Flexor Carpi Ulnaris Anatomy

Origin - two heads

Humeral head

• From the medial epicondyle of the humerus via the common flexor origin; it is the most ulnar (posterior) component of that origin.

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.
• This aponeurotic origin is shared with flexor digitorum profundus and extensor carpi ulnaris along the same subcutaneous border - the plane exploited in the posterior approach to the ulna.

The arch between the heads

• The two heads are bridged by a tendinous arch - the arcuate ligament of Osborne (the cubital tunnel retinaculum) - which spans from the medial epicondyle to the olecranon.
• The ulnar nerve passes beneath this arch, between the two heads, to enter the forearm.

Insertion

• Pisiform - the principal insertion. The pisiform is a sesamoid bone within the FCU tendon, articulating with the triquetrum.
• Force is transmitted onward by:
  • the pisohamate ligament to the hook of the hamate, and
  • the pisometacarpal ligament to the base of the fifth metacarpal.
• Some fibres continue into the flexor retinaculum and the abductor digiti minimi origin.
• Consequence: the pisiform acts as a sesamoid that displaces the tendon volarly, increasing 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.

Muscle belly

• Unlike FCR, FCU remains muscular almost to the wrist: fleshy fibres persist along the tendon to within a few centimetres of the pisiform.
• Practical consequence: an FCU transfer or graft carries muscle bulk with it, making subcutaneous routing bulkier than an FCR or palmaris longus transfer, and making pure tendon harvest impossible.

Nerve supply

• 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.

Motor entry point - the transposition-critical detail

• The motor branches leave the ulnar nerve at or just distal to the cubital tunnel, typically 1 to 4 cm distal to the medial epicondyle, and there are usually two to four of them.
• They enter the deep surface of the proximal muscle, running obliquely and distally.
• During anterior transposition these branches are the principal distal tether. They must be mobilised by intraneural dissection and traced proximally, not divided. Sacrificing a branch to gain a few millimetres of translation weakens the wrist stabiliser you were trying to protect, and 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; if so it should be the most proximal one only, with the remaining branches preserved.

Localising an ulnar neuropathy

The dorsal cutaneous branch of the ulnar nerve leaves the main trunk about 5 to 8 cm proximal to the ulnar styloid - proximal to Guyon canal. Preserved dorsal ulnar hand sensation with intrinsic weakness therefore localises the lesion distal to that branch, at the wrist.

Arterial supply - a segmental pattern that permits flaps

• Proximal third: the posterior ulnar recurrent artery, arising from the ulnar artery just distal to the elbow and ascending behind the medial epicondyle with the ulnar nerve. This is the dominant proximal pedicle.
• Middle and distal thirds: multiple muscular branches of the ulnar artery, which lies deep to the muscle and radial to the ulnar nerve.
• Inferior ulnar collateral artery contributes at the common flexor origin.

Flap applications

• Proximally based FCU muscle flap: raised on the posterior ulnar recurrent pedicle and rotated to cover exposed olecranon, medial elbow hardware, or an infected elbow wound. The pedicle enters within the proximal third, so the distal two-thirds of the muscle can be mobilised.
• Distally based FCU flap: possible on distal ulnar artery perforators for coverage of 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.
• Nerve consideration: the same operation that raises the flap threatens the FCU motor branches, so partial (split) muscle harvest preserving the innervated proximal portion is preferred where possible.

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; this is the theoretical basis for preferring in-situ decompression when the nerve is stable.

At the elbow - the cubital tunnel

• Roof: the arcuate ligament of Osborne spanning the two heads of FCU. It tightens in elbow flexion, reducing tunnel volume and raising intraneural pressure - the reason symptoms are nocturnal and flexion-provoked.
• Floor: the elbow joint capsule and the posterior band of the medial collateral ligament, with the medial trochlea.
• Medial wall: the medial epicondyle. Lateral wall: the olecranon.
• Distal continuation: the nerve then runs under the deep flexor-pronator aponeurosis (the FCU fascia) between FCU and flexor digitorum profundus - the compression point most often left unreleased.

