related talks: hip dysplasia, FAI (femoro-acetabular impingement), snapping hip, coxa vera, sports hernia
The Hip is a ball-and-socket joint. The femur (thigh bone) is the ball, and the pelvis (specifically the acetabulum) is the socket. In a normal hip the two match perfectly, the stress of daily activities is distributed evenly across the cartilage, and the cartilage can therefore survive for a person's entire life. Arthritis is the condition of cartilage wearing out. When the cartilage is gone, there is no lubrication in the joint (think of motor oil for a car engine) and therefore friction increases dramatically and people experience significant pain with motion.
HIP DYSPLASIA is important because the ball and socket doesn't match perfectly. The socket is abnormal. In can be shallow, meaning the socket does not full cover the ball...and as a result, the stress of daily activities is concentrated over a smaller surface area of cartilage, causing this cartilage to experience significantly greater pressure (P = F ÷ SA), causing it to wear out early. In other cases the socket can be too deep, which causes the ball and socket to impinge (called CAM Impingement), and as a result, the stress of daily activity can be focused on small areas of cartilage, leading these areas to wear out earlier than other areas, and cause a lot of pain.
But the issue is more complicated because hips, like ice cream, come in so many varieties. Hips are not binary (normal vs. abnormal), but rather they fall along a broad spectrum of ball-and-socket symmetry. Because of this variety, there isnt a perfect link between Arthritis (cartilage wearing out) and the shape of a hip. Doctors cannot say that a socket thats off by X-number-of-degrees will definitely develop arthritis (although they use a technical measurement on x-ray, Lateral Center Edge Angle < 20 degrees, to diagnose Hip Dysplasia in adults).
So why cant the doctors just say hip dysplasia causes hip arthritis, it seems obvious. The problem is that so many things can cause arthritis. Being overweight also puts more stress on the hip (F = m*a) and increases risk for arthritis. Running mararthons every year puts more stress (F = m*a) on their hip and increases risk for arthritis. And there are lots of others reasons we dont understand, because there are thousands of skinny, healthy people developing arthritis every year...but why? The car analogy works well: think about all the ways your car can wear out over 10 years. The hip can also wear out in similar ways, but it takes many decades... so all of these little factors may play a role in causing arthritis for different people. As a result, its very difficult to say that the hip shape alone is responsible for causing arthritis.
Therefore doctors recognize hip shape as a risk factor for arthritis but they cannot say with confidence when hip arthritis will occur...or even if hip arthritis will occur at all.
Basically, its an area of interest, but doctors need to do a lot more research.
Looking at the hips of athletes is one way to study hip dysplasia.
One study looked at 47 dancers in the New York Ballet. Almost 90% (41 out of 47) had mild dysplasia, and over 50% (24/47) had true hip dysplasia. Thats a lot of dysplasia. Its possible that their stretching, flexibility and intense work out routines (which start at a very early age) cause the hip to become dysplastic. But the question then becomes, how many of these elite dancers go on to need hip replacements 30 years later? And how many of them are experiencing hip pain now?
Abnormal hip shape has also been reported in elite hockey players. Over 50% of 74 players had some type of abnormal hip seen on x-ray. About 20% reported some type of symptoms. But overall it had no effect on function or skill.
Its been reported in elite soccer players. Over 50% of 94 players showed some type of abnormal hip seen on x-ray. But another study comparing youth soccer players with kids of the same age not playing soccer, showed that both groups had over 50% of kids with some type of abnormal hip.
One review of multiple studies suggests that high-contact sports like hockey and football, may lead to some increased risk for hip deformity but the short term and long term effect is unknown. Therefore, it seems like a lot of athletes have some form of "abnormal" hips but the significance of these x-ray findings is unclear. If a person can complete as a high level athelete and then live the rest of their life without major issues...is it fair to even call this hip abnormal?
