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Abstract
Objectives To investigate the temporal trends and ethnic and socioeconomic disparities in cruciate ligament (CL) injury incidence and associated costs in New Zealand over a 14-year period.
Methods All CL injury claims lodged between 2007 and 2020 were extracted from the Accident Compensation Corporation (a nationwide no-fault injury compensation scheme) claims dataset. Age-adjusted and sex-adjusted incidence rates, total injury costs and costs per claim were calculated for each year for total population and subgroups.
Results The total number of CL injury claims increased from 6972 in 2007 to 8304 in 2019, then decreased to 7068 in 2020 (likely due to widespread COVID-19 restrictions; analysis is therefore restricted to 2007–2019 hereafter). The (age-adjusted and sex-adjusted) incidence rate remained largely unchanged and was 173 cases per 100 000 people in 2019. There was a 127% increase in total injury claims costs and a 90% increase in costs per claim. Pacific people had the highest incidence rate and costs per 100 000 people, while Asians had the lowest; European, Māori and ‘other’ ethnicities had similar incidence rates and total costs. Incidence rates and total costs increased with income and decreased with neighbourhood deprivation. Costs per claim differed little by ethnicity, but increased with income level.
Conclusion The number and costs of CL injury claims in New Zealand are increasing. There are ethnic and socioeconomic disparities in CL incidence rates and costs, which are important to address when designing CL injury prevention programmes and programmes aimed at improving equity of access to medical care.
- epidemiology
- socioeconomic status
- health disparities
- costs
Data availability statement
No data are available. The data used in this study are not publicly available due to the strict security provisions of the Integrated Data Infrastructure (IDI). Access to the IDI may be made available by Statistics New Zealand to approved researchers.
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WHAT IS ALREADY KNOWN ON THIS TOPIC
Cruciate ligament injuries are common injuries with substantial treatment costs, work loss and quality of life impacts.
Little is known on disparities in incidence rates or costs by ethnic group or socioeconomic status.
WHAT THIS STUDY ADDS
The number of cruciate ligament injury claims in New Zealand increased by 19% between 2007 and 2019, and the associated total costs more than doubled.
There are ethnic and socioeconomic disparities in the age-adjusted and sex-adjusted incidence rates, treatment costs and work loss.
HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY
Implementation of nationwide cruciate ligament injury prevention programmes, with vulnerable subgroups in consideration, may be warranted.
Introduction
Cruciate ligament (CL) injuries are serious conditions that lead to substantial treatment costs, reduction in health-related quality of life and participation in physical activity, and may be associated with early onset knee osteoarthritis.1–5 Occurring predominantly in young sports participants, CL injuries have the potential for significant and long-lasting impacts on morbidity and productivity.6 7 CL injury prevention strategies can, therefore, have large benefits for individuals, the health system and society. To establish a case for prevention strategies, it is necessary to identify the scope of the problem and vulnerable subgroups to guide the development, implementation and evaluation of these strategies.8
In addition to offering value for money, new investment in prevention strategies should also be equitable; ideally, reducing health (and other) inequities. For example, there are significant ethnic and socioeconomic health inequalities in New Zealand (NZ), with worse health outcomes for Māori and for lower socioeconomic groups.9 10 In addition, knee injuries are the most common site of ligament or tendon injury in Māori,11 and there is some evidence that Māori may disproportionately be exposed to CL injury12; however, there is not sufficient evidence to date.
While there is a number of studies on the incidence of CL injuries at a population level (eg, in the USA,6 NZ,7 Norway13), including separately for different genders and age groups, no previous studies have reported disparities in CL incidence rates by ethnicity or socioeconomic status. This paper describes CL injury incidence and associated costs in one country (NZ), both for total population and stratified by ethnic group and socioeconomic status over a 14-year period (2007–2020). NZ’s universal accident and injury insurance scheme and comprehensive linked national data offer a unique opportunity to study national injury rates, as well as treatment costs, with high precision and level of detail. One previous study has presented the temporal trends of tendon and ligament knee injuries in NZ between 2010 and 2016.11 This is the first study to report on trends for CL injury specifically.
