Multiple sclerosis should not have any impact on knee replacement surgery in a technical matter of speaking. The real problem is, what the benefit from knee replacement would be if the patient is already disabled because of the multiple sclerosis. If the patient's legs are immobilized because of the multiple sclerosis there won't be any benefit from the knee replacement. But if the multiple sclerosis is under control and hasn't caused leg paralyzes yet, knee replacement is worth to be done if knee condition requires that.