Human and Porcine Lumbar Endplate Injury Risk in Repeated Flexion-compression

Introduction:: Low back pain (LBP) is a highly prevalent condition, affecting 50-80% of adults at some point in their lifetimes¹. LBP is particularly a problem among active-duty service members with 20% of all musculoskeletal injuries attributed to lumbar injury alone². Unfortunately, the etiology of chronic LBP is not well understood, nor are the specific injuries contributing to pain. An overwhelming 90% of patients that report LBP are diagnosed with ‘nonspecific’ LBP³, a condition assigned when specific pathology cannot be determined using current clinical diagnostic modalities. This problem can be elucidated with a better understanding of injury risk to the lumbar spine.

Flexion and repeated compressive loading have both been shown as contributing risk factors for lumbar spine injury^4,5. Similarly, those exposed to repeated combined loading, such as helicopter pilots⁶ and motor vehicle operators⁷ that experience whole-body vibration⁸, have a higher risk of LBP. Lumbar spine injury risk due to repeated flexion-compression should be investigated in human cadavers and animal models to inform our understanding of chronic lumbar spine injury in humans in vivo. With a better understanding of the etiology of injury, factors leading to chronic pain may be better understood.


Materials and Methods:: This study used 16 human and 16 porcine cadaveric lumbar functional spinal units (FSUs) to investigate the lumbar spine injury risk due to long-term repeated lumbar flexion-compression. Pigs are a commonly used animal model for lumbar spine studies^9,10 due to anatomical and physiological similarities with human lumbar spines¹¹. Specimens were imaged with high-resolution computed tomography (Nikon XTH 225 ST, microCT) before testing. The applied loading configuration modeled the combined loading exposures observed in high-speed watercraft occupants¹². Specimens were rigidly secured in a biaxial test fixture that applied sinusoidal axial compression at 1 Hz and independently applied an offset flexion ramp at 1 Hz. Test specimens were surrounded by an environmental chamber that maintained 37°C and saturated humidity throughout loading to maintain in vivo conditions. After loading, specimens were imaged with microCT and compared to pre-test images to assess injury.

Force applied to human and porcine specimens during testing were scaled to an equivalent stress condition for comparison, based on cross-sectional endplates areas measured in pre-test microCT imaging. Endplate fracture was defined as failure in this test series. Specimens that did not show endplate fracture were considered ‘noninjuries’ and were right-censored. Specimens that showed at least one endplate fracture were left-censored. Accelerated life analyses were developed using Minitab software to determine the 50% injury risk and 95% confidence intervals for both species.


Results, Conclusions, and Discussions:: Six of 16 human specimens and 9 of 16 porcine specimens experienced at least one endplate fracture. The longest loading durations were 80 hours in the porcine series and 6 days in the human series. Figure 1a shows the 50% endplate injury risk curves for human and pig in terms of scaled force and cycle count with 95% confidence intervals.

The 95% confidence intervals for human and pig injury risk overlap below 4000 N of scaled force, and the risk functions have similar slopes showing similarity in injury risk between human and pig. However, the 50% risk curves between the two species show different injury tolerances for the same scaled force. For example, at 2000 N of scaled applied compressive force, the 50% injury risk occurs at 71,000 cycles for human and 1,200,000 cycles for pig. This indicates that the injury risk for endplate fracture requires further scaling beyond only stress.

If an additional constant scale factor is applied to the porcine risk curve and optimized for minimal sum of squared error, the two curves are nearly equivalent with a scale factor of 0.588 (Figure 1b). However, the source(s) influencing the difference between these two risk curves is undetermined but may be explained by strain and/or material differences.

Intervertebral disc (IVD) injuries were also observed in all injured specimens and in many noninjured specimens. Low-level IVD failure likely occurs before endplate fracture and may be a potential source of chronic pain. Continued investigation is needed to determine injury risk for this incipient injury. Further, the loading was applied continuously, and therefore, the given endplate fracture injury risk is a conservative model. Regular and adequate rest is likely to decrease injury risk for a given force and cycle count.

This is the first injury risk criteria for human and animal lumbar spine loading in long-duration combined flexion-compression. The given injury risk curves provide a direct comparison of risk in terms of scaled force and cycle count between the two species. These injury risks will be particularly useful for future in vivo testing and informing injury prevention efforts.


Acknowledgements (Optional): : The authors gratefully acknowledge funding from the Office of Naval Research and US Naval Air Systems Command – Patuxent River through MTEC (Southwest Research Institute-PI).


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