Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

Our study did not reveal changes in average astigmatic values after discontinuing contact lens wear. However, these values were found to be reduced in 18.2% of cases and increased in 18.2%, and there were axis modifications of more than 20º in 27.3% of cases.

In our study, we have found greater cylinder changes when soft contact lenses are discontinued (reduction in 55.5% and increase in 18.2 % of cases) than the ones found in RGP lens wearers (reduction in 18.2% and increase in 18.2% of cases). Wilson and Klyce3 report that astigmatic modifications are greater in RGP lens wearers in terms of a cylinder

increase when lens wear is discontinued, contrary to what happens with soft contact lenses.

There was an improvement of one to four Snellen lines of visual acuity in 38.5% of cases, coinciding with improved regularity and radial symmetry in the topographic image (SAI and SRI decreased in 92% and 61.5% of cases, respectively, reaching a final value ≤ 0.5 in all cases). Improvements of visual acuity, and of symmetry and regularity indices, has been described by other authors.2,3,8,10,16,18

Follow-up on central corneal thickness with ultrasound pachymetry showed an increase in corneal thickness after RGP lenses were discontinued in 90% of cases, with an average increase of 15.5 microns (range from 0 to 25), which was statiscally significant (p<0.05). No differences were observed concerning pachymetric modifications in soft and RGP contact lens wearers.

To summarize, RGP contact lenses may induce corneal warpage with topographic flattening (with keratoconus-like images when decentered) and decreased myopia, as well as central corneal thinning. If all factors are not taken into consideration before refractive surgery, the result will be undercorrection and the selection of a smaller optic zone.

CONCLUSION

It is essential to perform a critical evaluation of the corneal topography of all candidates prior to refractive surgery. The tendency to proceed immediately to surgery if the topography appears normal should be avoided because the initial topography may depart significantly from the one obtained before contact lens fitting.

Contact lens wear should be discontinued before refractive surgery for a period of 1 to 2 weeks for soft lenses, and 1 month for RGP lenses.

Section III: Clinical Applications of Topography

However, in the event that any topographic signs of corneal warpage are observed, they must be followed until they normalize and/or stabilize. Therefore, topography will dictate the timing for refractive surgery.

The presence of topographic signs of corneal warpage must lead us to suspect the possibility of corneal thinning as a result of contact lens wear, requiring close topographic as well as pachymetric follow-up in these patients.

Careful operative evaluation and management are likely to improve the quality and predictability of corneal surgery of individual surgeons. Perhaps more importantly, the overall efficacy of procedures such as LASIK might be enhanced once unpredictable variables of contact lens-induced warpage are controlled.

REFERENCES

1.Hartstein J: Corneal warping due to wearing of corneal contact lenses. Am J Ophthalmol. 1965;60:1103-4.

2.Wilson SE, Lin DTC, Klyce SD, et al: Rigid contact lens decentration: A risk factor for corneal warpage. CLAO J. 1990;16:177-82.

3.Wilson SE, Lin DTC, Klyce SD, et al: Topographic changes in contact lens-induced corneal warpage. Ophthalmology. 1990;97:734-44.

4.Rengstorff RH: Corneal curvature and astigmatic changes subsequent to contact lens wear. J Am Optom Assoc. 1965;36:996-1000.

5.Levenson DS: Changes in corneal curvature with long-term PMMA contact lens wear. CLAO J. 1983;9:121-5.

6.Levenson DS, Berry CV: Findings on followup of corneal warpage patients. CLAO J. 1983;9:126-9.

7.Koetting RA, Castellano CF, Keating MJ: PMMA lenses worn for twenty years. J Am Optom Assoc. 1986;57:459-61.

8.Wilson SE, Klyce SD: Advances in the analysis of corneal topography. Surv Ophthalmol. 1991;35:26977.

Introduction

The number of patients undergoing cataract surgery has increased significantly over the past years. Cataract surgery is usually performed with the aim of improving vision. Over the years, technological advances have enabled a greater and greater degree of improvement to be achieved. This is at least partly related to technical improvements in phacoemulsification resulting in greater intra-operative safety. Originally, surgeons concentrated on the removal of the opacified lens to enable light to enter the posterior portion of the globe. With the introduction of microsurgery and intraocular lenses patients could hope for a return of good best-corrected visual acuity. Following the most recent developments in cataract surgery, with phacoemulsification attention has turned to ensuring that light is brought to an optimum focus on the retina, in order to provide patients with good uncorrected vision. However, the lens contributes only one-third of the total focusing power of the eye. The remaining two-thirds arises from the convex shape of the anterior corneal surface. Whereas most patients who receive state-of-the-art sutureless cataract surgery have excellent unaided vision postoperatively, patients with significant preexisting astigmatism cannot achieve the same results without correction of their cylinder. Studies of astigmatism in cataract cases report a prevalence of 18% to 23% with 1.5 D or more of keratometric cylinder and 9% to 12% with 2 D or more of keratometric

Figure. 1. Astigmatism Effect in the Postoperative Visual Result

Moderate to significant resultant astigmatism is an important cause of limited visual acuity in the postoperative period after cataract surgery. The goal in modern cataract surgery is to even up the axes, modify the differing focal lines in the eye (A), and bring the focus to the retina.

cylinder.1 Many of these cases would benefit from reduction or elimination of their cylinder (Figure 1).

