Effect of bone quality on the forces generated by compression screws

Internal fixation of the fractured scaphoid bone is used to promote union between bone fragments and to decrease wrist immobilization. Headless screws are commonly used because they minimize interference with articular surfaces and reduce tissue irritation and immobilization. In the present experiment, compressive force was measured as a function of bone quality for two headless screw types, the Herbert and the Acutrak. Forty-seven cylindrical samples of cancellous bone were prepared from fresh, previously frozen human cadaveric distal femora. The compressive forces generated as the screws were advanced into the specimens were measured and correlated to the specimens' bone mineral density (BMD) and density. Over the range tested, the average compressive force for the Acutrack screw was approximately 42% higher than that of the Herbert. Statistical significance, however, could not established because of the low statistical power of the test due to the inherent spread in the data. For the Acutrak screw, force was best fit to BMD and to density by second-order polynomials. Regression analysis indicated that similar correlations did not exist between force and BMD or between force and density for the Herbert screw. The correlation shown by the Acutrak screw indicates that it may be a more predictable as well as more effective system and therefore there may be some advantage in selecting this system. Furthermore, results suggest that the Acutrak screw generates greater forces with increasing bone density and could be more effective for a younger population.