Methods
Data sources, study cohort and construction of variables
All data were obtained from the Integrated Data Infrastructure (IDI), a linked research database maintained by Statistics New Zealand containing individual-level data from government administrative datasets, Statistics New Zealand surveys and non-governmental organisations.14 Within the IDI, data on CL injuries and treatment costs were collected from the Accident Compensation Corporation (ACC) claims dataset, which covers all injury claims made to ACC since 1994. The ACC is the sole provider of personal injury insurance in NZ, which covers most injury costs (including treatment and rehabilitation, loss of income and help with daily activities) on a no-fault basis. As such, there is a strong incentive to make a claim and the dataset should include the vast majority of CL injuries sustained in NZ. For our analysis, we extracted data on all CL injuries (which include both anterior and posterior CL sprains, partial tears and complete tears, regardless of surgical treatment) sustained from 2007 to 2020, by both NZ and foreign citizens residing in NZ during the year of the injury. We excluded individuals who were not residing in NZ during the year of the injury (eg, short-term visitors). All injuries sustained by the same individual during the period of study were included in the analysis.
Population data were taken from the estimated NZ resident population dataset created by Statistics New Zealand and containing unique linkable identifiers of all individuals residing in NZ each year. Ethnicity information was sourced from the ethnicity indicator constructed by Statistics New Zealand. It classifies ethnicity into five groups (European (including NZ European), Māori, Pacific, Asian and other (all ethnicities not included in the specific groups above)), and allows individuals to indicate more than one ethnic affiliation. We included all reported ethnicities in the analysis (eg, an individual reporting both European and Māori ethnicity would be included in both the European and Māori subgroups15). Data on income during the 12 months prior to injury were taken from the Inland Revenue dataset. We classified individuals into five income subgroups: no income, <NZD20 000, NZD20 001–40 000, NZD40 001–60 000, >NZD60 000. The New Zealand Index of Deprivation was derived from individuals’ location of residence. It classifies small geographic areas (corresponding to neighbourhoods of approximately 100–200 people) into deciles based on a range of socioeconomic factors (where decile 10 represents areas with highest deprivation).16
Statistical analysis
Injury claim rates and costs per 100 000 people (weighted by age and sex to reflect the 2007 NZ total population) were calculated for each year of the study for the total population and for each ethnicity and socioeconomic subgroup. The costs per claim for each year and subgroup were also calculated. The costs are presented separately for medical costs and entitlement costs (which include income compensation and social and vocational rehabilitation costs). All costs are shown in 2020 NZ dollars excluding GST (1 NZD≈0.51 GBP). R statistical package V.3.6.0 was used to conduct the analysis.17
Ethics and data availability
Access to the anonymised data used in this study was provided by Statistics New Zealand under the security and confidentiality provisions of the Statistics Act 1975. Careful consideration has been given to the privacy, security and confidentiality issues associated with using linked administrative data in the IDI; see the full disclaimer at the end of this article for further details.
The data used in this study are not publicly available due to the strict security provisions of the IDI. Access to the IDI may be made available by Statistics New Zealand to approved researchers.
Patient and public involvement
Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Results
Total population
There were 8304 CL injury claims in NZ in 2019, an increase of 19% since 2007 (see table 1). The number of injury claims dropped in 2020, most likely due to widespread restrictions in response to the COVID-19 pandemic; all results hereafter are reported for the 2007–2019 period only. The (adjusted) incidence rate of CL injury claims remained unchanged during the period. Total injury claims cost more than doubled, and amounted to NZD94.3 million in 2019. The increase in costs was mostly driven by a rise in entitlement costs (of which the bulk is income compensation payments18): while medical costs increased by 63%, entitlement costs increased by 140.1%. Similarly, cost per claim amounted to NZD11 353 in 2019, increasing by 36.8% in medical costs and 102.3% in entitlement costs since 2007.
Annual number of claims and costs
Ethnic and socioeconomic subgroups
Pacific people had the highest (adjusted) CL injury claims incidence rate (214 per 100 000 people in 2019), while Asians had the lowest rate (108); European, Māori and ‘other’ ethnicities had similar incidence rates of 187, 182 and 181, respectively (figure 1). These inequalities are reflected in total costs; moreover, the disparities in the (adjusted) costs by ethnicity increased over time (figure 1).
Adjusted incidence rates of cruciate ligament injury claims by ethnicity, income group and social deprivation level. The incidence rates and costs are reported per 100 000 people and adjusted by age and gender. The costs are presented in 2020 NZ dollars.
