Palmaris Longus Anatomy

Origin

• The medial epicondyle of the humerus via the common flexor origin, lying between flexor carpi radialis (radially) and flexor carpi ulnaris (ulnarly).
• Additional fibres from the adjacent intermuscular septa and the deep antebrachial fascia.

Muscle belly and tendon

• A short, fusiform belly in the proximal third of the forearm giving way to a long, thin, round tendon that occupies the distal two-thirds.
• The tendon is typically 12 to 15 cm in usable length and 3 to 5 mm in diameter - dimensions that make it ideal as a free graft.

Insertion

• The flexor retinaculum (transverse carpal ligament) and, in direct continuity, the apex of the palmar aponeurosis.
• Some fibres continue into the thenar fascia and the abductor pollicis brevis region - the anatomical continuity exploited in the Camitz opponensplasty, where a strip of palmar aponeurosis is harvested in continuity with the tendon to gain length.

The defining anatomical fact

• Palmaris longus passes SUPERFICIAL to the flexor retinaculum. It is the only long flexor tendon of the forearm that is not a carpal tunnel structure.
• Practical consequences: a palmaris longus tendon is not decompressed by carpal tunnel release; a structure identified at wrist harvest that dives deep to the retinaculum is not palmaris longus; and a volar wrist mass superficial to the retinaculum may be an anomalous palmaris belly.

Nerve supply

• Median nerve, root values C7 and C8 (a C6 contribution is described in some texts).
• The branch arises in the proximal forearm, after the pronator teres and flexor carpi radialis branches and before the flexor digitorum superficialis branches.
• Motor entry: into the deep surface of the short proximal belly, in the proximal third of the forearm.

Order of median innervation in the forearm

1. Pronator teres
2. Flexor carpi radialis
3. Palmaris longus
4. Flexor digitorum superficialis

Then the anterior interosseous nerve to flexor pollicis longus, the radial half of flexor digitorum profundus, and pronator quadratus.

Clinical significance of the innervation

• Palmaris longus weakness is essentially never diagnosed clinically - it is too weak a muscle to assess reliably and too often absent for a negative finding to mean anything.
• Its relevance to nerve injury is entirely surgical: it is a median-innervated donor, so in a high median nerve palsy palmaris longus is denervated and unavailable as a transfer, exactly as flexor digitorum superficialis is. This is the reason the Camitz opponensplasty is an operation for low median nerve palsy only.

Arterial supply

• Ulnar artery muscular branches - the dominant supply to the belly.
• Posterior ulnar recurrent artery proximally, at the common flexor origin.
• Median artery, when persistent, may contribute.
• The tendon itself is relatively avascular, nourished by paratenon vessels and by diffusion.

Why this matters for grafting

• Because the tendon is thin, round and relatively avascular, it survives as a free graft by revascularisation from the recipient bed, in the same way as any other free tendon graft.
• The graft therefore must be laid in a healthy, vascularised bed - a scarred, irradiated or infected bed is the reason to stage the reconstruction with a silicone rod rather than to place a graft primarily.
• Harvesting technique should preserve the paratenon on the graft where possible; stripping it aggressively delays incorporation and increases adhesion.

At the wrist - the relations that matter

• Superficial to palmaris longus: only skin, subcutaneous fat and the antebrachial fascia. It is the most superficial tendon at the volar wrist.
• Deep and slightly RADIAL to palmaris longus: the MEDIAN NERVE. In some individuals the nerve lies directly beneath the tendon.
• Radial to palmaris longus: flexor carpi radialis, and in the interval between the two, the palmar cutaneous branch of the median nerve, which arises 5 to 8 cm proximal to the wrist crease and pierces the fascia near the crease.
• Ulnar to palmaris longus: flexor digitorum superficialis, then flexor carpi ulnaris and the ulnar neurovascular bundle.
• Deep to it distally: the flexor retinaculum, which it crosses superficially before blending with the palmar aponeurosis.

The order of structures across the volar wrist, radial to ulnar

Radial artery, flexor carpi radialis, palmar cutaneous branch of the median nerve, palmaris longus, median nerve (deep), flexor digitorum superficialis, flexor digitorum profundus, flexor carpi ulnaris, then the ulnar artery and nerve on the radial side of the FCU tendon.

In the forearm

• Lies between flexor carpi radialis and flexor carpi ulnaris in the superficial flexor layer, superficial to flexor digitorum superficialis.
• The median nerve is deep to flexor digitorum superficialis in the forearm and only becomes superficial - and adjacent to palmaris longus - in the distal few centimetres.

In the palm

• The tendon fans out into the palmar aponeurosis, whose longitudinal pretendinous bands are the structures involved in Dupuytren disease. The continuity between palmaris longus and the palmar fascia is direct and is why a tense palmaris longus can accentuate a Dupuytren cord clinically.

