Flexor Pollicis Longus Anatomy

Origin

• The middle third of the volar (anterior) surface of the radius, from just distal to the radial tuberosity and oblique line down to the proximal border of pronator quadratus.
• The adjacent interosseous membrane.
• A variable accessory head - the Gantzer muscle - arising from the medial (coronoid) process of the ulna or from the medial epicondyle via the common flexor origin. It is present in a substantial proportion of forearms in cadaveric series, is usually innervated by the anterior interosseous nerve, and is a recognised potential compressor of that nerve.

Course

• Descends on the radial side of the deep flexor compartment, radial to flexor digitorum profundus, with the anterior interosseous nerve and artery in the interval between the two muscles on the interosseous membrane.
• Becomes tendinous in the distal forearm and passes deep to pronator quadratus and then into the carpal tunnel.
• Within the carpal tunnel it is the most radial and the deepest of the nine tendons, surrounded by its own synovial sheath - the radial bursa.
• It pivots at the distal border of the septum that separates the flexor carpi radialis tunnel from the carpal canal.
• It then passes between the two heads of flexor pollicis brevis and through the thumb flexor sheath under the A1, oblique and A2 pulleys.

Insertion

• The volar base of the distal phalanx of the thumb.

The thumb pulley system - different from the fingers

• A1 pulley at the metacarpophalangeal joint.
• Oblique pulley over the proximal phalanx, running proximal-ulnar to distal-radial, continuous with the insertion of adductor pollicis.
• A2 pulley at the interphalangeal joint.
• A variable variable annular pulley (Av) between A1 and the oblique pulley is described.
• The oblique pulley is the biomechanically critical one. Losing it causes bowstringing and loss of interphalangeal flexion power; A1 can be divided freely - which is exactly what a trigger thumb release does.

Nerve supply

• Anterior interosseous nerve (AIN), a branch of the median nerve, root values C7 and C8 (a T1 contribution is described).
• The AIN arises from the dorsoradial aspect of the median nerve approximately 5 to 8 cm distal to the lateral epicondyle, just after the median nerve passes between the heads of pronator teres and deep to the flexor digitorum superficialis arcade.
• Motor entry: branches enter the deep and radial surface of FPL in its proximal third.
• The AIN is a purely motor nerve (it carries proprioceptive afferents from the wrist joint but no cutaneous sensation). This is the diagnostic anchor of anterior interosseous syndrome.

The three AIN muscles

1. Flexor pollicis longus
2. Flexor digitorum profundus to the index (and variably the middle) finger
3. Pronator quadratus

Localising an isolated FPL palsy

The single most useful bedside discriminator between palsy and rupture: in a nerve lesion the tendon is intact, so passive wrist extension still produces passive thumb interphalangeal flexion (the tenodesis effect). In a rupture that tenodesis effect is lost.

Muscle

• Anterior interosseous artery - the dominant supply, running with the anterior interosseous nerve on the volar surface of the interosseous membrane in the interval between FPL and flexor digitorum profundus.
• Radial artery muscular branches to the radial part of the belly.

Tendon

• Segmental vessels enter from the anterior interosseous system proximally and from the princeps pollicis and digital arteries distally, with synovial diffusion within the flexor sheath.
• The intra-carpal-tunnel segment and the segment crossing the volar rim of the radius are the relatively hypovascular watershed zones - which is the second reason (alongside mechanical abrasion) that attritional rupture happens exactly there rather than elsewhere.

The radial bursa and its infective significance

• The FPL tendon sheath is continuous with the radial bursa, extending from the thumb into the distal forearm.
• The little finger sheath is continuous with the ulnar bursa.
• The two communicate in a substantial proportion of hands through Parona space, deep to the flexor tendons and volar to pronator quadratus. A thumb flexor sheath infection can therefore track to the little finger and back - the horseshoe abscess - and both bursae must be drained.

