Surgical Correction of Astigmatism by Microwedge Resection of the Limbus

SURGICAL CORRECTION OF ASTIGMATISM BY MICROWEDGE RESECTION OF THE LIMBUS RONALD P. JENSEN, MD and

ALEXA C. JENSEN, MD GLENDALE, CALIFORNIA This paper presents a preliminary report on an approach to the surgical correction of corneal astigmatism, ie, the microwedge resection of the cornea, analyzes the quantitative aspects of the surgical treatment, and describes the Jensen double-bladed microsurgical knife designed for making calibrated-uniform microsurgical wedge resections of the cornea. This technique makes correction of even small increments of corneal astigmatism possible.

INTRODUCTION

BARNERl reviewed the following principal surgical approaches employed to correct high degrees of corneal astigmatism: corneal section, thermal cautery, and corneal or scleral lamellar resection with suturing of the wound producing a steepening effect at right angles to the resection. Based on a study of the literature, Barner states "only the crescentic wedge resection technique may have a future in eliminating or diminishing excessive corneal astigmatism."l Initially, Troutman 2 developed the corneal wedge resection for correction of astigmatism postkeratoplasty. As early as 1970, Troutman 2 reported excising a crescentic wedge at the border of the astigmatic graft, 90° of the circumference and approximately 1.5 mm at its greatSubmitted for publication Sept 2, 1978.

est width at the axis across the flattest meridian. The·final effect was a correction averaging 10.3 diopters in five eyes (ranging from 6.5 to 15.0 D).2 Troutman3 outlined the application of this principle in congenital and acquired astigmatism (in cataractous eyes as well), but specified that in actual practice the procedure should be used for correction of high astigmatic errors only. In a group of ten patients, the average astigmatism was 11.4 D (ranging from 6 to 16 D). Postwedge resection results of this group demonstrated astigmatic errors ranging from 2 to 6 D, the average being 2.8 D, which represents a 75% reduction after surgical treatment. Based upon Troutman's3 theoretical calculations, a 0.1 mm resection would achieve a net correction of 1 D, and correspondingly, a 1.0 mm wedge should correct approximately 10 D, and 1.5 mm should correct 15 D. He points out that because of anatomic and elasticity factors and "our present technical inability to cut the cornea precisely to within 0.1 mm," the ultimate correction will vary in individual cases. 3 For correcting smaller amounts of astigmatism, Troutman 4 developed an easier procedure which he called the "relaxing incision." This

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SURGICAL CORRECTION OF ASTIGMATISM

procedure can be performed in .the office with the use of a local topICal anesthesic. A single incision is employed that extends only partially through the cornea; this causes the steeper corneal meridian to elongate and thereby approximately balance out the observed astigmatism. 4 Barraquer5 reported using the semilunar corneal resection (crescent resection) for the surgical correction of congenital astigmatism and noted that at that time there was no surgical means available for resection of the minute amounts of tissue necessary for correcting astigmatic errors of 1.5 D. Barraquer states that "only in a curved cornea with very high astigmatism, we may hope to reduce the ametropia in a useful way."5 Jaffe and Clayman6 gave a statistical analysis of the effect of different types of sutures and suturing techniques on postoperative astigmatism and discussed various clinical applications of these findings in the control of this condition. Jaffe stated that the Troutman wedge resection might be a more effective method of altering postoperative astigmatism. 6 At the present time, the Troutman wedge resection appears to be the most generally accepted and effective method of controlling postoperative astigmatism. However, the application of this procedure ~as been limited mainly to cases WIth a high degree of astigmatic error. The Troutman relaxing technique has been employed to correct lower degrees of astigmatism, but results appear to be less predictable. With the development of the Jensen double-bladed microsurgical

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knife calibrated microsurgical wedge resedtion of the cornea can be performed yielding predictable correction of even small increments of astigmatic error.. MEANS AND METHODS

The Jensen double-bladed microsurgical knife was used to surgically correct corneal astigmatism. The double-bladed knife includes an elongated member that has a first and second end that can be adapted to be hand held by the surgeon. The double-bladed knife also includes a pair of flat-surface blades which may be brought together. The double-bladed knife includes a spacing device for spacing the pair of blades parallel to each other, and is mechanically coupled to the elongated member and an adjusting device that can adjust the distance between the pair of blades. The adjusting device is mechanically coupled to the spacing device, which includes a pair of prongs that are joined together at the end to form the elongated member, each of which is adapted to receive one pair of blades at the other end. The adjusting device includes a screw mechansim, which is disposed perpendicular to the pair of prongs to adjust the spacing between the pair of blades. The spacing can be mechanically set for uniform wedge resection beginning with the smallest wedge width of

.

