Numerical study of effect of elastomeric stress absorbers on stress reduction in bone-dental implant interface

Objective This paper focused on optimal stress distribution in the mandibular bone surrounding a dental implant and is devoted to the development of a modified Osteoplant® implant type in order to minimize stress concentration in the bone-implant interface. Material and Methods This study investigated 0.4 mm thick layers of two elastomeric stress barriers incorporated into the dental implant using 3-D finite element analysis. Results Overall, this proposed implant provoked lower load transfer in bone-implant interface due to the effect of the elastomers as stress absorbers. The stress level in the bone was reduced between 28% and 42% for three load cases: 75 N, 60 N and 27 N in corono-apical, linguo-buccal and disto-mesial direction, respectively. Conclusion The proposed model provided an acceptable solution for load transfer reduction to the mandible. This investigation also permitted to choose how to incorporate two elastomers into the Osteoplant® implant system

Numerical Study of the Effect of Rigid and Dynamic Posterior Attachment Systems on Stress Reduction in Cortical and Spongy Bones of the Lumbar Segments L4-L5

Posterior instrumentation is a common fixation method used in the treatment of spinal diseases. However, the role of different models of fixation system in improving fixation stability in these fractures has not been established. Comparative investigation between posterior rigid fixation (pedicle screw) and four models of posterior dynamic fixation (B Dyne, Elaspine, Bioflex, Coflex rivet) may elucidate the efficacy of each design. The purpose of this study was to investigate the biomechanical differences between rigid fixation and dynamic fixation implantation by using finite element analyses. The goal of the present study was to evaluate the efficacy of five fixation systems mounted on L4-L5 motion segment. In this numerical study, finite element model of an L4-L5 segment was developed from computed tomography image datasets. Five fixation devices were also implanted internally to the motion segment. Another model with an intact intervertebral disc was also analysed for comparison. Loads simulating the physiological flexion, extension and lateral bindings were applied to the superior surface of L4. Results showed that the Elaspine, Bioflex, Coflex rivet and pedicle screw fixation implantation could provide stability in all motions and reduce von Mises stress in the cortical and spongy bone at the surgical segment L4-L5. Moreover, maximal von Mises stress in the annulus disc was observed in dynamic systems but within the safe range. The greater movement of the motion segment was also appeared in dynamic fixations. Existence of the fixation systems reduced the stress on the intervertebral disc which might be exerted in intact cases. Use of the fixation devices could considerably reduce the load on the discs and prepare conditions for healing of the injured ones. Furthermore, dynamic modes of fixation conferred the possibility of movement to the motion segments in order to facilitate the spinal activities. The numerical results showed that the posterior fixation system (rigid and dynamic) played a very important role in the absorption and minimization of stresses. On the other hand, the tow systems (rigid fixation and dynamic fixation) played such a great role in reducing the stress compared to other synthetic discs. In general, the posterior fixation system gave a lower level of stress in the cortical bones and the spongy bones of the L4-L5 lumbar segment compared to the intact model