Intra-Articular Physiological Saline in Temporomandibular Disorders May Be a Treatment, Not a Placebo: A Hypothesis, Systematic Review, and Meta-Analysis

<b>Background:</b> Intra-articular injections reduce pain in patients with temporomandibular joint (TMJ) disorders who are unresponsive to conservative treatment. Hyaluronic acid, blood products, and medications provide rapid relief when administered this way, although their mechanisms of action remain unclear. In control groups, which are intended to be untreated, 0.9% NaCl is typically delivered. The hypothesis that "normal saline injections in TMJ cavities produce a therapeutic effect" is proposed, with an exploration of its potential verification, alongside a systematic review and meta-analysis of studies on intra-TMJ 0.9% NaCl. <b>Methods:</b> Randomized controlled trials (RCTs) on patients with TMJ internal derangement, arthritis, or degeneration were selected under PRISMA 2020 and assessed with RoB2. <b>Results:</b> Seven RCTs with 359 patients were included. Weekly follow-ups revealed a decrease in articular pain by 23.72% (SE: 0.84%; 95% CI: 24.38-21.06%; <i>p</i> < 0.01), and monthly follow-ups indicated a decrease of 34.01% (SE: 1.09%; 95% CI: 36.16-31.86%; <i>p</i> < 0.01) compared to the baseline values. These findings were grounded in low-risk-of-bias evidence on 267 patients in five RCTs and 222 patients in four RCTs, respectively. <b>Conclusions:</b> The hypothesis warrants further testing to determine whether, in addition to the known biological activity of typical injectables, the mechanical action also contributes to pain relief.