Background: Little is known about the loads acting on a vertebral body replacement within the first month postoperatively. Among other things, such data are required for choosing the optimal rehabilitation program for a patient and for evaluating the efficacy of aids like crutches and braces.
Methods: Telemeterized vertebral body replacements were implanted into three patients, replacing parts of their fractured vertebral body L1. Six load components were measured for several exercises in upright and lying positions within the first postoperative month. The fractured vertebra was additionally stabilized with an internal spinal fixation device.
Findings: The highest force component acted in the direction defined by a line connecting the centres of the adjacent vertebral bodies. The resultant force was always less than 100 N in a lying position. Standing and sitting caused forces between about 150 and 450 N. Support with the arms or hands reduced the force considerably. Flexion of the upper body caused resultant forces higher than 420 N in all three patients. Elevation of both arms in the sagittal plane to about 90 degrees also led to high resultant forces, especially when carrying an additional weight in the hands or when working against the resistance of a physiotherapist. In the latter case, forces higher than 700 N were measured. The force direction was close to the axial direction of the treated spinal segment in upright body positions and varied only slightly for most exercises. The highest resultant bending and torsional moments measured were less than 4 Nm. For most exercises the resultant moment was below 2 Nm.
Interpretation: High loads act on a vertebral body replacement during several exercises already in the first month postoperatively. These activities should be avoided, especially in patients with osteoporotic vertebrae in order to prevent implant subsidence. Using e.g. a wheeled invalid walker or supporting the upper body by hands when sitting was found to reduce implant loads.