The prevalence figures

• In a population study of 300 Caucasian subjects aged 18 to 40 (150 men and 150 women) assessed by clinical inspection:
  • Unilateral absence in 49 subjects (16 per cent)
  • Bilateral absence in 26 subjects (9 per cent)
  • Unilateral and bilateral absence were more common in males, but not statistically significantly so.
  • In those with unilateral absence, the right side was more commonly affected, again without reaching statistical significance.
• Roughly one quarter of people therefore lack a palmaris longus on at least one side.

Ethnic and population variation

• Reported agenesis rates vary widely between populations. Rates in European-descended populations are generally the highest, in the region reported above, whereas published series from East Asian and African populations report substantially lower rates of absence.
• The practical message is not the precise figure for any one group but the principle: the prevalence of absence differs meaningfully between populations, so a surgeon should quote local or study-specific data rather than assume a universal figure, and should always examine the individual patient rather than rely on a population rate.

Inheritance and associations

• Absence appears to be inherited, with reported patterns most consistent with autosomal dominant transmission with variable penetrance, and it is not associated with any functional deficit or with any specific syndrome in the general population.
• It is not associated with reduced grip strength, pinch strength or wrist flexion power in published comparisons - the finding that underpins its status as a free graft donor.

Why the number matters clinically

• A patient consented for a thumb ulnar collateral ligament reconstruction, an elbow medial collateral ligament reconstruction, a flexor tendon graft or a pulley reconstruction on the assumption of an available palmaris longus has roughly a one in four chance of the tendon being absent on the operative side if it is never checked.
• The examination takes ten seconds. Not doing it is indefensible.

Anatomical variants

• Reversed palmaris longus: the muscle belly lies distally and the tendon proximally - the opposite of normal. The distal belly can present as a volar wrist mass, and by occupying space at the wrist it is a recognised cause of carpal tunnel syndrome and, less commonly, of ulnar nerve compression in Guyon canal. It is also useless as a conventional graft.
• Digastric or bifid palmaris longus, with a central tendon between two bellies.
• Duplicated palmaris longus, occasionally providing two grafts.
• Palmaris profundus: an anomalous muscle running deep to the retinaculum, within the carpal tunnel, inserting into the palmar fascia or the thenar muscles. Because it lies inside the tunnel it is a genuine cause of median nerve compression and, unlike the normal palmaris longus, it is not superficial to the retinaculum - the reason the retinaculum test is the definitive identification step at harvest.
• Accessory insertions into the thenar or hypothenar fascia.

Volar wrist mass

• An anomalous or reversed palmaris longus belly is a differential for a volar wrist swelling, alongside ganglion, lipoma, a persistent median artery and giant cell tumour of the tendon sheath. Dynamic ultrasound distinguishes them: a muscle belly contracts and moves with wrist flexion.

Relationship to Dupuytren disease

• Palmaris longus inserts directly into the apex of the palmar aponeurosis, whose pretendinous bands become the cords of Dupuytren disease.
• A prominent palmaris longus can make a pretendinous cord appear more obvious on wrist flexion, but there is no established causal association between palmaris longus presence or absence and the development of Dupuytren disease. Be careful not to overstate this in an examination.
• At fasciectomy the tendon is encountered proximally at the wrist and is not routinely divided.

Injury

• Laceration: palmaris longus is the most superficial structure at the volar wrist and is divided in almost any volar wrist laceration. Repair is not required - the tendon is functionally expendable - though its division is a useful marker that the laceration reached the deep fascia and that the median nerve must be formally examined.
• Spontaneous rupture is essentially unheard of and of no clinical significance.

The role of palmaris longus in carpal tunnel surgery

• It is the landmark, not the pathology. The tendon lies superficial to the retinaculum and takes no part in the compression, but it defines where the incision goes.
• Incision: longitudinal, in line with the radial border of the ring finger, that is ULNAR to the palmaris longus axis, from the distal wrist crease (or just distal to it) toward Kaplan cardinal line. This keeps the incision away from the median nerve (deep and radial to palmaris) and from the palmar cutaneous branch (radial to palmaris).
• Do not cross the wrist crease transversely. Cross obliquely if at all - a transverse scar over the crease is a classic route to a palmar cutaneous neuroma and a tender scar.
• Divide the transverse carpal ligament at its ULNAR margin, immediately radial to the hook of hamate, keeping away from the median nerve and the recurrent motor branch.
• The Lanz variants matter here: in the original operative series of 246 explored carpal tunnels, variations in the thenar branch course, accessory distal branches, high division and accessory proximal branches were all described. The percentages everyone quotes alongside them are not Lanz's - the thenar branch was extraligamentous in 46 per cent, subligamentous in 31 per cent and transligamentous in 23 per cent in Poisel's 100 cadaver hands, which Lanz cites. Say whose data it is; an examiner who knows the paper is listening for it. The stated conclusion - approach the median nerve from the ULNAR side - is exactly the palmaris longus rule restated.

