Astigmatic keratotomy effect of single-hinge, clear corneal incisions using various preincision lengths

Astigmatic keratotomy effect of single-hinge, clear corneal incisions using various preincision lengths Alfredo Amigo, MD, PhD, Arthur W. Giebel, MD, Jose A. Muifios, MD

ABSTRACT Purpose: To investigate the astigmatic keratotomy effect of a modified cataract incision .

s~ng l e-h i nge

Setting: Department of Ophthalmology, San Juan de Dios Hospital , Tenerife, Spain . Methods: This prospective study evaluated the astigmatic changes resulting from preincision grooves of less than 40, 45, and 55 degrees in arc length used with a single-hinge, self-sealing cataract incision in 144 eyes that had against-the-rule corneal astigmatism preoperatively. The intended 90% depth preincision was centered on the steep meridian and lengthened according to the amount of preoperative astigmatism. Outcome measurements were obtained by calculating the surgically induced astigmatism vectors and the postoperative keratometry changes 1 day, 1 week, and 1, 3, and 6 months after surgery. Results: Three months postoperatively, astigmatism decreased by 0.03, 0.30, and 0.68 diopter in the less than 40, 45, and 55 degree incision length groups, respectively. The differences between the 45 and 55 degree groups and the less than 40 degree group was statistically significant (P < .05) . Similar results were observed 6 months after surgery. Conclusion: The astigmatic preincision modification produced statistically significant increasing reductions in preoperative astigmatism according to preincision length. J Cataract Refract Surg 1998; 24:765-771

T

he use of anastigmatic incisions does not control postoperative astigmatism in eyes with significant preoperative astigmatism. One way to reduce preexisting corneal astigmatism is to use the small keratotomy effect of some cataract incisions by placing them along the steep corneal meridian. 1•2 Kohnen et al. found a statistically significant reduction (0.19 diopter [D]) in Presented in part at the XIVth Congress of the European Society of Cataract & Refractive Surgeons, Gothenburg, Sweden, October 1996 Reprint requests to Alfredo Amigo, MD, PhD, Marina 7, of41, 38002 Tenerife, Spain.

pre-existing corneal astigmatism by centering 3.0 to 4.0 mm wide clear corneal incisions on the steep axis ("Change in Astigmatism with Small Cataract Incisions Placed On or Off the Steep Meridian," presented at the XIVth Congress of the European Society of Cataract & Refractive Surgeons, Gothenburg, Sweden, October 1996) . However, this small keratotomy effect is insufficient for more significant pre-existing astigmatism. This study evaluated the safety and efficacy of reducing pre-existing corneal astigmatism during cataract surgery using a modification of the preincision groove length. 3

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Patients and Methods This prospective study evaluated the astigmatic changes in 144 consecutive eyes with pre-existing againstthe-rule (ATR) corneal astigmatism (steepest meridian at 180 ± 30 degrees) having phacoemulsification using a self-sealing, temporal, clear corneal, single-hinge incision. To study the effect of different groove (preincision) lengths in the single-hinge incisions, eyes were divided into three groups based on preoperative corneal astigmatism (Table 1). Mean age of the patients was 67.5 years ± 11.3 (SO); 57% of eyes were in women, and 54% were right eyes. To obtain the preincision lengths to be used in surgery, chord lengths of the 45 and 55 degree arcs were calculated for various corneal diameters using the following formula: L

= r h(l - cosA)

where L = chord length of the preincision in millimeters; r = corneal radius along the surgical (steep)

Table 1. Study groups according to pre-existing ATR corneal astigmatism.

