MRI of the Hand and Wrist

1

What are some common indications for performing magnetic resonance imaging (MRI) of the hand and wrist?

MRI of the wrist is routinely used to assess a wide variety of osseous and soft tissues abnormalities, including radiographically occult fractures; tendon, ligament, or cartilage injuries; tunnel syndromes; palpable abnormalities; and wrist pain. This chapter will concentrate predominantly on MRI of the wrist.

MRI of the hand is obtained much less frequently than MRI of the wrist. It is technically demanding, requires special techniques and equipment not widely available, and consistent, high-quality images are more difficult to obtain. Moreover, ultrasonography (US) has emerged as a strong competitor with both higher resolution than MRI and the capacity to image small structures during the passive or active performance of maneuvers. The downside to US is the shortage of both technologists and radiologists qualified to perform and interpret the studies.

MRI of the hand is occasionally used for the assessment of injury to the flexor and extensor tendons, as well as injury to the complex pulley system used to stabilize the flexor tendons. MRI is also sometimes used to assess injury to periarticular soft tissues such as the joint capsule and collateral ligaments as well as the volar plates (focal thickenings of the joint capsule) of the metacarpophalangeal and interphalangeal joints. The only periarticular structure that is commonly studied with MRI is the ulnar collateral ligament (UCL) of the carpometacarpal joint of the thumb. Other uses of MRI include assessment of either bony or soft tissue infection as well as of soft tissue masses.

2

What is the anatomy of the hand and wrist?

The distal radius and ulna articulate with the proximal row of carpal bones, although the ulna does not articulate directly but is separated by the articular disc of the triangular fibrocartilage complex (TFCC). The proximal carpal row consists of scaphoid, lunate, triquetrum, and pisiform bones. The distal carpal row consists of the trapezium, trapezoid, capitate, and hamate bones.

The distal carpal row articulates with the bases of the metacarpal bones, which in turn articulate with the proximal phalanges. In the thumb, there are only two (proximal and distal) phalanges joined at the interphalangeal (IP) joint, whereas in the remainder of the digits there are three phalanges (proximal, middle, and distal) joined by the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints.

The ligaments of the wrist consist of both the intrinsic ligaments, which extend between the bones of the proximal carpal row, and the extrinsic ligaments, primarily on the volar and dorsal surface of the wrist, which extend over much greater distances, often crossing the radiocarpal and carpometacarpal articulations. The extrinsic ligaments provide the overall stability of the wrist, whereas the intrinsic ligaments maintain the relationship of the bones of the proximal row.

Multiple tendons cross the dorsal surface of the wrist, contained within six separate compartments. As they extend onto the dorsal surface of the fingers, the tendons divide up in a complex pattern to form the dorsal “hood.” The flexor tendons cross the wrist on the volar surface within the carpal tunnel and exit onto the fingers as the deep and superficial flexor tendons, where they are held close to the bone by a series of fibrous pulleys.

3

What is meant by the term “intercalated segment”?

The intercalated segment is the proximal row of carpal bones. The name derives from the fact that there are no tendinous insertions on the proximal carpal bones and that they are inserted (“intercalated”) between the radius and the distal carpal row. The use of this term will become evident in the rest of this chapter.

4

What is ulnar variance?

Ulnar variance describes the relationship between the lengths of the radius and ulna as seen at the wrist. Positive ulnar variance refers to the situation in which the ulna is longer than the radius and is associated with degenerative disease of the TFCC. Negative ulnar variance describes the situation in which the ulna is shorter than the radius, and is associated with Kienböck's disease. Both of these entities will be discussed below.

5

What is the normal MRI appearance of the wrist?

The predominant signal intensity of normal bone marrow is the same as that of the subcutaneous fat and is generally bright unless specific measures have been taken to suppress it. Cortical bone has very low signal intensity on all standard imaging sequences. Cancellous bone is seen as a fine network with very low signal intensity within the fatlike signal intensity of the bone marrow. Normal tendons, normal ligaments, and the TFCC demonstrate low signal intensity on all MRI sequences, although small foci of mildly increased signal intensity, believed to represent mild degeneration, are seen with increasing frequency in asymptomatic patients of increasing age. The clinical significance of such foci must be considered in the patient's clinical context. Nerves appear similar to skeletal muscle on T1-weighted and proton density (PD)-weighted images and brighter than skeletal muscle on T2-weighted images, although the degree of brightness on T2-weighted images is variable such that the distinction between normal and abnormal can be problematic ( Figure 54-1 ).

6

What are the boundaries and contents of the carpal tunnel?

The carpal tunnel is bounded by the bones of the wrist on its deep margin and a taut fibrous band, the flexor retinaculum, on its superficial margin. The major contents are the long flexor tendons of the fingers and thumb, the flexor carpi radialis tendon, and the median nerve (see Figures 54-1, C , and 54-1, D ).

7

What are the boundaries and contents of the ulnar tunnel (Guyon's canal)?

The ulnar tunnel (Guyon's canal) is located superficial to the carpal tunnel on the ulnar aspect of the wrist and contains the ulnar nerve, artery, and vein. The flexor retinaculum is the floor. The radial surface of the pisiform is the ulnar margin. A superficial fascial layer is the roof (see Figures 54-1, C , and 54-1, D ).

8

What is carpal tunnel syndrome (CTS)?

CTS is the most common of the peripheral nerve entrapment syndromes and is caused by increased pressure on the median nerve within the carpal tunnel. The clinical presentation is generally pain and paresthesias in the distribution of the median nerve.