Lumbrical Muscles of the Hand

Origin. Each lumbrical arises from the flexor digitorum profundus tendons in the palm, distal to the distal edge of the transverse carpal ligament, at approximately the level of the mid-palm to the distal palmar crease depending on the position of the digits. Because the origin sits on a moving tendon it migrates roughly 2-3 cm proximally and distally with finger flexion and extension, which is why it can enter the carpal tunnel in full flexion.

Insertion. Each lumbrical runs distally along the radial side of its digit, passes volar to the deep transverse metacarpal ligament, then turns dorsally around the radial side of the MCP joint. It inserts into the radial lateral band of the extensor hood at the level of the proximal phalanx, joining the interosseous contribution to form the conjoined lateral band and thence the terminal tendon. There is no bony insertion at all: that is the defining feature, and the reason the lumbrical is the purest interphalangeal extensor.

Compared with the interossei. The two intrinsic groups are built for opposite jobs.

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Variations. Absence of the fourth lumbrical is reported in a minority of hands, and the third and fourth may be fused. Insertion into the ulnar rather than the radial lateral band, or into both, is described, particularly at the little finger. In some individuals the muscle belly itself lies within the carpal tunnel in flexion; hypertrophy of the muscle or an unusually proximal origin is a recognised cause of dynamic carpal tunnel syndrome, covered under Clinical Relevance.

The dual innervation. The first and second lumbricals (index and middle) are supplied by the median nerve, through branches of the first and second common palmar digital nerves in the palm, not from the recurrent motor branch. The third and fourth (ring and little) are supplied by the deep motor branch of the ulnar nerve. Root values are predominantly C8 and T1 for all four. It is the same 2-2 split as the flexor digitorum profundus itself: the index and middle profundus are anterior interosseous (median), the ring and little are ulnar.

Why the split matters. In an isolated ulnar palsy the index and middle keep their median-innervated lumbricals, which insert exclusively into the radial lateral band. That is sufficient to prevent clawing in those digits, so only the ring and little claw. In a combined low median and ulnar palsy all four lumbricals and all seven interossei are paralysed, so all four fingers claw, the classic leprosy and combined wrist laceration picture.

The recurrent branch distinction. In an isolated recurrent motor branch injury, the carpal tunnel release complication, the thenar muscles are lost but the lumbricals are spared, because their branches come off the common digital nerves distally. A median nerve division at the wrist takes both. Examiners like this distinction.

Entry point. Each branch enters the lumbrical from its radial and palmar aspect in the mid-palm, close to the origin. The first common digital nerve to the first lumbrical lies immediately radial to the index FDP tendon in the palm, at risk in deep palmar exploration and in an A1 pulley release performed too far proximally in the index.

Variation. The third lumbrical is the one with variable innervation: it is reported to receive a median contribution in a proportion of hands, and dual innervation of the third lumbrical is a recognised variant. The fourth is consistently ulnar. The Riche-Cannieu and Martin-Gruber anastomoses can shift lumbrical innervation and confuse both electrodiagnosis and clinical localisation.

Arterial. The lumbricals draw on three sources along their length:

• The superficial palmar arch and the common palmar digital arteries, which run alongside the lumbricals in the palm
• The deep palmar arch, via the palmar metacarpal arteries, proximally
• Dorsal digital branches distally, where the lumbrical joins the extensor hood

Venous and lymphatic. Drainage follows the palmar venous network. The lumbrical canal is a lymphatic and infective pathway from the palm to the digit, which is why its anatomy is set out under Relations and its infections under Clinical Relevance.

The lumbrical canal. A fascial tunnel in the palm containing the lumbrical muscle and its tendon, running from the midpalmar (or thenar) space proximally to the web space and the digit distally. Within it the lumbrical lies immediately radial to its FDP tendon and volar to the deep transverse metacarpal ligament. The common digital nerve and artery lie in the same interval, immediately volar or volar-radial to the lumbrical as it approaches the web space, where they bifurcate into the proper digital neurovascular bundles, roughly at the level of the distal palmar crease.

• Boundaries: the flexor tendon sheath and the FDP tendon medially and dorsally, the deep transverse metacarpal ligament dorsally at the distal end, and the palmar fascia and the digital neurovascular bundle volarly
• Communications: proximally with the midpalmar space (third and fourth canals) and the thenar space (first and second, radial to the thenar septum); distally with the subcutaneous tissue of the web space

The canal is the anatomical route by which infection tracks between the deep palmar spaces and the digits in both directions: a web space (collar-stud) abscess can track proximally into the midpalmar space along it, and a deep palmar space infection can present as a swollen finger.

Volar and dorsal. Volar to the lumbrical lie the palmar aponeurosis, the superficial palmar arch and the common digital nerves and arteries. Dorsal to it lie the flexor tendons, the interossei, and the deep transverse metacarpal ligament at the level of the MCP joints. The lumbrical is therefore the muscle you see when you have gone too deep in a palmar approach.

The deep transverse metacarpal ligament. It connects the palmar plates of the second to fifth MCP joints and is the dividing plane between the two intrinsic groups: lumbricals pass volar to it, interossei dorsal. That makes the lumbrical the anatomical model for every anti-claw tendon transfer. A transfer must be routed volar to the ligament, exactly as the lumbrical runs, to flex the MCP joint; routed dorsal to it, it extends the MCP joint and worsens the claw.

Recognition. Ask the patient to make a fist and watch the affected digit: it extends at the PIP while the others flex. Ask about a previous amputation, flexor tendon injury, jersey finger or tendon graft, because the history usually gives the diagnosis. Then establish whether the FDP is divided distal to the lumbrical origin (laceration, avulsion, amputation) or effectively lengthened (an over-long tendon graft, a ruptured repair), because the mechanism determines the fix.

