Flexor Digitorum Superficialis Anatomy

Origin - two heads

Humero-ulnar head

• Medial epicondyle of the humerus via the common flexor origin.
• The ulnar collateral ligament of the elbow.
• The medial (sublime) tubercle of the coronoid process of the ulna.

Radial head

• The oblique line and proximal volar shaft of the radius, between the radial tuberosity and the insertion of pronator teres.

The FDS (sublimis) arcade

• The two heads are joined by a fibrous arch - the sublimis bridge or FDS arcade - running obliquely between the coronoid and the radius.
• The median nerve and the ulnar artery pass DEEP to this arcade to enter the deep forearm. It is the fourth and most distal of the four classical pronator syndrome compression points.

The two strata of muscle bellies

• Superficial (volar) stratum: the bellies and tendons to the middle and ring fingers.
• Deep (dorsal) stratum: the bellies and tendons to the index and little fingers.
• The index belly is the most anatomically distinct, arising largely from the radial head - the basis of truly independent index proximal interphalangeal flexion.
• The little finger belly is the most variable, often deriving fibres from, or interconnected with, the ring finger tendon or the small finger profundus.

Arrangement in the carpal tunnel

• Nine tendons cross the carpal tunnel: four FDS, four FDP and flexor pollicis longus.
• The middle and ring FDS lie superficial (volar); the index and little FDS lie deep, in the same two-stratum arrangement as in the forearm.
• The middle finger FDS is therefore the tendon lying directly deep to the median nerve - and the tendon a surgeon meets first if the dissection strays deep during carpal tunnel release.

Insertion

• At the level of the proximal phalanx the tendon flattens and splits into two slips.
• The slips rotate approximately 180 degrees, pass dorsal to the profundus tendon, and decussate at Camper chiasm.
• They partially reunite and insert onto the volar surface of the proximal half of the middle phalanx, on both its radial and ulnar sides.

Nerve supply

• Median nerve, root values C7, C8 and T1.
• FDS is the last muscle supplied by the main trunk of the median nerve in the forearm. Immediately after giving its FDS branches, the median nerve gives off the anterior interosseous nerve, which supplies flexor pollicis longus, the radial half of flexor digitorum profundus and pronator quadratus.
• Motor entry: branches enter the deep surface of the muscle as the nerve passes beneath the sublimis arcade, in the proximal third of the forearm.

Localising value - the sequence the examiner wants

The discriminator worth memorising: FDS is weak in pronator syndrome and normal in anterior interosseous syndrome. And the palmar cutaneous branch, arising 5 to 8 cm proximal to the wrist crease, is spared in carpal tunnel syndrome but involved in pronator syndrome.

Muscle

• Ulnar artery muscular branches - the dominant supply, the vessel running deep to the arcade with the median nerve and then between FDS and flexor digitorum profundus.
• Radial artery branches to the radial head of origin.
• Anterior interosseous artery branches to the deep surface.
• Median artery, when persistent, runs with the median nerve and contributes; a persistent median artery is also a recognised cause of carpal tunnel syndrome and of median nerve compression when thrombosed.

Tendon - the vincular system

Within the digital sheath the tendons are nourished by two mechanisms: synovial diffusion and vincular perfusion from the transverse digital arteries.

• Vinculum breve superficialis (VBS): a short triangular fold at the FDS insertion on the middle phalanx.
• Vinculum longum superficialis (VLS): arises from the floor of the sheath at the level of the proximal phalanx.
• Vinculum breve profundus (VBP): at the profundus insertion on the distal phalanx.
• Vinculum longum profundus (VLP): arises from the FDS at the region of Camper chiasm - the single most examinable vincular fact, and the reason chiasm damage compromises profundus nutrition.
• Clinical corollary: the dorsal (posterior) surface of the tendon is the vascular side; the volar surface is relatively avascular and depends on diffusion. Core sutures are therefore placed volarly (palmar-placed core suture) to avoid strangling the dorsal vincular supply.

Pedicle position for flap or graft

• FDS is not used as a vascularised flap in routine practice; it is used as a free tendon graft or transfer, in which case the tendon revascularises from its new bed. This is why a transferred FDS must be laid in a healthy, well-vascularised subcutaneous or muscular bed, not through scar.

