Key Features
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Importance of the corneal shape before operation.
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Value of corneal topography.
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Prevention of induced corneal astigmatism.
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Treatment of astigmatism.
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Intraoperation.
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By incisions.
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By implant choice.
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Postoperation.
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By corneal laser surgery.
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By toric lens implantation.
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By piggyback lens implantation.
Introduction
When Sir Harold Ridley implanted a human eye with a replacement lens (intraocular lens [IOL]) in 1949, he initiated a change in the role of cataract surgery. As IOL implantation technology matured over the following years, cataract surgery became more than just removing a clouding crystalline lens; it allowed for the replacement IOL to be adjusted to correct intrinsic refractive error, or ametropia. In other words, there are two strategies for surgical intervention: first removing the impediment of a cataractous lens and then simultaneously incorporating an IOL of measured dioptric power to neutralize existing ametropia.
Of course, there are many other aspects to the refractive aspects of cataract surgery. Accurate biometry is vital (refer to that aspect of cataract management in the discussions in this section). Cataract surgery in eyes that have previously undergone corneal refractive surgery require special formulas to calculate the correct IOL power after keratometric values have been changed by that surgery. Management of astigmatism is a fundamental refractive need in cataract surgery and will be considered here. With the advent of clinical aberrometers and their application in refractive surgery, cataract replacement is now taking advantage of the deeper understanding of the relationship, in a refractive sense, between the cornea and the lens. Near, intermediate, and distance vision needs have to be satisfied by lens replacement, a task fulfilled by emergent multifocal IOL technology, pseudo-accommodative IOLs, and the future fulfillment of truly accommodating IOLs. The bases for refractive correction, as an aspect of cataract surgery, are accurate biometry on the one hand and corneal topography on the other.
The focus of cataract surgery is to correct the immediate aphakia. Current techniques and implants offer the opportunity to individualize patients’ postoperative spherical and astigmatic errors and thus achieve overall patient satisfaction with regard to refraction.
Intraoperative techniques are the first to be applied (1) to ensure that astigmatism is not induced and (2) to neutralize it intraoperatively, if possible. Residual astigmatism after cataract surgery can be corrected by using four different techniques. These are (1) classic limbal relaxing incisions, which are easy to perform but have limited precision; (2) corneal laser refractive surgery (photorefractive keratotomy [PRK]); or (3) laser-assisted in situ keratomileusis (LASIK), additionally allowing for correction of spherical components; and (4) more recently, the use of a piggyback toric intraocular lens in the ciliary sulcus.
Value of Corneal Topography
Fig. 5.10.1 illustrates the importance of preoperative corneal topography in ensuring that the eye to be operated upon is fully understood: “Understand before you treat.”
Corneal topography contributes significantly to our understanding of the requirements of cataract refractive surgery. It enables adjustments for astigmatism, when required, either intraoperatively or after the operation, while also detecting serious corneal issues before the operation to avoid unexplained poor postoperative visual acuity.
Intraoperative Management of Preoperative Corneal Astigmatism to Prevent Induction of Corneal Astigmatism
Corneal Incisions
In a randomized clinical trial and noncomparative interventional case series, Tejedor and Murube investigated the best location for clear corneal incision (CCI) in phacoemulsification (“phaco”), depending on pre-existing corneal astigmatism.
In summary, for cataract CCIs:
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A superior incision is recommended for at least 1.5 D of astigmatism with a steep meridian at 90°.
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A temporal incision is recommended for astigmatism less than 0.75 D and steep meridian at 180°.
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A nasal incision is recommended for at least 0.75 D of astigmatism with a steep meridian at 180°.
Beltrame et al. compared astigmatic and topographic changes induced by different oblique cataract incisions in 168 eyes having phaco, which were randomly assigned to one of three groups: (1) 3.5 mm CCI, 60 eyes ( Figs. 5.10.1–5.10.7 for similar examples); (2) 5.5 mm sutured CCI, 54 eyes; and (3) 5.5 mm scleral tunnel, 54 eyes. Incisions lay on the 120° semi-meridian. Corneal topography was performed preoperatively and 1 week, 1 month, and 3 months postoperatively. Simulated keratometric readings were used to calculate astigmatism amplitude and surgically induced astigmatism (SIA). Postoperative topographic changes were determined by subtracting the preoperative numeric map readings from the postoperative numeric map readings. At 3 months postoperatively, the mean SIA in the right and left eyes, respectively, was 0.68 ± 1.14 D (SD) and 0.66 ± 0.52 D in the 3.5 mm CCI group, 1.74 ± 1.4 D and 1.64 ± 1.27 D in the 5.5 mm CCI group, and 0.46 ± 0.56 D and 0.10 ± 1.08 D in the scleral tunnel group. Right and left eyes showed similar SIA amplitude but different SIA meridian orientation. SIA was significantly higher in the 5.5 mm CCI group than in the other two groups 1 and 3 months postoperatively ( P < 0.01). All groups showed significant wound-related flattening and non-orthogonal steepening at two opposite radial sectors. Topographic changes were significantly higher in the 5.5 mm CCI group and significantly lower in the scleral tunnel group.
