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

Abstract

IntroductionDuring the last 40 years, surgical correction techniques applied to spinal deformity have progressed radically. Harrington [8], Luque [14], and then Cotrel and Dubousset [6] added successive contributions to these advances using different approaches. Harrington [8] chose a simple distraction, correcting deformation essentially in the frontal plane. Luque [14] obtained this same distraction by translating the spine toward the rods, which were fixed by laminar wires. Cotrel and Dubousset [6] introduced a threedimensional correction and fixation system using open hooks and rods. During the last few years, developments Abstract Disadvantages of thoracic posterior implants and developments in rod contouring in situ led to the design of a new spinal implant: the bipedicular spinal fixation device (BSF). The BSF is composed of two bifid hooks linked by a compression transverse connector and inserted into the costo-vertebral and costotransverse joints. The aim of this biomechanical study was to determine the loading tolerance of the BSF. Three strength tests -a pull-out test, a lateral load-to-failure test and a uniaxial transversal compression test to failure -were performed using six human thoracic spines on an Instron testing device. Specimen evaluation consisted of: bone mineral density (BMD) measurement with the dual-energy X-ray absorptiometry (DEXA) technique, cortical thickness measurements and a morphometric study. The mean values for load-to-failure in the posterior and lateral tests were 324 N and 400 N respectively. The mean value of the uniaxial compression was 988 N. The mean BMD estimated by DEXA was 0.557 g/cm 2 . The BSF loading tolerance was compatible with the in situ rod contouring technique requirements when we considered posterior and lateral pull-out tests. The transversal compression test determined the appropriate and efficient BSF tightening force.

Evaluation of pullout strength and failure mechanism of posterior instrumentation in normal and osteopenic thoracic vertebrae

“…It has been assumed that failure strength is determined not only by BMD and implant type, but also by pedicle strength, cortical thickness, and pedicle dimensions. 1 In Group B (normal bone quality) failure of the hook claws occurred with significantly less force (p = 0.02) compared with the other systems. These findings are consistent with the observations of Hackenberg et al 16 who noted that pedicle screws provided superior stability compared with hooks in thoracic spines with BMD > 100 mg/ml.…”

“…Nevertheless, there is limited data on thoracic instrumentation comparing various fixation systems and the effect of their performance related to BMD on fixation strength. [1][2][3][4]6,7,23 One of the most accepted methods to evaluate the strength of the implant-bone interface has been the pullout test because it is simple, reproducible, and allows for direct comparisons. [2][3][4]6,12,13,16,19,27,32,36,37,40,43 The aim of this study was to investigate the effect of BMD on the axial pullout strength of 4 widely used posterior thoracic fixation systems: pedicle screws, laminar hooks, sublaminar wires, and pedicle screws augmented with sublaminar wires.…”

ASTM F1717 standard for the preclinical evaluation of posterior spinal fixators: Can we improve it?

“…The description of the geometrical features of the entire experimental set-up was originally based on unpublished measurements taken on two-level constructs cited in a previous experimental work. 11 Even if many authors 20–53 reported a great amount of data describing the morphometric/anatomical features of the VBs (Table 1), only Chaynes et al 35 investigated the geometrical relationship describing a complete functional spine unit (FSU) in relation to pedicle screw fixation. These parameters determine the final geometrical configuration of a spinal fixator and it would be very useful to determine the features of the experimental set-up closer to the clinical use, in order to build up a preclinical experimental set-up really representative of the clinical condition.…”

A pedicle screw system and a lamina hook system provide similar primary and long-term stability: a biomechanical in vitro study with quasi-static and dynamic loading conditions

“…Most experiments are done with pull-out test to investigate the fixation strength of single pedicle screws [4,[11][12][13][14][15][16][17][18][19][20][21] or other posterior fixation systems [16,20,22]. Sometimes screws are also tested with so-called toggling tests where cyclic bending moments are used [23][24][25][26][27].…”