Hand — Surgical Anatomy, Approaches, and Z- Plasty

Introduction

The hand is the most anatomically intricate region of the orthopedic skeleton, with 27 bones, more than 30 joints, intricate ligamentous and tendinous structures, multiple muscular layers, and dense neurovascular distribution all packed into a small volume. The functional demands — fine prehension, power grip, opposable thumb, sensory and dexterous manipulation — are matched by an anatomical complexity that requires the orthopedic surgeon to know not only the bony skeleton but also the precise relationships of the flexor and extensor tendons, the pulley system, the intrinsic muscles, the lumbricals, the interossei, the digital nerves and arteries, and the deep palmar arch. The orthopedic surgeon’s surgical approaches to the hand must respect these structures, with the principles of incision design (including the Z-plasty technique for managing skin contractures across flexion creases) being as important as the principles of tendon and nerve management. This chapter draws on Orthopaedic Surgical Approaches, Netter’s Concise Orthopaedic Anatomy, Gray’s Anatomy, and Dutton’s Orthopaedic Examination.

Bony and Articular Anatomy

The hand skeleton comprises: 8 carpal bones in two rows (proximal: scaphoid, lunate, triquetrum, pisiform; distal: trapezium, trapezoid, capitate, hamate) — see Topic Trauma-18 for fracture and dislocation anatomy. 5 metacarpals numbered 1 through 5 from radial to ulnar. 14 phalanges: Two for the thumb (proximal and distal) and three for each of the fingers (proximal, middle, distal). The total is 14 phalanges in the hand. The principal joints include: Carpometacarpal (CMC) joints: The articulations between the distal carpus and the metacarpal bases. The first CMC joint (thumb) is a saddle joint between the trapezium and the first metacarpal, providing the unique opposability of the thumb. The second through fifth CMC joints are essentially gliding joints with limited motion (more motion at the fourth and fifth, contributing to the cupping of the hand). Metacarpophalangeal (MCP) joints: The “knuckle” joints between the metacarpal heads and the proximal phalangeal bases. The fingers have condyloid joints (allowing flexion- extension and limited abduction-adduction); the thumb has a hinge-type joint. Interphalangeal (IP) joints: The proximal IP (PIP) joints and distal IP (DIP) joints of the fingers; the thumb has a single IP joint. All are hinge joints permitting flexion-extension only.

The collateral ligaments of the MCP and IP joints have specific anatomical features. The MCP collateral ligaments are taut in flexion and lax in extension (a fact that determines the safe immobilization position — see below). The IP collateral ligaments are taut throughout the range, with both proper collateral ligaments (between adjacent bones) and accessory collateral ligaments (between the proximal bone and the volar plate).

Skin and Subcutaneous Tissue

The palmar skin is glabrous (hairless), thick, and tightly bound to the underlying palmar fascia by skin ligaments. The dorsal skin is thinner, more mobile, with less subcutaneous tissue. The contrast between the palmar and dorsal skin determines the principles of incision design — palmar incisions should respect the natural flexion creases and avoid crossing them at right angles; dorsal incisions have more flexibility but should respect the underlying tendon anatomy. The flexion creases of the palm and digits correspond approximately to the underlying joints but are not always directly over the joint: Distal palmar crease: Approximately overlies the MCP joints. Proximal palmar crease: Approximately overlies the metacarpal heads. Distal interphalangeal crease: Overlies the DIP joint. Proximal interphalangeal crease: Overlies the PIP joint. Wrist crease: Overlies the radiocarpal joint.

Palmar Fascia and Compartments

The palmar aponeurosis is a triangular fibrous structure with the apex at the wrist (continuous with the palmaris longus tendon when present) and the base distally toward the fingers. The structure has longitudinal, transverse, and oblique fibers that together provide a strong protective layer over the palmar structures. Disease of the palmar fascia produces Dupuytren’s contracture (covered in Topic Orth-29). The palmar compartments are arranged around the longitudinal axis of the hand: Thenar compartment: Contains the abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, and adductor pollicis (variously included in this or the adductor compartment). Provides the thenar eminence at the base of the thumb. Hypothenar compartment: Contains the abductor digiti minimi, opponens digiti minimi, flexor digiti minimi brevis. Provides the hypothenar eminence. Central compartment (mid-palmar): Contains the lumbricals, flexor tendons, and palmar arch. Adductor compartment: Contains the adductor pollicis.

