Femoroacetabular impingement has recently become an epidemic in youth sports. We are starting to see more and more young athletes suffering from groin/hip pain caused by these abnormal bony changes. At this point, this condition is still poorly understood in terms of cause and prevention. But the only way to find the root cause of a problem is to understand how it started in the first place. In this article, we’ll over some of the earliest changes to the human hip, the most recent thoughts on the development of CAM morphology (a criteria in the diagnosis of FAI) and a theoretical cause based on some of the research.
Evolution to Bipedalism
I decided to start by taking you waaaay back in order to get an understanding of how we became the species we are today and how the evolution of our species may have an influence on the development of CAM morphology across youth sports.
Our earliest bipedal ancestors (Australopithecus afarensis, from 3.5 million years ago) are thought to be the first bipedal (standing and walking on 2 limbs) species on the planet. This transition from quadruped to bipedalism has led to some modifications of the human body including a longer spine with a lordosis and a wider and shorter pelvis. One of the more interesting adaptations to this transition related to the hip is the difference in cortical bone thickness on the superior aspect of the femoral neck. When compared to apes, who still use their arms and hands to travel, humans have much thinner cortical bone on the superior part of the femoral neck, but the trabecular bone is still very similar.
Lovejoy et al. (2004)One of the theories that may have led to this adaption is the migration of the abductor mechanism from the posterior aspect of the hip to the lateral aspect of the hip. As quadrupeds, there was no need for an abductor mechanism to maintain single leg stance, therefore no need for abductors. The smaller glutes (med and min) are primarily hip extensors in chimpanzees. However, during the transition to bipedalism, the abductor mechanism migrated laterally in order to allow us to control the pelvis in single leg stance.
This lateral migration placed the abductors in line with the superior femoral neck in order for it to prevent lateral tilting of the pelvis. Therefore, in weightbearing, the contraction of the abductors decreases the amount of shear tension forces being placed on the superior aspect of the femoral neck. In turn, the decreased forces on the superior femoral neck reduces the need for thick cortical bone to bear weight during upright standing and walking. According to Wolff’s law, bone will adapt to the loads applied to it, or a lack thereof. Over time and evolution, the cortical bone has become thin from a lack of loading thanks to the new arrangement of the abductor mechanism.
Lovejoy et al. (2004)Now you might be wondering why your reading about the evolution of humans instead of FAI in sports, but it’s important and pertinent information. We’re getting there. It will all make sense in the end. Let’s come back to the present day and focus on the current development of CAM morphologies in youth athletes.
Development of CAM in Youth Athletes
There is evidence that suggests that youth athletes playing certain sports are at a higher risk of developing CAM morphology than others (Philippon et al, 2013). Sports like hockey, soccer and basketball are at the top of these lists. Again, although the reason why these sports have a higher incidence rate than others is yet to be fully understood. But what we do know is that youth athletes playing these sports show early signs of CAM development at young ages and this could be related to levels of physical activity.
A recent study by Palmer et al. (2017) looked at youth soccer players (9-18 years old) who played from a soccer academy and compared them to a control group. They found the earliest evidence of an increased alpha angle around 10 years old. They also found that those who played at the club, national or international level had overall higher alpha angles than those who played no sport. This study suggests that there is a dose-response relationship between sport activity during adolescence and increased alpha angles, so the more sport activity during adolescence will lead to greater alpha angles.
Another study by Philippon et al. (2013) found that youth hockey players (10-18 years old) have overall higher alpha angles (measurement taken on X-ray to diagnose FAI) when compared to youth skiers. Hockey players were 4.5 times more likely to have an alpha angle greater than 55o(criteria for FAI diagnosis) than were skiers. They also found a positive correlation between age of hockey players and alpha angle. This suggests that as hockey players got older, their alpha angles increased. They also demonstrated that Midget aged hockey players were 36 times more likely to have an alpha angle greater than 55ocompared to skiers. This study suggests that young hockey players may begin showing morphological signs of an increased alpha angle as young as 10 years old, and there is an age related and level of play related increase in the progression of this alpha angle in hockey players.
