Volume, complications inversely related for cataract surgeons

November 1, 2007

A retrospective cohort study revealed a consistent volume-outcomes relationship for adverse events in cataract surgery. Surgeons performing more than 1,000 procedures a year had the lowest complication rate among four volume classifications. The overall adverse event rate, however, was low as well: less than 1 in 200.

Key Points

Toronto-Researchers conducting a population-based analysis of cataract surgery in Ontario found a volume-outcomes relationship in which surgeons performing the highest number of procedures per year had the lowest complication rates. Overall, the selected adverse event rate for surgeons performing 50 or more cataract procedures was less than 1 in 200; surgeons who performed more than 1,000 procedures a year had a complication rate of 0.1%.

"The more you do, the better you get, and it continues," said Chaim Bell, MD, PhD, an assistant professor of medicine and health policy, management and evaluation at the University of Toronto, and St. Michael's Hospital, and adjunct scientist at the Institute for Clinical Evaluative Sciences, Toronto. "We expected a plateau rate, but we didn't find one, or any evidence of diminishing returns. There is something different about people who do a lot more surgeries, and it affects their complication rate."

Dr. Bell and colleagues found a statistically significant linear relationship between volume and outcome (p <0.001) in each of 3 study years in their analysis.

The researchers were aware that relationships between volume and outcome had been found for certain surgeries and treatments in specialties such as cardiology, with a doctor's performance of more procedures associated with better outcomes.

"The interesting thing that we found for most of those types of surgeries in the literature was more along the lines that doing 10 surgeries was far better than doing two surgeries a year, or doing 30 surgeries is better than doing five surgeries a year. The difference between a high- and a low-volume surgeon might be 10, 20, maybe 50 procedures," Dr. Bell continued.

Volume higher

Cataract surgery volumes, however, tend to be much higher. Whereas studies of surgeries in other specialties entailed single- or double-digit surgery volumes, cataract surgeons often have annual volumes in triple or quadruple digits.

With that large volume of cataract surgeries and an extensive, diverse sample of patients, surgeons, and hospitals available for analysis, the researchers had an excellent opportunity to study the association between surgeon volume and the risk of surrogate markers for important postoperative events within 2 weeks of surgery.

"We were wondering if there was a plateau effect in cataract surgery," Dr. Bell said. "Cataract surgery had not been well-described, particularly in this era of new surgical techniques."

He and his colleagues began their research by identifying patients aged at least 20 years who had undergone cataract surgery using the Ontario Health Insurance Plan physician claims database for the period between April 1, 2001, and March 31, 2004. Ophthalmologists were identified using their physician specialty codes and unique billing identifiers. Each surgeon's annual cataract surgery volume was calculated based on the number of claims submitted for each fiscal year.

Only those who performed 50 or more cataract surgeries in 1 year were included in the analysis. Those physicians were linked with their respective patients to form the cohort. The baseline comparison group consisted of physicians who each performed 50 to 250 procedures a year; additional groups were physicians who performed 251 to 500, 501 to 1,000, or more than 1,000 procedures per year. Physicians with exceptionally low numbers of procedures annually were excluded to lessen miscalculation bias.

The number of cataract surgeries performed in Ontario during the study period ranged from 89,556 in the 2001–2002 fiscal year to 99,333 in 2003–2004. The reviewers looked at more than 284,000 procedures during the study period after excluding surgeons who performed fewer than the established minimum number per year. They reviewed annual volumes and also performed an analysis over the entire 3-year period by pooling all the data.

Between 231 and 243 surgeons each performed more than 50 cataract procedures per year at 70 hospitals or eye-surgery centers. The median surgeon volume was 353 in the first year, 391 in the second, and 392 in the third.