INTRODUCTION:

The intervertebral disc remains an enigmatic anatomy; offering both constrained and unconstrained properties as well as viscoelastic deformation. Although the nucleus pulposus does not completely behave physiologically as confined or unconfined, experimental setups have been performed to investigate and compare the mechanical properties of native nucleus pulposus to a cryomilled nucleus pulposus for cushioning metrics.

METHODS: Extracted nucleus pulposus tissue was dehydrated and subjected to a cryomilling process. The nucleus pulposus was pulverized with a cryomill to attain a target size (≤250 µm), and then lyophilized to create a particulate. Benchmark testing for cushioning properties of the nucleus pulposus commonly includes assessment of swelling pressure and hydraulic permeability in confined compression, and complex shear modulus with dynamic mechanical analysis (DMA).[1] Inherent to the evaluation is water binding as a correlate of proteoglycan composition.

RESULTS: Water content for the cryomilled tissue was 87.1% ± 1.0%, and similarly it was 87.8% ± 2.1% for the native tissue (p > .05; t-test). The hydraulic permeability was the same between both groups as 0.005 x 10^-12 m⁴/Ns (p > .05; t-test). Also, the swelling pressure for the cryomilled tissue was 5.4 ± 0.8 kPa and similar to the native tissue at 7.9 ± 2.4 kPa (p > .05; t-test). Both the cryomilled nucleus pulposus tissue and the native nucleus pulposus tissue retained a similar overall shape and consistency post-incubation following testing. Representative examples of stress-relaxation curves were similar for native nucleus pulposus tissue and cryomilled nucleus pulposus tissue (Figure 1). In addition to confined compression, cryomilled and native nucleus pulposus tissue underwent dynamic mechanical analysis in shear. The complex modulus, loss modulus, storage modulus and tan delta were similar for the native nucleus pulposus tissue and the cryomilled nucleus pulposus tissue, indicating that the viscous and elastic properties were similar to that existing for the native tissue (Figure 2).

DISCUSSION: Cryomilled nucleus pulposus tissue might provide a suitable replacement for nucleus pulposus tissue lost due to degradation, In this evaluation, native tissue was sized to support delivery through a 22-gauge cannula. Supplemented nucleus pulposus tissue seems to perform the same basic function with regard to maintaining matrix-water binding, providing a functional benefit of mechanical cushioning.

[1] Nerurkar et al. Mechanical design criteria for intervertebral disc tissue engineering. J Biomech 2010;43(6):1017-1030.