Recommendations for standardised screw pull-out from polyurethane foam - The influence of density variations of the test foam and the insertion method.

Insertion and pull-out tests of synthetic test material are well established for the initial laboratory evaluation of screws. However, not all test parameters are sufficiently described. The influence of small density deviations of the test material, of tapping or of manual or machine insertion has not been fully examined. The aim of the present study was to examine the influence of these specimen preparation parameters on the measurement results in order to increase the reproducibility and reliability of screw pull-out tests. For this purpose, a commercial polyurethane foam and a clinically used type of screw are evaluated with insertion and pull-out tests. Within a foam apparent density grade, small deviations in apparent density led to significant and relevant differences in the measured values of insertion torque and pull-out strength. Furthermore, an influence on the measurement results was found during tapping and during manual or machine insertion of screws. For these reasons, specimens with the same apparent density should be used as far as possible and evenly distributed among the test groups. In addition, the reproducibility of the results can be increased by machine insertion of the screws.