The external spinal fixator does not reduce anterior column motion under axial compressive loads. A mechanical in vitro study.

We performed an in vitro study to investigate the effect of external spinal fixation on anterior column motion under physiological axial compression loading. The AO external spinal fixator (ESF) was applied to 5 human cadaveric lumbar spine specimens (L3-S1) at levels L4 to S1. All specimens were tested in 4 configurations: i) intact, ii) ESF in a neutral position, iii) ESF in distraction (12 mm), and iv) ESF in compression (8-12 mm). Cyclic sinusoidal axial compressive loads from 60 to 600 N were applied for 10 cycles in each test condition. The axial displacement of the load application point was recorded as an indicator of anterior column axial translation. The axial motion with the fixator in distraction was significantly greater than all other conditions, including intact. Compression of the fixator resulted in the least axial displacement. External fixation in the neutral position did not significantly affect the overall axial translation, when compared to the intact state. In conclusion, the external spinal fixator did not significantly reduce anterior column axial translation and, in distraction, this motion exceeded that of the intact specimen. Since pain relief is frequently observed during distraction of the painful segment/s with the external fixator, the mechanical basis of the pain relief is not well understood.