The possibility of taking precise measurements of total corneal astigmatism enable more patients to be spectacle-free following cataract surgery.
Patients judge the success of modern cataract and refractive surgery by the degree of spectacle independence they achieve. Distance vision after cataract surgery is taken for granted, so patients are usually disappointed when they need glasses for driving and watching TV.
The need for glasses after cataract surgery is often due to a residual refractive error, including spherical and astigmatic errors. However, accurate biometry with modern optical biometers can negate spherical errors. Astigmatism, mainly corneal in origin, is a common refractive error in the cataract age group.
Prior to surgery, corneal astigmatism of more than 0.75 D and of at least 1 D is found in 23.9% and 41.2% of patients, respectively.1 Astigmatism can be addressed at the time of cataract surgery by toric IOL implantation or astigmatic keratotomies.
Effect of astigmatism on IOLs
Patients are also starting to request adequate intermediate and near vision without glasses after cataract surgery. Several presbyopia-correcting premium IOLs are available to address this issue. However, astigmatism greatly influences the function of these lenses and may result in patient dissatisfaction.2,3
Astigmatism reduces quality of vision, whether the patient receives a monofocal or a multifocal lens. In one study comparing the effect of astigmatism on the visual function of both types of IOLs—with astigmatism of 1.50 D or more—the corrected distance visual acuity and the distance-corrected intermediate visual acuity at 0.5 m were significantly worse in the multifocal IOL group than in the monofocal IOL group.4
The appropriate cut-off postoperative astigmatism to achieve optimal visual function is considered to be 0.5 D or less. Nevertheless, subjective perception of halos and glare after multifocal IOL implantation is correlated with the amount of postoperative residual astigmatism.4
If residual astigmatism is between 0.25 and 0.75 D—less than 1 D—the patient is usually satisfied and the quality of vision is not disturbed. However, if the astigmatism is beyond 0.75 D, the quality of vision is significantly decreased.
Residual astigmatism following cataract surgery
One of the leading causes of residual astigmatism after cataract surgery is the inaccurate preoperative measurement of corneal astigmatism. Most Placido topographers and keratometers measure corneal astigmatism based on anterior corneal radius measurements and assume that the posterior corneal surface has a fixed ratio of 82.2% to the anterior corneal surface.5,6
However, the principal planes of the anterior and posterior surfaces differ from one another, even in normal eyes. Furthermore, the posterior corneal surface might contribute to total corneal astigmatism, or total keratometry (TK), more than we thought.
The contribution of posterior corneal astigmatism to TK was explored by Koch et al. in 2012.7 They showed continuous drift of the steep meridian on the anterior corneal surface from vertical to horizontal with increasing age, whereas the steep meridian on the posterior corneal surface remains substantially vertical and stable with age.
The magnitude of posterior corneal astigmatism showed no or weak correlation to the anterior corneal astigmatism if the axis on the anterior surface was oblique or horizontal. They concluded that anterior and posterior corneal astigmatisms are not necessarily correlated, and that posterior corneal astigmatism might alter the magnitude and axis of the TK astigmatism.
For accurate preoperative measurement of corneal astigmatism, it is essential to take posterior corneal astigmatism into account. Modern toric IOL calculators empirically predict posterior corneal astigmatism for more precise postoperative results. However, precision is achieved by measuring, not predicting.
Measuring posterior corneal astigmatism
Tomography devices based on Scheimpflug images can measure posterior corneal astigmatism with moderate accuracy. Given that measurements must be converted to equivalent standard keratometry readings before using a biometry formula, it seems to be impractical for everyday refractive/cataract surgery.5,6 I primarily rely on swept-source optical coherence tomography (OCT) biometry (IOLMaster 700; Carl Zeiss Meditec AG) because it allows for accurate measurement of corneal astigmatism on both the anterior and posterior surfaces.
This technology measures the curvature of the anterior corneal surface followed by corneal thickness: by fitting this measurement to the anterior corneal curvature, the posterior corneal curvature can be accurately calculated. Calibration is then applied to these posterior corneal surface curvature values to ensure that TK is compatible with the existing IOL power calculation formula and constants.8
Clinical case study
A 57-year-old man with astigmatism requested a presbyopia-correcting IOL. His delta K was +0.80 D, beyond the 0.75 D cut-off threshold noted in the previous paragraph (see Figure 1). This amount of astigmatism needs to be corrected prior to implanting a multifocal premium IOL to obtain spectacle independence for distance and for near vision; based on this delta K, I should implant a multifocal toric IOL.
However, if I consider the posterior corneal measurement, I can see that just to the right of this number, the delta TK is 0.56 D. In this case, the true amount of corneal astigmatism was less than expected when we measured the anterior corneal surface only. The delta K of 0.80 is not the true number; the true delta TK is 0.56 D.
Thus, I can implant a presbyopia-correcting IOL knowing that it will not interfere with the quality of vision of this patient’s eye. There is no harm in leaving this amount of astigmatism because it is very low and will not interfere or affect visual quality with the presbyopia-correcting lens.
Now that we have devices that calculate total corneal astigmatism, cataract and refractive surgeons can make more precise measurements, which translates to more patients who have spectacle independence and better quality of vision because we are targeting emmetropia closely. All in all, astigmatism should be measured, not predicted.
Prof. Ahmed Assaf, MD, PhD, FRCSEd
Prof. Assaf is based at the Faculty of Medicine, Ain Shams University, Cairo, Egypt. He has no financial interests in the subject matter.