Capsulotomy diameter as a potential refractive planning variable

Effective lens position (ELP) remains an important source of uncertainty in intraocular lens (IOL) power calculation because it cannot be directly measured before cataract surgery. Preoperative biometric parameters can be measured accurately and incorporated into IOL power calculations, but IOL formulas must still predict where the optical plane of the implanted lens will ultimately reside. Errors in predicted postoperative lens position remain an important contributor to refractive prediction error.^1,24

The final IOL position is influenced not only by preoperative anatomy but also by the postoperative configuration of the capsular bag, capsular contractile forces, and remodeling around the implant. These forces may cause the IOL to settle slightly anterior or posterior to the position predicted by IOL formulas, potentially resulting in an unexpected residual refractive error. Current IOL power formulas do not account for surgically created anatomy, including anterior capsulotomy diameter and capsule-optic overlap. Capsulotomy diameter may therefore represent an additional source of ELP variability and, if its effect can be quantified, a potentially modifiable refractive planning variable.

Rationale for studying capsulotomy diameter

A change in capsulotomy diameter changes the amount of anterior capsule remaining over the IOL optic. This difference in capsule-optic overlap could alter the balance of forces acting on the IOL as the capsular bag contracts and remodels, thereby affecting postoperative axial IOL position.

Prior studies have suggested that capsulorhexis size may influence postoperative anterior chamber depth (ACD) and IOL position.^3,4 However, manual capsulorhexis variability and extension outside the edge of the IOL optic, in addition to other study design limitations, make it difficult to isolate a diameter effect and therefore limit the clinical utility of these results.

Femtosecond laser technology now permits highly reproducible control of capsulotomy diameter, circularity, and centration.⁵ The present study examined whether 2 precisely created capsulotomy diameters, both maintaining 360° IOL optic overlap, could produce a measurable difference in ELP.

What is the study design?

A prospective, randomized, contralateral paired-eye design was used in 40 eyes of 20 patients. One eye received a 4.6-mm femtosecond laser capsulotomy, and the fellow eye received a 5.6-mm capsulotomy. Both eyes received the same single-piece acrylic IOL design, featuring a 6.0-mm optic and a planar, 0° optic-haptic angulation.

The paired-eye design reduced variability between the 2 cohorts, whereas laser-created capsulotomies tightly controlled the principal operative variable. At 3 months, postoperative ACD was measured with swept-source optical coherence tomography and used as a proxy for ELP. The change in ACD from preoperative to postoperative measurement was calculated for each eye and compared between the 2 capsulotomy-size cohorts.

Results and relationship to lens thickness

Across the full cohort, a 59-µm difference was observed. IOLs in eyes receiving the larger capsulotomy were positioned modestly more posteriorly than those in eyes receiving the smaller capsulotomy. The overall effect was small and did not reach statistical significance.

More pronounced directional trends emerged after stratification by preoperative crystalline lens thickness. In eyes with thinner crystalline lenses (<5.0 mm), IOLs in the 5.6-mm capsulotomy cohort were positioned approximately 119 µm posterior to those in the 4.6-mm cohort. This posterior positioning would be expected to shift the refractive outcome in a hyperopic direction.

In eyes with thicker crystalline lenses (>5.0 mm), the direction reversed. IOLs in the 5.6-mm capsulotomy cohort were positioned approximately 123 µm anterior to those in the smaller-capsulotomy cohort. This anterior positioning would be expected to shift the refractive outcome in a myopic direction (Figure 1).

These observations suggest that capsulotomy diameter may contribute to ELP and that preoperative lens thickness may influence the direction of the response. For a 20 D IOL, an axial shift of approximately 120 µm could correspond to a refractive effect of approximately 0.10 to 0.15 D, with a greater effect in shorter eyes receiving higher-power IOLs.

These findings should be interpreted as directional, proof-of-principle observations. Neither the overall comparison nor the lens-thickness-stratified trends reached statistical significance, and the study was not powered to establish a clinical nomogram or demonstrate a statistically significant manifest refractive shift. Confirmation in a larger, adequately powered study is required.

Potential biomechanical explanation

The capsule-IOL complex can be considered a 3-dimensional biomechanical system in which the anterior capsule, peripheral capsule, posterior capsule, zonules, and IOL optic-haptic junction generate forces on the IOL optic in multiple directions. Their net effect may shift the optic slightly anterior or posterior relative to prediction.

