Rotator cuff tears, herniated discs, torn knee ligaments and cartilages are just like this; the same abnormality that hurts some folks doesn't hurt others. Over 80% of asymptomatic adult volunteers (people with no pain at all) who let us do an MRI of their necks were found to have abnormalities like disc herniations and bone spurs that we commonly operate on in symptomatic patients. The rotator cuff, my particular expertise, is even more mysterious. When it's torn and symptomatic, there is measurable weakness. A big, symptomatic tear often makes it impossible even to raise one's arm to eye level, and this doesn't get better until you fix it. But then you have patients with asymptomatic tears and they can be really big tears who not only are pain free but have strong shoulders with full motion as well.
Why does the same problem hurt one person and not the next? Here is the greatest mystery of my profession. As doctors, especially in my field of orthopedic surgery, we like to act as if we understand things like this but we don't. It's a really interesting question because whoever answers it can turn a Sean into a Bob getting the pain to go away without doing the operation and this would be a minor miracle. We do have some tantalizing tips, though, about what makes things hurt.
The first is inflammation. An object lesson here was taught by an old drug. Those older than 50 might remember Zomax. It was an anti-inflammatory that was also an amazing pain reliever. I was just a medical student when it was pulled off the market because of side effects, but I clearly remember patients with broken legs or fresh post-ops who were treated for pain with Zomax alone. They were nearly pain-free and clear as a bell with no narcotic drowsiness. This "super Advil " worked along the same chemical pathways as all the other non-steroidal anti-inflammatory drugs just better. And it seemed to take away arthritis pain almost completely, making symptomatic cases asymptomatic for eight hours at least.
We are quite familiar with the pain-inflammation connection. The most powerful anti-inflammatory drugs we have are the corticosteroids like prednisone. They have too many side effects to be first-line arthritis drugs but many folks have to take them for other reasons. These patients might find them actually too effective against pain. Appendicitis, gall bladder disease or even bowel infarctions can occur in these patients without them feeling any pain at all. This is very dangerous the pain of early appendicitis is a good thing; it saves lives by bringing patients to the doctor before they start to die of abdominal infection.
The biochemistry of the inflammatory process, which we recognize by an increase in local blood flow (more blood equals more heat equals inflammation) creates, among other things, molecules that sensitize pain-associated nerve endings. This process is very complicated but it's a good bet that sooner or later safe anti-inflammatory drugs will solve most of the pain problem. I, for one, am looking forward to a safer version of Zomax both for my patients' sake and, in my creaking dotage, my own.
Neural blockade control of the various "gateways" through which pain signals reach our brains, is next. There' s no doubt that pain is ultimately a mental phenomenon whose survival value involves some kind of negative re-enforcement; we learn not to hammer our thumbs by punishing experience. There are times when there' s more survival value in not feeling pain though, and at those times, even injured, we often don't have pain. Anyone who has seen an action movie knows we can take quite a beating yet be oblivious to pain. This is neural blockade at the highest level in the brain. But it can happen all the way down to the nerve-ending level. There is a well-known pain gate in the spinal cord; regional anesthesia controls pain by temporarily blocking nerve transmission via chemicals injected around more peripheral nerves. Farther down toward the pain-associated nerve endings we create neural blockade with topical anesthetics (like what the dentist puts on your gum so you don't feel the novocaine needle) or even ice.
As an orthopedist who has seen a lot of tough older men with painless, yet arthritic joints, I'm suspicious of there being a peripheral pain blockade that sometimes occurs around joints that are simply not given a chance to rest when they hurt. These are the ones who when asked "Don't you have any pain? say something like "not really but what does it matter, I still have to work to put the food on the table." I don't think it can be a central blockade because it's always there; mental or brain level suppression would be more variable.
That the mind could produce a physical change in the peripheral nerves is not impossible, however. With my own eyes I have seen the red circle left by an ordinary quarter, on the arm of a fellow student under hypnosis in medical school. I told her it was a burning coal but that she couldn't move to take it off. We thought her grimacing seemed a little fake. But the very obvious inflammation of the skin touching the quarter was not. It showed that the brain's control of specific, tiny nerves in this case the tiny blood vessels in the skin is profound.
Finally, depression and hormones have a lot to do with pain in some people. Back and neck pain flare-ups in people under stress are commonplace. We have successfully treated many pain patients with anti-depressants. Back in the days of female hormone replacement therapy, samples of estrogen skin patches gave great relief of all sorts of joint pains suffered by certain peri-menopausal patients. Exercise, strangely enough, seems to have taken the place of the hormones we used to give with nearly the same pain relief. This might be a covert form of hormonal therapy itself; the "pleasure hormones" or endorphins that are released in the brain by exercise (and eating and sex too) are potent natural pain killers.
So we clearly have some leads; in 20 years we'll probably understand it, but today pain remains a tantalizing mystery. For now if you want to avoid that knee replacement, the best I can recommend is that you is cheer up, pop an Advil, keep working, go to the gym, eat something and buy your spouse a present.