Viscoelastic heating of insulated bovine intervertebral disc

Heat Conduction Simulation of Chondrocyte-Embedded Agarose Gels Suggests Negligible Impact of Viscoelastic Dissipation on Temperature Change

Agarose is commonly used for 3D cell culture and to mimic the stiffness of the pericellular matrix of articular chondrocytes. Although it is known that both temperature and mechanical stimulation affect the metabolism of chondrocytes, little is known about the thermal properties of agarose hydrogels. Thermal properties of agarose are needed to analyze potential heat production by chondrocytes induced by various experimental stimuli (carbon source, cyclical compression, etc). Utilizing ASTM C177, a custom-built thermal conductivity measuring device was constructed and used to calculate the thermal conductivity of 4.5% low gelling temperature agarose hydrogels. Additionally, Differential Scanning Calorimetry was used to calculate the specific heat capacity of the agarose hydrogels. Testing of chondrocyte-embedded agarose hydrogels commonly occurs in Phosphate-Buffered Saline (PBS), and thermal analysis requires the free convection coefficient of PBS. This was calculated using a 2D heat conduction simulation within MATLAB in tandem with experimental data collected for known boundary and initial conditions. The specific heat capacity and thermal conductivity of 4.5% agarose hydrogels was calculated to be 2.85 J/g°C and 0.121 W/mK, respectively. The free convection coefficient of PBS was calculated to be 1000.1 W/m 2 K. The values of specific heat capacity and thermal conductivity for agarose are similar to the reported values for articular cartilage, which are 3.20 J/g°C and 0.21 W/mK (Moghadam, et al. 2014). This suggests that in addition to 4.5% agarose hydrogels mimicking the physiological stiffness of the cartilage PCM, they can also mimic the thermal properties of articular cartilage for in vitro studies.

Ex vivo study of the intradiskal pressure in the C6-7 intervertebral disk after experimental destabilization and distraction-fusion of the C5-C6 vertebrae in canine cadaveric specimens

OBJECTIVE

To evaluate intradiskal pressure (IDP) in the C6-7 intervertebral disk (IVD) after destabilization and distraction-fusion of the C5-C6 vertebrae.

SAMPLE

7 cadaveric C4-T1 vertebral specimens with no evidence of IVD disease from large-breed dogs.

PROCEDURES

Specimens were mounted in a custom-made 6 degrees of freedom spinal loading simulator so the C5-C6 and C6-C7 segments remained mobile. One specimen remained untreated and was used to assess the repeatability of the IDP measurement protocol. Six specimens underwent 3 sequential configurations (untreated, partial diskectomy of the C5-6 IVD, and distraction-fusion of the C5-C6 vertebrae). Each construct was biomechanically tested under neutral, flexion, extension, and right-lateral bending loads. The IDP was measured with a pressure transducer inserted into the C6-7 IVD and compared between the nucleus pulposus and annulus fibrosus and across all 3 constructs and 4 loads.

RESULTS

Compared with untreated constructs, partial diskectomy and distraction-fusion of C5-C6 decreased the mean ± SD IDP in the C6-7 IVD by 1.3 ± 1.3% and 0.8 ± 1.3%, respectively. During motion, the IDP remained fairly constant in the annulus fibrosus and increased by 3.8 ± 3.0% in the nucleus pulposus. The increase in IDP within the nucleus pulposus was numerically greatest during flexion but did not differ significantly among loading conditions.

CONCLUSIONS AND CLINICAL RELEVANCE

Distraction-fusion of C5-C6 did not significantly alter the IDP of healthy C6-7 IVDs. Effects of vertebral distraction-fusion on the IDP of adjacent IVDs with degenerative changes, such as those in dogs with caudal cervical spondylomyelopathy, warrant investigation.

Effect of Strain Rates on Failure of Mechanical Properties of Lumbar Intervertebral Disc Under Flexion

Objective

To evaluate the strain‐rate‐dependent viscoelastic properties of the intervertebral disc by in vitro experiments.

Method

The biomechanical experiments were conducted from September 2019 to December 2019. The lumbar spines of sheep were purchased within 4–6 hours from the local slaughterhouse, and the intervertebral disc samples were divided into three groups. In rupture group, the samples were used to test the mechanical behavior of the intervertebral disc rupture at different strain rates. In fatigue injury group, the samples were used to test the mechanical behavior of fatigue injury on the intervertebral disc under different strain rates. In internal displacement group, the samples were used to test the internal displacement distribution of the intervertebral disc at different strain rates by applying an optimized digital image correlation (DIC) technique.

Results

Both the yielding and cracking phenomenon occurs at fast and medium loading rates, while only the yielding phenomenon occurs at a slow loading rate. The yield stress, compressive strength, and elastic modulus all increase with the increase of the strain rate, while the yield strain decreases with the increase of the strain rate. The logarithm of the elastic modulus in the intervertebral disc is approximately linear with the logarithm of the strain rate under different strain rates. Both before and after fatigue loading, the stiffness in the loading and unloading curves of the intervertebral disc is inconsistent, forming a hysteresis loop, which is caused by the viscoelastic effect. The strain rate has no significant effect on the internal displacement distribution of the intervertebral disc. Based on the experimental data, the constitutive relationship of the intervertebral disc at different strain rates is obtained. The fitting curves are well coupled with the experimental data, while the fitting parameters are approximately linear with the logarithm of the strain rate.

Conclusions

These experiments indicate that the strain rate has a significant effect on the mechanical behavior of the intervertebral disc rupture and fatigue injury, while the constitutive equation can predict the rate‐dependent mechanical behavior of lumbar intervertebral disc under flexion very well. These results have important theoretical guiding significance for preventing lumbar disc herniation in daily life.