Scapholunate Dissociation: Watson's Shift Test and the Terry-Thomas Sign
In short
Scapholunate dissociation represents the most common carpal instability; Watson's shift test identifies subluxation, preventing progressive SLAC wrist collapse.
In this guide (4 sections)
Anatomy and Mechanics of the Scapholunate Ligament
The Scapholunate (SL) interosseous ligament is the cornerstone static stabilizer of the proximal carpal row. It binds the scaphoid and lunate bones together, ensuring that they move as a harmonious synchronous unit during wrist motion. The dorsal subregion of the ligament is the thickest and mechanically strongest component.
Scapholunate dissociation represents the most frequent and debilitating form of carpal instability, typically occurring during a high-energy fall onto an outstretched hand (FOOSH) with the wrist in extension, ulnar deviation, and intercarpal supination. When the dorsal SL ligament ruptures, the scaphoid tilts forward into abnormal flexion while the lunate tilts backward into extension, establishing a Dorsal Intercalated Segment Instability (DISI) deformity.
Diagnostic Clinical Testing and Radiographic Markers
| Diagnostic Finding | Examination Technique / Feature | Positive Indication | Pathological Severity | |
|---|---|---|---|---|
| Watson's Scaphoid Shift Test | Examiner presses thumb against palmar scaphoid tuberosity while moving wrist from ulnar to radial deviation | Dorsal subluxation of scaphoid over radial rim, followed by a painful 'clunk' when thumb pressure releases | Gold standard clinical test for dynamic scapholunate instability | |
| Terry-Thomas Sign (X-Ray) | Clenched-fist anteroposterior radiograph of the wrist | Scapholunate joint space gap widening > 3 mm (resembling the gap between the comedian's front teeth) | Confirms complete structural dissociation of the dorsal ligament | |
| Cortical 'Ring Sign' (X-Ray) | Anteroposterior projection of the carpus | Scaphoid foreshortened, projecting a circular cortical ring | Indicates severe pathological palmar flexion of the scaphoid | |
| SLAC Wrist Progression | Long-term radiograph following untreated SL tear | Progressive Scapholunate Advanced Collapse (SLAC) arthritis | End-stage osteoarthritis between scaphoid, radius, and capitate |
Physiotherapy Rehabilitation and Dart-Thrower's Motion
- The Dart-Thrower's Motion Plane: Train wrist extension-radial deviation transitioning into flexion-ulnar deviation; biomechanical studies prove this functional plane produces virtually zero intercarpal motion between the scaphoid and lunate, allowing safe strengthening.
- Neuromuscular Proprioceptive Retraining: Activate the dynamic muscular stabilizers of the scaphoid: the flexor carpi radialis (FCR), extensor carpi radialis longus (ECRL), and abductor pollicis longus (APL), which counteract scaphoid rotary subluxation.
- Strict Avoidance of Pure Wrist Flexion: Prohibit pure sagittal wrist flexion and heavy loaded gripping early, as both force the scaphoid into hyper-flexion and widen the scapholunate diastasis.
Frequently Asked Questions
What is the 'Terry-Thomas sign' on a wrist X-ray?
The Terry-Thomas sign is an abnormal gap between two wrist bones (the scaphoid and lunate) that looks like the famous gap between the front teeth of British comedian Terry-Thomas, proving that the ligament connecting the bones has completely snapped.
What happens if a torn scapholunate ligament is ignored?
If ignored, the separated bones rub abnormally against your forearm bone, leading inevitably to severe, irreversible arthritis known as SLAC wrist (Scapholunate Advanced Collapse), which eventually requires wrist fusion surgery.
What is the Dart-Thrower's motion in wrist rehab?
The dart-thrower's motion is moving your wrist diagonally - from up-and-out to down-and-in, just like throwing a dart. This movement plane lets you strengthen your wrist muscles without pulling apart the injured ligament.
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