Both the health and the economic burden of LBP, NKP, OST and RHE in Belgium were estimated. The health burden was summarized in terms of prevalent cases and prevalence rates for the years 2013–2018 by sex and age group. Mortality estimates were considered only for RHE, following the below-described GBD methodology, from 2013 to 2018. Disability-Adjusted Life Years (DALY) were used to summarize the impact of the concerned diseases both in terms of mortality and morbidity. The economic burden included the direct and indirect health costs attributable to the selected MSK disorders for the years from 2013 to 2017 expressed in mean yearly attributable cost and total yearly cost.
Prevalence and mortality
Prevalence estimates for LBP, NKP and OST were derived from the Belgian health interview surveys (BHIS) conducted in 2013 and 2018 among a representative sample of the Belgian population (N = 10,829 participants in 2013 and N = 11,611 participants in 2018). Respondents were 15 years or older and were recruited following a multistage sampling design. The sampling and recruitment procedure are described in Demarest et al. [10]. Interviews were performed using a face to face paper and pencil interview supplemented with a self-administered questionnaire covering more sensitive topics [10]. The data included self-reported information on health status and related health behaviour and determinants. Socio-demographic information such as age, gender, household educational level (i.e. the highest educational level within the household), income level (based on the calculated quintiles of the household income) and behavioural risk factors were included from the BHIS database. To identify persons suffering from chronic diseases, respondents were asked the following; “Did you suffer from ’disease of interest’ in the past 12 months?”. In addition, LBP and NKP cases were assigned only if respondents also reported a very bad to fair subjective health. This was done to avoid overestimation since LBP and NKP are not necessarily chronic conditions, differently from OST and RHE. Prevalence was computed by gender, 5-years age group and region (i.e. Brussels, Flanders and Wallonia). Considering that the BHIS does not provide information on cases for the < 15 years old category, we assumed the prevalence to be zero. Prevalence estimates for the years between 2013 and 2018 were calculated by means of linear interpolation based on the estimates derived from BHIS-2013 and BHIS-2018.
Lack of specificity (many false positives) in the RHE data was suspected looking at the high prevalence rates from the BHIS. Therefore, a more accurate data source was used for the prevalence estimates of RHE. They were derived from the Intego network, which represents a registration network for general practices in Flanders (Belgium). It includes anonymised diagnoses, laboratory results and drug prescriptions from 55 general practitioners (GPs) [11], [12]. RHE cases were identified through International Classification of Primary Care (ICPC) code L88. Data was delivered by 10-year age groups and sex from year 2013 to 2018. In order to obtain prevalence proportions, numbers of cases were divided by the yearly contact group (i.e. the number of different patients seen by the 55 GPs), available from the Intego database. Since these estimates are available for only one of the three Belgian regions, national estimates were inferred in the following way: (1) compute the proportion of cases in the other regions compared to Flanders using RHE estimates within BHIS, i.e. prevalence of RHE in Brussels/Wallonia divided by prevalence of RHE in Flanders for 2013 and 2018; (2) multiply the proportions derived in (1) by the prevalence of RHE in Flanders observed within Intego for obtaining the new prevalence of RHE cases in Brussels and Wallonia. The prevalence estimates in Brussels and Wallonia between 2013 and 2018 were computed by means of linear interpolation as done for the other MSK disorders.
Number of deaths between 2013 and 2018 were retrieved from Statbel, the Belgian statistical office, for people with RHE (ICD-10 codes: M05, M06, M08) as underlying cause of death. This included also the deaths attributed to RHE during the redistribution process of ill-defined causes of death in Belgian BoD study. Following the GBD methodology, LBP, NKP and OST were assumed to not generate any deaths [13].
Disability-Adjusted Life Years
DALYs are calculated by summing the Years Lived with Disability (YLD), representing the morbidity component and the Years of Life Lost (YLL), the mortality component. In order to compute the YLDs, the number of prevalent cases (prevalence times population) were multiplied by the disability weights (DW) retrieved from the GBD-2019, using the disease models of the GBD study [13]. Population information was extracted by 5-year age groups and sex from Statbel for the years from 2013 to 2018. For RHE, the YLLs were calculated subtracting the individual age at death from the GBD-2019 standard life expectancy [14].
In order to account for the overlap among these disorders, we used a multiplicative approach to compute comorbidity-adjusted YLD. The prevalence estimates of reporting one, two, or three of this conditions were extrapolated from BHIS for LBP, NKP and OST. Secondly DW were adjusted using the following formula 1 − ∏i(1 − DWi), with DWi being the disease specific DW [15], [16]. Considering that LBP, NKP and OST were extrapolated from the same data source, these were considered as interdependent based on the observed correlations. For RHE, we assumed that it was independent of the other health conditions.
Costs
This analysis included the estimation of 1) direct costs, measured by reimbursed expenditures for medical services and medications associated with treatment and care; and 2) indirect costs, measured by cost of work absenteeism.
