When the hip socket does not form completely, the condition is called acetabular dysplasia. In normal hips, at least 80 percent of the femoral head is covered by the acetabulum (or socket). In dysplastic patients, less than 80 percent of the femoral head is covered by the acetabulum. The acetabulum is typically shaped more vertically than horizontally. To help support the femoral head, the labrum increases in size. However, since the labrum is serving a function it is not normally designed to do, a tear often results and can cause pain or locking and catching.
If left untreated, the decreased coverage of the femoral head can lead to osteoarthritis. As a result of having decreased surface area to disperse the force of weight bearing, the cartilage of the femoral head and acetabulum can also wear out faster, often at an early age.
The exact cause of hip dysplasia has yet to be fully understood, but several varying factors seem to play a role. There is a genetic component since family members often have the condition in common.
Typically, females are affected most often. Although there is newborn screening for patients at risk for hip dysplasia (first-born female, family history), patients will often not present until their teenage years or mid-20s when the hip starts developing more inflammation and pain.
Because the hip socket is too shallow, the surrounding structures have to compensate. In younger patients, the labrum may be bigger than normal, but it may not have a tear in it. For older patients, a tear in the labrum will cause pain in the groin with potential locking or catching of the hip during normal movements.
The muscles around the hip also have to work overtime to keep the hip stable. The muscles on the side of the hip – gluteus medius and minimus (also known as the hip abductors) – often get overworked. The result is pain at the side of the hip when walking or toward the end of the day as these muscles become fatigued from the constant strain.
The large muscle in the front of the hip – the psoas – also has to work to keep the femoral head inside the deficient socket and can become inflamed as a result. Because this condition is a deficiency in bony coverage, the motion of the hip is usually fairly well maintained compared to hips with impingement.