Is innervation of cartilage the driving force behind development of osteoarthritis and subsequent pain, or is the degeneration of joints in osteoarthritis affecting nerves and creating pain?
This was the question underpinning a fascinating debate at the OARSI 2021 World Congress, featuring two giants of the OA research community: Anne-Marie Malfait, MD, PhD, professor of medicine in the division of rheumatology at Rush Medical College, Chicago, and Stefan Lohmander, MD, PhD, professor emeritus of orthopedics at Lund (Sweden) University in Sweden.
At stake in the discussion is a greater understanding of the physiological processes that underpin both the development of OA in joints and the experience of pain in patients with OA.
Lohmander started by pointing out that, while pain is the primary symptoms of OA, it does not always overlap with the physiological processes of the disease, as measured by techniques such as MRI, x-ray, biomarkers, and gait analysis.
"This lack of complete overlap is often a problem when doing our clinical trials," Lohmander told the conference, sponsored by Osteoarthritis Research Society International. "When talking about osteoarthritis, we also need to remind ourselves every so often that we are speaking of either the symptoms or the disease and maybe not always the both of them."
While a healthy joint has pain receptors everywhere but the cartilage, studies have found that the osteoarthritic joint brings blood vessels, sensory nerves, and cells expressing nerve growth factor from the subchondral bone into even noncalcified articular cartilage, he said.
These nociceptor neurons are mechanosensitive, so mechanical injury to the joint triggers inflammation, and the inflammatory proteins themselves act on the nociceptors to generate pain signals in the brain, "so clearly, it is the joint that signals the brain," Lohmander said.
However, Malfait pointed out that there is a body of evidence from animal studies showing that the absence of sensory nerves in joints – either from disease or removal – is associated with the onset or worsening of OA.
"Healthy nerves are really important to ensure healthy joints," Malfait said. She said age-related loss of sensory nerves always preceded age-related OA, and was also associated with age-related loss of proprioception and vibratory perception.
Interestingly, animal studies suggest that removing intra-articular nociceptors can actually have a protective effect on the osteoarthritic joint, Malfait said. Studies in humans who have experienced neurologic lesions also suggests improvement in conditions such as rheumatoid arthritis.
She raised the idea of neurogenic inflammation: that peripheral neurons are releasing vasoactive mediators that contribute to inflammation in tissues. "These nerves and nerve products are talking to all the different cells in the joints," she said.
Defending his argument that joint pathology is the cause of pain, not the pain causing the joint pathology, Lohmander gave the example of studies that looked at radiographic abnormalities between two knees of the same patient who also had discordant pain measures for each knee. This research "showed strong association between radiographic osteoarthritis and knee pain, supporting the argument that structural abnormalities cause knee pain," he said.
Martin van der Esch, PhD, of the Amsterdam University of Applied Sciences, said the debate was one of the highlights of the conference because it addressed such an important and longstanding question in OA.
"Is osteoarthritis leading to a generalized pain, so involvement of the nervous system, but the source – the causality – is in the joint?" he said in an interview. "Or is it the other way around, so that means is there first a problem inside the nervous system – including also the vascular system – and which is presented in the joint?"
It is more than an academic discussion because the conclusions of that could mean different treatment approaches are needed for different groups of patients, and raises the different ways of thinking about OA, he said.
None of the sources for this story declared having any relevant conflicts of interest.
This article originally appeared on MDedge.com, part of the Medscape Professional Network.
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Cite this: Which Comes First in Osteoarthritis: The Damage or the Pain? - Medscape - May 14, 2021.