What Causes Pain In Knees While Bending Them?

Hi, My husband is 44 yrs of age. From the past couple of months he has been complaining of pain in his right knee. The pain is felt only when he kneels or bends his knees and not while walking or any other time. He consulted a doctor who said it could be the start of arthrites and suggested some calcium tablets. However there has been no improvement. Can there be any other reason for the pain?