Applied Surgical Anatomy of the Volar Aspect of the Wrist

Overview

The carpal tunnel is a fibroosseous canal on the volar surface of the carpus. Its base is formed by the deeply concave surface of the volar aspect of the carpal bones, and its roof is formed by the flexor retinaculum (Fig. 5-37). The ulnar nerve runs over the surface of the flexor retinaculum; it is enclosed in its own fibroosseous canal, the canal of Guyon (Fig. 5-38).

Landmarks and Incision

The four attachments of the flexor retinaculum all are palpable (Figs. 5-42 and 5-43A):

The pisiform. This is located on the ulnar border of the wrist. The pisiform is a mobile sesamoid bone lying within the tendon of the flexor carpi ulnaris muscle. The bone was sometimes used by artisans to tap nails into soft wood or leather. Historically stress fractures were noted in cobblers who used to use the pisiform for this purpose. Acute fractures have been described in contact sports athletes14,15 and pisotriquetral arthrosis may be a cause of ulnar wrist pain.16
The hook of the hamate. This is slightly distal and radial to the pisiform. To locate it, place the interphalangeal joint of the thumb on the pisiform, pointing the tip toward the web space between the thumb and the index finger, and rest the tip of the thumb on the palm. The hook of the hamate lies directly under the thumb. Because it is buried under layers of soft tissue, one must press firmly to find its rather shallow contours. The deep branch of the ulnar nerve lies on the hook, and neurapraxia of the nerve has been described in cases of fracture.17
The ridge of the trapezium. The trapezium lies on the radial side of the carpus where it articulates with the first metacarpal. To palpate the ridge, identify the joint between the trapezium and the thumb’s metacarpal bone by moving the joint passively. The ridge feels like a prominent lump on the volar aspect of the trapezium (see Fig. 5-43A).
The tubercle of the scaphoid. This small protuberance is barely palpable just distal to the distal end of the radius on the volar aspect of the wrist joint (see Figs. 5-42 and 5-43A).

On its radial side, the retinaculum also attaches across the groove on the trapezium, converting the groove into a tunnel through which the

tendon of the flexor carpi radialis muscle runs before it attaches to the base of the second and third metacarpals (see Figs. 5-42 and 5-43A).

Figure 5-37 Superficial anatomy of the wrist and palm. Note the course of the cutaneous branch of the median nerve. The longitudinal bands of the palmar aponeurosis are continuations of the palmaris longus tendon.

Superficial Surgical Dissection and Its Dangers

Three structures run across the surface of the flexor retinaculum (see Fig. 5-37):

Tendon of the palmaris longus. The palmaris longus is a vestigial muscle of no functional importance. Its tendon is used frequently for tendon grafting. It is important to test for the presence of this tendon before surgery, because it is absent in about 10% of the population. The tendon also is used as an anatomic landmark for the injection of steroid into the

carpal tunnel. If the patient is asked to flex the wrist against resistance while pinching his fingers together the tendon of the palmaris longus (if it is present) is easily palpable together with the thicker and more radially located tendon of the flexor carpi radialis. The easily defined gap between the two tendons is the site where the needle should be inserted for injection of the carpal tunnel. The needle should be inserted here dorsally and distally at an angle of almost 45 degrees. Note also that because the carpal tunnel is a distensible space, if problems are encountered in injecting it, then the tip of the needle either is still in the flexor retinaculum or is imbedded in one of the tendons in the tunnel.

Fluid inserted through correctly positioned syringes should enter the space without encountering much resistance to pressure.
Palmar cutaneous branch of the median nerve. The course of the palmar cutaneous branch of the median nerve may vary in four important ways (see Fig. 5-37)18,19,20:
1. Normally, the nerve branches off 5 cm proximal to the wrist. It runs along the ulnar side of the tendon of the flexor carpi radialis before crossing the flexor retinaculum. On rare occasions, the nerve actually may be enclosed by parts of the flexor retinaculum and, thus, may run in a tunnel of its own on the wrist.
  
