BLOG: Implications of lens volume on surgical decision-making
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At the recent American Ophthalmological Society meeting, I had the honor of presenting research on crystalline lens anatomy, including new parameters that can’t be measured with conventional biometry.
My colleagues and I used the intraoperative spectral domain OCT that is integrated into the Catalys femtosecond laser (Johnson & Johnson Vision) to measure lens thickness, lens diameter and lens volume in 293 eyes of 293 patients undergoing cataract or refractive lens surgery. I previously blogged about our overall findings.
The Catalys device measures lens thickness intraoperatively as the distance between the anterior and posterior lens capsules. Lens diameter is the distance between the intersections of the anterior and posterior lens surfaces, and lens volume is the volume calculated from the measured anterior and posterior lenticular surface curvatures extended to intersect in the periphery. We also obtained conventional biometry measurements with the IOLMaster 500 (Carl Zeiss Meditec).
Our study showed that while longer eyes tend to have lenses that are thinner and wider in diameter than shorter eyes, these correlations are weak. Lens volume varies considerably — from 119.9 mm3 to 312.4 mm3 in our sample — and most importantly, it is not at all correlated with axial length. In other words, long eyes are no more likely to have a more voluminous lens than short eyes. This challenges the conventional wisdom that large eyes have large lenses. Our sub-analysis of the 96 eyes with axial length greater than 24.5 mm showed that the assumption that these eyes would have thicker, larger diameter or more voluminous lenses was wrong the vast majority (80% to 93%) of the time (Table 1). In fact, in a significant percentage of cases (5% to 28% of the time), the opposite was true: The long eyes had smaller than average lenses.
These data have a number of implications for surgical decision-making. Even with the evolution of fourth-generation IOL formulas, including radial basis function-based formulae, short eyes still not uncommonly end up with myopic refractive surprises. We feel this is largely due to large capsular volumes and the inability to predict the anterior displacement of the final resting position in the larger capsular bag. Furthermore, it is often assumed that eyes with high axial myopia undergoing cataract surgery are at increased risk for toric IOL rotation. This may prompt surgeons to utilize a capsular tension ring, but our data would suggest that a capsular tension ring may not be indicated in many long eyes that have perfectly average capsule dimensions.
Incorporating contemporary lens geometry into the estimation of effective lens position in IOL power calculations could improve refractive predictability. It may also aid in the development of new IOLs designed to fit different-sized capsular bags in a more customized way. Such technologies, including modular designs, fluid-filled IOLs and capsule-filling materials, are already on the horizon. Their performance may well depend on better matching the IOL to the natural variation in lens size and shape, and that will require pre- or intraoperative measurement of lens anatomy.
Reference:
- Waring GO 4th, et al. Am J Ophthalmol. 2021;doi:10.1016/j.ajo.2020.12.021.
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