Likely arthritis or multi-system disease such as lupus. Rheumatologic evaluation essential.
It really could be any number of things from inflammatory (arthritis) to a ligamentous injury. I would start by seeing my pcp.
Prior surgery?Swelling? Fever? Med hx? Too many posibilities. Need an evaluation with xrays.
Have it evaluated:
Have it evaluated by a professional. Depending on the severity of the injury treatment options include: bracing, casting, physical therapy, injections, and surgical repair.
Have it evaluated.:
Locking and clicking are signs of soft tissue or meniscus damage. This needs to be evaluated and treated by a specialist to possibly prevent future complications.
Knee pain in a 16 year old female is often related to chondromalacia. This involves the joint behind the kneecap. See an orthopedist. Specific exercises often provide good benefit. Of course, other causes for the pain such as tendinitis, bursitis, arthritis of the knee itself are also possible, .
Yes, a knee replacement is almost certainly in your future. However, 55 is on the young side for this surgery. Knee sleeves, NSAIDs and injections such as Synvisc or Orthovisc may help and should be tried first.