  Figure 5-38 The palmar aponeurosis and fascia have been elevated to reveal the transverse carpal ligament. The fascia of the forearm and the expansions of the flexor carpi ulnaris (volar carpal ligament) are left intact where they form the roof of the tunnel of Guyon. The canal of Guyon looking from proximal to distal (inset). The transverse carpal ligament forms the floor of the tunnel of Guyon; the roof is formed by the volar carpal ligament, which is a condensation of the fascia of the forearm and expansions of the flexor carpi ulnaris tendon. The canal is formed
  
  medially by the pisiform bone and laterally by the hook of the hamate bone.
  
  Figure 5-39 The palmar aponeurosis has been resected further distally to expose the superficial palmar arterial arch. The transverse carpal ligament also has been resected. The median nerve lies superficial to the tendons of the profundus, but at the same level with the superficialis muscle tendons. Note the motor branch of the median nerve to the thenar muscles. The location of its division from the median nerve is quite variable.
  
  The nerve divides into two major branches, medial and lateral, while crossing the flexor retinaculum. The lateral is the larger branch. Both supply the skin of the thenar eminence.
2. Less often, the nerve arises from the median nerve in two distinct branches, which travel separately across the wrist.21
3. The palmar cutaneous branch may arise within the carpal tunnel and
  
  penetrate the flexor retinaculum to supply the skin of the thenar eminence.
4. The palmar cutaneous branch may be absent, replaced by a branch derived from the radial nerve, the musculocutaneous nerve, or the ulnar nerve.21
  
  The skin incision described above avoids cutting the nerve by angling across the distal forearm in an ulnar direction. One must be aware, however, that considerable variability exists in the course of the nerve. Because damage can result in the formation of a painful neuroma, transverse incisions on the volar aspect of the distal forearm must be avoided. (Compression lesions of the nerve have been reported, but these are rare.)22,23
Ulnar nerve and palmar cutaneous branch of ulnar nerve.24 The ulnar nerve runs down the volar surface of the distal forearm under cover of the flexor carpi ulnaris muscle (see Fig. 5-38). The ulnar artery lies on its radial side. The tendon of the flexor carpi ulnaris inserts into the pisiform, which then joins with the hamate and fifth metacarpal via ligaments. Just proximal to the wrist, the artery and nerve emerge from under the tendon to pass over the flexor retinaculum (the transverse carpal ligament) of the wrist (see Fig. 5-38).

Figure 5-40 The palmar aponeurosis has been elevated up to its attachment to the digital flexor sheaths. Its deeper attachments to the volar plate and bone have been cut. The flexor tendons and digital nerves are shown in continuity, as are the superficial palmar arch and the thenar and hypothenar muscles. Note that the digital nerves and vessels go deep or dorsal to the natatory ligaments.

Flexor Pollicis Brevis. Origin. The muscle’s superficial part arises from the distal edge of the flexor retinaculum and the tubercle of the trapezium, a bone in the wrist. It passes along the radial side of the tendon of the flexor pollicis longus. The deeper (and medial) portion of the muscle is very small, and arises from the ulnar side of the first metacarpal bone between the oblique part of the adductor pollicis and the lateral head of the first dorsal interosseous muscle. Insertion. The superficial head is inserted into the radial side of the base of the proximal phalanx of the thumb; in its

tendon of insertion there is a sesamoid bone. The deep head is inserted into the ulnar side of the base of the first phalanx with the adductor pollicis. Action. Flexor of the metacarpophalangeal joint of the thumb. Nerve supply. Superficial head—median nerve (motor or recurrent branch). Deep head—deep branch of ulnar nerve.

Abductor Pollicis Brevis. Origin. Flexor retinaculum and tubercle of scaphoid. Insertion. Radial side of base of proximal phalanx of thumb. Action. Abduction of thumb at metacarpophalangeal joint and rotation of proximal phalanx of thumb. Nerve supply. Median nerve (motor or recurrent branch).

