To What Extent Is Severe Osteoarthritis Preventable?

Occupational and Non-Occupational Risk Factors for Knee and Hip Osteoarthritis

Tea Kontio; Markku Heliövaara; Eira Viikari-Juntura; Svetlana Solovieva

Disclosures

Rheumatology. 2020;59(12):3869-3877. 

In This Article

Discussion

In this population-based study we found that baseline age and BMI, as well as prior injury were associated with the risk of first hospitalization due to knee and hip OA. Composite cumulative workload was associated (with a dose–response pattern) with hospitalization due to knee OA and showed an association with hip OA at a younger age only. Altogether, prior injury, high BMI and intermediate to high composite cumulative workload accounted for 70% of hospitalizations due to knee OA. High BMI alone accounted for 61% of hospitalizations due to hip OA. Only 6% of hospitalizations due to hip OA were attributed to prior injury of the hip. Moreover, the risk of hospitalization due to knee OA was lower in men than women. In contrast, the risk of hospitalization due to hip OA tended to be higher in men than women.

Comparison With Previous Studies

There is a large heterogeneity in the definition of both knee and hip OA used in epidemiological studies. In population-based studies OA has been typically defined radiographically or clinically.[22] The outcome of the current study was hospitalization due to knee or hip OA, which can be viewed as advanced OA. Therefore, our results are comparable to previous studies on severe clinically defined OA.

Longitudinal studies are consistent regarding findings linking overweight, obesity and knee injury to the risk of knee OA.[6,7,10] A systematic review of 12 studies (with only one cohort study) found moderate evidence for an association between obesity and clinical hip OA (e.g. symptomatic or need for total joint replacement surgery) and no association between obesity and radiographic hip OA.[8] Similarly, a meta-analysis of 14 studies found a relatively weak positive association between BMI and hip OA, showing a stronger association for clinical than radiographic hip OA.[9] Few studies found an increased risk of advanced hip OA for patients with prior injury.[23,24] We found that obesity and prior injury are independent risk factors for both knee and hip OA leading to hospitalization.

A dose–response relationship of cumulative years in certain occupations and tasks with knee and hip OA has been previously reported, however findings have varied by the OA outcome.[12–14,25–28] In our study, cumulative exposures to physically heavy work, manual handling of heavy loads, kneeling or squatting, and standing or walking, as well as composite cumulative workload were associated with hospitalization due to knee OA. Overall, no association between cumulative exposures and hospitalization due to hip OA was found in the entire population in our study. However, among persons who were ≤60 years at the end of follow-up, high composite cumulative workload increased the risk of hospitalization due to hip OA by 3.2-fold. In our study, persons hospitalized due to hip OA were older than those hospitalized due to knee OA. More than half of the hospitalizations due to hip OA took place at the age of 60–74 years, suggesting that the majority of these persons were exposure-free for several years prior to their hospitalization.

In male and female construction workers, floor- and bricklayers, as well as farmers and female health assistants an increased risk of hospitalization due to knee and hip OA was reported already after 5 years in the occupation.[27] Our results suggest an increased risk of hospitalization due to knee OA in the general working age population after 20 years of exposure to physically heavy work.

Our findings suggest that after controlling for age, gender and other modifiable risk factors, BMI alone accounted for almost two-thirds of the hospitalizations due to hip OA. For hospitalizations due to knee OA, both overweight/obesity and prior knee injury were of high importance. Furthermore, reduction of composite cumulative workload to a low level has a potential to prevent 27% of hospitalizations due to knee OA and 24% of earlier (before 60 years) hospitalizations due to hip OA.

The PAFs for obesity calculated in the current study were higher than previously reported. PAF estimates depend on both the prevalence of a risk factor in the population and the strength of the association between the risk factor and the outcome. It is well known that risk estimates for less common outcomes (e.g. hospitalization due to OA) will be generally higher than those for more prevalent outcomes (e.g. radiographic OA). This means that for a risk factor the PAF value will increase with the severity of the outcome. Indeed, a meta-analysis comparing obesity-related PAFs for symptomatic knee OA and knee OA requiring total joint replacement across different countries found that the PAF estimates for the more severe outcome were always higher than those for the less severe outcome.[15]

Strengths and Limitations

The strengths of our study include a relatively large, nationally representative study population with a follow-up period of a maximum of 15 years, objectively identified cases of OA using the Finnish National Hospital Discharge Register and information on the major modifiable risk factors of OA at baseline. Because of the random sampling of the general population and a register-based outcome, there was a low risk of selection bias and no risk of attrition bias. The Finnish National Hospital Discharge Register covers information on hospital admissions and discharges from every hospital in the country, and it has been reported to be a reliable and accurate source of information.[21] Furthermore, the study population was large enough to allow for adjustments for several relevant covariates and the calculation of the overall PAF.

However, some limitations should be taken into consideration. Our cases included persons hospitalized due to knee or hip OA, therefore the results can be generalized to clinically severe knee or hip OA only. A relatively small number of hospitalizations resulted in wide CIs and reduced the power to observe statistically significant associations. For the follow-up of maximum 15 years, the first hospitalization due to lower limb OA was defined as the event of interest irrespective of the affected site (whether knee or hip). Thus, patients with hip OA leading to hospitalization might be under-sampled. However, it is unlikely that this would have affected the main results of our study. In addition, since our outcome was incident hospitalization due to OA, we excluded persons with hospitalization prior to the participation in the Health 2000 Study. It is unlikely that exclusion of these persons would have affected the observed associations of our study. Another limitation could be that we used in-patient hospitalization records to identify previous injuries and may therefore have underestimated the prevalence of previous injuries, in particular those of the knee. The underestimation of the prevalence of previous injuries may have resulted in an underestimation of PAFs. Yet, the PAFs in the current study were considerably higher than that reported in a recent meta-analysis.[6] Furthermore, due to a relatively small number of cases in our cohort, it was not possible to carry out side-specific analyses.

The calculation of cumulative workload was based on self-reported data regarding exposures and years being exposed. However, due to random sampling and the fact that participants at the time of data collection were not aware of the OA-related topic of the current study, a possible recall bias of collecting work history data is unlikely differential. The information on occupational history was collected at baseline, while information on employment periods between 2000 and 2015 was obtained from a register and used to estimate the years at work. We assumed that the person continued to work in the occupation reported at baseline. This assumption was based on scrutinizing the occupational mobility in the Finnish labour force during the time period of the study (see Supplementary Material, available at Rheumatology online, for details). Finally, the assessment of lifestyle factors such as LTPA and smoking was crude and might have induced misclassification bias.

In conclusion, our findings suggest that overweight/obesity, prior injury and cumulative physical workload are the most important modifiable risk factors that need to be targeted in prevention of knee OA leading to hospitalization. Long-term occupancy in jobs involving physically heavy work, manual handling of heavy loads, kneeling or squatting, and standing or walking should be the focus when looking to prevent severe OA of the knee. A substantial proportion of hospitalizations due to hip OA can be reduced by controlling excess body weight.

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