18.1 Introduction

The carpal tunnel is an important anatomical passageway that conveys the median nerve and nine flexor tendons (flexor digitorum superficialis [FDS] and flexor digitorum profundus [FDP] to the index, middle, ring, and little fingers, as well as the flexor pollicis longus [FPL] tendon to the thumb) from the forearm to the hand. A thorough knowledge of the anatomy of the carpal tunnel and that of the surrounding structures provides a critical base to all practitioners managing common upper extremity problems.

18.2 Anatomy

A thin layer of fascia, called the antebrachial fascia, covers the whole forearm (▶Fig. 18.1). The fascia thickens as it approaches the palmar aspect of the wrist.

The carpal tunnel does form a distinct tunnel located on the flexor surface of the wrist. The carpal bones and intercarpal ligaments that create a gentle arch concave toward the palm form the dorsal surface of the tunnel, commonly referred to as the floor. The positioning of the carpal bones also forms the walls of the tunnel. The volar surface, or roof, is formed by a strong transversely oriented fibrous layer known as the flexor retinaculum (▶Fig. 18.2 a,b). The mean width of the tunnel is 24 mm at its proximal end (▶Fig. 18.3), 20 mm at the narrowest region, and 25 mm at its distal border. ¹ The depth of the tunnel also varies, being around 12 mm at the proximal end, 10 mm at the narrowest, and 13 mm at the distal end. At the proximal end, the depth of the carpal tunnel is 8 mm on the radial side, 12 mm in the center, and 10 mm on the ulnar side (▶Fig. 18.4). The narrowest area of the tunnel is found 2.1 mm distal to the proximal pole of the capitate. ²

Motion of the wrist changes the diameter and alters the pressures in the carpal tunnel (▶Fig. 18.5a–c). In flexion of the wrist, the cross section of the carpal tunnel decreases as the flexor retinaculum comes closer to the carpal bones. ³ Gelberman et al ⁴ measured carpal tunnel pressures in normal subjects and patients with carpal tunnel syndrome. In control subjects, the pressure was 2.5 mm Hg that rose to 31 mm Hg with wrist flexion and 30 mm Hg in wrist extension. In contrast, in patients with carpal tunnel syndrome, the mean carpal tunnel pressure was 32 mm Hg that rose to 94 mm Hg with the wrist in 90° flexion and 110 mm Hg with the wrist in extension.

In many anatomical writings, the terms transverse carpal ligament (TCL), palmar carpal ligament (PCL), and flexor retinaculum are used interchangeably. ^{1 , 5} Schmidt and Lanz ³ make an anatomic distinction between the PCL and the flexor retinaculum. According to them, the superficial reinforcing bands of the forearm fascia form the PCL (▶Fig. 18.6). This ligament extends from the insertion onto the tendon of flexor carpi ulnaris (FCU) to the ulnar border of palmaris longus. The PCL adheres very tightly to the flexor retinaculum radial to the palmaris longus (▶Fig. 18.7a–d).

In a detailed study of the anatomy of the flexor retinaculum, Cobb et al ¹ divide the covering of the palmar wrist into anatomical parts. The thin fascia covering the whole forearm is the antebrachial fascia. Distal to that, the flexor retinaculum has three components. The most proximal portion of the flexor retinaculum (called palmar carpal ligament [PCL] by Schmidt and Lanz ³ ) is a thickening of the antebrachial fascia. This superficial sheet covers the ulnar artery and nerve and courses deep to the FCU. The flexor carpi radialis (FCR) tendon pierces the flexor retinaculum to enter its fibro-osseous tunnel. The middle part of the flexor retinaculum is distal and deeper to the PCL and is termed the transverse carpal ligament (TCL). It is attached to pisiform, hamate, the tubercle of the trapezium, and the tubercle of the scaphoid. The very distal portion of the flexor retinaculum is an aponeurosis between the thenar and hypothenar muscles.

The descriptions in the current literature are very confusing. After many dissections, a thorough study of the literature, and discussions with renowned anatomists and hand surgeons, we present our schema.

The antebrachial fascia covers the whole forearm with a thin lining. There is distinct condensation of this fascia in the palmar portion of the distal forearm that extends to envelop the ulnar neurovascular bundle and the FCU (▶Fig. 18.7a–d). The palmaris longus is superficial to this portion of the fascial condensation. Schmidt and Lanz ³ have termed this fascial segment PCL. Anatomically, it is a part of the antebrachial fascia (▶Fig. 18.6a,b, ▶Fig. 18.7a–d).

This portion can cause compression of the median nerve, and prominent hand surgery teachers like Dr. Carroll and Dr. Kleinert advocated always dividing this ligament during carpal tunnel release.

The thick covering of the carpal tunnel is the flexor retinaculum (▶Fig. 18.2a,b). It is a thick sheet that is attached to the ridge of the trapezium and ridge of the scaphoid on the radial side and to the pisiform and hamate on the ulnar side. The Terminology for Hand Surgery, by the IFSSH (2001), recognizes only flexor retinaculum as a terminology, and there is no mention of TCL. Therefore, the term transverse carpal ligament (TCL) is confusing and should not be used. Additionally, we feel that it is artificial to divide the flexor retinaculum into different anatomical components and that it serves no useful function. Thus, we believe that the antebrachial fascia and the palmar carpal ligament cover the forearm.

The Palmar carpal ligament (PCL) is a distinct component of the antebrachial fascia. The distal part and the true covering of the carpal tunnel is the flexor retinaculum. There is an area in the distal part of the flexor retinaculum that consists of crisscrossing of muscle aponeurosis of the thenar and hypothenar muscles (▶Fig. 18.9a,b). However, this segment is just the very distal part of the flexor retinaculum from which these muscles are taking origin. This happens in many areas of the body, and these areas do not deserve unique terminologies.

The flexor retinaculum is supplied by branches from the superficial branch of the radial artery and from the ulnar artery directly ³ (▶Fig. 18.10).

At the distal margin of the flexor retinaculum, there is a fat pad ⁶ that separates the retinaculum from the median nerve and marks an anatomical landmark of the end of the flexor retinaculum and outlines the distal completion of carpal tunnel release. The end of the flexor retinaculum is within 2 mm of the fat pad.

The superficial palmar arch that is formed by the ulnar artery and the superficial branch of the radial artery lies about 12.5 mm from the distal margin of the flexor retinaculum ^{6 – 8} (▶Fig. 18.11). The motor branch of the median nerve is 6.75 mm away.