Introduction

Ophthalmic surgery typically restores or preserves vision, is associated with dramatic improvements in quality of life, and is highly cost effective compared with other surgical procedures [1, 2]. The World Health Organization recognizes ophthalmic surgery, in particular cataract surgery, as an important metric for global health development [3]. Cataract surgical intervention rate (SIR) is used as a proxy measure of access to services and healthcare delivery [4].

While the SIR for cataract surgery is commonly reported, there are few published reports for the relative and absolute SIRs of other ophthalmic procedures [5,6,7]. The limited data suggests that SIRs for many ophthalmic procedures are increasing worldwide. Improvements in technology and surgical techniques have reduced procedure duration, decreased patient recovery time and are associated with improved patient outcomes [8,9,10]. These advances have, in many cases, increased the indications for surgery and reduced threshold for surgical intervention. For example, improvements in technology used in vitreoretinal surgery and the development of minimally invasive glaucoma surgery (MIGS) devices have expanded the indications and role of surgery in managing less severe retinal disease and glaucoma [9, 11,12,13,14].

The current study aimed to calculate nationwide population-level ophthalmic SIRs for all major categories of ophthalmic surgery over a 10 year period in New Zealand. Secondary aims were to assess changes in SIRs and patient demographics over time.

Methods

Study design

A retrospective population-based study of all ophthalmic surgery in New Zealand. No single dataset summarises all ophthalmic surgery in New Zealand therefore multiple data sources were used to calculate and cross validate total surgical numbers over the study period. The current study was conducted according to the tenets of the Declaration of Helsinki. The Institutional Review Board (IRB)/Ethics Committee ruled that approval was not required for this retrospective study [15].

Inclusion and exclusion criteria

Surgical procedures were identified and grouped into categories based on International Statistical Classification of Diseases and Related Health Problems (ICD) codes. All ICD-10 coded ophthalmic surgical procedures completed in New Zealand from 2009 to 2018 fiscal years were included for analysis. Only the primary procedure code was included in the analysis for operations that were labelled with a secondary procedure code in addition to a primary code. Exclusion criteria included procedure codes not linked to specific primary procedures, procedures with an overall nationwide total of less than 1000 over the duration of the study period, and intravitreal injections.

Government-funded surgical procedures

Data for all government-funded ophthalmic procedures from 2009 to 2018 fiscal years were identified and extracted from the New Zealand Ministry of Health records. Procedure counts were tallied for all categories meeting the inclusion criteria. Demographic and regional data for each procedure were included for analysis. All data were de-identified prior to analysis.

Private-funded surgical procedures

Private-funded ophthalmic surgical procedures performed in New Zealand were quantified using several data sources to calculate total numbers as there is no single aggregate data source that tracks these metrics. Private-funded ophthalmic procedures were either private (main insurer) funded or private (other) funded.

Private (main insurer) funded surgical procedures

The private (main insurer) group consisted of de-identified data from the largest single private healthcare insurer in New Zealand, covering 62% of New Zealand’s privately insured population (personal communication). This private insurer provided data on the number of ophthalmic surgical procedures performed during the study period.

Private (other) funded surgical procedures

The private (other) funded group consisted of procedures that were either self-funded or funded by healthcare insurers other than that listed in the private (main insurer) group. The absolute and relative proportion of these procedures were calculated by cross referencing several data sources. The absolute total nationwide number of cataract, vitrectomy and keratoplasty procedures was calculated using industry records for intraocular lens implant sales (2016–2018), vitrectomy pack sales (2016–2018) and New Zealand National Eye Bank keratoplasty recipients numbers (2009–2018) (personal communications), respectively. Special lenses, such as piggyback or secondary intraocular lens, purchased were excluded.

The total number of private-funded surgeries (summation of private (main insurer) and private (other)) for cataract, vitrectomy and keratoplasty could then be calculated by subtracting the government-funded procedures from the total numbers in each category. Absolute private (other) funded totals for cataract, vitrectomy and keratoplasty procedures were then calculated by subtracting private (main insurer) funded totals from the combined private (main insurer) funded and private (other) funded totals. The relative frequency of procedures in the private (main insurer) funded group was used to estimate the absolute number of remaining private (other) funded procedures other than cataract, vitrectomy and keratoplasty.

