The inverse effects of load transfer and load sharing on axial compressive stiffness

Biomechanical analysis of spinal implants with different rod diameters under static and fatigue loads: an experimental study

Spinal implants are commonly used in the treatment of spinal disorders or injuries. However, the biomechanical analyses of them are rarely investigated in terms of both biomechanical and clinical perspectives. Therefore, the main purpose of this study is to investigate the effects of rod diameter on the biomechanical behavior of spinal implants and to make a comparison among them. For this purpose, three spinal implants composed of pedicle screws, setscrews and rods, which were manufactured from Ti6Al4V, with diameters of 5.5 mm, 6 mm and 6.35 mm were used and a bilateral vertebrectomy model was applied to spinal systems. Then, the obtained spinal systems were tested under static tension-compression and fatigue (dynamic compression) conditions. Also, failure analyses were performed to investigate the fatigue behavior of spinal implants. After static tension-compression and fatigue tests, it was found that the yield loads, stiffness values, load carrying capacities and fatigue performances of spinal implants enhanced with increasing spinal rod diameter. In comparison to spinal implants with 5.5 mm rods, the fatigue limits of implants showed 13% and 33% improvements in spinal implants having 6 mm and 6.35 mm rods, respectively. The highest static and fatigue test results were obtained from spinal implants having 6.35 mm rods among the tested implants. Also, it was observed that the increasing yield load and stiffness values caused an increase in the fatigue limits of spinal implants.

Vertebral Body Hounsfield Units are Associated With Cage Subsidence After Transforaminal Lumbar Interbody Fusion With Unilateral Pedicle Screw Fixation

OBJECTIVE

To assess the association between Hounsfield units (HU) measurement and cage subsidence after lumbar interbody fusion.

BACKGROUND

Transforaminal lumbar interbody fusion (TLIF) with unilateral fixation becomes a popular treatment modality for lumbar degenerative disease. Cage subsidence is a potentially devastating complication after lumbar interbody fusion with unilateral fixation. Recently, a new technique for assessing bone mineral density using HU values from computed tomography has been proposed. Bone quality is believed to be one of the important factors that cause cage subsidence after TLIF.

MATERIALS AND METHODS

Cage subsidence after single-level (L4/5) TLIF with unilateral fixation was prospectively documented at a single institution between 2013 and 2014. Patients with cage subsidence were matched 1:1 to a control cohort without cage subsidence on the basis of age and sex. HU values were measured from the preoperative computed tomography. All patients received computed tomographic scans at a minimum of 6 months postoperatively. Sagittal images were evaluated for evidence of cage subsidence.

RESULTS

Eighteen patients with cage subsidence were well matched 1:1 to a cohort without cage subsidence and had complete imaging data. The global lumbar HU values were significantly lower in patients with cage subsidence than in the controls (112.4±10.08 vs. 140.2±10.17; P=0.0015). Similarly, a regional assessment of HU across the fusion levels was significantly lower in patients with cage subsidence (113.4±10.47 vs. 127.9±8.13; P=0.0075). The areas under the receiver operating characteristic cure were 0.715 and 0.636 for global and regional assessment, respectively. The best cut-offs for global and regional assessment were 132 (sensitivity: 83.3%; specificity: 61.1%) and 122 (sensitivity: 72.2%; specificity: 55.6%), respectively.

CONCLUSIONS

Lower preoperative HU values is associated with cage subsidence after TLIF with unilateral fixation. HU measurement may be used as a predictor of cage subsidence after unilateral fixation, which also should be incorporated in preoperative planning.

ISO 12189 standard for the preclinical evaluation of posterior spinal stabilization devices--II: A parametric comparative study

The International Standardization Organization (ISO) 12189 standard was recently introduced to preclinically evaluate and compare the mechanical properties of posterior stabilization devices. This scenario presents some new significant steps ahead over the vertebrectomy model recommended by American Society for Testing and Materials (ASTM) F1717 standard: the modular anterior support allows for describing a closer scenario to the effective clinical use as well as to test very flexible and dynamic posterior stabilization devices. Despite these significant advantages, ISO 12189 received little attention in the literature. Anatomical parameters depending on the spinal level were compared to the published data or original measurements on biplanar stereoradiography on 13 patients. Other mechanical variables, describing the test set-up design, were considered and all parameters were investigated using a numerical parametric finite element model. Stress values were calculated by also considering their worst-case combination. The standard set-up represents quite well the anatomy of an instrumented average thoracolumbar segment. The parametric comparative analysis demonstrates a significant (even beyond +350%) maximum increase in the stress on the device, compared to the standard currently in use. The anterior support stiffness plays the most detrimental effect (maximum stress increases up to 396%). The initial precompression step has an important role in determining the final stress values achieved at peak load (up to +76%). Moreover, when combining these two contributions, an even higher stress increase may be achieved (up to 473%). Despite the other anatomical parameters playing a secondary role, their worst-case combination demonstrates that a device could potentially undergo higher stresses than those reached according to standard suggestions (maximum increase of 22.4% at L1). Any user/designer should be aware of these effects when using ISO 12189 standard for the preclinical evaluation of posterior spinal stabilization devices.

ISO 12189 standard for the preclinical evaluation of posterior spinal stabilization devices--I: Assembly procedure and validation

The International Standardization Organization introduced standard 12189 for the preclinical evaluation of the mechanical reliability of posterior stabilization devices. The well-known vertebrectomy model formalized in standard F1717 by the American Society for Testing and Materials was modified with the introduction of a modular anterior support made up of three calibrated springs, which allows to describe a more realistic scenario, closer to the effective clinical use, as well to test even very flexible and dynamic posterior stabilization implants. Despite these important improvements, ISO 12189 received very little attention in the literature. The aim of the work is to provide a systematic procedure for the assembly and validation of a finite element model capable of describing the experimental test according to ISO 12189. The validated finite element model is able to catch very well the effective stiffness of the unassembled and assembled constructs (percentage differences <2% and <10%, respectively). As concern the assembled construct, the experimentally measured and predicted strains were found in a good agreement (R2 > 0.75, root mean square error < 30%), but the procedure without precompression lead to much better results (R2 > 0.96, root mean square error < 10%). Given the prediction errors of the assembled construct fall within the experimental range of repeatability, the finite element model can be systematically implemented to support the mechanical design of a variety of spinal implants, to quantitatively investigate the load-sharing mechanism, as well as to investigate the loading conditions set by ISO 12189 standard.