Figure 5-41 Portions of the thenar and hypothenar muscles have been resected to reveal their layering. The ulnar nerve passes between the origin of the abductor digiti minimi and the flexor digiti minimi. In the thenar region, the course of the flexor pollicis longus is seen as it crosses between the two heads of the flexor

pollicis brevis. Portions of the long flexors of the fingers have been resected to show their layering. The superficial palmar arch runs superficial to the tendons, whereas the deep palmar arch is immediately deep to the tendons. Note that potential spaces develop on the undersurface of the flexor tendons and their sheaths, and on the deep intrinsic muscles of the hand, the interosseous on the hypothenar side and the adductor pollicis on the thenar side. A septum that runs from the undersurface of the flexor tendons to the third metacarpal divides the two spaces. More distally, the superficial transverse ligament has been resected, revealing the course of the lumbricals and the digital vessels that run superficial or palmar to the deep transverse metacarpal ligaments.

Adductor Pollicis. Origin. Oblique head from bases of second and third metacarpals, trapezoid, and capitate. Transverse head from palmar border of third metacarpal. Insertion. Ulnar side of base of proximal phalanx of thumb via ulnar sesamoid. Action. Adduction of thumb. Opposition of thumb. Nerve supply. Deep branch of ulnar nerve.

Opponens Pollicis. Origin. Flexor retinaculum. Insertion. Radial border of thumb metacarpal. Action. Opposition of metacarpal bone of thumb. Nerve supply. Median nerve (motor or recurrent branch).

Figure 5-42 The deepest layer of the palm is revealed. The deep palmar arterial arch lies deep to the long flexor tendons and superficial to the interosseous muscles. It crosses the palm with the deep branch (motor branch) of the ulnar nerve. The nerve supplies all the interosseous muscles. More distal, the interosseous muscles are seen running deep (dorsal) to the deep transverse ligament. The deep transverse metacarpal ligaments attach to the palmar plate, which is seen on the fifth metacarpal. The pulleys of the thumb are seen in relationship to the digital nerves.

Figure 5-43 A: The bones of the wrist and palm and the proximal metacarpals are seen in relationship to the creases of the wrist. The necks of the metacarpals are at the level of the distal palmar crease. The distal wrist crease runs from the proximal

portion of the pisiform to the proximal portion of the tubercle of the scaphoid and marks the proximal level of volar carpal ligament. The proximal transverse palmar crease is at the radiocarpal joint. B: Kaplan’s cardinal line. Used to locate the motor branch of the median nerve to the thenar muscles.

At this level, the anatomic arrangement of these structures can be remembered by the mnemonic “ANT”: The artery is the most lateral structure, then the nerve, and, finally, the tendon of the flexor carpi ulnaris (see Fig. 5-38).

The palmar cutaneous branch arises in the forearm. It travels lateral to the ulnar artery in 75% of patients. The nerve perforates the fascia of the anterior forearm just proximal to the distal wrist crease. In the palm, the nerve travels superficial to the superficial palmar arch in most cases. It is therefore at risk during decompression of the ulnar nerve in the Canal of Guyon.

At the wrist, the nerve is particularly vulnerable to damage by lacerations. The grim triad of lacerations of the tendon of the flexor carpi ulnaris, the ulnar artery, and the ulnar nerve is a common sequela of falling through a window with the ulnar border of the wrist flung forward to protect the face.

As the nerve crosses the flexor retinaculum, it is covered with a tough fibrous tissue that is continuous with the deep fascia of the forearm, the volar carpal ligament. The tunnel thus formed, the canal of Guyon, has four boundaries: A floor, the flexor retinaculum (transverse carpal ligament); a medial wall, the pisiform; a lateral wall, the hamate; and a roof, the volar carpal ligament (distal fascia of the forearm; see Fig. 5-38).

Around the pisiform, the ulnar nerve divides into two branches. The superficial branch supplies the palmaris brevis muscle and the skin of the small finger and ulnar half of the ring finger. The deep branch supplies all the small intrinsic muscles of the hand, except those of the thenar eminence and the radial two lumbricals (see Figs. 5-39 to 5-42).

Deep Surgical Dissection and Its Dangers

Median Nerve

The median nerve crosses the volar aspect of the distal forearm deep to the flexor digitorum superficialis muscle. Just above the wrist, it becomes superficial and lies between the tendons of the palmaris longus and flexor carpi radialis muscles. It enters the palm by traversing the carpal tunnel (see Fig. 5-38).

