Therapeutic ultrasound (US) and photobiomodulation (PBM) are promising treatment modalities for restoring articular cartilage. Owing to their different mechanisms of action, individual or combined stimulation may elicit different bioeffects on chondrocytes. This study aimed to evaluate the potential of US and PBM, applied alone or combined, in augmenting chondrocytes' responses toward cartilage matrix synthesis and reverting their catabolic activities, with or without interleukin-1 (IL-1) incubation. Human chondrocytes were cultured under basal or proinflammatory conditions (1ng/ml IL-1) and stimulated daily for 6 days with US, PBM, US followed by PBM, and PBM followed by US. Chondrocyte activity, along with protein and mRNA expression of cartilage synthesis- and degradation-related markers, was analyzed. IL-1 incubation did not significantly affect chondrocytes' metabolic activity or sulfated glycosaminoglycans (GAGs) production. Both US and PBM, alone or combined, increased the protein and mRNA of collagen type II (COL II) and aggrecan (ACAN) under basal and proinflammatory conditions. PBM had a stimulatory effect on GAG synthesis. While PBM did not influence the expression of cartilage destruction markers, US potentiated the chondrocytes' response to inflammation by increasing metalloproteinases and IL-1 mRNA levels. The sequential combination of US and PBM induced distinct bioeffects depending on the application order, but these were not superior to the effects of individual treatments. As this study used chondrocytes from a single donor, generalizability may be limited. While both US and PBM stimulated matrix synthesis, PBM showed a comparatively greater ability to mitigate chondrocyte degradation and may be of interest for cartilage repair strategies.

Photobiomodulation (PBM) within red to near-infrared spectrum has been proposed as a method to restore articular cartilage injuries, due to its ability to modulate effectively chondrocyte activity, namely viability, proliferation and extracellular matrix synthesis. This in vitro study aimed to determine the most appropriate PBM dosage on human chondrocytes. Firstly, human chondrocytes were stimulated daily or every other day with light-emitting diodes (LEDs) at wavelengths of 600940 nm and power densities of 417 mW/cm2, in continuous or pulsed mode for up to 4 min with temperature monitored during stimulation. After 3 days, the PBM most effective protocols enhancing metabolic activity and proliferation were further applied for seven days to assess their effect on cartilage protein production using immunocytochemistry, western blot, and alcian blue staining. Near-infrared LEDs (850 and 940 nm) at power densities of 417 mW/cm2, in continuous or pulsed mode, significantly increased normalized chondrocyte metabolic activity, regardless of the periodicity or operation mode applied. A dose-dependent response was demonstrated, with variations among the PBM parameters. Particularly, PBM with 17 mW/cm2 at 940 nm consistently promoted collagen type II, aggrecan, and glycosaminoglycans deposition in the chondrocyte matrix, while preserving their normal phenotype in monolayer cultures. Stimulation with LEDs did not cause a significant temperature increase. Herein, stimulation with near-infrared LEDs effectively increased the viability and secretion of cartilage proteins in human chondrocytes. This approach holds promise as a non-invasive therapeutic modality for articular cartilage repair in future preclinical and clinical research.