Proximal half angle of the screw thread is a critical design variable affecting the pull-out strength of cancellous bone screws

Abstract

Background Screws with strong pull-out strength have been sought for the treatment of cancellous bone. We hypothesized that an obliquely angled screw thread has advantages over conventional vertical thread with a minimal proximal half angle. Methods Metal and bone screws were made of stainless steel and porcine cortical bone. Their proximal half angle was set at 0°, 30°, or 60°. The screws were inserted into porcine cancellous bone. At 0°, the thread faced the recipient bone vertically. Pullout tests at a rate of 30 mm/min ( n = 40, each screw type) and microcomputed tomography ( n = 6) were conducted. Findings The pull-out strength of the screws was maximal at 30°; 348.8 (SD, 44.1) N with metal and 326.6 (39.4) N with bone. It was intermediate at 0°; 301.9 (35.9) N with metal and 278.2 (30.6) N with bone. It was minimal at 60°; 126.5 (39.0) N with metal and 174.8 (29.7) N with bone. Cancellous bone was damaged between the threads at 30°, while intact cancellous bone was preserved between the threads at 0°. Interpretation A proximal half angle of around 30° is appropriate because the pullout force is applied to the recipient bone evenly. Commercial cancellous screws can be improved by changing the thread shape to minimize the damage to recipient bone.