There are a number of reasons that an individual will make the choice to have hip or knee replacement surgery. They can include ongoing severe pain and limited mobility due to arthritis, osteoporosis, rheumatoid arthritis or trauma/severe injury to the area after an accident or prolonged sporting injuries and strains. Arthritis is the main cause of replacement surgery due to the nature of the disease in which the cartilage of the joint becoming warn and causing increasing pain by the ends of the joint bones moving against each other.