Introduction: Chronic inflammation has the potential to alter mechanical properties of the intervertebral disc. This has been shown in rat tail functional spine units [1], however it is unknown how an isolated, intact rat tail disc may be affected in response to an inflammatory-induced environment. Mechanical testing of an intact annulus provides a more comprehensive quantification of disc mechanics, offering a holistic approach [2]. Decorin, a class 1 small leucine-rich protein (SLRP), has the potential to stimulate an inflammatory environment by promoting the release of pro-inflammatory cytokines, such as IL-6, through the TLR2/4 pathway [1]. An inflammatory environment can alter disc mechanics, however there is limited knowledge regarding the role of the nucleus pulposus during incubation, and whether its presence impacts the disc’s integrity post-incubation. The purpose of this study was to determine the short-term mechanical effects of decorin on isolated annulus rings and to determine if nucleus presence during incubation alters the response.
Methods: Thirty-two annulus fibrosus rings were dissected from eight female Sprague Dawley rat tails. All samples were randomized into one of four culture conditions: control media with nucleus present (n=8), control media without nucleus present (n=8), 5µg/mL decorin media with nucleus present (n=8), and 5µg/mL decorin media without nucleus present. Samples were cultured for 24 hours. Following the culture period, annular rings were mechanically tested in tension to failure at a rate of 2%/sec [2]. Mechanical outcomes of interest included Young’s modulus, end-of-toe-region stress and associated strain, initial failure stress and associated strain, and maximum stress and associated strain.
Results: No significant effect of either treatment (decorin versus control) or condition (with nucleus versus without nucleus) was observed for any mechanical property. However, medium to high effect sizes (Cohen’s D ranging from 0.51 to 0.89) were noted between non-inflamed annular controls and decorin treated annular samples such that decorin resulted in reduced stiffness as well as lower toe-region and failure strain. Medium to high effect sizes (Cohen’s D ranging from 0.53 to 0.83) were also noted such that in the non-inflammatory control samples, culturing with the nucleus present resulted in lower failure stress and maximum stress as well as lower strain at failure.
Discussion: Though not significant, the findings of this study indicate that short term exposure to decorin decreases stiffness of the annulus and increases tensile deformation. It is likely if the culture period was longer, these observed changes would have been larger and statistically significant. Although inflammatory changes can be detected at the cellular level within 24-hours [1], it is probable that mechanical changes continue to manifest days later. Future studies will focus on understanding the time-course of mechanical changes at the disc level. The findings of this study also suggest that the presence of the nucleus pulposus during culture may affect the strength of the disc, suggesting the cells of the nucleus play a key role in how the annulus responds to stimuli mechanically. Depending on the research question, it may be appropriate to remove the nucleus prior to culturing.