Group 1*

Group 2

Group 3

Pre-existing corneal astigmatism (D)

< 0.5

0.50 to 1.25

>1.25

Preincision length (degrees of arc)

< 40t

45

55

53

30

21

Characteristic

Number of eyes *Control group

t3.4 mm

meridian in millimeters (i.e., one half the corneal diameter); A = the angular size of the preincision in degrees of arc. Table 2 was created and used intraoperatively as a reference. For example, the table shows that when a steep corneal meridian (corneal diameter in the steep meridian) is 11.5 mm long, the chord length of a 45 degree preincision is 4.4 mm. All surgeries were performed by one surgeon (A.A.) using peribulbar anesthesia. The surgical meridian (steep axis) was identified with a Mendez marker (#9-700, Duckworth & Kent, Ltd.) and marked with a Sinskey hook (Huco #4.6701, Huber & Co.) with the patient lying down; temporal limbal asymmetry was used as a reference. In eyes without obvious temporal limbal asymmetry or with high preoperative corneal astigmatism, the 3 and 9 o'clock positions were marked with a pen with the patient seated and after topical anesthesia was administered. The corneal diameter was measured at the surgical meridian with a caliper (#9-650, Duckworth & Kent, Ltd.). The intended preincision length in millimeters (from Table 2) was then marked with the caliper centered on the initial surgical meridian mark. After ultrasonic pachymetry (Pocket, B.V. International) of the peripheral "limbal" cornea at the incision site, a square, trifaceted diamond knife with a calibrated micrometer and footplates (Huco, Huber & Co.) was set to 90% of the pachymetry reading and used to make the preincision groove. The groove was placed in clear cornea, just anterior to the insertion of the conjunctiva, and made to the appropriate length, derived from Tables 1 and 2.

Table 2. Preincision lengths at various corneal diameters. Arc Length (Degrees)

Corneal Diameter (mm) at Incision Meridian

9.0

9.5

10.0

10.5

11.0

11.5

12.0

12.5

3.4

30 35 40

3.5

3.6

3.8

3.9

3.4

3.6

3.8

3.9

4.1

4.3

4.4

45

3.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

50

3.8

4.0

4.2

4.4

4.6

4.9

5.1

5.3

5.5

55

4.2

4.4

4.6

4.8

5.1

5.3

5.5

5.8

6.0

60

4.5

4.8

5.0

5.3

5.5

5.8

6.0

6.3

6.5

Incisions smaller than 3.4 mm left blank

766

13.0

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After a hinge incision was made using a modification of a previously described technique, 4 a 3.4 mm wide and approximately 2.0 mm long corneal tunnel was dissected from half-depth stroma to a deeper level using a 2.0 mm wide crescent blade (#8065-940002, Alcon Surgical, Inc.). A stab incision (paracentesis) was made 75 degrees to the left of the surgical meridian with a 1.4 mm, 20 gauge lancet (#8065-912001, Alcon Surgical, Inc.). After the anterior chamber and lamellar dissection were filled with viscoelastic, a 3.2 mm keratome was used to angle down and enter the anterior chamber through the lubricated tunnel. To improve wound stabilization when a 45 or 55 degree preincision length was used, the corneal tunnel was placed at the edge of the preincision closest to the 180 degree axis (Figure 1). In cases in which the surgical meridian was centered on the 180 degree axis, the tunnel was placed on the right side of the preincision. Next, continuous curvilinear capsulorhexis, nuclear hydrodissection, and phacoemulsification were performed. After cortical cleanup, the capsular bag was inflated with viscoelastic and the wound enlarged to 3.4 mm with the crescent blade. A three-piece, foldable, silicone intraocular lens (IOL) (SI-30NB PhacoFlex® II, Allergan) was inserted into the bag with a folder forceps (Fine II, Rhein Medical). After the residual viscoelastic was aspirated with the irrigation/ aspiration handpiece, the anterior chamber was deepened with balanced salt solution through the paracentesis. The wound was checked for watertightness by drying and pressing its scleral side with a cellulose spear. The wound was sutured with a single radial 10-0 nylon stitch only when the eye appeared soft, had a shallow chamber, or showed obvious leakage. All eyes were patched until they were examined the following day.

Manual keratometry was performed preoperatively and 1 day, 1 week, and 1, 3, and 6 months postoperatively by one of three physicians (A.A., A.W.G., J.A.M.) using one of two Javal-Schiotz keratometers. The keratometers were calibrated before the study. Surgically induced astigmatism (SIA) was calculated by vector analysis using Jaffe's method. 5 The arithmetic difference between preoperative and postoperative keratometric astigmatism at each follow-up was also calculated to determine the efficacy of the procedure in reducing pre-existing astigmatism. A standard analysis of variance was done; a P-value of 0.05 was considered significant.

