Opposing flanks vs parallel flanks - The influence of screw design on compression and pull-out strength

Screws generate and maintain compression against distracting forces when performing osteotomy or fusion surgery. Headless screws have been recently designed with opposing flank angles (OFA) between the proximal and shaft threads and are purported to achieve better compression. The aim 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 strength (POS).

This was a biomechanical, in-vitro sawbone study comparing four screw designs: two opposing flank headless designs (Screw A_O, Screw B_O), one parallel flank headless design (Screw C_P) and one headed design (Screw D_H). All screws were 4.0 mm diameter x 50 mm length, partially threaded, cannulated screws typically used in foot and ankle surgery (five screws tested for each design). Screw B_O had a shorter and narrower proximal threaded portion than Screws A_O and C_P. A custom apparatus was designed for measuring compression and POS. An osteotomy was created in sawbone blocks of 0.32 g/cm ³ density which simulated cancellous bone. The divided segments were apposed with four strain-gauged load cells in between. Screws were inserted per manufacture recommendations, perpendicular to the osteotomy and maximum compression was measured. To measure POS, block segments were distracted in increasing increments until the construct failed.

There was a significant difference in compression between all screws: Screw A_O, 311.0 ± 4.2 N; Screw C_P, 201.4 ± 28.6 N; Screw B_O, 168.6 ± 15.9 N; Screw D_H, 119.8 ± 13.2 N (p < 0.001). There was no significant difference in POS between Screw A_O (466.0 ± 29.0 N) and Screw C_p (399.0 ± 46.0 N, p = 0.089), but the other screws had significantly lower POS: Screw B_O, 310.0 ± 22.0 N; Screw D_H, 183.0 ± 12.9 N (p < 0.001).

One of the OFA designs exhibited the highest compression and POS in this sawbone model. However, the variable results between designs suggests other screw design features may have a greater influence than the opposing of flank angles.