A new bending stiffness measurement device to monitor the influence of different intramedullar implants during healing period.

To manage fractures in long bones, intramedullar implants, plates or external fixators are often used. In many cases, the implants are removed after bone consolidation. X-ray images are normally used to monitor bone formation and to determine the point of return to full load bearing and removal of the implants. However, plain radiographs give only inaccurate information about the degree of healing progress. Known quantitative methods as QCT, DEXA, etc. provide information about bone density which certainly contributes to the mechanical properties of healing bone, but they do not provide a direct measurement of the stiffness of the healing callus. In this study we present an in vivo 4-point-bending stiffness device for small animals which is designed to directly monitor the progression of the healing process. The device was tested in a bone-defect model with different test-specimens chosen to simulate the stiffness of bone at different stages of healing. To verify the results, it was tested in an animal fracture study in rabbits during the healing period with and without an intramedulary implant. Both the test-specimen and bones of the in vivo study were compared with data in a materials testing system (MTS) in four-point bending. The device was found to have a high precision and significant in vitro and in vivo correlation with the MTS. The results suggest that this measurement device has the ability to monitor the healing process of bone and to analyse the influence of degradable implants on the mechanical behaviour of bone or bone metabolism effecting pharmaceutics.