In the forearm

• Superficial to the ulnar nerve and artery: the FCU muscle belly. The nerve lies 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 the nerve from there distally.
• Radial to FCU: flexor digitorum superficialis and, further radially, palmaris longus and FCR.
• Ulnar and dorsal: the subcutaneous border of the ulna, and beyond it extensor carpi ulnaris.

At the wrist and Guyon canal

• At the wrist the ulnar nerve and artery lie RADIAL to the FCU tendon and pisiform. This is the operative rule: find the FCU tendon and the bundle is on its radial side.
• Guyon canal boundaries:
  • Ulnar wall: the pisiform and the FCU tendon.
  • Radial wall: the hook of the hamate.
  • Floor: the transverse carpal ligament and the pisohamate ligament (and the hypothenar muscles distally).
  • Roof: the volar carpal ligament and palmaris brevis.
• The canal is approximately 4 to 4.5 cm long and is divided into three zones: zone 1 proximal to the bifurcation, zone 2 around the deep motor branch, and zone 3 around the superficial sensory branch.

Cubital tunnel syndrome - the FCU perspective

• 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 tunnel cross-section falls as the arcuate ligament tightens and the medial collateral ligament bulges into the floor; intraneural pressure rises several-fold. 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, later intrinsic wasting with the first dorsal interosseous the most visible. Sensory loss involves the dorsal ulnar hand because the dorsal cutaneous branch arises proximal to the wrist.
• The ulnar paradox: clawing of the ring and little fingers is LESS marked in a high (elbow) lesion than in a low (wrist) lesion, because the ulnar half of flexor digitorum profundus is also denervated and cannot flex the distal interphalangeal joints.

Investigation

• Nerve conduction studies with inching across the elbow; a conduction velocity drop across the elbow segment supports the diagnosis. Note that 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 for cubitus valgus, osteophytes, a prominent medial epicondyle or an old fracture (tardy ulnar palsy).

FCU as the landmark of Guyon canal

• The pisiform and the FCU tendon form the ulnar wall of the canal, and the hook of hamate the radial wall. Every safe exposure of the canal begins by identifying the FCU tendon and working along its radial border.
• Zones and syndromes (after Gross and Gelberman):
  • Zone 1 - proximal to the bifurcation. Compression produces combined motor and sensory loss.
  • Zone 2 - around the deep motor branch. Produces pure motor loss of the intrinsics; whether the hypothenar muscles are involved depends on the exact level.
  • Zone 3 - around the superficial branch. Produces pure sensory loss.
• Causes: 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, and repetitive external pressure such as prolonged cycling.
• Key discriminator from cubital tunnel syndrome: in a Guyon canal lesion FCU is normal, the ulnar half of flexor digitorum profundus is normal, and dorsal ulnar hand sensation is normal, because all three are supplied proximal to the canal.

Calcific tendinitis at the pisiform

• Presentation: 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.
• Diagnosis: 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; joint aspiration is negative for crystals and organisms.
• 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. Recognising it prevents an unnecessary washout.

FCU insertional tendinopathy

• Chronic ulnar wrist pain in racquet and stick sports, rowers and manual workers. Tenderness lies on the tendon proximal to the pisiform rather than over the pisotriquetral joint.
• Frequently coexists with pisotriquetral arthritis; ultrasound distinguishes tendon thickening and sheath fluid from joint pathology.
• Treated with rest, splinting and eccentric loading; a peritendinous (not intratendinous) injection may help. Refractory cases with established pisotriquetral arthritis are managed by pisiform excision.

Pisotriquetral arthritis and pisiform excision

• Presentation: pain leaning on the heel of the hand, pain on resisted wrist flexion, tenderness and crepitus on pisiform shear.
• Excision technique and hazards: 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 are 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, a mobile pisiform, and weak ulnar deviation. Repair is worthwhile in high-demand patients given FCU's stabilising role.

