Pano lesions are not always seen on an X-ray even though the dog may be showing clinical signs of lameness and pain. 

No definitive reason is really known for the cause of Pano.  It has been attributed to high protein levels in food, high fat intake, growth spurts, lack of different vitamins or minerals, too much of certain vitamins or minerals, environmental causes, bacterial infections, parasite infections and many other non conclusive reasons.  People have reported that high doses of Vitamin C will help to alleviate the onset of Pano, or the symptoms when they occur, and others swear by antibiotic treatments, holistic and natural approaches and even acupuncture and chiropractic treatments.  For the most part, Pano will occur and run it's course; it is a self limiting disease and will subside on it's own regardless of treatments.   Analgesics can be given, but in a lot of cases the relief from pain is minimal.  Some report cessation of symptoms after neutering, which may be an indication of association between stress and hormonal changes, but this is unproved as well.   Maintaining good nutrition during bouts of Pano is important, as many dogs who are feeling pain will have a depressed appetite too.   Ligaments and muscles are still growing and forming in young dogs.  I have found that ligamentation and conditioning can be compromised considerably during the painful stages of Pano due to the lack of exercise.  Dogs should be encouraged to exercise lightly between episodes, but hard running and very long walks should be excluded until the Pano is gone for good, and is wise to limit in young dogs even if they are not afflicted with Pano.