Flexor Carpi Ulnaris Anatomy

Humeral head. From the medial epicondyle of the humerus via the common flexor origin, of which it is the most ulnar (posterior) component.

Ulnar head. From the medial margin of the olecranon and, by an aponeurosis, from the proximal two-thirds of the subcutaneous posterior border of the ulna. That aponeurotic origin is shared with flexor digitorum profundus and extensor carpi ulnaris along the same subcutaneous border, which is the plane exploited in the posterior approach to the ulna.

The arch between the heads. A tendinous arch, the arcuate ligament of Osborne (the cubital tunnel retinaculum), spans from the medial epicondyle to the olecranon and bridges the two heads. The ulnar nerve passes beneath it, between the heads, to enter the forearm.

View larger

Insertion. The principal insertion is the pisiform, a sesamoid bone within the FCU tendon that articulates with the triquetrum. The pull continues through the pisohamate ligament to the hook of the hamate and the pisometacarpal ligament to the base of the fifth metacarpal, and some fibres run on into the flexor retinaculum and the origin of abductor digiti minimi. As a sesamoid the pisiform displaces the tendon volarly and increases the FCU flexion moment arm; it also means an FCU transfer must be detached from the pisiform, which changes the pull of the pisohamate complex.

The muscle belly. Unlike FCR, FCU remains muscular almost to the wrist, with fleshy fibres persisting along the tendon to within a few centimetres of the pisiform. A transfer or graft therefore carries muscle bulk with it, which makes subcutaneous routing bulkier than an FCR or palmaris longus transfer and makes pure tendon harvest impossible.

Nerve supply. The ulnar nerve, root values C7 and C8 with a variable T1 contribution. The C7 component is the reason FCU can be relatively preserved in a pure C8-T1 lower trunk lesion. FCU and the ulnar half of flexor digitorum profundus are the only muscles the ulnar nerve supplies in the forearm; everything else it supplies is in the hand.

The motor branches. They leave the ulnar nerve at or just distal to the cubital tunnel, typically 1 to 4 cm distal to the medial epicondyle, there are usually two to four of them, and they enter the deep surface of the proximal muscle running obliquely and distally. During anterior transposition they are the principal distal tether and must be mobilised by intraneural dissection and traced proximally, not divided; a kinked or tethered branch produces post-transposition ulnar-sided forearm pain. The most proximal branch may occasionally require division to permit tension-free transposition, and if so it should be the most proximal one only, with the remaining branches preserved.

Localising an ulnar neuropathy. The forearm muscles and the dorsal cutaneous branch are all supplied proximal to the wrist, which is what bedside localisation exploits:

A segmental pattern. The proximal third is fed by the posterior ulnar recurrent artery, which arises from the ulnar artery just distal to the elbow and ascends behind the medial epicondyle with the ulnar nerve; this is the dominant proximal pedicle. The middle and distal thirds receive multiple muscular branches of the ulnar artery, which lies deep to the muscle and radial to the ulnar nerve, and the inferior ulnar collateral artery contributes at the common flexor origin. Because the dominant pedicle enters within the proximal third, the distal two-thirds of the muscle can be mobilised on it, which is what permits the flaps described under Surgical Relevance.

The nerve's own blood supply. The ulnar nerve at the elbow relies on a longitudinal extraneural plexus fed by the superior and inferior ulnar collateral and posterior ulnar recurrent arteries. Circumferential mobilisation for transposition strips this plexus, which is the theoretical basis for preferring in-situ decompression when the nerve is stable.

At the elbow: the cubital tunnel. The roof is the arcuate ligament of Osborne spanning the two heads of FCU. The floor is the elbow joint capsule and the posterior band of the medial collateral ligament, with the medial trochlea; the medial wall is the medial epicondyle and the lateral wall the olecranon. Distal to the tunnel the nerve runs under the deep flexor-pronator aponeurosis (the FCU fascia) between FCU and flexor digitorum profundus.

