Temporal Changes in the Age at Diagnosis of Osteonecrosis of the Femoral Head and Potential Associated Factors: A Multicenter Sentinel Monitoring System in Japan.

Background: In Japan, a multicenter sentinel monitoring system was implemented as a patient registry for osteonecrosis of the femoral head (ONFH). Although recent monitoring data suggest that the age at diagnosis of ONFH is increasing, it is unclear whether this trend is due to Japan's aging population. We aimed to evaluate the temporal trends in ONFH incidence by age group using other epidemiological data.

Methods: From the monitoring system up to 2023, patients who had diagnoses between 2002 and 2021 were categorized into four 5-year periods (2002 to 2006, 2007 to 2011, 2012 to 2016, and 2017 to 2021). To determine the incidence by sex and 10-years age groups, the sex- and age-specific proportions of newly diagnosed (incident) ONFH cases were calculated from the monitoring system. Assuming this distribution applies to all ONFH patients in Japan, the numerator of the sex- and age-specific annual incidence rates of ONFH was calculated by multiplying each proportion by the number of incident ONFH patients in the country, as estimated from previous nationwide epidemiological surveys. Japan's sex- and age-specific total population was used as the denominator.

Results: The incidence rate (per 100,000 population) for men aged under 29 years decreased over the four periods (1.08, 1.06, 1.02, and 0.90, respectively), as did the rate for those aged 30 to 39 years (4.40, 3.70, 3.73, and 3.30, respectively). Conversely, an increase was observed in men aged 60 to 69 years (1.69, 1.97, 2.07, and 2.78, respectively). For women, the incidence rate decreased in those under 29 years (1.12, 0.86, 0.78, 0.82), but increased in those aged 50 to 59 years (1.58, 1.23, 2.20, 2.06) and 60 to 69 years (1.28, 1.50, 1.85, 2.03).

Conclusions: The age at diagnosis of ONFH is increasing among both men and women, even when considering Japan's aging population. The basic characteristics of ONFH may be changing.