.....

Fig I.-Jensen astigmatism knife.

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0.1 mm and increasing in mcrements of 0.025 mm (Fig 1). The microwedge resection technique, using the Jensen astigmatism knife, was performed at the time of cataract surgery in eight of nine cases presented and as the primary procedure in the ninth case, a pseudophakic patient with postoperative astigmatic error. A 90 0 double incision was made, centered on the flattest meridian and extending through approximately one-half the thickness of the cornea. Using a Beaver blade, the parallel incisions were V'd micro surgically to the level of the Descemet's membrane. The anterior chamber was entered with a cataract knife, and scissors were used to remove the wedge tissue. With the exception of the one case previously mentioned, the incision was enlarged to approximately 135 0 for an extracapsular lens extraction and lens implantation. The routine extracapsular procedure recommended for cataract extraction and Binkhorst lens implantation was carried out, and the wound was closed with 10-0 nylon or 10-0 prolene interrupted sutures that were placed deeply in the corneal wound but without penetration (Fig 2). RESULTS

Nine cases are presented in which the micro wedge resection technique was performed using the Jensen double-bladed microsurgical knife. The principal effect of the procedure is to steepen the flatter meridian and flatten the steeper meridian. Preoperative astigmatism ranged from 0.37 to 6.25 D, the average being 3.28 D. Microsurgical resections of 0.1 to 0.4 mm were made. Sutures were removed by six months

Fig 2.-Jensen microwedge resection (front view). Double-bla de 90° incision at the surgical limbus centered on fl attest meridian.

postoperatively, and postoperative central peratometric readings were taken at 1 week, 3 weeks, 6 weeks, 3 months, and 6 months (Table 1). In all but one case, astigmatic error was reduced to 1 D or less. The postwedge reading of 2 D in case 8, was taken at six weeks; this correction should approach 1 D after six months. DISCUSSION

Based on the results obtained using the micro wedge resection technique with the double-bladed surgical knife, it is evident that astigmatic error can be corrected in a predictable controlled manner. Even small increments of againstthe-rule astigmatism can be corrected at the time of cataract extraction; whereas, prior to the development of this procedure, the against-the-rule astigmatic error would probably have increased. In an astigmatic eye, it is exceedingly important to attempt to prevent the development of againstthe-rule astigmatism as a complica-

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TABLE 1 CASE RESULTS

CASE NO.

1 2 3 4 5 6 7 8 9

PREOPERATIVE KERATOMETRY

45.00/41.62X97 46.00/43.12X90 44.50/41.00X92 44.75/43.75X90 43.75/46.75x1l5 44.62/44.25X63 46.25/43.25x90 49.50/43.38x90 47.00/40.75x90

AMOUNT OF RESECTION (IN MM)

0.20 0.15 0.20 0.10 0.20 0.10 0.20 0.40 0.40

POSTOPERATIVE KERATOMETRIC READINGS FIRST WK

THIRD WK

42.75/44.50X103 43.75/45.50X100 40.62/44.75x90 42.25/46.00x95 46.00/45.00X105

...