In the forearm

• Radial to FPL: the radius and, superficially, the radial artery and brachioradialis.
• Ulnar to FPL: flexor digitorum profundus, with the anterior interosseous nerve and artery lying between them on the interosseous membrane - the plane the surgeon must identify before any deep forearm dissection.
• Superficial to FPL: flexor digitorum superficialis and the superficial flexor layer; the median nerve lies on the deep surface of FDS, that is, superficial to FPL.
• Deep to FPL: the radius, interosseous membrane and, distally, pronator quadratus.

At the wrist - the danger zone

• FPL is the most radial and the deepest of the nine carpal tunnel tendons.
• It lies within a few millimetres of the volar cortex of the distal radius at the watershed line, of the scaphoid tubercle, of the scaphotrapezial joint and of the trapezium.
• It pivots at the distal border of the septum separating the FCR tunnel from the carpal canal - a fixed point around which the tendon is dragged.
• The median nerve lies volar and radial to the FDS tendons, superficial to FPL, so FPL is not the tendon encountered first at carpal tunnel release - but it is the tendon lying deepest and most radial once the canal is open.

In the thumb

• Passes between the two heads of flexor pollicis brevis and volar to the metacarpophalangeal joint.
• The radial digital nerve of the thumb crosses obliquely, from ulnar-proximal to radial-distal, directly OVER the FPL sheath at the level of the A1 pulley. At that point it lies superficially and close to the midline - the reason it is the classic casualty of a percutaneous or careless open trigger thumb release.
• The princeps pollicis artery and the digital vessels accompany the nerves on each side.

Rupture over a prominent volar plate

• Mechanism: the plate edge sits at or distal to the watershed line - the most volar transverse ridge of the distal radius, just proximal to the volar rim - where the flexor tendons lie closest to bone. FPL, being most radial and deepest, contacts the plate first.
• The Soong grading of plate prominence on a true lateral radiograph, using a line tangential to the most volar extent of the volar rim parallel to the volar cortex of the shaft:
  • Grade 0: the plate does not extend volar to the line.
  • Grade 1: the plate is volar to the line but proximal to the volar rim.
  • Grade 2: the plate lies directly on or distal to the volar rim.
• Grade 2 prominence carries the tendon risk. In the original comparative series, three flexor tendon ruptures occurred in a group where 63 per cent of plates were grade 2, and none in a group with no grade 2 plates.
• Timeline: rupture typically 6 to 24 months after fixation, usually preceded by volar radial wrist pain and crepitus.
• Prevention: place the plate proximal to the watershed line, repair pronator quadratus over it where possible, and remove a prominent plate in a symptomatic patient before the tendon goes.
• Treatment of the established rupture: remove the plate at the same operation as the tendon reconstruction - leaving the offending metal in place guarantees a repeat.

The Mannerfelt lesion

• Attritional rupture of FPL within the carpal tunnel over a bony spur, classically arising from the scaphoid (the scaphotrapezial region), in rheumatoid arthritis.
• The mechanism is the same as the plate: a hard prominence in a confined space, with the most radial and deepest tendon taking the abrasion.
• Presentation: painless loss of thumb interphalangeal flexion in a rheumatoid patient, often attributed by the patient to their arthritis. Frequently there is preceding volar wrist swelling from tenosynovitis.
• Progression: if untreated, the index profundus is the next tendon to rupture over the same spur.
• Treatment: flexor tenosynovectomy, excision of the offending spur and repair of the volar capsule over it, plus reconstruction of the tendon. Removing the spur is not optional.

Other causes of rupture

• Scaphoid nonunion with a volar prominence or a trapezial osteophyte, producing the same attrition in a non-rheumatoid patient.
• Closed traumatic rupture in climbers and weightlifters, and after repeated corticosteroid injection for trigger thumb.
• Chronic tenosynovitis of any inflammatory or infective cause, including atypical mycobacterial infection, which weakens the tendon before it parts.

