[Evaluation of the mechanical properties of surgical screws using torsion tests]

The torsion test for cortical and cancellous screws introduced by DIN and adopted by ISO is applied to control the tolerances defined in these standards. Both standards contain wants of clearness and the essential fault to tolerate products which achieve the claimed torsional moment only by plastic deformation during the test. The torsional moment determined in this way simulates a higher torsional resistance as it is characteristic for a rigid junction. This defect is demonstrated with screws of different tensile properties and compared with a lot of samples. In order to be able to recognize and eliminate insufficient products the torsion moment at a small angle of torsion is proposed as characteristic value instead of the maximal torsion moment at an undefined angle of torsion.