42.00/48.50x90 44.25/46.50X150 37.75/47.50X80

TABLE 1 -

42.75/44.25X1l5 44.12/45.50X90 42.00/44.00x90 43.00/46.50X90 45.50/44.50X1l5 41.50/47.50X90 42.00/48.00x90 45.00/46.75x90 40.25/47.00x77

SIXTH WK

44.00/44.12X75 44.75/44.75x90 42.50/43.25X90 42.62/45.87X90 45.75/45.00X60 41.75/47.38x97 42.75/46.50x73 45.00/47.00X90 41.50/45.25X45

Continued

CASE RESULTS

CASE POSTOPERATIVE KERATOMETRIC READINGS SIXTH MO NO. THIRD MO

1 2 3 4 5 6 7 8 9

... ... 42.50/43.25X100 44.50/45.12x90 45.25/46.83x102 43.25/46.75X110 43.75/45.00X50 * 42.75/44.00x75

44.00/43.38x90 45.00/44.00X83 43.12/42.75X85 44.50/44.50X90 45.50/46.00X110 45.00/44.87X90 43.87/44.00X90 * 43.38/43.38x90

AMOUNT OF ASTIGMATISM PREWEDGE

3.38 2.88 3.50 1.00 3.00 0.37 3.00 6.12* 6.25

POSTWEDGE

0.62 1.00 0.37 0.00 0.50 0.13 0.13 2.00* 0.00

CORRECTION

2.76 1.88 3.13 1.00 2.50 0.24 3.13 8.12* 6.25

·Patient unavailable for follow·up.

tion of the operative procedure. This is believed to be best accomplished by replacing the cornealscleral wound in correct apposition with wound edges sutured tightly together and held until wound healing is secure; this is best accomplished by the overtightening of sutures.

cases presented in the early postoperative phase. In astigmatism correction surgery, the keratometer reading should initially demonstrate a positive cylinder astigmatism in the direction of the tightened stitches. It is not until the eye is healed and the stitches are removed that the final reading is obtained.

It is the overtightening of sutures that accounts for the apparent overcorrection of the astigmatism of the

It is recommended that the minimum amount of resection be 0.10 mm, which yields a correction of

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1 D. Case 6 appears to demonstrate a correction of 0.24 D with a 0.10 mm resection. However, this value is somewhat misleading in that, since an overcorrection was suspected, sutures were loosened early allowing the wound to separate a minute amount resulting in an excellent outcome. The use of this technique is not recommended, as the final result can be inconsistent. Amounts of resection should be calculated pre surgically using the method described in this paper and sutures be left in place for a minimum of four months.

QUANTITATIVE ASPECTS OF THE CORNEAL MICROWEDGE RESECTION

When a circumferential incIsIOn is made in the cornea for the purpose of inserting an intraocular lens, an ideal opportunity is given to correct any preoperative astigmatism using a corneal microwedge resection. This resection is similar to the wedge resection pioneered by Troutman to correct postoperative astigmatism that frequently follows keratoplasty. The main difference is that the corneal micro wedge resection does not have the wedgeshaped cross section discussed by Troutman, because a specialized double-bladed surgical knife that makes a parallel-sided incision either partially (microwedge resection) or entirely (block resection) through the cornea is used. Troutman 3 set forth approximate criteria for determining the maximum width of the crescent-shaped section that was to be removed from the cornea; however, this wedge procedure has generally been limited to patients with reasonably large astigmatism ranging 6 D or more.

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The purpose of the present analysis is to sufficiently quantify the resection technique so it can be used for both small and large astigmatic corrections. To accomplish a precise but small resection requires exceptional dimensional control, which is allowed only by the doublebladed method. The selection of the correct width of the double-bladed incision requires a more exacting analysis than the approximations presented by Troutman. 3 The following is a detailed analysis for the incision width based on Troutman's physical model for the corneal deformation (which we believe to be essentially correct). The analysis leads to the determination of a precise correction, assuming that the cornea is inelastic. Provisions are made to incorporate an elastic correction factor if statistical follow-up data on patients suggest that such a correction is appropriate. Physical Model

An astigmatic cornea has two principal meridians, one of highest dioptric power and one of lowest dioptric power. These principal meridians are orthogonal and can be identified with a keratometer. The microwedge resection is always made to reduce the length of the flatter or low diopter meridian; this tends to increase its curvature (Fig 3, a). A less obvious point that Troutman 3 has previously made, is that removal of the thin corneal section causes the li~bus to distort (Fig 3, b), and thereby reduce the curvature of the orthogonal meridian (Fig 3, c). The analysis proceeds by specifying the length of the principal meridians and their associated cord lengths before and after the re-

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~ ---------- ----~~~r-

~""

~'i-~-

"',

(a) Section A-A

.. ------ ...

