Familial risks of five types of osteoarthritis in first-, second- and third-degree relatives - A nationwide Swedish family study.

Objective: Osteoarthritis (OA) is a common degenerative disease with a genetic contribution. However, no large nationwide family study concerning the heredity of OA has been published. This first nationwide study aimed to determine the familial risks of the main types of OA in twins, full-siblings, half-siblings, and cousins in Sweden. Design: The Swedish Multigeneration register was linked to the National Patient Register (NPR) to investigate the heredity of OA (poly-, hip-, knee-, first carpometacarpal joint-, and other-OA) between 1997 and 2018. Offspring born by Swedish parents were included. The adjusted familial hazard ratios (HRs) with 95 ​% confidence interval (CI) were determined for OA among twins, full-siblings, half-siblings, and cousins. Adjustments were made for birth year, sex, educational level, and comorbidities (chronic obstructive pulmonary disease, alcoholism, and obesity). Results: A total of 6 547 966 individuals (48.77 ​% women) were included with mean age 41.44 years (range 0-86.96 years) at end of follow-up. Familial HRs were increased for all five types of OA (even in cousins) and correlated to degree of genetic resemblance between relatives. For instance, adjusted HRs among full-siblings were for poly-OA 2.29 (95 ​% CI 2.09-2.51), hip-OA 2.04 (95 ​% CI 1.98-2.07), knee-OA 1.75 (95 ​% CI 1.73-1.77), thumb-OA 2.60 (95 ​% CI 2.45-2.76), and other-OA 1.52 (95 ​% CI 1.48-1.56). Conclusions: Heredity is an important predictor of future risk of OA for all five types of OA in the Swedish population. Strongest heredity was observed for first carpometacarpal joint -OA followed by poly-OA and hip-OA. Weakest heredity was observed for knee-OA and other-OA.