The mimics. A fixed PIP extension contracture, a swan-neck deformity and intrinsic tightness all produce an extended PIP. Only the lumbrical-plus finger extends more when the patient tries to flex, so watch the finger during active effort.

Treatment. Division of the lumbrical tendon at its insertion into the lateral band is the definitive treatment, and it is simple and reliable. It removes the abnormal linkage between the retracting profundus stump and the extensor mechanism; the finger loses a small amount of interphalangeal extension power, which the interossei amply compensate, and the paradoxical extension abolishes immediately. Division or excision of the retracted profundus stump is an alternative, and where the finger is otherwise reconstructable, tightening or revising an over-long graft addresses the cause.

The trap. Do not treat a lumbrical-plus finger by tightening or repairing the profundus unless you can restore a functioning distal insertion. In an amputation stump there is nothing distal to attach it to, so the only options are lumbrical release or excision of the stump.

Presentation. An incomplete fist and weak grip in uninjured fingers, months after an injury or operation to a different digit; patients describe a "weak hand" rather than a stiff finger. Record the fingertip-to-distal-palmar-crease distance for each digit in maximum active flexion, and look for a consistent shortfall in the digits that share the profundus belly with the injured one. Then confirm passively: each affected digit should have full passive flexion, and if passive flexion is also limited the problem is joint stiffness, not quadriga.

Causes, and how to avoid each. Every cause fixes the length or tethers the excursion of one profundus tendon.

Treatment. Tenolysis where adhesion is the cause, once the wound is mature and the joints supple. Division of the tethering tendon, for example an FDP that has been tenodesed over an amputation stump, immediately restores excursion to the others, and a short graft is lengthened or revised. Results are good when the cause is a discrete tether and the joints are supple, and poor when there is diffuse scarring.

The collar-stud abscess. A web space infection with a superficial palmar or dorsal component connected through the web by a narrow track, also called a dumb-bell abscess. The dorsal component is often larger because the dorsal tissues are looser, so the swelling looks dorsal while the source is palmar. The adjacent fingers are held abducted by the web space swelling, a useful and specific finding.

Spread along the canal. Infection tracks proximally from the web into the midpalmar space (third and fourth canals) or the thenar space (first and second canals, radial to the thenar septum), so a neglected web space abscess becomes a deep palmar space infection. When a patient with a digital infection develops palmar swelling and loss of the palmar concavity, the infection has tracked along the lumbrical canal and requires deep space drainage, not just digital drainage.

Drainage. A combined palmar and dorsal incision is usually required, because a single incision decompresses only one half of the collar stud. Longitudinal or curved palmar incisions that do not cross the web at right angles, plus a dorsal incision, allow through-and-through drainage. Never make a transverse incision across the web itself: the resulting scar contracture is worse than the infection.

Antibiotics. Cover Staphylococcus aureus and streptococci, and consider MRSA cover according to local prevalence. In a bite injury, cover Eikenella corrodens (human) or Pasteurella multocida (animal) with co-amoxiclav.

The deep palmar spaces. Three spaces matter to the surgeon draining a hand:

• Midpalmar space: deep to the flexor tendons of the middle, ring and little fingers, ulnar to the thenar septum; presents with loss of the palmar concavity and pain on passive extension of the middle, ring and little
• Thenar space: radial to the thenar septum; presents with a swollen, abducted thumb and a full first web
• Parona's space: the potential space in the distal forearm between pronator quadratus and the flexor tendons, the route by which a radial and an ulnar bursal infection communicate to form a horseshoe abscess

Dynamic carpal tunnel syndrome. With full finger flexion the lumbrical origins migrate proximally into the carpal tunnel, and Cobb's cadaver study showed that incursion to be a normal occurrence, not an anomaly. In individuals with a proximal origin or hypertrophied lumbricals, seen in musicians, keyboard users and manual workers whose symptoms appear only with sustained grip, the incursion is proposed as the mechanism by which sustained grip could raise carpal tunnel pressure; the study measured position, not pressure, so that step remains a hypothesis. The presentation is median paraesthesia that occurs only with prolonged gripping, typing, playing an instrument or using a tool, resolves on release, and comes with normal standard nerve conduction studies at rest.

Managing it. Activity modification and ergonomic change come first. Carpal tunnel release is effective where symptoms are disabling and other causes are excluded, and the lumbrical incursion is visible at operation with the fingers flexed.

Dupuytren surgery. The lumbrical is a useful depth landmark in the palm, but the danger in Dupuytren disease is the spiral cord, which displaces the digital neurovascular bundle superficially, proximally and toward the midline, into the plane the surgeon expects to be safe and directly into the line of the approach. Identify the bundle proximally in normal tissue and follow it distally; never divide a cord before you have seen the nerve, and never dissect a cord distal to unknown anatomy.

Intrinsic reconstruction. Every anti-claw tendon transfer replicates the lumbrical course, inserting into the radial lateral band for a non-correctable claw, or into the A1 or A2 pulley or the proximal phalanx for a correctable claw. The lumbrical is the anatomical template, which is why the operation is sometimes described as creating an "artificial lumbrical".

Trigger finger. A proximal lumbrical origin or muscle belly entering the A1 pulley region is an occasional cause of triggering that does not resolve with A1 release. Recognise it at operation: the muscle belly, not a nodule, is the obstruction, and the treatment is debulking or partial excision rather than further pulley division.