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

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 degrees , 30 degrees , or 60 degrees . The screws were inserted into porcine cancellous bone. At 0 degrees , 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 degrees ; 348.8 (SD, 44.1)N with metal and 326.6 (39.4)N with bone. It was intermediate at 0 degrees ; 301.9 (35.9)N with metal and 278.2 (30.6)N with bone. It was minimal at 60 degrees; 126.5 (39.0)N with metal and 174.8 (29.7)N with bone. Cancellous bone was damaged between the threads at 30 degrees , while intact cancellous bone was preserved between the threads at 0 degrees.

INTERPRETATION

A proximal half angle of around 30 degrees 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.