J. Tremblay

Biomechanical assessment of the stabilization capacity of monolithic spinal rods with different flexural stiffness and anchoring arrangement.

BACKGROUND Spinal disorders can be treated by several means including fusion surgery. Rigid posterior instrumentations are used to obtain the stability needed for fusion. However, the abrupt stiffness variation between the stabilized and intact segments leads to proximal junctional kyphosis. The concept of spinal rods with variable flexural stiffness is proposed to create a more gradual transition at the end of the instrumentation. METHOD Biomechanical tests were conducted on porcine spine segments (L1-L6) to assess the stabilization capacity of spinal rods with different flexural stiffness. Dual-rod fusion constructs containing three kinds of rods (Ti, Ti-Ni superelastic, and Ti-Ni half stiff-half superelastic) were implanted using two anchor arrangements: pedicle screws at all levels or pedicle screws at all levels except for upper instrumented vertebra in which case pedicle screws were replaced with transverse process hooks. Specimens were loaded in forward flexion, extension, and lateral bending before and after implantation of the fusion constructs. The effects of different rods on specimen stiffness, vertebra mobility, intradiscal pressures, and anchor forces were evaluated. FINDING The differences in rod properties had a moderate impact on the biomechanics of the instrumented spine when only pedicle screws were used. However, this effect was amplified when transverse process hooks were used as proximal anchors. INTERPRETATION Combining transverse hooks and softer (Ti-Ni superelastic and Ti-Ni half stiff-half superelastic) rods provided more motion at the upper instrumented level and applied less force on the anchors, potentially improving the load sharing capacity of the instrumentation.

In-vitro assessment of the stabilization capacity of monolithic spinal rods with variable flexural stiffness: Methodology and examples.

The concept of a monolithic Ti-Ni spinal rod with variable flexural stiffness is proposed to reduce the risks associated with spinal fusion. The variable stiffness is conferred to the rod using the Joule-heating local annealing technique. To assess the stabilization capacity of such a spinal rod, in vitro experiments on porcine spine models are carried out. This paper describes the methodology followed to evaluate the effect of Ti-Ni rods compared to conventional titanium rods. Validation of the methodology and examples of results obtained are also presented.

Factors affecting intradiscal pressure measurement during in vitro

ObjectivesTo assess the reliability of intradiscal pressure measurement during in vitro biomechanical testing. In particular, the variability of measurements will be assessed for repeated measures by considering the effect of specimens and of freezing/thawing cycles.MethodsThirty-six functional units from 8 porcine spines (S1: T7-T8, S2: T9-T10, S3: T12-T11, S4: T14-T13, S5: L1-L2 and S6: L3-L4) have been used. The intervertebral discs were measured to obtain the frontal and sagittal dimensions. These measurements helped locate the center of the disc where a modified catheter was positioned. A fiber optic pressure sensor (measuring range: -0.1 to 17 bar) (360HP, SAMBA Sensors, Sweden) was then inserted into the catheter. The specimens were divided into 3 groups: 1) fresh (F), 2) after one freeze/thaw cycle (C1) and 3) after 2 freeze/thaw cycles (C2). These groups were divided in two, depending on whether specimens were subjected to 400 N axial loading or not. Ten measurements (insertion of the sensor for a period of one minute, then removal) were taken for each case. Statistical analyses evaluated the influence of porcine specimen and the vertebral level using a MANOVA. The effect of repeated measurements was evaluated with ANOVA. The difference between freeze/thaw cycles were analysed with U Mann-Whitney test (P≤0.05).ResultsWithout axial loading, the F group showed 365 mbar intradiscal pressure, 473 mbar for the C1 group, and 391 mbar for the C2 group. With 400N axial load, the F group showed intradiscal pressure of 10610 mbar, the C1 group 10132 mbar, the C2 group 12074 mbar. The statistical analysis shows a significant influence of the porcine specimen (p<0.001), with or without axial loading and of the vertebral level with (p=0.048) and without load (p<0.001). The results were also significantly different between the freeze/thaw cycles, with (p<0.001) and without load (p=0.033). Repeated measurement (without load p = 0.82 and with p = 0.56) did not show significant influence.ConclusionsThe results tend to support that freezing/thawing cycles can affect intradiscal pressure measurement with significant inter-specimen variability. The use of the same specimen as its own control during in vitro biomechanical testing could be recommended.

Braided tubular superelastic cables provide improved spinal stability compared to multifilament sublaminar cables.

This study investigates the use of braided tubular superelastic cables, previously used for sternum closure following sternotomy, as sublaminar fixation method. It compares the biomechanical performance of spinal instrumentation fixation systems with regular sublaminar cables and proprietary superelastic cables. A hybrid experimental protocol was applied to six porcine L1–L4 spinal segments to compare multifilament sublaminar cables (Atlas, Medtronic Sofamor Danek, Memphis, TN) with proprietary superelastic cables. First, intact total range of motion was determined for all specimens using pure moment loading. Second, pure moments were imposed to the instrumented specimens until these intact total ranges of motion were reproduced. Compared to the intact specimens, the use of superelastic cables resulted in stiffer instrumented specimens than the use of multifilament cables for all the loading modes except axial torsion. Consequently, the superelastic cables limited the instrumented segments mobility more than the multifilament cables. Spinal instrumentation fixation systems using superelastic cables could be a good alternative to conventional sublaminar cables as it maintains a constant stabilization of the spine during loading.