In Vitro Measurements of Porcine Anterior Column Units Under Free Swelling

Biomechanical Effects of Human Lumbar Discography: In Vitro Experiments and Their Finite Element Validation

STUDY DESIGN

An experimental and computational finite element analysis of human lumbar spine discography and its resulting effects on disk biomechanics.

OBJECTIVE

To characterize the changes in stress and displacement of the human lumbar spine disks after puncture due to discography.

SUMMARY OF BACKGROUND DATA

Discography of the intervertebral disk (IVD) may be used to diagnose pathology of the disk and determine whether it may be a source for chronic back pain. It has recently been suggested that discography may lead to IVD degeneration, and has been a cause of controversy among spine care physicians.

MATERIALS AND METHODS

Both in vivo experiment using cadaveric specimens and a finite element model of the same L3-L5 lumbar spine was developed using computed tomography scans. Discography was simulated in the model as an area in the disk affected by needle puncture. The material properties in the nucleus pulposus were adjusted to match experimental data both before and after puncture.

RESULTS

Puncture of the IVD leads to increased deformation and increased stresses in the annulus fibrosis region of the disk. Pressure in the nucleus pulposus was found to decrease after puncture. Experimental and computational results correlated well.

CONCLUSIONS

Puncturing the IVD changes disk biomechanics and hence may lead to progressive spine degenerations in particular in the punctured disks.

Effect of Hydration on Healthy Intervertebral Disk Mechanical Stiffness

The intervertebral disk has an excellent swelling capacity to absorb water, which is thought to be largely due to the high proteoglycan composition. Injury, aging, degeneration, and diurnal loading are all noted by a significant decrease in water content and tissue hydration. The objective of this study was to evaluate the effect of hydration, through osmotic loading, on tissue swelling and compressive stiffness of healthy intervertebral disks. The wet weight of nucleus pulposus (NP) and annulus fibrosus (AF) explants following swelling was 50% or greater, demonstrating significant ability to absorb water under all osmotic loading conditions (0.015 M-3.0 M phosphate buffered saline (PBS)). Estimated NP residual strains, calculated from the swelling ratio, were approximately 1.5 × greater than AF residual strains. Compressive stiffness increased with hyperosmotic loading, which is thought to be due to material compaction from osmotic-loading and the nonlinear mechanical behavior. Importantly, this study demonstrated that residual strains and material properties are greatly dependent on osmotic loading. The findings of this study support the notion that swelling properties from osmotic loading will be important for accurately describing the effect of degeneration and injury on disk mechanics. Furthermore, the tissue swelling will be an important consideration for developing biological repair strategies aimed at restoring mechanical behavior toward a healthy disk.

Estimating the compressive strength of the porcine cervical spine: an examination of the utility of DXA

STUDY DESIGN

The failure strength of porcine spinal units was correlated with vertebral size and bone mineralization. The accuracy of the resulting predictive equations was tested with an independent sample of spinal units.

OBJECTIVES

To determine if dual energy x-ray absorptiometry (DXA)-obtained measures of bone mineralization can be used to accurately predict the compressive tolerance of porcine spinal units.

SUMMARY OF BACKGROUND DATA

Porcine spinal units are often used in place of cadaveric tissues, and normalization is used to improve the transferability of model results. In compressive testing, normalization can be performed to the estimated compressive strength. Bone mineralization measures have been shown to be positively correlated with compressive tolerance and have been used to predict the tolerance of human spinal units. However, the accuracy of these predictive equations has not been assessed with an independent sample.

METHODS

Twenty porcine cervical spinal units were scanned (DXA) to obtain measures of bone mineral content (BMC) and bone mineral density (BMD). The units were compressed to failure, and the failure loads were correlated with the measured bone mineralization and endplate area of the spinal unit. The regression equations were used to predict the compressive tolerance of an independent sample of spinal units.

RESULTS

BMC (P = 0.078) and BMD (P = 0.2834) were not significantly correlated to compressive strength. Endplate area was the most highly correlated variable, with an r of 0.5329. The use of a predictive equation including BMC on the second independent sample resulted in errors of estimation of 1.4 +/- 1.2 kN, corresponding to 13% of the average compressive strength. In comparison, the use of an equation employing endplate area alone resulted in estimation errors of 11%.

CONCLUSIONS

Measures of BMC/BMD did not enhance predictions of compressive strength and will not reduce errors in compressive load normalization in a porcine model. The poor correlations found between BMC and compressive strength may be due to the non-load-bearing anterior processes of the porcine cervical spine.