[Experimental studies of the biomechanics of scaphoid screws]

The purpose of this study was to determine the anchoring forces of four commonly used scaphoid screws in synthetic polyurethane foam. A linear relation was found between the length of the probe and the anchoring forces. The leading thread of the Herbert screw showed a anchoring force of 54 N/mm, the trailing thread 80 N/mm, and the Ulrich screw 75 N/mm. This linear relation of the shaft screws was only relevant upto a border probe length no longer that of the thread. Furthermore, it is valid for screws with continuous threads up to a probe length of 12 mm respectively. The cortical screw developed an anchoring force of 59 N/mm and the cancellous screw, 63 N/mm. With short probes of 4 and 6 mm, the trailing threads of the Herbert and Ulrich screws have the best anchoring forces, with values of 319 N/399 N and 307 N/435 N, respectively. The forces necessary for extraction of the tested scapoid bone screws placed in 8 mm polyurethane cubes were between 404 N and 527 N and were narrowly distributed. An additional conclusion of this study was that the application of screws with flat ends (Herbert and Ulrich screws) which were placed perpendicular to the dense material layer, significantly increased the anchoring forces.