Hi,
From history it seems that as x-ray of knee joint is clear you might be having lax and flabby quadricep muscles giving rise to this problem.
Go for Quadricep and patellar muscle exercise, 10 minutes in the morning and evening.
Take calcium, vitamin A and D supplements.
Ok and take care.

Hi, this is Dr Nanda,Knee joint is a major wieght baring joint and quite complexed in structure ,it is good to know that your X-ray reports are normal,that leaves us with strenthening the muscles and tendons around the knee.
curative measures;1)Start with a hot pack for half an hour in the morning.
2)Toning up exercise to the knee;a)five half sit ups of three sets.
b)do knee rotations ,streaches and contractions sitting on a high chair position.
3)Mix 5 drops of citronella oil and five drops of camphor oil ,apply the mixture and leave it for 30mnts in the evening.
4) Repeat the hot pack before retiring to bed.
Preventive measures: Always keep a tab on your weight gain,adopt simple walking ,yogaas the part of the life style