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Keratorefractive aspects of the scleral pocket incision and closure method for cataract surgery
Keratorefractive aspects of the scleral pocket incision and closure method for cataract surgery Samuel Masket, M.D.
B TRA \I CUlT nt cataract inci 'ion induce tran ient and permanent chan e in corneal a tigmati. m, 'f picall a h o-pha. e a tigmalic I'e. ponse i. ob 'el' ed· an initial, ith-the-I'ule han e is folio, db an e entual and permanent again t-the-l'lIle hift from pI' per'ati e a tigmati m, The earliel'literatul'e uggests that the ma nitude of the net a tigmatic s' in appro imate. 6.0 diopter fOl' hu'ge limbal ,ound closed, ith intelTupted 'utUl'e, Ho' e er appl ing the t n ts of kenltorefracti e sur el' to the catal'act inci ion and it · clo 'ure allo\ , the ur eon to limit po top I'ati e iatro enic a. ti matic ., ing , The reduced phaco mul ification inci ion ize in combination , ith a cleral pocket clo ed , ith a continuou in Ie knott d 10-0 monofilament n Ion , uture under tonometr'ic and keratometric control ignificantly dampen ' the change. in corneal a tigmati 'm durin the earl and late po 'topel'ati e period ,~1 publi h d I'eport a aluated in the pr nt tud, I' al that the net a ti matic S' in ma be reduc d to les ' than 1.5 diopter. ther b affol'din rapid and table optical re ult ,
Ke
Word
against-th -rlll appo~itional Slit.!!!:.. c\oslIr " cataract utur' c\o. III' ~ intraop ' rath
Control of iatrogenic changes in postoperative astigmatism remains a challenge to the cataract surgeon . Since 90% of cataract patients have less than 2.0 diopters (D) of corneal astigmatism prior to surgery, which they tolerate well, the majority of them would benefit by experiencing no change in astigmatism after surgery. Unfortunately, however, all current cataract incisions induce transient and permanent changes in corneal astigmatism. 2 Typically, early after surgery, a with-the-rule astigmatic change is observed; this is followed by an against-the-rule astigmatic decay,l-6 The initial with-the-rule change appears to be caused by suture-induced wound compression, and the against-the-rule drift, by eventual subclinical wound
ectasia. 2 ,6,' Since there are two operant astigmatic changes, the challenge to the surgeon is to induce a minimal initial with-the-rule change while providing a stable wound environment that precludes a profound and prolonged against-the-rule astigmatic decay. The traditional large limbal wound (Figure 1) closed with interrupted sutures is known to exhibit large astigmatic changes postoperatively.3,4,5 Typically, an initial 4.0 D with-the-rule change from presurgical astigmatism is noted during the early postoperative phase; a subsequent astigmatic decay results in a 2.0 D against-the-rule change from the preoperative cylinder (Figure 2), the result of a net 6.0 D postsurgical astigmatic swing. 2 For modern cataract management,
Presented in part at the Symposium on Cataract , IOL and Refractive Surgery, Los Angeles, March 1988. Reprint requests to Samuel Masket , M.D ., Suite 204, 7230 Medical Center Drive, Canoga Park, California 91307-1957. 70
J CATARACT REFRACT SURG-VOL 15. JANUARY 1989
.6
WITH
RULE
+4
.2
..................... -2
AGAINST
WEEKS
8 ..............................
-4
RULE
Fig, 2.
Fig. l.
(Masket) Schematic representation of a large superior limbal wound for extracapsular cataract extraction ; note wound closure by inte rrupted sutures.
the prolonged visual recovery time for this wound method in addition to the large eventual change from presurgical astigmatism seems unsatisfactory. Fortunately, however, the advent of keratorefractive surgery has added to the understanding of the astigmatic behavior of planned ocular incisions and has afforded the cataract surgeon the opportunity to apply the principles of radial and astigmatic keratotomy to cataract wound incision and closure, since the latter are potentially powerful keratorefractive tools. The work of Rowsey8 is a guideline for inte rpreting the astigmatic relationships of corneal incisions. One may extrapolate from it to understand the astigmatic behavior of cataract incisions.