An interesting find by Siebenrock et al. (2013) as well as Palmer et al. (2017) is that youth elite athletes (basketball and soccer, respectively) had increased epiphyseal extension towards the metaphysis, which has been shown to occur after chronic repetitive trauma in rabbits. This abnormal growth plate shape is associated with cam-type deformities in adults. Both studies found that while the athletes had open growth plates, an increase in epiphyseal extension may be correlated to cam-type deformities. Palmer et al. (2017) also found cartilage deposits at the head-neck junction in athletes as young as 10 years old, showing evidence of early development of cam-type deformities preceding epiphyseal changes. These findings suggest that CAM morphologies may not be caused by reactive bone formation, but rather an accelerated growth of the epiphyseal plate stimulated by high volumes of sport participation.
Up to this point, we’ve gone over how the evolution of the abductor mechanism has led to thinner cortical bone on the superior aspect of the femoral neck due to the shift of the abductors to the lateral aspect of the pelvis has taken some of the load off the femoral neck. More recent research in youth sports suggests that those who play high level or elite sports (those who tend to participate in a higher volume of their sport) show extension of the growth plate and early evidence of CAM morphologies as young as 10 years old. Based on this information, we can speculate that those who play higher volumes of sport are subject to greater loads on their hips. The abductor mechanism was previously thought to have led to decreased load on the hip, in turn decreasing bone formation on the superior aspect of the femoral neck. But now with high volumes of sport, we are seeing greater loads on the hip and an increase in abnormal bone formation.
Is the Abductor Mechanism to Blame?
Running puts significant load on the femoral head/neck. Edwards et al. (2015) measured peak loads at different areas of the femur during self-selected running speed in young experienced runners. They found that the femoral neck took almost 4x body weight in shear forces. Remember, this is in running. In athletes who are sprinting and quickly changing directions, these forces can only be greater. Young athletes who are playing team sports (soccer, basketball, hockey, etc.) that require quick changes in direction might be exposing their hips to loads greater than 4x their body weight. And we also have to take into consideration that these young athletes are repeatedly subjecting their hips to these loads, day after day for weeks and months at a time.
Now knowing the abductor mechanism’s role in decreasing cortical bone thickness of the femoral neck due to decreased load, it seems logical to think that maybe the loads from sports are too much for the abductor muscles to handle. And because the abductors aren’t relatively strong enough to handle the high loads of sports, we are putting excessive loads on the femoral neck with all these activities. In other words, the abductor mechanism can’t absorb enough of the load and the femoral neck is repeatedly being exposed to shear forces that it is no longer built to tolerate from an evolutionary adaption to bipedal locomotion. These high shear forces are then stimulating extension of the growth plate and cartilage hypertrophy at the femoral head/neck junction, creating early signs of CAM in youth athletes.
The other aspect of this is that it has been suggested that sedentary lifestyles lead to weak glutes. Most kids these days, although they may be active in their sports, are spending a lot of time being sedentary between school and at home activities (TV, video games, etc.). So, if kids are sedentary and their glutes (med/min – main hip abductors) get weak, their abductor mechanism will be able to tolerate even less load, exposing the femoral head/neck junction to even more shear forces when they are active. A change to an overall more active lifestyle, including a variety of sports and activities, could also have an impact in the development of FAI in youth sports.
References
Edwards, W. B., Gillette, J. C., Thomas, J. M., & Derrick, T. R. (2008). Internal femoral forces and moments during running: implications for stress fracture development. Clinical Biomechanics, 23(10), 1269-1278.
Lovejoy, C. O. (2005). The natural history of human gait and posture: Part 2. Hip and thigh. Gait & posture, 21(1), 113-124.
Philippon, M. J., Ho, C. P., Briggs, K. K., Stull, J., & LaPrade, R. F. (2013). Prevalence of increased alpha angles as a measure of cam-type femoroacetabular impingement in youth ice hockey players. The American journal of sports medicine, 41(6), 1357-1362.
Palmer, A., Fernquest, S., Gimpel, M., Birchall, R., Judge, A., Broomfield, J., … & Glyn-Jones, S. (2018). Physical activity during adolescence and the development of cam morphology: a cross-sectional cohort study of 210 individuals. Br J Sports Med, 52(9), 601-610.
Siebenrock, Klaus A., et al. “Growth plate alteration precedes cam-type deformity in elite basketball players.” Clinical Orthopaedics and Related Research® 471.4 (2013): 1084-1091.