INFLUENCING FACTORS ON THE HOLDING-POWER OF SPINE SURGERY SCREWS - EXPERIMENTAL EXAMINATIONS

Introduction and Aim of the Study: The primary assumption for successf ul internal fixation of the spine is a stable screws fixation. Aim of this experimental examination was to point out the influences on axial pull-out strength of screws and to suggest improvements for its incre ase. Methods: Eight specifically manufactured screw-prototypes were te sted in standardized pull-out tests in comparision by pairs. An isotro pic, synthetic hard-foam was used as test material. It resembled to th e spongiose bone and reduced the deviation of the results. To show the transferability of the results on bone there were made pull-out tests out off calf vertebra. Results: A significant increase of axial pull- out strenght can be achieved by a wide external diameter, a big insert ion depth, a flat angel of tooth profile and a small thread pitch. Thi s examination could not prove a significant advantageous influence on axial pull-out strength by fast pull-out, asymmetrical ''Butress-threa d'' or proportion of core- and external diameter difference. Conclusio ns: Referring to the aim of this examination a wide outside-diameter c an be recommended. Likewise the depth of insertion should be as deep a s the anatomic conditions permit. The influence of these two factors a re considerable. In contrast to other author's statements the maximum axial stability increases significantly by a small thread pitch and a flat angel of tooth profile.