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high diopter meridian (H) is increased by the W; so (3) 2rHsinOH + W = 2r~sinO~. However, the length of this meridian remains unchanged by surgical treatment, so that (4) 2rHOH = 2r~O~. Subscript H in these last two equations refers to the high diopter meridian .

L.e

(b)

Fig 3.-Corneal shape before (dotted lines) and afte: .(solid lin~s) the resection. Lowest diopter ~endIan that IS reduced in length by W (a), lImbus (b), and highest diopter meridian whose cord length increases by W resulting from the distortion of limbus (c).

moval of a wedge of width (W). From simple geometrical considerations, the length of the meridian of a circular section is simply 2rO where r is the radius of curvatur~ and 0 is the half angle of the section (Fig 4). The associated cord length in Fig 4 is 2r sinO. When a wedge of width (W) is removed from the low diopter meridian we find, to a good approximation, that both the length of the original meridian and its cord are reduced by this amount. That is (1) 2rLOL - W = 2r~O~ and (2) 2rL sinO - W = 2r~sinO~. In these equations, the subscript L refers to parameters for the low diopter meridian while the prime superscript t) indicates values after the removal of the corneal section. The unprimed rand 0 are the initial values of these parameters before the resection. Because of the limbal distortion (Fig 3, b), the cord of the

Fig 4.-Geometry of cornea showing meridional length of 2re and associated cord length of 2r sine.

The four previous equations can be simultaneously solved for the desired W if the initial diopter values of the high and low meridians and their associated cord lengths are known. The solution is somewhat complicated because the equations are transcendental in nature and must be solved by a proGedure of succe~sive approximations. Once a solution for W is found, it is straightforward to verify its validity by direct substitution into the previous equations. A correct value for W should always result in an equality between rL and rH; that is, the final diopter values of the principal meridians should be equal after surgical treatment. Example Calculation

Let us assume that the principal meridians have been found by keratometer readings to have values of 40 and 44 D. These D values can be

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converted into radii of curvature using the simple relationship 337.4 (5) rL = 40 = 8.435 mm and 337.4 (6) rH = 44 = 7.67 mm. Let us further assume that both cords associated with these principal meridians are initially 12 mm long so that the low diopter cord length = 2rLsin(h = 12 mm and high diopter cord length = 2rHsinOH = 12 mm. The previous two equations can be simultaneously solved for the two unknowns, Land H, by substituting the values of rL and rH determined previously. One finds that (7) OL = 45.34° and (8) OH = 51.49°. By a procedure of successive approximations, it has been determined that for this example (9) W = 0.22 mm. Although the procedure for the solution is somewhat complicated, as well as tedious, the correctness of this value for W can be verified by straightforward substitution of equations 5 through 9 into equations 1 through 4. This results in (10) 2rLOL - W = 13.13 mm - 2r'LO~, (11) 2rLsinOL - W = 11.78 mm = 2r~sinO~, (12) 2rHsinOH + W = 12.22 mm - 2r~sinO~, and (13) 2rHOH = 13.78 mm = 2r~O~. Equations 10 and 11 have a selfconsistent solution of r~ = 8.23 mm and O~ = 45.7°, while equations 12 and 13 lead to r~ = 8.225 mm and O~ = 48.0°. The fact that ri. and r~ are essentially equal confirms the correctness of the above choice of W. The final focusing power of both principal meridians will be 337.4 _ 337.4 - 41 D r' - 8.23Comparison with Troutman's Approximation for W Troutman 3 indicated that a reasonable approximation for the re-

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section width was 0.1 mm/D difference. For the previous example where the astigmatism was 4 D, this would imply an approximate resection width of 0.4 mm, which is almost twice as large as the exact value that was calculated to be 0.22 mm in the example calculation section. As the magnitude of the astigmatism increases, Troutman's approximation,3 when applied to the basic block resection, becomes better but generally tends toward ov,ercorrection of the asigmatism. In actuality, there may not be as great a discrepancy between Troutman's approximation and our exact calculation if W is interpreted to be the average width of the incision. Since Troutman employs a wedgeshaped incision, its average width is exactly one half of the maximum width at the exterior of the cornea. One half of 0.4 mm is 0.2 mm, which compares quite favorably with the exact calculated value of 0.22 mm. Because we do not employ a simple wedge-shaped incision, we will discuss correction factors for our incision shapes.

