The effect of screw trajectory for the reduction and association of the scaphoid and lunate (RASL) procedure: a biomechanical analysis

The purpose of this study was to determine if screw placement in the reduction and association of the scaphoid and the lunate (RASL) procedure affected the ability of the scapholunate joint to withstand force. After completely disrupting of the scapholunate ligament in 29 fresh-frozen cadaveric wrists, we placed the RASL screw either distal or proximal to the lateral aspect of the dorsal ridge of the scaphoid and into the dorsal or volar aspect of the lunate. Specimens were subjected to repeated cycles of transcarpal axial force, mimicking clenched-fist loading, until failure. Screw placement distal to the lateral aspect of the dorsal scaphoid ridge was significantly associated with failure when examined manually, radiographically (1.8 vs. 0.5 mm) and using real-time motion capture (diastasis: 1.6 vs. 0.4 mm; Euler angle: 4.5 ° vs. 0.8 °). The lateral aspect of the dorsal ridge is a reliable radiographic landmark on the scaphoid and provides surgeons with a convenient starting point to achieve the most biomechanically stable RASL construct, and, therefore, enhances the potential for an optimal clinical outcome.