The Contribution of Age and Obesity to the Number of Painful Joint Sites in Individuals Reporting Osteoarthritis

A Population-Based Study

Elizabeth M. Badley; Jessica M. Wilfong; Calvin Yip; Dov B. Millstone; Anthony V. Perruccio

Disclosures

Rheumatology. 2020;59(11):3350-3357. 

In This Article

Discussion

This population-based study of individuals with OA with information on the number of painful joint sites showed the vast majority of participants (84%) had two or more painful joint sites, with nearly half having four or more. The frequency and distribution of painful joint sites was similar to that of the limited number of clinical studies that have looked at this in patients with OA[12,15,38,39] and in population studies of arthritis.[10,40] Contrary to our hypotheses, neither age nor BMI were associated with the number of joint sites reported as painful.

The increase with age of the prevalence of OA[1–3,5] has led to suggestions that at least some phenotypes of OA are related to cellular and other processes of ageing of the musculoskeletal system.[31,41,42] If OA is associated with cellular processes of ageing, one might expect these processes should affect all joints, so it is surprising that we did not find a greater number of painful joint sites at older ages. Data from a population-based survey of a primary care population asking about the number of painful joint sites (up to seven) showed no indication of a higher number with age,[9] with similar findings from a community survey for the population aged ≥45 years.[40] A potential explanation of the null finding is that OA can onset at any age, so that at any given age there is a range of durations of symptoms. It might, thus, be expected that the relationship between the number of joint sites should be one with duration rather than age. In our regression analyses we found that a longer duration of joint symptoms was associated with a higher number of painful joint sites in the multivariable analysis (Table 3). While this might be interpreted as being consistent with ageing processes, the magnitude of the RR from this analysis (RR = 1.16) gives a somewhat misleading impression. As can been seen from Figure 2, there is only a very modest increase of just over one extra site between durations of 0–5 and 20+ years across quartiles of duration. Moreover, the number of joint sites affected within each duration category was similar for each age group. Cushnaghan et al.[12] also found only a weak correlation (r = 0.29) for symptom duration with increasing age. One provoking interpretation of these findings is that the onset of OA, regardless of age, can be at several joint sites with only a modest increase in number of joint sites over time. A potential implication is that OA does not inevitably progress with the involvement of more painful joints over time, although this would need to be confirmed with longitudinal data.

The lack of association of BMI with the number of painful joint sites was also an unexpected finding. Few studies have considered the relationship of BMI and multiple joint involvement in OA. Hoogeboom et al.[43] found no difference in mean BMI in patients with hip or knee OA with and without pain in other joints. There is growing interest in a possible systemic component to OA, and in this context Bruyere et al.[31] suggested that multi-site OA is a feature of OA comorbid with inflammation or metabolic syndrome. That obesity does not appear to be associated with the number of painful joint sites does not fit with this nor with those speculations about metabolic or other mechanisms for OA that are grounded in the association with overweight and obesity, particularly in the hands, a non-weight-bearing joint.[28,29,32] As the knee is one of the most frequently affected joints and the joint that is most strongly associated with BMI[23] we further examined how knee pain was distributed by number of joint sites. We found respondents with knee pain at all levels of painful joint site count. We therefore suggest the lack of association of the number of joint sites with BMI is likely a reflection of the effect of the distribution of painful knees. The role of obesity in multi-joint OA clearly requires further exploration.

Consistent with other studies, women were more likely to have pain at multiple joint sites than men.[8] The number of painful joint sites was associated with having two or more comorbidities, independently of age. As noted above, current theories of OA suggest a role for low-grade inflammation. Chronic inflammation is implicated in the progression of many chronic diseases including heart disease, diabetes, bowel disease and asthma.[44] The association of number of joint sites with the number of comorbidities could thus be a reflection of overall inflammatory load or other systemic processes. This finding needs further investigation, along with the lack of association with obesity.

Major strengths of this study are that it utilized data from a nationally representative survey on arthritis that meant we were able to focus on number of painful joints sites in respondents reporting OA. However, it is also necessary to bear in mind several limitations. The cross-sectional nature of the data means we were limited to looking at associations. We have no information on the site of back pain, whether lumbar or thoracic, nor on the specific joint sites for pain in the foot and hand. As with most population-based surveys, OA diagnosis was self-reported. While self-report may introduce misclassification, self-report of arthritis in population-based studies has been found to be adequate for surveillance purposes including for OA.[45–47] Moreover, a review of the effect of OA definition on prevalence showed similar estimates for self-reported and symptomatic OA definitions.[48] There is also uncertainty about whether all the painful joint sites are attributable to OA as there may be other pathologies affecting the soft-tissues such as tendonitis or bursitis. A study of older women (96% had OA) showed that while 80% also had soft-tissue pathology most (85%) had OA at multiple sites.[49] We therefore presume that a high proportion of the painful joint sites in our studies are likely associated with OA. Our replication study in a clinical population with confirmed severe hip or knee OA similarly found no association between number of painful joint sites and increasing age or BMI, supporting the generalizability of our findings.

The findings from this study raise new questions about the role of age and BMI (obesity) in the development of multi-joint OA, especially as this and other studies show that most people with OA have multiple joint sites involved. Studies of OA that focus on only one primary joint site may be misleading in that they do not adequately represent the totality of OA. Neglect of the possibility of multiple joint involvement in studies of OA could potentially lead to the false attribution of particular risk factors or adverse outcomes to the joint under study, giving rise to potentially misleading conclusions. Understanding OA as a multi-joint disease is critical to making progress with defining disease phenotypes and identifying potential etiological mechanisms as well as the provision of care. Multiple joint site involvement may make compliance with management strategies such as exercise difficult. Furthermore, the involvement of joint sites other than a primary joint may also contribute to limiting the success of otherwise successful procedures such as total joint replacement surgery.[50] Further research is needed to elucidate the frequency and characteristics of multi-joint OA, with implications for understanding phenotypes, and the development of prevention and treatment strategies.

Comments

3090D553-9492-4563-8681-AD288FA52ACE

processing....