Experimental and Computational Comparison of Intervertebral Disc Bulge for Specimen-Specific Model Evaluation Based on Imaging.

Finite element modelling of the spinal unit is a promising preclinical tool to assess the biomechanical outcome of emerging interventions. Currently, most models are calibrated and validated against range of motion and rarely directly against soft-tissue deformation. The aim of this contribution was to develop an in vitro methodology to measure disc bulge and assess the ability of different specimen-specific modelling approaches to predict disc bulge. Bovine bone-disc-bone sections (N = 6) were prepared with 40 glass markers on the intervertebral disc surface. These were initially magnetic resonance (MR)-imaged and then sequentially imaged using peripheral-qCT under axial compression of 1 mm increments. Specimen-specific finite-element models were developed from the CT data, using three different methods to represent the nucleus pulposus geometry with and without complementary use of the MR images. Both calibrated specimen-specific and averaged compressive material properties for the disc tissues were investigated. A successful methodology was developed to quantify the disc bulge in vitro, enabling observation of surface displacement on qCT. From the finite element model results, no clear advantage was found in using geometrical information from the MR images in terms of the models' ability to predict stiffness or disc bulge for bovine intervertebral disc.

Development of robust finite element models of porcine tibiofemoral joints loaded under varied flexion angles and tibial freedoms.

The successful development of cartilage repair treatments for the knee requires understanding of the biomechanical environment within the joint. Computational finite element models play an important role in non-invasively understanding knee mechanics, but it is important to compare model findings to experimental data. The purpose of this study was to develop a methodology for generating subject-specific finite element models of porcine tibiofemoral joints that was robust and valid over multiple different constraint scenarios. Computational model predictions of two knees were compared to experimental studies on corresponding specimens loaded under several different constraint scenarios using a custom designed experimental rig, with variations made to the femoral flexion angle and level of tibial freedom. For both in vitro specimens, changing the femoral flexion angle had a marked effect on the contact distribution observed experimentally. With the tibia fixed, the majority of the contact region shifted to the medial plateau as flexion was increased. This did not occur when the tibia was free to displace and rotate in response to applied load. These trends in contact distribution across the medial and lateral plateaus were replicated in the computational models. In an additional model with the meniscus removed, contact pressures were elevated by a similar magnitude to the increase seen when the meniscus was removed experimentally. Overall, the models were able to capture specimen-specific trends in contact distribution under a variety of different loads, providing the potential to investigate subject-specific outcomes for knee interventions.

An In Vitro Study of the Intervertebral Disc Structure Using 3 T Magnetic Resonance Imaging.

STUDY DESIGN: An in vitro magnetic resonance imaging (MRI) study. OBJECTIVE: Investigate the potential of high-field MRI for producing higher quality images of the intervertebral disc (IVD) to better distinguish structural details. SUMMARY OF BACKGROUND DATA: Higher spatial and contrast resolution are important advantages when imaging the complex tissue structures in the spine such as the IVDs. However, at present it is challenging to capture the substructural details in the IVD such as the lamellae. METHODS: Three MRI sequences; two-dimensional proton-density-weighted Turbo-Spin-Echo (PD-TSE), 2D T2-weighted Turbo-Spin-Echo (T2W-TSE) with fat-saturation (FS), and 3D Spoiled-Gradient-Echo (3D-GE), were modified based on the image quality and scan duration. IVDs of three intact cadaveric lumbar-spines (T12-S1, Age 83-94 yr) were imaged using these optimized sequences. Thereafter each IVD was transversely sectioned and the exposed surfaces were photographed. Landmark observations from corresponding MRI slices and photographs were compared to confirm the MRI captured morphology. The image quality was evaluated using signal-to-noise ratio (SNR), and relative-contrast values. Finally, the underlying tissue structures, including specific pathological features, were qualitatively compared between the MR images and photographs. RESULTS: Observations from photographs and corresponding MRI slices matched well. The PD-TSE sequence had better overall SNR, but the relative contrast between the tissue types was relatively poor. The 3D-GE sequence had higher relative contrast between the IVD and bone, but not between annulus and nucleus regions. The T2W images provided the best relative contrast between the annulus and nucleus, however the standard deviations here were high. Structural details including fissures, vascular and granular tissue proliferation, and pathologies in the endplate region, were identifiable from the MR images obtained using the optimized sequences. CONCLUSION: The results demonstrate the potential of high-field MRI to capture the IVD structural details. Since the acquisition durations were within clinically acceptable levels, these methodological improvements have the potential to enhance clinical diagnostics. LEVEL OF EVIDENCE: 4.

Methodology to Produce Specimen-Specific Models of Vertebrae: Application to Different Species.

Image-based continuum-level finite element models have been used for bones to evaluate fracture risk and the biomechanical effects of diseases and therapies, capturing both the geometry and tissue mechanical properties. Although models of vertebrae of various species have been developed, an inter-species comparison has not yet been investigated. The purpose of this study was to derive species-specific modelling methods and compare the accuracy of image-based finite element models of vertebrae across species. Vertebral specimens were harvested from porcine (N = 12), ovine (N = 13) and bovine (N = 14) spines. The specimens were experimentally loaded to failure and apparent stiffness values were derived. Image-based finite element models were generated reproducing the experimental protocol. A linear relationship between the element grayscale and elastic modulus was calibrated for each species matching in vitro and in silico stiffness values, and validated on independent sets of models. The accuracy of these relationships were compared across species. Experimental stiffness values were significantly different across species and specimen-specific models required species-specific linear relationship between image grayscale and elastic modulus. A good agreement between in vitro and in silico values was achieved for all species, reinforcing the generality of the developed methodology.

