[The double-cup carpus: a demonstration of the variable geometry of the carpus].

OBJECTIVES: The variable frontal geometry of the carpus has been known for many years, however there is no unanimity as to whether to describe the dynamic model of the carpus as comprising row or columnar functional units. The place of the scaphoid is also discussed. This study attempts to understand the organization and the composition of the functional units of the carpus.

METHODS: We took radiographs of 40 normal right wrists in radial and ulnar deviation and measured the displacement in the coronal plane of each carpal bone except the pisiform. We measured the angular movements of each carpal bone compared to a vertical axis passing through the geometric centre of the carpus. This axis is parallel to the radial axis which is defined as the line joining the midpoints of the radius at 2 and 5 cm proximal to the radial articular surface. We studied the movement of each row and each column.

RESULTS: Recorded angular movements were the followings: scaphoid 26 degrees, lunate 28 degrees, triquetrum 29 degrees, trapezium 44 degrees, trapezoid 50 degrees, capitate 50 degrees, hamate 56 degrees. Average angular movement within the first row is 27 degrees, within the second row is 50 degrees. Average angular movement within the radial column is 38 degrees, middle column is 39 degrees, ulnar column is 42 degrees.

CONCLUSIONS: The amplitude of movement are similar for the bones of each row, and different for the bones of each column. The bones of each row tend to move together and can alone account for all movements of the wrist. The movements measured between each column are torsional intrarow movements, allowing congruence between the two rows and the glenoid surface of the radius. The scaphoid movements are superposable with those of lunate and triquetrum. Scaphoid kinematics joins the first row. Radio-ulnar deviation of the wrist is shared equally between the radiocarpal and midcarpal joints. This sharing of wrist movement between the two rows constitutes for us a double cup carpal model.