Imaging characteristics of rabbit femoral cartilage in the early post-trauma by µMRI and PLM

INTRODUCTION: Joint injuries, with or without associated disruption of the articular surface, frequently lead progressively toward a severe unbearable condition, post-traumatic osteoarthritis (PTOA). The detection of at-risk OA individuals could enable effective interventions before any major structural damage has occurred or before the pain becomes chronic in the pre-symptomatic stage. Despite the knowledge of the potential pathogenic mechanisms occurring in the injured joint have increased in the past years, specific predictive markers are lacking, in particular, non-invasively. This study aimed to study the characteristics of femoral cartilage in the rabbit model for six weeks quantitively using µMRI and PLM.

METHODS: A total of 12 female New Zealand rabbits were equally grouped into two sets: 0 weeks and 6 weeks. In both groups, the femur of one knee from each animal was impacted through a surgical approach and the contralateral knee was kept intact. For the 0-week group, the animal was sacrificed immediately after the impact, and a cartilage bone plug of 2 x 3 x 2 mm was excised from the femur at the impact site and imaged using µMRI at a resolution of 11.7µm/pixel. Histological sections were made on the cartilage tissue and imaged using Polarized Light Microscopy (PLM) at a resolution of 1µm/pixel. The 6-week group had the animal sacrificed six weeks after the one-time impact and similar imaging procedures were performed.

RESULTS: T2 relaxation values generated from the µMRI with the surface of the cartilage oriented at 55˚ to the external magnetic field B_o showed an increase in the 6-week group’s bulk cartilage samples, with a statistical difference of p-value = 0.06 . The major contribution towards this T2 increase came from the superficial (SZ) and transitional zone (TZ). The optical Angular and Retardation images and depth-dependent profiles generated from the PLM, which demonstrates a reduced SZ in the 6-weeks of impacted cartilage.

CONCLUSION: An increase in T2 values in 6 weeks of cartilage samples explains the increase in hydration in the early progression of PTOA. Reduction in the superficial zone thickness from the angular and retardation measurements demonstrates subtle disruption of the cartilage surface.