Technology on knee replacements is changing rapidly. My wife has had two, with no related back pain.

If I were younger I would go ahead for the best life possible, gambling that in 15 years (life of a knee replacement today????) or so, the technology would be so more advanced that fixing it again would be no problem. Also, there are new alternatives to total knee replacement. And, who knows when one is going to die? Enjoy life to the fullest when you can, IMHO.

I have (had) atrial fibrillation, for which the current ultimate cure is a heart ablation - burning some of the tissue in the heart. I had it fixed within a year of 24/7 occurrence. Others I know are waiting and waiting and waiting for something better. In the meantime I have enjoyed almost 7 years of normal sinus rhythm. They are still waiting.

I also have Trigeminal Neuralgia (extreme pain), which is best cured by a near-brain surgery. I made the same decision regarding the TN as the AFib. I now enjoy almost a pain-free life -with the help of some minor meds - while others are still waiting for something better. If, at 73, I die today, I will have lived the fullest life possible.