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Published on: October 9, 2014
Oblique screws at the plate ends increase the fixation strength in synthetic bone test medium
Karl Stoffel1, Gwidon Stachowiak, Thomas Forster
1Department of Orthopaedic Surgery, Fremantle Hospital, Fremantle, Western Australia, Australia. stoffel@cyllene.uwa.edu.au
Objective:
To test the hypothesis that oblique screws at the ends of a plate provide increased strength of fixation as compared to standard screw insertion.
Design:
Biomechanical laboratory study in synthetic bone test medium.
Methods:
Narrow 4.5-mm stainless steel low-contoured dynamic compression plates were anchored with cortical screws to blocks of polyurethane foam. The fixation strength in cantilever bending (gap closing mode) and torsion was quantified using a material testing system. Different constructs were tested to investigate the effect of the screw orientation at the end of the plate (straight versus oblique at 30 degrees), the plate, and bridging length as well as the number of screws.
Results:
An oblique screw at the plate end produced an increased strength of fixation in all tests; however, the difference was more significant in shorter plates and in constructs with no screw omission adjacent to the fracture site. Both longer plates and increased bridging length produced a significantly stronger construct able to withstand higher compression loads. Under torsional loading, the fixation strength was mainly dependent on the number of screws.
Conclusions:
The current data suggest that when using a conventional plating technique, plate length is the most important factor in withstanding forces in cantilever bending. With regard to resisting torsional load, the number of screws is the most important factor. Furthermore, oblique screws at the ends of a plate increase fixation strength.
