Objective quantification of intervertebral disc volume properties using MRI in idiopathic scoliosis surgery

The use of MRI in the study of patients with idiopathic scoliosis: a systematic review of the literature

Objective. To analyze the frequency of hidden neuraxial pathology in idiopathic scoliosis (IS), to substantiate the need for MRI in IS and to identify promising areas for the use of MRI in the examination of patients with IS.Material and Methods. The literature review was carried out using the PubMed and Google Scholar databases. Of the 780 papers on the research topic, 65 were selected after removing duplicates and checking for inclusion/exclusion criteria. As a result, 49 original studies were included in the analysis. Level of evidence – II.Results. According to modern literature, the main direction of using MRI in idiopathic scoliosis is the search for predictors of latent pathology of the spinal cord and craniovertebral junction. The frequency of neuraxial pathology in idiopathic scoliosis is 8 % for adolescent IS and 16 % for early IS. The main predictors of neuraxial pathology are male sex, early age of deformity onset, left-sided thoracic curve and thoracic hyperkyphosis. MRI in IS may be a useful addition to radiological diagnostic methods to identify risk factors and to study degenerative changes in the spine.Conclusion. MRI of the spine should be performed in the early stages of IS to detect latent spinal cord tethering. In type I Chiari anomalies, there is a possibility that early neurosurgery can prevent the development of scoliosis. The main signs of latent neuraxial pathology in IS are early progression of spinal deformity, left-sided thoracic curve, male gender and thoracic kyphosis over 40° according to Cobb.MRI can be used as an effective non-invasive tool in research aimed at identifying risk factors for IS, including helping to track early degeneration of intervertebral discs.

Modeling multiaxial damage regional variation in human annulus fibrosus

In the present article, a fully three-dimensional human annulus fibrosus model is developed by considering the regional variation of the complex structural organization of collagen network at different scales to predict the regional anisotropic multiaxial damage of the intervertebral disc. The model parameters are identified using experimental data considering as elementary structural unit, the single annulus lamellae stretched till failure along the micro-sized collagen fibers. The multi-layered lamellar/inter-lamellar annulus model is constructed by considering the effective interactions between adjacent layers and the chemical-induced volumetric strain. The regional dependent model predictions are analyzed under various loading modes and compared to experimental data when available. The stretching along the circumferential and radial directions till failure serves to check the predictive capacities of the annulus model. Model results under simple shear, biaxial stretching and plane-strain compression are further presented and discussed. Finally, a full disc model is constructed using the regional annulus model and simulations are presented to assess the most likely failed areas under disc axial compression. STATEMENT OF SIGNIFICANCE: The damage in annulus soft tissues is a complex multiscale phenomenon due to a complex structural arrangement of collagen network at different scales of hierarchical organization. A fully three-dimensional constitutive representation that considers the regional variation of the structural complexity to estimate annulus multiaxial mechanics till failure has not yet been developed. Here, a model is developed to predict deformation-induced damage and failure of annulus under multiaxial loading histories considering as time-dependent physical process both chemical-induced volumetric effects and damage accumulation. After model identification using single lamellae extracted from different disc regions, the model predictability is verified for various multiaxial elementary loading modes representative of the spine movement. The heterogeneous mechanics of a full human disc model is finally presented.

A microstructure-based model for a full lamellar-interlamellar displacement and shear strain mapping inside human intervertebral disc core