Outcome measures and calculation of intervention rates

The primary outcome measure was the SIR for each surgical category that met the inclusion criteria. SIRs were calculated only for procedures/surgical categories with available government-funded and private (main insurer) data and were estimated only for categories other than cataract, vitrectomies or keratoplasties. Individual SIRs were defined as the number of procedures in each surgical category/100,000/year. The population value used was the mean of the New Zealand population reported in June of each year from 2009 to 2018 by Stats NZ, New Zealand’s official data agency. The June population estimates were chosen as these were closest to the middle of the year and likely best reflected the median population in each year [16]. The timing of this census was midway through the study period. Patient demographic variables for each government-funded surgical category were analysed as secondary outcome measures.

Statistical analysis

All statistical analyses were performed using R Version 3.4.2 (R Foundation for Statistical Computing, Vienna, Austria). Pearson correlation was used to determine statistically significant changes in procedure summed totals over time, with a p value <0.05 considered statistically significant. A choropleth map of regional cataract SIRs was created based on the 10 year average government-funded cataract SIR.

Results

Analysis of the government-funded procedure codes excluded 145,223 codes not linked to specific primary procedures (e.g., “Ophthalmological examination”), procedures with an overall nationwide total of less than 1000 over the duration of the study period, and intravitreal injections. Intravitreal injections were excluded due to inconsistent and regional variations in coding that prevented accurate reconciliation. The remaining 210,830 government-funded ophthalmic procedures (Supplementary Table 1) met the inclusion criteria and comprised 10 categories of surgical procedures: cataract, vitrectomy, strabismus, conjunctival lesion excision, glaucoma, dacryocystorhinostomy, keratoplasty, non-keratoplasty cornea (cornea), lid repair, and eye removal (Fig. 1). The ‘conjunctival lesion’ category included procedure codes for pterygia and conjunctival lesion excisions. The ‘glaucoma’ category included procedure codes for trabeculectomy, fistulation, insertion or removal of aqueous shunts, trabeculoplasty, destruction of the ciliary body, and other procedures for glaucoma. The ‘cornea’ category included procedure codes for non-keratoplasty procedures for corneal collagen crosslinking, refractive, corneal graft augmentation, and epithelial debridement procedures. The ‘lid’ category included procedure codes for entropion and ectropion repairs. The ‘eye removal’ category included procedure codes for enucleation, evisceration, and exenteration procedures.

Fig. 1: Summary of all included ophthalmic procedures performed per surgical category in New Zealand from 2009 to 2018 according to source of funding.
figure 1

*Surgical intervention rate (SIR) calculated only for operations with both government-funded and private-funded (main insurer) data.

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There were 86,082 private (main insurer) procedures in seven surgical categories: cataract, vitrectomy, strabismus, pterygium only (conjunctival lesion excision data not available), glaucoma, dacryocystorhinostomy, and keratoplasty. Data were not provided for the remaining private (main insurer) surgical categories: non-keratoplasty cornea, lid corrections, and eye removal surgery.

An estimated 113,187 private (other) procedures were performed. The total number of intraocular lenses and vitrectomy packs distributed across New Zealand from 2016 to 2018 were 163,281 and 10,544, respectively. The total number of patients receiving corneal graft tissue from 2009 to 2018 was 2,889.

The combined total number of included government-funded and private-funded (summation of private (main insurer) and private (other)) surgical procedures performed from 2009 to 2018 was 410,099. A majority of the procedures were government-funded (n = 210,830 (51%)), followed by private (other) (n = 113,187 (28%)) then private (main insurer) (n = 86,082 (21%)). The annual number of surgical procedures performed in all three categories increased over the 10-year study period (p < 0.01) (Fig. 2).

Fig. 2: Frequency of ophthalmic procedures over time by funding source.
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Data includes the total number of all major ophthalmic surgical procedures completed in New Zealand from 2009 to 2018.

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Patient gender and ethnicity were compared to the average New Zealand population during the 10 year study period Out of 210,830 government-funded surgical procedures, female surgical patients (n = 113,704, 54%) were overrepresented compared to the New Zealand national population (n = 2,312,410, 51%). Male surgical patients (n = 97,126, 46%) were underrepresented compared to the New Zealand national population (n = 2,234,380, 49%).

Age distribution of each included government-funded surgery varied (Fig. 3). Strabismus surgeries were predominantly performed in paediatric populations while cataract, eye removal, and lid surgeries were mostly performed in elderly populations. Between 2009 and 2018 there was a 43% increase in all surgical categories for patients aged 65 years or older. Percentage increases in ophthalmic procedures for the 65 and older age group for cataract, vitrectomy and glaucoma surgeries were greater than the national population increase over the study period. In contrast, keratoplasty procedures in patients 65 years of age and over decreased relative to population growth over the same period (Fig. 4).

