Cam morphology in Femoroacetabular Impingement Syndrome (FAIS)
In a typical hip, there's a slight concave curve (sometimes described as a ‘scoop’) where the neck of the femur meets the head, and the socket sits angled slightly forward - a feature known as anteversion.
This shape allows the femoral head to move through its full range without catching on the socket's rim.
Cam morphology describes a change to this shape: extra bone forms at the front and top of the femoral head-neck junction and the femoral head loses some of its natural roundness. This often occurs around age 13 – 15.
Clinically, this is measured using something called the alpha angle, visible on X-ray or MRI. The larger the angle, the more pronounced the cam shape.
An alpha angle of greater than 60 means the presence of cam morphology. An alpha angle of greater than 78 is considered a pathological cam deformity associated with end stage osteoarthritis. An alpha angle of 83 degrees leads to 10 times higher odds of end stage osteoarthritis within 5 years.
For every degree of increase in alpha angle, the risk of end stage osteoarthritis increases by 5%.
Cam morphology is actually very common.
Research pooling imaging data across large groups of people has found it in around two-thirds of athletes (independent of whether they had symptoms at all), in around half of people presenting with hip pain, and in roughly one in five people with no hip symptoms whatsoever.
This means that a meaningful share of the population has a cam-shaped hip and no pain to show for it.
This points to an important distinction. Cam morphology itself isn't the pathology but a structural variation that can, under the right circumstances, create excessive or repetitive contact between the femur and the socket during movement, and it's that contact that eventually produces symptoms.
Sport appears to influence how the shape develops in the first place.
Primary cam morphology, the form most often seen clinically, develops during adolescence (age 13-15) while the growth plate is still open in otherwise healthy hips. This is likely in response to, or to adapt to, repetitive high-impact loading.
Secondary cam morphology arises from prior hip disease or trauma, such as slipped capital femoral epiphysis or Perthes disease.