Data of the respondents to the BHIS-2013 was linked with national health insurance data compiled by the Intermutualistic Agency (IMA) 2013–2017. Health insurance is compulsory in Belgium covering more than 99% of the population. The IMA database comprised reimbursed total health care costs, for every payment modality (directly paid by the health insurance, patients out-of-pocket and supplements). These expenditures included 1) ambulatory care (over-the-counter pharmaceuticals excluded), 2) hospital care and 3) reimbursed medicines purchased through pharmacies. Available information on hospital care only included variable costs. However, in Belgium, the national health insurance also pays a fixed amount to the hospitals per admitted patient, depending on the type of hospital and treatment. Precise information on these costs was not directly available in the dataset. In order to estimate the fixed part of the total hospital care cost, the hospitalizations per patient per year were multiplied with the average annual 100% per diem cost publicly available by type of hospitalization (per diem costs available through: https://www.riziv.fgov.be/nl/themas/kost-terugbetaling/door-ziekenfonds/verzorging-ziekenhuizen/Paginas/verpleegdagprijzen-ziekenhuizen.aspx).
Costs of absenteeism were estimated based on the question available in the BHIS-2013: “Have you been absent from work during the past 12 months due to health problems? In doing so, take into account any conditions, injuries or other health problems you may have had and which resulted in an absence from work”. Followed by the question: “How many days in total have you been absent from work for the past 12 months due to health problems? If you are unable to indicate this number of days correctly, please give an estimate”. However, since respondents might have included weekend days in their answer, the maximum number of working days per year, i.e., 226 days, was considered. If the answer given exceeded this number, the maximum number of working days was used. The productivity loss was valued using the human capital approach, assuming that each day of absence corresponds to the average gross daily wage [17]. The number of days absent from work was then multiplied by the national yearly average labour cost per hour multiplied by an average of 7.6 hours (38 hours per week) per working day [18], [19]. The labour cost includes compensation for employees as well taxes without subsidies. The individual cost of absenteeism over the 2013–2017 period was obtained by multiplying the annual individual cost for the days absent from work based on the days reported in the BHIS-2013 and the national yearly average labour cost of the corresponding years. Only the working population was included in the calculation (respondents who stated having a paid job at the time of the survey).
Figure 1 shows the sample selection process, including data merging, used for the computation of cost estimates between 2013 at 2017. People who were not continuously insured throughout the observed time period (2013–2017) were excluded. People who died within the observational period were excluded for their direct and indirect costs in the years after their death. The final sample resulted in 9,814 respondents.
Statistical analyses
Prevalence and mortality estimates were presented as number of cases and as percentages of cases over the total population per age group, sex and year. In order to have an estimate for the overall Belgian population, the survey design (weights, strata and cluster) of the BHIS-2013 and BHIS-2018 was duly taken into account. The detailed computation of the weights is described elsewhere [10], [20]. Comorbidity adjusted DALYs were summed up by age group and sex. Confidence intervals (95% CI) were calculated for prevalence estimates based on the standard errors resulting from the survey analysis. Using the sampling distribution of the DW and of the proportion of people in each health states derived from GBD 2019 literature [21], uncertainty was propagated to DALYs using 1,000 Monte Carlo simulations.
Univariate and multivariable regressions with negative binomial distribution and log link were used to explore the extent to which average yearly cost was associated with MSK disorders. The first was used to estimate an unadjusted incremental cost with only the disease as independent variable. Multivariable regressions additionally included socio-demographic characteristics and lifestyle factors: age-groups, gender, household educational level, mean household income level, high body mass index (BMI), smoking, alcohol misuse, unhealthy eating behaviour (i.e. drinking sugared soft drinks daily) and physical inactivity. A “double-selection” approach was used for the selection of the variables to be included in the final multivariable model [22], i.e., the variables were identified by the means of two regression models, finding those that predict the dependent variable (average costs) and those that predict the independent variables (each MSK disorder separately). The final linear regression included all MSK disorders and all the covariates significant in the models above-described after a backwards elimination at a 10% level of acceptance.
Incremental costs were estimated at the individual level using the method of recycled predictions, also known as G-computation or direct standardization, that allows to estimate the marginal effect of each MSK disorder on health care costs [23], [24]. The latter was computed with a multistep approach: (1) the coefficients of the regression model (described above) were used to predict health care costs for each respondent using the observed disease distribution as reported in BHIS-2013; (2) the same coefficients were then used to predict health care cost assuming that none of the respondents had the concerned disease, keeping all other characteristics as observed (including the disease status of the other disorders); (3) the difference of the above-described predictions represents the individual incremental cost of the concerned disorder; (4) finally, we calculated the attributable cost of LBP and NKP as the survey weighted average of these individual incremental costs. Considering that LBP and NKP are also symptoms for OST and RHE and that the cost attributable to OST and RHE might be partially mediated through LBP and NKP, the above calculated costs represented only the costs directly attributable to the considered disorder, and ignoring other disorders. In order to calculate the total costs attributable to OST and RHE, inspired by causal mediation analyses [25], we first used the disease models of LBP and NKP (LBP/NKP as dependent variable, OST, RHE and covariates as independent variables) to stochastically impute the disease status of LBP and NKP in the absence of OST and RHE. We next applied the above-described four steps, where in step 2 we set LBP and NKP to these imputed disease statuses. Because the latter were stochastically imputed, this Monte Carlo simulation process was repeated 1,000 times and results were averaged across simulations.
95%CIs were computed with a bootstrap approach with 1,000 replicates. Total disease costs were computed by multiplying the previously calculated survey weighted average cost for the disease by the prevalence of the disease. Total costs for all MSK disorders were estimated by multiplying the mean per capita incremental cost by the prevalence of at least one MSK disorder in the general Belgian population for the direct costs and in the working Belgian population according to BHIS-2018.
All analyses were performed in R version 1.4.1717 [26]; the survey weighted analyses were performed using the survey package [27]