20 Anatomy of the Wrist Joint

Nathan Polley and Amit Gupta

20 Anatomy of the Wrist Joint

20.1 Bones of the Wrist

The wrist is generally described as a joint with motion between the eight individual carpal bones and between the proximal and distal rows. These eight carpal bones can be divided into two rows, the proximal and distal carpal rows. The proximal row consists of the scaphoid, lunate, triquetrum, and pisiform. The distal row is comprised of the trapezium, trapezoid, capitate, and hamate. (▶Fig. 20.1, ▶Fig. 20.2, ▶Fig. 20.3). ¹

**Fig. 20.1** Dorsal and palmar view of the bones of the right wrist.

**Fig. 20.2** Bones of the right wrist. Dorsal View.

**Fig. 20.3** Bones of the right wrist. Volar View.

20.1.1 Extensor Retinaculum

The extensor retinaculum forms the dorsal strap of the wrist, helping anchor the extensors of the wrist and fingers in separate compartments. Unlike descriptions in anatomical texts with diagrams, the extensor retinaculum is a three-dimensional structure (▶Fig. 20.4a). The first compartment contains the extensor pollicis brevis (EPB), the abductor pollicis longus (APL), and frequently multiple tendons of accessory APL. This compartment may be subdivided into two, with the EPB running in its own compartment.

**Fig. 20.4** (a) The dorsal compartments of the extensor retinaculum with the tendons removed showing different levels of the compartments. (b) The dorsal compartment of the wrist with the extensor tendons in place.

The second compartment contains the extensor carpi radialis longus and the extensor carpi radialis brevis. The third compartment contains the extensor pollicis longus tendon. The fourth compartment contains the extensor digitorum communis (EDC) and the extensor indicis tendons. A more superficial fifth compartment contains the extensor digiti minimi tendon. (▶Fig. 20.4b).

The sixth compartment is very special. It divides into two layers to enclose the extensor carpi ulnaris tendon. The deep layer of the compartment is intimately related to the triangular fibrocartilage (TFC) and forms the peripheral layer of the TFC. Whereas the other five compartments are fixed to the radius, the sixth compartment moves with the ulna in pronation and supination, staying relatively fixed by the subsheath to the ulna.

20.1.2 Flexor Retinaculum

The carpus is curved on the palmar surface. The space between the scaphoid and trapezium on the radial side and the hook of the hamate and pisiform on the ulnar side is the area of the carpal tunnel and is bounded on the palmar aspect by the flexor retinaculum (▶Fig. 20.5a). The flexor retinaculum consists of three fibrous bands that extend between the trapezium and the hamate, the trapezium and the pisiform, and the scaphoid and the hamate (▶Fig. 20.5b,c).

**Fig. 20.5** (a) End-on view of the carpus showing the curved carpus with scaphoid and trapezium on the radial side and the hamate especially the hook on the ulnar side. (b) The flexor retinaculum and the curved carpus. (c) The relationship between the volar ligaments of the carpus and the flexor retinaculum. (d) The extensor and flexor retinaculum encircle the wrist like a watch strap and meet at their insertion into the pisiform. (**e–g**) The space between the carpus and the flexor retinaculum is maximum in the neutral position of the wrist (e); the space and thus the volume decreases in flexion (f) and extension (g) of the wrist.

The pisiform forms a key structure. The extensor and flexor retinaculum encircle the wrist like a watch strap and meet at their insertion into the pisiform (▶Fig. 20.5d).

In the neutral position, the carpal tunnel volume and the space between the flexor retinaculum and the carpus are at their maximum. These parameters decrease in flexion and extension especially in hyperextension (▶Fig. 20.5e,f,g).

20.1.3 Ligaments of the Wrist

There are countless articles, academic texts, and cadaveric and anatomic studies attempting to define the complex anatomy, function, and orientation of the ligaments of the wrist.

Verbal communication from Taleisnik to Berger was discussed in an article by Berger in 2001, stating that “anatomy does not change, only our descriptions of anatomy change.” ² Taleisnik defined the wrist ligaments as extrinsic and intrinsic, which are based on anatomic location. ³ Extrinsic ligaments originate proximal to the carpal bones and course distally to insert onto the carpal bones (▶Fig. 20.6), while intrinsic ligaments are found solely within the boundaries of the carpal bones (▶Fig. 20.7). ¹ Berger and Landsmeer consider the majority of the wrist ligaments to be intracapsular ligaments, which essentially means they are confined between the synovium and fibrous layer of the joint capsule. ⁴

**Fig. 20.6** Dorsal extrinsic ligaments of the right wrist. DICL: Dorsal Intercarpal Ligament, DRCL: Dorsal Radiocarpal Ligament.