Within the tunnel, the nerve lies superficial to the tendons of the flexor digitorum profundus and flexor pollicis longus muscles. The superficialis tendons lie toward the ulnar side of the nerve. At the distal border of the flexor retinaculum, the nerve divides into two branches (see Figs. 5-39 and 5-40).

The medial branch sends cutaneous branches to the adjacent sides of the ring and middle fingers, and to the adjacent sides of the middle and index fingers.
The lateral branch sends cutaneous branches to the radial side of the index finger and to both sides of the thumb. The lateral branch usually also sends off the motor, or recurrent, nerve (see Fig. 5-39), which is the key surgical landmark and major surgical danger in carpal tunnel decompression.

The motor nerve supplies the muscles of the thenar eminence. Its course may take any one of eight significant variations22,25:

The classic course (seen in 50% of patients). The branch arises from the volar radial aspect of the median nerve distal to the radial end of the carpal tunnel. The nerve hooks radially and upward to enter the thenar muscle group between the flexor pollicis brevis and abductor pollicis brevis muscles.
The position of the motor branch can be estimated by drawing one vertical line from the web space between the middle and index fingers, drawing another from the radial origin of the first web space, then connecting to the hook of the hamate (Kaplan’s cardinal line). The intersection of these two lines marks the entrance of the motor branch into the thenar muscles (see Fig. 5-43B).26
A variation that occurs in about 30% of patients. The branch arises from the anterior surface of the nerve within the carpal tunnel. It passes through the tunnel with its parent nerve and hooks around the distal end of the flexor retinaculum to enter the thenar group between the flexor pollicis brevis and abductor pollicis brevis muscles.
A variation that occurs in about 20% of patients. The branch arises from the anterior surface of the nerve within the carpal tunnel. It travels radially to pierce the flexor retinaculum and enter the thenar group of muscles between the abductor pollicis brevis and flexor pollicis brevis muscles.27
A rare variation. The branch arises from the ulnar side of the median

nerve.28 It crosses the median nerve within the tunnel, then hooks around the distal end of the flexor retinaculum to enter the thenar muscle group. It also may pass through the flexor retinaculum and lie anterior to it.29
Another rare variation. The nerve arises from the anterior surface of the median nerve within the carpal tunnel. At the distal end of the flexor retinaculum, the branch hooks radially over the top of the retinaculum. The nerve crosses the distal part of the retinaculum almost transversely before entering the thenar group of muscles.
A very rare variation (multiple motor branches).30 Double nerves may follow any of the courses described above.
A third rare variation (high division of the median nerve).31 The nerve may divide into medial and lateral branches high up in the forearm. The thenar branch, originating from the lateral branch, may leave the carpal tunnel either in the conventional manner or by piercing the flexor retinaculum on its radial side.

All these variations should be considered when the nerve is exposed. If the tunnel is opened on the ulnar side of the nerve, the motor branch will be preserved unless it lies on the same side. Patients with exceptionally rare variations usually have large palmaris brevis muscles, which should alert the surgeon to the possibility during the approach.22

Flexor Digitorum Superficialis

Within the carpal tunnel, the tendons to the middle and ring fingers are superficial to the tendons of the index and little fingers. This arrangement dictates correct repair in cases of multiple tendon laceration (see Figs. 5-38 and 5-41).

Flexor Digitorum Profundus

The tendons of the flexor digitorum profundus lie deep to the tendons of the flexor digitorum superficialis. The tendon to the index finger is separate; the other three still may be attached partially to each other as they pass through the carpal tunnel (see Fig. 5-41).

Flexor Pollicis Longus

The tendon of the flexor pollicis longus lies deep to that of the flexor carpi radialis and is found on the most radial aspect of the canal at the same depth as the profundus tendons (see Fig. 5-41).

Flexor Carpi Radialis

The flexor carpi radialis tendon perforates the flexor retinaculum to lie in the groove of the trapezium before it inserts into the bases of the second and third metacarpals. It does not pass through the carpal tunnel (see Fig. 5-42).