Population-wide incidence estimates for soft tissue knee injuries presenting to healthcare in southern Sweden: data from the Skane Healthcare Register

Introduction: Soft tissue knee injury is a well-established and potent risk factor for development of knee osteoarthritis. However, there is a paucity of epidemiological data from the general population. Our aim was to estimate the annual person-level incidence for a wide spectrum of clinically diagnosed soft tissue knee injuries, and their distribution by age, sex, and season. Methods: In Sweden, in-and outpatient health care is registered using each individuals' unique personal identifier including International Classification of Diseases (ICD) 10 diagnostic code(s) as determined by physicians' clinical examination. For the calendar years 2004-2012, we studied the population in southern Sweden, Skane region (approx. 1.3 million). We identified residents who had at least one visit to a physician with clinically diagnosed knee ligament, meniscal, or other soft-tissue injury (S80.0, S83 and all subdiagnoses). We then calculated the mean annual incidence over the 9-year period. As a secondary objective, we investigated potential seasonal variation. Results: The annual incidence for males and females was 766 (95% CI: 742, 789) and 676 (649, 702) per 100,000 persons/year respectively. For males and females, the peak rate occurred in 15 to 19 year-olds (1698 per 100,000 men and 1464 per 100,000 women, respectively). In women, rates were lowest in the 25 to 34 year-old age range before rising again between the ages of 35 and 49 years. We found substantial seasonal variation, greatest in men, with peaks in March-May and August-October. Conclusions: The incidence of clinically diagnosed soft-tissue knee injury peaks in adolescence and emerging adulthood. However, a range of knee injuries continue to occur across the adult lifespan including at ages when osteoarthritis is typically diagnosed and managed. The potential cumulative effect on osteoarthritis progression of these injuries may warrant further investigation.