The ligamentous anatomy of the wrist is complex due to the stabilization required for the numerous carpal bones and extensive motion (
Figs. 11.17,
11.18,
11.19). The orientation of the ligaments about the wrist is also complex, making it difficult to include all of the dorsal or volar ligaments in any one orthogonal MR image plane.
30,50,51,52,53,56,57,65,66,67,68,69 MR images must be obtained using thin sections (= 1 to 2 mm), a small FOV (= 10 cm), and 256 × 256 or 256 × 192 matrix. Three-dimensional Fourier techniques are preferred for thinner contiguous sections, and reformatting can be accomplished. Volar, dorsal, and interosseous ligaments can be defined most consistently with these techniques or MR arthrography.
30,50,51,52,53,54,70
The distal radioulnar joint is primarily stabilized by the TFCC. This complex consists of several components that blend with one another and include the triangular fibrocartilage, the ulnocarpal meniscus, the ulnar collateral ligament (UCL), and the palmar and dorsal distal radioulnar ligaments (
Figs. 11.17B,
11.18B, and
11.20). The articular disc is composed of fibrocartilage. The disc attaches to the ulnar margin of the radius with a broader ulnar portion attaching to the ulnar styloid, ulnar fovea, and deep lamina of the antebrachial fascia. The deep lamina is separated from the superficial lamina by the ECU tendon and its sheath.
56,65 The triangular fibrocartilage (TFC) is most easily identified on coronal images (
Fig. 11.18), with the dorsal and volar ligaments most easily seen on axial or three-dimensional images.
30,50,51,52,53,54,71,72 The TFC is normally of low signal intensity on MR images. However, degeneration, especially on the ulnar aspect, is common on patients over 50 years of age creating areas of increased signal intensity.
73,74,75 Additional support of the distal radioulnar joint is provided by the interosseous membrane between the radius and ulna, the ECU tendon, and the concavity of the sigmoid notch of the radius (
Figs. 11.14 and
11.20).
21,56,65
Palmar Ligaments
The palmar ligaments consist of two concentric arches originating 1 to 2 mm from the volar margin of the radius and inserting into the proximal carpal row and TFCC (
Figs. 11.20 and
11.21).
58,65 The radioscaphocapitate ligament (
Fig. 11.21A) is most lateral, extending from the radial styloid to the scaphoid waist and capitate. At the capitate it joins the ulnocapitate ligament to form the arcuate ligament. The long radiolunate ligament lies medial to the radioscaphocapitate ligament (
Fig. 11.21A). The radioscapholunate ligament (ligament of Testut) extends vertically between the short and long radiolunate ligaments to insert on the lunate and medial scaphoid. The short radiolunate ligament extends from the medial radius to the lunate forming the floor of the radiolunate space.
56,65,67
The palmar midcarpal ligaments include the scaphotrapezium trapezoid, scaphocapitate, triquetrocapitate, triquetrohamate, and pisohamate ligaments (
Fig. 11.21A). These ligaments are contiguous, with the radiocarpal and ulnocarpal ligaments joining to form the nearly contiguous palmar capsule.
56,65
The ulnocarpal ligaments (ulnolunate, ulnocapitate, and lunotriquetral) (
Figs. 11.20 and
11.21A) originate primarily from the palmar radiolunate ligament and the TFC.
65,67
Smith
53 was able to define six of the eight palmar ligaments –
1, radioscaphocapitate;
2, radiolunotriquetral;
3, radiolunate;
4, ulnolunate;
5, ulnotriquetral; and
6, triquetroscaphoid – in 95% of wrists. The radioscaphoid and
radioscapholunate were demonstrated in 66% and 26%, respectively. Three-dimensional techniques were used.
Interosseous Ligaments
The scapholunate and lunotriquetral ligaments are C-shaped (
Fig. 11.22), extending from dorsal to proximal to palmar surfaces of the joints.
65,68,69 The scapholunate ligament is thicker dorsally.
68,69 Both the dorsal and palmar portions of the lunotriquetral ligament are thicker than the proximal portion.
The scapholunate ligament is low signal intensity in 63% and has areas of intermediate signal intensity in 37% of patients. The ligament may be triangular (90%) or linear (10%) in its configuration (
Fig. 11.23).
52 Signal intensity was low and uniform (type 1) in 49% of wrists. Areas of intermediate signal intensity were noted in 51%. Type 2 central increased signal intensity in the ligament occurred in 14%, type 3 distal increased signal intensity occurred in 16% and proximal type 4 signal intensity occurred in 2% of cases. Intermediate signal intensity extended through the ligament (type 5) in 19% of patients (
Fig. 11.23).
52
Similarly, the lunotriquetral ligament may also have a linear or triangular configuration with variations in signal intensity in asymptomatic individuals.
50,56 The lunotriquetral ligament is triangular in 63% and more linear in 37%
of patients. An amorphous appearance was noted in a few patients by Smith and Snearly (
Fig. 11.24).
50 Signal intensity is not always uniformly low, similar to variations described in the scapholunate ligament (
Fig. 11.24).
