Effect of the intervertebral disc on vertebral bone strength prediction: a finite-element study

Abstract

BACKGROUND CONTEXT

Osteoporotic vertebral fractures (OVFs) are a prevalent skeletal condition in the elderly but the mechanism behind these fractures remain unclear due to the complex biomechanical interplay between spinal segments such as the vertebra and intervertebral discs (IVDs).

PURPOSE

To investigate the biomechanical influence of IVDs by (1) comparing finite element (FE)-predicted failure load with experimentally measured failure load of functional spinal units (FSUs) and (2) comparing this correlation with those of FE-predicted failure load and bone mineral density (BMD) of the single central vertebra with experimentally measured failure load.

STUDY DESIGN

A computational biomechanical analysis.

PATIENT SAMPLE

Ten thoracic FSUs consisting of a central vertebra, the adjacent IVDs, and the upper and lower halves of the adjacent vertebrae were harvested from formalin-fixed human donors (4 males, 6 females; mean age of 82±9 years).

OUTCOME MEASURES

The outcome measures included the prediction of vertebral strength and determination of BMD in FSUs and the single central vertebra and the correlation of both measures with experimentally measured vertebral strength of the FSUs.

METHODS

The FSUs underwent clinical multidetector computed tomography (MDCT) (spatial resolution: 250×250×600 μm³). BMD was determined for the FSUs from the MDCT images of the central vertebrae. FE-predicted failure load was calculated in the single central vertebra of the FSUs alone and the entire FSUs. Experimentally measured failure load of the FSUs was determined in a uniaxial biomechanical test.

RESULTS

BMD of the central vertebrae correlated significantly with experimentally measured failure load (R²=0.66, p<.02), whereas FE-predicted failure load of the central vertebra showed no significant correlation with experimentally measured failure load (p=.07). However, FE-predicted failure load of FSUs best predicted experimentally measured failure load of FSUs (R²=0.93, p<.0001).

CONCLUSIONS

This study demonstrated that routine clinical MDCT images can be an accurate and feasible tool for prediction of OVFs using patient-specific FE analysis of FSU models.

CLINICAL SIGNIFICANCE

Improved management of OVFs is essential amidst current clinical challenges. Implementation of a vertebral strength assessment tool could result in more accurate prediction of osteoporotic fracture risk and aid clinicians with better targeted early treatment strategies.

Introduction

Osteoporotic vertebral fractures (OVFs) are one of the most common complications of osteoporosis and are associated with an increased risk of subsequent fractures, loss of daily abilities, and high mortality, especially in the aging population. Currently, the gold standard for diagnosis of osteoporosis is bone mineral density (BMD) assessment via dual-energy X-ray absorptiometry [1]. However, it is inherently limited by the acquisition of two-dimensional projections, leading to blindsiding of important structural knowledge that could be used to identify patients with impending fractures. In addition, majority of fractures occur in patients classified in the osteopenia category than in the osteoporotic category, which evidently shows that BMD should be cautiously treated as primary factor in the calculation of fracture risk [2]. Since dual-energy X-ray absorptiometry is not capable of providing three-dimensional (3D) data of the spine and the association of BMD to OVFs is still inconclusive, it is of paramount importance to extract more holistic information, related to the early diagnosis and prediction of impending OVFs. Due to such challenges involved in diagnosing fractures before they occur, patient-specific nonlinear finite element (FE) analyses have shown potential in predicting bone strength noninvasively [3], enabling the possibility of early intervention.

Although FE analyses have recently been widely established in the prediction of bone strength and consequently fractures, challenges remain in the modeling of the spine. This is because the human spine is structurally complex, unlike long bones. Consequently, many FE studies have focused on modeling of vertebral bodies in isolation in the prediction of various biomechanical properties [4], [5], [6], [7]. For example, a study conducted by Buckley et al. showed that quantitative computed tomography-based FE-predicted strength measures correlated significantly with experimentally measured strength and was better predictive of compressive strength than BMD [4]. These studies demonstrated that FE-predicted stiffness and strength were predictive of experimental parameters and could act as a possible diagnostic tool for the evaluation of biomechanical and fracture properties of bone.

Since intervertebral discs (IVDs) also influence the load applied to the spine, FE models of isolated vertebral bodies alone provides limited information on vertebral strength. FE analysis incorporating the IVDs could be critical in predicting vertebral strength and impending fractures of the spine.

A recent study by Groenen et al. evaluated failure behavior in functional spinal units (FSUs), which consists of three vertebral bodies and two interspersed IVDs [8]. However, their study found that only stiffness, and not failure load, had significant correlations with experimentally measured failure load. Therefore, the objectives of our study were (1) to compare FE-predicted failure load of FSUs with experimentally measured failure load of FSUs and (2) to compare this correlation with those of FE-predicted failure load and BMD of the single central vertebra with experimentally measured failure load of FSUs.