Daniel J. Cook

Reliability of computer-assisted lumbar intervertebral measurements using a novel vertebral motion analysis system

BACKGROUND CONTEXTnTraditional methods for the evaluation of in vivo spine kinematics introduce significant measurement variability. Digital videofluoroscopic techniques coupled with computer-assisted measurements have been shown to reduce such error, as well as provide detailed information about spinal motion otherwise unobtainable by standard roentgenograms. Studies have evaluated the precision of computer-assisted fluoroscopic measurements; however, a formal clinical evaluation and comparison with manual methods is unavailable. Further, it is essential to establish reliability of novel measurements systems compared with standard techniques.nnnPURPOSEnTo determine the repeatability and reproducibility of sagittal lumbar intervertebral measurements using a new system for the evaluation of lumbar spine motion.nnnSTUDY DESIGNnReliability evaluation of digitized manual versus computer-assisted measurements of the lumbar spine using motion sequences from a videofluoroscopic technique.nnnPATIENT SAMPLEnA total of 205 intervertebral levels from 61 patients were retrospectively evaluated in this study.nnnOUTCOME MEASURESnCoefficient of repeatability (CR), limits of agreement (LOA), intraclass correlation coefficient (ICC; type 3,1), and standard error of measurement.nnnMETHODSnIntervertebral rotations and translations (IVR and IVT) were each measured twice by three physicians using the KineGraph vertebral motion analysis (VMA) system and twice by three different physicians using a digitized manual technique. Each observer evaluated all images independently. Intra- and interobserver statistics were compiled based on the methods of Bland-Altman (CR, LOA) and Shrout-Fleiss (ICC, standard error of measurement).nnnRESULTSnThe VMA measurements demonstrated substantially more precision compared with the manual technique. Intraobserver measurements were the most reliable, with a CR of 1.53 (manual, 8.28) for IVR, and 2.20 (manual, 11.75) for IVT. The least reliable measurements were interobserver IVR and IVT, with a CR of 2.15 (manual, 9.88) and 3.90 (manual, 12.43), respectively. The ICCs and standard error results followed the same pattern.nnnCONCLUSIONSnThe VMA system markedly reduced variability of lumbar intervertebral measurements compared with a digitized manual analysis. Further, computer-assisted fluoroscopic imaging techniques demonstrate precision within the range of computer-assisted X-ray analysis techniques.

Anterior lumbar interbody fusion with integrated fixation and adjunctive posterior stabilization: A comparative biomechanical analysis

BACKGROUNDnInterbody fusion cages with integrated fixation components have become of interest due to their ability to provide enhanced post-operative stability and mitigate device migration. A recently approved anterior lumbar interbody fusion cage with integrated fixation anchors has yet to be compared in vitro to a standard polyetheretherketone cage when used in combination with an interspinous process clamp.nnnMETHODSnTwelve human cadaveric lumbar segments were implanted at L4-L5 with a Solus interbody cage (n=6) or standard polyetheretherketone cage (n=6) following Intact testing and discectomy. Each cage was subsequently evaluated in all primary modes of loading after supplementation with the following posterior constructs: interspinous process clamp, bilateral transfacet screws, unilateral transfacet screw with contralateral pedicle screws, and bilateral pedicle screws. Range of motion results were normalized to Intact, and a two-way mixed analysis of variance was utilized to detect statistical differences.nnnFINDINGSnThe Solus cage in combination with all posterior constructs provided significant fixation compared to Intact in all loading conditions. The polyetheretherketone cage also provided significant fixation when combined with all screw based treatments, however when used with the interspinous process clamp a significant reduction was not observed in lateral bending or axial torsion.nnnINTERPRETATIONnInterbody cages with integrated fixation components enhance post-operative stability within the intervertebral space, thus affording clinicians the potential to utilize less invasive methods of posterior stabilization when seeking circumferential fusion. Interspinous process clamps, in particular, may reduce peri-operative and post-operative comorbidities compared to screw based constructs. Further study is necessary to corroborate their effectiveness in vivo.

Variability of manual lumbar spine segmentation.

Background The application of kinematic data acquired during biomechanical testing to specimen-specific, three-dimensional models of the spine has emerged as a useful tool in spine biomechanics research. However, the development of these models is subject to segmentation error because of complex morphology and pathologic changes of the spine. This error has not been previously characterized. Methods Eight cadaveric lumbar spines were prepared and underwent computed tomography (CT) scanning. After disarticulation and soft-tissue removal, 5 individual vertebrae from these specimens were scanned a second time. The CT images of the full lumbar specimens were segmented twice each by 2 operators, and the images of the individual vertebrae with soft tissue removed were segmented as well. The solid models derived from these differing segmentation sessions were registered, and the distribution of distances between nearest neighboring points was calculated to evaluate the accuracy and precision of the segmentation technique. Results Manual segmentation yielded root-mean-square errors below 0.39 mm for accuracy, 0.33 mm for intrauser precision, and 0.35 mm for interuser precision. Furthermore, the 95th percentile of all distances was below 0.75 mm for all analyses of accuracy and precision. Conclusions These findings indicate that such models are highly accurate and that a high level of intrauser and interuser precision can be achieved. The magnitude of the error presented here should inform the design and interpretation of future studies using manual segmentation techniques to derive models of the lumbar spine.

