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As you have mentioned that your father had some injury 10 years back, Probably it was some kind of soft tissue injury
, which in the last 10 years developed Osteo-arthritis
of the knee joint. Which in my views, is the cause of pain, Swelling & muscle wasting.
Now the most important is whether the knee has been damaged to the extent where total knee replacement
is the only option left or any other modalities can be applied, it all depends on the physical examination & X-rays.
Consult an orthosurgeon as soon as possible. For the time being give him some pain-killer, Hot-cum-cold application, knee brace
& comlete bed rest