Cyclic loads do not compromise functionality of the interspinous spacer or cause damage to the spinal segment: an in vitro analysis.

STUDY DESIGN: A biomechanical study to evaluate the effects of interspinous spacer under cyclic complex loading. OBJECTIVES: To determine the risk of device migration and to assess damage on the device and specimen under extreme coupled motion. Another objective was to evaluate the effect on vertebral foramen and canal dimensions after spacer implantation. SUMMARY OF BACKGROUND DATA: Interspinous spacers are a relatively new treatment option that are clinically exposed to complex loads. However, the biomechanical performance of these spacers has not been well characterized. METHODS: Six human cadaveric motion segments were used for this study. The interspinous spacer (SuperionTM, Vertiflex Inc, California) was tested for 5 degrees extension/10 degrees flexion coupled with an axial rotation of +/-3 degrees. CT images were taken for specimens in neutral, 5 degrees extension, and 10 degrees flexion before and after the implantation of the spacer. Vertebral foramen and canal dimensions were quantified. RESULTS: There was no device migration or subsidence. Specimens did not sustain any significant injury during testing. Canal area was minimally altered and foramen height, width, and area increased in extension and were statistically significant as compared to intact. CONCLUSION: Interspinous spacer effectively prevents the motion at the implanted level and does not change the anatomy significantly.

Pedicle Screw Configuration for Thoracolumbar Burst Fracture Treatment: Short versus Long Posterior Fixation Constructs with and without Anterior Column Augmentation.

STUDY DESIGN: An in-vitro study. PURPOSE: The current study is aimed at investigating the differences in stability between short posterior fixation (SPF), hybrid posterior fixation (HPF), and long posterior fixation (LPF) with and without anterior column augmentation using calcium phosphate bone cement (CaP) for treating burst fractures (BFs). OVERVIEW OF LITERATURE: The ideal treatment for thoracolumbar BF is controversial regarding the use of short or LPF constructs. METHODS: Seven human thoracolumbar spines (T9-L4) were tested on a six degree of freedom spine simulator in three physiologic planes, flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Tested surgical constructs included the following: intact, injury (BF), SPF (T12-L2), HPF (T11-L2), LPF (T11-L3), SPF+CaP, HPF+CaP, LPF+CaP, and CaP alone (CaP). Range of motion (ROM) was recorded at T12-L2 in FE, LB, and AR. RESULTS: THE REDUCTION IN MEAN ROM TRENDED AS FOLLOWS: LPF>HPF>SPF. Only LPF constructs and HPF with anterior column augmentation significantly reduced mean ROM in FE and LB compared to the intact state. All instrumented constructs (SPF, HPF, and LPF) significantly reduced ROM in FE and LB compared to the injured condition. Furthermore, the instrumented constructs did not provide significant rotational stability. Injecting CaP provided minimal additional stability. CONCLUSIONS: For the injury created, LPF and HPF provided better stability than SPF with and without anterior column augmentation. Therefore, highly unstable fractures may require extended, long or hybrid fusion constructs for optimum stability.

Biomechanical evaluation of a novel posterior integrated clamp that attaches to an existing posterior instrumentation for use in thoracolumbar revision.

STUDY DESIGN: An in vitro biomechanical study. PURPOSE: To evaluate the biomechanics of a novel posterior integrated clamp (IC) that extends on an already implanted construct in comparison to single long continuous bilateral pedicle screw (BPS) and rod stabilization system. OVERVIEW OF LITERATURE: Revision surgery in the thoracolumbar spine often necessitates further instrumentation following a failed previous back surgery. Stability of these reconstructed constructs is not known. METHODS: Six osteoligamentous T12-L5 calf spines were tested on a spine motion simulator in the following configurations: intact, four level constructs (T13-L4), three level constructs (L1-L4), and two level constructs (L2-L4), by varying the ratio between BPS and IC. A load control protocol of 8 Nm moments was applied at a rate of 1°/sec to establish the range of motion value for each construct in flexion-extension, lateral bending, and axial rotation. Statistical analysis was performed on raw data using repeated measures analysis of variance and significance was set at p<0.05. RESULTS: On an average, the reduction in motion for the four level continuous pedicle screw and rod construct (67%) was similar to those extended with integrated clamps (64%). Furthermore, for three level and two level constructs, no significant difference was observed between continuous pedicle screw constructs and those revised with the integrated clamps (regardless of the ratio between BPS and IC). CONCLUSIONS: The novel posterior IC showed equivalent biomechanical rigidity to continuous pedicle screw rod constructs in revision scenarios. Clinical studies on posterior rod adjunct systems are necessary to confirm these results.

In vitro biomechanical study to quantify range of motion, intradiscal pressure, and facet force of 3-level dynamic stabilization constructs with decreased stiffness.

STUDY DESIGN: An in vitro biomechanical study. OBJECTIVE: To perform in vitro biomechanical testing on a lumbar spine using a 6-degree-of-freedom machine. To compare the range of motion (ROM), intradiscal pressure, and facet force of different 3-level dynamic stabilization constructs with traditional rigid constructs. To determine the effect of decreasing the stiffness of the dynamic construct on the various parameters. SUMMARY OF BACKGROUND DATA: Dynamic stabilization systems are a surgical option that may minimize the development of adjacent segment disease. METHODS: Seven T12-S1 specimens were tested at ± 7.5 Nm in flexion-extension, lateral bending, and axial rotation. The testing sequence was (1) intact, (2) intact with facet sensors, (3) L3-S1 rigid (3R), (4) L3-L4 dynamic and L4-S1 rigid (1D-2R A), (5) L3-L5 dynamic and L5-S1 rigid (2D-1R A), and (6) L3-S1 dynamic (3D A). Constructs 1D-2R A, 2D-1R A, and 3D A were tested again with the specialized designs of B and C of decreased stiffness. ROM, intradiscal pressure, and facet force were measured. RESULTS: In all loading modes there was a trend of increasing motion with decreased stiffness. Significant differences were seen with more dynamic stabilization levels but no significance was seen with only decreasing the stiffness. 3R facet force at the caudal instrumented level significantly decreased compared with intact and dynamic stabilization constructs during axial rotation. CONCLUSION: Biomechanical testing resulted in a trend of increased ROM across instrumented levels as the stiffness was decreased. Dynamic stabilization increased the ROM across instrumented levels compared with rigid rods. These results suggest that decreasing the stiffness of the construct may lessen the probability of adjacent-level disease. Although the specialized devices are not commercially available, clinical data would be necessary for a clearer understanding of adjacent level effects and to confirm the in vitro biomechanical findings. LEVEL OF EVIDENCE: N/A.