Camitz opponensplasty

• Indication: severe carpal tunnel syndrome with marked thenar wasting and loss of palmar abduction, at the time of carpal tunnel release.
• Principle: palmaris longus is harvested with a strip of palmar aponeurosis in continuity, which provides the extra length needed to reach the thumb without a separate graft.
• Technique: the palmaris longus with its fascial extension is tunnelled subcutaneously across the palm to the abductor pollicis brevis insertion, tensioned with the thumb in palmar abduction.
• What it restores: palmar abduction, not true opposition. It provides no meaningful pronation of the thumb. It is chosen for simplicity and for the fact that it can be done through the same operation, not for the quality of the vector.
• Modifications that improve it: adding a pulley at the ulnar remnant of the flexor retinaculum improves thumb flexion and pronation as well as abduction; a minimally invasive modification passing the transferred tendon beneath the abductor pollicis brevis fascia through palm and thumb incisions only - avoiding an incision across the wrist crease - has been reported to reduce painful wrist scars and bowstringing of the transferred tendon compared with the conventional technique.
• Availability rule: palmaris longus is median-innervated, so Camitz is an operation for LOW median nerve palsy only. In a high median palsy, use extensor indicis proprius or an abductor digiti minimi (Huber) transfer.

Preoperative steps

1. Confirm the tendon is present on the intended donor side with at least two clinical tests; use ultrasound if equivocal.
2. Consent for the alternative graft (plantaris, superficialis slip, extensor retinaculum, long toe extensor).
3. Mark the tendon with the patient awake, using the Schaeffer test.

Technique

• Distal incision: a short transverse incision at the distal wrist crease, directly over the tendon. Incise skin only and spread bluntly - the tendon is immediately subcutaneous.
• Confirm identity before dividing. The three checks:
  1. It is superficial to the flexor retinaculum - the definitive test. Any structure diving deep to the retinaculum is not palmaris longus.
  2. Traction on it tensions the palmar aponeurosis and does not move a finger.
  3. It is a tendon: firm, glistening white, without fascicular striations or longitudinal vasa nervorum. In an awake patient, traction produces no paraesthesiae.
• Divide distally, place a holding suture, and deliver the tendon proximally either:
  • through one or two additional small transverse counter-incisions at intervals up the forearm, or
  • with a closed tendon stripper advanced proximally along the tendon to the musculotendinous junction.
• Preserve the paratenon on the graft where possible; it aids incorporation.
• Take 12 to 15 cm and confirm the graft length against the requirement before closing.

Hazards and how to avoid them

• Harvesting the median nerve. The reported catastrophe. Prevented absolutely by the retinaculum test and by never using a blind stripper before positive identification.
• Palmar cutaneous branch injury from a transverse incision crossing the crease radially - keep the incision small, over the tendon, and cross the crease obliquely if extension is needed.
• Injury to a superficial vein producing a haematoma - ligate rather than avulse.
• Blind stripper injury to the median nerve or to muscle if the stripper deviates from the tendon; advance it gently, in line, without force.
• Incomplete harvest where the tendon avulses at the musculotendinous junction - use a counter-incision if resistance is felt.

What the graft is used for

• Flexor tendon reconstruction: single-stage grafting and the second stage of a two-stage silicone rod reconstruction.
• Thumb ulnar collateral ligament reconstruction for chronic gamekeeper thumb.
• Elbow medial collateral ligament reconstruction - palmaris longus is the classic donor for the docking and figure-of-eight techniques.
• Pulley reconstruction (Kleinert-Weilby, Bunnell loop, Karev belt-loop) for A2 or A4 loss.
• Scapholunate and other carpal ligament reconstructions, interposition arthroplasty and static suspension procedures.
• Outside orthopaedics: eyelid slings for facial palsy, lip augmentation, tympanoplasty grafts and temporomandibular joint reconstruction - all exploiting the same expendability.

Palmaris longus as a tendon transfer

Camitz opponensplasty (low median nerve palsy).

• Harvested with a strip of palmar aponeurosis in continuity for length, tunnelled subcutaneously to the abductor pollicis brevis insertion.
• Restores palmar abduction; provides little pronation.
• Performed at the time of carpal tunnel release in severe thenar wasting.
• Modifications using a flexor retinaculum pulley improve flexion and pronation; a minimally invasive variant avoiding an incision across the wrist crease reduces scar pain and bowstringing.

Palmaris longus to extensor pollicis longus (radial nerve palsy).

• The standard third limb of the classical transfer set: pronator teres to extensor carpi radialis brevis, flexor carpi radialis to extensor digitorum communis, and palmaris longus to a rerouted extensor pollicis longus.
• The extensor pollicis longus is rerouted out of the third compartment to give a straight line of pull and to combine extension with palmar abduction of the thumb.
• Where palmaris longus is absent, the ring finger superficialis substitutes as the thumb extensor motor.

Palmaris longus as an anatomical landmark