Anterior Interosseous Nerve Anatomy

Course

• Origin: Leaves the Median nerve 5-8 cm distal to lateral epicondyle, as it passes between the two heads of Pronator Teres.
• Path: Travels initially parallel to the Median nerve, then dives deep.
• Relations: Runs between FPL (radially) and FDP (ulnarly) on the anterior surface of the Interosseous Membrane.
• Vessels: Accompanied by the Anterior Interosseous Artery (branch of Common Interosseous).
• Termination: Passes deep to Pronator Quadratus, supplying it, and ends as sensory branches to the radiocarpal, midcarpal, and CMC joints.

The nerve lies strictly on the interosseous membrane.

Innervation

1. Flexor Pollicis Longus (FPL): Flexes the IPJ of the thumb.
2. Flexor Digitorum Profundus (FDP): To Index (and often Middle) finger. Flexes DIPJ.
3. Pronator Quadratus (PQ): Primary pronator of the forearm.

Note: The FDP to Ring and Little is innervated by the Ulnar nerve.

Sites of Compression

• Gantzer's Muscle: Accessory head of FPL (present in ~45-50% of people). Runs from medial epicondyle to FPL tendon. AIN passes deep to it.
• Pronator Teres: Deep head.
• FDS Arch: Proximal edge of FDS.
• Vascular Leash: Aberrant radial recurrent vessels.
• Lacertus Fibrosus: Thickened bicipital aponeurosis.

Order of release is typically proximal to distal.

Martin-Gruber Anastomosis (MGA)

• Definition: A crossover of motor fibers from Median (AIN) to Ulnar nerve in the forearm.
• Prevalence: ~15% of population.
• Types:
  • Type I: To Thenar muscles (simulating high ulnar palsy if median injured?).
  • Type II: To First Dorsal Interosseous (FDI).
  • Type III: To Hypothenar.

The pattern of innervation guides the diagnosis.

• Significance: In a proximal Ulnar nerve lesion, intrinsic function may be spared (because fibers travel via Median and cross over distally). Sensation is NOT affected by MGA.

Always check MGA in conflicting EMG findings.

Richie-Cannieu Anastomosis

• Definition: Crossover from Ulnar to Median in the PALM (Thenar).
• Significance: All-ulnar hand or All-median hand variants.
• Contrast: MGA is in the Forearm. RCA is in the Palm.

RCA is often associated with a "Cannieu-Richie" hand (All ulnar).

Kiloh-Nevin Syndrome (AIN Palsy)

• Weakness: Loss of FPL (Thumb IP flexion) and FDP Index (DIP flexion).
• OK Sign: Patient cannot make a perfect "O". Instead, they make a "Teardrop" or pinch pinch (pad-to-pad) due to lack of DIP/IP flexion.
• Pronator: Weakness of pronation in the fully flexed elbow (PQ is the primary pronator here; PT is relaxed).
• Sensation: NORMAL. No numbness (different from Pronator Syndrome).

If numbness is present, it is NOT isolated AIN.

Examination

• Pinch Strength: Reduced tip-to-tip pinch.
• Froment's Sign: Negative for Adductor Pollicis (Ulnar), but appearance of "Jeanne's sign" (Thumb MCP hyperextension) may occur due to compensatory forces? Actually, usually Froment's tests Ulnar. In AIN, Adductor is intact.
• Tenodesis Effect: Check if passive wrist extension causes finger flexion to rule out tendon rupture.

Always compare with the contralateral side.

Electrodiagnostics

• EMG: Gold standard. Shows active denervation (fibrillations, positive sharp waves) in FPL, FDP I/II, and PQ.
• Timing: Changes appear 3-4 weeks after onset.
• Differential: Brachial Plexitis (Parsonage-Turner) will show patchy involvement of other nerves (e.g., Suprascapular, Long Thoracic).

Look for "denervation edema" on MRI if EMG is equivocal.

MRI

• Masses: To rule out ganglion cyst, tumor, or lipoma compressing the nerve.
• Neuritis: "Hourglass" constrictions or hyperintensity of the nerve (Parsonage-Turner).
• Muscle: Denervation edema (bright on T2) in PQ is a subtle sign of AIN pathology.

Ultrasound can also show nerve thickening.

Conservative (Standard)

• Indication: Spontaneous palsy (Neuritis/Viral) without a mass.
• Duration: 3-12 months. Most resolve.
• Therapy: Maintain PROM to prevent contractures.
• NSAIDs: For pain (if Neuralgic Amyotrophy).

Avoid surgical release in the acute inflammatory phase.

Surgical Decompression

• Indication: Space-occupying lesion (tumor, cyst) OR failure to recover after 12 months (controversial).
• Technique: Exploration of the proximal forearm. Release of Lacertus, Superficialis Arch, Deep Pronator head, and Gantzer's muscle.
• Results: Unpredictable if diagnosis was Parsonage-Turner.

Ensure release involves all 5 potential sites.

Tendon Transfer

• Indication: Permanent paralysis.
• FDP: Side-to-side suture of Index FDP to Middle/Ring FDP (creates a Mass Action grip).
• FPL: Transfer of Brachioradialis or FDS-Ring to FPL.
• Fusion: IPJ fusion of thumb for stability.

Tendon transfer is the reliable salvage.

Henry's Approach

• Incision: Curvilinear incision crossing the elbow crease obliquely (Lateral to Medial).
• Interval: Between Pronator Teres (Median N) and Brachioradialis (Radial N).
• Superficial: Protect the Lateral Cutaneous Nerve of the Forearm (MCN).

The MCN is often at risk during the incision.

Decompression Steps

1. Lacertus Fibrosus: Divide the bicipital aponeurosis.
2. Pronator Teres: Release the tendinous deep head.
3. FDS Arch: Release the proximal fibrous arch of FDS.
4. Gantzer: Identify and release any accessory FPL head (Gantzer's).
5. Vascular: Ligate crossing radial recurrent vessels (Leash of Henry).

Release must be complete to avoid recurrence.