New means in spinal pedicle hook fixation

Abstract

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)

Biomechanical evaluation of a bipedicular spinal fixation device: three different strength tests

“…Nevertheless, the measurement analyses showed differences among all the studies. Overall, our measurements were similar to other studies on fresh specimens [2,16], but at the upper limits. This observation could probably be explained by our having only male adult cadavers and using fresh cadaver segments without washing, boiling or chemical treatment.…”

“…Several studies have been performed to investigate the pull-out force of different screws and hooks in thoracic and thoraco-lumbar spine [4,5,18,19], but relatively few authors limited their studies to the thoracic spine [2,7,9]. Berlemann et al [2] tested: -The Universal Spine System (USS) pedicle hook with one screw -A prototype pedicle hook with two screws -The Cotrel-Dubousset (CD) pedicle hook with a screw 5.5 mm long and 3 mm in diameter, and -A pedicle screw 5-mm USS in nine thoracic cadaver spines (T2-T12) with a mean age at time of death of 55.8 years. The mean values for the constrained pull-out strengths were 910 N, 1530 N, 740 N and 1650 N respectively for the four implants.…”

“…Many authors [2,4,5,7,9,19] have shown a linear correlation between BMD estimated by the usual methods (DEXA or CT) and pull-out tests with pedicular screws in thoracic or thoraco-lumbar spines. In contrast, hook pullout forces seem to be independent of the BMD [2,5]. In strength tests on human cadaveric spines, spinal BMD is probably one of the more important predictive factors contributing to the pedicular screw loading tolerance.…”

“…The mean age at time of death (67.8±8.4 years) and the BMD values of our specimens were far from those of the young scoliotic population for whom this fixation device was primarily intended, but this is a common criticism of in vitro biomechanical studies. Berlemann et al [1] found the mean BMD estimated by DEXA to be between 0.643 and 0.988 g/cm 2 . In a thoraco-lumbar comparative study, Breeze et al [4] found the mean BMD estimated by DEXA to be between 0.297±0.103 g/cm 2 (bicortical screws) and 0.308±0.113 g/cm 2 (unicortical screws).…”

A biomechanical analysis of the self-retaining pedicle hook device in posterior spinal fixation

“…However, these instruments disturb the surgical field and tend to dislodge the hook as a result of the long lever arm. Augmentation of the pedicle hook to the lamina and pedicle by a small screw has been proposed for pedicle hook fixation [1,2,[5][6][7]. These special pedicle hooks solve these disadvantages of regular hooks partially, however, fixation of a small screw in the same pedicle that is used for the pedicle hook fixation weakens the pedicle and increases the risk of pedicle fracture [6,10].…”