There was a clear gradient in injury claims incidence rate and costs by income (figure 1). The (adjusted) incidence rate in 2019 was 142, 205, 200, 254 and 274 injury claims per 100 000 people for individuals with no income, NZD1–20 000, NZD20 001–40 000, NZD40 001–60 000 and >NZD60 000, respectively. There were similar, and increasing, disparities in (adjusted) costs (figure 1). Similarly, injury claim rates and costs were higher for areas with lower socioeconomic deprivation level, although the disparities were not as large as between-income groups (figure 1).
Costs per claim differed little by ethnicity. Medical costs varied between around NZD1400 for Europeans and Māori and under NZD1600 for Asians and ‘other’ ethnicities in 2019; however, medical costs for Pacific people increased at a higher rate during the study period than for other ethnicities (figure 2). Entitlement costs were highest for Māori (NZD11 485 per claim in 2019) and lowest for Asians (NZD9326).
Costs per claim by ethnicity, income group and social deprivation level. The costs are presented in 2020 NZ dollars.
The differences in costs per claim were more pronounced by income groups. Both medical costs and entitlement costs were lowest for individuals with no income (medical costs NZD1149 per claim in 2019, entitlement costs NZD5305) and highest for individuals with income of NZD40 001–60 000 (NZD1681 and NZD15 953). This pattern was also reflected in the results by deprivation level for medical costs, but not for entitlement costs.
Full results by ethnic and socioeconomic subgroups are presented in the online supplemental appendix 1.
Supplemental material
Discussion
Total population
The total number of injury claims in NZ increased by 19% between 2007 and 2019. A similar increase of 16% has been reported for tendon and ligament knee injuries in NZ over the period 2011–2016.11 However, the (age-adjusted and sex-adjusted) incidence rate remained unchanged, indicating that the rise in number of claims is mainly driven by population growth. Total medical costs associated with CL injury claims increased by 63% during the study period. One potential explanation for the greater increase in medical costs compared with the increase in the number of injury claims is an increase in the rate of CL reconstructions. Sutherland et al reported that the incidence rate of primary anterior cruciate ligament (ACL) reconstructions in NZ increased by 11% between 2009 and 2016.19 Similarly, a US study found that while the (age-adjusted and sex-adjusted) incidence rate of ACL tears decreased for males and remained unchanged for females during 1990–2010, the rate of ACL reconstructions increased significantly over the same period,7 suggesting that the increase in surgeries was driven by changes in medical treatment rather than in the incidence of CL injuries. Other studies have reported similar increases in rates of ACL reconstructions across several countries (eg, the UK,20 Australia,21 the USA,22 Italy,23 Canada24). Information on surgical treatment was not available in the data used in this study. The increase in medical costs of CL injuries may also partly reflect medical cost inflation in excess of the broader inflation measure used for price adjustment in this study.
Ethnic and socioeconomic subgroups
Our results demonstrate disparities in injury claims rates by ethnicity, with Asians having the lowest and Pacific people having the highest incidence rate. One of the possible explanations for these disparities is the difference in sports participation rates by ethnic groups. A nationwide survey on sports participation in NZ reports that Asians have the lowest levels of participation in play, active recreation and sport in 2019.25 It also shows that, among the eight sports where the most sports-related ACL injuries occur,7 Pacific people have the highest participation rate in seven, while Asians have the lowest participation rate in four.26 Another possible factor contributing to the ethnic disparities is difference in ethnic predisposition to CL injury. There is some evidence pointing to such differences between African-American and white individuals27 28; however, no such studies exist for Māori, Pacific people or Asians. We suggest future research on ethnic disparities by cause of injury (eg, sports, occupational, road accident) may provide further insight into the causes of these observed disparities.
We also found a socioeconomic gradient in CL incidence, with individuals with lower income or from higher deprivation areas having lower rates of CL injury claims. Again, this could be due to differences in sports participation, as individuals from higher deprivation areas in NZ were shown to participate in fewer sports and spend less time being physically active.25 Differences in access to medical care to receive a CL injury diagnosis (and hence lodge an ACC claim) may also contribute to the discrepancy in recorded injury incidence rates.