The determinant role of the annulus fibrosus interlamellar zones in the intervertebral disc transversal and volumetric responses and hence on their corresponding three-dimensional conducts have been only revealed and appreciated recently. Their consideration in disc modeling strategies has been proven to be essential for the reproduction of correct local strain and displacement fields inside the disc especially in the unconstrained directions of the disc. In addition, these zones are known to be the starting areas of annulus fibrosus circumferential tears and disc delamination failure mode, which is often judged as one of the most dangerous disc failure modes that could evolve with time leading to disc hernia. For this latter reason, the main goal of the current contribution is to incorporate physically for the first time, the interlamellar zones, at the scale of a complete human lumbar intervertebral disc, in order to allow a correct local vision and replication of the different lamellar-interlamellar interactions and an identification of the interlamellar critical zones. By means of a fully tridimensional chemo-viscoelastic constitutive model, which we implemented into a finite element code, the physical, mechanical and chemical contribution of the interlamellar zones is added to the disc. The chemical-induced volumetric response is accounted by the model for both the interlamellar zones and the lamellae using experimentally-based fluid kinetics. Computational simulations are performed and critically discussed upon different simple and complex physiological movements. The disc core and the interlamellar zones are numerically accessed, allowing the observation of the displacement and shear strain fields that are compared to direct MRI experiments from the literature. Important conclusions about the correct lamellar-interlamellar-nucleus interactions are provided thanks to the developed model. The critical interlamellar spots with the highest delamination potentials are defined, analyzed and related to the local kinetics and microstructure.

Localization and Segmentation of 3D Intervertebral Discs in MR Images by Data Driven Estimation

This paper addresses the problem of fully-automatic localization and segmentation of 3D intervertebral discs (IVDs) from MR images. Our method contains two steps, where we first localize the center of each IVD, and then segment IVDs by classifying image pixels around each disc center as foreground (disc) or background. The disc localization is done by estimating the image displacements from a set of randomly sampled 3D image patches to the disc center. The image displacements are estimated by jointly optimizing the training and test displacement values in a data-driven way, where we take into consideration both the training data and the geometric constraint on the test image. After the disc centers are localized, we segment the discs by classifying image pixels around disc centers as background or foreground. The classification is done in a similar data-driven approach as we used for localization, but in this segmentation case we are aiming to estimate the foreground/background probability of each pixel instead of the image displacements. In addition, an extra neighborhood smooth constraint is introduced to enforce the local smoothness of the label field. Our method is validated on 3D T2-weighted turbo spin echo MR images of 35 patients from two different studies. Experiments show that compared to state of the art, our method achieves better or comparable results. Specifically, we achieve for localization a mean error of 1.6-2.0 mm, and for segmentation a mean Dice metric of 85%-88% and a mean surface distance of 1.3-1.4 mm.

A method for quantitative measurement of lumbar intervertebral disc structures: an intra- and inter-rater agreement and reliability study

Background

There is a shortage of agreement studies relevant for measuring changes over time in lumbar intervertebral disc structures. The objectives of this study were: 1) to develop a method for measurement of intervertebral disc height, anterior and posterior disc material and dural sac diameter using MRI, 2) to evaluate intra- and inter-rater agreement and reliability for the measurements included, and 3) to identify factors compromising agreement.

Methods

Measurements were performed on MRIs from 16 people with and 16 without lumbar disc herniation, purposefully chosen to represent all possible disc contours among participants in a general population study cohort. Using the new method, MRIs were measured twice by one rater and once by a second rater. Agreement on the sagittal start- and end-slice was evaluated using weighted Kappa. Length and volume measurements were conducted on available slices between intervertebral foramens, and cross-sectional areas (CSA) were calculated from length measurements and slice thickness. Results were reported as Bland and Altman’s limits of agreement (LOA) and intraclass correlation coefficients (ICC).

Results

Weighted Kappa (K_w (95% CI)) for start- and end-slice were: intra-: 0.82(0.60;0.97) & 0.71(0.43;0.93); inter-rater: 0.56(0.29;0.78) & 0.60(0.35;0.81). For length measurements, LOA ranged from [−1.0;1.0] mm to [−2.0;2.3] mm for intra-; and from [−1.1; 1.4] mm to [−2.6;2.0] mm for inter-rater. For volume measurements, LOA ranged from [−293;199] mm³ to [−582;382] mm³ for intra-, and from [−17;801] mm³ to [−450;713] mm³ for inter-rater. For CSAs, LOA ranged between [−21.3; 18.8] mm² and [−31.2; 43.7] mm² for intra-, and between [−10.8; 16.4] mm² and [−64.6; 27.1] mm² for inter-rater. In general, LOA as a proportion of mean values gradually decreased with increasing size of the measured structures. Agreement was compromised by difficulties in identifying the vertebral corners, the anterior and posterior boundaries of the intervertebral disc and the dural sac posterior boundary. With two exceptions, ICCs were above 0.81.

