Femur begins to grind away in the socket. Body's ability to replenish the cartilage reduced with lack of motion. Wearing away of the cartilage in the socket, acetabulum. Pitting and scaring of the head of the femur. Alignment problem including the spine.
Sedentary lifestyle contributes in some cases.
Most of the pain, especially early on, is muscular pain. Cartilage will be replenished when there is proper motion. Body is a living organism which replenishes muscle vs. depletion. Body pain indicates muscular requirement for hip movement. Pain signals broader problems.