Results Of the initial group of 144 eyes, 27 were excluded because they had an acrylic lens implanted through a larger 4.0 mm tunnel (n = 21) or intraoperative problems that led to wound modifications (n = 6). An acrylic lens (AcrySof® MA30BA, Alcon Surgical, Inc.) was used in diabetic and young eyes and in those with pseudoexfoliation or a history of inflammation. Thirteen of the remaining 117 eyes were excluded because they received a radial suture, leaving 104 eyes for the astigmatism analysis (Table 1). The mean SIA in all three groups, as determined by vector analysis using the Jaffe method, is shown in Tables 3 to 5 and Figure 2. There was a statistical difference (P < .05) between the 45 degree preincision

Diopters 2.5 2

1.5

0

50

100

1--+- <40 - - 45" - • Figure 1. (Amigo) Astigmatic preincision; the corneal tunnel is placed at the preincision edge to improve intraoperative wound stability.

150

sso 1

Figure 2. (Amigo) Surgically induced astigmatism (vectors) by preincision length.

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Table 3.

Induced astigmatism by vector analysis and induced axis in the control group (preincisions less than 40 degrees or 3.4 mm;

n =53). Postop Interval

1 day

1 week

1 month

3 months

6 months

WTR

~61

Table 4.

Postop Interval

1 day

1 week

1 month

3 months

6 months

WTR

~61

Induced Axis (ATR) Number of Eyes

Induced Axis (WTR) Number of Eyes

0.67 ± 0.48

27

8

5

(0.04-2.18}

(67.5)

(20.0)

(12.5}

0.65 ± 0.41

30

6

12

(0.04-1.97}

(62.5)

(12.5)

(25.0)

0.53 ± 0.39

30

11

10

(0.02-1. 75}

(58.8)

(21.6)

(19.6)

0.43 ± 0.31

24

13

9

(0.01-1.65)

(52.2)

(28.3)

(19.6)

(%)

(%)

(%)

48

51

46

11

4

3

(0.05-0.74)

(61.1}

(22.2}

(16.7)

and ,;;120 degrees; oblique

~31

and ,;;60 or

~121

and ,;;150 degrees; ATR >0 and ,;;30 or

Total Eyes

40

0.30 ± 0.23

18

and ,;;210 degrees

~151

Induced astigmatism by vector analysis and induced axis in the 45 degree preincision group (n = 30). Induced Axis (ATR) Number of Eyes

Induced Astigmatism (D) Mean ± SO (Range)

Induced Axis (WTR) Number of Eyes

Induced Axis (Oblique) Number of Eyes

(%)

(%)

1.25 ± 0.69

15

3

(0.11-2.50)

(78.9)

(15.8}

(5.3)

1.30 ± 0.97

19

5

3

(0.05-3.89)

(70.4)

(18.5)

(11.1)

0.98 ± 0.64

18

5

4

(0.12-2.27)

(66.7)

(18.5)

(14.8)

0.78 ± 0.49

18

6

4

(0.01-1.83}

(64.3)

(21.4}

(14.3)

0.62 ± 0.31

14

2

(0.04-0.91)

(82.4}

(11.8)

and ,;;120 degrees; oblique

~31

and ,;;60 or

~121

and ,;;150 degrees; ATR >0 and ,;;30 or

length group and the control group (less than 40 degree length) at every postoperative visit except 6 months, at which time there was a drop in follow-up for both groups. The difference between the 55 degree preincision group and the control group was statistically significant at all follow-ups. Mean keratometric astigmatism in the control group remained statistically unchanged (Table 6, Figure 3). The 45 degree group had a statistically significant reduction in keratometric astigmatism over the control group at 3 months. The 55 degree group had a 768

Induced Axis (Oblique) Number of Eyes

Induced Astigmatism (D) Mean ± SO (Range)

(%)