50,52
The interosseous ligaments in the second carpal row consist of dorsal and palmar transverse interosseous bands (
Fig. 11.25). The trapeziocapitate and capitohamate interosseous ligaments have deep ligaments between the articulating surfaces.
56,65
Ligamentous anatomy of the metatarsophalangeal and interphalangeal (IP) joints is similar with collateral and volar ligaments incorporated into the joint capsule
(
Figs. 11.18E and
11.26).
21,76 These ligaments are tight in extension and relax with flexion of the joints. The collateral ligaments are seen on axial and coronal MR images (
Figs. 11.17 and
11.18). The palmar plate is clearly seen on axial and sagittal images.
76
Muscular Anatomy
Many of the muscles and tendons that cross the wrist originate at the elbow and forearm. These myotendinous units are discussed in
Chapter 10. The muscles of the forearm, which are largely responsible for flexion and extension of the wrist, are thoroughly discussed in
Chapter 10. Therefore, except for essential anatomy, they will not be reviewed here.
77 This section will primarily deal with those muscles directly related to bones of the hand and wrist with regard to their origins and insertions (
Table 11.3).
The chief flexors of the wrist are the flexor carpi radialis and flexor carpi ulnaris. The palmaris longus is a minor flexor of the wrist (
Fig. 11.27).
46,56 Extension of the wrist is largely due to the extensor carpi radialis longus and brevis and the ECU (
Fig. 11.27). During radial deviation of the wrist, primary muscles involved are the abductor pollicis longus and extensor pollicis brevis. Ulnar deviation of the wrist is accomplished primarily by the ECU.
46,56,78,79
There are typically four lumbrical muscles that arise from the flexor digitorum profundus tendons and extend along the radial aspects of the second through fifth metacarpals to insert in the extensor aponeurosis of the proximal phalanx on the radial side. The muscles can be identified in the axial and coronal planes (
Figs. 11.17 and
11.18). The lumbricals are seen as a tissue of muscle signal intensity between the flexor digitorum profundus tendons proximally and along the radial aspect of the metacarpals adjacent to the interosseous muscles more distally.
46,56 Insertions are not usually clearly defined on MRI. The flexor pollicis longus is discussed in
Chapter 10; however, its function is important in the hand and wrist, so certain aspects of its anatomy need to be repeated. As noted in
Table 11.3, the muscle originates from the anterior aspect of the middle third of the radius.
56,80 The tendon passes through the radial side of the carpal tunnel (
Fig. 11.17) radial to the superficial and deep flexor tendons (
Figs. 11.17,
11.18, and
11.27). A synovial sheath of the flexor pollicis longus tendon begins just proximal to the flexor retinaculum and extends distally to near the insertion of the tendon on the distal phalanx of the thumb (
Table 11.3).
46,56
The interosseous muscles form the deepest layer of the muscles in the hand and are divided into palmar and dorsal groups (
Fig. 11.17). The palmar group consists of three muscles that take their origin on the radial aspect of the fifth and fourth metacarpals and the ulnar aspect of the second metacarpal. The muscles pass distally between the metacarpophalangeal (MCP) joints to insert on the extensor aponeurosis. The dorsal interossei originate from adjacent metacarpals, the first arises from the first and second metacarpals, the second from the second and third, the third from the third and fourth, and the fourth from the fourth and fifth metacarpal diaphysis. The muscles pass dorsally and distally to insert with a palmar and dorsal slip into the bases of the proximal phalanges. The interosseous muscles, both palmar and dorsal, are innervated by the deep branch of the ulnar nerve. The interosseous muscles aid in abduction and adduction of the fingers of the hand (
Table 11.3).
56
The thenar eminence or muscle group is comprised of the abductor pollicis brevis and superficial head of the flexor pollicis brevis that overlie the opponens pollicis (
Fig. 11.17). The abductor pollicis brevis arises from the flexor retinaculum and has deeper origins from the trapezium and trapezoid. This somewhat triangular muscle extends distally to insert in the radial aspect of the proximal phalanx of the thumb. It serves as the primary abductor of the thumb. The flexor pollicis brevis has two heads, one superficial and the other deep. The superficial head arises from the trapezium and flexor retinaculum and the deep head from the
trapezoid. The muscle extends distally to form a tendon that inserts on the radial flexor side of the base of the proximal phalanx of the thumb. The primary function is flexion and rotation of the thumb. The opponens pollicis is partially covered by the abductors and flexors of the thumb and arises from the flexor retinaculum and trapezium to insert on the radial surface of the diaphysis of the first metacarpal. The adductor pollicis arises with both oblique and transverse heads. The transverse head arises from the ulnar surface of the third metacarpal diaphysis and the oblique head from the base of the third metacarpal and flexor aspects of the trapezium, trapezoid, and capitate. The triangular muscle extends to insert at the base of the proximal phalanx of the thumb. This muscle serves to adduct the metacarpal and flex the MCP joint of the thumb (
Table 11.3).