This has two important implications for cataract surgery. First, knowledge of the power contributed by the cornea is essential to accurately calculate the power of intraocular lens to be inserted. Second, very small changes in corneal shape can have a dramatic effect on the precision with which light rays are brought to a focus on the retina.

Therefore, incisions made in the cornea or anterior sclera during cataract extraction have the potential to change the refraction of the eye. Assessments of corneal topography can be used to minimize the adverse results of these incisions and even utilize their effects to advantage. It has long been known than the surgical removal of cataracts can be associated with marked changes in corneal curvature that can limit visual rehabilitation postoperatively2.

Over recent decades there have been numerous changes that have resulted in great improvements in efficacy. Intraocular lenses (IOLs) have been developed and refined, as have suture materials, surgical microscopes, and micro instrumentation. Techniques have evolved from extracapsular procedures to small-incision techniques and phacoemulsification with foldable lens implants.

Figure 2. Placido Rings Topography System

The Placido rings from corneal topography shows the reflected shape of the cornea. The reflective lights images on the surface of the cornea produce a perfect concentric circular pattern. The data obtained is analyzed by the computer.

Section III: Clinical Applications of Topography

The aim of cataract surgery is to return patients to good uncorrected vision. This requires that their final refraction should be within 1D of emmetropia or a predetermined ametropic result and that preexisting and surgically induced astigmatism should be minimized3. In order to achieve this, the refractive element of each stage of surgery has to be optimized. This is facilitated, particularly in difficult cases, by the use of corneal topography (Figure 2).

Surgical Incisions

Preoperative Evaluation and Planning

of the Incision

Knowledge of the amount, location and type of preexisting astigmatism is important if it is to be reversed prior or during cataract surgery. Topography is very useful in confirming the precise location of the steep semimeridian. Because we center our cataract incisions on the steep meridian and adjust the incision size as a function of the preoperative astigmatism, it is important to precisely identify the steep meridian. Vector analysis can be used to calculate the induced astigmatism which needs to be added to the existing astigmatism in order to produce the desired spherical end result4. This may be achieved by three alternative methods5. First, astigmatic keratotomy or relaxing incisions may be performed prior to or during cataract surgery6,7. Second, a toric intraocular lens can be implanted. Third, the astigmatism may be addressed by using the appropriate placement and construction of the incision8. This is done by centring the incision on the steep meridian and using a wound construction-closure combination that will produce the required astigmatic decay9. The effect can be further titrated against the topography by selective suture removal postoperatively. Experience with temporal cataract surgery has enabled some surgeons to rotate the cataract incision to the steep axis, using an incision designed to flatten at the incision site, in an attempt to reduce preexisting astigmatism (Figure 3). One can monitor the effects of wound manipulation on resultant astigmatism using corneal topography.

Chapter 10: Corneal Topography in Cataract Surgery

Astigmatism in Cataract Surgery

A diamond knife (D) blade enters the first incision and is then directed slightly oblique to the iris plane and advanced into the anterior chamber. This forms the internal aspect of a self-sealing incision (arrow).

Incision Type and Location

Surgically induced changes in corneal contour are less following more peripheral (posterior) incisions in the sclera or limbus than those involving the cornea10. Some authors have claimed that for smaller incision surgery, some incision sites (for example, superotemporal or temporal) cause less astigmatism than others11. Do clear-corneal incisions compare favorably with scleral-tunnel incisions? Corneal topography allows direct comparison of these two techniques. A randomized, prospective study was conducted comparing topographic images of temporal clear-corneal and superior scleral-tunnel incisions12.

Both techniques evidenced either no induced corneal changes or mild flattening at the wound site. There were no significant differences between

groups in incidence of flattening at the wound site, and this mild corneal change generally regressed. Under topographic analysis, both techniques appear to be essentially astigmatically neutral.

