INFLUENCE OF MUSCLE FORCES ON LOADS IN INTERNAL SPINAL FIXATION DEVICES

Study design. The loads acting on an internal spinal fixation device w ere measured in vivo. Objectives. To determine the influence of muscle forces on implant loads. Summary of Background Data. Only limited inf ormation exists regarding the loads acting on spinal implants in vivo. Though the muscles greatly influence spinal load, they have been negl ected in most studies. Methods. Telemeterized internal spinal fixation devices were used to study the influence of muscle forces on the impl ant loads in three patients before and after anterior interbody fusion . Results. Contracting abdominal or back muscles in a lying position w as found to significantly increase implant loads. Hanging by the hands from wall bars as well as balancing with the hands on parallel bars r educed the implant loads compared with standing; however, hanging by t he feet with the head upside down did not reduce implant loads compare d with lying in a supine position. When lying on an operating table wi th only the foot end lowered so that the hips were bent, the patient h ad different load measurements in the conscious and anesthetized state before anterior interbody fusion. The anesthetized patient evidenced predominately extension moments in both fixators, whereas flexion mome nts were observed in the right fixator of the conscious patient. After anterior interbody fusion had occurred, the differences in implant lo ads resulting from anesthesia were small. Conclusions. The muscles gre atly influence implant loads. They prevent an axial tensile load on th e spine when part of the body weight is pulling, e.g., when the patien t is hanging by his hands or feet. The implant loads may be strongly a ltered when the patient is under anesthesia.