Effects of high-frequency, low-magnitude mechanical stimulus on bone healing.

Recent studies have shown osteogenic effects of high-frequency mechanical stimuli. The purpose of this study was whether externally applied, high-frequency, low-magnitude interfragmentary movements affect the process of bone healing. In 12 sheep, a transverse osteotomy with a 3 mm gap was created in the right metatarsus and externally stabilized by a rigid circular fixator. External stimulation was performed in six sheep with the use of ground-based vibration. The sheep were standing with their hind limbs on a platform that produced vertical movements resulting in interfragmentary movements of approximately 0.02 mm magnitude at 20 Hz frequency. The other six sheep remained rigidly stabilized by external fixation during the 8-week study and served as a control group. Healing was assessed postmortem by densitometric and mechanical examinations. No significant differences were found between the two groups, although callus formation was slightly enhanced (11%) in the stimulated group compared with the control group. Mechanical stimuli attributable to weightbearing in the control group were sufficient enough to initiate callus formation even under rigid, external fixation. Thus, external mechanical stimulation with the stimulation design described in the current study might not be indicated for improvement of bone healing.

Analysis of inter-fragmentary movement as a function of musculoskeletal loading conditions in sheep.

It is well accepted that inter-fragmentary movement influences the fracture healing process. Small axial movement can stimulate callus formation whereas larger shear movement delays the healing process. It is, therefore, essential for optimal fracture healing to minimize shear and to control axial movement. Unfortunately, the complex gap movements are mostly unknown under the large variety of clinical as well as experimental conditions of fracture fixation. To further understand the complex interactions of musculoskeletal loading and inter-fragmentary movements in bones and to reduce the need for animal experiments, a three-dimensional (3D) musculoskeletal model of the left hind limb of a sheep was developed. From 3D ground reaction forces and inverse dynamics, resultant joint loading was determined over a gait cycle. Muscle and joint contact forces were derived from an optimization routine and internal loads in the tibia and metatarsus from beam theory. Finally, inter-fragmentary movements were calculated from the bony loading condition and experimentally determined stiffness matrices of monolateral AISF external fixator constructs. Both the joint contact forces at the hip and gap movement of a mid-shaft tibial fracture agree with in vivo data reported in the literature. The bones proved to be mainly axially loaded with slightly increasing shear forces toward their ends. The results suggest that inter-fragmentary movement of metatarsal fractures is fairly independent of the fracture location whereas the movement increases in proximal tibial fractures compared to those in the distal and diaphyseal tibia. Considerable shear movement was found for all locations and external fixator mountings. However, shear movement could be minimized with a cranio-lateral rather than a cranio-medial shift from the cranial fixator plane.