Published today in the New Zealand Medical Journal, the Dunedin School of Medicine study looks at district health board operations between 2006 and 2013. Since 2007, there had been no significant increase in hip and knee surgery, the study found.
People in smaller DHBs, such as West Coast and Whanganui, had better access to hip and knee replacement surgery.
Southern District Health Board, which covers Otago and Southland (220,000 people), ranked 14th of the country's 20 DHBs. Boards with lower rates of surgery include big-population boards Auckland DHB and Capital and Coast.
``In general, there were higher rates of the provision of publicly funded hip and knee total joint replacement procedures among the smallest DHBs in New Zealand, by population, compared with the largest population DHBs. The finding that rates vary between DHBs, even when [adjusted for age and ethnicity], suggest equity among DHBs is not being achieved nationally,'' the study says.
About 65% of joint replacements are publicly funded. The study did not examine privately funded joint replacement surgeries.
``These findings indicate that those living within the largest DHBs (by population) may be disadvantaged in terms of access to publicly funded hip and knee total joint replacement surgery.
``We cannot determine the reasons behind these findings.''
It could be because larger centres had more private-sector hospitals.
``It has also been suggested that higher rates of private surgery could lead to lower rates of publicly funded surgery due to surgeons not being available for public work.
``Other factors such as high acute loads and complex tertiary referrals - which are likely to be more common in larger DHBs - may also influence access to publicly funded procedures.''
Procedure numbers had increased but boards were struggling to keep up with the growing population.
Procedures were up by 6% but surgery per head of population decreased by 0.6%, the study shows.
Study co-author Associate Prof David Gwynne-Jones, who is an orthopaedic surgeon at Dunedin Hospital, told the Otago Daily Times it was ``absolutely'' correct to describe the situation as a postcode lottery.
While other boards' rankings had changed over the period of the study, Southern remained a consistently poor performer, due to a ``backlog'' of arthritis cases in the South, Prof Gwynne-Jones said.
The southern board's rank also stayed the same when adjusted for age and ethnicity.
Small boards fared better because orthopaedic services needed a minimum number of surgeons in order to run, and that ended up benefiting boards with modest populations, Prof Gwynne-Jones suggested.
And bigger boards, including Southern, had to deal with trauma and complex patients transferred from other areas.
One positive was a finding that Maori and low-income people fared comparatively well in terms of access. Researchers had hoped to examine the role of the private sector more closely but the data had been insufficient to do so, he said.
Ministry of Health prioritisation clinical leader Dr Chris McEwan, in response to questions from the ODT, said differences existed between DHBs but they had reduced over the period of the study.
``The significance of the finding of lower intervention rates in some DHBs will be in part addressed by a new national orthopaedic clinical prioritisation tool, introduced by all DHBs this year, which will put us in a better position to address this. The tool is endorsed by the New Zealand Orthopaedic Association.
``The study shows there are higher intervention rates for total joint replacement for Maori and those from high-deprivation areas - this is helping to address the traditionally lower rates for both these groups.''
Dr McEwan said surgery numbers were keeping pace with population increase.
``The rate of provision of total joint replacement has been maintained - an achievement, given the population increase amongst older groups.''
