‘Sag sign’—A simple radiological sign for detecting injury to the thumb ulnar collateral ligament

Abstract

Aim: To describe a new radiological sign after rupture of the thumb ulnar collateral ligament. Introduction: Rupture of the thumb ulnar collateral ligament is a commonly missed injury, with delayed diagnosis leading to considerable morbidity. Stress radiographs and MRI scans have been used to diagnose chronic (gamekeepers thumb) or acute (skiers thumb) injuries to this ligament. The former often causes discomfort and the latter are often not readily available. We describe a new radiological sign seen on the lateral radiographs of the thumb, which has previously not been described in the literature. The ‘sag sign’ is volar subluxation of the proximal phalanx in relation to the metacarpal at the metacarpal phalangeal joint Method: Between 2001–2006, radiographs of nineteen patients who had undergone repair of thumb ulnar collateral ligaments were retrospectively reviewed. There were 12 male and 7 female patients with an average age of 44. These were compared to a control group of normal thumb radiographs. The ‘sag sign’ was present on all the lateral radiographs of thumbs with ulnar collateral ligament tears. Once the ligament was repaired, the metacarpophalangeal joint alignment returned to normal. The sign was validated by senior house officers and registrars in orthopaedics training. Conclusion: The sag sign is a reliable indicator of an underlying injury to the thumb ulnar collateral ligament. Many studies have looked at the radiological diagnosis of this commonly missed injury. Stress radiography and ultrasound require straining an injured thumb which can extend the lesion and cause discomfort. MRI and MR arthrography are both sensitive and specific, but are costly and time consuming. Our sign is evident on plain film, is easily available, and does not require additional apparatus.