BASIC INCISION CONCEPTS 1. The keratometric effect of partial or full thickness corneal, limbal, or scleral incisions is an eventual flattening of the surgical meridian .2 , i ,8 2. Transverse or arcuate incisions placed close to the corneal apex induce greater meridional flattening than do more peripheral incisionsY As an example, in creating T cuts for astigmatic keratotomy, the smaller the central clear zone the greater the effect.l° 3. The longer a given transverse or arcuate incision, the greater the keratometric flattening in the surgical meridian. ll , 12 With long incisions, an additional "coupling" effect or steepening of the opposite meridian is observed. 8 4. Radial incisions induce less keratometric flattening of the surgical meridian than do transverse or circumferential incisions . From these principles, one may assume that the astigmatic effect of the cataract wound is a keratometric J CATARACT REFRACT
(Masket) Schematic postoperative astigmatic decay curve commonly observed for large limbal wounds with interrupted suture closure. Time course for astigmatic decay from with-the-rule (positive change) to against-the-rule (negative change) varies with suture material used andlor the cutting of sutures .
flattening of the surgical meridian, that more peripheral and shorter incisions induce less astigmatic change, and that radially oriented incisions cause less change in corneal cylinder than circumferential incisions. Unfortunately, however, current technology does not permit radial entries for cataract surgery. (Hirschman [Welsh Cataract Congress, Houston , September 1986] has suggested that a triangular trab eculectomy flap incision for cataract surgery offers a radial wound approach; however, because the chamber entrance in such a wound is circumferential at the limbus, the trabeculectomy flap is only half scleral thickness.) Therefore , it appears that the short phacoemulsification incision combined with a scleral pocket or tunnel incision (Figure 3) as originally proposed by Kratz and co-workers l .'3.14 and modified by others lS - li offers the most physically stable wound concept to reduce the incisional component, or againstthe-rule aspect, of iatrogenic astigmatism.
ASTIGMATIC EFFECTS OF SUTURE CLOSURE While the cataract wound causes an eventual keratometric flattening of the surgical meridian, or an against-the-rule change in corneal cylinder in the common case of a superiorly placed incision, suture closure of the wound induces a transient keratometric steepening or with-the-rule change for superior incisions, which is the initial aspect of the two-phase postsurgical iatrogenic astigmatic swing. The degree of suture-induced transient meridional steepening varies with suture orientation , suture tension, and suture material. Suture-induced steepening of the surgical me ridian is due to wound tissue compression , the "calling card" effece (Figure 4). The greater the suture-induced wound compression, the more the initial transient with-the-rule iatrogenic cylinde r. The more centrally the suture is placed and the tighter the SURe-VOL 1.5, JANVARY 1988
71
Fig. 4. Fig. 3.
(Masket) Clinical photograph of a completed scleral pocket incision with continuous, double running, single knotted monofilament suture closure. The suture is oriented to place the knot within the scleral pocket.
sutures are pulled, the greater the change. The scleralpocket incision method assures that the sutures will be peripherally oriented, and a continuous, double running, single knotted suture assures that the suture tension will be equally distributed along the full length of the incision. An analysis of my studies1,17-22 for the key factors of suture-induced astigmatism with closure of a standard 6.5 mm scleral pocket incision showed the following: 1. Suture-induced with-the-rule astigmatism is transient and will decay with time and/or cutting of sutures .1,8,18 Figure 5 demonstrates the mean induced postoperative change in corneal cylinder for 50 eyes that had phacoemulsification with posterior chamber lens implantation through a 6.5 mm scleral pocket incision and closed with a running 10-0 nylon suture without the aid of operative keratometry.18 Note that the with-the-rule change abates with time. In one eye from that series (Figure 6), a 3.0 D induced vertical steepening abated fully by four months after surgery. Another study 1 of induced with-the-rule change after surgery demonstrates that the greater the suture-induced with-therule change, the greater the eventual degradation of induced astigmatism (Table 1). Accordingly, with regard to the scleral pocket method, the surgeon cannot expect to induce a permanent keratometric steepening 72
(!'vlasket) The "calling card" effect of suture-induced wound compression. The schematic cross-sectional figure of the unsutured cornea is on the left. The figure to the right is a schematic cross-sectional view of the cornea after suture-induced compression superiorly. Note that the central corneal curvature is increased or steepened as a result of peripheral compression.
of the surgical meridian. Therefore, to provide an early useful optical result of cataract surgery, the surgeon must induce little with-the-rule cylinder; it appears that an intraoperative 1.5 D with-the-rule change from presurgical astigmatism is appropriate .14,21 It is also essential to create a physically sound wound to prevent
Fig. 5.
(Masket) Mean induced astigmatic change over time for 50 eyes that had phacoemulsification with posterior chamber lens implantation performed through a scleral pocket incision closed with a running appositional suture. Note that positive changes are with the rule, whereas negative changes are against the rule.
J CATARACT REFRACT SURG-\'OL 15, JANUARY 1989
Table 1. Final astigmatic change related to induced astigmatic change within first postoperative week. The two groups demonstrate similar final change, indicating that large initial with-the-rule changes abate with time. Induced Astigmatic Change
One Week
Four Months
> 2.0 D
+3.40 D .31 SEM
+ .20 D .15 SEM
::; 2.0 D
+
- .38 D .09 SEM
.ilO D .15 SEM
From Masket l
Fig. 6.
