RESPONSE OF BONE ON STATIONARY LOAD FROM THE INPLANT

Background Residual stress remains in bone tissues after press-fit-fixation of a joint prosthesis, recently employed for joint arthroplasty. The response of bone tissues to the residual stress is, however, unknown because it is not physiological. This unnatural stimulus may have adverse effects on bone tissues, including causing thigh pain or bone resorption. In the present study, we designed an experimental method to apply a stationary load from inside an animal femur using a loop spring of titanium alloy with super elasticity. The femoral response was assessed based on the migration of the wire into bone twelve weeks after implantation. As the results, wire migration was noted in 10 of 11 cases. Methods We developed a method using a loop spring made of super elastic titanium alloy, which can maintain sufficient stress in a rat femur for a prolonged period. This titanium alloy, which contains 43.94% titanium and 56.06% nickel, was supplied as a wire (WDL1, Actment Co., Ltd., Kasukabe, Japan). In the present study, an experimental method was designed to apply a stationary load from inside a rat femur by inserting a loop spring made of super elastic wire. Results Ten weeks after implantation, migration of the spring wire into the cortical bone was noted in 10 of 11 cases. To assess spring migration in cortical bone, we measured the distance from the endocortical surface to the tip of springs on micro-CT images. The line of the endocortical surface was extrapolated from the adjacent to the wire contact area. The estimated load was distributed from 1.19 to 3.28 N. The migration depth on anterior and posterior sides was not associated with the estimated load. Discussion In the present study, we developed a method of generating a stationary stress field in a rat femur using a loop spring made from Ni-Ti alloy with super ermore, implantation of the pin was presumed to be able to interpretation by the thelasticity. The load that originated from elastic deformation was large enough to apply mechanical stimulation to bone tissue. The estimated load was distributed from 1.19 to 3.28 N. Migration of the implanted loop spring in the femur was observed in ten of the eleven. The migration depth apparently did not increase with the increase in estimated load. Therefore, regardless of the load, that is stationary load is applied has been suggested to be involved in the migration of the pin. Furtheory of bone remodeling. n a state where the pin load of is applied, the bone implanted pin to which was destroyed by osteoclasts, was presumed to occur is bone formation in the implanted state. Interpretation The present findings suggest that an excessive stationary load at the implant surface induces endosteal bone resorption together with the migration or protrusion of a prosthesis.