A Nondestructive Method to Distinguish the Internal Constituent Architecture of the Intervertebral Discs Using 9.4 Tesla Magnetic Resonance Imaging.

STUDY DESIGN: An in vitro study of the intervertebral disc (IVD) structure using 9.4T magnetic resonance imaging (MRI). OBJECTIVE: Investigate the potential of ultrahigh-field strength MRI for higher quality 3-dimensional (3D) volumetric MRI datasets of the IVD to better distinguish structural details. SUMMARY OF BACKGROUND DATA: MRI has the advantages of being nondestructive and 3D in comparison to most techniques used to obtain the structural details of biological tissues, however, its poor image quality at higher resolution is a limiting factor. Ultrahigh-field MRI could improve the imaging of biological tissues but the current understanding of its application for spinal tissue is limited. METHODS: 2 ovine spinal segments (C7-T1, T2-T3) containing the IVD were separately imaged using 2 sequences; 3D spin echo (multislice-multiecho) pulse sequence for the C7-T1 sample and 3D gradient echo (fast-low-angle-shot) pulse sequence for the T2-T3 sample. The C7-T1 sample was subsequently decalcified and imaged again using the same scanning parameters. Histological sections obtained from the decalcified sample were stained followed by digital scanning. Observations from corresponding MRI slices and histological sections were compared as a method of confirmation of morphology captured under MRI. The signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and relative-contrast values were calculated for quantitative evaluation of image quality. RESULTS: Measurements from histology sections and corresponding MRI slices matched well. Both sequences revealed finer details of the IVD structure. Under the spin echo sequence, the annulus lamellae architecture was distinguishable and the SNR and CNR values were higher. The relative contrast was considerably higher between high (nucleus) and low (bone) signal constituents, but between the nucleus and the annulus the relative contrast was low. Under the gradient echo sequence, although the relative contrasts between constituents were poor, the fiber orientation was clearly manifested. CONCLUSION: The obtained positive results demonstrate the potential of ultrahigh-field strength MRI to nondestructively capture the IVD structure. LEVEL OF EVIDENCE: N/A.

Derivation of inter-lamellar behaviour of the intervertebral disc annulus.

The inter-lamellar connectivity of the annulus fibrosus in the intervertebral disc has been shown to affect the prediction of the overall disc behaviour in computational models. Using a combined experimental and computational approach, the inter-lamellar mechanical behaviour of the disc annulus was investigated under conditions of radial loading. Twenty-seven specimens of anterior annulus fibrosus were dissected from 12 discs taken from four frozen ovine thoracolumbar spines. Specimens were grouped depending on their radial provenance within the annulus fibrosus. Standard tensile tests were performed. In addition, micro-tensile tests under microscopy were used to observe the displacement of the lamellae and inter-lamellar connections. Finite elements models matching the experimental protocols were developed with specimen-specific geometries and boundary conditions assuming a known lamellar behaviour. An optimisation process was used to derive the interface stiffness values for each group. The assumption of a linear cohesive interface was used to model the behaviour of the inter-lamellar connectivity. The interface stiffness values derived from the optimisation process were consistently higher than the corresponding lamellar values. The interface stiffness values of the outer annulus were from 43% to 75% higher than those of the inner annulus. Tangential stiffness values for the interface were from 6% to 39% higher than normal stiffness values within each group and similar to values reported by other investigators. These results reflect the intricate fibrous nature of the inter-lamellar connectivity and provide values for the representation of the inter-lamellar behaviour at a continuum level.

Vertebroplasty: Patient and treatment variations studied through parametric computational models.

BACKGROUND: Vertebroplasty is increasingly used in the treatment of vertebral compression fractures. However there are concerns that this intervention may lead to further fractures in the adjacent vertebral segments. This study was designed to parametrically assess the influence of both treatment factors (cement volume and number of augmentations), and patient factors (bone and disc quality) on the biomechanical effects of vertebroplasty. METHODS: Specimen-specific finite element models of two experimentally-tested human three-vertebral-segments were developed from CT-scan data. Cement augmentation at one and two levels was represented in the respective models and good agreement in the predicted stiffness was found compared to the corresponding experimental specimens. Parametric variations of key variables associated with the procedure were then studied. FINDINGS: The segmental stiffness increased with disc degeneration, with increasing bone quality and to a lesser extent with increasing cement volume. Cement modulus did not have a great influence on the overall segmental stiffness and on the change in the elemental stress in the adjoining vertebrae. However, following augmentation, the stress distribution in the adjacent vertebra changed, indicating possible load redistribution effects of vertebroplasty. INTERPRETATION: This study demonstrates the importance of patient factors in the outcomes of vertebroplasty and suggests that these may be one reason for the variation in clinical results.