Knee pain – and central sensitisation


Last week I started to discuss central sensitisation indicators in people with osteoarthritic knees, based on a paper by Lluch, Nijs, Courtney, Rebbeck, Wylde & Baert, et al (2017). I’m going to continue with this topic this week, because with the rise of osteoarthritis in the general population and particularly the impact of an aging population, I think we will all need to think hard about how we conceptualise osteoarthritis, and what we do for management. While efforts within my own Department (CReaTE – tissue engineering) involve developing new ways to remodel knee-joint tissues, we know that it will be some years before this approach is widely available (human trials haven’t started yet), and given the relative lack of funding for joint replacements, I think developing effective assessment and rehabilitation for painful knees is a real area of development.

So last week I discussed using simple measures such as >5 on a 0 – 10 VAS (NRS), pain drawings/maps showing radiating pain or widely distributed pain, the pattern of pain fluctuation (during activity, with an increase after activity), and using a couple of fairly simple questionnaires to help identify those most likely experiencing more than the “simple” OA pain we’ve learned about. And as always, identifying psychosocial factors which can lead to increased disability and distress is important.

Along with the clinical interview, we usually incorporate physical examination or physical performance testing. There are some indicators that might be useful such as inconsistent responses to our usual physical examination (ie testing increases pain even though some of them shouldn’t do so) – this should not be interpreted as a sign that the person is “faking bad” or exaggerating their experience. I can’t emphasise this enough! It’s possible that anxiety on the part of a person can wind the nervous system up – leading to what is usually non-nociceptive input being interpreted as nociceptive (Courtney, Kavchak, Lowry et al, 2010).

Another indicator is the presence of widespread hypersensitivity to mechanical stimuli – it’s a common finding in people who have central sensitisation and includes increased response to pressure and touch. You could, as a clinician, use a pressure algometer both close to the knee, and further away, to establish over-excitability of the nociceptive pathways. Interpreting findings using pressure algometry is not straightforward because there is overlap between those with OA and those without, but it’s possible to use norms from the general population (such as Nesiri, Scaramozzino, Andersen et al, 2011). It’s a bit of a challenge because of the overlap between the two populations, but can add to the clinical picture. Pain (allodynia) on light touch or being stroked with a cottonwool ball around the knee, is definitely a clue that something’s up.

Both thermal hyperalgesia and tactile hypoaesthesia (reduced sensitivity to von Frey fibre testing) have been associated with central sensitisation – if you don’t have formal testing apparatus, the back of a warmed teaspoon placed on the skin for 10 seconds should be experienced as hot but not painful in someone who isn’t tending to central sensitisation, and you can use cottonbuds (or cottonwool) to identify loss of sensation acuity, provided you do so in a systematic way (the authors suggest starting where it’s most painful and stimulating the skin in a wheel spoke pattern, gradually widening out).

Putting it all together

Any single test, on its own, is unlikely to be a good predictor of central sensitisation, but when combined with the information you obtain from the person, along with the relevant questionnaires, should begin to help develop a picture of who is likely to have a less-than-ideal response to planned trauma. What we do about reducing the potential for central sensitisation is still  begin hotly debated but we DO know that giving good information about pain mechanisms, and encouraging graded exposure and graded activity can be helpful. Given that exercise is a good approach for reducing the impact of osteoarthritis in the knee, for those with the additional burden of central sensitisation, I think swimming or hydrotherapy could also be helpful, as could mindfulness and even mindful movement like tai chi, yoga or xi gong.

Conclusion

People living with OA in their knees often spend many years having difficulty managing their pain before they are able to have surgery. From recent research in New Zealand, I don’t think many people are offered a pain “education” approach, and indeed, I’d bet there are a lot of people who don’t get referred for movement-based therapy either. Misunderstanding is rife in OA, with some people uncertain of the difference between osteoarthritis and rheumatoid arthritis, and others very worried that they’re going to “wear the joint out” if they exercise. While OA isn’t as sexy as low back pain, doesn’t have the economic cost of low back pain, and has a reasonable surgical option – it is still a significant problem for many people. Helping those people be more confident to move, helping reduce their uncertainty about the effect of movement on their joints, and giving them an opportunity to think differently about their knee pain would be a real step forward. Surgery, while helpful for many, is either not available or unsuccessful for others, and it’s time we attended to their needs as well.

 

Courtney CA, Kavchak AE, Lowry CD, et al. (2010). Interpreting joint pain: quantitative sensory testing in musculoskeletal management. Journal of Orthopaedic Sports Physical Therapy. 40:818–825.

Lluch Girbes E, Meeus M, Baert I, et al. (2015) Balancing “hands-on” with “hands-off” physical therapy interventions for the treatment of central sensitization pain in osteoarthritis. Manual Therapy. 20:349–352.

Lluch, E., Nijs, J., Courtney, C. A., Rebbeck, T., Wylde, V., Baert, I., . . . Skou, S. T. (2017). Clinical descriptors for the recognition of central sensitization pain in patients with knee osteoarthritis. Disability and Rehabilitation, 1-10. doi:10.1080/09638288.2017.1358770

Neziri AY, Scaramozzino P, Andersen OK, et al. (2011). Reference values of mechanical and thermal pain tests in a pain-free population. European Journal of Pain. 15:376–383.

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