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Biomechanical Analysis of Screw Trajectory in the Reduction and Association of the Scaphoid and Lunate (RASL) Procedure
Christina M. Beck, PhD; Philip Nasser, BS; Michael R. Hausman, MD; Steven M. Koehler, MD
Mount Sinai Medical Center, New York, NY

Introduction: The reduction and association of the scaphoid and lunate (RASL) procedure is a technique for treating scapholunate (SL) ligament disruptions, achieving sufficient functional outcomes and restoring proper wrist kinematics to prevent the development of scapholunate advanced collapse. The goal of this cadaveric study was to determine if placement of the screw affects the ability of the SL joint to withstand mechanical force, thereby providing insight into the importance of screw trajectory in the outcome of the RASL procedure.

Methods: The RASL procedure was performed by the senior author on 27 fresh-frozen cadaveric wrists after complete disruption of the SL ligament. Using fluoroscopic guidance, a headless compression screw was placed either distal (15 screws) or proximal (12 screws) to the scaphoid waist and into the dorsal (13 screws) or volar (14 screws) aspect of the lunate. Specimens were mounted onto a custom-made jig and the wrist was subjected to repeated cycles of transcarpal axial force, increasing in a step-wise fashion from 100N to 200N, covering the spectrum of physiologic compression forces during clenched-fist motion. Failure was defined as any amount of diastasis between the scaphoid and lunate, as determined using radiographs between each force interval, direct visual inspection, and real-time infrared camera displacement monitoring using a Vicon Nexus motion capture system. Statistical significance was determined by a two-tailed t test or Fisher's exact test setting a confidence level of 95% (? ? 0.05).

Results: The average cadaver age was 52 (range 18-64). No specimens were reported to have used steroids, have a history of fragility fractures, or osteoporosis. Cadaver age (p=0.082), BMI (p=0.958), physical activity level (p = 0.596), smoking status (p=0.678), gender (p=0.695), and hand dominance (p=0.695) were not significant predictors of failure for the procedure. When examining screw trajectory, placement in the dorsal versus volar poles of the lunate (p=0.706) were not significant predictors of failure. However, screw placement distal to the scaphoid waist was significantly (p<0.001) associated with failure of the RASL procedure.

Conclusion: This study offers the first biomechanical analysis of treatment of a SL ligament rupture with the RASL procedure. The results presented here suggest screw placement proximal to the scaphoid waist is optimal for success of the RASL procedure, and that accurate positioning in the lunate does not affect the outcome of the RASL. The scaphoid waist is a reliable radiographic landmark, providing surgeons with the means to achieve a consistent successful screw starting point.

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