Sorry to hear about your knee problems, Jack. They are doing wonderful things with joint replacement these days. When your day comes I'm sure it will go just fine for you.

Two possible solutions have come to mind for metal detecting:

1. Dig standing up. I have hunted on multiple occasions with someone who does this. They just bend at the waist.

2. Carry a bucket to sit on while digging, or a small portable chair or stool.

I'm not sure if these options would be viable for you, but it may be worth a try. Best of luck!