The medial antebrachial cutaneous nerve. Its posterior branches cross the operative field roughly 1.5 to 3.5 cm distal to the medial epicondyle, and an anterior branch crosses more proximally.

In the forearm. The FCU belly lies superficial to the ulnar nerve and artery: the nerve runs deep to FCU, in the plane between FCU and flexor digitorum profundus. The ulnar artery joins the nerve in the middle third of the forearm and lies radial (lateral) to it from there distally. Radial to FCU are flexor digitorum superficialis and, further radially, palmaris longus and FCR; ulnar and dorsal lie the subcutaneous border of the ulna and, beyond it, extensor carpi ulnaris.

At the wrist. The ulnar nerve and artery lie radial to the FCU tendon and pisiform. That is the operative rule: find the FCU tendon and the bundle is on its radial side.

Guyon canal. Approximately 4 to 4.5 cm long, and bounded by:

• Ulnar wall: the pisiform and the FCU tendon
• Radial wall: the hook of the hamate
• Floor: the transverse carpal ligament and the pisohamate ligament, with the hypothenar muscles distally
• Roof: the volar carpal ligament and palmaris brevis

Gross and Gelberman divided it into three zones: zone 1 proximal to the bifurcation, zone 2 around the deep motor branch and zone 3 around the superficial sensory branch.

The anatomy is the disease. The ulnar nerve traverses five potential constriction points around the elbow, three of which are FCU-related or FCU-adjacent:

1. The arcade of Struthers, roughly 8 cm proximal to the medial epicondyle, a fascial band from the medial head of triceps to the intermuscular septum
2. The medial intermuscular septum
3. The medial epicondyle and retro-condylar groove itself
4. The arcuate ligament of Osborne between the two heads of FCU, the roof of the tunnel proper
5. The deep flexor-pronator aponeurosis distally, where the nerve passes between FCU and flexor digitorum profundus

Why flexion provokes symptoms. In elbow flexion the arcuate ligament tightens and the medial collateral ligament bulges into the floor, the tunnel cross-section falls, and intraneural pressure rises several-fold, which is why symptoms are nocturnal and flexion-provoked. Add wrist extension and shoulder abduction and the nerve is placed on maximal excursion, the basis of the elbow flexion provocative test.

Clinical pattern. Paraesthesiae in the ring and little fingers, nocturnal symptoms, clumsiness, weak grip and pinch, and later intrinsic wasting with the first dorsal interosseous the most visible. Sensory loss involves the dorsal ulnar hand. An elbow-level palsy weakens FCU and the ulnar half of flexor digitorum profundus, and the profundus weakness is why clawing of the ring and little fingers is LESS marked in a high (elbow) lesion than in a low (wrist) lesion, the ulnar paradox: the denervated profundus cannot flex the distal interphalangeal joints into a claw.

Investigation. Nerve conduction studies with inching across the elbow: a conduction velocity drop across the elbow segment supports the diagnosis, though studies may be normal in early or purely dynamic compression. Ultrasound shows nerve enlargement proximal to the compression and dynamically demonstrates subluxation over the epicondyle. Radiographs look for cubitus valgus, osteophytes, a prominent medial epicondyle or an old fracture (tardy ulnar palsy).

What compresses the nerve here. The causes:

• A ganglion arising from the pisotriquetral or hamatocapitate joint, the commonest
• Hook of hamate fracture or nonunion
• Ulnar artery thrombosis or aneurysm (hypothenar hammer)
• Anomalous muscles, including an accessory abductor digiti minimi or an aberrant palmaris brevis
• Repetitive external pressure such as prolonged cycling

Zones and syndromes (after Gross and Gelberman). Compression in zone 1, proximal to the bifurcation, produces combined motor and sensory loss. Compression in zone 2, around the deep motor branch, produces pure motor loss of the intrinsics, and whether the hypothenar muscles are involved depends on the exact level. Compression in zone 3, around the superficial branch, produces pure sensory loss.