There is a huge quantity of literature now supporting the theory that smaller incisions are associated with less surgically induced changes in corneal contour. Since the introduction of foldable intraocular lenses, it has been shown that an unsutured 3.2 mm incision is usually associated with less than 0.5D against-the-rule shift in corneal astigmatism (Figure 4). Corneal topography, with its ability to show changes across the entire cornea, can demonstrate the actual differences between different incision sizes (2.8, 3.2 and 6.0 mm) that keratometry may miss.

Figure 4. Correcting Ametropia by Changing the Corneal Curvature

A transverse incision of the cornea (A) can also be used to correct astigmatism when performing cataract surgery, specifically phacoemulsification. This technique may be done employing also the phacoemulsification incision over the steepest meridian (C). (Art from Highlights of Ophthalmology collection of medical illustrations).

Wound Closure

Unsutured Wounds

The use of well-constructed self-sealing incisions avoids suture-related complications, and postoperative astigmatism is no longer the frequent and serious problem it used to be13,7,14. Unsutured small incision wounds typically show only a mild "against- the-rule" astigmatic shift (<1.00D) which tends to decay with time15,16,17,18. If flattening occurs, it tends to remain localized to the area of the wound, and does not necessarily reduce uncorrected visual acuity unless it encroaches on the central cornea.

Sutured Wounds

If the wound requires suturing, monofilament nylon is commonly used. This is an inert nonabsorbable suture with a relatively high tensile strength, which allows minimal natural decay of

Section III: Clinical Applications of Topography

induced astigmatism, until the suture is removed19,20.

To minimize surgically induced astigmatism, radial sutures should be relatively deep and of moderate length to prevent tissue compression or wound gape21,22. For tunnel incisions, some studies suggest that horizontal, triangular, or mattress sutures are associated with less wound related steepening than either radial or cross sutures23.

Surgical Correction of Postoperative

Astigmatism

Postoperative astigmatism persisting after suture adjustment can be addressed either by revision of the original incision or a separate corneal refractive procedure24. Excimer laser ablation or astigmatic keratotomy (Figure 5) can be performed across the steep meridian and wedge resection or compressive sutures can be used to reverse flattening.

Figure 5. Astigmatic Keratotomy for the Correction of Astigmatism-ATR

The relaxing incisions (K) of the acuate keratotomy, flatten the corneal curvature in the detected astigmatic meridian. In the case shown below, the effect is directed to the corneal curvature in the astigmatic horizontal meridian. Myopic astigmatism. (Art from Highlights of Ophthalmology collection of medical illustrations).

Chapter 10: Corneal Topography in Cataract Surgery

Postoperative Corneal Curvature

Corneal Steepening

Wound-related corneal steepening (with-the- rule astigmatism for a superior incision) occurs secondary to compression of tissue at the wound site (Figure 6). This is commonly a result of the overtightening of sutures, or edema of the wound margin. It may also be due to vertical wound misalignment in which the central edge underrides the peripheral edge, or due to cautery causing tissue contraction15. The compression of tissue at the limbus depresses the limbal cornea towards the centre of the globe, thereby increasing the curvature of the central cornea (that is a reduction in the radius of curvature). There is a

small area of flattening immediately within the area of the suture and a secondary flattening in the meridian perpendicular to the suture, as a result of coupling.

Corneal Flattening

Wound-related corneal flattening (againstthe- rule astigmatism for a superior incision) occurs as a result of wound gape25 (Figure 7). This is sometimes seen to a small extent in unsutured woundsl5,18, but more commonly if sutures are too loose, either at the time of surgery or if there is subsequent cheese-wiring, knot slippage, suture-related inflammation, degradation, or removal.

Sutures which are placed too superficially may result in posterior wound gape, which has a similar topographic appearance. Vertical misalignment of the wound with the central edge of the incision, flattens the incisional meridian21.

Irregular Astigmatism

If wound-related flattening or steepening is due to either a single or a uniform structural defect, regular astigmatism is most likely and is relatively easy to correct either optically or surgically. However, more complex anatomical changes can result in irregular astigmatism, which produces greater visual dysfunction and is more difficult to correct3. Bi-oblique astigmatism (non-perpendicular axes) may occur if non-adjacent sutures are overtightened. A torsional effect results from a horizontal

misalignment of the wound, whether due a mismatching of its edges or non-radial suture bites.

Corneal Topography in the Evaluation

of Cataract Surgery

Corneal topography is a useful tool for monitoring the postoperative course of cataract removal and intraocular lens implantation in cases of preexisting or induced astigmatism. For the clinician, corneal topography may reveal problems not apparent on routine examination. For the researcher, it is an essential adjunct to the investigation of the relative merits of sutureless vs. sutured surgery, smallvs. large-incision surgery, temporal vs. superior surgery, and clear-corneal vs. scleral-tunnel surgery. Corneal topography makes important contributions to interpreting the visual rehabilitative results of