Practical Determination of the Resection Width A series of calculations for the resection width using the formulas developed in the physical model section are expressed in Fig 5. The physician locates on the abscissa of this graph the measured value of the patient's astigmatism, then traces directly upward to the intersection of the curve labeled with the lowest dioptric power for the patient. From this point, the resection width can be found by translating horizontally to the ordinate, which is a direct measure of the resection width in millimeters.

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1.1

1.0 0 .9 0 .8 --:0 .7 ~ ~

- 0.6 I

~

a

0 .5

3: 0.4 Z

o-

~

0.3

u

~0.2

w a::

FOR 12 MM DIAMETER LIMBUS

0.1

o

2

3

4

5

6

ASTIGMATISM

7

8

9

10

II

12

( DIOPTERS)

Fig 5.-Resection width vs astigma tism for three different values of lowest diopter meridia n . Astigmatism is differ ence between hi ghest and lowest dioptric powers of initial cornea.

In developing Fig 5, the assumption was made that the principal cords of the limbus were both 12 mm long. Further calculations have revealed that if the average diameter of the limbus is less than 12 mm, the resection width must be decreased somewhat, while the opposite is true for larger limbal diameters. A simple factor to correct the resection width extracted from Fig 5, for variations in limbal diameter, is listed in Table 2. Another important correction must be made to the value of W obtained

TABLE

2

VARIATIONS IN LIMBAL DIAMETER

AVERAGE DIAMETER OF LIMBUS (MM)

CORRECTION FACTOR (MULTIPLY THE VALUE OF W OBTAINED IN FIG 3 BY THIS AMOUNT)

11 12 13

0.94 1.0 1.08

from Fig 5 if the double-bladed incision does not have straight parallel walls penetrating entirely through the cornea. By definition,

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this incision is the compound or microwedge incision as differentiated from the simple block incision. In the section on surgical procedure, the microwedge resection technique was employed in all cases presented. The double-bladed incision extended only about one half the way through the cornea, followed by a "V" shaped cut that penetrated the remaining distance through the cornea. The average width of such a compound incision is only 75% of the double-blade width. To correct for this factor, the double-blade spacing had to be increased one third over the width obtained from Fig 5. Alternatively, the value of W obtained from Fig 5 should be multiplied by a second correction factor that is listed in Table 3. TABLE

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developed by Troutman 3 is shown. Its value of 2.0 corrects for the fact that the average width is only one half of the opening at the top. CORRECTION FOR CORNEAL ELASTICITY

The assumption that the cornea is inelastic was implicit in the analysis of the corneal deformation in the previous sections. That is, the elastic deformations of the cornea associated with the resection were assumed to be much smaller than· the section thickness. This appears to be a reasonable assumption; however, in the event that the calculated values for wedge thickness consistently under or over correct for the observed astigma3

VARIATIONS IN INCISION SHAPE

INCISION SHAPE

Block Wedge Compound microwedge

~ ~. ~~ .~

~~/

Since it is difficult to control the exact profile of the compound microwedge incision, we have switched to the better defined parallel walled block incision in our more recent work. In this case, the correction factor is unity and the values for W obtained from Fig 5 can be directly employed. In Table 3, for the purpose of comparison, the correction factor for the wedge incision

CORRECTION FACTOR (MULTIPLY THE VALUE OF W OBTAINED IN FIG 3 BY THIS AMOUNT)

1.0

2.0 1.33

tism, it is recommended that an empirical elasticity correction factor be applied to W in addition to the two factors previously discussed. For example, if the values of wedge thickness determined in this section result in a statistically consistent overcorrection in the astigmatism (ie, 25%), then the value of W should be reduced by multiplying it by an elasticity correction

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factor (ie, 0.80). Developing such an empirical correction factor may take several years or more of careful collection and analyses of postoperative data.

