Posttraumatic osteoarthritis (PTOA) is typically initiated by momentary supraphysiologic shear and compressive forces delivered to articular cartilage during acute joint injury and develops through subsequent degradation of cartilage matrix components and tissue remodeling. PTOA affects 12% of the population who experience osteoarthritis and is attributed to over $3 billion dollars annually in healthcare costs. It is currently unknown whether articulation of the joint postinjury helps tissue healing or exacerbates cellular dysfunction and eventual death.We hypothesize that post-injury cartilage articulation will lead to increased cartilage damage. Our objective was to test this hypothesis by mimicking the mechanical environment of the joint during and post-injury and determining if subsequent joint articulation exacerbates damage produced by initial injury. We use a model of PTOA that combines impact injury and repetitive sliding with confocal microscopy to quantify and track chondrocyte viability, apoptosis, and mitochondrial depolarization in a depth-dependent manner. Cartilage explants were harvested from neonatal bovine knee joints and subjected to either rapid impact injury (17.34 ± 0.99 MPa, 21.6 ± 2.45 GPa/s), sliding (60 min at 1 mm/s, under 15% axial compression), or rapid impact injury followed by sliding. Explants were then bisected and fluorescently stained for cell viability, caspase activity (apoptosis), and mitochondria polarization.Results show that compared to either impact or sliding alone, explants that were both impacted and slid experienced higher magnitudes of damage spanning greater tissue depths.

Purpose: Facet joint osteoarthritis (FJOA) is widely prevalent in middleaged and older populations and is thought to be a common cause of back pain. The etiopathogenesis of FJ OA remains unclear. NFAT1 (NFATc2) is a member of the family of nuclear factor of activated T cells (NFAT/Nfat) transcription factors. Previous studies revealed that mice with a global deletion of the Nfat1 gene displayed osteoarthritic changes in appendicular synovial joints. However, whether NFAT1 deficiency affects synovial joints of the spine remains unknown. This study aimed to test our hypothesis that NFAT1 deficiency causes FJOA in mice because NFAT1 is essential for maintaining homeostasis of articular cartilage and synovium of the facet joints. Methods: Lumbar spine tissue samples were harvested from Nfat1 knockout (Nfat1-/-) mice and wild-type (WT) mice as negative controls at the age of 2, 6, and 12 months for histopathological and molecular biological analyses. L3-S1 lumbar spine samples were fixed in 2% paraformaldehyde. Tissue sections prepared with decalcification and embedded in paraffin were used for safranin-O and fast green staining to identify cartilage cells and matrices. Hematoxylin and Eosin (H&E) staining was performed for structural/cellular analysis of bone and soft tissues. Total RNA isolated from articular cartilage and synovium (with a thin layer of fibrous capsular tissue) of L3-S1 facet joints were reversetranscribed for gene expression analysis using quantitative real-time RT-PCR (qPCR). All animal procedures were approved by the Institutional Animal Care and Use Committee. Statistical analyses were performed with the Student t-test and ANOVA. Results: At 2 months of age, the intensity of safranin-O staining (for proteoglycans) in articular cartilage of the lumbar facet joints was substantially reduced in Nfat1-/-mice, compared to that of the WT mice. OA-like structural changes (e.g. articular surface fibrillations/ clefts and osteophyte formation) were not evident. At 6 months, mild to moderate OA structural changes occurred in the lumbar facet joints of Nfat1-/-mice, but not in WT mice. At 12 months, severe OA structural changes were observed in the lumbar facet joints of Nfat1-/-mice, but not in WT mice. Subchondral bone changes were not remarkable in the Nfat1-/-facet joints at any time points. qPCR gene expression analysis demonstrated significantly decreased expression of mRNA for aggrecan (Acan) with significantly increased expression of mRNA for interleukin-1b (Il1b) in the Nfat1-/-facet joint cartilage at 2 months. Significantly decreased expression of mRNA for collagen-II (Col2a1) and significantly increased expression of mRNAs for collagen-10, matrix metalloproteinase-3 (Mmp3), a disintegrin and metalloproteinase with