Total Eyes

19

27

27

28

17 (5.9) ~151

and ,;;210 degrees

statistically significant reduction in keratometric astigmatism over the other two groups that persisted from the first week follow-up to the end of the study. Early in the study, it was noticed that placing the corneal tunnel at an edge of the enlarged preincision resulted in better wound stability during surgery (Figure 1). Other than a microstep at the wound edge that occasionally appeared more prominent in the larger preincisions, there were no other wound-related surgical complications from the enlarged preincisions. There was one preincision perforation caused by an incor-

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Table 5. Induced astigmatism by vector analysis and induced axis in the 55 degree preincision group (n Postop Interval

Induced Astigmatism (D) Mean ± SD (Range)

1 day

1 week

1 month

3 months

6 months

WTR

~61

Induced Axis (WTR) Number of Eyes (%)

Induced Axis (Oblique) Number of Eyes (%)

Total Eyes

14

2.13 ± 1.16

14

0

(0.35-3.71)

(100)

(0.0)

1.69 ± 0.95

17

2

(0.04-3.59)

(89.5)

(10.5)

1.47 ± 0.84

16

3

(0.39-3.13)

(84.2)

(15.8)

1.26 ± 0.62

16

3

(0.40-2.22)

(84.2)

(15.8)

1.22 ± 0.44

10

4

(0.40-1.88)

(71.4)

(28.6)

and o:;120 degrees; oblique

~31

and o:;6Q or

~121

19

19

19

4

and o5150 degrees; no ATR astigmatism was induced

reedy set diamond knife, and there was one case of endophthalmitis in the control group. However, both cases had been previously excluded from analysis because the wounds had been sutured. There were no cases oflate wound instability requiring a suture and no other cases of late endophthalmitis. Mean corneal diameter at the surgical meridian was 11.7 ± 0.47 mm (range 10.5 to 13.0 mm). Mean pachymetry at the temporal incision sites was 736 + 49 J.llll (range 630 to 850 J.llll).

single-hinge incision described by Langerman. 4 We chose this incision because of its improved stabilicy6 and because it uses a deep preincision that has the same architectural characteristics as a transverse relaxing Table 6. Mean reduction in pre-existing corneal astigmatism by preincision length. Prelnclslon Length < 40 Degrees (3.4 mm)

45 Degrees

55 Degrees

Astigmatism (D) -0.09 ± 0.57

-0.30 ± 0.65

-0.16 ± 0.86

19

14

Postop Interval

1 day

Discussion To increase the keratotomy effect of our temporal approach, we modified the self-sealing, corneal tunnel,

Number of Eyes

40

1 week Astigmatism (D) -0.09 ± 0.46

Diopters

Number of Eyes

0.7 - r - - - - - - - - - - - - - -

Astigmatism (D) -0.05 ± 0.46 Number of Eyes

0.4-t-----0.3 t---..::-r-----

50

Astigmatism (D) -0.03 ± 0.31

0.1

Number of Eyes

0

1week

-0.10 ± 0.83 -0.46 ± 0.46* 27

19

-0.24 ± 0.65 -0.56 ± 0.55t 27

19

3 months

t--t'·~---

1 day

48

1 month

+-----------;FW--0.5 t - - - - - - - - - 0.6

0.2

= 21).

1 month

3 months

8 months

46

-0.31 ± 0.49t -0.68 ± 0.49t 28

19

6 months Astigmatism (D) -0.03 ± 0.26 -0.35 ± 0.34* -0.61 ± 0.42*

10<40°

Number of Eyes

Figure 3. (Amigo) Mean reduction in preoperative astigmatism by preincision length (*significant difference compared with <40° group, P < .05; ** significant difference compared with <40° group, P < .01 ).