Conclusions

Length measurements and calculated CSAs of disc morphology and dural sac diameter from MRIs showed acceptable intra- and inter-rater agreement and reliability. However, caution should be taken when measuring very small structures and defining anatomical landmarks.

Moderate-intensity running causes intervertebral disc compression in young adults

BACKGROUND

Decreased intervertebral disc (IVD) volume can result in diminished load-carrying capacity of the spinal region. Although moderate-intensity running is generally advocated for apparently healthy adults, running causes a loss in stature that is thought to reflect IVD compression. The aim of this investigation was to use magnetic resonance imaging (MRI) to quantify the influence of moderate-intensity treadmill running on IVD height and volume in the thoracic and lumbar regions of the vertebral column.

METHODS

A clinic-based repeated-measures design was used in eight healthy young asymptomatic adults. After preliminary measurements and familiarization (day 1), participants reported to the clinic on two further occasions. MRI scans and stature measurements were completed at baseline (day 2), preexercise (day 3), and after 30 min of moderate-intensity treadmill running (postexercise, day 3). Mean height and volume were derived for all thoracic and lumbar IVDs from digitized MRIs, and stature was determined with a stadiometer.

RESULTS

Moderate-intensity running resulted in 6.3% ± 0.9% reduction in mean IVD height and 6.9% ± 1.0% reduction in calculated IVD volume. The day-to-day variation in mean IVD height and volume were 0.6% ± 0.6% and 0.4% ± 0.6%, respectively.

CONCLUSIONS

This is the first study to quantify the influence of moderate-intensity running on IVD height and volume. Changes in IVD height and volume were observed throughout the thoracic and lumbar vertebral regions. These findings suggest that future studies evaluating the influence of various loading activities and recovery techniques on IVD structure should consider thoracic as well as lumbar regions of the spine.

Quantitative evaluation of an automatic segmentation method for 3D reconstruction of intervertebral scoliotic disks from MR images

Background

For some scoliotic patients the spinal instrumentation is inevitable. Among these patients, those with stiff curvature will need thoracoscopic disk resection. The removal of the intervertebral disk with only thoracoscopic images is a tedious and challenging task for the surgeon. With computer aided surgery and 3D visualisation of the interverterbral disk during surgery, surgeons will have access to additional information such as the remaining disk tissue or the distance of surgical tools from critical anatomical structures like the aorta or spinal canal. We hypothesized that automatically extracting 3D information of the intervertebral disk from MR images would aid the surgeons to evaluate the remaining disk and would add a security factor to the patient during thoracoscopic disk resection.

Methods

This paper presents a quantitative evaluation of an automatic segmentation method for 3D reconstruction of intervertebral scoliotic disks from MR images. The automatic segmentation method is based on the watershed technique and morphological operators. The 3D Dice Similarity Coefficient (DSC) is the main statistical metric used to validate the automatically detected preoperative disk volumes. The automatic detections of intervertebral disks of real clinical MR images are compared to manual segmentation done by clinicians.

Results

Results show that depending on the type of MR acquisition sequence, the 3D DSC can be as high as 0.79 (±0.04). These 3D results are also supported by a 2D quantitative evaluation as well as by robustness and variability evaluations. The mean discrepancy (in 2D) between the manual and automatic segmentations for regions around the spinal canal is of 1.8 (±0.8) mm. The robustness study shows that among the five factors evaluated, only the type of MRI acquisition sequence can affect the segmentation results. Finally, the variability of the automatic segmentation method is lower than the variability associated with manual segmentation performed by different physicians.

