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Chapter 27 – Refractive Aspects of Cataract Surgery

Chapter 27 – Refractive Aspects of Cataract Surgery

 

MARIA REGINA CHALITA

RONALD R. KRUEGER

BRIAN S. BOXER WACHLER

 

INTRODUCTION

The precise postoperative refractive outcome of cataract surgery became a main focus during the 1990s.[1] Small incision surgical techniques such as clear corneal or posterior limbal tunnel incisions,[2] combined with astigmatic keratotomy and the use of foldable intraocular lenses (IOLs), have led to early stable refractive results. Visual rehabilitation in most patients, however, is limited by residual refractive error. The common goal of modern cataract surgery is not only to perform a safe and flawless procedure but also to achieve postoperative emmetropia with improved uncorrected visual acuity.[3]

In this chapter, the history and principles of surgically induced astigmatism are described, as well as the devices and methods of measuring astigmatism. In addition, the astigmatic effects created and modified by incision parameters (size, configuration, construction, location, and closure) in cataract extraction are discussed, along with the use of transverse and arcuate keratotomy in cataract surgery. Issues concerning IOL calculation and selection to achieve spherical emmetropia are dealt with elsewhere in this book.

HISTORY

The mathematics for calculating surgically induced refractive changes was first described in 1849.[4] In 1975, Jaffe and Clayman[5] properly applied this method to analyze the relationship between cataract surgical technique and the refractive results in 1557 eyes undergoing cataract extraction. The wounds were closed using various techniques, including 10-0 monofilament nylon sutures that were either started at the 2:30 or 9:30 positions and tied at the 12 o’clock position (Troutman suture technique) or started at the 12 o’clock position and tied at the 2:30 or 9:30 positions (Willard suture technique). Knots tied superiorly were associated with with-the-rule (WTR) astigmatism, whereas knots tied close to the horizontal meridian were associated with against-the-rule (ATR) astigmatism. This early report facilitated an understanding of the dynamics of suture tension and corneal astigmatism in cataract surgery. It applied vector analysis using graph paper, rectangular coordinates, and the law of cosines and sines to determine surgically induced astigmatism. In 1979, Cravy[6] expanded the principles of Jaffe and Clayman by developing an approximate method for categorizing postoperative astigmatism as WTR or ATR to help guide suture removal and understand the dynamics of various types of wounds.

PRINCIPLES

The principles of corneal biomechanics are the foundation for understanding surgically induced refractive changes. Jaffe and Clayman[5] found that the steepness of the corneal meridian is related to suture knot placement; Cravy[6] emphasized that total central corneal power is conserved in cataract surgery, not decreased or increased, as long as tissue is neither added nor removed.

Using a corneoscope, Rowsey[7] established the principles that govern our ability to understand refractive power changes and coupling in the cornea. The first six of the ten “caveats” are listed here, as they are most applicable to the dynamics encountered in cataract surgery:

• The normal cornea flattens over any incision.

• Radial corneal incisions flatten the adjacent cornea and the cornea 90° away.

• The flattening effect of radial incisions on the cornea increases as incisions approach the visual axis.

• The cornea flattens directly over any sutured incision.

• The cornea flattens adjacent to loose limbal sutures, flattens 180° away, and steepens 90° away.

• The cornea steepens adjacent to tight limbal sutures, steepens 180° away, and flattens 90° away.

Wound slippage that results from a sutureless limbal cataract incision leads to the opposite curvature effect of central meridional flattening.

DEVICES FOR MEASURING ASTIGMATISM IN THE CLINIC

Manual Keratometer

By doubling a projected ring image onto the cornea, the keratometer best measures regular astigmatism in the central 3.0?mm of the cornea. Irregular astigmatism and prolate to oblate corneal changes (from refractive surgery) are conditions in which the keratometer may not perform optimally.

Corneal Topography (Computerized Videokeratography)

This examination remains the gold standard for astigmatism measurement. Various technologies exist that analyze the curvature and elevation of the cornea, extending to the periphery. Many machines employ Placido disc imagery but differ in terms of the proprietary algorithms that process the Placido images. Some devices contain vector analysis software to measure surgically induced astigmatism based on simulated keratometry.

Van Loenen Keratoscope

The Van Loenen keratoscope (JedMed Inc., St. Louis, MO) is a simple, handheld device used to assess qualitatively the corneal shape by projecting rings along a translucent cylinder onto the cornea and observing their reflection ( Fig. 27-1 ). In the clinic, this is done by the illumination and magnification of a slit-lamp microscope. Corneal astigmatism creates an oval Placido disc reflection, where the steep axis corresponds to the short axis of the oval. The Van Loenen keratoscope is useful in the clinic to assess higher degrees of astigmatism.

 

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Figure 27-1 Van Loenen keratoscope. The concentric circles are projected onto the cornea with the aid of the slit lamp or operating microscope.

 

 

Figure 27-2 Maloney keratometer. Constructed of metal, this keratoscope is used to assess qualitatively the intraoperative corneal curvature.

