'Terry Thomas' sign and the 'ring sign' of scapholunate instability.

Scapholunate instability (SLI) (also known as scapholunate dissociation or rotary subluxation of the scaphoid) is caused by a tear in the interosseous ligaments of the lunate, scaphoid and the capitate bone with a tear in the dorsal radiocarpal ligaments by acute dorsifexion injury or fractures of the distal radius. (1,2) The patient complains of wrist pain, weakness of grip and a clicking sensation in the wrist.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

SLI is diagnosed with a gap or separation between the scaphoid and the lunate bones on a posterior anterior wrist X-ray. More than 2 mm is suspicious and 4 mm or more is diagnostic of SLI. (1,2) This scaphoidlunate gap is also referred to as the 'Terry Thomas' sign (Fig. 1) referring to the distinctive gap between the upper incisors of the late British comedian.

There is in addition foreshortening of the scaphoid, causing the cortical margin of the distal pole to become parallel to the central axis of the X-ray and appear as a ring called the the 'ring sign' (Fig. 1). The gap is better demonstrated in ulnar deviation or clenched-hand position (Fig. 2). (1,2) Treatment is dependent on the symptoms, ranging from rest to carpal fusion.

(1.) Rogers LF. Radiology of Skeletal Trauma, 2nd ed. Edinburgh: Churchill Livingstone,.1992: 927-929.

(2.) Hudson TM, Caragol WJ, Kaye JJ. Isolated rotatory subluxation of the carpal navicular. AJR 1976; 126: 601-611.

A S Bajwa, MB BS, FCS (SA) Ortho

Brenthurst Life Health Clinic, Parktown, Johannesburg

A K Bajwa, MB BS, FCRad (Diag) SA

Department of Diagnostic Radiology, Chris Hani Baragwanath Hospital and

University of the Witwatersrand, Johannesburg