In the forearm

• Superficial to FDS: pronator teres, flexor carpi radialis, palmaris longus and flexor carpi ulnaris - the superficial flexor layer.
• Deep to FDS: flexor digitorum profundus, flexor pollicis longus and, distally, pronator quadratus - the deep flexor layer.
• The median nerve lies on the DEEP surface of FDS, adherent to its underside, for most of the forearm. It emerges at the wrist between FDS and flexor carpi radialis, lying deep to palmaris longus.
• The ulnar nerve and artery run between FDS (radially) and flexor carpi ulnaris (ulnarly), on the surface of profundus.

At the wrist

• The median nerve becomes superficial and lies volar and radial to the FDS tendons, just deep to palmaris longus - which is the reason palmaris longus is the surface landmark for the nerve.
• The FDS to the middle finger is the tendon immediately deep to the nerve.

In the digit - zone II relations

• FDS lies volar (superficial) to FDP proximally, then splits and passes dorsal to it at the proximal phalanx.
• Both tendons run within the fibro-osseous digital sheath, constrained by the pulley system.
• The neurovascular bundles lie volar-lateral on each side of the sheath and are the structures at risk in any Bruner incision or lateral approach.

The pulley system

• Five annular pulleys (A1 to A5) and three cruciate pulleys (C1 to C3), plus the palmar aponeurosis pulley proximal to A1.
• Odd-numbered annular pulleys overlie joints: A1 at the metacarpophalangeal joint, A3 at the proximal interphalangeal joint, A5 at the distal interphalangeal joint.
• Even-numbered annular pulleys overlie the phalanges: A2 on the proximal phalanx, A4 on the middle phalanx - classically taught as the biomechanically critical pair whose loss produces bowstringing.

The Verdan zones

Why zone II is difficult

• Two tendons in one canal. FDS and FDP must glide independently within a rigid fibro-osseous sheath. Repair bulk, adhesion between the two repairs, and adhesion to the sheath all conspire against motion.
• Camper chiasm. The FDS slips wrap around profundus, so the tendon relationships change over just a few millimetres of tendon length - a repair at the chiasm is technically demanding and prone to a bulky, catching result.
• The vinculum longum profundus arises here, so damage carries a nutritional cost as well as a mechanical one.
• The pulleys are unforgiving. A repair that will not pass under A4 must be trimmed, or the pulley vented.

Principles of zone II repair

• Strong core suture: a four- to six-strand core repair with adequate purchase (approximately 7 to 10 mm from the cut end), placed volarly to spare the dorsal vincular supply, combined with a circumferential epitendinous suture which adds strength, smooths the repair and resists gapping.
• Slight repair-site shortening (a few per cent) is acceptable and reduces gapping; a gap of more than 3 mm predicts rupture and adhesion.
• Pulley venting. Contemporary practice accepts partial or complete venting of A4 and of the distal part of A2 to allow a bulky repair to glide, provided a sufficient length of the pulley system is retained overall. Published series have shown venting is needed in the majority of zone II repairs and does not lead to clinically relevant bowstringing.
• The FDS decision. Repair FDS where it can be done without bulk - this preserves PIP volar restraint and prevents late swan neck deformity, and the FDS repair also preserves the vinculum longum profundus origin. Where both slips cannot be repaired without crowding the canal, repair one slip and excise the other.
• Early active mobilisation under supervision is now standard, and repair strength must be chosen with that regimen in mind.

The FDS arcade and pronator syndrome

• Four classical median nerve compression points in the proximal forearm, proximal to distal:
  1. The ligament of Struthers, running from a supracondylar process to the medial epicondyle (present in a small minority of arms).
  2. The lacertus fibrosus (bicipital aponeurosis).
  3. Between the two heads of pronator teres.
  4. The FDS (sublimis) arcade - the fibrous arch between the humero-ulnar and radial heads.
• Clinical pattern: aching volar forearm pain aggravated by activity, paraesthesiae in the median digits that are typically not nocturnal, and - the discriminator from carpal tunnel syndrome - sensory disturbance in the palmar cutaneous territory over the thenar eminence, because that branch arises proximal to the wrist.
• Provocative tests: pain and paraesthesiae on resisted middle finger proximal interphalangeal flexion implicate the FDS arcade specifically; resisted pronation with the elbow extended implicates the pronator heads; resisted elbow flexion with the forearm supinated implicates the lacertus.
• Investigation: electrodiagnostic studies are frequently normal, so the diagnosis is clinical and anatomical. Imaging excludes a mass or a supracondylar process.
• Management: activity modification, splinting and time first; surgical release addresses all four sites in sequence through a single anterior approach, because leaving one uninvestigated is the commonest cause of failure.