Corneoscope

Before corneal topography became commonplace, the corneoscope provided qualitative and quantitative measurements of corneal curvature out to the periphery. It is rarely used today.

DEVICES FOR MEASURING ASTIGMATISM INTRAOPERATIVELY

Terry Keratometer

This device, which also uses a doubling of a projected ring, attaches to the head of the operating microscope. In a study by Lindstrom and Destro,[8] the group of patients that had cataract wounds closed with the aid of the Terry keratometer experienced significantly better postoperative astigmatism control than did the group for which the keratometer was not used.

Van Loenen Keratoscope

This device, described earlier, can also be used to project rings onto the cornea intraoperatively using the illumination and magnification of the operating microscope.

Maloney Keratometer

Similar to the Van Loenen keratoscope, the Maloney keratometer (Storz, St. Louis, MO) differs in that it is manufactured from metal ( Fig. 27-2 ). It is used to assess qualitatively corneal shape and astigmatism.

METHODS OF CALCULATING ASTIGMATISM

Simple Method

In this method, only magnitude is subtracted; axis is disregarded. It is invaluable to gain information about the final outcome of a procedure performed in patients who have minimal preoperative astigmatism. Because the axis is ignored, this method is inherently inaccurate. Compared with the vector

 

 

Figure 27-3 Graphic vector analysis of surgically induced astigmatism as plotted with a protractor. By drawing preoperative (K1 ) and postoperative (K3 ) astigmatism, surgically induced astigmatism (K2 ) can be easily deduced (20?mm = 1D; see text for details).

 

 

 

Example of Vector Method of Calculation*

 

PREOPERATIVE

46.50D @ 035°

 

44.00D @ 125°

 

 

POSTOPERATIVE

46.00D @ 050°

 

43.75D @ 140°

 

 

ASTIGMATISM

K1 :2.50D @ 035°

 

 

ASTIGMATISM

K3 :2.25D @ 050°

 

 

* See Figure 27-3 .

 

 

 

method, the simple method grossly underestimates surgically induced astigmatism[9] and can produce errors in the range of 33–166%.[10] Hence, the simple method is not recommended.

Vector Method

Originally described by Stokes in 1849 and later by Jaffe and Clayman,[5] the vector method is perhaps the most straightforward method to incorporate both magnitude and direction. The vector method relies on geometrical principles and graphing of the direction and amplitude of preoperative and postoperative astigmatism (or cylinder) on paper to determine the surgically induced vector. Because the graph covers 360°, the axis must be doubled before drawing the vector on paper. Figure 27-3 demonstrates this method for determining corneal power. The method may be used for positive or negative cylinder as long as the preoperative and postoperative cylinders are of the same sign. By convention, the example uses the Jaffe and Clayman notations: K1 (preoperative astigmatism), K2 (surgically induced astigmatism), and K3 (postoperative astigmatism).

For the example shown in Box 27-1 , K2 , the surgically induced change, is to be determined. The astigmatism vectors are drawn using a protractor and denoting the magnitude with a convenient scale (see Fig. 27-3 ). The scale can be adjusted as long as consistency is maintained for each problem. A triangle is formed by creating a third side, which is K2 . The magnitude is measured with the millimeter scale and is found to be 1.25D. The axis is determined using the protractor after a dashed parallel line is drawn at the end of vector K1 (186° in Fig. 27-3 ). The axis is always measured around K1 , not K3 . This value is halved to yield the surgically induced meridian of the astigmatism (93° in this example). Use of polar coordinate paper eliminates the

 

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need for a protractor and ruler. In the example here, surgically induced astigmatism is 1.25D @ 93°.

Law of Cosines and Sines

Using trigonometric equations, surgically induced astigmatism can also be derived as described by Jaffe and Clayman.[5] Even a rough sketch of vector forces (see Fig. 27-3 ) facilitates comprehension of the trigonometry of this method. The law of cosines and sines is perhaps the most sensitive of all methods used to detect changes in surgically induced astigmatism. Because all induced cylinder has a positive value, this method gives the highest calculated value for astigmatism,[11] with rare exceptions.[12] The law of cosines is used to determine the magnitude of the surgically induced astigmatism, K2 (K = magnitude, and k = angle opposite K), as in equation 1 . For the example shown in Figure 27-3 and Box 27-1 , the calculation is given in equation 2 .

 

 

 

 

 

 

 

The law of sines is used to calculate the angle, as in equation 3 , from which equation 4 is obtained. To determine the meridian of K2 : 70 – 64.2 = 5.8; add 5.8 to 180 to yield 185.8° (see Fig. 27-3 ). This angle is halved to find the meridian of the surgically induced astigmatism: 92.9°. So, in the example above, surgically induced astigmatism is 1.25D @ 92.9°.

 

 

 

 

 

 

 

Polar Values

Naeser[13] simplified the vector method by breaking down astigmatism into WTR and ATR components and formulating a single value, KP, which represents the polar value of net astigmatism (see equation 5 , where M is magnitude of the astigmatism). Naeser points out advantages and disadvantages of this method. The single number represents the astigmatism and incorporates a balance between WTR and ATR astigmatism. However, a single value loses specificity, since different combinations of meridians and magnitudes may have the same polar value. Additionally, there may be a discrepancy between astigmatic magnitudes and polar values; some polar values may be 0 (45° meridian) when there is truly magnitude to the astigmatism.

