Effect of bone quality on the forces generated by compression screws

Internal fixation of the fractured scaphoid bone is used to promote union b etween bone fragments and to decrease wrist immobilization. Headless screws are commonly used because they minimize interference with articular surfac es and reduce tissue irritation and immobilization. In the present experime nt, compressive force was measured as a function of bone quality for two he adless screw types, the Herbert and the Acutrak. Forty-seven cylindrical sa mples of cancellous bone were prepared from fresh, previously frozen human cadaveric distal femora. The compressive forces generated as the screws wer e advanced into the specimens were measured and correlated to the specimens ' bone mineral density (BMD) and density. Over the range tested, the averag e compressive force for the Acutrack screw was approximately 42% higher tha n that of the Herbert. Statistical significance, however, could not establi shed because of the low statistical power of the test due to the inherent s pread in the data. For the Acutrak screw, force was best fit to BMD and to density by second-order polynomials. Regression analysis indicated that sim ilar correlations did not exist between force and BMD or between force and density for the Herbert screw. The correlation shown by the Acutrak screw i ndicates that it may be a more predictable as well as more effective system and therefore there may be some advantage in selecting this system. Furthe rmore, results suggest that the Acutrak screw generates greater forces with increasing bone density and could be more effective for a younger populati on. (C) 1999 Elsevier Science Ltd. All rights reserved.