Comparison of quantitative computed tomography-based measures in predicting vertebral compressive strength

Abstract

Patient-specific measures derived from quantitative computed tomography (QCT) scans are currently being developed as a clinical tool for vertebral strength prediction. QCT-based measurement techniques vary greatly in structural complexity and generally fall into one of three categories: 1) bone mineral density (BMD), 2) "mechanics of solids" (MOS) models, such as minimum axial rigidity (the product of axial stiffness and vertebral height), or 3) three dimensional finite element (FE) models. There is no clear consensus as to the relative performance of these measures due to differences in experimental protocols, sample sizes and demographics, and outcome metrics. The goal of this study was to directly compare the performance of QCT-based assessment techniques of varying degrees of structural sophistication in predicting experimental vertebral compressive strength. Eighty-one human thoracic vertebrae (T6 -T10) from 44 donors cadavers (F = 32, M = 12; 85 + 8 y.o., max = 97 y.o., min = 54 y.o.) were QCT scanned and destructively tested in uniaxial compression. The QCT scans were processed to generate FE models and various BMD and MOS measures, including trabecular bone mineral density (tBMD), integral bone mineral density (iBMD), and axial rigidity. Bone mineral density was weakly to moderately predictive of compressive strength (R 2 = 0.16 and 0.62 for tBMD and iBMD, respectively). Ex vivo vertebral strength was strongly correlated with both axial rigidity (R 2 = 0.81) and FE strength measurements (R 2 = 0.80), and the predictive capabilities of these two metrics were statistically equivalent (p > 0.05 for differences between FE and axial rigidity). The results of this study indicate that non-invasive predictive measures of vertebral strength should include some level of structural sophistication, specifically, gross geometric and material property distribution information. However, for uniaxial compression of isolated vertebrae, which is the current biomechanical testing paradigm for new non-invasive strength assessment techniques, QCT-based FE and axial rigidity measures are equivalent predictors of experimental strength. However, before abandoning the FE method in favor of more simplistic techniques, future work should investigate the performance of the FE method versus MOS measures for more physiologically representative loading conditions, e.g., anterior bending or in situ loading with intervertebral discs intact.

Effect of once-yearly zoledronic acid on the spine and hip as measured by quantitative computed tomography: results of the HORIZON Pivotal Fracture Trial

“…In this study, we modeled Ea as BMD 2 where BMD is the integral BMD of a region and where NCSI equals BMD 2 multiplied by MNCS and TCSI equals BMD 2 multiplied by MXCS. TCSI has been validated against femoral strength in vitro by Lotz et al [8] and for vertebrae by Buckley et al [9]. Although prospective studies are needed, our results suggest a significant treatment effect with zoledronic acid on all three of these key indices, and the increase in volumetric BMD is consistent with expectation of an increase in the compressive strength.…”

Locally measured microstructural parameters are better associated with vertebral strength than whole bone density

“…Strength obtained from FE analyses predicted 76 % of the variation in bone strength, which also is less than the local measures. The correlation between FE and experimental results found here is in accordance with previous studies [28,[36][37][38][39][40]; Dall'Ara et al (2010) found a determination coefficient of 0.79 between FE-predicted vertebral strength and experimental values. It should be emphasized that the locations that we identified as the best predictor for vertebral strength represent the microstructure of the bone in a 4-mm spherical region around the element centroid.…”

Compressive strength of elderly vertebrae is reduced by disc degeneration and additional flexion

“…The discrepancy can be explained by distinct experimental set-ups and distinct levels of disc degeneration. While embedding of isolated vertebrae ensures well-defined loading conditions (Buckley et al 2007), the test of the spinal segments is not as controlled. Moreover, the degeneration grading of the IVDs does not necessarily reflects its mechanical properties (Maquer et al 2014b).…”