Opposing flanks versus parallel flanks – the influence of headless screw design on compression and pull-out resistance

R. Tan, A. Morillo, S. Taylor, A. Bernasconi, S. Patel, K. Malhotra

Introduction: Screws generate and maintain compression against distracting forces when performing osteotomy or fusion surgery. Headless screws with opposing flank angles (OFA) between the threads of the head and shaft are purported to achieve better compression. The purpose of this study was to compare OFA designs against traditional parallel flank angle (PFA) headless screws and headed screws to determine differences in compression and pull-out resistance (POR).

Methods: In this biomechanical in-vitro study, four screw designs were compared: headless screws with OFA (Screw_A and Screw_B), headless screws with PFA (Screw_C), and headed screws (Screw_D). All screws were 4.0x50mm partially threaded cannulated screws. Screw_B had a 1.4mm shorter head length and 0.5mm narrower head thread diameter than Screw_A and Screw_C, which were similar. A custom apparatus was designed for measuring compression and POR. Osteotomies were created on synthetic bone blocks (density 0.32g/cm3) simulating cancellous bone. Screws were inserted perpendicular to osteotomies in accordance with manufacturer guidance and maximum compression recorded. Increasing distracting forces were then applied to the blocks until the screws pulled out. For each screw type, five screws were tested.

Results: There was no significant difference in maximum compression between screw designs: Screw_ A=38.7N±14.2N, Screw_B=48.7N±15.6N, Screw_C=51.9N±36.4N, Screw_D=32.0N±9.2N; p=0.921. When assessing POR, all screws failed at the head-bone interface (screw heads subsided into block). Pull-out forces significantly differed between all groups: Screw_A=466N±29.0N, Screw_B=310N±22.0N, Screw_C=399N±46.0N, Screw_ D=183N±12.9N; p<0.001 (ANOVA). Screw_A had the highest POR but the other OFA design (Screw_B) had significantly lower POR.

Conclusion: Screw design, whether headless (OFA or PFA) or headed did not appear to influence compression generated. However, headed screws had significantly lower POR than headless designs. Within headless designs, OFA may increase POR, but other screw head features (number / diameter of threads) had an apparently greater influence than flank angle.