Approach to Guyon's Canal (Ulnar Nerve at the Wrist)

What it exposes. The volar, ulnar-sided wrist approach gives direct, extensile access to Guyon's canal (the ulnar tunnel) and its contents — the ulnar nerve, the ulnar artery, and both terminal branches of the nerve within and beyond the canal. It is the only approach that allows full decompression of the canal, excision of an offending mass, management of the ulnar artery, and excision of the hook of hamate through a single incision. The canal lies immediately ulnar to the carpal tunnel on the volar surface of the wrist. Why this approach is chosen. Guyon's canal sits ulnar to the carpal tunnel, so a volar ulnar incision gives the only route to the deep motor branch as it curves around the hook of hamate. A coexistent median nerve compression is common, and the incision can be extended radially to release the carpal tunnel in the same setting. Primary indications. - Ulnar tunnel syndrome (Guyon's canal syndrome) — compressive neuropathy of the ulnar nerve within the canal, with motor, sensory, or mixed deficit depending on the zone affected.

• Space-occupying lesions within the canal — ganglion cyst (most common), lipoma, anomalous muscle (accessory abductor digiti minimi), or fibrous band.
• Ulnar artery pathology — thrombosis or aneurysm (hypothenar hammer syndrome) requiring exploration, resection, or reconstruction.
• Hook of hamate pathology — symptomatic fracture nonunion (os hamuli syndrome) requiring excision of the hook.
• Iatrogenic or post-traumatic compression — following carpal tunnel release, distal radius fixation, or local trauma. Contraindications. Active infection or compromised skin over the ulnar volar wrist; a symptom pattern fully explained by cubital tunnel or cervical disease without electrodiagnostic or imaging confirmation of canal pathology; medical unfitness for elective hand surgery; and predominantly proximal ulnar nerve pathology better addressed at the elbow or forearm. Alternative and combined approaches. A carpal tunnel approach (extend radially for combined release); a medial forearm (FCU-sparing) approach to the ulnar nerve for proximal extension into the distal forearm when pathology tracks proximally; and a dorsal approach to the hook of hamate for isolated hook excision, though the volar route is preferred because it also decompresses the canal. ### Anatomy you must know before you cut The canal is a soft osseofascial space (not a tight fibrosseous tunnel). Its boundaries are the high-yield anatomy: | Boundary | Structure | |----------|-----------| | Roof (volar) | Volar (palmar) carpal ligament (superficial lamina of the flexor retinaculum), reinforced proximally by palmaris brevis | | Floor (dorsal) | Transverse carpal ligament (deep lamina) and the pisohamate ligament, over the pisometacarpal ligament and opponens digiti minimi | | Ulnar (medial) wall | Pisiform (proximally) and the pisohamate ligament / abductor digiti minimi | | Radial (lateral) wall | Hook of hamate (distally) and the transverse carpal ligament | Contents. The ulnar nerve enters lateral to the FCU tendon and medial to the ulnar artery, then bifurcates within the canal. The ulnar artery lies RADIAL to the nerve, accompanied by vene comitantes. The superficial (sensory) branch supplies palmaris brevis and sensation to the hypothenar and the ulnar one and a half digits. The deep (motor) branch passes lateral to the hook of hamate, pierces the hypothenar muscles, and crosses the palm with the deep palmar arch to supply all intrinsics (interossei, adductor pollicis, hypothenar, medial two lumbricals). The bifurcation and the three zones. The ulnar nerve divides into its superficial sensory and deep motor branches within the canal, typically at the level of the hook of hamate (distal to the pisiform). This division defines the zones:

The zone predicts the lesion: the clinical and electrodiagnostic picture tells you which zone to prioritise at surgery. ### Examination and localisation before surgery Localising the level is essential before operating, because the differential includes cubital tunnel and cervical disease. - Motor — test first dorsal interosseous, adductor pollicis (Froment's sign), and finger abduction; intrinsic wasting indicates motor branch involvement.

• Sensory — test the volar ulnar one and a half digits (superficial branch) VERSUS the dorsoulnar hand (dorsal cutaneous branch). A SPARED dorsoulnar dorsum localises the lesion to Guyon's canal, because the dorsal cutaneous branch arises proximal to the canal and is therefore spared.
• Vascular — palpate the ulnar pulse, perform Allen's test to assess ulnar artery patency, and look for a hypothenar mass or callus.

