- Top of page
- MATERIALS AND METHODS
- AUTHOR CONTRIBUTIONS
- Supporting Information
Lifetime risk of total knee replacement (TKR) refers to the probability of having knee replacement surgery over an individual's lifetime. Although data on the incidence of TKR may be instructive for researchers, the estimation of “lifetime risk” produces more tangible information for health professionals and policymakers to assess utilization and unmet need in the community and make decisions about the allocation of resources. Examining changes in lifetime risk over time can inform population health strategies and potentially facilitate the uptake of primary and secondary prevention strategies within populations. Where studies from the US have estimated the lifetime risk of symptomatic hip and knee osteoarthritis (OA) ([1, 2]), 2 recent studies also have investigated the lifetime risk of TKR. Using a primary care database, Culliford et al () estimated the lifetime risk of TKR in the UK to range from 8–11% for women ages 50–70 years and from 6–8% for men in this age group. Over a 15-year period from 1991–2006, lifetime risk of TKR increased substantially for women (+6.9%) and men (+4.4%) (), although potential explanations for this finding were not provided. Weinstein et al () used national health survey data from 2005–2008 and estimated that the cumulative lifetime risk of a 25-year-old to have a TKR in the US was 7.0% (95% confidence interval [95% CI] 6.1–7.8%) for men and 9.5% (95% CI 8.5–10.5%) for women. Because these studies represent the only available data on lifetime risk of TKR, whether risk of knee replacement surgery is similar in other developed countries and how risk may change over time remain unknown.
A range of factors could affect access to TKR over time and potentially contribute to changes in lifetime risk of knee replacement surgery. These include insurance coverage or health care setting (), socioeconomic status (SES) ([6, 7]), and geographic location ([8, 9]). The aims of this retrospective, population-based longitudinal cohort study were to quantify the lifetime risk of having a TKR in Victoria, Australia over time and describe temporal changes in the incidence of TKR according to health care setting (public versus private hospitals), SES, and geographic location.
Box 1. Significance & Innovations
- Lifetime risk is a measure of disease risk with potential utility for the public, health professionals, and policymakers. For this study, lifetime risk of total knee replacement (TKR) was used to estimate the probability of having knee replacement surgery over an individual's lifetime.
- Lifetime risk of TKR increased over the 9-year study period to 10.4% for men and 11.9% for women.
- Sustained disparities in access to TKR relating to health care setting and socioeconomic status were observed, although the incidence of people receiving knee replacement surgery in more remote areas increased over time.
- Top of page
- MATERIALS AND METHODS
- AUTHOR CONTRIBUTIONS
- Supporting Information
Estimating lifetime risk of TKR is a relatively new approach, with only 2 other recent studies using this technique (). Incorporating comprehensive data from both public and private hospital settings in Australia, our study has shown a clear increase in mortality-adjusted lifetime risk of TKR surgery over a 9-year period, most notably for women. We have also provided age- and sex-specific estimates. These data, considered in combination with estimates of the OA burden over time, can be helpful in policy settings to inform population health strategies and motivate uptake of primary and secondary prevention strategies. For example, increases in the lifetime risk of TKR without concomitant increases in the burden of severe OA can signal changes in patient preferences, surgical decision making, or the effects of health care policies. Gaining insight into these population-wide fluctuations, which are independent of disease burden, can be useful for budget priority setting and assessing the impact of policy changes on health care utilization. These data may also support advocacy activities for policy and funding changes. International comparisons of TKR lifetime risk could provide insight into the extent of unmet need within a country or region.
Our lifetime risk estimates for TKR were significantly higher than estimates reported for the UK () and the US (). This is unlikely to relate to differences in risk factors for OA or OA prevalence between countries, but rather to differences in health care provider characteristics (for example, variation in thresholds at which surgery is offered [[22, 23]]) and international variation in health systems (such as the availability of TKR surgery and the proportion of the population covered by health insurance). Of the 3 countries, Australia and the UK have the most in common with regard to the structure of their health care systems. Both Australia and the UK offer universal health care, where interventions such as TKR can be accessed through taxpayer-funded systems (public hospitals in Australia and the NHS in the UK). Both countries also have parallel private health systems that can be accessed by those who hold private health insurance or by those who can afford to pay for the costs associated with treatment. However, the proportion of people with private health insurance in Australia is considerably higher than in the UK (40–45% versus 10%) (). In Australia, the private health system offers 2 main advantages for patients considering TKR: choice of surgeon and avoidance of lengthy waiting times for consultation and surgery. In the US, publicly funded health care is only provided for specific groups (for example, people ages ≥65 years, those with very low income, and people who are severely disabled). Health care in the US is provided by a range of private, public, and nonprofit organizations and treatment can be associated with significant expenses (). There was also a sharper temporal increase in lifetime risk for men in Australia when compared to the UK. It is unknown if this is related to sex differences in the uptake of TKR between countries (). Although limited data are currently available, similar analyses from other countries will be useful for exploring international patterns of lifetime TKR risk and differences in unmet need.