 

 

 

Cravy Method

This method details the derivation of an astigmatism ratio. This ratio serves both as an indicator of wound apposition, suture placement, and tension and as a means of categorizing WTR or ATR astigmatism.[6] At best, this method yields approximate results, except when the preoperative and postoperative astigmatisms are not oblique.

Alpins Method

Alpins[10] introduced the concepts of targeted induced astigmatism, magnitude and axis of error, coefficient of adjustment, and index of success to measure the ability to achieve a desired outcome and refine surgical technique.

Holladay, Cravy, Koch Method

Ten steps are outlined that incorporate both refractive and keratometric readings as a means of measuring and following postoperative refractive changes. [14] Olsen[15] reported a slight modification of this technique in 1993.

In 1998, Holladay et al.[16] recognized that his former calculation method could be improved by incorporating the magnitude and axis of the surgically induced refractive change into the calculations.

Azar Sinusoidal Method

This method uses a single sinusoidal equation to calculate surgically induced refractive changes as applied to cataract and refractive surgery.[17]

Summary

Before trigonometry was applied to astigmatism analysis, the simple method was the standard; however, this is now known to be inaccurate and should not be used. Before using any of the methods described previously, it is important to remember to adjust refractive data for statistical analysis. Holladay et al.[18] introduced this concept in 2001, showing many sources of error in refractive outcome statistics, such as the use of multiple lens systems in the Phoroptor, errors in vertex calculations, difficulty in accurately defining the axis of astigmatism, and failure to consider measurement errors when working with keratometric data. They pointed out that refractive data must be adjusted for vertex distance before comparison to topographical or keratometric data. Descriptive statistics such as means, standard deviations, shape factor (?), standard error of the mean, and correlation coefficients can be calculated only after converting polar to Cartesian values.

Regardless of the method used, as long as surgeons take the time to understand the effects of their surgery by using astigmatism analysis, they will be more successful in achieving their surgical goals by modifying their plans as indicated by such analysis.

ASTIGMATISM IN EXTRACAPSULAR CATARACT EXTRACTION

Extracapsular expression of the nucleus requires an incision with a chord length of 9.0–11.0?mm. These long incisions can induce initial large amounts of WTR astigmatism. Neumann et al. [9] reported aggregate surgically induced astigmatism of 1.29D at 3 months and 1.08D at 6 months. Minassian et al.[19] compared extracapsular cataract extraction (ECCE) with small incision phacoemulsification (phaco) and found that whereas the phaco group attained a good, stable level of visual acuity quickly (by the third postoperative week), the vision in the ECCE group continued to improve for up to 6 months. Analysis by multiple regression indicated that the poorer results with ECCE were due to higher levels of astigmatism after surgery.

Even with meticulous wound closure, these wounds are keratometrically unstable. The postoperative shift toward ATR astigmatism reflects wound slippage that occurs for as long as 2 years. This incorporates the premise of “coupling,” in which an increase in corneal power in one meridian is accompanied by a decrease in the orthogonal meridian, with preservation of average corneal power. Although the corneal surfaces changed dramatically over time in the study by Talamo et al.,[20] the average corneal power determined by keratometry and by spherical equivalent manifest refraction showed a high degree of stability over 4 years ( Fig. 27-4 ).

Another important point is determining the optimal time to remove sutures after ECCE. Talamo et al.[20] recommend suture removal at the 8–10 week visit if significantly more than 3D of WTR astigmatism is present 3–5 weeks postoperatively. Other authors have suggested removing sutures at 12 weeks postoperatively

 

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Figure 27-4 Postoperative extracapsular cataract extraction course of manifest refraction and keratometry without suture cutting. Mean spherical equivalent refraction and keratometry (mean for all patients) correlate well and change little over time, which indicates that total central corneal refractive power is conserved through coupling, despite against-the-rule astigmatism drift. The upper and lower bars represent one standard deviation from the mean. (Adapted with permission from Talamo JH, Stark WJ, Jottesch JD, et al. Natural history of corneal astigmatism after cataract surgery. J Cataract Refract Surg. 1991;17:313–8.)

and prescribing glasses 1 month after suture removal. Beware that suture cutting may turn WTR astigmatism into unwanted ATR astigmatism over time.[21]

Preoperative astigmatism also has predictive value for postoperative astigmatism. Talamo et al.[20] showed that patients tend to have less postoperative than preoperative WTR astigmatism. Preoperative ATR astigmatism showed the opposite trend. These results imply that the cornea tends toward the original magnitude and direction of the initial astigmatism, except when more than 2D of ATR astigmatism exists preoperatively.