Anterior interosseous nerve syndrome - the contrast

• The AIN arises just distal to the FDS branches. Its palsy produces weak flexor pollicis longus, weak index flexor digitorum profundus and weak pronator quadratus, giving the square pinch or abnormal OK sign, with NO sensory loss and normal FDS.
• A tight FDS arcade or an accessory head of FPL (Gantzer muscle) can compress the AIN as well as the main trunk.
• Many cases are now recognised as neuralgic amyotrophy (Parsonage-Turner) rather than true compression - a severe pain prodrome followed by painless palsy points to that diagnosis and to a conservative course.

Swan neck deformity

• Mechanism: loss of the volar restraint of the PIP joint. FDS division, FDS harvest without leaving a slip, attritional rupture in rheumatoid disease, or volar plate attenuation allow PIP hyperextension; the lateral bands then subluxate dorsally, the extensor force concentrates at the PIP, and the DIP drops into flexion.
• Prevention in surgery: when harvesting an FDS for transfer, leave one slip attached distally (or perform a formal FDS tenodesis) in any finger with pre-existing laxity.
• Treatment options include FDS tenodesis (using one slip left attached distally and sutured to the proximal phalanx or looped around A2), spiral oblique retinacular ligament reconstruction, and PIP volar plate advancement.

Flexor tenosynovitis and trigger finger

• Trigger finger is a mismatch between the flexor tendons and the A1 pulley. On sheath entry the FDS slips are frequently the source of the catching, and a nodule is often found on FDS rather than FDP.
• Persistent triggering after an adequate A1 release is classically due to an FDS slip catching; the standard salvage is excision of one slip of FDS (usually the ulnar slip), which decompresses the canal without producing a swan neck because the other slip remains.
• Pyogenic flexor tenosynovitis (Kanavel signs: fusiform swelling, semiflexed posture, tenderness along the sheath, pain on passive extension) tracks along the same sheath; the little finger and thumb sheaths communicate with the ulnar and radial bursae respectively, permitting a horseshoe abscess.

Rheumatoid and attritional disease

• Flexor tenosynovitis within the carpal tunnel is a common rheumatoid presentation, producing median nerve compression and reduced tendon excursion with an intact tendon - a "pseudo-rupture" pattern where the patient cannot flex but passive motion is full.
• True attritional rupture over carpal bone spurs affects flexor pollicis longus first (the Mannerfelt lesion) but can progress to the index profundus and superficialis.

Anatomical variants worth knowing

• Absent or non-independent little finger FDS in a substantial minority - the single most commonly encountered variant.
• Interconnections between the little finger FDS and either the ring FDS or the little finger FDP explain the variable clinical testing pattern.
• Accessory slips of FDS to the palm or to the lumbricals are described and can present as a mass or as a cause of carpal tunnel syndrome when a muscle belly extends distally into the canal.

Exposure of the flexor sheath in the digit

• Bruner (volar zig-zag) incision: the apices of the zig-zag must reach the mid-lateral line at the flexion creases; angles that are too acute produce tip necrosis. Full-thickness flaps are raised down to the sheath, keeping the neurovascular bundles in the flap.
• Midlateral incision: placed at the dorsal limit of the flexion creases, dorsal to the neurovascular bundle. It leaves a less conspicuous scar and avoids a scar directly over the repair, but exposure of the volar sheath is more limited.
• Sheath windows: open the sheath between pulleys, through the cruciate segments, to preserve annular pulley substance.

Retrieval of a retracted tendon

• A divided FDP with an intact vinculum stays close to the injury; a divided tendon with ruptured vincula retracts to the palm.
• Retrieve atraumatically: flex the wrist and fingers and milk the tendon distally; if that fails, make a separate palmar incision at the A1 pulley and pass the tendon distally through the sheath using a small catheter or a suture shuttle.
• Never grasp the tendon repeatedly with toothed instruments - each bite damages the gliding surface and guarantees adhesion.

Proximal forearm release for pronator syndrome

• Anterior approach with a lazy-S incision crossing the antecubital fossa obliquely.
• Identify and protect the lateral antebrachial cutaneous nerve at the lateral edge of the biceps tendon.
• Release in sequence: lacertus fibrosus, then the superficial head of pronator teres (or its deep fascia), then the FDS arcade, tracing the median nerve beneath the arch.
• Look proximally for a supracondylar process and ligament of Struthers - a bony spur on radiographs is the clue.

