Measurement of mechanical properties on gap healing in a rabbit osteotomy model until the remodeling stage

Adaptive Image Segmentation Reveals Substantial Cortical Bone Remodeling During Early Fracture Repair

The goal of this study was to develop an image analysis algorithm for quantifying the effects of remodeling on cortical bone during early fracture healing. An adaptive thresholding technique with boundary curvature and tortuosity control was developed to automatically identify the endocortical and pericortical boundaries in the presence of high-gradient bone mineral density (BMD) near the healing zone. The algorithm successfully segmented more than 47,000 microCT images from 12 healing ovine osteotomies and intact contralateral tibiae. Resampling techniques were used to achieve data dimensionality reduction on the segmented images, allowing characterization of radial and axial distributions of cortical BMD. Local (transverse slice) and total (whole bone) remodeling scores were produced. These surrogate measures of cortical remodeling derived from BMD revealed that cortical changes were detectable throughout the region covered by callus and that the localized loss of cortical BMD was highest near the osteotomy. Total remodeling score was moderately and significantly correlated with callus volume and mineral composition (r > 0.64, p < 0.05), suggesting that the cortex may be a source of mineral needed to build callus.

An innovative Mg/Ti hybrid fixation system developed for fracture fixation and healing enhancement at load-bearing skeletal site

Magnesium (Mg) is a potential biomaterial suitable for developing biodegradable orthopaedic implants, especially as internal fixators for fracture fixation at non-load bearing skeletal sites. However, Mg alone cannot provide sufficient mechanical support for stable fracture fixation at load bearing sites due to its rapid degradation in the early stage after implantation. In consideration of the strengths and weaknesses of Mg, we developed an innovative magnesium/titanium (Mg/Ti) hybrid fixation system for long bone fracture fixation and investigated the fixation efficacy. The finite element analysis (FEA) results indicated that the Mg/Ti hybrid fixation system provided sufficient mechanical support for fracture fixation at load-bearing skeletal site. As a proof-of-concept, we performed a "Z-shaped" open osteotomy at the mid-shaft of rabbit tibia. For comparison, the animals were divided into two groups: Mg/Ti group (fixated with Mg screws and Ti fixators) and Ti control group (fixated with Ti screws and Ti fixators). The radiographic, four-point bending mechanical test, histological and histomorphometric analysis were postoperatively performed in a temporal manner up to 12 weeks. Both X-ray and micro-CT images of the Mg/Ti group showed a larger callus (14.7% at 3rd week and 24.8% at 6th week, n = 5-7, p < 0.05) in the regions of interest (ROIs) over time, especially at the opposite cortex of the fixation plate. At the 12th week post-operation, the biomechanical test result indicated that the rabbit tibia in the Mg/Ti group healed better and the overall mechanical strength was approximately 3-fold higher (n = 8, p < 0.05) than that at 6th week. Furthermore, the FEA revealed that the Mg/Ti group had a higher mechanical strength (19.5% at week 6 and 31.5% at week 12) at the specified ROI and resulted in an earlier and faster endochondral ossification (68.0% at week 3 and 71.4% at week 6) with a higher expression of osteocalcin (54.0%) and collagen I (34.2%) than the Ti control group (n = 4, p < 0.05). Further evaluation suggested that a higher expression of calcitonin gene-related peptide (CGRP), a known osteogenic neuron peptide, in the fracture callus of the Mg/Ti group might be a major underlying mechanism of enhanced fracture healing attributed to the release of Mg ions during the degradation of Mg screws.

Biomechanical methods for the assessment of fracture repair

The progress of fracture healing is directly related to an increasing stiffness and strength of the healing fracture. Similarly the weight bearing capacity of a bone directly relates to the mechanical stability of the fracture. Therefore, assessing the progress of fracture repair can be based on the measurement of the mechanical stability of the healing fracture. However, fracture stability is difficult to assess directly due to various obstacles of which shielding of the mechanical properties by the fracture fixation construct is the most relevant one. Several assessment methods have been proposed to overcome these obstacles and to obtain some sort of mechanical surrogate describing the stability of the fracture. The most direct method is the measurement of the flexibility of a fracture under a given external load, which comprises the challenge of accurately measuring the deformation of the bone. Alternative approaches include the measurement of load share between implant and bone by internal or by external sensors. A direct 3 dimensional measurement of bone displacement is provided by radiostereometric analysis which can assess fracture migration and can detect fracture movement under load. More indirect mechanical methods induce cyclic perturbations within the bone and measure the response as a function of healing time. At lower frequencies the perturbations are induced in the form of vibration and at higher frequencies in the form of ultrasonic waves. Both methods provide surrogates for the mechanical properties at the fracture site. Although biomechanical properties of a healing fracture provide a direct and clinically relevant measure for fracture healing, their application will in the near future be limited to clinical studies or research settings.