Although cartilage doesn't have nerve endings, bone, synovium, the joint capsule, and the surrounding muscles, tendons, and ligaments do. As cartilage becomes severely worn, pressure and stress accumulate in the surrounding structures, creating friction and inflammation. As it degenerates and inferior cartilage is created, water gradually fills the void. This increased aqueous presence in the joint does not have the same shock-absorbing or sponge-like properties, or the icy smoothness of cartilage. As the cartilage becomes further damaged, the ligaments, joint capsule, tendons, muscles, and bones are forced to bear undue weight, and the nerve endings in these structures begin to transmit pain.
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