Biomechanical evaluation of a bipedicular spinal fixation system: a comparative stiffness test

Increasing loads and diminishing returns: a biomechanical study of direct vertebral rotation

STUDY DESIGN

Biomechanical simulation of DVR and pure-moment testing on thoracic spines.

OBJECTIVES

Characterize load-deformation response of thoracic spines under DVR maneuvers until failure, and compare to pure-moment testing of same spines. Despite reports of surgical complications, few studies exist on increase in ROM under DVR torque. Biomechanical models predicting increases from surgical releases have consistently used "pure-moments", a standard established for non-destructive measurement of ROM. Yet, DVR torque is not accurately modeled using pure moments and, moreover, magnitudes of torque applied during DVR maneuvers may be substantially higher than pure-moment testing.

METHODS

Cadaveric thoracic spines (N = 11) were imaged, then prepared. Polyaxial pedicle screws were implanted at T7-T10 after surgical releases. Bilateral facetectomies and Ponte osteotomies were completed at T10-T11. A custom apparatus, mounted into an 8-dof MTS load frame, was used to attach to pedicle screws, allowing simulation of surgical DVR maneuvers. Motions of vertebrae were measured using optical motion tracking. Torque was increased until rupture of the T10-T11 disc or fracture at the pedicle screw sites at any level. The torque-rotation behavior was compared to its behavior under pure-moment testing performed prior to the DVR maneuver.

RESULTS

Under DVR maneuvers, failure of the T10-T11 discs accompanied in most cases by pedicle screw loosening, occurred at 13.7-54.7 Nm torque, increasing axial rotation by 1.4°-8.9°. In contrast, pure-moment testing (4 Nm) increased axial rotation by only 0.0°-0.9°.

CONCLUSIONS

DVR resulted in substantially greater correction potential increases compared to pure-moment testing even at the same torque. These results suggest increased flexibility obtained by osteotomies and facetectomies is underestimated using pure-moment testing, misrepresenting clinical expectations. The present study is an important and necessary step toward the establishment of a more accurate and ultimately surgically applied model.

LEVEL OF EVIDENCE

III.

Biomechanical cadaver study of proximal fixation in a minimally invasive bipolar construct

STUDY DESIGN

Biomechanical human cadaver study.

OBJECTIVE

To determine the three-dimensional intervertebral ranges of motion (ROMs) of intact and hook-instrumented thoracic spine specimens subjected to physiological loads, using an in vitro experimental protocol with EOS biplane radiography. Pedicle screws are commonly used in thoracic instrumentation constructs, and their biomechanical properties have been widely studied. Promising clinical results have been reported using a T1-T5 thoracic hook-claw construct for proximal rod anchoring. Instrumentation stability is a crucial factor in minimizing mechanical complications rates but had not been assessed for this construct in a biomechanical study.

METHODS

Six fresh-frozen human cadaver C6-T7 thoracic spines were studied. The first thoracic vertebrae were instrumented using two claws of supra-laminar and pedicle hooks, each fixed on two adjacent vertebrae, on either side of a single free vertebra. Quasi-static pure-moment loads up to 5 Nm were applied to each specimen before and after instrumentation, in flexion-extension, right and left bending, and axial rotation. Five steel beads impacted in each vertebra allowed 3D tracking of vertebral movements on EOS biplanar radiographs acquired after each loading step. The relative ranges of motion (ROMs) of each pair of vertebras were computed.

RESULTS

Mean ROMs with the intact specimens were 17° in flexion-extension, 27.9° in lateral bending, and 29.5° in axial rotation. Corresponding values with the instrumented specimens were 0.9°, 2.6°, and 7.3°, respectively. Instrumentation significantly (P < 0.05) decreased flexion-extension (by 92-98%), lateral bending (by 87-96%), and axial rotation (by 68-84%).

CONCLUSION

This study establishes the biomechanical stability of a double claw-hook construct in the upper thoracic spine, which may well explain the low mechanical complication rate in previous clinical studies.

LEVEL OF EVIDENCE

Not applicable, experimental cadaver study.

