A systematic review and meta-analysis was conducted with the goal of examining the existing literature regarding the effectiveness of anodal transcranial direct current stimulation (a-tDCS) applied to the primary motor cortex (M1) as a standalone method or as a priming technique in alleviating pain among individuals with knee osteoarthritis.

As per the PRISMA statement, a systematic literature search across CENTRAL, Embase, CINAHL, and MEDLINE was carried out. The electronic databases were queried using a combination of search terms tailored for each database. These terms included “Knee” OR “Knee osteoarthritis” OR “Osteoarthritis” AND “transcranial direct current stimulation” OR “tDCS” OR “anodal transcranial direct current stimulation” OR “cathodal transcranial direct current stimulation” OR “Non-invasive brain stimulation”.

The major endpoint was pain intensity scales post-intervention, evaluated through Knee Injury and Osteoarthritis Outcome Score (KOOS), Western Ontario and McMaster Universities Arthritis Index (WOMAC), numerical rating scale (NRS), and visual analogue scale (VAS). Two reviewers initially screened abstracts and titles for identifying potential studies, and subsequently, the full text of selected studies underwent independent review by the same reviewers to determine eligibility.

Assessment of the studies' risk of bias was done independently by two reviewers by employing the Cochrane risk of bias tool (RoB 2.0). With the utilization of RevMan 5 software (version 5.2), the results were pooled.

The analysis encompassed 14 studies involving 740 subjects dealing with osteoarthritis of the knee. Within the meta-analysis, six studies contrasted the effects of a-tDCS alone against sham stimulation, while five studies examined the impact of a-tDCS in combination with other interventions versus sham stimulation.

The results indicated a favorable effect of a-tDCS alone on pain in those suffering from knee osteoarthritis (standard mean difference [SMD] -0.52; I2 = 69%). Additionally, the combination of a-tDCS with other treatments demonstrated a positive effect (SMD -1.23; I2 = 48%) on knee osteoarthritis pain. However, the certainty of this evidence is limited because of the heightened risk of bias and imprecision observed in the studies.

a-tDCS may be regarded as both an independent and supplementary approach in mitigating knee osteoarthritis pain. Subsequent randomized studies must tackle issues related to the quality of research, such as limited sample sizes, in order to bolster the overall confidence in the results.