Introduction

Increasing attention has been paid to endplate(EP) injury as having a crucial role in the etiopathogenesis of painful intervertebral disc (IVD) degeneration due to associations of Modic changes (MCs) and pain in clinical^[1]. EP injuries are described to occur through peripheral or central accumulating microfracture, and accumulation of EP microfractures are involved in all types of EP injury^[2]. Models of EP microfracture that progresses to painful conditions are highly needed to better understand pathophysiological mechanisms and screen therapeutics. This study established an EP microfracture model and characterized associated IVD degeneration, vertebral remodeling, and pain-related behaviors.

Methods

Male Sprague-Dawley rats (5 months of age) were randomly divided into 3 groups: Sham (n=6), EP injury+PBS (n=6), EP injury+TNFα (n=7). For the EP injury groups, the proximal EPs of L4-5 and L5-6 were punctured obliquely through the vertebral body using a 0.6 mm K-wire (Fig. 1A). A total of 2.5 ul of PBS or TNFα (0.25 ng in 2.5 ul) were then injected into each IVD following the EP injury. IVD height was quantified using radiography pre-operatively, and at 8 weeks. Behavioral tests (i.e. hind paw von Frey and forepaw grip test) were performed biweekly. Lumbar spines were dissected for MRI and μCT analyses.

Results

EP injures induced IVD degeneration with decreased IVD height and MRI T2 values compared to Sham (Fig. 1B, D). EP injury showed MC type1-like changes around the EP defects on MRI with hypointensity on T1-weighted images and increased T2 of bone marrow being visible in the EP+TNFα group. EP microfracture similarly showed trabecular bone remodeling on μCT, and EP+TNFα further showed secondary damage in cartilage EP adjacent to the injury (Fig. 2). EP injures caused significantly decreased paw withdrawal threshold & reduced grip forces (Fig.3), suggesting increased pain sensitivity. 

Discussion

Decreased grip force and paw withdrawal threshold following EP microfracture suggest increased pain-like behaviors consistent with axial mechanical pain. EP-driven axial pain is consistent with our biomechanical findings that EP microfracture cause axial instability^[3]. Interestingly, we created an EP injury with size of only  ~2% of the average rat lumbar EP surface area^[4], yet this minimal injury appears sufficient to induce axial pain since two injuries groups had identical behavioral results. Most importantly, this EP microfracture model induced Modic-like changes and the EP+ TNFα group appeared to amplify bone marrow/IVD crosstalk to stimulate MC type1-like changes, and these results are consistent with Dudli et al. who found proinflammatory stimulus was critical to induce MC1-like changes^[1]. This EP injury model might therefore be a clinically relevant model involving IVD pro-inflammatory conditions and EP microfracture accumulation to better understand and screen EP pathophysiology. Further studies will associate pain-related behaviors with signal intensity of MC1-like bone marrow changes to further compare with the human condition, and will assess nervous system changes to inform pain pathomechanisms.

References:

[1]. Dudli+The Spine Journal, 2018 [2]. Lotz +Global Spine, 2013 [3]. Wang+ORS, Poster1432, 2020 [4]. Jaumard+Spine,2015 

Acknowledgment: Funded by NIH grants R01AR057397, R01AR078857, and R01EB019980.