How it differs from cubital tunnel syndrome. In a Guyon canal lesion FCU, the ulnar half of flexor digitorum profundus and dorsal ulnar hand sensation are all normal, because all three are supplied proximal to the canal.

Calcific tendinitis at the pisiform. Abrupt, severe, disproportionate ulnar wrist pain with redness, warmth and swelling over the pisiform, often with a low-grade systemic response. It convincingly mimics septic arthritis, gout or cellulitis, and patients are not infrequently admitted for intravenous antibiotics. The giveaway is a radiograph: a supinated oblique or carpal tunnel view shows amorphous calcification just proximal to or at the pisiform, within the FCU tendon. Inflammatory markers may be mildly raised, and joint aspiration is negative for crystals and organisms.

Its management. It is self-limiting. Splinting, non-steroidal anti-inflammatory medication and, where needed, a local corticosteroid injection settle it within days to a few weeks; surgery is essentially never required, and recognising it prevents an unnecessary washout.

FCU insertional tendinopathy. Chronic ulnar wrist pain in racquet and stick sports, rowers and manual workers, with tenderness on the tendon proximal to the pisiform rather than over the pisotriquetral joint. It frequently coexists with pisotriquetral arthritis, and ultrasound distinguishes tendon thickening and sheath fluid from joint pathology. Treatment is rest, splinting and eccentric loading; a peritendinous (not intratendinous) injection may help, and refractory cases with established pisotriquetral arthritis are managed by pisiform excision.

Pisotriquetral arthritis and pisiform excision. Pain leaning on the heel of the hand, pain on resisted wrist flexion, and tenderness and crepitus on pisiform shear. The pisiform is shelled out from within the FCU tendon through a longitudinal incision along the radial border of the tendon and the tendon is repaired; the ulnar nerve and artery lie immediately radial and the deep motor branch curves around the hook of hamate just distal and radial, both within millimetres of the field. Outcomes are generally good, with little measurable loss of wrist flexion power once the tendon is repaired.

Rupture. Attritional or traumatic FCU rupture is uncommon. It presents with loss of the tendon cord on resisted testing, weak ulnar deviation and a palpable, mobile pisiform; attritional rupture at the pisiform leaves the sesamoid without its tendon anchor and it migrates proximally. Repair is worthwhile in high-demand patients given FCU's stabilising role.

In-situ decompression. A longitudinal incision of 6 to 8 cm posterior to the medial epicondyle, centred on the cubital tunnel; keeping it posterior to the epicondyle reduces the chance of a painful scar directly on the point of the elbow. Identify and protect the branches of the medial antebrachial cutaneous nerve crossing the field, then release from proximal to distal:

1. Divide the deep fascia and, if tight, the medial intermuscular septum proximally
2. Divide the arcuate ligament of Osborne, the roof of the tunnel between the FCU heads
3. Continue distally and divide the deep flexor-pronator aponeurosis between the FCU heads and over the nerve as it passes between FCU and flexor digitorum profundus

When the release is adequate. The nerve is free for approximately 5 to 8 cm proximal and distal to the epicondyle with no residual band on passive elbow flexion. Then flex the elbow through a full arc: if the nerve now dislocates anteriorly over the epicondyle, transposition is required.

Anterior transposition. Indicated for ulnar nerve subluxation or dislocation, prior failed in-situ release, cubitus valgus or a deformed medial epicondyle, and nerve tethering by scar. Beyond the in-situ release:

• Excise the medial intermuscular septum rather than merely divide it
• Mobilise the nerve circumferentially, preserving the longitudinal vascular plexus where possible
• Mobilise the FCU motor branches by intraneural dissection so the nerve translates anteriorly without tethering
• Place the nerve anterior to the medial epicondyle and hold it there subcutaneously (a fascial sling), submuscularly (under the flexor-pronator mass, requiring a Z-lengthened or step-cut reattachment) or intramuscularly
• Confirm a smooth path with no kinking through full flexion and extension before closure

Medial epicondylectomy. An alternative that removes the fulcrum without devascularising the nerve. Only a partial epicondylectomy should be performed.