The general rules of FDS harvest

• Choose the ring finger as the default donor. It is expendable, has reliable independent function, and its loss is least noticed.
• Preserve the index FDS: independent index proximal interphalangeal flexion is essential to precision pinch.
• Avoid the little finger FDS: it is functionally absent or non-independent in a large minority of hands.
• Leave one slip attached distally - or perform a formal tenodesis - in any finger with pre-existing hyperlaxity, to prevent a post-harvest swan neck deformity.
• Divide the tendon in the digit at the level of the A1 to A2 region and deliver it proximally through a palmar or distal forearm incision; do not avulse it.

Opponensplasty

• Bunnell FDS opponensplasty: the ring finger FDS is passed around a pulley created from the distal edge of the flexor carpi ulnaris tendon at the pisiform, then across the palm to the abductor pollicis brevis insertion and the extensor pollicis longus expansion at the thumb metacarpophalangeal joint. The pulley position - ulnar and proximal - creates the correct vector of palmar abduction with pronation.
• Royle-Thompson modification: uses the distal (ulnar) border of the transverse carpal ligament as the pulley - simpler, but the more distal and radial pulley gives a vector closer to pure palmar abduction with less pronation.
• When to choose FDS over palmaris longus: an FDS opponensplasty provides more power and is used in a low median nerve palsy where the patient needs strength, or when palmaris longus is absent. Camitz (palmaris longus) opponensplasty is chosen when a simple, quick abduction restoration is required at the same sitting as a carpal tunnel release in severe thenar wasting.
• In a HIGH median nerve palsy, FDS is not available as a donor (it is denervated) - extensor indicis proprius or abductor digiti minimi (Huber) transfer must be used instead. This is a favourite examination trap.

Zancolli lasso for claw deformity

• Correcting ulnar (or combined) claw requires restoring metacarpophalangeal flexion so that extensor force reaches the interphalangeal joints.
• The lasso divides an FDS slip within the sheath, passes it around the A1 pulley and sutures it back to itself. The tendon then acts as a metacarpophalangeal flexor.
• The lasso is a static-into-dynamic conversion that requires supple joints and a passively correctable deformity - test with the Bouvier manoeuvre (holding the MCP joints flexed corrects the interphalangeal extension lag) before selecting it.

Other established FDS transfers

• FDS ring to extensor pollicis longus when palmaris longus is absent in radial nerve palsy reconstruction.
• FDS to adductor pollicis (usually the middle finger FDS routed through the second or third intermetacarpal space) to restore key pinch in ulnar palsy.
• FDS to flexor digitorum profundus for an irreparable profundus, or a sublimis-to-profundus (STP) transfer in the spastic or contracted hand, where all superficialis tendons are divided and used to lengthen the profundus mass.
• FDS as an interposition or as a free graft in reconstruction of the flexor system.

FDS in pulley reconstruction

• Indication: loss of A2 or A4 with bowstringing, most often after infection, in the rheumatoid hand, in climbers, or during staged flexor tendon reconstruction.
• Graft choices: a slip of FDS, palmaris longus, a segment of extensor retinaculum, or a discarded tendon graft.
• Techniques:
  • Kleinert-Weilby: the graft is woven through the residual rim of the old pulley and sutured to itself - it does not encircle the phalanx.
  • Bunnell loop: the graft encircles the phalanx, volar to the extensor mechanism at the proximal phalanx (A2) and dorsal to the extensor mechanism at the middle phalanx (A4). Getting these two relationships the right way round is a classic examination point.
  • Karev belt-loop: the graft is passed through a window created in the volar plate.
• Tension: the reconstructed pulley must sit snugly against the phalanx without strangling the tendon; test glide before closing.

FDS at carpal tunnel release

• The middle finger FDS is the first tendon encountered deep to the median nerve - useful as an orientation landmark, and the structure a surgeon injures if the knife strays deep.
• Divide the transverse carpal ligament at its ULNAR margin, immediately radial to the hook of hamate, precisely to keep away from both the median nerve and the recurrent motor branch.
• Anomalous FDS muscle bellies extending distally into the canal are a recognised cause of carpal tunnel syndrome in young patients; if found, the distal belly may be resected.
• Persistent median artery and a bifid median nerve commonly occur together; both raise the risk of injury with a limited-incision or endoscopic technique and should be sought on preoperative ultrasound where available.
• After release, an intact FDS mass bulging into the released canal is normal; it does not indicate incomplete release.