Anterior interosseous nerve palsy

• Presentation: inability to flex the thumb interphalangeal joint and the index distal interphalangeal joint, with no sensory loss whatsoever, and weakness of pronator quadratus (tested by resisting pronation with the elbow fully flexed to minimise pronator teres).
• The sign: the OK sign, or Kiloh-Nevin sign - asked to make a circle with thumb and index, the patient produces a square pinch with both terminal joints hyperextended and the pulps meeting along their length rather than tip to tip.
• Superficialis must be normal - the FDS branches leave the median nerve just proximal to the AIN origin. If FDS is also weak, the lesion is at the pronator level or higher.
• Causes: compression by the deep head of pronator teres, the FDS arcade, a Gantzer accessory head of FPL, aberrant vessels, or a mass; supracondylar fracture and forearm fracture or its fixation; and - increasingly recognised as the majority - neuralgic amyotrophy (Parsonage-Turner syndrome).
• The history that changes management: a severe pain prodrome in the shoulder and arm followed by a painless motor palsy points to neuralgic amyotrophy, which is managed expectantly with analgesia, therapy and time, most patients recovering over months. A palsy without a pain prodrome that fails to recover by around three to six months, with electrodiagnostic evidence of denervation, warrants exploration and release of all potential compressive structures.
• Late salvage if recovery does not occur: brachioradialis transfer to FPL restores thumb interphalangeal flexion, and a side-to-side tenodesis of the index profundus to the middle finger profundus restores index terminal flexion.

The Linburg-Comstock anomaly

• Anatomy: an anomalous tendon slip connecting flexor pollicis longus to flexor digitorum profundus, most often to the index.
• Prevalence: in the original description, clinical examination of 194 patients suggested the anomaly in at least one extremity in 31 per cent and bilaterally in 14 per cent; dissection of 43 cadavers found it in at least one extremity of 25 per cent and bilaterally in 6 per cent. Two things follow. First, the clinical figure is higher than the anatomical one, which is the wrong way round if the test detected only real tendon slips - so a positive test picks up something else in a proportion of people. Second, and more useful at the bedside: of those 194 patients, four had symptoms attributed to the anomaly. About one person in three carries it and roughly one in fifty of those examined had a problem from it, so finding it is almost never the answer to a painful wrist.
• Clinical picture: the patient cannot flex the thumb interphalangeal joint without simultaneously flexing the index distal interphalangeal joint. Most people are asymptomatic and unaware. A minority - typically those in occupations demanding independent thumb and index control, such as carpenters, musicians and surgeons - develop pain and chronic tenosynovitis at the distal volar forearm.
• Diagnosis: the clinical pattern is characteristic; dynamic high-resolution ultrasound demonstrates the tethering slip.
• Treatment: reassurance for the asymptomatic. For the symptomatic, surgical division of the tendinous interconnection is effective, and in the original series was performed for chronic tenosynovitis in all four symptomatic cases described.
• Examination trap: do not report a Linburg-Comstock pattern as an FPL palsy or a tendon adhesion. The thumb flexes - it just does not flex alone.

The Gantzer muscle

• An accessory head of FPL arising from the coronoid process or the medial epicondyle, present in a substantial proportion of forearms in cadaveric series.
• Usually supplied by the anterior interosseous nerve.
• Clinical relevance: it may cross and compress the anterior interosseous nerve, and it is a structure that must be recognised and released during exploration for AIN palsy.

Trigger thumb (stenosing tenosynovitis at A1)

• Adult trigger thumb: catching, locking and pain at the metacarpophalangeal level with a palpable nodule on the tendon at the A1 pulley. Associated with diabetes, which also predicts a poorer response to injection and a higher recurrence rate.
• Management: splinting and corticosteroid injection first; surgical release of the A1 pulley where these fail. The oblique pulley must be preserved - dividing it causes bowstringing and weak interphalangeal flexion.
• The nerve at risk: the radial digital nerve of the thumb crosses obliquely over the FPL sheath at the A1 pulley level, lying superficially and close to the midline. This is the reason percutaneous release is discouraged in the thumb and why open release with direct visualisation of the pulley is the safe standard.