Interosseous compartments: Four dorsal interosseous and three palmar interosseous muscles between the metacarpals. Carpal tunnel at the wrist contains the median nerve and the nine flexor tendons (FPL, four FDS, four FDP). Compartment syndrome of the hand requires recognition of these multiple compartments and decompression through specific incisions.

Tendinous Anatomy

Flexor Tendons The flexor tendons pass through the carpal tunnel and into the digits through the flexor sheaths. The flexor digitorum superficialis (FDS) tendons insert on the middle phalanges; the flexor digitorum profundus (FDP) tendons pass through the FDS chiasm (Camper’s chiasm) and insert on the distal phalanges. The flexor pollicis longus (FPL) is the deep flexor of the thumb, inserting on the distal phalanx. The flexor pulley system maintains the tendons against the bone, preventing bowstringing during flexion. The annular pulleys (A1-A5) and the cruciate pulleys (C1- C3) form the pulley system. A2 (over the proximal phalanx) and A4 (over the middle phalanx) are the principal pulleys for biomechanical function; their preservation during surgery is essential to prevent bowstringing. The A1 pulley is the site of trigger finger (stenosing flexor tenosynovitis); release of A1 is the surgical treatment. The flexor tendon zones (Verdan) describe the location of injury for prognostic and treatment purposes: Zone I: Distal to the FDS insertion. Only FDP involved. Zone II (“no man’s land”): From the proximal A1 pulley to the FDS insertion. Both FDS and FDP within the synovial sheath. The most technically demanding zone for repair because of the synovial environment and risk of adhesions; outcomes have historically been disappointing but have improved with early protected motion protocols. Zone III: Between the carpal tunnel and the A1 pulley. Lumbrical region. Zone IV: Carpal tunnel. Zone V: Proximal to the carpal tunnel (distal forearm). Extensor Tendons The extensor tendons pass through the six dorsal extensor compartments at the wrist (from radial to ulnar): Compartment 1: Abductor pollicis longus (APL) and extensor pollicis brevis (EPB). Site of de Quervain’s tenosynovitis.

Compartment 2: Extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB). Compartment 3: Extensor pollicis longus (EPL). Passes around Lister’s tubercle and is at risk in distal radius fractures (delayed rupture is a recognized complication). Compartment 4: Extensor digitorum communis (EDC) and extensor indicis proprius (EIP). Compartment 5: Extensor digiti minimi (EDM). Compartment 6: Extensor carpi ulnaris (ECU). In the digits, the extensor tendons form a complex extensor hood mechanism with the lumbricals and interossei. The central slip inserts on the dorsum of the middle phalangeal base, and the lateral bands continue distally to form the terminal extensor tendon inserting on the dorsum of the distal phalangeal base. The extensor tendon zones (Verdan) parallel the flexor zones: Zone I: DIP joint and distal. Site of mallet finger. Zone II: Middle phalanx. Zone III: PIP joint. Site of boutonnière injury (central slip disruption). Zone IV: Proximal phalanx. Zone V: MCP joint. Site of “fight bite” injuries. Zone VI: Dorsum of the hand. Zone VII: Wrist (extensor retinaculum). Zone VIII: Distal forearm.

Intrinsic Hand Muscles

Thenar Muscles Abductor pollicis brevis (APB): Abducts the thumb. Innervated by the median nerve. The classical test for median nerve function. Opponens pollicis: Opposes the thumb. Innervated by the median nerve. Flexor pollicis brevis (FPB): Flexes the thumb MCP. Has dual innervation (median nerve for the superficial head, ulnar nerve for the deep head). Adductor pollicis: Adducts the thumb. Innervated by the ulnar nerve. The classical test for ulnar nerve function (Froment sign). Hypothenar Muscles Abductor digiti minimi (ADM): Abducts the small finger. Innervated by the ulnar nerve.

Opponens digiti minimi: Opposes the small finger. Flexor digiti minimi brevis: Flexes the small finger MCP.

Lumbricals The lumbricals (4 muscles) originate from the FDP tendons in the palm and insert into the radial side of the extensor expansion of each finger. Function: flex the MCP and extend the IP joints. Innervation: the radial two lumbricals (to the index and middle fingers) are innervated by the median nerve; the ulnar two lumbricals (to the ring and small fingers) are innervated by the ulnar nerve.