36

17

14

*Significant difference compared with eyes in <40 degree group (P< .05) tSignificant difference compared with eyes in <40 degree group (P< .01)

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keratotomy. Both are very deep corneal incisions of a specific length placed at a certain distance from the center of the cornea. In his original description, 4 Langerman showed his incision had no effect on corneal astigmatism, suggesting a poor relaxing effect on the surgical meridian. Like other anastigmatic clear corneal designs, the Langerman incision does not claim to have a refractive effect. Thus, it does not include refractive incisional parameters such as accurate incision depth. However, the predictability of a relaxing incision strongly depends on the accuracy of certain parameters such as precise depth, exact length, and the distance to the corneal center or optical zone. In our study, we chose to control these three factors to predictably obtain the desired refractive effect. We deepened the preincision groove by setting the micrometer at 90% of the corneal thickness based on intraoperative pachymetry. The wide range of pachymetry values in our study (from 630 to 850 f.Ull) strongly supports the importance of this measurement for safety and predictability. We used a square diamond micrometer knife designed for astigmatism surgery to obtain a precise and constant depth. To minimize the variation that different corneal diameters could have on the effective optical zone size, we expressed the preincision length in degrees of arc. Our corneal diameters ranged from 10.5 to 13.0 mm, which shows the importance of this measurement in extreme cases. To illustrate, a preincision of 55 degree arc length would translate into a 4.8 mm chord length for a 10.5 mm cornea and a 6.0 mm chord length for a 13.0 mm cornea (Table 2). This represents a clear difference when trying to compare the effect of limbal keratotomies. Despite these considerations, vector analysis showed that a preincision length smaller than 40 degrees, or the 3.4 mm used in our control group, behaved anastigmatically and induced a mean astigmatism comparable to that in other studies using similar temporal clear corneal incisions without a deep preincision.?-9 The mean effect on the pre-existing corneal astigmatism was practically nil in this group. There are several approaches to augment the keratotomy effect of the small clear corneal incision. One is to use a smaller optical zone by placing the incision closer to the corneal center, an approach used by 770

Kershner in his keratolenticuloplasty technique. 10 However, there is some concern about the endothelial risk with this technique because the phaco tip is placed closer to the corneal center. 11 •12 Inspired by Kershner's keratolenticuloplasty design (corneal tunnel functioning as an astigmatic keratotomy), we chose another well-known variable of astigmatic refractive surgery: incision length. ModifYing this parameter, however, presents its own restrictions as serious complications have been correlated with an increase in the length of even self-sealing corneal tunnels.U· 14 Thus, we modified only the length of the preincision and tried to keep the tunnel at the accepted safe dimensions of 3.4 X 2.0 mm. 6 Finally, we placed the incision close to the temporal limbus at the maximum distance from the central endothelium. For improved wound stability, the corneal tunnel and the deep groove were created at different levels in the corneal stroma according to Langerman's description.4 This single-hinge design is another difference from Kershner's technique, 10 in which a tunnel is created at the groove's bottom. To clearly differentiate and accurately describe the incision characteristics in this study, we suggest the following, albeit cumbersome, term: self-sealing, dear-corneal tunnel, singlehinge incision with 90% depth limbal preincision of variable arc length or astigmatic preincision. With the preincision arc length increased to 45 degrees, there was a statistically significant increase in mean SIA over that in the control group (less than 40 degree length); when the length was increased to 55 degrees, the increase in SIA was significantly greater than in the other two groups. This proportional increase in induced astigmatism resulted in a proportional mean reduction in pre-existing corneal astigmatism. It appears as though limbal keratotomies (preincisions), at least in the horizontal meridian, must exceed a minimum length to have a consistent relaxing effect. Shorter preincisions (i.e., 3.4 mm or less than 40 degrees), regardless of depth or other variations in intended corneal tunnel dimensions induced by technique or surgical maneuvers, behaved anastigmatically in our hands. At first glance, there may appear to be a disparity between the values calculated by vector analysis and those found by arithmetic subtraction. This disparity highlights the importance of using both methods to

