To investigate the mechanism of shed syndecan-4 (sSDC4) in temporomandibular joint osteoarthritis (TMJOA) in rats, aiming to provide experimental evidence for its prevention and treatment.

This study was approved by the Institutional Animal Ethics Committee. Twelve 6-week-old female Sprague Dawley (SD) rats were randomly divided into two groups. They received a single intra-articular injection into the bilateral superior cavity of temporomandibular joint, which consisted of either 50 μL of 4 mg/mL monosodium iodoacetate (TMJOA model group) or 50 μL of phosphate-buffered saline (PBS, control group). After 4 weeks, the mandibular condylar cartilage was harvested for hematoxylin & eosin (H&E) staining, Safranin O-fast green (SO) staining, and type Ⅱ collagen (Col-Ⅱ) immunohistochemical staining to assess the degree of cartilage degeneration. The synovium of the temporomandibular joint was collected for immunohistochemical staining to detect the expression levels of interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) to evaluate the degree of synovial inflammation. Synovial fluid from the temporomandibular joint cavity was collected to measure sSDC4 levels by enzyme-linked immunosorbent assay (ELISA). In addition, 12 6-week-old female SD rats were randomly divided into a His-SDC4 group and a control group, receiving injections into the bilateral superior cavity of temporomandibular joint of either 100 ng/mL (50 μL) of His-SDC4 protein or 50 μL of PBS once every 3 days for a total of 28 days. The same experimental procedures were performed for H&E staining, SO staining, and immunohistochemical staining (Col-Ⅱ IL-6, TNF-α) to observe condylar cartilage degeneration and detect synovial inflammation. Rat synovial fibroblasts and condylar chondrocytes were cultured in vitro and randomly divided into a His-SDC4-stimulated (10 ng/mL) group and control group. Perform CCK-8 cytotoxicity assays and observe cellular morphology under optical microscopy, the mRNA expression levels of IL-6 and TNF-α were detected by real-time quantitative polymerase chain reaction (RT-qPCR), and the levels of IL-6 and TNF-α in cell culture supernatants were measured by ELISA.

Compared with the control group, the TMJOA group showed decreased condylar cartilage thickness, percentage of SO-positive area, and percentage of Col-Ⅱ-positive area (all P<0.001); an increased synovitis score (P<0.001) and increased percentages of IL-6- and TNF-α-positive cells in the synovium (all P<0.001); and a significant increase in sSDC4 levels in the synovial fluid (P=0.011). Following intra-articular injection of His-SDC4, condylar cartilage thickness, percentage of SO-positive area, and percentage of Col-Ⅱ-positive area all decreased (all P<0.001); the synovitis score increased (P=0.006), and the percentages of IL-6- and TNF-α-positive cells in the synovium increased (all P<0.001). In vitro experiments showed that His-SDC4 stimulation significantly upregulated the expression levels of IL-6 and TNF-α in both synovial fibroblasts and condylar chondrocytes (all P<0.01), and the levels of these two cytokines in the culture supernatants also significantly increased (all P<0.01).

During TMJOA progression, the level of sSDC4 in the synovial fluid is significantly elevated, which can directly stimulate synovial fibroblasts and condylar chondrocytes to secrete more pro-inflammatory cytokines, forming a vicious cycle that accelerates TMJOA progression.

To explore the biological effects of electromagnetic pulse (EMP) with different high intensities on condylar cartilage in rats.

SD rats were randomly divided into a sham group (Sham) and an irradiation group (EMP1: 500 kV/m, 10 Hz; EMP2: 270 kV/m, 10 Hz). Then, they were sacrificed at 1 h, 3 h, 12 h, 24 h and 3 d after irradiation. The degree of cartilage degeneration was evaluated by HE, safranine O-fast green, type Ⅱcollagen immunohistochemistry and TUNEL staining. Immunohistochemistry and western blot were performed to detect the expression of the matrix degradation factors: matrix metalloproteinase-13 (MMP-13), a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS-5) and the apoptosis key factor cleaved-cysteinyl aspartate specific proteinase (cleaved-Caspase3) in condylar cartilage.

HE staining showed that, compared with the Sham group, a small amount of exfoliation was found on the fibrous surface layer of the cartilage after irradiation in the EMP1 and EMP2 groups. Compared with the Sham group, the percentage of safranine O-fast green-positive area decreased significantly at 12 h and 24 h (both P＜0.01) in the EMP1 group and 12 h and 24 h in the EMP2 group (both P＜0.05); the percentage of type Ⅱcollagen-positive area decreased significantly at 3 h and 12 h (P＜0.05, P＜0.001) in the EMP1 group. In addition, the number of TUNEL-positive apoptotic cells increased significantly at 1 h, 3 h, 12 h, and 24 h in the EMP1 group and 1 h, 3 h, and 12 h in the EMP2 group (P＜0.05). Moreover, at different timepoints (except at 3 d) in the EMP1 group and EMP2 group, the percentage of MMP-13, ADAMTS-5- and cleaved Caspase3-positive chondrocytes and their protein levels in condylar cartilage increased significantly after irradiation (P＜0.05).

EMP with a certain degree of high-intensity can induce early transient damage to condylar cartilage. This effect is dose-and timedependent.