Challenging the Conventional Standard for Thoracic Spine Range of Motion: A Systematic Review

BACKGROUND

Segmental motion is a fundamental characteristic of the thoracic spine; however, studies of segmental ranges of motion have not been summarized or analyzed. The purpose of the present study was to present a summary of the literature on intact cadaveric thoracic spine segmental range of motion in each anatomical plane.

METHODS

A systematic MEDLINE search was performed with use of the terms "thoracic spine," "motion," and "cadaver." Reports that included data on the range of motion of intact thoracic human cadaveric spines were included. Independent variables included experimental details (e.g., specimen age), type of loading (e.g., pure moments), and applied moment. Dependent variables included the ranges of motion in flexion-extension, lateral bending, and axial rotation.

RESULTS

Thirty-three unique articles were identified and included. Twenty-three applied pure moments to thoracic spine specimens, with applied moments ranging from 1.5 to 8 Nm. Estimated segmental range of motion pooled means ranged from 1.9° to 3.8° in flexion-extension, from 2.1° to 4.4° in lateral bending, and from 2.4° to 5.2° in axial rotation. The sums of the range of motion pooled means (T1 to T12) were 28° in flexion-extension, 36° in lateral bending, and 45° in axial rotation.

CONCLUSIONS

The pooled ranges of motion were similar to reported in vivo motions but were considerably smaller in magnitude than the frequently referenced values reported prior to the widespread use of biomechanical testing standards. Improved reporting of biomechanical testing methods, as well as specimen health, may be beneficial for improving on these estimations of segmental cadaveric thoracic spine range of motion.

Flexibility of thoracic spines under simultaneous multi-planar loading

PURPOSE

The corrective potential of two posterior-only destabilization procedures for scoliosis deformity was quantified under single and multi-planar loading using cadaveric spines.

METHODS

Ten full-length human cadaveric thoracic spines were mounted in an 8-df servohydraulic load frame. Cyclic, pure moments were applied in: (1) flexion-extension, (2) lateral bending, (3) axial rotation, (4) flexion-extension with axial rotation, and (5) lateral bending with axial rotation at 0.5°/s, to ±4 Nm. Each specimen was tested intact, and again after nine en bloc bilateral total facetectomies, and one, two, three, and four levels of Ponte osteotomies. Motion was measured throughout loading using optical motion tracking.

RESULTS

Under single-plane loading, facetectomies and Ponte osteotomies increased thoracic spine flexibility in all three planes. Compared to total facetectomies, higher per-level increases were seen following Ponte osteotomies, with increases in total range of motion (total ROM) of up to 2.7° in flexion-extension, 1.4° in lateral bending, and 3.1° in axial rotation following each osteotomy. Compared to the facetectomies, four supplemental osteotomies increased total ROM by 23 % in flexion (p < 0.01) and 8 % in axial rotation (p < 0.01). Increases in lateral bending were smaller. Under multi-planar loading, each Ponte osteotomy provided simultaneous increases of up to 1.4°, 1.6°, and 2.2° in flexion-extension, lateral bending, and axial rotation.

CONCLUSIONS

Ponte osteotomies provided higher per-level increases in ROM under single-plane loading than total facetectomies alone. Further, Ponte osteotomies provided simultaneous increase in all three planes under multi-planar loading. These results indicated that, to predict the correction potential of a surgical release, multi-planar testing may be necessary.

Correlations between short-time Fourier- and continuous wavelet transforms in the analysis of localized back and hip muscle fatigue during isometric contractions

The aims of the current study were to examine the stationarities of surface electromyographic (EMG) signals obtained from eight bilateral back and hip muscles during a modified Biering-Sørensen test, and to investigate whether short-time Fourier (STFT) and continuous wavelet transforms (CWT) provided similar information with regard to EMG spectral parameters in the analysis of localized muscle fatigue. Twenty healthy subjects participated in the study after giving their informed consent. Reverse arrangement tests showed that 91.6% of the EMG signal epochs demonstrated no significant trends (all p>0.05), meaning 91.6% of the EMG signal epochs could be considered as stationary signals. Pearson correlation coefficients showed that STFT and CWT in general provide similar information with respect to the EMG spectral variables during isometric back extensions, and as a consequence STFT can still be used.

Mechanical testing of a smart spinal implant locking mechanism based on nickel-titanium alloy

STUDY DESIGN

Development and testing of a new spinal implant-locking mechanism based on the special properties of nickel-titanium alloy.