46,56
The hypothenar muscle group consists of one superficial and three deep muscles. The superficial muscle is the palmaris brevis that arises from the ulnar side of the palmar aponeurosis and extends medially to attach into the skin along the medial border of the palm. This muscle is superficial to the ulnar nerve and artery.
56 The deep muscles include the abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi (
Figs. 11.17 and
11.18). The abductor digiti minimi is the most superficial of the three deep muscles. It arises from the distal surface of the pisiform and passes distally along the medial aspect of
the hand to insert along the ulnar side of the base of the fifth proximal phalanx. This muscle abducts the little finger at the MCP joint. It acts along with the dorsal interosseous muscle to assist in abduction or spreading of the fingers. The flexor digiti minimi brevis arises more distally than the abductor digiti minimi and takes its origin from the hook of the hamate and flexor retinaculum. Thismuscle passes more obliquely and medially and inserts in the same position as the abductor. The main function of this muscle is as flexor of the fifth MCP joint. The third and final muscle of the deep hypothenar group is the opponens digiti minimi. This muscle is the deepest and arises deep to the abductor and flexor from the flexor retinaculum and distal hook of the hamate, taking an oblique course to insert along the ulnar aspect of the fifth metacarpal diaphysis. This muscle draws the fifth metacarpal anteriorly. All of the hypothenar muscle groups is innervated by the deep branch of the ulnar nerve (
Table 11.3).
46,56
Numerous muscular variations have been described.
81,82
The accessory abduction digit minimi has been reported in up to 24% of patients. The extensor digitorum manus muscle is reported in 1% to 3% of the general population. The origin of the lumbrical muscles (
Table 11.3) may vary, with the origin arising in the carpal tunnel in 22% of patients. The palmaris longus is typically seen only as a tendon at the level of the wrist. In up to 13% of patients, the muscle may be absent. There are numerous other variations, including palmaris longus inversus (muscle distally, tendon proximally), non-tendinous variation (muscle from origin to insertion), central tendon with muscle tissue proximally and distally, and a bifid variant with two tendinous insertions distally.
56,82 A more complete discussion of muscle variants and clinical implications is included in the Pitfalls section of this chapter.
Neurovascular Anatomy
The neurovascular anatomy of the hand and wrist is complex (
Fig. 11.28). Because there are numerous causes of nerve compression in this region, it is especially essential to understand the anatomy and relationship of these structures in the hand and wrist (
Figs. 11.28 and
11.29).
54,55,56,83,84,85,86,87,88 MR evaluation of neurovascular anatomy is most easily accomplished by following these structures from proximal to distal on axial images (
Figs. 11.17 and
11.29). On the ulnar side of the distal forearm proximal to the carpal tunnel, the ulnar artery, nerve, and the accompanying veins lie deep to the flexor carpi ulnaris (
Fig. 11.17).
54,56 The nerve is generally medial to the artery at this level. At the level of the pisiform, these structures pass along the lateral or radial side of the pisiform, passing deep to the volar carpal ligament and then distally into the palm of the hand anterior to the flexor retinaculum but deep to the palmaris brevis muscle (
Fig. 11.29B).
83 At the level of the pisiform, the ulnar nerve typically divides into superficial and deep branches (
Fig. 11.29B). Also, at
the pisiform level, the nerve and accompanying vascular structures lie between the volar carpal ligament and flexor retinaculum in a space commonly known as the Guyon’s canal.
54,56 Lesions proximal to or within the canal can produce both sensory and motor abnormalities in the ulnar nerve distribution.
54,56
The two flexor digitorum muscles (superficial and profundus) are lateral to the ulnar nerve and vessels at the level of the wrist (
Fig. 11.29). The tendon of the palmaris longus lies superficially. These structures are most easily identified on axial MR images (
Figs. 11.17 and
11.29). The midline volar structures of the wrist, as they enter the carpal tunnel, tend to form three layers. The most superficial or anterior layer is formed by the flexor digitorum superficialis. The middle layer is formed by the superficial flexor of the index and middle fingers, and the most posterior or deepest layer is formed by the flexor digitorum profundus tendons. All tendons have a common sheath just before they pass under the flexor retinaculum. The palmaris longus tendon is the most superficial and midline structure at the wrist level (
Figs. 11.17 and
11.29).
46,56
The median nerve lies deep to the flexor digitorum superficialis through much of the forearm (
Figs. 11.17 and
11.29). Just proximal to the wrist, it emerges on the radial side of the superficial flexor and passes forward and medially to lie in front of the flexor tendons in the carpal tunnel (
Figs. 11.17 and
11.29). At the distal margin of the flexor retinaculum, the median nerve divides into five or six branches. These small branches are difficult to identify, even when thin axial MR sections are obtained.
21,55,88
The muscle planes and fascial compartments of the palm basically divide the palm into three compartments – the thenar, hypothenar, and central compartments (
Figs. 11.17 and
11.28). These compartments, along with the tendon sheaths of the flexor tendons, are anatomically important in the spread of inflammatory and infectious diseases.
56