INFLUENCE OF CEMENT AUGMENTATION ON PEDICULAR SCREW MIGRATION INSERTED IN OSTEOPOROTIC VERTEBRAE

Posterior internal fixation systems undergo internal constraints resulting in high load bearing requirement for the pedicular screw/bone interface. Only few studies deal with the impact of the vertebral augmentationon the migration of pedicular screws. In this study, the impact of the pedicular screw augmentationhas been investigated under physiological load for osteoporotic vertebras. The data have been proceeded to reduce the influence of vertebral geometry, which generally leads to results devoid of statistical meaningIn 8 osteoporotic vertebrae, two screws have been inserted in each vertebra: a non- augmentedon one side and an augmentedone on the contralateral side. Compression tests have been performed (two consecutive 50 cycles load steps -100N and 200N-) to observe the displacement of the screw’s head. Two different setups have been employed: a free connection (FC) and a blocked connection (BC). A load step is successful if the migration between two consecutive cycles tends to zero. To reduce the impact of the vertebras’ geometry, the screws’ migration have been compared contra-laterally using the migration ratio (MR). MR of vertebraeis defined as the division of the augmentedscrew’s migration with the non- augmentedscrew’s migration. All the augmentedscrews survived both test setups whereas the non- augmentedfailed the 200N FC load step. Significant differences are observable only for the highest successful load steps for each test setup: T-tests (P=0.039 and P=0.007 respectively) put into evidence that the results are statistically smaller than one. It is observable as well, that the BC induced fewer loads into the vertebrae: even non- augmentedscrew can withstand 200N load step. As expected, augmentationof pedicular perforated screws increases their stability in osteoporotic vertebrasundergoing large physiological load. This could be explained by the fact that the presence of PMMA increases the load transfer interface improving screw/PMMA complex bearing capacity. Smaller loads induce only small differences that are not significant.