Introduction: Adult acquired flatfoot deformity (AAFD) is a disabling condition that may require complex surgical treatment. Little is known about the incidence of AAFD in the general population and specifically of AAFD requiring specialist care. We aimed to describe the incidence of AAFD referred to specialist care in the Swedish general population.

Methods: We conducted a nation-wide epidemiological register study to estimate the incidence of referred AAFD in the general population. We retrieved data from the Swedish National Patient Register. All individuals aged 16 years or older, with a first-time diagnosis of AAFD (ICD-10 code M214) between 2007 and 2018 were identified. Total incidences, change over time, and gender-specific and age-specific incidences per 100,000 person-years were calculated using population size data from Statistics Sweden. Incidences were compared using the Poisson test.

Results: The incidence rate of referred AAFD in the general population was 23.0 (95% CI 22.7-23.3) per 100,000 person-years. The incidence rate in women was 30.4 (95% CI 29.9-30.8) and in men was 15.4 (95% CI 15.1-15.8). The highest incidence rates were found in the age Group 61-75 years. The incidence rates varied significantly across the 21 regions in Sweden. The age-standardized and sex-standardized incidence rates ranged from 8.3 (95% CI 7.2-9.4) to 69.1 (95% CI 62.4-75.8).

Conclusion: AAFD requiring referral to specialist care is common in the general population. Women had nearly twice the incidence of AAFD compared to men. Large unexplained regional variations in the incidence rates exist.