Abstract
Persistent post-surgical pain (PPSP) remains a problem after knee replacement with some studies reporting up to 20% incidence. Pain is usually felt by those who do not operate to be a monolithic entity. All orthopaedic surgeons know that this is not the case. At its most basic level, pain can be divided into two categories, mechanical and non-mechanical.
Mechanical pain is like the pain of a fresh fracture. If the patient does not move, the pain is less. This type of pain is relieved by opiates. Mechanical pain is seen following knee replacement, but is becoming less frequent. It is caused by a combination of malrotations and maltranslations, often minor, which on their own would not produce problems. The combination of them, however, may produce a knee in which there is overload of the extensor mechanism or of the medial stabilizing structures. If these minor mechanical problems can be identified, then corrective surgery will help.
Non-mechanical pain is present on a constant basis. It is not significantly worsened by activities. Opiates may make the patient feel better, but they do not change the essential nature of the pain. Non-mechanical pain falls into three broad groups, infection, neuropathic and perceived pain. Infection pain is usually relieved by opiates. Since some of this pain is probably due to pressure, its inclusion in the non-mechanical pain group is questionable, but it is better left there so that the surgeon always considers it. Low grade chronic infection can be extremely difficult to diagnose. Loosening of noncemented knee components is so rare that when it is noted radiologically, infection should be very high on the list of suspicions. The name neuropathic pain suggests that we know much more about it than we do in reality. Causalgia or CRPS-type two is rare following knee replacement. CRPS-type one or reflex sympathetic dystrophy probably does exist, but it is probably over-diagnosed. The optimum treatment I have found is lumbar sympathetic blocks. Lyrica, Gabapentin and Cymbalta may also help. Perceived pain is the largest group. It does not matter what you tell the patient, some believe a new knee should be like a new car, i.e. you step into it and drive away. The fact that they have to work to make it work is horrifying. Perceived pain is widespread. The classic treatise, Dr. Ian McNabb's book “Backache”, should be studied by all who wish to understand pain complaints.
Any experienced knee surgeon will have his list of red flags or caveats. I will list only a few. If the patient comes in with a form asking for a disability pension on the first visit. If the patient's mother answers the questions. If the patient comes in taking massive doses of opiates. If the patient is referred to you by a surgeon who does more knee replacements than you do.
There are other issues such as good old fibromyalgia, which appears to have gone the way of the dodo. It has been replaced by something equally silly called central sensitization. The theory of central sensitization is that if one has pain somewhere or other for three months or six months or whatever, there are going to be changes in the brain and spinal cord. It then does not matter what happens to the original pain, i.e. whether or not it goes away, the pain will persist because of the changes in the brain, hence, the title of the pain in the brain syndrome. If this theory was correct, we might as well all go home because we have all been wasting our time for the last 30 years because none of our patients would get any better. After all, all of our patients have had pain for a lot longer than three months, many of them have been involved in trauma and sometimes, compensation is at issue. The pain in the brain theory, therefore, sounds about as realistic as the flat earth society or the treatment of Galileo.