Capsulotomy diameter directly changes the amount of anterior capsule tissue remaining and the degree of capsule-optic overlap. A smaller capsulotomy leaves more anterior capsule and greater overlap, whereas a larger capsulotomy leaves less. Changing the diameter alters capsule-optic overlap and the net distribution of forces acting on the IOL, potentially affecting the axial position of the optic (Figure 2).

The influence of this change may depend on the broader context of capsular bag volume and geometry. Preoperative lens thickness—an easily measured biometric parameter—may help identify the biomechanical context in which a particular directional response is more likely.

Immediate clinical implications: Consistency before customization

Most importantly, the findings of this study suggest that variability in capsulotomy diameter may not be refractively neutral, even when 360° capsule overlap of the IOL optic is maintained.

Considering this principle, these findings support a recommendation to use a consistent, repeatable anterior capsulotomy diameter, evaluate postoperative refractive outcomes, and optimize surgeon- and IOL-specific formula constants. Femtosecond laser technology, such as the ALLY from Lensar, is well suited to this objective because it permits highly reproducible capsulotomy sizing and centration.

Future clinical implications: Capsulotomy diameter as a refractive planning variable

The longer-term implication is the potential to move from consistency to patient-specific customization. Even highly accurate IOL formulas do not always provide an available IOL power that predicts the exact desired refractive target. In a patient with a refractive goal of plano, an IOL formula may recommend an 18.0 D IOL, predicting a +0.22 D outcome, whereas the next available 18.5 D IOL may predict a −0.13 D outcome. In current practice, the IOL power representing the best compromise is generally selected.

A validated surgical variable capable of producing a small, predictable adjustment around the selected IOL power could narrow this interval. An 18.0 D IOL predicting slight hyperopia might be paired with a capsulotomy diameter associated with a small anterior or myopic offset. Conversely, an 18.5 D IOL predicting slight myopia might be paired with a capsulotomy diameter associated with a small posterior or hyperopic offset. The objective would be to refine the predicted outcome around the selected IOL power and hedge the result toward the desired refractive target.

Capsulotomy diameter applications

A larger, adequately powered prospective study is needed to confirm the interaction between capsulotomy diameter and ELP and to define the role of preoperative lens thickness and other biometric variables. Prespecified subgroup analyses, evaluation across multiple IOL designs, and independent or multicenter validation would strengthen the evidence required for clinical applications.

If the relationship is validated and quantified, capsulotomy diameter could be incorporated into a new IOL formula, applied as an add-on nomogram to an existing formula, or integrated into a femtosecond laser, digital microscope overlay, or surgical planning software. Conventional formulas could continue to provide baseline IOL power recommendations, while a capsulotomy-based model could provide a predicted ELP or refractive offset around the selected power to finesse refractive outcomes.

Conclusion

Capsulotomy diameter may represent an underrecognized surgically created variable that contributes to postoperative ELP. The present findings suggest that the magnitude and direction of this effect may depend on preoperative lens thickness, with opposite directional trends observed between the 4.6-mm and 5.6-mm capsulotomy cohorts in thicker vs thinner crystalline lenses. Although these observations require confirmation, they support consistent capsulotomy diameter in current practice and provide a rationale for investigating capsulotomy diameter as a future patient-specific refractive planning variable.

>>> Interested in this topic? Watch the full interview with David Folden, MD, on capsulotomy diameter here.

References
1. Norrby S. Sources of error in intraocular lens power calculation. J Cataract Refract Surg. 2008;34(3):368-376. doi:10.1016/j.jcrs.2007.10.031
2. Li T, Stein J, Nallasamy N. AI-powered effective lens position prediction improves the accuracy of existing lens formulas. Br J Ophthalmol. 2022;106(9):1222-1226. doi:10.1136/bjophthalmol-2020-318321
3. Çekiç O, Batman C. The relationship between capsulorhexis size and anterior chamber depth relation. Ophthalmic Surg Lasers. 1999;30(3):185-190. doi:10.3928/1542-8877-19990301-06
4. Findl O, Hirnschall N, Draschl P, Wiesinger J. Effect of manual capsulorhexis size and position on intraocular lens tilt, centration, and axial position. J Cataract Refract Surg. 2017;43(7):902-908. doi:10.1016/j.jcrs.2017.04.037
5. Friedman NJ, Palanker DV, Schuele G, et al. Femtosecond laser capsulotomy. J Cataract Refract Surg. 2011;37(7):1189-1198. doi:10.1016/j.jcrs.2011.04.022