it is due to osteoarthritis in knee, its age onset senile degenerative disease, better avoid knee bending , avoid sitting cross legged, avoid stairs climbing, consult orthopedic surgeon get evaluated and treated, can be treated

Osteoarthritis occurs when the cartilage that cushions the ends of bones in your joints gradually deteriorates. Cartilage is a firm, slippery tissue that permits nearly frictionless joint motion. In osteoarthritis, the slick surface of the cartilage becomes rough. Eventually, if the cartilage wears down completely, you may be left with bone rubbing on bone. Consult Orthopedician...