Presents a formula for reliably predicting holding power of bone screws, something which has not previously been reported. This formulation is of value to the orthopaedic clinician in general and osteosynthesis research in particular. In this study, based on the experimental data and some biomechanical considerations, the authors proposed an analytical formula for the pullout strength of bone screws as a function of important design and clinical parameters. The formula was tested by comparison to the measured results of pullout testing in vitro. Pullout force is predicted to be nonlinearly related to design variables (major diameter, minor diameter and pitch) and linearly related to the clinical variables (depth of insertion, bone density). This formula was consistent with the mean pullout strength of all different screws tested within an average error of 11%. The importance of pitch design on the pullout strength was emphasized and an explicit formula which was consistent in predicting the pullout strength within 6% of mean value was determined. >