ASTIGMATISM IN PHACOEMULSIFICATION

Nuclear Expression versus Phacoemulsification

Phacoemulsification offers the advantage of cataract removal through a smaller wound, which decreases the surgically induced astigmatism and the decay of induced astigmatism compared with ECCE.[19] Lindstrom and Destro[8] showed that in a group of patients who had 6.5?mm incision phacoemulsification, 5% had greater than 3D and 65% had less than 1D of astigmatism at 6–10 weeks postoperatively. This compared favorably with the results in the 10?mm ECCE group: 24% with greater than 3D and 27% with less than 1D of astigmatism. The longer incision and increased number of sutures in ECCE undoubtedly result in the greater degree of astigmatism ( Table 27-1 ).[8]

Wound Length with Sutured Closure

Sutures cause corneal steepening in the meridian of the suture. However, greater surgically induced astigmatism results with a longer wound, even when the same number of sutures is used. A 7?mm long scleral pocket incision causes more postoperative keratometric cylinder than a 4?mm wound (1.33D and 1.03D, respectively) up to 6 weeks postoperatively.[22] In a larger wound, more tissue can be affected by suture tension. Small wounds have less surgical edge surface area and, therefore, are more resistant to the mechanical forces of the sutures.

Sutured wounds that differ by 1.5?mm are similar in the amount of surgically induced astigmatism. In a large cohort of 276 cases, Davison [23] found no difference for up to 1 year in patients who had 4.0?mm and 5.5?mm incisions using a standardized double “X” suture closure. At 1 year, the induced mean was

 

TABLE 27-1 — COMPARISON OF SURGICALLY INDUCED CYLINDER BY VECTOR ANALYSIS FOR NUCLEAR EXPRESSION AND PHACOEMULSIFICATION

 

 

Follow-up Visit

Study Group

 

3 Months

6 Months

Phaco/silicone

Mean (SD)

1.29 (0.96)

1.08 (0.69)

 

Range

0–3.25

0–2.67

Phaco/PMMA

Mean (SD)

1.06 (1.09)

1.06 (1.15)

 

Range

1–7.42

0–7.42

ECCE/PMMA

Mean (SD)

2.27 (1.28)

1.74 (1.16)

 

Range

0.5–7.0

0–4.61

The nuclear expression/polymethyl methacrylate (NE/PMMA) results are significantly different from those of the two phacoemulsification groups (by one-way ANOVA at 3 months, p = 0.0001; at 6 months, p = 0.014). No difference occurred between the phacoemulsification groups. (Adapted with permission from Lindstrom RL, Destro MA: Effect of incision size and Terry keratometer usage of postoperative astigmatism. J Am Intraocul Implant Soc. 1985;11:469–73.)

 

 

 

 

TABLE 27-2 — EVALUATION OF ASTIGMATISM IN 4.0?MM SUTURED CATARACT SURGERY WOUNDS

 

Period

Study

1 Week

4 Weeks

12 Weeks

Shepherd[24]

+0.13 (0.67)

+0.02

-0.22 (0.47)

Steinert et al. [11]

+0.07 (1.18)

+0.05 (1.11)

-0.21 (0.93)

Uusitalo et al. [25]

-0.35 (0.79)

-0.35 (0.07)

-0.53 (0.69)

Astigmatism decay over a 3-month period is similar for three studies that used the Cravy method (+, with-the-rule astigmatism; -, against-the-rule astigmatism).

 

 

0.3D and 0.2D of ATR astigmatism in the 4.0?mm and 5.5?mm groups, respectively.

The 4.0?mm sutured incision allows for a short rehabilitation period. Not only is less astigmatism induced, but also less variability occurs in the amount of induction. Using the Cravy method, Shepherd[24] illustrated this with 4.0?mm incisions. At 1 week, 0.13D of WTR astigmatism was induced, compared with 0.22D of ATR astigmatism at 3 months. Steinert et al.[11] showed similar findings for 4.0?mm incisions. The work of Uusitalo et al.[25] also supports minimal decay in 4.0?mm wounds. Table 27-2 shows the similarities of the three studies.

Larger wounds produce greater amounts of WTR astigmatism. In addition, the astigmatism induced by these wounds decays faster over time and requires more time for stabilization ( Fig. 27-5 ). [24] Uncorrected visual acuity also parallels the amount of residual cylinder. Patients who have longer wounds must wait longer until they receive spectacles, and during that longer time, they have relatively reduced uncorrected visual acuity compared with patients who have shorter incisions. Longer sutured incisions prolong visual rehabilitation in patients.

Wound Length with Sutureless Closure

SCLERAL POCKET APPROACH.

Sutureless closure offers the efficiency of decreased operating time as well as reduced material cost and less ocular tissue manipulation.

The ability to detect changes in astigmatism seems to be influenced by which instrument is used to evaluate the sutureless technique.[26] [27] The difference map of corneal topography detected flattening along the wound and central and paracentral steepening.[26] Similar-sized incisions of 3.2?mm, 4.0?mm, and 5.0?mm were evaluated for 6 months by Hayashi et al.[27] ; for these incisions, an automated keratometer and corneal topography showed good consistency. The 3.2?mm wound induced significantly less astigmatism (0.38D) compared with the 4.0?mm and

 

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Figure 27-5 Degradation of postoperative induced astigmatism for various wound lengths. Longer incisions induce more astigmatism and require a longer period to reach stabilization. (Adapted with permission from Shepherd JR. Induced astigmatism in small incision cataract surgery. J Cataract Refract Surg. 1989;15:85–8.)

