For common sports injuries, it has long been thought that external factors played the central role in the type and severity of the damage. But now, a new study from investigators at Lund University in Sweden is adding a new factor to that traditional mindset: genetics. The Swedish researchers just published new findings that suggest a high rate of heritability for a very common sports knee injury—rupture of the anterior cruciate ligament (ACL). Results from the new study were recently released in the British Journal of Sports Medicine through an article titled, “High genetic contribution to anterior cruciate ligament rupture: Heritability ~69%.”
By analyzing data from the Swedish Twin Register, along with data from the Swedish National Patient Register, researchers were able to see how many identical twins and fraternal twins had been diagnosed with anterior cruciate ligament rupture.
“We used a twin study approach, linking the Swedish Twin Register with national healthcare data to form a 30-year, population-wide, longitudinal twin cohort,” the authors wrote. “We studied ACL rupture in this cohort of 88,414 identical and fraternal twin pairs, aged ≥17 years, to determine the familial risk and heritability of ACL rupture.”
The opportunity to observe injuries among both identical and fraternal twins enables conclusions to be drawn about heredity. “Identical twins have totally identical genes, while fraternal twins, like other siblings, share half of their genes with each other,” explained lead study investigator Karin Magnusson, PhD, an associate researcher and expert in twin studies within the field of orthopedics at Lund University. “We can estimate heritability by studying how often cruciate ligament injury occurs in both twins in a set; that is, we compare the ‘double’ prevalence in identical and fraternal twins. This way, we can draw conclusions about the importance of heredity and environment.”
The study authors noted that this is the first study that has quantified the genetic contribution to ACL rupture using the twin approach.
“Our results show that genes appear to contribute more than we thought. To put it very simply, we can say that out of all cruciate ligament injuries, 69% can be explained by genetics,” Magnusson remarked. “This should not be interpreted to mean an individual’s risk of suffering the injury is 69%, rather that it is significant in terms of the wider population. Heredity is easier to understand when compared with other illnesses or conditions. The genetic risk of suffering from cancer is 33%, for example. This means that 33% of variation within the population in terms of causes of all cancer cases is due to genetic variation, while the rest is due to other factors—such as environment or lifestyle.”
Interestingly, the Lund University team found that the genetic risk was equally high in men and women.
“The incidence rate of ACL rupture was 70 (95% CI 66 to 74) per 100,000 person-years,” the authors wrote. “The familial risk, which is the excess risk ratio (RR) of the second twin having ACL rupture given that the first twin has had such a rupture, was higher in identical twin pairs (RR=8.6, 95% CI 6.2 to 11.0) than in fraternal twin pairs (RR=1.9, 95% CI 0.9 to 3.0). The overall heritability of ACL rupture was high, 69% (95% CI 47 to 91), increasing from 60% at age 17 years to 80% at age 60 years. Women and men had similar familial risk and heritability of ACL rupture.”
The researchers are optimistic about their findings and hope the data can be used to help prevent future injuries from occurring.
“In the past, genetic factors have not been taken into account when working to prevent anterior cruciate ligament injury, for example, among athletes,” concluded senior study investigator Martin Englund, MD, PhD, a professor at Lund University, and physician at Skåne University Hospital. “This study does not provide us with an answer as to whether heredity entails anatomical or physiological conditions that could affect the risk of a cruciate ligament injury. However, if we know that there are many cases of this injury within a family, then it is worth being extra cautious and doing more preventive training, such as landing after jumping when playing handball, and so on.”