Males in the most disadvantaged quintile appeared less likely to have a TKR compared to less disadvantaged males; however, this was only significant between SES quintiles 1 and 4. No association was observed for females. The highest rates of TKR utilisation for both sexes were observed in those aged >79 years; increased TKR utilisation was again observed for males in quintile 5 compared to quintile 1, and this pattern was also observed for females.
Although it may be suggested that the need for a TKR does not discriminate between individuals based on their SES but instead on disease severity at end-stage OA, many lifestyle risk factors associated with OA severity, such as obesity,31
are predominantly associated with SES in both sexes.32
Therefore, greater end-stage disease may be more likely in the lower SES groups. However, our findings suggested the opposite situation, whereby increased TKR utilisation may actually exist for those of upper SES. We speculate that there may be various reasons for differences in the uptake of TKR surgeries between different socioeconomic quintiles,34
especially for the elderly, which may include patterns of referral; inequitable accessibility between the public and private health system36
or perhaps that a combination of these issues exist.
While healthcare is relatively equitable for all socioeconomic groups within Australia due to the availability of both private and public health systems, studies have suggested that social determinants of TKR utilisation may still include differential access to healthcare.23
For instance, in a study of Italian and Greek migrants within Australia, similar numbers of non-Australian-born people underwent joint replacement in the public system compared to Australian-born individuals; however, differences between Australian-born and non-Australian-born patients were observed for those using the private health system.23
Given that we observed a significant difference between quintile 4 and the most disadvantaged SES quintile for males in the total population, and disparities between SES quintiles for females when limited to patients aged >79 years, we speculate public versus private healthcare usage may remain a determinant of TKR utilisation as populations age.
In addition to accessibility, differing social factors and beliefs in those from different socioeconomic quintiles may influence health-seeking behaviour, or the willingness to undergo a TKR procedure. Given that these issues are beyond the scope of this analysis, our discussion concerning the patient's decision to undertake or refuse TKR, and decision-making processes of the prescribing health professional, are purely speculative. Functional motivations have been suggested as a key driver for uptake of TKR where access to health care is equitable, with patients who self-report a higher range of movement (ROM) more likely to postpone surgery.37
It may be possible that individuals of lower SES have a greater threshold for functional limitations such as ROM, or perhaps given that individuals of lower SES are less likely to be physically active,32
they are less likely to perform movements associated with full ROM.
Interestingly, it has been shown that TKR is more likely to be performed for males compared to females;40–42
however, we observed the rate of TKR for females to exceed that of males in the BSD. Although speculation, there may be two explanations for this observation. Higher body mass index (BMI) is a significant predictor for the development of knee OA;43–45
strong inverse relationship between BMI and SES is observed in a random sample of women shown as representative of the BSD population, and of the broader Australian population.32
However, this may not fully explain the differences between sexes in our sample, as a similar relationship between BMI and SES has also been observed in males in the same geographic region.33
Given this, we suggest that as knee OA is more common in women compared to men,4
our findings suggest that SES may play a role in exacerbation or mediation of disparities between TKR and sexes.
A cross-sectional association has been demonstrated in NHANES between the demands of occupational knee bending and knee OA in persons aged 55–64 years,47
and consistent with this was the Framingham study, which showed a relationship between occupational physical labour, knee bending and later OA, especially among males.48
These data are supported by a case–control study in the UK, which showed an increased risk of knee OA in subjects whose main job entailed more than 30 min/day of squatting or kneeling, or climbing more than 10 flights of stairs per day, or where the job entailed heavy lifting.49
These data suggest a role for at least one parameter of SES in knee OA. However, occupational knee bending is an unmeasured confounder in this analysis. Furthermore, it is unclear as to why the differences in TKR between upper and lower SES quintiles in men became more significant in the older age group, for whom physically demanding occupations may or may not have been a confounder, although given that the number of TKR performed does not indicate disease prevalence, but instead symptomatic disease, the need for TKR is in part a product of activity level as well as disease severity. Nevertheless, as a greater number of TKR are performed in older individuals compared to their younger counterparts, this observation may be of little consequence unless there was a concomitant increase or decrease in the magnitude of the effect, which did not appear to be the case in this study.
One strength of this study was that the TKR were ascertained from a comprehensive national registry that has been validated against health department unit record data using a sequential multilevel matching process, and, coupled with the retrieval of unreported records, is the most complete set of data relating to hip and knee replacement in Australia. Of the TKR cases identified for 2006–2007 in the BSD, 6% could not be coded for SES, which may have influenced the association with SES. However, given that the spread of these patients was relatively equal between sexes (49% male), and represented equal proportions of the population at risk for each sex (both 0.02%), any potential disparity would be non-differential. The BSD region has been shown to be representative of the broader Australian population and thus provides an excellent location for epidemiological research. However, these observations cannot be assumed to exist in other geographic regions of Australia, or relate to the country as a whole. We were unable to examine functional determinants of perceived need for TKR, uptake and/or referral patterns for TKR, or lifestyle risk factors in our population. Furthermore, the AOANJRR database does not collect information regarding comorbidities, medications or other factors that may influence associations between SES and TKR, thus we were unable to account for these factors in our analyses. Given that sample size limited our analyses, it is imperative that this analysis be performed on a larger scale, to determine whether SES is associated with TKR for the broader Australian population.
Further investigation is warranted on a larger scale to investigate the role that SES may play in TKR utilisation, and whether this may be related to differences in referral for TKR, differences in OA prevalence or in health-seeking behaviour.