Injuring a knee in youth dramatically increases the risk for osteoarthritis, researchers say. 

Knees injured at age 25 to 34 years were six times more likely to develop osteoarthritis within 11 years, reported Barbara Snoeker, from Lund University in Sweden, and colleagues in an article published online December 11 in the British Journal of Sports Medicine. 

The risk for osteoarthritis varied with the type of injury. "Of all injury types, cruciate ligament injuries, meniscal tears and intra-articular fractures yielded the highest estimates of increased risk," they write. 

Previous studies have identified knee injury during adolescence and young adulthood as an important risk factor for osteoarthritis. But most of these studies used retrospective analysis, and those that relied on patients' recall may be particularly unreliable, the researchers say. 

There are too few studies in young people to yield reliable estimates of the magnitude of risk, they add. In addition, the existing studies have focused mostly on cruciate ligament and meniscal tears, leaving open questions about other types of knee injury. 

To fill this gap, Snoeker and colleagues analyzed data from the Skåne Health Register, which includes all healthcare consultations involving the 1.3 million people who live in Skåne, Sweden. They identified 5500 people who were 25 to 34 years of age when they were diagnosed with a knee injury for the first time from 1999 to 2007. The study excluded people with existing osteoarthritis. 

The researchers compared this cohort with 143,788 people of the same age range who had not injured a knee during this period but were in the database as a result of unrelated medical consultations. 

After 19 years, 11.3% of the people with injured knees were diagnosed with knee osteoarthritis, compared with 4.0% of the people who had no knee injuries. 

After adjusting for age, sex, residential area, education, income, diabetes, obesity, and hypertension, the hazard ratio (HR) for osteoarthritis in the injured knees compared with the uninjured knees was 5.7 11 years after the injury. The adjusted HR ratio after 11 years was 5.3 for men and 6.5 for women. 

For people younger than 30 years, the adjusted HR was 7.6, whereas for people older than 30 years, it was 4.7 after 11 years. This reflects the lower baseline risk for osteoarthritis in younger people, the researchers explain. 

The adjusted HRs after 11 years varied with the type of knee injury. The adjusted HRs were 8.2 for cruciate ligament injury, 7.6 for meniscal tear, 7.0 for a fracture of the upper end of the tibia/patella, 6.5 for injury to multiple structures, 5.9 for a dislocation, 5.2 for a cartilage tear/other injury, 4.9 for collateral ligament injury, and 3.2 for contusion.

On average, people with a cruciate ligament injury developed osteoarthritis 16 months faster than people without knee injuries who also developed osteoarthritis. For meniscal tears, the difference was 12 months, and for fractures, 8 months. 

The researchers were surprised there was not a bigger difference in disease-free time. They speculate that those people without injuries who developed osteoarthritis must have had some other risk factor, such as a genetic predisposition or obesity.

"To the best of our knowledge, our study is the first that used a cohort from the general population of young adults to estimate the risk of a wide variety of knee injuries on the development of clinically-evident knee [osteoarthritis]," the researchers write.