# Transcranial Direct Current Stimulation in the Treatment of Chronic Knee Pain: A Scoping Review
## 1 Introduction  
Chronic knee pain is a pervasive and debilitating medical condition that affects a significant portion of the global population, particularly among older adults and those with underlying health conditions such as osteoarthritis and rheumatoid arthritis [1; 2; 3]. The impact of chronic knee pain extends beyond physical discomfort, often leading to reduced mobility [4], psychological distress, and a diminished quality of life [5]. A wide choice of therapeutic options is available for chronic knee pain ranging from conservative (e.g., patients' education, physical therapy, topical and systemic non-steroidal anti-inflammatory drugs, opioids, and intra-articular treatments) [6] to surgical interventions like knee replacement, with varying degrees of success [7; 8; 9]. The impact of chronic knee pain extends beyond individual suffering; it imposes substantial burdens on healthcare systems worldwide, including increased healthcare costs, frequent medical consultations, and the necessity for long-term management strategies. Given the widespread prevalence and the profound personal and societal impacts of chronic knee pain, there is an urgent need for effective and accessible treatment options. This scoping review aims to evaluate the efficacy of Transcranial Direct Current Stimulation (tDCS) as a potential non-invasivetreatment for chronic knee pain in older adults. By systematically examining existing research, we seek to provide a comprehensive overview of tDCS's effectiveness, safety, and underlying mechanisms. This review will address critical gaps in current research and offer insights to guide future studies and clinical practices. To underline the importance of this review, it is crucial to highlight the limitations of conventional treatments for chronic knee pain, such as potential side effects, high costs, and limited long-term efficacy. Therefore, exploring alternative modalities like tDCS, which promises a favorable safety profile and potential neuromodulatory benefits, is of paramount importance. Our goal is to contribute to the ongoing discussion on innovative, non-invasive treatments for chronic knee pain, ultimately aiming to enhance patient outcomes and reduce the overall burden on healthcare systems [10]. However, these treatments often come with their own set of limitations, including:  
1. Potential side effects, such as gastrointestinal issues from NSAIDs or addiction risks from opioids. 2. High costs associated with long-term medication use or surgical interventions. 3. Limited long-term efficacy, as some treatments may only provide temporary relief. Therefore, there is growing interest in exploring alternative non-invasive treatment modalities that can offer effective pain relief without the associated drawbacks of conventional therapies. One such promising avenue is the use of Transcranial Direct Current Stimulation (tDCS) [11; 12; 13]. tDCS is considered promising because it is non-invasive, has a favorable safety profile, and has shown potential in modulating neural pathways involved in pain perception and modulation. Additionally, preliminary studies indicate that tDCS can improve pain and functional outcomes in various neurological and psychiatric conditions [14]. Transcranial Direct Current Stimulation involves the application of a low electrical current to the scalp through electrodes to modulate neuronal activity [15]. The technique has been explored in a range of medical conditions, from depression and anxiety to chronic pain syndromes. Preliminary studies suggest that tDCS could offer a novel approach to managing chronic knee pain by targeting the neural pathways involved in pain perception and modulation [16; 17; 18; 19; 20; 21; 22; 23; 24]. This review aims to identify and address specific gaps in current research on tDCS for chronic knee pain, including its efficacy, safety, and mechanisms of action, to guide future research directions.