In-situ decompression

• Incision: longitudinal, posterior to the medial epicondyle, centred on the cubital tunnel, 6 to 8 cm. Keeping the incision 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, typically 1.5 to 3.5 cm distal to the medial epicondyle.
• Release sequence, 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.
• The single most common technical failure is stopping at Osborne ligament. The distal FCU fascia must be released; adequate decompression means the nerve is free for approximately 5 to 8 cm proximal and distal to the epicondyle with no residual band on passive elbow flexion.
• Check for subluxation after release by flexing the elbow through a full arc. If the nerve now dislocates anteriorly over the epicondyle, transposition is required.

Anterior transposition

• Indications: ulnar nerve subluxation or dislocation, prior failed in-situ release, cubitus valgus or a deformed medial epicondyle, and nerve tethering by scar.
• Additional steps beyond in-situ release:
  • Excise the medial intermuscular septum rather than merely divide it - the transposed nerve will otherwise angulate over its edge, creating a new compression point.
  • Mobilise the nerve circumferentially while preserving the longitudinal vascular plexus where possible.
  • Mobilise the FCU motor branches by intraneural dissection so the nerve can translate anteriorly without tethering. Divide only the most proximal branch, and only if unavoidable.
  • 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 - excessive resection destabilises the origin of the medial collateral ligament and can produce valgus instability.

Exposure of Guyon canal

• Incision: a longitudinal or gently curved incision along the radial border of the FCU tendon and pisiform, extending distally into the palm across the wrist crease obliquely (never transversely).
• Sequence:
  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, passing 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, note that 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. Still the reference operation.

Rationale. The overactive FCU is the dominant deforming force. Transferring it dorsally converts a deforming flexor into a wrist extensor, improving resting posture, appearance, hygiene and, in selected patients, grasp and release.

Patient selection - the determining factor.

• Voluntary control of the FCU is essential: the patient must be able to activate wrist flexion on command. An FCU that fires only as part of a mass spastic pattern will not become a useful extensor.
• Adequate passive wrist extension must be present; a fixed contracture requires release or carpal-level correction first.
• The ability to release objects with the wrist extended must be tested preoperatively by passively holding the wrist extended and asking the patient to open the hand. If finger extension is inadequate in that position, transfer to extensor digitorum communis rather than ECRB is preferred, or the operation is reconsidered.
• Preserved sensation and stereognosis, and cognitive ability to participate in therapy, predict a functional as opposed to purely cosmetic gain.

Technique.

• Detach FCU from the pisiform through a longitudinal ulnar incision, preserving as much tendon length as possible.
• 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.
• Route the tendon subcutaneously around the ulnar border of the forearm to the dorsum - superficial to the extensor carpi ulnaris, in a wide smooth tunnel to avoid an angular pull.
• Weave into extensor carpi radialis brevis (for wrist extension with a neutral deviation vector) using a Pulvertaft weave, tensioned with the wrist in about 30 degrees of extension and the elbow flexed.
• Immobilise in a long-arm cast in wrist extension for 4 weeks, then a removable splint and graded therapy.

Choice of insertion.

• ECRB - the standard: a central, balanced wrist extension vector without radial deviation.
• ECRL - risks radial deviation deformity and is generally avoided.
• EDC - chosen when finger extension is inadequate with the wrist held extended.

Pitfalls. Over-tensioning produces a fixed extension deformity that is harder to live with than the original flexion posture; under-tensioning achieves nothing. Transferring an FCU without voluntary control converts a flexion deformity into an extension deformity.

FCU in radial nerve palsy

• The historical Jones transfer used FCU to extensor digitorum communis. Modern practice prefers FCR to EDC to preserve FCU as the wrist stabiliser during grip. Know both and be able to defend the modern preference.

FCU muscle flap

• A proximally based FCU flap on the posterior ulnar recurrent pedicle covers the olecranon, medial elbow and exposed hardware. Elevate from distal to proximal, preserving the proximal third and its motor branches.
• Donor morbidity is the same as for a transfer - loss of ulnar-sided wrist flexion - so a split flap preserving part of the innervated muscle is preferred where the defect allows.