Characterization of facet joint cartilage properties in the human and interspecies comparisons.

The facet joint, a synovial joint located on the posterior-lateral spine, is highly susceptible to degenerative changes and plays a significant role in back-related morbidities. Despite its significance, the facet is rarely studied and thus current treatment strategies are lacking. This study aimed to characterize, for the first time, the properties of human, pig, monkey, and rabbit lumbar facet cartilage providing much-needed design criteria for tissue engineering approaches. In this study, where possible, the facet's morphological, histological, mechanical, and biochemical properties were evaluated. Comparisons between the properties of the inferior and superior facet surfaces, as well as among spinal levels were performed within each species. In addition, interspecies comparisons of the properties were determined. The human facet joint was found to be degenerated; 100% of joint surfaces showed signs of pathology and approximately 71% of these were considered to be grade 4. Joint morphology varied among species, demonstrating that despite the mini-pig facet being closest to the human in terms of width and length, it was far more curved than the human or any of the other species. No notable differences were found in the mini-pig, monkey, and rabbit mechanical and biochemical properties, suggesting that these species, despite morphological differences, may serve as suitable animal models for studying structure-function relationships of the human facet joint. The characterization data reported in this study may increase our understanding of this ill-described joint as well as provide the foundation for the development of new treatments such as tissue engineering. STATEMENT OF SIGNIFICANCE This work provides the first comprehensive description of the properties of lumbar facet joint cartilage. Importantly, this work establishes that histological, biochemical, and mechanical properties are comparable between bipedal and quadrupedal animals, helping to guide future selection of appropriate animal models. This work also suggests that the human facet joint is highly susceptible to pathology. The mechanical properties of facet cartilage, found to be inferior to those of other synovial joints, provide a greater understanding of the joint's structure-function relationships as well as the potential etiology of facet joint pathology. Lastly, this work will serve as the foundation for the development of much-needed facet joint treatments, especially those based on tissue engineering approaches.