Tearing the anterior cruciate ligament (ACL) is one of the worst injuries an athlete can sustain. When a player in any sport suffered this injury, they will be sidelined for at least a year, and their career often times comes into question. However, for what a serious injury this is, it seems to happen way too often. ACL tears have become so common that just last last year two football players suffered tears during sack celebrations. While there are many jokes to be made about Steven Tulloch and Lamar Houston, both of wh
om suffered a torn ACL’s while outlandishly celebrating a sack of the quarterback, they both serve as excellent examples of how common this injury has become. In the 2013-2014 NFL season, 63 players tore their ACL. There are 53 players on a team, and 32 teams in the league. This means out of the 1590 players active on opening day, about 4% of them tore their ACL.Another way to think about that statistic is you coul
d field an entire NFL team with the players who tore their ACL. 4% is a very large percent and is clearly indicative of large problem. It is commonly known that athletes are at risk to injure the ACL after a hard change of direction, landing awkwardly from a jump, or from physical contact to the knee. These causes are very general, and athletes do them all the time, and not all athletes tear their ACL. This leads to the question of what else could be causing these injuries. In this post I will explore the idea that some people are born more likely to tear their ACL’s than others.
In a study published September of 2010 by the British Journal of Sports Medicine, researchers looked into two young girls who were twins. They videotaped the girls cutting, jumping, and running while wearing special markers to track them. While they were taped both the girls were totally healthy, but within a year of the tape, they had both suffered ACL tears while playing their respective sports. It is important to note that the twins had shown the following risk factors prior to the injury “increased knee abduction angles, decreased knee flexion angles, increased general joint laxity, decreased H/Q ratios and femoral intercondylar notch width”(as reported by the study). Not only that, but it was reported in this article that the older sister of the twins had also torn her ACL in this time frame. Clearly this does not even come remotely close to providing good evidence towards the alternative hypothesis, but it did get the conversation going. This report was actually part of a much larger study looking into ACL injury risk and possible causes among thousands of young athletes. The researches involved were observing many different risk factors and just happened to come upon this one. This means the report was largely due to the Texas sharpshooter problem. So this report, while interesting, does not hold much water and was effected by Texas sharpshooter problem.
As I mentioned, this small report had gotten the conversation started about family genetics and ACL tears. In December of 2010, another study was published in the British Journal of Sports Medicine examining the correlation between a specific gene and ACL tears. This study took a simple random sample 129 people with a previous ACL tear and a control of 219 people with no history of the injury. In the two groups there were 38 and 83 females respectively. The gene tested for was COL12A1, and the results showed a strong correlation between ACL tears and this gene among females. The correlation did not exist amongst males, or the control group (male or female). For the female group with a previous injury to the ACL, the researchers calculated a .048 p-value, which at the standard 5% significance level would lead to a rejection of the null hypothesis. Given that this study was properly conducted, can we now officially say with confidence that ACL injuries are hereditary? The answer to that question would be no for a few reasons. First, based off this study we would have to believe that it is only women who have a predisposition, which would be hard to believe. Second, it is impossible to say anything with reasonable confidence based off one study, especially one where the main results were only found with 39 women. It would certainly be useful to not only see more studies done on this, but also studies with a larger sample size. Third, the p-value was .048, and as it is standard practice to use a 5% significance level, this only barely makes the cut. At basically any lower significance level the researchers would have failed to reject the null hypothesis. Given what we know about this study, the chances of a type one error are high. Once again the results in this study were encouraging, but there is still no solid evidence here to show that ACL tears are hereditary. It is still close however, and it does show the potential for one day possibly being able to predict these types of injuries.
Since 2010 there have been a few more studies that have found similar results (such as this one) but there still are simply not enough. In order to determine with more certainty that genetics can determine ACL tears there have to be more studies done that all show similar results. At the base of this question however, there seems to be a very simple answer. If a person wants to know if they are at risk of an injury to their ACL, simply looking at their family history seems like a pretty good way of finding out. In a New York Times article, Dr. Timothy Hewett, director of sports medicine research at Ohio State and lead researcher for the study with the twins, was quoted saying, “If you have a first-generation relative, a sibling or parent, who has torn an A.C.L., you are likely to be at above-average risk.” For now, there is not enough evidence to say ACL injuries are hereditary and there is no mechanism to predict them, but the future seems bright.