Exposure. A longitudinal or gently curved incision along the radial border of the FCU tendon and pisiform, extended distally into the palm across the wrist crease obliquely, never transversely. Then:

1. Identify the FCU tendon and pisiform, the ulnar wall
2. Divide the volar carpal ligament, the roof, and reflect palmaris brevis
3. Identify the ulnar artery radial to the nerve and follow the nerve to its bifurcation into the superficial sensory and deep motor branches
4. Follow the deep motor branch as it curves dorsally and radially around the hook of the hamate, between the origins of abductor digiti minimi and flexor digiti minimi and beneath the fibrous arch of the hypothenar muscles, a discrete compression point in its own right that must be released

Combined release. Where a carpal tunnel release is also being performed, dividing the transverse carpal ligament decompresses the floor of Guyon canal and often relieves zone 1 symptoms without a separate ulnar release.

Look for the cause. A ganglion, a hook of hamate nonunion, a thrombosed or aneurysmal ulnar artery, or an anomalous muscle. Simply releasing the roof without addressing a space-occupying lesion fails.

The Green transfer: FCU to ECRB in cerebral palsy. Described by Green for the flexed, pronated, ulnar-deviated spastic wrist, and still the reference operation. Transferring the overactive FCU dorsally converts a deforming flexor into a wrist extensor, improving resting posture, appearance, hygiene and, in selected patients, grasp and release.

Patient selection is the determining factor. What to establish before offering it:

• Voluntary control of the FCU: the patient must be able to activate wrist flexion on command, because an FCU that fires only as part of a mass spastic pattern will not become a useful extensor
• Adequate passive wrist extension: a fixed contracture requires release or carpal-level correction first
• The ability to release objects with the wrist extended, tested preoperatively by passively holding the wrist extended and asking the patient to open the hand
• Preserved sensation and stereognosis, and the cognitive ability to participate in therapy, which predict a functional as opposed to purely cosmetic gain

Technique. From the pisiform to the cast:

1. Detach FCU from the pisiform through a longitudinal ulnar incision, preserving as much tendon length as possible
2. Mobilise the muscle proximally along the ulna, protecting the ulnar nerve and artery, which lie deep and radial to it, and preserving the proximal motor branches so the transferred muscle remains innervated
3. Route the tendon subcutaneously around the ulnar border of the forearm to the dorsum, superficial to extensor carpi ulnaris, in a wide smooth tunnel to avoid an angular pull
4. Weave into extensor carpi radialis brevis with a Pulvertaft weave, tensioned with the wrist in about 30 degrees of extension and the elbow flexed
5. Immobilise in a long-arm cast in wrist extension for 4 weeks, then a removable splint and graded therapy

Choice of insertion. ECRB is the standard: a central, balanced wrist extension vector without radial deviation. ECRL risks a radial deviation deformity and is generally avoided. EDC is chosen when finger extension is inadequate with the wrist held extended, the alternative being to reconsider the operation.

FCU in radial nerve palsy. The historical Jones transfer used FCU to extensor digitorum communis. Modern practice prefers FCR to EDC so that FCU is left in place; know both and be able to defend the modern preference.

FCU muscle flap. A proximally based flap on the posterior ulnar recurrent pedicle covers an exposed olecranon, medial elbow hardware or an infected elbow wound. Elevate it from distal to proximal, preserving the proximal third with its pedicle and motor branches; the same dissection threatens those branches, and donor morbidity is the same as for a transfer, loss of ulnar-sided wrist flexion, so a split flap preserving part of the innervated proximal muscle is preferred where the defect allows. A distally based flap on distal ulnar artery perforators can cover the distal forearm and ulnar wrist, but the ulnar artery must be confirmed non-dominant with an Allen test and imaging before any distally based ulnar-artery-dependent flap.