dear Sir I have understood your concern see you mentioned injury 18 years before so chances happened to your bone since then which is gradually may due to negligence of proper care to your knee joint at the time after injury this problem persists... now we can do a lot if you agree to do this. first need to get x-ray done to confirm about bony change, as you mentioned popping sound present at time of sqaut this indicate some degenerative changes happening in your joints. if x-ray is normal go for MRI study which confirm soft tissue injury in your knee. if you go to physical therapist, they can evaluate by some special test done. see nobody can prevent changes happened to bone and joint but we can surely prevent it from further spreading so by physical therapy like ultrasound and IFT modalities your pain will be relieved then at home apply alternative cool and warm water pouring. in center learn strengthening exercises for knee muscles then only after dedicated effort for 1year you will recover better. I hope you satisfied with this answer, still have any questions feel free to ask me. thanks dr Hemang Jani.