Principal findings
Overall, the presence of arthritis is common in the municipality of Chankom, which aligns with what has been reported in other Mexican [
31] and international reports [
6]. The disability prevalence ratio between the arthritis and non-arthritis populations is 2.8:1, as calculated by Zhang’s method [
32]. This means that people living with arthritis in this community are 2.8 times as likely to have disability as the people living without arthritis after controlling for age, gender, and number of comorbidities. Consequently, this group of chronic conditions have important disabling effects in this community, as has been observed in other populations [
7].
The results from the evaluation of associations between modifiable risk factors and the overall prevalence of arthritis and its main types show that this group of chronic diseases are linked with factors that either increase the vulnerability or increase the loading of the joint, as has been previously suggested [
26]. On the one hand, social factors, such as low level of wealth, may have increased joints’ vulnerability to be affected by degenerative and/or inflammatory processes. On the other hand, physical and behavioral factors, such as BMI or doing repetitive movements, may have increased the loading within the joints facilitating the manifestation of joint damage.
Being wealthier was associated with less probability of presenting overall arthritis in this community, similar to what has been reported in a population-based study conducted in Brisbane, Australia [
24]. Chankom is a Mexican indigenous community, where the people face health inequities [
11]. These inequities impede the delivery of timely and appropriate care for solving initial MSK problems for all community members, increasing vulnerability to develop arthritis. Having less wealth in Chankom could also be associated with inadequate nutritional intake, which may foster the progression of joint degeneration and/or inflammation.
Factors that increase joint loading, such as BMI, were significantly associated with a higher prevalence of overall arthritis in this community. This association only held for the prevalence of lower extremity OA, which has been consistently reported in other epidemiologic studies [
33]. A person with a BMI of 29 was 1.5 times more likely to present with lower extremity OA than a person with a BMI of 24.
The lack of a significant association observed between BMI and hand OA prevalence does not support the suggested systemic effects of obesity in OA [
34]. Consequently, it is possible that in our analysis BMI acted only as a joint loading factor and not as a systemic factor that increased joint vulnerability through serologic inflammatory markers, as has been proposed in the literature [
35].
Results related to cumulative mechanical joint stress, the other joint-load increasing factor addressed in this study, were inconsistent and conflict with what has been reported in the literature. On the one hand, static cumulative joint mechanical stress was associated with a lower prevalence of overall arthritis. This association only held for the prevalence of RA, and persons in the RA group reported doing regular repetitive climbing, standing, kneeling, and sitting-to-standing less often than people without RA (Table
3). A recent cross-sectional study conducted in Colombia found that people with RA usually performed low levels of physical impact work [
36]. This implies that our findings could be related to a lower engagement in physically demanding activities by the RA group, which can be considered a case of “inversed causality” [
37].
On the other hand, the OA subgroup analyses showed that for men, repetitive hand jolting and sit-to-stand movements approached a significant association with hand OA prevalence. In fact, all men with hand OA reported doing these repetitive activities, supporting the notion that dynamic cumulative mechanical joint stresses were linked to the manifestation of this condition. However, repetitive lifting was significantly associated with a lower prevalence of hand OA and repetitive standing for longer than 30 min was significantly associated with a lower prevalence of low-extremity OA in men; while the latter has been linked with a higher prevalence of hip and knee OA [
38].
The inconsistent findings observed in the cumulative joint mechanical stress analyses could be related to the high frequency with which participants reported doing repetitive movements during their main occupation. More than 60 % of the population reported doing ≥2 static and dynamic cumulative repetitive movements (data not shown). This indicates some homogeneity among the occupations performed by these community inhabitants; usually, men do the same type of agricultural work while women do similar housework activities. This homogeneity makes it difficult to explore differences between groups. Consequently, we cannot conclude anything solid about the role that joint mechanical stress has on the manifestation of arthritis in this population.
Interestingly, smoking behavior was not significantly associated with RA prevalence in this study, which contradicts several reports in the literature [
25,
27]. It has been suggested that only heavy smoking and therefore the dosage, and not just the presence of smoking, is associated with the “seropositive” type of RA [
39]. We could not determine whether serologic markers were present in participants with RA. However, we are sure that none of the participants with confirmed RA in Chankom, where smoking is uncommon, reported this behavior. Considering that the prevalence of RA observed in this community (1 %) aligns with the prevalence reported worldwide [
27], we could argue against the existence of a real association between smoking and the manifestation of RA.
Strengths and limitations
The main strengths of this study are related to the methods used for screening and defining arthritis cases and the use of locally grounded measurement instruments. The census strategy, involving the majority of adults living in Chankom, allowed us to conduct a comprehensive analysis of the OA and RA problems in this community. The COPCORD methodology we followed has been validated and used with success in detecting MSK diseases, including OA and RA at the community level in Mexico [
40]. This methodology involved a duplicate assessment of cases, including diagnostic confirmation by specialists, which increases our confidence in the validity of the prevalence estimates observed. Finally, the use of a cross-culturally validated instrument, which involved the participation of local people in its development, increased confidence about the local relevance of observed results. For example, people who lived in Chankom decided the properties on which to differentiate levels of wealth among community members, increasing the cultural relevance of the measurements “level of wealth.”
The main limitations of this work are related to the cross-sectional design, the measurement of disability, and the measurement of regular repetitive movements performed in the main occupation. The cross-sectional nature of this study precludes us from establishing causal associations between known risk factors and arthritis incidence. For instance, the accuracy of the counterintuitive associations observed between mechanical joint stress and a lower prevalence of hand and low extremity OA in men can only be established through longitudinal data. The cross-sectional design also prevents the further assessment of “non-specific arthritis” cases, limiting the possibility of observing how these cases progress over time and with which type of arthritis (inflammatory or degenerative) they will ultimately be diagnosed. Responding to this design-related limitation, we initiated a longitudinal surveillance of this population and results will be available in the future.
The measurement of disability is complex and it has been suggested that considering only one dimension of physical function, such as what people think they can do from a pre-defined list of activities contained in a questionnaire is not enough to understand the whole disabling effects of an illness [
41]. Therefore, we may not have detected the entire disabling effects of arthritis for people living in Chankom. This is a common limitation in epidemiologic studies of arthritis-related disability. Finally, we did not incorporate a measurement of the actual time (hours, days, months or years) spent on doing the repetitive movements explored in the main occupation. The lack of this temporal component limits our analysis and interpretations about the role that cumulative joint mechanical stress plays in the presentation of arthritis in this community.
Implications for research
The disabling effects of arthritis need to be further assessed by incorporating measurements of other dimensions of physical function such as the execution of standardized tasks or the limitations conditioned by the disease on the performance of real life activities. In addition, it is important to evaluate the presence of modifiable factors linked with the progression of arthritis in this community and how these relate with its disabling effects. The association between social factors, such as the level of wealth, and the prevalence of arthritis in Mayan rural communities, should be further explored using quantitative and qualitative methods. Finally, there is a need for longitudinal studies that explore possible causal associations between the significant factors detected in this study and the prevalence of the various arthritis types, especially for those unexpected and counterintuitive associations (i.e., cumulative mechanical joint stress analyses).