Quantitative Analysis of the Nonlinear Displacement–Load Behavior of the Lumbar Spine

There is currently no universal model or fitting method to characterize the visco-elastic behavior of the lumbar spine observed in displacement versus load hysteresis loops. In this study, proposed methods for fitting these loops, along with the metrics obtained, were thoroughly analyzed. A spline fitting technique was shown to provide a consistent approximation of spinal kinetic behavior that can be differentiated and integrated. Using this tool, previously established metrics were analyzed using data from two separate studies evaluating different motion preservation technologies. Many of the metrics, however, provided no significant differences beyond range of motion analysis. Particular attention was paid to how different definitions of the neutral zone capture the high-flexibility region often seen in lumbar hysteresis loops. As a result, the maximum slope was introduced and shown to be well defined. This new parameter offers promise as a descriptive measurement of spinal instability in vitro and may have future implications in clinical diagnosis and treatment of spinal instability. In particular, it could help in assigning treatments to specific stabilizing effects in the lumbar spine.

Verification of pure moment testing in a multi–degree of freedom spine testing apparatus

Background Pure moment testing is a common method used in cadaveric spine testing. The fundamental basis for the widespread acceptance of applying a pure moment is uniform loading along the column of the spine. To our knowledge, this protocol has not been experimentally verified on a multi–degree of freedom testing apparatus. Given its ubiquitous use in spine biomechanics laboratories, confirmation of this comparative cadaveric test protocol is paramount. Methods Group A specimens (n =13) were used to test the pure moment protocol, by use of 3 constructs that changed the number of involved vertebrae, orientation, and rigidity of the spine construct. Group B specimens (n = 6) were used to determine whether potting orientation, testing order, or degradation affected the range of motion (ROM) by use of 8 constructs. Each group was subjected to 3 cycles of flexion-extension, lateral bending, and axial torsion. The data from the third cycle were used to calculate the ROM for each method. Results Group A testing resulted in significant differences in ROM across the 3 constructs for lateral bending and axial torsion (P < .02) and trended toward a difference for flexion-extension (P = .055). Group B testing showed an increase in ROM across 8 constructs (P < .04) but no significant difference due to the orientation change. Conclusion The increased ROM across constructs observed in both groups indicates that the cause is likely the testing order or degradation of the specimens, with orientation having no observed effect. The data do not invalidate pure moment testing, and its use should persist.

Facet Joint Complex Considerations for Biomechanics of the Lumbar Functional Spinal Unit: An Improved Model Based Method for Investigating Facet Articulation

The biomechanics of facet kinematics is an important consideration for the native and pathologic spine. Moreover when evaluating new implants and techniques designed for motion preservation from either an anterior or posterior approach, the facet joint complex requires measurement techniques that do not violate the joint integrity to accurately characterize the contributions to the overall functional spinal unit response. Three-dimensional modeling of the lumbar spine can be used to derive a wealth of data beyond range of motion without invasive procedures. We have developed a method to apply kinematic data to rigid body models of each vertebral body reconstructed from high resolution CT scans. The method described has been shown to reduce registration errors from previous techniques. To portray the robustness of this method, we have computationally defined the articular surfaces of the L3-L4 facet joint and tracked minimum distance (MD) between each surface. The MD between intact and rigid rod instrumented conditions has been compared. The application of the rigid rod construct has shown to constrain the facet surfaces spatially closer than in the intact condition.

A consideration for the utility of the post-operative Oswestry Disability Index for measuring outcomes after sacroiliac joint fusion

Sacroiliac joint (SIJ) dysfunction and its surgical treatment remain a controversial topic in spine surgery. Determining success after SIJ fusion may be difficult due to preexisting back pain, lumbar fusion (LF), and functional disability. We examine the utility of Oswestry Disability Index (ODI) as a measure of clinical outcomes after minimally invasive SIJ fusion. A retrospective review of 24 patients with at least 12- months follow-up. Patients were divided into two groups based on presence of previous LF. Their post-operative ODI was compared with overall satisfaction, pain reduction, and return to work status. No difference in demographics was found in patients with and without prior LF with 92% of patients reporting lower post-operative pain and 96% being satisfied. Presence of LF did not show any statistically significant differences in pain or satisfaction. However, patient with prior LF reported lower ODI than those without LF at 1-year post-operatively (P=0.015). Postoperative ODI may give a falsely pessimistic impression of outcomes in SIJ fusion patients with prior LF, and its use and limitations should be carefully considered in future studies.

Erratum to ‘Variability of manual lumbar spine segmentation’ [International Journal of Spine Surgery 6 (2012) 167–173]

[This corrects the article DOI: 10.1016/j.ijsp.2012.04.002.].

Combinations of Biomechanical Metrics Involving Single and Multi Level Lumbar Posterior Dynamic Spinal Treatments

The subtle effects of motion preservation devices are comparatively more difficult to detect based wholly on the range of motion (RoM) parameter, despite the historical success in characterizing fixation devices. Additional approaches such as descriptive facet techniques, i.e., facet translations analysis, and newly defined metrics, i.e., interpedicular displacement (ID) must first be established, verified and standardized. Ideally, more complete biomechanical parameters would facilitate clinical understanding of specific new devices. The recent focus in our lab has been to measure the RoM in conjunction with other parameters, including ID, to more completely characterize a lumbar spine treated with a posterior dynamic systems (PDS). Furthermore, understanding the construct and which components dominate overall tissue response is important for PDS systems.Copyright

Analyzing the Hysteresis of the Lumbar Spine

Significant advances have been made in the field of spine biomechanics with the introduction of continuous testing machines and new testing protocols. [1] Despite all the technological achievements, range of motion (RoM) continues to be the only widely agreed upon, standardized metric for data analysis. In load-controlled flexibility testing, displacement is typically recorded over three loading cycles between established limits in specific modes of loading. The first two cycles are intended for preconditioning, and the data from the third cycle is used for analysis. [2] Plotting displacement versus load defines the hysteresis inherent to the motion segment.Copyright