Abstract
INTRODUCTION
It is generally accepted that strong hammering is necessary for the press fit fixation of a joint prosthesis. In this regard, large stress must remain within bone tissues for a long period. This residual stress is, however, some different from the feasible mechanical stimuli for bone tissues because that is stationary, continuous and directed from within outward unlike physiological conditions. The response on this residual stress, which may induce the disorder of the fixation of implant, has not been discussed, yet.
In the present study, we designed an experimental method to exert a stationary load from inside of a femur of a rat by inserting a loop spring made from a super elastic wire of titanium alloy. Response of the femur was assessed by bone morphology mainly about the migration of the wire into the bone twelve weeks after the implantation.
MATERIALS AND METHODS
We developed a method using a loop spring made of super elastic wire of titanium alloy, which can maintain sufficient magnitude of stress in a rat femur during the experimental period. The loop spring was fabricated with a wire of 0.4 mm diameter before the quenching process. Eleven Wistar rats of ten weeks old were used for the experiments. The loop spring was inserted the right femur, as shown in Figure 1. The left femur was remained intact. The compressive load was added from within outward of bone marrow when the spring was compressed with the insertion into a bone marrow of a rat femur, as shown in Figure 2. The average contact stress was calculated by dividing the elastic force by the spring and bone contact area. The contact stress was distributed from 62 to 94 MPa, which are sufficiently lower than the yield stress of cortical bone [1]. The assessment of bone morphology around the implanted loop spring was performed by micro-CT imaging after the twelve weeks of cage activity.
RESULTS
To assess the migration of the spring in the femur, we measured the distance from the endocortical surface to the periphery of the spring, on the micro CT image, as shown in Figure 3. Apparent migration of the spring wire was observed on nine specimens among the eleven. Deep migration over 0.3 mm was observed at three cases.
DISCUSSION
The migration of the wire in cortical bone was accompanied with the bone resorption on the surface of the wire toward outside. Therefore, the present findings suggest that stationary load at the implant surface can induce endosteal bone resorption and prosthesis dislocation and protrusion.
CONCLUSION
We developed a method for configuring a stationary stress field in a rat femur using a loop spring with the super elasticity. It was found that stationary stress about 70 MPa can induce bone resorption.