The observed increase in lifetime risk of TKR over the study period could be partly attributed to the aging population, with more people ages >80 years receiving TKRs, increased rates of sporting injuries (), and rising rates of obesity (all risk factors for knee OA incidence [[27-30]]), and an increasing prevalence of severe joint disease ([28-30]). As we identified increased utilization of TKR in the private hospital setting, other potential drivers of growth could include financial incentives to having private health insurance, timelier access to TKR surgery in private hospital settings, and greater awareness of effective surgical interventions as these become increasingly common. An increase in knee arthroscopies for OA within the Australian private health system over this time period also has been demonstrated (). According to recent data from the AOANJRR, 69.4% of knee replacements in Australia are now performed in the private health system (). Since significant health insurance reforms were introduced in Australia more than a decade ago, these initiatives may have contributed to the observed rise in incidence of TKR in private hospitals. In 1999–2000, initiatives introduced by the Australian government to boost the number of people with private health insurance (including substantial rebates and incremental loadings on insurance premiums according to age) saw the proportion of Australians with private health insurance increase from 38% in 1998 to 51% in 2001 (). In our data, rates of TKR increased in the private system over the time period, whereas the rates of TKR in the public system remained constant. The shifting of patients into the private system may have improved access to the public system for individuals who previously would not have received care. However, in 1999/2000, 12.4% of patients in public hospitals waited more than 365 days for a TKR (), while 14.9% waited 365 days or more in 2008/2009 (). This suggests there still may be considerable unmet need that may be related to the increasing growth in the aging population over this time period. Improved access and utilization of care have been demonstrated in other countries after health care systems encouraged the uptake of private health insurance; however, inequities in care may become more pronounced ().
Lower SES was associated with a reduced incidence of TKR during the study period. In Australia, people with low SES generally have reduced access to the private hospital system. Our finding is consistent with studies from the UK and Canada that have reported reduced utilization of TKR by lower SES groups ([7, 35, 36]). In contrast to these studies, a cross-sectional Australian study reported higher rates of TKR for Australians living in the most disadvantaged areas compared with those living in the least disadvantaged areas (). However, this study assessed TKR in 1 year (2006–2007) and used the socioeconomic index for measuring disadvantage (i.e., low income and unskilled occupations). We used the Index of Economic Resources, a measure of income, because we considered this measure would more closely reflect ability to afford health insurance. However, we also found that the lower SES group in our study had higher rates of comorbidities (such as chronic heart failure and diabetes mellitus), which may contraindicate surgery. Other studies have shown that comorbidities are a major barrier to accessing joint replacement for the poorest individuals ().
On a positive note, we found an increasing incidence of TKR over time for people residing in outer regional and remote areas, suggesting improved access to TKR surgery for these individuals. However, it should be noted that the incidence of TKR in regional areas was already greater than in metropolitan areas at the beginning of the study period. This builds on the findings from an earlier Australian cross-sectional study that demonstrated higher rates of knee replacements in regional areas of Australia compared to major cities over a 2-year period from 2005–2007 (). The increased incidence of TKRs in outer regional and remote areas could reflect a greater need for TKR surgery due to an older population in these regions, supported by median age data for regional Victoria compared with metropolitan Melbourne (the capital of Victoria) (). People living in outer regional and remote areas also may be at greater risk for knee OA due to occupational risk factors such as manual labor. The larger increase in incidence for these individuals could also relate to greater previously unmet need and the increased provision of orthopedic services in regional areas over the past decade.
A major strength of our study design was the inclusion of public and private hospital data to improve the accuracy of our estimates and the external validity of the study. We acknowledge that this study utilized administrative data collected primarily for hospital reimbursement purposes, and although all procedures were primary TKRs, we do not have information on whether patients had previously received a primary TKR for the contralateral knee. This could overestimate the age at first TKR. Although the majority of the cohort had a diagnosis of OA (96%), we also utilized data from people who had other diagnoses (such as RA), and this may conflate the risk estimates due to the different course of each disease. However, our sensitivity analysis (limited to people with OA) produced similar findings for lifetime risk. We relied on area-based measures of socioeconomic status and rurality, which are known to have limitations (). These indices were based on patients' residential locations rather than where they received treatment. There were 15.6% of cases with missing SLA data due to missing address information within the hospital admission records, where we imputed economic resource scores and rurality codes. We conducted a sensitivity analysis where we excluded cases with missing data, and this did not alter the trends we identified. We also acknowledge that changes in population factors (for example, age, injury rates, and SES distribution) may have contributed to temporal changes in lifetime risk and TKR incidence, but population data on these factors were not available for our analyses. Comorbidity data were obtained from the hospital episode records for the TKR admission and any admission within the year prior to the TKR. However, comorbidities have been shown to be underreported within hospital administrative data (). Only 2.2% of the cohort was found to be obese, which is well below the population estimates of obesity for Australia (20.5%) (); therefore, these data were not included in our analyses. Since comorbidities were not included in the analysis of lifetime risk, this is unlikely to change our estimates. Finally, although we did not have access to national hospital data, as Australia's second most populous state, it is reasonable to expect the Victorian data to be representative of other large Australian states. However, there may have been state-based policy initiatives to improve access to TKR (for example, in regional areas) during the study period of which we are not aware.
Increases in lifetime risk of TKR were evident over a 9-year period, although our understanding of contributing factors remains limited. While growth in risk factors for OA and greater disease severity may have partly driven this increase, government incentives to encourage the uptake of private health insurance also may have improved access to care for some people. Although the rising incidence of TKR for those living in more remote areas is encouraging, the observed disparities relating to health care setting and SES are concerning and warrant further investigation.