP flexion); dorsal ulnar cutaneous nerve is involved (dorsoulnar numbness); digital clawing is deceptively MILD because lack of FDP pull eliminates active DIP flexion.
• Low Lesion (Guyon Canal / Wrist): FCU is normal; FDP 4/5 is intact (strong active DIP flexion); dorsal sensation is preserved (DUCN branches 5–7 cm proximal to wrist); digital clawing is SEVERE and EXAGGERATED because intact FDP exerts unchecked flexion on denervated, hyperextended digits.

Differentiation from Proximal Neurologic Mimics

• C8-T1 Cervical Radiculopathy: Involves median-innervated thenar muscles (abductor pollicis brevis) and radial-innervated C8 muscles (extensor indicis proprius), which are spared in isolated ulnar neuropathy. Sensation is impaired along the medial forearm (T1 dermatome), whereas ulnar entrapment spares medial forearm sensation (supplied by medial antebrachial cutaneous nerve). Spurling test is frequently positive.
• Neurogenic Thoracic Outlet Syndrome (TOS): True neurogenic TOS (lower trunk brachial plexus compression) produces the “Gilliatt-Sumner hand” with prominent thenar atrophy (APB > FDI) combined with hypothenar wasting, medial forearm sensory loss, and positive elevated arm stress testing (Roos test).

Triage & Management Algorithm

• Mild Neuropathy (McGowan I / Dellon Mild): Normal motor power (MRC 5/5), absent atrophy, intermittent paresthesias. First-line management: rigid night splinting in 45° of elbow flexion, daytime ergonomic pads, and nerve gliding exercises. Re-evaluate at 6 to 12 weeks.
• Moderate Neuropathy with Motor Axon Loss (McGowan II / Dellon Moderate): Measurable intrinsic weakness (MRC 3–4/5), early first web space atrophy, motor conduction velocity <50 m/s. Indication: surgical decompression (in situ open/endoscopic release, or anterior transposition if dynamic subluxation is documented).
• Severe Neuropathy with Active Denervation <12 Months (McGowan III): Profound intrinsic paralysis (MRC 0–2/5), severe wasting, active fibrillations on needle EMG confirming viable motor endplates. Indication: surgical decompression combined with Supercharged End-to-Side (SETS) Anterior Interosseous Nerve (AIN) to deep ulnar motor nerve transfer in the distal forearm to rescue hand intrinsics before irreversible endplate fibrosis occurs.
• Chronic Irreversible Paralysis with Positive Bouvier Test: Longstanding denervation (>12 to 18 months) with complete muscle fibrosis. Decompression or nerve transfer is ineffective. Proceed with Zancolli lasso procedure (FDS looped through A1/A2 pulleys) to restore dynamic MCP flexion, combined with ECRL/BR transfer to adductor pollicis for key pinch.
• Chronic Irreversible Paralysis with Negative Bouvier Test: Fixed PIP flexion contractures or extensor central slip attenuation mandate contracture release or reconstructive tendon transfers routed directly into the lateral bands (Stiles-Bunnell or Brand transfer), combined with EIP to FDI transfer for index finger abduction.

9. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

10. References

• Caliandro P, La Torre G, Padua R et al. Treatment for ulnar neuropathy at the elbow. Cochrane Database Syst Rev. 2016. doi:10.1002/14651858.CD006839.pub4 [Level I: Systematic Review / Meta-Analysis]
• Novak CB, Lee GW, Mackinnon SE, Lay L. Provocative testing for cubital tunnel syndrome. J Hand Surg Am. 1994. doi:10.1016/0363-5023(94)90193-7 [Level II: Prospective Diagnostic Study]
• Boone S, Gelberman RH, Calfee RP. The Management of Cubital Tunnel Syndrome. J Hand Surg Am. 2015. doi:10.1016/j.jhsa.2015.03.011 [Level V: Expert Review & Diagnostic Synthesis]
• Shah CM, Calfee RP, Gelberman RH et al. Outcomes of rigid night splinting and activity modification in the treatment of cubital tunnel syndrome. J Hand Surg Am. 2013. doi:10.1016/j.jhsa.2013.02.039 [Level IV: Prospective Cohort Study]
• Burahee AS, Sanders AD, Shirley C et al. Cubital tunnel syndrome. EFORT Open Rev. 2021. doi:10.1302/2058-5241.6.200129 [Level V: Narrative Review & Clinical Synthesis]

11. Master Class Pro-Tip

When evaluating suspected ulnar motor neuropathy or distinguishing high cubital tunnel compression from low Guyon canal lesions, the examiner must systematically eliminate false-positive and false-negative traps arising from compensatory mechanics and anomalous neural crossovers. During the Froment key pinch test, always manually stabilize the thumb metacarpophalangeal (MCP) joint in 15 degrees of flexion while the patient pinches against resistance; this prevents pseudonormal pinch stability achieved by volar capsular loading and unmasks subtle, early adductor pollicis weakness that would otherwise be concealed. To definitively isolate the deep motor branch distal to Guyon canal, evaluate the first dorsal interosseous (FDI) muscle with the index finger held in slight hyperextension and full radial abduction against resistance while directly palpating the contractile bulk of the first dorsal web space. If profound clawing of digits 4 and 5 is present but FDI strength remains inexplicably normal (MRC grade 5/5) with normal motor conduction across the elbow, suspect a Martin-Gruber anastomosis with median-to-ulnar crossover in the proximal forearm; confirm this crossover by performing comparative stimulation of the median nerve at the antecubital fossa while recording compound muscle action potentials from the first dorsal interosseous. Finally, in advanced ulnar clawing, never commit to a dynamic Zancolli lasso procedure without performing the Bouvier maneuver: if manual stabilization of the MCP joints in 30 to 45 degrees of flexion fails to produce immediate, active distal interphalangeal and proximal interphalangeal extension, the patient has an uncorrectable extensor central slip attenuation or fixed joint contracture requiring dynamic tendon transfer directly into the lateral bands rather than an isolated MCP flexion block.

Master Class Clinical Pearl: Precision Diagnostics in Ulnar Nerve Motor Evaluation