The design of a fracture movement transducer
The literature on bone growth, fracture healing, fracture treatment and the effects of forces and movements on these is reviewed. Some methods of assessing the progress of fracture union are considered. The conclusion from this is that a fracture movement transducer is needed for use on an external fracture fixator, and the design criteria are outlined. The possible types of transducers are considered and a system of light falling on lateral effect photodetectors was used. The linearizing algorithms applicable to the employed detectors are discussed and tested. The electronic circuits were designed and revised, initially to produce a reasonable power consumption, and then to give a system with reduced noise levels and improved linearity. Isolation amplifiers were required to prevent leakage currents reaching the patient from the mains powered computer. The algorithms relating the movements of the transducer to the movements at the fracture site were derived, including the use of elasticity theory to calculate the bending of the fixator pins. The engineering descriptions of the patient tests and the methods of calculating the fracture stiffness are considered. The transducer was calibrated, and consideration was given to the effect of the fixator pins loosening in the bone. Seven patients, all with tibial fractures, six injured in road traffic accidents and one footballing injury, were tested and their clinical histories were reviewed. Comparison was made between the results from the displacement transducer and a strain gauge transducer, related to the clinical history. It is concluded that this transducer, although useful, has more relevance as a research than as a clinical tool, but that further tests would lead to increased further understanding of fracture healing.