Influence of lunate type on scaphoid kinematics.

PURPOSE: To assess the influence of lunate type on scaphoid kinematics.
METHODS: One hundred normal wrists had fluoroscopic assessment of the wrist in maximal radial, neutral, and ulnar deviation. The shortest distance in a neutral position between the capitate and triquetrum, C-T distance, determined lunate type. A type I lunate had a C-T distance of < or =2 mm, a type II lunate > or =4 mm, and an intermediate group lay between these values. Scaphoid flexion and translation in radial and ulna deviation was measured.
RESULTS: There were 18 subjects with a type I lunate, 19 with an intermediate lunate, and 63 with a type II lunate. There was no statistically significant difference between lunate type, subject age, or hand dominance. There was a statistically significant higher proportion of women with a type I lunate. Subjects with a type II lunate had a statistically greater amount of flexion during radioulnar deviation as determined by CR index (0.79 vs 0.91) and scaphoid flexion index (0.21 vs 0.09). Subjects with a type II lunate had statistically less translation during radioulnar deviation as determined by translation ratio (0.22 vs 0.31) and scaphoid inclination index (0.18 vs 0.23). The average scaphoid kinematic index in subjects with a type II lunate was 1.24, intermediate 0.86, and type I 0.42. A scaphoid kinematic index of greater than 1 indicates the scaphoid has more flexion during radioulnar deviation than translation.
CONCLUSIONS: Wrists with a type I lunate show statistically greater scaphoid translation with radial deviation. Wrists with a type II lunate show statistically greater scaphoid flexion with radial deviation. Intermediate lunates have intermediate scaphoid mechanics. This allows the surgeon to determine the likely wrist scaphoid mechanics based on the lunate type determined from a single posterior-anterior x-ray.