## Why Scapholunate ligament tear leading to DISI and eventual SLAC wrist arthropathy is right The structure marked **D** is the scapholunate joint space, which normally measures 1–2 mm on PA radiograph. Widening to >3 mm is pathognomonic for the "Terry Thomas sign" (or "David Letterman sign"), indicating scapholunate ligament tear. This disrupts the normal carpal kinematics, causing dorsal intercalated segment instability (DISI), where the lunate assumes a dorsally tilted posture. Over time, this malalignment leads to predictable degenerative changes — scapholunate advanced collapse (SLAC wrist) — a progressive arthropathy affecting the radial wrist. The disruption of Gilula Arc III in this case reflects secondary changes from the underlying ligamentous injury. Per Maheshwari Orthopedics 10e and Sutton Radiology 7e, scapholunate interval widening is the cardinal radiographic sign of this injury and mandates early recognition to prevent irreversible arthropathy. ## Why each distractor is wrong - **Lunate dislocation with perilunate progression and immediate carpal tunnel compression**: Lunate dislocation presents with the "pie-crust" or "spilled teacup" sign on lateral radiograph and acute loss of carpal alignment on PA. The scapholunate space itself is not primarily widened in pure lunate dislocation; rather, the lunate is displaced volarly. This is an acute high-energy injury requiring emergency reduction, not a chronic presentation with isolated scapholunate widening. - **Scaphoid waist fracture with nonunion and secondary osteoarthritis**: While scaphoid fractures are the most common carpal fractures (60–70%), they do not characteristically widen the scapholunate joint space. A scaphoid fracture line would be visible as a discrete fracture, and nonunion would show sclerosis and gap at the fracture site, not scapholunate interval widening. The mechanism and radiographic findings do not match. - **Distal radius fracture with malunion and ulnar impaction syndrome**: Distal radius fractures (Colles, Smith, Barton, Chauffeur) affect the radius itself and may alter ulnar variance, but they do not directly widen the scapholunate space. Ulnar impaction is related to ulnar variance and TFCC pathology, not scapholunate ligament integrity. The scapholunate widening is not explained by this diagnosis. **High-Yield:** Scapholunate joint space >3 mm = Terry Thomas sign = scapholunate ligament tear → DISI → SLAC wrist (predictable arthropathy); always measure this space on PA wrist radiographs in trauma. [cite: Maheshwari Orthopedics 10e Ch 38; Sutton Radiology 7e]
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