We found no evidence of ethnic disparities in medical costs per claim. However, there is a clear gradient in medical costs per claim by income, with individuals with no income having the lowest cost. These disparities may indicate inequities of access to care by different socioeconomic groups, for example, due to differing ability to meet indirect costs of treatment not covered by ACC. Previous studies, including from countries with publicly funded healthcare systems, have shown that individuals with lower socioeconomic status have lower likelihood of undergoing surgery after an ACL injury.29–31 This is also borne out by the gradient in medical costs per claim by neighbourhood socioeconomic deprivation, which may reflect inequality in access to local health services. (There is no strong gradient in entitlement costs per claim by neighbourhood deprivation, suggesting this is not merely a proxy for differences in individual income.)
Finally, our results show that entitlement costs per claim increase with individuals’ income, as is expected as ACC weekly compensation is proportional to income. We also found that Māori have the highest entitlement costs per claim. While Māori have lower average incomes than the non-Māori population, this appears to be more than offset by their higher rates of employment in types of work that are likely to be more significantly affected by a CL injury (eg, 19% of Māori work as labourers, while only 8% of Europeans and 9% of Asians do so).32
Strengths and limitations
The major strength of this study is the use of the comprehensive ACC dataset, which allowed us to explore trends in CL injury rates on a national level and identify vulnerable subgroups. Most published studies on CL injuries focus on ACL reconstructions13 20–24 (thus underestimating the true burden of CL injury) and/or are limited to short study duration,7 and none have explored differences by ethnic or socioeconomic groups. It is, however, important to be aware of potential limitations of the ACC data, which could be caused, for example, by individuals writing incorrect information on data collection sheets, not making a claim (although there is a high incentive to do so as ACC provides insurance on a ‘no-fault’ basis), or not going to a health provider for his/her injury at all (noting there is no time limit on when an individual can make a claim to ACC). Another limitation of the data is that it does not distinguish between anterior and posterior CL injuries, which would allow for greater insight into the nature of CL injuries. We leave this for future research. The income bands used for analysis were held constant over the 13-year period of analysis; due to inflation and increases in incomes, these may not be equally appropriate throughout the period, however they should still capture the patterns of differing CL injury burden across varying income groups within each year.
Policy implications
The rising medical and economic costs of CL injuries over the last 14 years in NZ highlight the need for implementing CL injury prevention strategies. The results reported in this paper are necessary for constructing a business case for such strategies and investigating the potential cost-effectiveness in future modelling studies.
Injury prevention and treatment strategies should also aim to reduce inequities in health outcomes. Our results suggest that non-targeted strategies may have the greatest reach in the highest-income communities (with higher incidence rates and medical costs). To avoid increasing existing health and socioeconomic inequities, potential CL injury prevention strategies would be best targeted at equitably reaching ethnic groups with highest CL incidence rate (eg, Pacific communities in NZ). Strategies should also focus on improving access to medical services for the most economically derived and lowest income groups.
Data availability statement
No data are available. The data used in this study are not publicly available due to the strict security provisions of the Integrated Data Infrastructure (IDI). Access to the IDI may be made available by Statistics New Zealand to approved researchers.
Ethics statements
Patient consent for publication
Ethics approval
This study was approved by the University of Otago Human Research Ethics Committee (Health) (HD20/074). The study uses routinely collected administrative data, made available by Statistics New Zealand.
References
Footnotes
Contributors YP: concept, methodology, analysis, writing and editing paper, funding acquisition. RW: concept, methodology, analysis, editing paper. JHA: concept, methodology, editing paper, supervision, guarantor.
Funding The funding for this study was provided by the HRC Health Delivery Research grant (20/1164). YP was supported by a postdoctoral fellowship grant from Lottery Health Research (2021-152330).
Disclaimer These results are not official statistics. They have been created for research purposes from the Integrated Data Infrastructure (IDI), which is carefully managed by Statistics New Zealand. For more information about the IDI, please visit https://www.stats.govt.nz/integrated-data/. The results are based in part on tax data supplied by Inland Revenue to Statistics New Zealand under the Tax Administration Act 1994 for statistical purposes. Any discussion of data limitations or weaknesses is in the context of using the IDI for statistical purposes, and is not related to the data’s ability to support Inland Revenue’s core operational requirements.
Competing interests None declared.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.
Provenance and peer review Not commissioned; externally peer reviewed.
Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.