Loads on an internal spinal fixation device during physical therapy

Modified internal spinal fixation devices allow the measurement of the forces and moments acting on the implants. The aim of this study was to measure the loads on internal fixation devices for selected body positions and movements during physical therapy.

Loads on an internal spinal fixation device were measured in 10 patients with degenerative instability or compression fractures using a telemeterized implant.

Relatively low implant loads were found in the recumbent body positions. Most exercises performed in a lying position caused implant loads less than that measured for standing and are therefore not likely to increase the risk of screw breakage. Fixation device loads were lower for sitting relaxed than for standing. The highest implant loads (128% of the value for standing) were measured during walking. Standing up, sitting down, and lateral bending and axial rotation of the upper body while standing led to fixation device loads between 111% and 120% related to the value for standing. Even higher fixation device loads were measured for ventral flexion and extension of the upper body while standing. Kneeling on hands and knees, and flexing and extending the back in this position, caused implant loads that were lower than for standing.

Standing up, sitting down, and lateral bending and axial rotation of the upper body while standing may slightly increase the risk of pedicle screw breakage, whereas ventral flexion and extension of the upper body while standing may increase this risk considerably if the region bridged by the implant is distracted (the distance between upper and lower screws was increased) during surgery. However, walking is the exercise that plays the major role concerning pedicle screw breakage because it causes the highest bending moments of all exercises studied and it loads the fixation devices most frequently.