Interossei The dorsal interossei (4 muscles) abduct the fingers from the midline of the hand. The palmar interossei (3 muscles) adduct the fingers toward the midline. All interossei are innervated by the ulnar nerve. Loss produces clawing and intrinsic minus posture.

Neurovascular Anatomy

Digital Nerves and Arteries The digital nerves and arteries run along the lateral and medial aspects of each digit, with the artery typically dorsal and the nerve typically volar in the proximal aspect of the digit, then the relationship reverses with the nerve volar and the artery slightly dorsal in the distal aspect. The pattern is: Median nerve supplies the digital nerves to the radial three and a half digits (thumb, index, middle, and radial half of ring finger) — the typical median nerve sensory distribution. Ulnar nerve supplies the digital nerves to the ulnar one and a half digits (small finger and ulnar half of ring finger). Radial nerve (superficial branch) supplies sensation to the dorsum of the radial three and a half digits proximal to the DIP joints. Arteries The deep palmar arch (formed by the radial artery with contribution from the ulnar artery) and the superficial palmar arch (formed by the ulnar artery with contribution from the radial artery) supply the digital arteries. The arches anastomose at the level of the metacarpal bases and provide rich vascular supply with redundancy. The Allen test assesses the patency of the radial and ulnar arteries by occluding both, exsanguinating the hand by clenching, then releasing one artery and observing the rate of refill. The test is performed before radial artery harvest or before procedures that may compromise one of the arteries.

Principles of Hand Incisions

The principles of hand incision design are essential to good outcomes: Avoid crossing flexion creases at right angles: This produces scar contracture across the crease, with subsequent loss of motion. Incisions should either parallel the creases or cross them at acute angles (typically less than 45 degrees), forming a “Z” or “step” pattern. Avoid longitudinal incisions on the volar surface of the digits: A straight longitudinal incision contracts and produces flexion contracture. The Bruner zig-zag incision (alternating diagonal incisions creating “Z” pattern over the volar digit) provides exposure while avoiding contracture. Mid-axial incisions on the digits: These straight longitudinal incisions along the lateral or medial mid-axis of the digit avoid crossing the volar creases entirely and provide good exposure for flexor tendon sheath and neurovascular work. The incision is placed at the most dorsal point of the volar flexion creases. Dorsal incisions: Generally have more flexibility but should respect the underlying extensor tendon and the venous and lymphatic drainage. Wrist incisions: The transverse wrist crease is generally avoided as a longitudinal incision site; the incision should either curve around the wrist or be placed proximal or distal to the crease.

Z-Plasty

The Z-plasty is a fundamental hand surgery technique used to lengthen contracted scars across flexion creases or contractures. The principle is to transpose two triangular flaps to redirect tension along a different axis, with consequent elongation along the original axis of contracture. Technique The classical 60-degree Z-plasty uses two triangular flaps with 60-degree angles, producing a 75 percent gain in length along the central axis of the original contracture. Variations include 45-degree Z-plasty (50 percent gain) and 90-degree Z-plasty (more length gain but with substantial tension and skin compromise). The procedure begins with the central limb of the “Z” placed along the contracted scar or along the axis where lengthening is desired. The two oblique limbs are placed at the chosen angle (typically 60 degrees) at each end of the central limb. The skin is incised along these three lines, the two triangular flaps are elevated, and the flaps are transposed (the flap that was lateral is brought across to occupy the medial position, and vice versa). The central limb of the Z then runs perpendicular to its original orientation, producing elongation along the original contracture axis.

Applications The Z-plasty is used for:

Burn scar contracture release: Across flexion creases. Dupuytren’s contracture release: At the level of the palmar fascia and skin. Surgical incision design: When the planned incision would cross a flexion crease, the Z- plasty can be incorporated to avoid contracture. Web space deepening: For first web space contractures. Modifications: Multiple Z-plasties (in series along a longer contracture), planimetric Z- plasty, and Y-V advancements are variations used for specific situations.

