A Viable Animal Model of Neuroarthropathic Changes in Wild-Type Rodents

CATEGORY: Diabetes, Hindfoot, Midfoot/Forefoot INTRODUCTION/PURPOSE: Neuroarthropathy is a progressively debilitating disease commonly affecting the lower extremity, which can lead to profound bone and joint destruction, limb deformity, and even amputation. While there are several causes, long- standing neuropathy is the most common. This leads to loss of protective sensation to the foot, which combined with repetitive micro-trauma as a result of physical activity can lead to bone and joint destruction of the lower extremity as well as major deformity. Currently there are no animal models of this disease process. The purpose of this study was to develop a novel rodent model of neuroarthropathy through the induction of the underlying neuropathic state combined with a treadmill running protocol that closely resembles local micro-trauma seen in humans. METHODS: Following IAUCUC approval, 8 diet-induced obesity (DIO) C57BL/6 mice were obtained along with 6 non-obese C57BL/6 control mice. DIO mice were fed a high-fat diet (60% fat by kcal) ad libitum starting at the age of 6 weeks. Control mice were administered a standard low-fat diet (10% fat by kcal) ad libitum from birth. Starting at 14 weeks-old, all mice underwent a controlled, high-intensity cardiovascular training protocol using a rodent treadmill four times per week at 30 minute intervals. This was carried out for seven weeks including a one-week acclimation period. Speed, distance and time were held constant between groups. Weekly hind-paw sensory assessments were performed including von Frey filament and hotplate testing. Anteroposterior and mediolateral X-Rays were obtained at the beginning and conclusion of the study and radiographic analysis was performed bilaterally for all specimens. Lastly, histopathologic analysis was performed to assess musculoskeletal destructive changes to the hind-paw. RESULTS: Baseline hotplate testing revealed that obese and non-obese mice did not differ in thermal response latency (p=0.53), however following the seven-week running protocol, obese mice demonstrated increased thermal response latency compared to controls (p=0.006). Von Frey filament testing in obese mice depicted significantly reduced hind-paw withdrawal reflexes compared to controls, both prior-to and following treadmill running (p<0.001). Comparison of initial and final radiographs for obese (figures 1.A and 1.B) and non-obese mice (figures 1.D and 1.E) showed an average increase in talo-first metatarsal angle of 7.66 degrees (p=0.007) and 0.66 degrees (p=0.75) respectively. The difference in change between groups was also significant (p<0.05). Preliminary histopathological analysis revealed significant abnormal bone remodeling in obese mice following treadmill running (figure 1.C) compared to controls. CONCLUSION: Obese, neuropathic animals, when challenged with a controlled exercise regimen designed to induce micro-trauma develop reliable sensory, radiographic, and histopathologic parameters reminiscent of neuroarthropathy in humans. Conversely, non-obese control animals, treated with the same exercise regimen, do not develop these neurodegenerative and musculoskeletal changes. Perhaps wild-type obese mice can be further characterized as a viable model for the development of idiopathic neuroarthropathy.