In vitro biomechanical analysis of three anterior thoracolumbar implants.

OBJECT The goal of this study was to evaluate the comparative efficacy of three commonly used anterior thoracolumbar implants: the anterior thoracolumbar locking plate (ATLP), the smooth-rod Kaneda (SRK), and the Z-plate. METHODS In vitro testing was performed using the T9-L3 segments of human cadaver spines. An L-1 corpectomy was performed, and stabilization was achieved using one of three anterior devices: the ATLP in nine spines, the SRK in 10, and the Z-plate in 10. Specimens were load tested with 1.5-, 3-, 4.5-, and 6-Nm in flexion and extension, right and left lateral bending, and right and left axial rotation. Angular motion was monitored using two video cameras that tracked light-emitting diodes attached to the vertebral bodies. Testing was performed in the intact state in spines stabilized with one of the three aforementioned devices after the devices had been fatigued to 5000 cycles at +/- 3 Nm and after bilateral facetectomy. There was no difference in the stability of the intact spines with use of the three devices. There were no differences between the SRK- and Z-plate-instrumented spines in any state. In extension testing, the mean angular rotation (+/- standard deviation) of spines instrumented with the SRK (4.7 +/- 3.2 degrees) and Z-plate devices (3.3 +/- 2.3 degrees) was more rigid than that observed in the ATLP-stabilized spines (9 +/- 4.8 degrees). In flexion testing after induction of fatigue, however, only the SRK (4.2 +/- 3.2 degrees) was stiffer than the ATLP (8.9 +/- 4.9 degrees). Also, in extension postfatigue, only the SRK (2.4 +/- 3.4 degrees) provided more rigid fixation than the ATLP (6.4 +/- 2.9 degrees). All three devices were equally unstable after bilateral facetectomy. The SRK and Z-plate anterior thoracolumbar implants were both more rigid than the ATLP, and of the former two the SRK was stiffer. CONCLUSIONS The authors' results suggest that in cases in which profile and ease of application are not of paramount importance, the SRK has an advantage over the other two tested implants in achieving rigid fixation immediately postoperatively.

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