An in vivo feasibility study investigating the fixation of osteotomised rabbit tibia using a bioresorbable composite plate and a comparison of the effects of the bioresorbable plate and a metal plate on intact bone

Background: Modern fracture management uses surgically implanted metal plates to fix fractured bones to restore pre-injury anatomy. Following healing, the plate may have to be removed requiring a second operation. Reactive thinning of the bone beneath the metal plate (stress shielding) can increase the theoretical risk of re-fracture once the plate is removed. Bioresorbable plates offer the possibility of reduced stress shielding due to plate degradation causing a progressive reduction in mechanical properties and obviating the need for a second operation to remove the plate. Aim: To determine the feasibility of using a novel bioresorbable composite plate to fix an in vivo osteotomised bone in an animal model. Following the failure of this experiment, the study aimed to determine the in vivo effects of the composite plate on intact bone in comparison to a conventional steel plate. A secondary aim was to determine the in vitro mechanical properties of both types of plate compared to bone. Methods: A reproducible in vivo osteotomy technique was developed using a rabbit tibia model and fixed with the composite plate. Following the failure of this experiment, subsequent in vivo experiments compared the effects of the composite plate with a metal plate on intact bone. Twenty-five rabbits underwent surgical application of the composite plate to the intact right tibia. In parallel, 25 rabbits underwent surgical application of the metal plate to the right tibia. In each study arm, groups of five rabbits were sacrificed at 5 sequential time points - 2, 6, 12, 26 and 52 weeks. At sacrifice, the right plated tibia and the left unplated bone, which acted as a control, were retrieved. The outcomes assessed included structural changes (radiographs, Micro CT, electron microscopy); mechanical changes (flexural testing) and histological changes (paraffin slides). In vitro mechanical tests investigated the mechanical properties of different plate designs, intact bone and osteotomised bone. Results: The single in vivo osteotomy experiment resulted in composite plate failure within a few hours of implantation. No further in vivo osteotomy experiments were conducted. The intact bone experiments demonstrated new bone growth around both types of plate, but greater bone loss occurred beneath the steel plated bones compared with the composite plated bones as the duration of implantation increased. The relative augmentation in mechanical characteristics when comparing the plated with control bones reduced to a greater extent in the metal plated bones compared with the composite plated bones. Few inflammatory cells were seen in both types of implanted bone at the surveyed time points. In vitro studies demonstrated reduced mechanical properties in the degraded composite plates compared with polymer and metal plates. Conclusion: The mechanical properties of the composite plate are insufficient to enable application as a fracture fixation device. Improved outcomes in terms of bone preservation when comparing the composite plate with the steel plate in an in vivo intact bone model reflect differences in the mechanical properties of the two types of plate