5.0?mm wounds (0.56D and 0.6D, respectively). In the 3.2?mm group, no coupling occurred based on the different corneal topography maps, but longer incisions showed coupled steepening. The astigmatism decay was negligible for all incision groups.

CLEAR CORNEA APPROACH AND CORNEAL TOPOGRAPHY.

Analysis of astigmatism after temporal clear cornea surgery shows the limitations of keratometry compared with corneal topography. The 3.0?mm temporal incision induced between 0.28 and 0.53D of temporal flattening, with no effect on the nasal corneal curvature ( Fig. 27-6 ).[28] Coupled vertical steepening was also absent. These subtle changes were not detected by keratometry. Greater flattening is seen with longer corneal incisions.

Corneal topography can be used to detect curvature changes in the periphery; these provide a more complete understanding of incisional effects, especially when the changes are subtle. Topography has demonstrated that the temporal flattening of a 5.0?mm incision is in the 0.50–1.75D range, with less dramatic nasal flattening of 0.25–0.75D and vertical steepening of 0.25–0.75D ( Fig. 27-7 ).[29] Clear corneal entries do not induce equal and opposite coupling in curvature, especially in the periphery. The astigmatic influence of clear corneal incisions disproportionately affects the temporal meridian. Automated keratometry and corneal topography have shown good consistency in sutureless scleral incisions.[30]

 

 

Figure 27-6 Temporal 3?mm incision. A, Surgically induced topographical change 1 month following temporal 3.0?mm clear corneal cataract surgery. Note the temporal flattening. B, Paired Wilcoxon test results of significance (p = 0.05) for surgically induced topographical change 1 month following temporal 3.0?mm clear corneal cataract surgery. Note the absence of vertical coupling. (Reprint permission granted by The American Journal of Ophthalmology; C. Vass, MD; and Ophthalmic Publishing Company. From Vass C, Menapace R. Computerized statistical analysis of corneal topography for the evaluation of changes in corneal shape after surgery. Am J Ophthalmol. 1994;118:177–84.)

 

 

Figure 27-7 Temporal 5?mm incision. A, Surgically induced topographical change 3 months following temporal 5.0?mm clear corneal cataract surgery. Note the nasal and temporal flattening. B, Paired Wilcoxon test results of significance (p <0.01) for surgically induced topographical change 3 months following temporal 5.0?mm clear corneal cataract surgery. Note the vertical coupling, which was not present in 3.0?mm wounds. (Adapted with permission from Vass C, Menapace R, Amon M, et al. Batch by batch analysis of topographic changes induced by sutured and sutureless clear corneal incisions. J Cataract Refract Surg. 1996;22:324–30.)

 

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Effects of Other Incision Types

FROWN INCISION.

Singer[31] was the first to introduce this technique, which was named for its appearance to the surgeon during surgery ( Fig. 27-8 ). Although the incision is curved, the arc length is either 6?mm or 7?mm, and the apex of the frown is 1.5?mm posterior to clear cornea. A single horizontal vertical mattress suture of 10-0 nylon secures the central one third of the wound. This wound can accommodate insertion of a 6.0?mm or 7.0?mm diameter optic IOL. Of Singer’s 62 cases, one patient had a self-resolving filtering bleb, and a second patient had a wound leak that required additional radial sutures. [31]

This technique has two advantages. First, it induces less surgical astigmatism compared with standard scleral pocket incisions (0.82D versus 1.30D at 1 year). Only 0.28D of ATR astigmatism was reported at 6 months postoperatively. [32] The second advantage is its high degree of stability. Although a modest degree of induced astigmatism results, there is only a minimal change in the actual amount of astigmatism induced. Therefore, this incision can be considered decay resistant. The curve of this incision integrates radial components, which are more stable than for a transverse and antifrown incision.

RADIAL TRANSVERSE INCISION.

Siepser[33] developed this incision technique, which incorporates a “pita pocket” dissected through a radial incision ( Fig. 27-9 ). The 3.5?mm radial incision is made 1.5?mm posterior to the vascular arcade. A 2.0?mm microsclerotome is used to create pita pockets on both sides of the radial incision for the insertion of a 6.0?mm diameter IOL. No sutures are used for closure. Lens insertion is technically more difficult, which explains the rate of conversions to other techniques (19/124). At 3 months postoperatively, 0.2D of cylinder was induced, and the wound stabilized at 1 week.

The architecture of this radial wound may preserve scleral support of the cornea. It is possible that a traditional horizontal incision disrupts the scleral fibers that maintain tension on the cornea over the length of the incision, thereby allowing the cornea to relax in that meridian. By suturing the wound, tension is put back on the cornea in that meridian. However, the suture does not prevent eventual slippage. The sutureless transverse radial incision allows for insertion of large, optic, rigid IOLs, but with insignificant long-term induced astigmatism.

