Hi Karen and welcome to HW.
THere are a lot of informative people here, and hopefully one will come around with some answers for you. I really have no idea what to tell you.
Is the pain a sciatic nerve or something different? I notice when I get pains like that, my knees and feet act up terribly....it seems to be a viscious cirlce on which came first - did the joint pain come from tweaking my walking due to the nerve pain or did nerve pain come from tweaking my back due to joint pain. Who knows!
I can't imagine they'd replace a joint due to hyperextension unless it was damaged or causing it. You'd think they'd brace it or something else. But, I'm not knowledgable in that area.
I will suggest seeing a podietrist as well - adjusting posture via orthotics saved me from a ton of agony!
Let us know what happens at the doctor next week. Take care and good luck***