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evaluate astigmatic data. Although induced astigmatism was greatest during the first week postoperatively, the mean reduction in pre-existing corneal astigmatism was minimal. This paradox is explained by the large percentage of early overcorrections that resulted in little "net" reduction in pre-existing corneal astigmatism. This was particularly noticeable in the longest preincision (55 degree) group. However, as the induced astigmatism regressed over the first month and the number of overcorrections decreased, a higher mean reduction in astigmatism occurred. The ATR induced axis seen in some cases in the less than 40 and 45 degree groups seems to contradict any temporal incision relaxing effect. This highlights the limits of accuracy and the measurement error in using the Javal-Schiotz manual keratometer to pick up subtle changes (less than 0.2 D) in what is or is close to an anastigmatic incision. The amount of induced astigmatism in the 55 degree group far outweighed any measurement error, resulting in no cases of apparent ATR induced axis. The mean reduction effect obtained was low and would be inadequate in eyes with more significant preexisting corneal astigmatism. However, in the senile cataract population, only 5% of patients have preexisting corneal astigmatism greater than 2.5 D, and somewhere between 36 and 45% of patients have preexisting corneal astigmatism greater than 1.0 D.' 5 Therefore, a wound architecture that reduces up to 1.5 D of astigmatism combined with an appropriate IOL can provide 95% of senile cataract cases with postoperative astigmatism of less than 1.0 D. In conclusion, this wound modification is another tool to bring us closer to the objective of every surgery for senile cataract: satisfactory uncorrected vision.

3. Amigo A, Muifios JA. Correlaci6n entre longitud de preincisi6n y astigmatismo inducido en la facoemulsificaci6n por cornea clara. Microcirugfa Ocular 1997; 5(1):65-70 4. Langerman DW Architectural design of a self-sealing corneal tunnel, single-hinge incision. J Cataract Refract Surg 1994; 20:84-88 5. Jaffe NS, Clayman HM. The pathophysiology of corneal astigmatism after cataract extraction. Trans Am Acad Ophthalmol Otolaryngol1975; 79:0P615-0P630 6. Ernest PH, Fenzl R, Lavery KT, Sensoli A. Relative stability of dear corneal incisions in a cadaver model. J Cataract Refract Surg 1995; 21:39-42 7. Kohnen T, Dick B, Jacobi KW Comparison of the induced astigmatism after temporal dear corneal tunnel incisions of different sizes. J Cataract Refract Surg 1995; 21:417-424 8. Pfleger T, Skorpik C, Menapace R, et al. Long-term course of induced astigmatism after dear corneal incision cataract surgery. J Cataract Refract Surg 1996; 22:72-77 9. Long DA, Monica LM. A prospective evaluation of corneal curvature changes with 3.0- to 3.5-mm corneal tunnel phacoemulsification. Ophthalmology 1996; 103:226-232 10. Kershner RM. Keratolenticuloplasty: arcuate keratotomy for cataract surgery and astigmatism. J Cataract Refract Surg 1995; 21:274-277; correction p 488 11. Masket S. Arcuate keratotomy for cataract surgery and astigmatism (letter). J Cataract Refract Surg 1995; 21:597 12. Fine IH, Hoffman RS. Refractive aspects of cataract surgery. Curr Opin Ophthalmol 1996; 7:21-25 13. Menapace R. Delayed iris prolapse with unsutured 5.1 mm dear corneal incisions. J Cataract Refract Surg 1995; 21:353-357 14. Davis PL. PMMA implants via temporal dear corneal incisions: concern replaces confidence. Eur J Implant Refract Surg 1994; 6:205-210 15. Sanders DR, Grabow HB, Shepherd J, Raanan MG. STAAR AA 4203T toric silicone IOL. In: Martin RG, Gills JP, Sanders DR, eds, Foldable Intraocular Lenses. Thorofare, NJ, Slack, 1993; 237-250

References 1. Koch DD, Lindstrom RL. Controlling astigmatism in cataract surgery. Semin Ophthalmol1992; 7:224-233 2. Vass C, Menapace R, Amon M, et al. Batch-by-batch analysis of topographic changes induced by sutured and sutureless dear corneal incisions. J Cataract Refract Surg 1996; 22:324-330

None of the authors has a proprietary or financial interest in any material or method mentioned. The Department of Ophthalmology, Hospital San juan de Dios, Tenerife, Spain, and Rita Giebel Tustin, California, USA, helped prepare the study.

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