THE “W” INCISION.

Hennekes’s[34] one-stitch “W” incision utilizes intrinsic scleral support for rapid stabilization ( Fig. 27-10 ). The length of the “W” is 7.0?mm, and its base is 2.0?mm from the limbus. A single Vicryl suture is used to secure the apex of the

 

 

Figure 27-8 Frown incision. This technique preserves scleral support, which helps prevent wound slippage, while being able to accommodate a rigid polymethyl methacrylate intraocular lens. (Adapted with permission from Singer JA. Frown incision for minimizing induced astigmatism after small incision cataract surgery with rigid optic intraocular lens implantation. J Cataract Refract Surg. 1991;17[suppl]:677–88.)

central flap. This incision may be used in conjunction with filtering surgery for glaucoma or a pars plana approach for posterior segment surgery. One case of hypotony occurred in a series of 82 eyes.

Induced astigmatism was 1.18D at 1 week, 1.03D at 1 month, and 0.9D at 3 months. The increased surface area of the scleral triangular flap allows for more rapid wound healing and stabilization. The radial arms of the “W” provide additional support against wound slippage.

Effects of Suture Technique

The horizontal (tangential) closure induces less astigmatism than radial closures do. “Tight” and “loose” radial running sutures (four-cross shoelace) for 6.5?mm scleral pocket incisions induce more initial WTR refractive cylinder than a horizontal figure-eight does.[35] Steinert et al.[11] showed that induced astigmatism in the first 1–2 weeks after a 6.5?mm wound is dependent on suture technique. The horizontal closure is remarkably resistant to decay up to 6–12 months, whereas the running closure results in decay. For larger incisions, horizontal suture closure offers the

 

 

Figure 27-9 Radial transverse incision. This wound is highly stable, but lens insertion is technically difficult. (Adapted with permission from Siepser SB. Sutureless cataract surgery with radial transverse incision. J Cataract Refract Surg. 1991;17:716–8.)

 

 

Figure 27-10 “W” incision. This wound preserves scleral support and may be versatile for other concomitant surgical procedures. (Adapted with permission from Hennekes R. A high-stability, one-stitch W incision for cataract surgery. J Cataract Refract Surg. 1996;22:407–10.)

 

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Figure 27-11 Double-paired transverse keratotomy incisions. A, Cross section of the scleral pocket incision closed with a continuous suture in apposition. Note that the suture passes within the wound space superficial to the internal layer of the scleral pocket. B, Cross section of the scleral pocket incision closed with a deeply placed suture. Note that the suture passes deep to the wound space and incorporates the internal layer of the scleral pocket. (Adapted with permission from Masket S. Deep versus appositional suturing of the scleral pocket incision for astigmatic control in cataract surgery. J Cataract Refract Surg. 1987;13:131–5.)

advantages of minimal initial induced astigmatism and long-term stability. Buzard and Shearing[36] demonstrated no difference in surgically induced astigmatism among 5.0, 6.0, and 6.5?mm wounds with a central, one third width horizontal suture. Smaller 4.0?mm wounds show no significant difference if they are closed with a horizontal suture or left sutureless.[37] A 4.0?mm wound closed with a single vertical suture induces minimal astigmatism, as does a sutureless closure, as demonstrated by corneal topography.[38] The induced astigmatism of 3–4?mm scleral pocket incisions is not affected by suture technique.[39]

The vector forces of a radial suture preferentially affect tissue and thus corneal curvature along the axis of the suture. The advantage of the horizontal (tangential) suture closure is that the horizontal vector forces have much less effect on the cornea, and hence less induced astigmatism. The reason for minimal decay compared with that found with radial closures may be the absence of significant induced astigmatism.

Although horizontal closures induce less astigmatism than radial closures, the latter may be constructed to minimize the astigmatic effect by modulating the depth of passage of the suture needle. A needle passed deep through the wound results in a more stable wound with less induced astigmatism than for an appositional closure ( Fig. 27-11 ).[40] Incorporating the deep tissue helps anchor the anterior scleral flap, thereby making it less vulnerable to the intraoperative superior movement that induces WTR astigmatism and less prone to subsequent decay.

Effects of Suture Materials

Mersilene suture is made of nonbiodegradable polyester that is less elastic than nylon. As a result, the initial wound edema from scleral incisions causes almost twice as much WTR astigmatism in Mersilene as in nylon closures.[41] The elasticity of nylon allows the suture to partially accommodate the wound edema and minimize subsequent changes in corneal curvature. Conversely, it is the elasticity of nylon that accounts for its long-term instability in ECCE. The use of nylon sutures results in a slow drift to ATR astigmatism, and at 2 years postoperatively, there is a rapid ATR astigmatism change as a result of spontaneous rupture of the sutures. Mersilene does not rupture, but there is also a slow drift toward ATR astigmatism with its use.[42]

Other Operative Factors Affecting Astigmatism

INTRAOCULAR PRESSURE.

