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.

The pull-out force of thoracic spinal pedicle hooks secured by long fixation screws engaging the posterior portion of the vertebral endplate was measured. The performance of these hooks was compared with that of hooks using a shorter screw and different screw orientation such that the vertebral endplates were not perforated. The longer and differently angulated screws, engaging the endplate, significantly enhanced the fixation potential of the hooks.

Pedicle hooks which are used as an anchorage for posterior spinal instrumentation may be subjected to considerable three-dimensional forces. In order to achieve stronger attachment to the implantation site, hooks using screws for additional fixation have been developed. The failure loads and mechanisms of three such devices have been experimentally determined on human thoracic vertebrae: the Universal Spine System (USS) pedicle hook with one screw, a prototype pedicle hook with two screws and the Cotrel-Dubousset (CD) pedicle hook with screw. The USS hooks use 3.2-mm self-tapping fixation screws which pass into the pedicle, whereas the CD hook is stabilised with a 3-mm set screw pressing against the superior part of the facet joint. A clinically established 5-mm pedicle screw was tested for comparison. A matched pair experimental design was implemented to evaluate these implants in constrained (series I) and rotationally unconstrained (series II) posterior pull-out tests. In the constrained tests the pedicle screw was the strongest implant, with an average pull-out force of 1650 N (SD 623 N). The prototype hook was comparable, with an average failure load of 1530 N (SD 414 N). The average pull-out force of the USS hook with one screw was 910 N (SD 243 N), not significantly different to the CD hook's average failure load of 740 N (SD 189 N). The result of the unconstrained tests were similar, with the prototype hook being the strongest device (average 1617 N, SD 652 N). However, in this series the difference in failure load between the USS hook with one screw and the CD hook was significant. Average failure loads of 792 N (SD 184 N) for the USS hook and 464 N (SD 279 N) for the CD hook were measured. A pedicular fracture in the plane of the fixation screw was the most common failure mode for USS hooks.(ABSTRACT TRUNCATED AT 250 WORDS)

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