During the cam impingement, changes primarily occur at the neck of the femur (cam comes from a Dutch word meaning ‘cog’). This means that a thickening will occur on the neck of the femur, which will then lead to impingement against the joint socket.
This MRI shows clearly that the neck of the femur is much broader and not tapered compared to the opposite side. As a result, impingement occurs predominantly during bending, yet the mechanism is exactly opposite of the pincer impingement.
This animation will help you to understand how impingement results in progressive joint damage.
The thickened neck of the femur impinges against the socket and first destroys the labrum, then subsequently the cartilage.