Congenital trigger thumb

• Presents in infancy or early childhood as a fixed flexion of the interphalangeal joint (pollex flexus) with a palpable Notta node on the tendon at the A1 pulley - not usually as true triggering.
• Important distinction: it must be differentiated from congenital clasped thumb (in which the thumb is held flexed and adducted with a deficient extensor mechanism) and from an absent or hypoplastic FPL, which occurs in thumb hypoplasia.
• A proportion resolve spontaneously; persistent cases are treated by A1 pulley release, with excellent results.

Zone I to III of the thumb (Verdan)

Why FPL laceration is technically hard

• It retracts far. FPL has no interconnections with adjacent tendons and no substantial vincular tether, unlike the finger profundus tendons. A divided FPL commonly retracts into the thenar eminence or into the distal forearm.
• Retrieval: a proximal counter-incision at the carpal tunnel or the distal forearm is frequently required, and the tendon is shuttled distally through the sheath with a catheter or suture. Blind repeated grasping guarantees adhesion.
• Repair: a multi-strand core suture with an epitendinous suture; A1 may be released to allow the repair to glide, but the oblique pulley is preserved.
• Rehabilitation: protect in a thumb spica dorsal blocking splint with a supervised early motion programme; the thumb tolerates immobilisation better than the fingers but stiffens at the interphalangeal joint.

Exposure of the FPL in the forearm

• Volar (Henry) approach: the interval between brachioradialis (with the radial artery) laterally and the flexor carpi radialis medially. Deep dissection exposes flexor pollicis longus lying on the radius, and pronator quadratus distally.
• The anterior interosseous nerve and artery lie in the interval between FPL and flexor digitorum profundus on the interosseous membrane. Identify this interval before any deep dissection.
• The FPL origin can be elevated subperiosteally from the radius to expose the middle third of the bone - the standard exposure for a radial shaft plate through Henry.

Exposure at the wrist

• The FCR approach exposes the volar distal radius; FPL is encountered deep and is retracted ulnarly with the flexor mass after the floor of the FCR sheath is opened.
• The watershed line is the distal limit for plate placement. Confirm on a true lateral.
• Carpal tunnel extension: where FPL must be retrieved or a Mannerfelt lesion addressed, the carpal tunnel is opened with an incision in line with the radial border of the ring finger, staying ulnar to the palmar cutaneous branch of the median nerve and dividing the transverse carpal ligament at its ulnar margin.

Exposure in the thumb

• Trigger thumb release: a transverse or short longitudinal incision at the level of the metacarpophalangeal flexion crease, at the proximal edge of the sesamoids. Spread bluntly down to the sheath. Identify the radial digital nerve, which crosses obliquely over the sheath, before dividing A1 under direct vision. Preserve the oblique pulley.
• Zone II FPL repair: a Bruner-type incision along the volar thumb, opening the sheath between pulleys, with the same nerve-protection discipline.

Choosing a reconstruction for a lost FPL

The decision rests on three questions: is the interphalangeal joint supple, what is the quality of the bed, and what does the patient actually need?

Technical points for the FDS ring transfer to FPL

• Harvest the ring finger superficialis: divide it in the digit at the A1 to A2 region and deliver it proximally through a palmar incision.
• Leave one slip attached distally in a lax finger, or perform a formal tenodesis, to prevent a donor swan neck.
• Route the donor through the carpal tunnel or subcutaneously across the thenar eminence into the thumb, in a straight line of pull.
• Tension with the wrist in neutral so that the thumb interphalangeal joint rests in slight flexion and the passive tenodesis effect is restored - the thumb should flex as the wrist extends and extend as the wrist flexes.
• Always address the cause first. Remove the prominent plate; excise the scaphoid spur and cover the raw bone with capsule. A reconstruction laid across an unaddressed prominence will simply rupture again.

Interphalangeal arthrodesis - do not undersell it

• Position the interphalangeal joint in approximately 15 to 20 degrees of flexion.
• It gives a stable pinch post with predictable results, requires no therapy programme, and is entirely appropriate in a rheumatoid patient, an older patient, a manual worker who needs strength rather than dexterity, and after a failed reconstruction.
• Choosing arthrodesis is a positive decision, not a failure of ambition, and stating so with reasons is what an examiner is looking for.