IOP is a hidden variable that seldom receives much attention in terms of refractive effect. Securing the wound in a soft eye has been postulated to result in greater induced postoperative astigmatism, because greater tension is placed on the suture after the IOP increases to the normal range in the postoperative period. Trying to suture at physiological IOPs obviates this effect. A postsuture inflation technique is advocated that utilizes a snug first suture throw, followed by filling of the anterior chamber with balanced salt solution to give a pressure of 7–23?mmHg, and then the final throws.[43]

INCISION DISTANCE FROM THE LIMBUS.

A more posterior incision is believed to be more stable, as it minimizes any “slip” the cornea may experience from a more anterior (closer to the limbus) incision.[44] Posterior incisions benefit from greater scleral support, which counters wound slippage.

LENGTH OF SUTURE.

Long sutures are thought to induce more steepening (with the incision) than short bites, because of the greater forces needed to secure the former.[45] For a given incision that can be closed adequately with a certain number of short sutures, fewer long sutures are required to close the same wound because the greater vector forces generated by each long suture oppose a wider amount of tissue margin.

POSTOPERATIVE CORTICOSTEROIDS.

Manipulation of the duration of action of corticosteroids has been advocated to tailor the postoperative course to a desired astigmatic end point. Prolonged use of steroids in selected cases may allow great wound slippage to help treat preexisting WTR astigmatism. Likewise, a short course of postoperative steroids may help minimize astigmatic decay from a superior scleral pocket incision in a patient who has preoperative ATR astigmatism.

ASTIGMATIC KERATOTOMY IN PHACOEMULSIFICATION

Wound manipulation and adjustment of architecture are ways to modulate astigmatism. A near astigmatically neutral cataract extraction may be achieved with a small incision phacoemulsification. Incorporation of astigmatic keratotomy intraoperatively may then provide the surgeon with the most

 

235

 

 

Figure 27-12 Schematic of cornea with double-paired transverse keratotomy incisions. This technique may be used to correct astigmatism intraoperatively or postoperatively. (Adapted with permission from Maloney WR, Grindle L, Senders D, Pearcy D. Astigmatism control for the cataract surgeon: comprehensive review of surgically tailored astigmatism reduction (STAR). J Cataract Refract Surg. 1989;15:45–54.)

 

 

TABLE 27-3 — COMPARISON OF THE EFFECT OF TRANSVERSE ASTIGMATIC KERATOTOMY (TAK) AT 7.0?MM, 6.0?MM, AND 5.5?MM OPTICAL ZONES (GROUPS 2, 3, AND 4, RESPECTIVELY)

 

 

 

 

 

 

Average Surgically Induced K (K3 ) (D)

Group

Preoperative Astigmatism (D)

Number of Eyes

TAK Optical Zone (mm)

Average Preoperative K (K1 ) (D)

Average Postoperative K (K2 ) (D)

Total

TAK Only (estimate)

1

<1.00 (control)

105

0.53 0.43

0.82 0.63

0.78 0.57

— —

2

1.00–2.00

30

7.0

1.24 0.33

0.74 0.56

1.57 0.76

2.04 0.74

3

2.00–3.00

19

6.0

2.36 0.44

0.91 0.81

2.59 0.93

2.85 0.90

4

>3.00

12

5.5

3.62 0.63

1.76 1.00

3.49 1.60

3.52 2.10

Averages and standard deviations of preoperative and postoperative astigmatism, total change in astigmatism, and estimated change in astigmatism from TAK incisions alone are shown. (Adapted with permission from Hall GW, Campion M, Sorenson CM. Reduction of corneal astigmatism at cataract surgery. J Cataract Refract Surg. 1991;17:407–14.)

 

 

predictable and stable means of treating a patient for cataract and astigmatism.

Transverse Astigmatic Keratotomy

In addition to its use for postkeratoplasty astigmatism, trapezoidal astigmatic keratotomy was used by Lavery and Lindstrom[46] to correct astigmatism in cataract surgery. Later, their work showed that two transverse corneal incisions produced the greatest degree of flattening, and the radial component was not necessary. This led to the development of transverse astigmatic keratotomy (TAK), which is a method used to manage astigmatism during or after cataract surgery ( Fig. 27-12 ).[44]

Critical factors that influence the amount of meridional flattening are:

• Diameter of the optical zone in which the incisions are made

• Length and depth of the transverse incisions

• Single versus dual paired incisions

Age, gender, and IOP are other factors that determine the way the cornea responds to relaxing incisions.

