Loads on a telemeterized vertebral body replacement measured in two patients

Study Design. The loads acting on a vertebral body replacement ( VBR) were measured in vivo. Objective. To measure the implant loads for different activities within the first 6 months after surgery. Summary of Background Data. Mathematical models exist for predicting spinal loads for various activities. The intradiscal pressure has been measured in vivo for many activities. Loads on internal spinal fixation devices have been measured in 10 patients. However, only little information exists regarding the loads acting on a VBR. Methods. Telemeterized VBRs were implanted into 2 patients with a fractured L1 vertebral body. The implant allows the in vivo measurement of 3 force and 3 moment components acting on the implant. For several activities, implant loads were measured in the first 6 months after surgery. Results. One month after surgery, the resultant force during standing was about 270 N in 1 patient and 300 N in the other. When the patients were lying in relaxed positions, resultant forces were less than 30% of the values during standing. In one patient, implant loads were slightly lower during sitting than during standing whereas in the other patient higher loads were measured during sitting. In both patients, flexion of the upper body and walking upstairs caused implant loads, which were more than twice as high as those during standing. Force direction varied only slightly for forces higher than 100 N. Conclusion. High forces may act on a VBR especially in the first postoperative month. Flexion of the upper body and going upstairs cause high implant loads and should be avoided in the first few months after stabilizing the spine.