Right and left eyes showed similar SIA amplitudes but different SIA meridian orientations and topographic modifications, probably because of the different supero-temporal and supero-nasal corneal anatomic structures. The 5.5 mm CCI induced significantly higher postoperative astigmatism, SIA, and topographic changes.
Corneal Incisions
In a randomized clinical trial and noncomparative interventional case series, Tejedor and Murube investigated the best location for clear corneal incision (CCI) in phacoemulsification (“phaco”), depending on pre-existing corneal astigmatism.
In summary, for cataract CCIs:
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A superior incision is recommended for at least 1.5 D of astigmatism with a steep meridian at 90°.
- •
A temporal incision is recommended for astigmatism less than 0.75 D and steep meridian at 180°.
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A nasal incision is recommended for at least 0.75 D of astigmatism with a steep meridian at 180°.
Beltrame et al. compared astigmatic and topographic changes induced by different oblique cataract incisions in 168 eyes having phaco, which were randomly assigned to one of three groups: (1) 3.5 mm CCI, 60 eyes ( Figs. 5.10.1–5.10.7 for similar examples); (2) 5.5 mm sutured CCI, 54 eyes; and (3) 5.5 mm scleral tunnel, 54 eyes. Incisions lay on the 120° semi-meridian. Corneal topography was performed preoperatively and 1 week, 1 month, and 3 months postoperatively. Simulated keratometric readings were used to calculate astigmatism amplitude and surgically induced astigmatism (SIA). Postoperative topographic changes were determined by subtracting the preoperative numeric map readings from the postoperative numeric map readings. At 3 months postoperatively, the mean SIA in the right and left eyes, respectively, was 0.68 ± 1.14 D (SD) and 0.66 ± 0.52 D in the 3.5 mm CCI group, 1.74 ± 1.4 D and 1.64 ± 1.27 D in the 5.5 mm CCI group, and 0.46 ± 0.56 D and 0.10 ± 1.08 D in the scleral tunnel group. Right and left eyes showed similar SIA amplitude but different SIA meridian orientation. SIA was significantly higher in the 5.5 mm CCI group than in the other two groups 1 and 3 months postoperatively ( P < 0.01). All groups showed significant wound-related flattening and non-orthogonal steepening at two opposite radial sectors. Topographic changes were significantly higher in the 5.5 mm CCI group and significantly lower in the scleral tunnel group.
Right and left eyes showed similar SIA amplitudes but different SIA meridian orientations and topographic modifications, probably because of the different supero-temporal and supero-nasal corneal anatomic structures. The 5.5 mm CCI induced significantly higher postoperative astigmatism, SIA, and topographic changes.
To Treat Preoperative Corneal Astigmatism
Astigmatic Incisions
The cornea does not have axes but has only meridia, and all references to corneal incisions for the relief of astigmatic error in the cornea should be made to the “steep meridian.”
Limbal Relaxing Incisions
Kaufmann et al. compared limbal relaxing incisions (LRIs) with placement of the corneal cataract incision on the steepest keratometric meridian for the reduction of pre-existing corneal astigmatism at the time of cataract surgery. In a prospective single-center study, patients having 1.5 D or more of keratometric astigmatism were randomly assigned to two surgical techniques: on-steep meridian incisions (SMIs) consisting of a single CCI centered on the steepest corneal meridian; or LRIs consisting of two arcuate incisions straddling the steepest corneal meridian and a temporal CCI. After 6 months, the flattening effect was 0.35 D (range 0–0.96 D) and 1.10 D (range 0.25–1.79 D), respectively ( P ≡ 0.004), thus confirming that the amount of astigmatism reduction achieved at the intended meridian was significantly more favorable with use of the LRI technique and remained consistent throughout the follow-up period.
Opposite Clear Corneal Incisions
Lever and Dahan were the first surgeons to demonstrate that in cataract surgery, the CCI has a small flattening effect on corneal curvature, which can be used to reduce pre-existing astigmatism (PEA). Adding an identical, penetrating CCI opposite the first one enhances the flattening effect. The extent of flattening affecting the optical zone of the cornea is dependent on the width of the clear corneal tunnel incision and the way it is constructed. Although a general algorithm can be devised, in general, it is incumbent upon each surgeon to devise his or her own algorithm, as the location of the incision, the knife used, and the length of the tunnel are difficult to standardize. Suffice it to say, the wider the incision and the more centrally it is placed, the greater will be its effect. The local flattening of the incision only has a central effect if it is wide enough. Figs. 5.10.2–5.10.15 illustrate all the incisions, their effects, and the healing process as depicted by corneal topography. It is recommended that surgeons wishing to utilize the technique should study the effects of their own CCIs through the medium of corneal topography and thereby derive a personal nomogram.
