Preclinical evaluation of dose-dependent effects of photobiomodulation therapy on persistent inflammation in the temporomandibular joint.

BACKGROUND: Low-level laser therapy (LLLT) is a non-invasive, innovative approach for alleviating pain and improving function in temporomandibular joint (TMJ) disorders. This study aimed to compare the effect of three different doses of photobiomodulation (PBM) (2.5, 5 or 10 J/cm2) on the reduction of nociceptive events of rats' TMJ with persistent inflammation and inflammatory profile modulation. METHODS: Male Wistar rats (n = 240) were submitted to a model of temporomandibular inflammation induced by induced by Complete Freund's Adjuvant (CFA) and treated with PBM on days 1, 3, 5, 7 and 10 post-CFA or saline (SAL) injection. Assessments included orofacial mechanical sensitivity through von Frey test, quantification of Evans blue plasma extravasation, myeloperoxidase activity analysis (MPO) and neutrophil/leukocyte infiltration in synovial fluid and TMJ tissues. RESULTS: Regarding the analgesic effect of LLLT, an inverted U-shaped curve was observed, as despite the effectiveness of all three tested doses, the 5 J/cm2 dose completely prevented the reduction in the mechanical allodynia threshold induced by temporomandibular inflammation. Regarding the evaluation of inflammatory parameters, all three doses studied effectively reduced plasma extravasation, neutrophils and MPO, with more pronounced effects observed at the 5 and 10 J/cm2 doses. Interestingly, the lowest dose evaluated, 2.5 J/cm2, partially prevented mechanical allodynia, plasma extravasation, MPO activity and neutrophil count, both in the acute application and with repeated treatment for up to 10 days. Total leukocyte levels, however, were reduced with LLLT at the 5 and 10 J/cm2 doses. CONCLUSIONS: This study highlights the dose-dependent efficacy of LLLT, with an intermediate dose (5 J/cm2) producing the most pronounced analgesic effects and the 5 and 10 J/cm2 doses demonstrating significant therapeutic benefits in reducing acute and persistent TMJ inflammation. Importantly, even the lowest dose effectively modulated key components of the innate immune response, indicating its potential to reduce early inflammatory processes.