The standard covers various dimensions for some different types of screws, and details tests to establish the mechanical strength of metallic medical screws. Measuring the torque developed while inserting the screw, and the pullout force are also prescribed
A motorised torque testing system together with a force testing stand plus suitable loadcells
Statistically significant quantities of the screws are tested by clamping in a torque testing system. Screws are also driven into reference material to simulate real bone, then subjected to an axial load to determine pullout forces
Refer to the standard and the required annexes for details of the test to be performed and apparatus involved. Several tests are given in the standards. Mecmesin can supply test equipment for:
Torsional yield strength
Medical screws are firmly held then subjected to rotational force. By monitoring the torsional load and angle, a torsional yield strength can calculated. (N.B. some manual calculation may be involved)
The maximum torque is determined from the torque / angle curve.
The clamped screws are tested to the point of failure, and the angle at which the screw breaks is recorded
Insertion and removal torque
Screws are driven into reference material to simulate real bone. The maximum torque is reported during the initial four revolutions on insertion, and the removal torque on the maximum encountered while reversing the direction.
Screws are driven into reference material to a specified depth. An axial load is then applied until the screw fails or releases from the test block. The maximum load is reported.