Perhaps because it's so common and makes its sufferers so cranky, arthritis is one of those diseases that's especially prone to rumor and hearsay. Not only do unsubstantiated "cures" and therapies circulate endlessly on the Internet, but some people (including scientists who should know better!) continue to believe that arthritis bedeviled even the mighty dinosaurs.
Why is this so? As with many such widely held beliefs, dinosaur arthritis is based on a grain of truth: paleontologists have, in fact, unearthed two fossils of Iguanodon that show unmistakable signs of arthritis in the ankles (not to get too technical, but this evidence involves bone structures called osteophytes which impede the movement of the joint).
As detailed by Bruce M. Rothschild in the indispensable book The Complete Dinosaur, these arthritic Iguanodons account for two individuals out of a total population of 39. But just as the media will abstract specific scientific results to the population at large - say, recommending daily doses of powdered zinc because one species of lab rat showed a two percent increase in average life span on this regimen - it quickly became received knowledge that, well, all dinosaurs had arthritis.
Rothschild, an M.D. with an interest in paleontology, demonstrates exactly the reverse. As he writes, pretty much the only dinosaurs proven to have human-type arthritis were those two Iguanodons - the condition has not been found in "hadrosaurs, ankylosaurs, theropods, stegosaurs…or sauropods." He goes on to point out that, while human obesity can lead to arthritis, the same isn’t true of dinosaurs - even huge herbivores like Diplodocus were relatively healthy.
If you've been happily picturing a young, brash, Triceratops thumbing its nose at an arthritic T. Rex, don't worry - dinosaurs may not have had arthritis, but they were susceptible to other bone and joint diseases. Rothschild discusses evidence of such conditions as diffuse idiopathic skeletal hyperostosis and vertebral fusion--perhaps not as aggravating as arthritis, but likely to make a dino every bit as cranky.
Of course, part of the problem with identifying disease in long-dead dinosaurs is finding sufficient fossil evidence to make a diagnosis. It's possible that, at some point in the future, an Apatosaurus will be found with unmistakable evidence of arthritis - but until then, you should take this urban legend with a pinch of powdered zinc!