In this study, we assessed the risk for major intra and postoperative complications and the clinical outcomes of a large cohort of adult cataract patients, operated with pupil expansion by one of four methods: sphincterotomy, stretching, iris hooks or pupil expansion rings. To the best of our knowledge, this is the first study to compare all four principal techniques for mechanical pupil expansion. Despite several other differences between the groups, our data do not support an association between the method of pupil expansion and PCME. Notably, zonular dialysis was more commonly associated with the use of iris hooks and pupil expansion rings.

During the last decade, pupil expansion rings gradually became the predominant mechanical expansion method by residents and specialists alike, as seen in our data and several other recent studies [2, 3]. While small pupils have been associated with several intraoperative complications, such as iris trauma and bleeding or postoperative inflammation, it should be noted that mechanical techniques to manage a small pupil are also not complication-free [5, 7, 10,11,12,13]. Previous reports have noted several key differences between mechanical expansion techniques. For instance, Akman et al. showed iris retracting hooks and polymethyl methacrylate pupil dilator rings to be more time-consuming but more stable throughout surgery when compared to manual stretching [9]. Regarding intraoperative complications, for example, it has been suggested that manual stretching may lead to micro-tears in the iris sphincter, potentially resulting in permanent mydriasis, as well as hyphema in susceptible eyes. Therefore, sphincterotomies may be indicated in very small or fibrotic irides [14]. Wang et al. found corneal endothelial cell density (ECD) to be higher after pupil expansion using iris hooks and Malyugin ring than bimanual stretching and radial cutting [15]. In contrast, Wilczynski et al. found more ECD loss using iris hooks than the Malyugin ring [16].

A study by Wilczynski et al. found that pupil expansion with Malyugin ring manifested in better postoperative BCVA than expansion using iris hooks [16]. Similarly, BCVA in our study was best among eyes operated with iris stretching (P = 0.001), followed by pupil expansion rings, sphincterotomy and iris hooks. However, it is important to note that these results follow the same trend as the baseline BCVA in our study, which differed significantly and was also best in the iris stretching group. BCVA gain ranged between 0.42 and 0.45 logMAR units with no significant differences between the pupil expansion methods. A large study by Balal et al. recently compared iris hooks, Malyugin ring and intracameral phenylephrine as management options in small pupil cases. It found no difference in VA improvement between the different methods when independent variables were controlled for [2].

A study by Nderitu and Ursell found that the use of Malyugin ring resulted in postoperative anterior uveitis and corneal edema, compared to iris hooks or no pupil expansion [3]. Substantial pupil expansion with iris hooks has also been associated with PCME and chronic inflammation, among other postoperative complications [17]. In our study, however, all postoperative complications were observed to be similar between the groups. This was further substantiated in multivariable regression analysis, as neither PCME nor postoperative uveitis were independently associated with any specific mechanical pupil expansion method. There was a higher proportion of consultant-level surgeons in the sphincterotomy and stretching groups than in the iris hooks and expansion ring groups. Fellows used iris hooks and expansion rings more commonly, while trainees used expansion rings more than any other method.

Regarding the confounders assessed in our study, multivariable analysis showed patient age to be an independent risk factor for postoperative uveitis, while surgeon seniority was a significant protective factor against it. In addition, male sex was found to be a risk factor for zonular dialysis. Although it was previously suggested that some of the pupil expansion methods (namely iris hooks and pupil expansion rings) might also assist the stabilization of the capsular bag, our results do not emphasize protection from zonular dialysis [14]. In fact, zonular dialysis in our study tended to be higher among eyes operated with iris hooks and pupil expansion rings. Of note, iris hooks were also associated with the highest incidence of posterior capsular rupture, which was insignificant in multivariable analysis. In addition, the iris hooks and expansion ring groups had a lower proportion of consultant-level surgeons. Still, there were also significant differences in patient age and diabetes as systemic comorbidity between the groups, which could have accounted for the difference.

This study includes a few limitations that must be considered. The study had a retrospective, non-randomized design and several differences were found between the groups’ baseline characteristics, which may confound the results. Moreover, selection bias should be considered, as both preoperative risk stratification and surgeon preference, which may affect the selection of the expansion method and lead to risk for complications independent from the expansion method selected, were not taken into account in the study. To mitigate this concern, multivariable analysis controlling for age, sex, diabetes mellitus and surgeon seniority was performed to assess the independent effect of specific pupil expansion methods on cataract surgery complications. Finally, the study is based on data from a single center with a homogenous population, which may lower the results’ external validity.

In conclusion, our analysis of 1266 eyes operated with different mechanical pupil expansion methods found no difference in the risk for PCME or PCR between the groups, and suggests that all techniques may be equally considered. Optimal pupil expansion method selection may be based on local clinical practices, surgeon preference, additional operative time required and available resources.