Fig. 3: Age distribution for ophthalmic government-funded procedure categories in New Zealand.
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Relative age distribution for each surgical procedure.

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Fig. 4
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Increase of selected government-funded procedure categories in the 65 years and older age group compared to the New Zealand 65 years and older national population [16].

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The annual number of all included government-funded, private-funded private (main insurer) and private (other) procedures were counted for each surgical category (Supplementary Table 2). Cataract surgery made up 78% (n = 318,564) of all included procedures. The number of procedures in each surgical category increased over the study period for all categories except conjunctival lesion, dacryocystorhinostomy and eye removal procedures. The proportion increase was statistically significant for vitrectomy (p < 0.001), cataract (p = 0.001), keratoplasty (p = 0.002) and lid (p = 0.032) surgical categories.

The national SIR was calculated for the seven surgical categories with comprehensive government and private-funded data (Fig. 5A). A choropleth map of the 10-year average cataract SIR showed geographic variation in each New Zealand geographic region (Fig. 5B).

Fig. 5
figure 5

Left panel: Nationwide surgical intervention rates. Standardised rates per 100,000 population per year using the mean New Zealand population reported in June of each year from 2009 to 2018 [16]. Right panel: Choropleth map of 10 year average standardised cataract surgical intervention rate. Lighter shading shows higher regional surgical intervention rates.

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Discussion

The current study is the first to evaluate nationwide SIRs for all major categories of ophthalmic surgery in New Zealand in the government- and private-funded sectors. Best possible approximates were made for the private (other) group as no central repository exists with this data. Substantial numbers of ophthalmic surgical procedures were completed in 10 categories with the cataract category comprising 78% of all included procedures. The mean government-funded nationwide ophthalmic SIR was 465/100,000/year and the total SIR for all ophthalmic procedures, regardless of funding, was 906/100,000/year or approximately 1 ophthalmic procedure for every 110 people per year.

Government-funded surgeries accounted for 51% of all surgeries during the study period. The number of procedures performed increased by 25.3% from 2009 to 2018, with the greatest increase in government-funded procedures (39.5%) followed by private-funded procedures (12.3%). The healthcare system in New Zealand, not limited to ophthalmic services, is approximately 80% government-funded [17]. In 2018, New Zealand spent 9% of its gross domestic product on healthcare, an amount less than the United States (16.7%), the United Kingdom (9.9%), and Australia (9.2%) [18]. The demand for ophthalmic surgeries will likely continue to increase in New Zealand and globally. Understanding service utilisation distribution will be critical for both workforce and health system planning.

Patients aged 65 years or older underwent more government-funded ophthalmic surgeries than any other age group, accounting for 71% of all surgical interventions. This ageing population demographic grew nationally by 34% during the study period and most government-funded surgical procedures followed the same trend [19]. The number of patients aged 65 years or older undergoing cataract, vitrectomy, and glaucoma surgery increased by 41%, 91%, and 83% respectively and in all cases exceeded total population growth over the study period. Effective planning is required to meet the growing utilisation of ophthalmic surgeries by an ageing population.

The New Zealand cataract SIR was less than other OECD countries and nearly half of all surgeries were completed in the private sector. The current study of aggregated government- and private-funded data calculated an average 10-year cataract SIR of 703/100,000/year. In 2018 the New Zealand cataract SIR was 764/100,000/year. Cataract SIRs in 2018 for other OECD countries included 1111/100,000/year in Australia, 1123.3/100,000/year in Sweden, and 731.1/100,000 population in the United Kingdom (government-funded only) [5]. The relatively high proportion of private-funded cataract surgery may indicate poor access to government-funded surgery. Poor access to government-funded cataract surgery may be associated with inequities in access for those who cannot access private-funded surgery due to financial barriers or lack of private medical insurance. Cataract SIRs continue to increase globally as populations grow and age and with improvement surgical techniques [20, 21].

The current study reports a high vitrectomy SIR (67/100,000 population/year). The vitrectomy category had the second highest average SIR in this study and the total number of surgeries increased by 50% from 2009 to 2018. As far as the authors are aware, this study is the first to report a nationwide SIR for vitrectomy procedures, regardless of indication. Compared to cataract surgery, vitrectomy procedures require greater resources and specialist surgical training [22, 23]. Surgical advances and evolution of small gauge vitrectomy surgery have reduced surgical risks and threshold for surgery [24]. Indications for vitrectomy surgery include retinal detachments, macular disease and diabetic retinopathy [25,26,27,28]. Increasing vitrectomy SIR may be in part due to an ageing population, with increased risk of diabetic retinopathy and retinal detachment. The increasing prevalence of myopia, a risk factor for retinal detachment may also be a contributing factor [29]. There is the potential for further increasing demand for vitrectomy procedures in younger, economically active age groups as projections suggest 10.2% of the global population, or 578 million people, will be diabetic by 2030 [30, 31].

