Patterns of Load-to-Strength Ratios Along the Spine in a Population-Based Cohort to Evaluate the Contribution of Spinal Loading to Vertebral Fractures

Abstract

Vertebral fractures (VFx) are common among older adults. Epidemiological studies report high occurrence of VFx at mid-thoracic and thoracolumbar regions of the spine; however, reasons for this observation remain poorly understood. Prior reports of high ratios of spinal loading to vertebral strength in the thoracolumbar region suggest a possible biomechanical explanation. However, no studies have evaluated load-to-strength ratios (LSRs) throughout the spine for a large number of activities in a sizeable cohort. Thus, we performed a cross-sectional study in a sample of adult men and women from a population-based cohort to: 1) determine which activities cause the largest vertebral LSRs, and 2) examine patterns of LSRs along the spine for these high-load activities. We used subject-specific musculoskeletal models of the trunk to determine vertebral compressive loads for 109 activities in 250 individuals (aged 41 to 90 years, 50% women) from the Framingham Heart Study. Vertebral compressive strengths from T₄ to L₄ were calculated from computed tomography-based vertebral size and bone density measurements. We determined which activities caused maximum LSRs at each of these spinal levels. We identified nine activities that accounted for >95% of the maximum LSRs overall and at least 89.6% at each spinal level. The activity with the highest LSR varied by spinal level, and three distinct spinal regions could be identified by the activity producing maximum LSRs: lateral bending with a weight in one hand (upper thoracic), holding weights with elbows flexed (lower thoracic), and forward flexion with weight (lumbar). This study highlights the need to consider a range of lifting, holding, and non-symmetric activities when evaluating vertebral LSRs. Moreover, we identified key activities that produce higher loading in multiple regions of the spine. These results provide the first guidance on what activities to consider when evaluating vertebral load-to-strength ratios in future studies, including those examining dynamic motions and the biomechanics of VFx. © 2020 American Society for Bone and Mineral Research (ASBMR).

Keywords: BIOMECHANICS; BONE QCT; FRACTURE PREVENTION; FRACTURE RISK ASSESSMENT; OSTEOPOROSIS.

Fig 1.

The procedure to develop subject-specific models for everyone in this study. Briefly, a musculoskeletal (MSK) model is adjusted for sex, height/weight, muscle morphology, and spinal curvature (A). Then we applied external loading and postures to simulate an activity (126 activities in this study) (B). Finally, static optimization was used to predict muscle forces, and then a joint analysis tool in OpenSim was used to calculate spinal joint loading.⁽²⁰⁾ Spinal loading comes from MSK modeling, while vertebral strength comes from a regression equation developed based on QCT-based finite-element analysis (FEA) (C, D) to predict vertebral strength from CSA and vBMD of each vertebra. For those missing levels (where QCT data was missing), we used a multiple imputation approach to fill the missing vertebral strength data along the spine. We then calculated load-to-strength ratio (LSR) at each vertebral level using both vertebral loads and vertebral strength (E).

Fig 2.

Association between vertebral strength from CT-based finite-element analysis and the product of vertebral cross-sectional area (CSA) and integral volumetric bone mineral density (BMD) (R² = 0.895).

Fig 3.

The proportion (%) of maximum load-to-strength ratios at each vertebral level (A) and median values of load-to-strength ratios (B) for nine key activities producing maximum load-to-strength ratios. These nine activities produced 95.8% of the maximum load-to-strength ratios overall among 109 activities examined and at least 89.6% at each vertebral level.

Fig 4.

Box plot showing load-to-strength ratios for the top four activities at each level. The four activities shown at each level account for 69.6% to 98.4% of the maximum load-to-strength ratios for that level in the population-based cohort. Three distinct regions of the spine can be found based on the activities contributing to maximum load-to-strength ratios: upper thoracic (T₄ to T₆), lower thoracic (T₈ to T₁₁), lumbar (T₁₂ to L₄). For better visibility, outliers of load-to-strength ratio >1 are shown compressed between the gray bars; actual outlier values >1 are found in Supplemental Fig. S1.