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Application of Analysis to Cases

ing was 44.25 in case 6. The W should be 0.015x1.33 = 0.020 mm (Fig 5). The actual incision was 0.1 mm, which resulted in an llndercorrection of 0.13 D. (We would have predicted an overcorrection of approximately 1 D.)

In case 1, the observed astigmatism was 3.38 D and the lowest diopter reading 41.62. From Fig 5, W should be 0.18X1.33 = 0.24 mm. The actual incision was 0.2 mm, and the actual correction was 0.62 D under value, which was qualitatively consistent with the analysis.

In case 7, the observed astigmatism was 3 D and the lowest diopter reading was 43.25. The W should be 0.16x 1.33 = 0.21 mm (Fig 5). The actual incision was identical to the calculated value of 0.2 mm, and results were consistent with the analysis.

In case 2, the observed astigmatism was 2.88 D with the lowest diopter reading being 43.12. From Fig 5, W should be 0.15x1.33 = 0.2 mm. The actual incision was 0.15 mm with a resulting undercorrection of 1 D.

The observed astigmatism in case 8 was 6.12 D and the lowest diopter reading 43.38. The W should be 0.40X1.33 = 0.53 mm (Fig 5). The actual incision was 0.4 mm. After five weeks the astigmatism appeared to be overcorrected by 2 D; however, the analysis would suggest that after six months the astigmatism would be undercorrected by approximately 1 D.

The observed astigmatism was 3.5 D with the lowest diopter reading being 41 in case 3. From Fig 5, W should be 0.19X1.33 = 0.25 mm. The actual incision was 0.2 mm, and the correction was 0.37 D under value, which was qualitatively consistent with the analysis. The observed astigmatism in case 4 was 1 D with the lowest diopter being 43.75. From Fig 5, W should be 0.04X1.33 = 0.053 mm. The actual incision was 0.1 mm resulting in an exact balance. (We would have predicted an overcorrection of 1 D.) Case 5 had an observed astigmatism of 3 D with the lowest diopter reading being 43.75. The W should be 0.18x1.33 = 0.24 mm (Fig 5). The actual incision was 0.2 mm resulting in an undercorrection of 0.5 D. The observed astigmatism was 0.37 D and the lowest diopter read-

In case 9, the observed astigmatism was 6.25 D and the lowest diopter reading 40.75. The W should be 0.3x 1.33 = 0.4 mm (Fig 5). The actual incision was identical to the calculated value of 0.4 mm, and the correction was exact. CONCLUSION This paper presented the microwedge resection technique, whereby even small increments of astigmatic error can be corrected at the time of cataract surgery. The use of the Jensen double-bladed microsurgical knife, which can provide a means for making controlled uniform amounts of resection from 0.1 mm increasing in 0.025 mm increments of wedge thickness, was

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introduced. Finally, a quantitative analysis of the surgical treatment has been given, and the theoretic findings have been applied to the actual results obtained in the nine cases presented.

ACKNOWLEDGMENT The authors acknowledge the assistance of Douglas A. Pinnow, PhD, with the mathematical analyses, and W. Edward Johansen, MS, JD, with ancillary services in the research and development of the Jensen double-bladed knife.

Key Words: Astigmatism; microwedge resection; double-bladed microsurgical knife; Jensen astigmatism knife.

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REFERENCES 1. Barner SS: Surgical treatment of corneal astigmatism. Ophthalmic Surg 7:43-48, 1967.

2. Troutman RC: III Symposium on the International Ophthalmic Microsurgery Study Group, Mexico. S. Basel, Karger, 1970. 3. ___: Microsurgical control of corneal astigmatism in cataract and keratoplasty. Trans Am Acad Ophthalmol Otolaryngol 77:0P-563-0P-572, 1973. 4. _ _ _: Microsurgery of the Anterior Segment of the Eyes. St Louis, CV Mosby Co, 1977, vol 2, p 274.

5. Barraquer J, in discussion of Pierse D, Kersley HJ (eds): Microsurgery of cataract, vitreous, and astigmatism. Adu Ophthalmol 33:217-221, 1976. 6. Jaffe NS, Clayman HM: The pathophysiology of corneal astigmatism after cataract extraction. Trans Am Acad Ophthalmol Otolaryngol 79:0P-615-0P-630, 1975.