[Experimental study of improved threading of bone screws]

In order to improve the holding force between the bone and the screw, we have been carrying out experimental investigations of the stability of bone and screw connection. Especially Schanz screws with a diameter of 6 mm, used in connection with the Wagner leg lengthening device, have been taken into consideration. By use of a loading model we determined the sizes of the essential kinds of loadability of the bone-screw connection and therefrom derive directions for dimensioning. The influence of the thread parameters (e.g. flank angle, pitch, pitch angle) on the loadability of the bone-screw connection has been investigated experimentally for 17 different thread profiles. Summarizing up the results, a pitch angle of 7,5 degrees and a flank angle between 12 degrees and 15 degrees proved to enable the maximum loadability. Corresponding to the results of our trials with the loading model, the improvement of the thread was carried out on the premises of simultaneous failure of both the cortical thread and the Schanz screws by pressure or shear. The loosening behaviour of the improved thread has been tested together with the conventional thread form. The results showed a reduction of about 20% in loosening of the screws and an increase of about 30% in pressure and traction loadability.