Hallux valgus (HV) is a prevalent foot deformity characterized by the lateral deviation of the great toe at the first metatarsophalangeal joint [[1], [2], [3]]. This condition is frequently encountered in medical practice and is often associated with significant functional disability and foot pain [4,5]. Epidemiological studies have shown that HV affects approximately 23.0 % of adults aged 18–65 years and up to 35.7 % of those over 65 years [6]. This deformity impacts not only the elderly population but also the relatively younger working-age population [[7], [8], [9]].

The etiology of HV is multifactorial, involving genetic predisposition, foot structure, and external factors such as footwear [10]. Gender is one of the most critical factors, with epidemiological studies showing that HV is three times more prevalent in women than in men in the general population [6,11,12]. Other factors, such as obesity, flatfoot, and a history of wearing high-heeled shoes, have also been suggested; however, no definitive conclusions have been reached [13]. Most of the studies included in this review are small-scale, with sample sizes of fewer than 300 participants, and assessments of HV in large cohort studies are often based on self-reported diagnoses rather than objective evaluations. To identify factors associated with HV, large-scale investigations with objective assessments of HV are needed.

Compared to Western countries, Japan has a relatively short history of adopting Western-style footwear but has a long-standing and unique tradition of traditional sandals made from straw and wood [14]. Differences in shoe selection and usage may influence the development of HV, but further research is required to substantiate this hypothesis. In Japan, reports on HV prevalence have been limited to specific populations, such as older adults living in rural areas (Nishimura et al., 23 %, Matsumoto et al., 39 %) [15,16] and female university students (Okuda et al., 30 %) [17]. Large-scale studies encompassing a wide age range are essential to comprehensively determine the prevalence of HV and identify its associated risk factors.

In light of this context, we conducted the first-ever prospective population-based cohort study (HAllux valgus and Pain ePIdemiology study: HAPPI study) in Japan to clarify the incidence of HV and investigate its associated risk factors. This study aimed to contribute to the development of effective preventive and therapeutic strategies tailored to the Japanese population. The purpose of this study was to elucidate the incidence of HV in Japan and to identify its risk factors through a robust epidemiological approach.