Last summer I had some persistent debilitating knee pain that finally required an MRI to diagnose. The verdict was at age 37 I was already suffering from the family curse of osteo-arthritis; which had contributed to severe degeneration of the cartilage in my knees.
I was advised to use only light weight and no more deep squatting of any kind...So for the past year nearly all of my 'leg work' has been on a stair mill or elliptical machine, or occasional rear lunges [which don't seem to hurt my knees much].
I switched to front squats almost exclusively a few years ago as I really liked the way they felt...especially for my low back; which always felt a little insecure doing heavy back squats [maybe because of my long torso 6' tall with only a 30" inseam].
I can't remember where I read it, but I thought I saw somewhere that lifting weights actually stimulates cartilage growth. Anyone know some more about that? See I'm wondering if I did some specific low wear and tear movements for my legs if I could actually be helping my bum knees...???