In this condition, the blood supply to the head of femur (The ball of the ball and socket hip joint) gets reduced. Which causes the ball to loose its normal smooth and rounded shape. Since the ball becomes irregular in shape, the friction inside the joint increases, causing stiffness and pain.
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When the hip is painful due to deformed head, hip joint replacement restores painless movements.

A 35 years old man who abused alcohol was unable to walk with pain in both his hips.

His x-rays showed deformed femoral head on both sides. He was diagnosed as having bilateral avascular necrosis of hips

He underwent staged hip replacements