Patient Specific Material Mapping for Functional Outcome Prediction and Planning in Spinal Surgery

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Abstract

Background The research effort on spinal disorders and treatments has been increased during the last decades and a rapid progression in implant development has occurred. In spite of the effort, postoperative complications such as screw loosening, implant subsidence, pseudo-arthrosis still pose a relevant problem. Despite the diversity of these complications, changes of the mechanical loads acting on the anatomical structures after fusion surgery are considered to play an essential role.

Numerical simulations can help predict the stress (re-) distribution after a surgical intervention. Since, the load distribution is strongly dependent on individual factors such as anatomical and structural characteristics, the recent trend has been heading towards patient specific simulations. While patient specific geometries can be derived from radiological data by manual segmentation or with machine learning approaches, material mapping strategies are largely lacking. The lack of knowledge collides with the growing demand for patient specific simulations and leads to the goal of this dissertation:

Goal The objective of this doctoral thesis is to develop a material mapping strategy that allows individualized bone and soft tissue mapping for patient-specific computational models.

Biomechanical Fundamentals An array of experiments was performed to establish a biomechanical basis, which can serve as a fundament to build on for the development of a material mapping strategy. As a main project, a stepwise reduction study was conducted with spinal segments originating from human lumbar cadavers. The biomechanical relevance of each of the various passive spinal structure was characterized and associations with radiological data were synthesized.

It was found that spinal segments show a large inter-specific variability in stiffness, which mirrors in a large variability of range of motions of intervertebral discs and a large variability of load-displacement curves of spinal ligaments. However, this large interspecific variability stands in contrast to a very high consistency of the contribution patterns between the different anatomical structures. This consistency exists across different levels and throughout a very heterogeneous cohort. This “principle of contribution conservation” indicates that the characteristics of the passive structures are accurately matched to one another and that they are adapted to the specific range of motion of the spinal segment.

It was further found that in the degenerative situation, these contribution patterns are slightly altered due to structural degenerative changes. In the load cases in which the facet joints act as a major contributor, an increase in their contribution is observed, while some load is released from the intervertebral disc. Pretension, which exists in all structures is continuously reduced as the segment develops towards a more degenerative state.

As another sign of degeneration, spondylophytes develop. However, in contrast to the general opinion, spondylophytes (< grade 4) do not support the segment as a propping structure but increase stiffness at the spondylophyte-affected side during tensile loading. Only severe spondylophytes with osseous bridging (grade 4) show large contribution to the segmental stability. A clinical classification system based on the biomechanical data was introduced.

It was also found that shear wave elastography, which was developed for stiffness measurements, shows a decent effectiveness as an assessment tool for intervertebral disc stiffness. Data derived from the cadaveric study were used to derive a generic stiffness distribution function which allows bulk material mapping for numerical modelling.

A very intriguing finding, is a surprisingly high correlation between the intervertebral disc stiffness and the average HU-value acquired from clinical computed tomography (CT) of the intervertebral disc. While clinically established MRI based grading systems (e.g. Pfirrmann) barely show a correlation with the biomechanical properties of the intervertebral disc, the CT value is a good predictor for the mechanical condition of a specific intervertebral disc.

Soft tissue mapping On the basis of a the newly discovered correlation between the CT-intensity of the intervertebral disc and its biomechanical properties in combination with the high conformity of contribution patterns of the paraspinal structures, a novel approach for soft tissue mapping is proposed. Three features with the highest correlation (intervertebral disc area, intervertebral disc level and CT annulus intensity) were used to train a machine learning approach to predict the load deflection curves of the intervertebral disc. With the predicted range of motion during flexion-extension and the average contribution pattern, the load deflection curves of the other spinal soft-tissue structures are predicted. Besides an approach for the prediction of soft tissue parameters, a numerical material which is capable of rendering the spinal tissue realistically is another essential requirement. A tetrahedral-based, fiber reinforced, explicit Mooney Rivlin material for finite element modelling was developed and validated.

Bone tissue mapping Finite element analysis can serve as tool to preoperatively assess the performance of screw fixations in the bone. For this purpose, the individual bone density distribution must be well represented in the numerical material model of the spine. The great advantage of bone mapping in contrast to soft tissue mapping is that the apparent density on the CT has shown to be directly relate to the Young’s modulus of the bone. In order to achieve a bone mapping strategy which allows for the accurate simulation of a patient-specific screw-bone-interface, finite element pull-out simulations were performed and validated against cadaveric experiments.

Synthesis The current doctoral thesis presents a novel technique for material mapping (soft and bone tissue) in computational, spinal modelling. The required biomechanical fundamentals are established and an advanced insight in the biomechanical characteristics of the spine in the healthy and degenerated situation are elaborated within the frame of this work. The proposed material mapping strategy represents an essential milestone in person-specific computational modelling

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Cited By (1)

Year	Entry
2023	Minster P-H, Lafon Y, Beillas P. Implications of range of motion requirements for the laxity of ligaments in a lumbar finite element model. J Biomech. February 2023;148:111460.