Biomechanical testing of three newly developed transpedicular multisegmental fixation systems

Abstract

A series of 216 biomechanical tests with 36 calf spines were performed to evaluate the rigidity of three newly developed prototypes of transpedicular fixation systems (Spine Fix, AO/ASIF prototype 1, AO/ASIF prototype 2) as compared to the already established Cotrel-Dubousset (CD) system. The Spine Fix system follows the same principle of spinal fixation as the CD system, while the two prototypes of the AO/ASIF group introduce a new concept of spinal reduction and fixation technique, using a three-dimensional adjustable fastening system of transpedicular screws to a longitudinal rod. This allows for correction and fixation of the instrumented vertebra segments in any position. During the tests the main point of interest was whether the newly gained degrees of freedom are associated with a loss of stiffness in the construct. Furthermore, the study evaluated whether transpedicular systems should be optimized from the technological point of view, or whether the stability and rigidity of these systems is determined mainly by the quality of pedicular anchorage. Load displacement was measured using a calf spine model with a precisely defined three-column lesion. Each implant was loaded up to 15 Nm in flexion, extension, lateral bending, and axial rotation. In all tests, the construct behaved in a highly linear fashion (r2> 0.94). By continously measuring the forces and moments at the cranial end of the spine specimen high accuracy of the tests was achieved (standard deviation: x-axis, 1.74%; y-axis, 1.36%; z-axis, 1.21%). In general, the stifness was found to be highest in lateral bending, followed by flexion/extension and axial rotation. Spine Fix was the stiffest implant in flexion/extension, AO/ASIF prototype 1 in lateral bending, and AO/ ASIF prototype 2 in rotation. In comparison to the CD system (stiffness of CD = 100%), differences in stiffness ranged from 77.3% prototype 1 to 140.8% Spine Fix in flexion, from 78.2% prototype 2b to 134.7% Spine Fix in extension, from 108.1% prototype 2b to 213.5% prototype 1 in lateral bending, and from 80.3% prototype 1 to 110.6% prototype 2 in axial rotation. The Spine Fix and prototype 2 systems showed equal or higher stiffness coefficients compared to the CD system. Prototype 1 is significantly more flexible, except in lateral bending, than the CD. From the technical point of view, the two AO/ ASIF prototypes allow the correction and fixation of an instrumented vertebra in any position. Prototype 2, despite the additional joint between transpedicular screws and longitudinal rods, shows stiffness comparable to that of the CD system.