Radiology Corner – SLAC Wrist

By: Terry R. Yochum, DC, DACBR, Fellow, ACCR, and Chad J. Maola, DC

Case History 

This adult male patient presents with pain in the wrist. Observe the widened scapholunate space. What may this be indicative of?

Figure 1. What may this widened space indicate?

Figure 1. What may this widened space indicate?

Diagnosis

The widening of the scapholunate space is characteristic of scapholunate disassociation creating the classic “Terry Thomas Sign.” This is frequently associated with calcium pyrophosphate deposition disease (CPPD). The term “SLAC wrist” represents scapholunate advance collapse (SLAC.)

General Considerations

Pyrophosphate arthropathy refers to the structural joint changes that occur as a sequel to the presence of CPPD crystals within a joint. The radiographic features are similar to degenerative joint disease (osteoarthritis), with loss of joint space, subchondral sclerosis, cyst formation, osteophytes, loose bodies, and joint deformity. Chondrocalcinosis may or may not be visible despite these changes.

CPPD crystal deposition disease may be distinguished from DJD, however, by five recognizable features:1

Unusual articular distribution. Features of degenerative joint disease without history of previous trauma or surgery may be the result of CPPD crystal deposition; for example, at the wrist, elbow, and glenohumeral joints.

Unusual intra-articular distribution. Involvement of selective joint compartments within an articular complex suggests pyrophosphate arthropathy. The most common sites include degenerative changes in the radiocarpal, trapezioscaphoid, talocalcaneal, and patellofemoral compartments.

Prominent subchondral cysts (geodes). Intraosseous synovial fluid extrusions often result in numerous and large subchondral cysts.

Severe, destructive subchondral bone changes. Rapidly progressive subchondral fragmentation and collapse, with loose body formation, may simulate neuropathic joint disease.

Variable osteophyte formation. Osteophytes may be large or absent, despite severe joint space changes.

Knee

The knee is the most frequent joint for both clinical and radiologic changes to be visible. The most distinctive radiographic features are chondrocalcinosis and co-existing or isolated unusual compartmental involvement.

Chondrocalcinosis of the hyaline cartilage and menisci is characteristic. Up to 7% of asymptomatic individuals over the age of 60 years, however, may have chondrocalcinosis. Hyaline chondrocalcinosis is most visible on the femoral condyles and posterior surface of the patella. Meniscal fibrocartilage chondrocalcinosis is seen on the AP projection as triangular calcifications with the apices directed medially in the medial and lateral compartment. The lateral meniscus may be slightly denser than the medial.1

Intra-articular compartmental involvement most commonly is found in the medial femorotibial region, followed by patellofemoral and lateral femorotibial portions of the joint. Isolated patellofemoral compartmental changes have been stressed as an important sign of CPPD crystal deposition disease. Other accompanying features include subchondral sclerosis and cysts, articular fragmentation, and loose body formation.

Wrist

As in the knee, the wrist is a frequently affected articulation, with characteristic sites of calcification and arthropathy. Calcification of the triangular cartilage in the ulnocarpal joint space is frequently visible. Hyaline chondrocalcinosis may be seen adjacent to any bones within the carpus. Other structures that may calcify include the synovium and various ligaments of the wrist, especially between the scaphoid and the lunate bones.1

Arthropathy distinctly has a predilection for the radiocarpal joint. Signs of involvement include loss of joint space between the scaphoid and distal radius, subchondral sclerosis, cysts, and osteophytes. Disruption of the intervening ligament between the scaphoid and the lunate results in a widened scaphoid-lunate articular space (Terry Thomas’ sign, scapholunate dissociation). In addition, the scaphoid may move proximally and alter the articular contour of the radius, while the lunate moves distally toward the capitate, creating a stepladder appearance in the radiocarpal joint alignment. The combination of diminished radiocarpal space, scapholunate dissociation, and rotary subluxation of the scaphoid allows the capitate to migrate proximally, producing the scapholunate advanced collapse (SLAC) deformity. Severe degenerative signs are also common in the trapezioscaphoid joint. There is always a lack of articular alterations in the radioulnar compartment, unlike rheumatoid arthritis.1

Reference

1. Yochum TR, Rowe LJ. Essentials of Skeletal Radiology, 3rd ed. Baltimore: Williams & Wilkins, 2005:513.