Effect of cutting flute design on cortical bone screw insertion torque and pullout strength

Objective: To determine the effect of the number and length of cutting flutes on the insertion torque and pullout strength for self-tapping 4.5-millimeter cortical bone screws. Design: Screws were self-tapped in the diaphysis of human cadaver femurs. Each of the six screw types studied had different designs with varying cutting flute lengths and numbers. Bone mineral density, insertion torque, and pullout strength were measured. Setting: The study was conducted at an experimental biomechanics laboratory associated with a university medical center. Outcome Measurements: Insertion torque and pullout strength were normalized by the local bone mineral density. Results: The mean normalized insertion torque of the design with four full-length cutting flutes was less than the design with three full-length flutes and the two designs with one-third length flutes (p < 0.05). The mean normalized pullout strength of the screw with four full-length flutes was significantly greater than that of all screws with fewer than three flutes (p < 0.05). Conclusions: Priorities for a cutting flute design should ideally include ease of screw insertion, minimal soft tissue irritation, and maximal screw holding power. Screws with more than two flutes were easier to insert and did not cause cortical damage during insertion. The screw with four full-length flutes showed a trend toward being the easiest to insert and having the greatest holding strength.