OBJECTIVE

To develop a new self-tightening locking mechanism to reduce fretting corrosion at implant junctions.

SUMMARY OF BACKGROUND DATA

All current implant locking involves tightening of a nut against the rod and screw head to form a coupling. Particulate debris is generated, and the coupling becomes loose because of wear between the rod and locking mechanism (fretting). To avoid this fretting, a new locking mechanism with an automatic retightening effect based on the superelastic and shape-memory properties of nickel-titanium alloy has been developed.

METHOD

The new coupling made of nickel-titanium alloy will tightly lock the rod when temperature increases to 50 degrees C (shape-memory effect). If fretting occurs, the coupling will further tighten itself around the rod (superelastic effect). This new coupling is mechanically tested against 4 current implant couplings.

RESULTS

In axial compression, conventional couplings failed between 570 and 740 N, while the new coupling reached 800 N without loosening. In axial rotation, conventional devices failed between 1.8 and 5.3 Nm, while the new coupling reached 6.5 Nm without failure. During testing, the retightening effect could be seen on the force versus displacement plot.

CONCLUSIONS

To our knowledge, the self-tightening coupling is a new concept not previously described and is attributable to the superior superelastic effect of the new coupling. This implant coupling has the potential to be used as a very low profile system and also in nonfusion technologies in which demands on the coupling would not higher without the protection of spinal fusion.

Statistical modelling of fatigue-related electromyographic median frequency characteristics of back and hip muscles during a standardized isometric back extension test

The purpose of the present study was to evaluate which statistical model - linear, logarithmic, quadratic or exponential - best described the fatigue-related electromyographic (EMG) changes of back and hip muscles. Twenty healthy volunteers performed a modified Biering-Sorensen test. The EMG activity of the latissimus dorsi (LD), longissimus thoracis pars thoracis (LTT) and lumborum (LTL), iliocostalis lumborum pars thoracis (ILT) and lumborum (ILL), multifidus (MF), gluteus maximus (GM) and biceps femoris (BF) was measured bilaterally using surface electrodes. Higher R(2) values were found for the quadratic models (p<0.05 for all muscles), and lower R(2) values for the logarithmic models (p<0.05 for LTT, LTL, ILL, MF and GM). The exponential models generated higher R(2) values compared to the linear ones for the LTT, LTL and MF (all p<0.05). Further analyses revealed, however, that these models did not add useful additional information, and therefore would only increase the complexity. The findings of the current study validate the use of simple linear regression techniques when studying fatigue-related EMG median frequency characteristics of back and hip muscles during isometric contractions.

Surgical correction of scoliosis by in situ contouring: a detorsion analysis

STUDY DESIGN

A detorsion analysis of the scoliosis surgical correction by means of in situ contouring technique (ISC).

OBJECTIVE

To describe the technique of ISC. To measure the vertebral and intervertebral axial rotation in thoracic and lumbar curves and their correction obtained by ISC.

SUMMARY AND BACKGROUND DATA

The vertebral and intervertebral axial rotation allows to evaluate the severity of the curves. However, the intervertebral axial rotation is barely studied and the vertebral axial rotation is a controversial point of the surgical correction.

METHODS

Twenty patients with thoracic and lumbar scoliosis were operated on with ISC. Vertebral axial rotation at the apex and the sum of intervertebral axial rotations all along the curve were computed before and after surgery from the three-dimensional stereoradiographic reconstruction of the spine and the pelvis. All the measurements were made in the standing position.

RESULTS

Correction of the axial rotation was obtained at the apex of both thoracic and lumbar curves of idiopathic and degenerative scoliosis. The mean values of correction (in terms of axial rotation) were 8 degrees to 19 degrees (62%-67%). The percentage of correction of the sum of intervertebral axial rotations all along the curve, proposed as a "detorsion index" (preoperative - postoperative/preoperative), was found at 57% to 92%. No significant differences were found for the correction (in terms of axial rotation and detorsion) between idiopathic and degenerative curves.

CONCLUSIONS

The axial rotation was measured in clinics on standing patients with scoliosis from three-dimensional stereoradiographic reconstruction and demonstrated a reliable detorsion obtained by ISC.

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

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 costo-transverse 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.