[Screw-in performance of threaded cups].
BACKGROUND
For threaded cups, cup diameter, the shape of the threads and the shape of the cup have a major influence on the screw-in performance. The designs of standard implants vary greatly, making it difficult to carry out comparative studies to provide clear-cut results on the effect of individual parameters on the insertion process.


METHODS
15 different prototype cups were manufactured for this study. Three sizes, three shapes and five thread designs were tested using artificial bone. Screw-in performance was measured with the slope of insertion torque before and after reaching the seating point, the torque at the seating point, the insertion angle and the change of slopes at the seating point.


RESULTS
Modified trapezoid threads have the lowest insertion resistance and the clearest seating point as well as the most distinct change of slope at the seating point. Flat threads show lower insertion resistance and more increased slopes at the seating point than sharp threads. The slope of insertion torque before and at the seating point is higher for sharp threads in comparison to the other threads, the change of slop is however low. Larger cups show a higher insertion resistance.


CONCLUSIONS
Screw-in performance is decisively influenced by the design of the threads. Modified trapezoid threads are the easiest to insert with excellent manual control, thus reducing the risk of intra-operative overturning. Flat threads also have a low screw-in resistance. Sharp threads have an unsatisfactory screw-in performance because the seating point cannot be "felt".
Effenberger
H
H
Orthopädische Universitätsklinik und Poliklinik Friedrichsheim, Frankfurt a. M. effenberger@implantat-atlas.com
Bösl
O
O
Witzel
U
U
Linke
B
B
D000077:Acetabulum / Q000601:surgery*; D001696:Biomechanical Phenomena; D001863:Bone Screws; D019544:Equipment Failure Analysis; D006622:Hip Prosthesis; D006801:Humans; D011474:Prosthesis Design; D017755:Prosthesis Fitting