The answer to your question is a technical one that depends on the cause of your knee problem. In general, knee replacement is considered only when other, less drastic options have been exhausted. For example, acute and serious trauma to the knee may cause severe disability that must be aggressively addressed. On the other hand, the pain of arthritis can usually be controlled for years with medicine, physical therapy and special exercises. Joint replacement is not advised until the situation has degenerated to the point where medicine is no longer effective, or the mechanics of joint mobility have deteriorated to produce a distinct handicap.
Artificial joints have a varying life expectancy.
I suggest that you discuss your situation with your orthopedic surgeon. If, later, you are still unsure about the operation, ask for a referral for a second opinion. I cannot maintain a comfortable body temperature. For example, I awaken as often as 10 times a night because I am hot and sweaty. Once the covers are off, I'm too cold.