In 1989, Davison[47] evaluated both single and double pairs of 3.5?mm transverse incisions tangential to a 7.0?mm optical zone and to 7.0?mm and 9.0?mm optical zones, respectively. Patients who had preoperative WTR and ATR astigmatism in the single-pair group experienced a decrease of 1.3D and 1.1D, respectively. Patients who had the same preoperative WTR and ATR astigmatism in the double-pair group showed a more striking

 

 

Figure 27-13 Scatterplot of the axis of the attempted effect versus the axis of the surgically induced cylinder. In patients who had 1.0–1.9D of preexisting astigmatism and who received double-paired keratotomy incisions, a high correlation existed between attempted and achieved axes of correction. (Adapted with permission from Maloney WF, Senders DR, Pearcy DE. Astigmatic keratotomy to correct pre-existing astigmatism in cataract surgery. J Cataract Refract 1990;16:297–304.)

decrease of 1.9D and 4.1D, respectively. Larger optical zones were associated with a decreased astigmatic effect from paired transverse incisions.[48] In addition, relaxing incisions placed at smaller optical zones cause increased irregular astigmatism.[49]

Although many patients experience a reduction of preoperative astigmatism, a tendency exists to cause overcorrection. For 1–2D of astigmatism, Maloney et al.[50] showed that paired 3.0?mm incisions at the 7.0?mm optical zone flattened the cornea on average by 1.96D, which led to overcorrection in 25% of patients; dual incisions at 7.0?mm and 8.0?mm optical zones led to a similar level of overcorrection. In patients who had greater than 2D of astigmatism, less overcorrection occurred. On average, 7% were undercorrected and 14% were overcorrected with single and dual paired incisions, respectively, and overcorrections occurred in both groups. Both single and double transverse pairs showed good correlation of intended and surgical axes ( Fig. 27-13 ).[50] Placing more shallow incisions, Hall et al.[51] found overall undercorrection derived from 3?mm double-paired transverse incisions. More importantly, they calculated the effect of TAK independent of the superior cataract incision by using 105 control patients who had 6.0?mm scleral pocket incisions without TAK; on average, these patients had 0.78D of ATR astigmatism change. The flattening from the incision tended to negate the effect of TAK, which was done mostly for ATR astigmatism. At 7.0?mm, 6.0?mm, and 5.5?mm optical zones, the average TAK flattening was 2.04D, 2.85D, and 3.52D, respectively ( Table 27-3 ).[51]

 

236

In astigmatic keratotomy, it is generally thought to be more prudent to undercorrect than to overcorrect. Maloney and Shapiro[52] published a nomogram to guide surgeons.

Arcuate Astigmatic Keratotomy

An arcuate incision creates corneal relaxation equidistant from the optical center along the entire length of the incision, whereas a tangential incision actually encompasses a series of optical zones that increase toward the ends of the incision. Therefore, arcuate incisions provide greater relaxation per chord length than do transverse incisions. Several authors have outlined nomograms for arcuate incisions to manage preoperative astigmatism[53]

The conventional method of astigmatic keratotomy dictated the use of a relatively deep incision fixed around 90% of corneal thickness. The length of incision has been the main factor manipulated in controlling the degree of astigmatism correction. Based on patients who underwent astigmatic keratotomy with short incisions and developed undesirable corneal changes, Akura et al.[54] developed a new method. This method uses a relatively long incision (90° in length in regular astigmatism) covering the full arc of the steep area and controls the degree of astigmatic correction by varying the incision depth. They noted that controlling the level of correction by varying the incision depth allows the surgeon to use long incisions covering the entire steep area, minimizing the undesirable changes induced by conventional deep and narrow incisions and resulting in an ideal corneal sphericity after surgery.[54]

Cataract patients may experience an increased depth of focus after corneal relaxing incisions. Gills et al.[53] reported anecdotally good undercorrected near as well distance acuity in patients who had more than 2D of preoperative astigmatism. This effect may occur because residual steep areas, found on topography, contribute to a bifocal effect. Good uncorrected distance and near vision has also been noted after standard cataract extraction with residual myopic astigmatism.[55]

Limbal Relaxing Incisions

Limbal relaxing incisions were first described by Gills and Johnson [56] as a useful method to reduce astigmatism. These incisions are believed to produce more homogeneous coupling and less irregular astigmatism, since they are placed far from the center of the cornea. With a diamond blade set at 600?µm, a single 6.0?mm incision placed anterior to the palisades of Vogt corrects up to 1D of astigmatism. For 1–2D of astigmatism, paired incisions placed at the steep axis may be used, and for 2–3D, each incision is extended to 8.0?mm. For astigmatism greater than 3D, the limbal incisions may be combined with corneal relaxing incisions. These incisions may be used to prevent overcorrection, as their effect is weaker than that of corneal relaxing incisions.

FUTURE TRENDS

Until the 21st century, the methods used to evaluate and correct refractive errors were spectacles, contact lenses, IOLs, and refractive surgeries. But these methods allow the measurement and correction of only the spherical and cylindrical components of refractive errors. With the development of wave front sensing, the method of analyzing refraction and quality of vision has changed. Higher-order aberrations are of some importance in this new paradigm. How cataract surgery changes higher-order aberrations in the human eye is still not fully known. Mierdel et al.[57] published the first pilot study to evaluate the effects of cataract surgery on ocular higher-order aberrations. They found that the averaged Zernike coefficients exhibited no significant differences from normal values, except for the coefficient K5 (astigmatism at 0° and 90°). However, coefficients showed a significant high variability, especially the coefficients for spherical aberration or astigmatism.

Cataract surgery is a procedure that may considerably increase the ocular higher-order aberrations. These aberrations are not predictable and can affect visual acuity, despite optimal spherocylindrical correction, especially under mesopic conditions.

 

 

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