Investigations. Electrodiagnostics (NCS/EMG) confirm compression and localise wrist versus elbow (slowing across the wrist with a preserved dorsal cutaneous response) and grade severity. Ultrasound is first-line imaging for a ganglion, aneurysm, or anomalous muscle. MRI characterises soft tissue masses, hook of hamate fracture or nonunion, and ulnar artery pathology. CT (carpal tunnel view) is hook-of-hamate-specific when fracture or nonunion is suspected but not seen on plain films. Plain wrist radiographs including a carpal tunnel view assess the pisiform, hook of hamate, and degenerative pisotriquetral change. ### Position and landmarks Supine with the affected arm abducted onto a radiolucent hand table, the limb exsanguinated and a pneumatic tourniquet on the upper arm for a bloodless field (essential, because the ulnar artery is the key danger). The hand is fully supinated (palm up). The surgeon sits cephalad with the assistant opposite; loupe magnification and fine instruments are recommended — this is peripheral nerve and vessel surgery. Mark the pisiform at the distal wrist crease, the hook of hamate 1 to 2 centimetres distal and radial to the pisiform, the palmaris longus tendon (incision placed just ulnar to it), and the FCU tendon as the ulnar border.

Work down through the layers ulnar to the palmaris longus, protecting the dorsal cutaneous branch in the proximal flap, then open the volar carpal ligament (the roof) onto the nerve and artery and trace the deep motor branch around the hook of hamate. There is no muscle plane to develop — this is a direct subcutaneous exposure.

Structures at risk, by layer.

Extensile options. Extend proximally along the radial side of the FCU tendon into the distal forearm to expose the ulnar nerve proximal to Guyon's canal (the medial forearm approach) when pathology tracks proximally or for proximal neurolysis. Extend distally into the palm following the deep motor branch toward the deep palmar arch for full motor-branch decompression and access to the superficial branch to the digits. The incision can be connected to a carpal tunnel incision for combined median and ulnar nerve decompression when both are confirmed. Closure. Release the tourniquet before closure and achieve meticulous haemostasis, particularly around the ulnar artery, to avoid a haematoma compressing the nerve. The volar carpal ligament does NOT require reconstruction — leaving the roof open is standard and does not cause bowstringing, because the canal is a soft osseofascial space rather than a tight fibrosseous tunnel. Close the skin with interrupted non-absorbable or subcuticular sutures, everting the zig-zag corners to avoid trapdoor scarring, and apply a bulky dressing with a volar wrist splint.

• Ulnar nerve decompression at Guyon's canal — release of the volar carpal ligament and the hypothenar fascial edge; the principal operation through this exposure.
• Ganglion excision — typically a pisotriquetral joint ganglion projecting into zone 2.
• Hook of hamate excision — for symptomatic nonunion (os hamuli syndrome).
• Ulnar artery exploration, resection, or reconstruction — for hypothenar hammer syndrome (thrombosis or aneurysm), with reversed interposition vein graft reconstruction when the palmar arch is incomplete.
• Mass excision — lipoma, anomalous muscle (accessory abductor digiti minimi), or fibrous band.
• Ulnar nerve neurolysis — external neurolysis of the nerve and its branches.
• Combined carpal tunnel release — by extending the incision radially when median nerve compression coexists.

Guidelines, registries and global practice. Ulnar tunnel syndrome and the volar approach to Guyon's canal are managed worldwide on the same anatomical and surgical principles, examined across contemporary training systems. The zone-based classification is near-universal and drives surgical decision-making. International hand-surgery consensus decompresses the canal by releasing the volar carpal ligament (roof) while preserving the transverse carpal ligament (floor) unless a concurrent carpal tunnel release is performed. AO trauma practice treats symptomatic hook of hamate nonunion by excision through the volar approach, because rigid fixation is rarely required. Vascular hand-surgery practice images ulnar artery lesions (hypothenar hammer syndrome) with Doppler plus CT or MR angiography and uses resection with reversed interposition vein graft when the palmar arch is incomplete. In high-resource settings ultrasound and MRI routinely identify a compressive mass before surgery and microvascular reconstruction is available; in resource-limited settings the diagnosis is often clinical with nerve conduction studies where available, and ganglion excision and canal decompression are performed through the same volar approach with basic instrumentation. Consent (globally applicable). Discuss the small risk of injury to the ulnar nerve and artery, residual or recurrent symptoms (particularly if the cause is not fully resectable), wound complications, complex regional pain syndrome, and the possibility that coexisting cubital tunnel or cervical disease contributes to the picture and may need separate management.