Nocebo Confession: this image makes my knee hurt!
The recently published prospective (March 2014) Future directions in painful knee osteoarthritis: Harnessing complexity in a heterogeneous population really breaks-down some wonderful concepts about pathoanatomy and pain perception.
The article reports that 50% of individuals with osteoarthritis-like knee pain have positive radiographs for osteoarthritis (OA). Interestingly enough, 50% of those with positive radiographs for OA have knee pain. Citing the NIH they quote “it is important to separate conceptually the disease process of OA and the syndrome of musculoskeletal pain and disability.” They also refer to N. Hadler’s paper Knee Pain is the Malady, Not Osteoarthritis, which is a wonderful title indeed!
The authors propose a new conceptual model which is outstanding in breadth and scope. They delineate the disease process into three contributing categories: 1) Knee pathology 2) Psychological distress 3) Pain neurophysiology.
Knee pathology involves the regular and commonly held positions about OA. They include the radiographic (X-Ray) evidence, the chondral changes, osteophytes, the joint narrowing, the “bone-on-bone”, the “you have the knees of a 90 year old.” This part of the puzzle obviously contributes, but the relationship is NOT 1-to-1 for pain and dysfunction. – Try explaining that to someone who has knee pain, has been told they have OA and been told they have the knees of a 90 yr old. It will be a long discussion. Of course there are the biomechanical stresses on the knee including increased body mass and joint positioning that are implicated in the knee pathology puzzle piece.
The psychological distress part has to do with fear of movement (moving could cause pain!!) and catastrophizing (ruminating over the situation and winding it up into a hypersensitive syndrome) and other factors (don’t want to give it all away, read the article!) similar to other chronic pains. It is shown that the better the coping mechanisms and the better the positive belief strategies that the patient can employ the better the outcomes.
The pain neurophysiology aspect is… well, please allow me to quote:
“Pain is a perception, a construct of the nervous system, not equivalent to (although likely influenced by) sensory nociceptive input from the periphery. Nociception is known to undergo modulation at multiple locations in afferent pathways, and there are opportunities at each of these locations for clinically relevant changes in the neurophysiological processing of pain. In knee OA, the areas of the central nervous sytem that appear to play a role in the perception and modulation of pain include (but probably are not limited to) the spinal cord, brain stem, thalamus, somatosensory and motor corticies, prefrontal cortex, cingulate cortex, and limbic system.” (pg. 427)
Please see my post on touch sensation and somatosensory changes with pain.
Central Sensitization is also implicated in the neurophysiology of knee pain. This is when there is a feed-back loop type of response where a nociceptive input comes in and the spinal cord spins that back out to the periphery, causing more noxious chemicals and processes to occur and be released and a viscous cycle begins, similar to a muscle spasm. Check this video for some easily digestible details:
Also seen in research is an impaired descending control (the endogenous opioid release) mechanism in these patients. The way I understand it this is typical of chronic pain disease states.
Note: osteoarthritis is not categorized as an inflammatory process (although it ends in -itis..). Be skeptical of those needing anti-inflammatory meds for their non-inflammatory disease process. That being said the authors point out that some cytokines and inflammatory mediators are seen in elevated levels in those with knee OA. Specifically Interleukin 6 and 8 (however IL-6 has been shown to have anti-inflammatory properties as well). IL-1 and TNF-alpha are also seen in cartilage deterioration and nociceptor irritation. There is also often joint swelling (tumor!: a sign of inflammation). So this area seems to be not so clear.
Next post I will report on the future direction of diagnosis, prognosis and treatment. Read it here.