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. 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.INTRODUCTION
MATERIALS AND METHODS
Surgical intervention is rarely indicated in the osteoporotic patient with compression fractures and kyphosis. In rare instances, the vertebral fracture is of the burst type, with spinal canal compromise and neurologic deficits, including paraplegia. These patients must be considered for surgical intervention. Reconstruction of such a spine poses technical challenges, because of concerns about adequacy of fixation and source of autogenous bone which is also osteoporotic. In addition, these patients frequently have serious medical conditions that increase the possibility of perioperative complications. Spinal shortening is a surgical procedure in which circumferential resection of vertebra is followed by closure of two adjacent vertebrae and fusion. It is mechanically more stable than augumentative spinal reconstruction and needs less bone graft. Eight spinal shortenings were performed in eight patients for the treatment of paralysis due to osteoporotic vertebral collapse. Patients are ranged from 68 to 83 (average 74 years). Affected vertebrae were L1 in four, Th12 in three and Th9 in one case. After bone resection of affected vertebra from posteriorly through transpedicular route, shortening and correction of kyphosis was performed. Osteotomy was fixed by long segment instrumentation and short segment bone graft with Hartshill rectangular rod, sublaminar wiring and laminectomized local bone. Paraparesis which was present before surgery disappeared and spinal stability was obtained. Bony union was observed after six months. Surgical complication was seen in one case with hepatisis. A massive bleeding necessitating clamp of drain tube saved her life in the expense of neurological deterioration. We now consider this patient was out of indication for spinal shortening. With the follow-ups ranged from 9 to 36 months (average 19 months), neural function was preserved. It was concluded that spinal shortening using instrumentation is a safe and effective procedure for the treatment of osteoporotic vertebral collapse with paralysis.