**Fig. 20.7** The intrinsic ligaments of the wrist (scapholunate ligament) and the extrinsic volar radiocarpal ligaments from the dorsal wrist with the wrist hyper flexed. SL: Scapholunate ligament, RCL: Radial Collateral Ligament, RSC: Radioscaphocapitate Ligament, LRL: Long Radiolunate Ligament, RSL: Radioscapholunate ligament, UCL: Ulnar carpal Ligaments.

20.2 Extrinsic Ligaments

The volar extrinsic ligaments consist of the radioscaphocapitate (RSC), long radiolunate (LRL), radioscapholunate (RSL), short radiolunate (SRL) and the ulnocarpal ligaments (UCL). The ulnocarpal ligaments will be discussed in Chapter 24. The dorsal radiocarpal ligament makes up the dorsal extrinsic system. In regard to origin and insertion, we will refer to origin as proximal and insertion as distal. The volar extrinsic ligaments are difficult to see when viewed from a volar approach. The volar wrist appears as a homogenous layer (▶Fig. 20.8). In situ, these ligaments can be identified through a dorsally inserted arthroscope (▶Fig. 20.9) or dorsal capsulotomy with hyperflexion (▶Fig. 20.10).

**Fig. 20.8** The volar carpal ligaments appear as a homogeneous layer when viewed from the volar aspect.

**Fig. 20.9** The volar carpal ligaments seen through an arthroscope.

**Fig. 20.10** The volar carpal ligaments can also be seen from the dorsal side after hyper flexing the right wrist. SL: Scapholunate Ligament, RSL: Radioscapholunate Ligament.

20.3 Radioscaphocapitate Ligament

The radioscaphocapitate (RSC) ligament (▶Fig. 20.11, ▶Fig. 20.12) has also been termed the radioscaphoid, radiocapitate, arcuate, and deltoid ligament. ⁵ The name radioscaphocapitate is largely recognized and used in most literature, thereby adequately describing the ligamentous attachments to the respective carpal bones. The RSC ligament originates from the radial styloid of the distal radius with the radial most fibers inserting onto the waist of the scaphoid. Although debated, this portion is referred to as the collateral component of the lateral wrist: radial collateral ligament (RCL) (▶Fig. 20.13). The remaining fibers are oriented obliquely and ulnarly toward the capitate. Radially, these fibers will coalesce with the scaphocapitate ligament ⁶ and some fibers will largely interdigitate with fibers of the ulnocapitate ligament and meet at the base of the capitate (▶Fig. 20.13, ▶Fig. 20.14).

**Fig. 20.11** Volar extrinsic ligaments showing the radioscaphocapitate ligament, the long radiolunate and the ulnocarpal ligaments (left wrist). LRL: Long Radiolunate Ligament, RSC: Radioscaphocapitate Ligament, SRL: short radiolunate ligament, UCL: Ulnar carpal Ligaments.

**Fig. 20.12** The relationship of the carpal tunnel to the volar extrinsic ligaments (left wrist). LRL: Long Radiolunate Ligament, RSC: Radioscaphocapitate Ligament, SRL: short radiolunate ligament, UCL: Ulnar carpal Ligaments.

**Fig. 20.13** The radioscaphocapitate (RSC) ligament, the long radiolunate ligament (LRL) and the collateral ligament of the right wrist. RCL: radial collateral ligament, SRL: short radiolunate ligament, UCL: ulnocarpal ligaments.

**Fig. 20.14** The volar carpal ligaments of the left wrist showing the details of the radioscaphocapitate ligament. LRL: long radiolunate ligament, RSC: radioscaphocapitate ligament, SRL: short radiolunate ligament, UCL: Ulnar carpal Ligaments.

This coalescence of the RSC with the ulnocapitate ligament, gives the classic arch or inverted V shape and has been historically termed the “arcuate” or “deltoid” ligament. Controversy exists over whether there is direct attachment of the RSC to the capitate (▶Fig. 20.15). A small percentage of fibers attach to the waist of the capitate, but whether these are directly from the RSC itself, are coalescences from the scaphocapitate ligament and RSC, or are merely synovial reflections from the RSC is debated. These anatomical variations may be purely academic, and it is unknown whether these contribute to any significant biomechanical stability, as this attachment, being at the proximal nonarticular neck of the capitate, is thought to act only as a “sling” for support of the capitate. ¹ ^, ⁵ This insertion completes what Berger and Landsmeer described as three major components of the RSC ligament: radial collateral, radioscaphoid, and radiocapitate. ⁴ The ligament runs on the waist of the scaphoid and acts as a fulcrum for flexion of the scaphoid (▶Fig. 20.16, ▶Fig. 20.17, ▶Fig. 20.18, ▶Fig. 20.19).