Introduction: The use of self-tapping screws has become increasingly popular since it allows for a rapid screw placement avoiding the tapping step during ORIF of fractures.. While sharing the same basic principle of cutting flutes and partial threads at the tip, at least four types of screw design is currently available, varying in the number and shape of cutting flutes. The purpose of this biomechanical study was to research for any significant difference between the various self-tapping screws

Material and Methods: Three different designs of 4.5-mm self-tapping screws and one standard 4.5 screw serving as control were compared for pull-out strength after insertion into an adult human non-embalmed cadaveric humeri. All specimens were machined to a 5 mm uniform cortical thickness. Four equidistant 3.2 mm holes were drilled into each specimen by an MTS mounted drill. All screws were inserted randomly in one of the four positions using a hand screwdriver. The cortical bone specimen was secured between two metal plates to the base of a MTS machine while a uniaxial tensile force was applied to the jig for screw removal at a rate of 0.833 mm/sec until holding power had decreased to 25 % of the maximum. Load displacement curves were recorded. Resulting data was analyzed using paired student-t tests. P values of less then 0.05 were considered statistically significant.

Results: The mean load-to-failure was 97.4167N (S.D. 13.29924) for the Synthes control screw, 69.2333N (S.D. 4.48360) for the Synthes self-tapping screw, 67.15 (S.D. 11.23864) for the Stryker self-tapping screw, and 55.0667 (S.D. 8.59271) for the ODI self-tapping screw. A significant difference was found between the mean pull-out strength of the Synthes control screw when compared to each of the three self-tapping screws (Pairs 1–3, P < 0.05). Furthermore, the mean pull-out strength of the ODI self-tapping screw was found to be significantly less than Stryker self-tapping screw (Pair 6, P < 0.05). There was no significant difference between Synthes self-tapping screws and Stryker self-tapping screws (Pair 5, P < 0.05).

Discussion and conclusion: Self tapping screws with three short cutting flutes performed better than those with two long cutting flutes. Despite of the different designs and length of the cutting flutes in self-tapping screws, they all have less pull out strength than regular screws