Bending inward like this is referred to as flexion. When the femur is implanted directly into the acetabulum, the hip no longer functions as a ball-and-socket joint.
This is further explained below.
Generally, In vertebrate anatomy, a ball-and-socket joint, also known as a spheroidal joint, is a joint in which the rounded surface of one bone slides inside a depression on another bone. This kind of joint allows for a larger range of motion than any other type of joint.
In conclusion, This motion is known as flexion. A pivot joint, in which the femur is directly planted into the acetabulum, is used in lieu of the hip's natural ball-and-socket joint during hip replacement surgery.
Read more about the ball-and-socket joints
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