Musculotendinous Anatomy of the Wrist

INTRODUCTION

∗ The author wishes to recognize the contributions of Charles A. Goldfarb, MD, and Martin I. Boyer, MD, and their collaborative efforts in creating the anatomic dissections used in this chapter.

The clinical evaluation and treatment of conditions afflicting the wrist are assisted greatly through a detailed knowledge of local and regional anatomy. Similar to art, anatomy must be understood to appreciate its beauty, but it must be appreciated to understand its intricacies and variations. At the wrist, musculotendinous anatomy involves motor units acting directly on the wrist and those that cross or originate at the wrist to act indirectly on the radiocarpal and distal radioulnar joints. Disorders of the musculotendinous system, such as tendinopathy and deficit due to trauma, may affect wrist function; however, pathologic conditions of the wrist may influence musculotendinous function, such as tendon rupture in the rheumatoid patient with caput ulnae syndrome.

Variations in musculotendinous anatomy are not uncommon and should be considered during the diagnostic evaluation of patients and during surgical exposure. Appreciation for anatomic variation compels the clinician to query the diagnostic dilemmas—unexplained clinical findings not supported by “normal” anatomy—and to recognize the implications of such variations, such as the radiologist’s reading of “longitudinal tearing” of the abductor pollicis longus tendon in the presence of multiple tendon slips as opposed to a single tendon, and the influences of such variations on postinjury rehabilitation programs.

Comprehension of anatomy about the wrist enhances our abilities as clinicians and as surgeons, providing a foundation on which we are able to accurately evaluate and treat disorders of the wrist.

DORSAL WRIST

The extrinsic muscles crossing the wrist originate from one of three groups: the mobile wad of three, the superficial extensor, and the deep extensor muscle groups. Generally, these musculotendinous structures are divided into six dorsal wrist fibro-osseous compartments at the level of the wrist. Each compartment is separated by a vertical fibrous septum originating from the periosteum of the distal radius or ulna and extending to the extensor retinaculum ( Figs. 3-1 and 3-2 ). The dorsal compartments are numbered from 1 to 6, beginning at the midsagittal aspect of the radial wrist. The extensor retinaculum is a 2-cm fibrous band that acts as a pulley to restrict tendon bowstringing and to maximize biomechanical efficiency for digital and wrist extension ( Fig. 3-3 ).

The first dorsal compartment contains the abductor pollicis longus (APL) and extensor pollicis brevis (EPB) tendons ( Fig. 3-4 ). Both muscular origins are from within the deep extensor compartment and are innervated by the posterior interosseous nerve. The APL and EPB obliquely cross the second dorsal compartment approximately 7 cm proximal to the wrist joint before entering the fibro-osseous compartment along the midsagittal aspect of the distal radial metaphysis. The APL arises from the dorsal surface of the radius and inserts into the thumb metacarpal base, and it has a variable insertion into the trapezium and into the thenar musculature (opponens pollicis and abductor pollicis brevis [APB]). The APL, which may have one or many tendon slips (two slips being the most common, approximately 70% of the time), is located palmar and radial to the EPB tendon and acts to abduct and extend the thumb metacarpal ( Fig. 3-5 ).

Typically, the EPB is one tendon slip and may exist within its own subcompartment (30%). It originates from the dorsal surfaces of the radius and the interosseous membrane and has a variable insertion into the dorsal base of the thumb proximal phalanx and/or into the extensor mechanism of the thumb. The EPB acts to extend the thumb at the metacarpophalangeal (MCP) joint and may extend, but not hyperextend, the thumb interphalangeal joint.

The brachioradialis arises from the mobile wad; however, it does not cross the wrist joint because it has a broad insertion along the radial border of the distal radial metaphysis. It is identified deep to the APL and EPB tendons at the wrist. The brachioradialis originates from the upper supracondylar ridge of the lateral distal humerus and inserts into the radial aspect of the distal radial metaphysis and the radial styloid. It is innervated by the radial nerve.

The extensor carpi radialis longus (ECRL) and the extensor carpi radialis brevis (ECRB) make up the contents of the second dorsal compartment at the wrist and arise from the mobile wad. The longitudinally oriented tendons cross the radiocarpal joint immediately radial to Lister’s tubercle. The ECRL originates at the lower supracondylar ridge of the lateral distal humerus and inserts into the dorsal base of the index metacarpal. The ECRL is innervated by the radial nerve and acts to extend and radially deviate the wrist. The ECRB originates from the lateral epicondyle, the elbow capsule, and the annular ligament of the radiocapitellar joint and inserts into the dorsal base of the long finger metacarpal ( Fig. 3-6 ). It is innervated by one of the radial nerves (30%), posterior interosseous nerves (45%), or superficial branch of the radial nerve (25%) and acts to extend and to radially deviate the wrist. The extensor carpi radialis indicis is an anatomic variation described in up to 10%, which runs in the interval between the ECRL and ECRB.