Surgical Approaches to Specific Hand Pathologies

Carpal Tunnel Release The carpal tunnel release is the most commonly performed hand procedure. The approach uses a longitudinal incision in the palm parallel to the thenar crease, approximately 2 to 3 cm distal to the distal wrist crease. The incision is in line with the radial border of the ring finger, avoiding the superficial palmar arch distally and the palmar cutaneous branch of the median nerve proximally (which crosses the wrist crease at the radial side of the palmaris longus tendon). The palmar fascia is incised, and the transverse carpal ligament is identified and divided completely. The median nerve is identified and protected. The release proceeds proximally and distally until the ligament is completely divided. The procedure can also be performed endoscopically through a small proximal incision and an endoscopic blade, with reduced post-operative pain but similar long-term outcomes. Trigger Finger Release The A1 pulley release is performed through a small transverse or oblique incision at the distal palmar crease, over the affected finger’s MCP joint. The neurovascular bundle (radial and ulnar) is identified and protected. The A1 pulley is identified and divided longitudinally, with care to preserve the A2 pulley (which begins immediately distal to A1). Mallet Finger Repair Operative repair of mallet finger uses a dorsal incision over the DIP joint, with options for K-wire fixation, suture anchor repair, or tension-band wiring depending on the fracture pattern. Boutonnière Reconstruction Reconstruction of boutonnière deformity uses a midline dorsal incision over the PIP joint, with various techniques to reconstruct the central slip and reposition the lateral bands (Curtis, Matev, or other techniques).

Jersey Finger Repair The Jersey finger (FDP avulsion from the distal phalanx) is approached through a Bruner zig-zag incision along the volar finger and palm, with retrieval of the retracted tendon and reinsertion to the distal phalanx with suture anchor or transosseous suture. Skier’s Thumb (UCL Tear) The ulnar collateral ligament repair of the thumb MCP joint uses a dorsal-ulnar incision over the thumb MCP joint, with identification of the dorsal sensory branch of the radial nerve (which crosses the operative field and is at risk of injury and neuroma formation), the adductor aponeurosis, and the ulnar collateral ligament. The Stener lesion (displacement of the avulsed ligament superficial to the adductor aponeurosis) is identified and the ligament is repaired or reattached to bone. Tendon Repair Flexor tendon repair uses a Bruner zig-zag incision to expose the tendon sheath, with windowing of the sheath at the level of the laceration. The modified Kessler technique (4- strand core suture with epitendinous repair) or stronger 6-strand or 8-strand techniques provide repair strong enough for early protected active motion rehabilitation protocols (the Strickland or Duran protocols). Extensor tendon repair uses dorsal incisions appropriate to the zone of injury. Simpler than flexor tendon repair because of the more accessible location and less restrictive sheath. The chapter has addressed the principal hand anatomical and surgical considerations. The summary returns to the central themes.

Summary and Take-Home Points The hand is the most anatomically intricate orthopedic region, with 27 bones, complex tendon and pulley systems, intrinsic muscles, and dense neurovascular anatomy. The principles of hand surgical anatomy include the flexor tendon zones (with zone II “no man’s land” between the proximal A1 pulley and the FDS insertion being the most challenging for repair), the extensor tendon zones, the A2 and A4 pulleys as the critical biomechanical pulleys, the intrinsic compartments (thenar, hypothenar, central, adductor, four dorsal interossei, three palmar interossei), the digital neurovascular bundles along the lateral and medial aspects of each digit, and the palmar and dorsal arches with their digital branches. The principles of hand incision design are essential: avoid crossing flexion creases at right angles (using Bruner zig-zag or mid-axial incisions on the digits), respect the underlying tendon anatomy on the dorsum, and use the Z-plasty technique to lengthen contractures across flexion creases (with 60-degree Z-plasty providing 75 percent length gain along the central axis).

The principal hand surgical approaches include the carpal tunnel release (longitudinal palmar incision), trigger finger release (transverse or oblique distal palmar crease incision), mallet finger and boutonnière reconstructions (dorsal IP joint incisions), Jersey finger repair (Bruner incision with tendon retrieval), and Skier’s thumb UCL repair (dorsal- ulnar thumb MCP incision with recognition of Stener lesion). The median nerve, ulnar nerve, and radial sensory nerve provide the sensory and motor innervation of the hand, with the median nerve in the carpal tunnel supplying the thenar muscles (APB, opponens, FPB superficial head) and the radial two lumbricals; the ulnar nerve in Guyon’s canal supplying the hypothenar muscles, interossei, ulnar two lumbricals, and adductor pollicis; the radial sensory branch providing dorsal sensation to the radial three and a half digits proximal to the DIP joints. The chapter that follows turns to the pelvis, beginning the lower extremity anatomical sequence.