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 augmentation on the migration of pedicular screws. In this study, the impact of the pedicular screw augmentation has 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 meaning In 8 osteoporotic vertebrae, two screws have been inserted in each vertebra: a non-augmented on one side and an augmented one 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 vertebrae is defined as the division of the augmented screw’s migration with the non-augmented screw’s migration. All the augmented screws survived both test setups whereas the non-augmented failed 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-augmented screw can withstand 200N load step. As expected, augmentation of pedicular perforated screws increases their stability in osteoporotic vertebras undergoing 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.
Movement between an implant surface and overlying soft tissue gives rise to fibrous capsule formation with a liquid filled void. Clinically, this situation is more prevalent with electropolished stainless steel (EPSS) implants compared to commercially pure titanium (CpTi) implants. We hypothesise this is mainly due to lack of microtopography on the EPSS. Four experimental EPSS surfaces with varying microtopographies were selected by a combination of morphological analysis using the scanning electron microscope and quantitative roughness analysis using laser profilometry. Standard treated EPSS (ISO 5832/1) and CpTi (ISO 5832/2) surfaces were also used. The plates had only one screw hole at either end so that the interaction of the tissue with an intact surface could be evaluated. Six plates of each type were implanted on both the left and right tibia, randomly, of 18 white New Zealand rabbits under the muscle for 12 weeks. After sacrifice samples underwent standard histological processing. Briefly, fixation, dehydration, embedding in methyl methacrylate, sectioning at 250μm slices (with implant), grinding to 50μm and staining with Giemsa. Digital images were taken with a light microscope and the size of thickening of connective tissue on the implant surface and the presence or absence of a liquid filled void was observed. Results showed no voids present on the CpTi samples. The standard EPSS had 3/6 plates with a void. The experimental EPSS surfaces were in-between these results. There was no relationship between quantitative measurements of average roughness (Ra) and the presence or absence of a void. There was a relationship between lack of fine microroughness of a surface (as seen with the SEM) and the presence of a void. The size of capsular thickening was not related to the Ra of the surface. These results support that void formation is mainly due to lack of microtopography on the plates.