Conclusions

This comprehensive evaluation of the automatic segmentation and 3D reconstruction of intervertebral disks shows that the proposed technique used with specific MRI acquisition protocol can detect intervertebral disk of scoliotic patient. The newly developed technique is promising for clinical context and can eventually help surgeons during thoracoscopic intervertebral disk resection.

Evaluation of lumbar disc and spine morphology: long-term repeatability and comparison of methods

Establishing the long-term repeatability of quantitative measures of lumbar intervertebral disc and spinal morphology is important for planning interventional studies. We aimed to examine this issue and to determine to what extent a smaller number of measurements per disc or vertebral level could be used to save operator time without compromising measurement precision. Twenty-one healthy male subjects were scanned at baseline and 1.5 years later. On sagittal MR-scans intervertebral disc cross-sectional area, anterior disc height, posterior disc height, intervertebral angle and intervertebral length were measured. The repeatability of the average value from all sagittal images or from 1, 3, 5 or 7 images centred at the spinous process was evaluated. Bland-Altman analysis showed all measurements to be repeatable between testing days. Intervertebral length was the most precise measurement (coefficients of variation [CVs] between 1.2% and 1.5%), followed by disc cross-sectional area (CVs between 2.9% and 3.6%). Variance component analysis showed that using 7 images, but not 1, 3 or 5 images, resulted in a similar level of measurement error as when measurements from all images were included.

In vitro disc pressure profiles below scoliosis fusion constructs

STUDY DESIGN

Biomechanical human cadaveric study comparing straight and scoliotic spines with healthy and degenerated L4/5 discs.

OBJECTIVE

To describe the biomechanical environment of discs under various spinal alignments by measuring the coronal intradiscal pressure profiles.

SUMMARY OF BACKGROUND DATA

Abnormal loading of the lumbar discs in the concavity of scoliotic curves may accelerate disc degeneration, which may be related to pain.

METHODS

Eight intact human cadaver spines (T1-S1; mean donor age 47 years old) underwent radiographs, DEXA, and MRI and were graded for disc degeneration. Each specimen was instrumented in a normal (straight coronal) spinal alignment from T4-L4. Intradiscal pressure profiles for the L4/5 disc and resultant moments were obtained under axial follower loads up to 1500 N. Testing was repeated for bilateral 3-cm decompensation. Posterior instrumentation was used to induce scoliosis (thoracic and lumbar curve average = 25 degrees, fractional lumbosacral curve average = 5 degrees), and testing was repeated for all load states.

RESULTS

MRI found 4 healthy (grade I and II) and 4 degenerated (grade III to V) L4/5 discs. Scoliosis and decompensation significantly increased coronal moments (P < 0.003). Disc pressures increased linearly with greater applied loads for all specimens. Healthy L4/5 discs exhibited uniform pressure profiles with normal spinal alignment and minimal effect with simulated scoliosis or decompensation. For degenerated discs, there was a relative pressure profile depression in the nucleus relative to the anulus region; with spinal malalignment, either due to scoliotic curvature, decompensation, or both, there was disc pressure profile asymmetry. The ratio of maximum intradiscal pressure at the concavity relative to the convexity was 1.1 (range, 1.0-1.2) for healthy discs and 3.6 (range, 2.2-4.4) for degenerated discs in the scoliotic specimens (P = 0.008).

CONCLUSION

Disc pressure profilometry below long spinal constructs found asymmetric loading with the greatest loads at the concave inner anulus, especially in the presence of disc degeneration, scoliosis, and decompensation. For the degenerated cases, there was substantial disc pressure profile asymmetry despite only mildly severe scoliotic curvatures. These results suggest that scoliosis surgeons should minimize end-vertebra tilt, maximize lumbar curve, and balance correction at the time of surgical intervention. These results combined with prior animal studies suggest a compounding effect of asymmetric loading and progression of disc degeneration.