The median nerve originates from nerve roots that comprise the lateral and medial cords of the brachial plexus. As the nerve enters the arm, it does not branch until it passes below the elbow ultimately to innervate the numerous wrist and digital flexors. As it enters the hand, the median nerve innervates the LOAF muscles, which include the first and second lumbricales, the opponens pollicis, the abductor pollicis brevis, and the flexor pollicis brevis. The recurrent motor branch, which may arise in the carpal tunnel, can reach the thenar muscles either by looping around the distal end of the transverse carpal ligament (most common type) or through the ligament anywhere along its length. The motor branch innervates the abductor pollicis brevis, the opponens pollicis, and the superficial head of the flexor pollicis brevis. The palmar cutaneous branch of the median nerve exits the median nerve prior to its entry into the carpal tunnel and then travels superficially alongside the median nerve into the palm, where it divides into a medial and lateral branch supplying the skin over the median eminence and extending medially to the fourth metacarpal bone. The branch most commonly originates about 2 cm proximal to the proximal border of the flexor retinaculum but may have a variable origin and course. The sensory supply of the median nerve extends to the radial 372 digits of the hand via the common palmar digital branches. The rest of the sensory innervation of the hand is supplied by the sensory branch of the ulnar nerve as it branches from the main trunk in the distal forearm and immediately radial to the tendon of the flexor carpi ulnaris.
The floor of the carpal tunnel is comprised of the carpal bones, the ligamentous extensions between the carpal bones, and the overlying radiocarpal ligament. The roof of the tunnel is formed by the transverse carpal ligament as it extends radially from the tuberosity of the scaphoid and the crest of the trapezius to its ulnar attachments at the hook of the hamate and the pisiform bone. Proximally, the transverse carpal ligament blends with the fibers of the antebrachial fascia at the distal wrist crease. The ligament extends distally into the palmar approx 3.0 ± 0.25 cm to end near Kaplan's cardinal line. Kaplan's cardinal line is a line defined to be parallel to the distal palmar crease; it runs along the base of the fully extended thumb. In relation to an endoscopic approach to the carpal tunnel, several key anatomical features need to be highlighted.
Access to the tunnel is made ulnar to the palmaris longus, whereas the median nerve is located radial to the palmaris longus, thereby keeping it well away from the endoscopic instrumentation. Although the recurrent motor branch of the median nerve has a variety of branching patterns, the most common is the extraligamentous and recurrent type extending radially to the thenar musculature. Again, this anatomical arrangement keeps the motor branch away from the endoscopic approach. By placing the proximal incision immediately ulnar to the palmaris longus, the surgeon stays away from both the main ulnar nerve and its cutaneous sensory branch. By aiming the distal incision to the third web space, entrance into Guyon's canal is avoided as well as any ulnar neurovascular injuries. By placing the distal incision immediately distal (< 1cm) to the distal end of the transverse carpal ligament, the vascular palmar arch is easily avoided. Therefore, an anatomically safe corridor exists between the proximal and distal incisions located immediately radial to the hook of the hamate, where the transverse carpal ligament can be safely and completely sectioned to release the compromised carpal tunnel (Fig. 1).
Fig. 1. Diagram demonstrates key anatomical landmarks. A proximal incision is made 1-2 cm proximal to the distal wrist crease on the ulnar side of the palmaris longus tendon. A distal wound is made within a 1-cm-radius circle with an epicenter at 4 cm distal to the wrist crease along the third web space. Line A depicts Kaplan's cardinal line. The intersection of line B with line A points to the location of the hook of the hamate. Median nerve is located on the radial side of the palmaris longus tendon.
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