Osteoarthritis is the most common joint disease in adults worldwide, with the knee being most frequently affected [1]. The main symptoms of knee osteoarthritis (KOA) included knee pain and functional limitations, which seriously affect the quality of life [2]. The incidence of KOA increases with factors such as age, gender, obesity, excessive physical activity and previous knee trauma [3,4]. It could reduce patient employability, increase healthcare costs and place a heavy financial burden on individuals and society [3].

Acupuncture is widely used in clinical management of KOA [5], [6], [7], [8], [9], [10]. The guidelines of both American College of Rheumatology (ACR) and the American Academy of Orthopedic Surgeons (AAOS) conditionally recommended the use of acupuncture [11,12]. Acupuncture treatment has a low incidence of adverse effect and is cost-effective [13,14]. The effect of acupuncture is influenced by many stimulation parameters, such as treatment frequencies and intervention duration. Our team had proved that 3 sessions per week of acupuncture immediately improved knee pain and dysfunction compared with 1 session per week of acupuncture [15]. However, there is a lack of evidence comparing different duration of acupuncture for KOA. Determining a reasonable duration of intervention is vital to the effect of acupuncture [16]. Our previous study “Efficacy of intensive acupuncture versus sham acupuncture in knee osteoarthritis: a randomized controlled trial” (ATKOA) found that 8-week acupuncture treatment improved pain and joint function, and the effect maintained until week 26 [6]. Researches by other teams suggested that 4-week acupuncture relived pain and improved joint function in patients with KOA [17], [18], [19]. But it is unclear whether 4-week acupuncture could achieve the same effect as 8-week acupuncture for KOA. This study aims to evaluate the influence of acupuncture duration on the effect in treating KOA through a secondary analysis based on intra-group comparison. Secondary analysis helps to draw new findings from the various data sources cited, providing a solid basis for further research, and it is also time- and resource-saving. This study could provide a view for KOA patients to formulate a more reasonable and effective duration of acupuncture.