Glaucoma SIRs remained stable in New Zealand over the study period. The average glaucoma SIR across all three sources of funding was 33/100,000/year; if the private (other) funded category is excluded then the glaucoma SIR for government- and private (main insurer) funded was 13/100,000/year and 8/100,000/year, respectively. Glaucoma SIRs declined worldwide in the 1990s with the introduction of new pharmacologic treatment options, laser treatments and new and evolving surgical techniques [32,33,34]. An audit of Australian glaucoma national practices reported an increase in the number of glaucoma laser and surgeries performed from 2003 to 2017 [33]. These alternative treatment modalities, improved diagnostic and monitoring imaging technology, and the rise of nurse and optometrist-led glaucoma monitoring may explain the stable rate of glaucoma procedures during the study period [32, 35].

The average keratoplasty SIR was 6/100,000/year with a bimodal distribution of patients aged 15–39 years (n = 812, 44%) and 60–79 years (n = 416, 22%). Bimodal patterns of keratoplasty similarly were reported in the United Kingdom and Australia [36, 37]. Indications for corneal transplantation in New Zealand include keratoconus (34.5%), repeat transplantation (23.1%), Fuchs endothelial corneal dystrophy (17%), bullous keratopathy (10.8%) and keratitis (4.4%) [38,39,40]. Keratoconus has a high prevalence in New Zealand and is the leading cause for corneal transplantation [39, 41, 42]. The stable keratoplasty SIR may be due to increases in corneal collagen crosslinking to halt progression of keratoconus and/or limited supply of corneal tissue for transplantation.

The SIR for strabismus surgery fluctuated during the study period. The average strabismus SIR across all sources of funding was 20/100,000/year; following the exclusion of private (other) funded category the strabismus SIR for government- and private (main insurer) funded was 15/100,000/year and 2/100,000/year, respectively. Most strabismus procedures were performed in younger age patients, consistent with other studies [43, 44]. Potential explanations for the variable strabismus SIR frequency include increased conservative management of strabismus, such as treatment of associated refractive error, and improved surgical techniques possibly decreasing the need for re-operation [45, 46].

Some limitations exist in the current study. The study lacked accurate data for intravitreal anti-VEGF injections and refractive surgeries; and insufficient details on the number of repeat or multiple procedures for a single patient. There are some limitations inherent to big data studies, including relying on the accuracy of coding data [47, 48]. Finally, no central repository exists aggregating private ophthalmic surgical data in New Zealand. Best possible estimates were made to impute the number of surgeries performed in the private (other) group using actual patient data from multiple sources. Intraocular lenses were used as a proxy of the number of cataract surgeries performed. Vitrectomy packs were used to estimate the number of vitrectomy surgeries performed as vitrectomies cannot be performed without them. Data on the number of patients undergoing keratoplasty surgeries were obtained from the New Zealand National Eye Bank, the single organisation responsible for the acquisition, storage and distribution of donated corneal, scleral and amniotic membrane tissue throughout New Zealand [49, 50]. The organisation has stringent, standardised protocols and maintains a comprehensive database of donors and transplant recipients around New Zealand.

The current study is the first to provide an overview of the absolute and relative supply of ophthalmic surgical procedures in a developed nation, aggregating data from both the government and private-funded sectors. The demand for ophthalmic surgery is steadily increasing, most notably for cataract and vitrectomy surgeries. The increase in SIRs cannot be explained by population growth alone. It is essential that the supply of ophthalmologists and funding for ophthalmic surgery is scaled appropriately to meet this increasing demand.

Summary

What was known before

  • Surgical intervention rates (SIRs) provide a proxy measure of disease burden, service availability, and evolution of ophthalmic surgery.

  • While the SIR for cataract surgery is commonly reported, there are few published reports for the relative and absolute SIRs of other ophthalmic procedures.

What this study adds

  • This study provides a comprehensive nationwide overview of all major ophthalmic procedures performed in the government and private-funded sectors in New Zealand from 2009 to 2018.

  • Analysis of 410,099 ophthalmic procedures in New Zealand demonstrates steady increases in the SIRs over the past decade. These findings can help create an atlas of variation for ophthalmic services in New Zealand.