Article
Comparison of Screw loosening and pull-out resistance between a double-threaded Screw Geometry and a standard Polyaxial Screw with and without polymethylmethacrylate-augmentation in polyurethane foam
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Authors
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Kurt Wiendieck
- Neurochirurgische Universitätsklinik des Universitätsklinikums Erlangen
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Björn Sommer
- Neurochirurgische Universitätsklinik des Universitätsklinikums Erlangen
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Michael Buchfelder
- Neurochirurgische Universitätsklinik des Universitätsklinikums Erlangen
| Published: | June 2, 2015 |
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Outline
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Objective: Screw loosening or failure is a common clinical complication of pedicle screw fixation, especially in patients with osteoporosis. Pedicle Screw augmentation is becoming a popular solution to this problem. However, this technique can cause complications. The objective of this study is to investigate whether a novel pedicle screw design offers a better loosening and pull-out resistance compared to a standard pedicle screws with and without augmentation.
Method: The screws were inserted in rigid foam blocks that represented osteoporotic cancellous bone. The tests were divided into 4 groups. The pull-out after insertion as well as the loosening displacement and the pull-out after a cycle loading with a load range of 270N by 1 500 000 cycles were measured. All screws had Ø6,5mm diameter and 45mm length. We tested a standard polyaxial screw , the double threaded screw and then both with an additional augmentation.
Results: The mean values of loosening displacement for Group 1 to 4 were 0,35mm, 0,06mm, 0,13mm and 0,004mm respectively. By performing a significance t-Test all P-Values reveal statistical significance between the mean values of all groups. The mean values of maximum Pull-out after first insertion for group 1 to 4 were 1372N, 1296N, 1972N and 2008N. The mean values of maximal Pull-out after cycle loading for group 1 to 4 were 874N, 1321N, 1800N and 1681N respectively. In this case the t-Test showed for each group statistical significance.
Conclusions: Our results confirmed that loosening of the the double threaded screw is significantly lower than that of the standard polyaxial screw and even lower than the one of the augmented fenestrated polyaxial screw. This may be due to the increment of fixation of the screw at the material due to its wider inner diameter that compacts the material around its threads giving a much higher fixation. The results also confirm that the loss of maximal pull-out between the first insertion and after the cycle loading is very significant for the standard polyaxial screw and inexistent for double threaded screw. However, the pull-out resistance of the augmented polyaxial screw is significantly compared to the non- augmented double threaded screw. It presented also no reduction after cycle loading. In order to deepen this investigation, further studies in cadaveric human bone will be performed as well as a histological evaluation of the bone displacement.
