A biomechanical study on preloaded compression effect on headless screws

Commonly used headless design screws such as Acutrak and Herbert screws have limited compressive effect on short fragment fixation. Therefore, we investigated whether preloaded compression techniques could be helpful in improving the compression effect. A prototype of interfragmentary compression screw was designed with a constant pitch to avoid self-compressive effect. Preloaded compression was achieved with a sleeve device before the insertion of the trailer side (the leading side was inserted). Prototype screw test results were compared with those of Acutrak and Herbert screws. Eight pullout and eight compression tests were performed for each of the three screws using homogenous synthetic bone blocks. The mean pullout forces of the Acutrak, Herbert and prototype screws were 67.21, 45.90 and 61.88 N, respectively. Preloaded compression (18.15 N) was higher than the compression produced by Acutrak (15.27 N) and Herbert (15.11 N) screws (p < 0.05). However, at the end of full insertion, half of the power obtained by preloaded compression was lost. Preloaded compression technique was successful in attaining maximum compression rate. During trailing-side insertion, constant pitch design failed to keep compression rate. Variable pitches can be useful to preserve or increase compression rate. In addition, sleeve usage may be more advantageous in thin fragment fixation.