The Development of a Murine Model of Compression Induced Vertebral Degeneration with Associated Bone Marrow Edema

Purpose. The absence of predictable radiographic changes that correlate with painful vs. non-painful structural changes pose the greatest obstacle to progress in the effective evidence based treatment of chronic low back pain (LBP). Currently clinicians utilize Magnetic Resonance Imaging (MRI) as the most sensitive indicator of degeneration, using criteria developed by Michael Modic1;2 in which Modic Type 1 (MT1) MRI changes are associated with the early stages of chronic low back pain and used to designate those individuals who most likely benefit from surgical intervention.3 However, the nature of these Modic changes remains unknown. Thus, to the end of elucidating the cellular and biomechanical changes that lead to MT1 we propose the development of a mouse model utilizing chronically loaded tail vertebrae that closely resembles the radiology and histopathology of vertebral degeneration seen in humans.;Methods. Our murine model utilizes an Ilizarov-type device to deliver compressive forces via titanium pins implanted in the tail vertebrae. A novel surface coil for high-resolution (∼100mum) contrast enhanced (CE)-MRI developed to characterize bone marrow edema in the mouse knee was then used about the region of interest (ROI) in the tail. A T1-weigthed FLASH sequence was used to acquire images in a 3T clinical MRI. Using commercially available software we adapted previously developed techniques for quantifying biomarkers of bone marrow edema. These biomarkers were used in several longitudinal studies including the natural history of arthritis as well as the induction and treatment of vertebral degeneration. Micro-Computed tomography (Micro-CT ∼10microm) was employed to quantify bony structural changes. Histological sectioning (3mum) and immunohistochemistry were also performed on harvested specimen to assess cellular changes, via histomorphometry.;Results. Longitudinal studies of wild type mice and transgenic mice genetically modified to overproduce tumor necrosis factor (TNF-Tg) demonstrated significant differences in normalized marrow contrast enhancement (NMCE) values. Wild type mice showed a significant decrease in NMCE values from 8 to 12 weeks of age, corresponding to decreased levels of cellularity found during histomorphometric investigation. TNF-Tg mice maintained elevated NMCE values through 24 weeks of age. These elevated NMCE values corresponded to increased cellularity and vascularity found in the analysis of orange G and alcian blue (OG/AB) stained slides. Longitudinal studies of loaded vertebrae demonstrated elevated NMCE values corresponding with an increase in cellularity and vascularity seen in histological analysis similar to that seen in the TNF-Tg mice. A transgenic knock out line of mice lacking the tumor necrosis factor receptor type 1 and type 2 (TNFR1R2 -/-) failed to produce an increase in NMCE values when the same loading was applied. However, wild type mice placed under compressive loading that went on to develop bone marrow edema, failed to respond to anti-TNF therapy as NMCE values were not significantly different from the placebo treated mice. These early experiments shed some light on the importance of TNF as a mediator of load induced BME changes in the vertebrae adjacent to degenerating discs, and enhance our understanding of BME as a biomarker of LBP.;Conclusions. This study demonstrates the utility of a murine model in the investigation of the inflammatory processes leading to debilitating spinal degeneration. The translational component of this work cannot be overstated. This model provides the ability to investigate diagnostic techniques and treatment options directly applicable to the human population. It also allows us to better understand the initiation and developmental stages of such degenerative cascades, so that we might diagnose earlier and develop superior evidence based treatments.