The knee is one of the joints most commonly affected by osteoarthritis (OA), and pain is the most commonly reported symptom. In England, 1 in 4 individuals aged 55 or over report knee pain, which is often linked to osteoarthritis (OA). In individuals with OA-related knee pain, mechanisms within the central nervous system (brain or spinal cord), also referred to as ‘central mechanisms’ may intensify the experience of OA knee pain[2,3]. Indeed, 20 to 40% of individuals receiving targeted treatment (e.g. total knee replacement) do not report pain relief, possibly due to central mechanisms of OA knee pain.
Quantitative Sensory Testing (QST) and imaging techniques can be used to identify aspect of central mechanisms, such as central sensitization across individuals. QST involves applying mechanical pressure and/or thermal stimuli to examine changes in the sensitivity of an individual’s nerve fibre.
For this study, we wanted to identify a set of questions that measure person-specific characteristics linked to central mechanisms of OA knee pain. A simple questionnaire would clearly be more practical, and less resource- or time consuming than QST and fMRI approaches to identify people with central mechanisms that could be significantly contributing to their pain experience. Being able to easily identify such individuals could be important in helping to determine their most appropriate management plan and could encourage the development of novel treatments that target central mechanisms in these individuals.
In order to identify characteristics that are linked to central pain mechanisms, the Knee Pain and Related Health in the Community (KPIC) study recruited 9506 individuals from their general practices in the East Midlands region (UK) using a postal survey. One hundred and four questions measuring various characteristics associated with the knee pain were included in the postal survey. Ninety eight individuals without knee pain and 322 with knee pain were invited for knee X-rays, and QST – application of a pressure probe to the unaffected shin. Knee OA and pain experts (e.g. consultant rheumatologists, psychologists, etc.) rated on a survey, how highly each question was rated as a likely measure of central mechanisms. Using statistical tests for association, we then looked at the relationship between each characteristic, and the QST objective measure of central pain mechanisms.
We found that eight different characteristics were related to central pain mechanisms. These characteristics were anxiety, depression, having other painful areas in the body (pain distribution), how one thinks about pain (catastrophic thinking), how pain affects attention to a task (cognitive impact), sleep disturbance, fatigue, and pain that feels tingly or electrical in nature (neuropathic like pain). Eight questions (one representing each of these 8 characteristics) were combined in a new short questionnaire called the Central Aspects of Pain in the Knee (CAP-Knee) Scale. For a full account, see our extended report in PAIN .
What were the limitations of our study?
Our study was not without limitations. The KPIC survey included a comprehensive, but not exhaustive question set, and other questions not included within the KPIC survey may also be relevant to the central mechanisms of OA knee pain. Experts who rated the questions were all members of the Arthritis Research UK Pain Centre and it could be argued that this may have introduced bias for question selection. However, the breadth of expertise from the participating experts is reflective of the multidisciplinary approach involved in the treatment of OA knee pain. In addition, other approaches were employed to inform the question selection, such as the relationship between QST and each question.
Although the QST measure used here is a highly reliable method for assessment of pain mechanisms, this study would have benefited from the use of other QST measures (such as heat or cold), or imaging approaches.
What are the future directions and implications of this study?
Combining 8 characteristics in a brief question set is an effective alternative to identify central pain mechanisms in people with OA knee pain than longer questionnaires addressing any single characteristic alone. Including questions that represent each of these characteristics within one easily completed and inexpensive questionnaire might allow clinicians to identify the need to investigate treatments that target central pain mechanisms in those individuals, potentially improving pain relief for the individual. Our study improves understanding of the relationships between key characteristics linked to central mechanisms in OA knee pain, and our findings here may have important implications for other chronic pain conditions such as back pain, or cancer pain. Employing other QST- approaches for identifying central mechanisms, such as temporal summation, will be useful to confirm the study findings, as this study only employs one QST modality.
Other questions stem from our findings. For example, how effective will the CAP-Knee Scale be in predicting individuals with knee pain who are more likely to benefit from treatment aimed at the central nervous system? How do individuals with OA knee pain interpret the questions included in the CAP-Knee Scale? These remain to be assessed in future studies.
About Kehinde Akin-Akinyosoye
Kehinde is a graduate student in the Academic Rheumatology PhD program, and a member of the Arthritis Research UK (ARUK) Pain Centre of excellence at the University of Nottingham. Her research seeks: (1) to better understand how processes within the central nervous system influence the pain experience, and (2) to identify biological or psychosocial markers linked to mechanisms which drive pain within the central nervous system. Her overall research aims to improve pain relief in individuals with chronic pain, in particular, knee pain due to osteoarthritis.
Her PhD project focuses on creating a valid and feasible tool to identify subgroups of individuals with knee pain who might benefit from treatment targeted towards the central nervous system in clinical practice. Studies within her PhD project mainly involve questionnaire and clinical assessment of individuals with knee osteoarthritis pain. For more information, see https://www.nottingham.ac.uk/paincentre/index.aspx
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