(Masket) Astigmatic decay curve of one eye. Note that the initial with-the-rule change of 3.0 D degrades almost completely by four months after surgery.
a prolonged against-the-rule drift. The scleral-pocket incision method offers a large surface area, which provides for wound stability. 2. A deeply placed suture limits initial with-the-rule astigmatic change and creates a physically stable wound. A controlled study l7 comparing two methods of continuous suturing of the scleral pocket incision determined that when the deep layer of the scleral bed was included with the suture (Figure 7), the initial
with-the-rule cylinder change was significantly less than that exhibited for an appositional method of suture closure (Figure 8). The data of that investigationl7 (Figure 9) indicated that the deep suture method induced approximately 1.0 D of iatrogenic with-therule change at one week after surgery compared to approximately 2.0 D observed for the appositional group. Incorporation of the deep layer of the bed prevents overtightening of the suture and unintentional sliding of the outer layer of the scleral pocket. At four months after surgery, the healed astigmatic change was clinically similar for the two groups, thereby discounting any need to induce a large degree of initial with-the-rule cylinder to preserve a final low astigmatic change from presurgical corneal astigmatism. 2o Moreover, the range of induced astigmatic errors observed at one week after surgery for the deep suture group was markedly lower than that for the appositional group, as noted in Table 2.20 An additional
DEEP
Fig. 7.
(Masket) Schematic cross-sectional representation of the scleral pocket incision closed in "deep" fashion to include the deeper layer of the scleral bed. Care is taken to avoid penetration of the ciliary body.
APPOSITIONAL
Fig. 8.
(Masket) Schematic cross-sectional representation of the scleral pocket incision closed in appositional fashion. The suture apposes the layers of the scleral pocket but does not include the deep scleral layer.
J CATARACT REFRACT SURG-vOL 15. JAKUARY 1989
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+20
MEAN ASTIGMATIC CHANGE WITH TIME
+10
ot-------~~----~~~~--------------------
- 1L-__- .__- .____. -__- .____. -__. -__- .____. -__- . 2 4 6 8 10 12 14 16 18
WEEKS
Fig. 9.
(Masket) Graph illustrating the mean induced astigmatic decay curves for the appositional (upper tracing) and the deep (lower tracing) suture groups. The deep suture group demonstrates significantly less initial with-the-rule astigmatic change, whereas the final healed results are clinically similar for the two groups.
benefit of the deep suture method is a reduced tendency for unintentional filtering bleb formation, noted in three of 50 eyes closed appositionally and in none of 35 cases closed with the deep method. 17 3. Quantitative intraoperative keratometry, when properly performed, allows suture tension adjustment Table 2. Range of induced astigmatic change at one week after surgery. Appositional Suture Group
Deep Suture Group
N=50 -0.50 to +6.12 Net range 6.62
N=35 -1.00 to + 2.S7 Net range 3.S7
to prevent large, unexpected and undesired astigmatic changes in the early postoperative period. 21 It is essential to establish physiologic levels of intraocular pressure prior to keratometry and/or suture tension adjustmen t. Surgical keratometry has long been proposed to aid the surgeon in control of astigmatism after cataract and corneal surgery.13,14,23,24 However, because of the variables in performing operative keratome try, evidence of its benefits has been inconclusive and the published information from Lindstrom l l and Binder25 ,26 and their coworkers failed to establish statistically valid results for quantitative (Terry) surgical keratometry with cataract surgery. A more recent controlled study21 demonstrated the positive value of intraoperative keratometry in preventing large early undesired postoperative astigmatic changes. In that study,21 as noted in Table 3, it was determined that intraoperative quantitative (Terry) keratometry was statistically similar to one day postsurgical routine office keratometry when necessary variables, including intraocular pressure, were strictly controlled at surgery; when lOP was not controlled, postoperative and intraoperative keratometric readings were statistically dissimilar. As in that study, operative intraocular pressure readings may be determined with a modified Barraquer surgical applanation tonometer (Figure 10) or with a combined applanation tonometer and qualitative placido disc keratometer (Figure 11). Although that study evaluated only a scleral pocket incision method, it is essential to establish intraocular pressure at physiologic levels prior to final tying of sutures with any wound method. Should eye pressure be too low when the sutures are tied, the suture will act with
Values represent change in diopters of corneal cylinder from preoperative readings; positive values are with the rule; negative values are against the rule. From Masket20
Table 3. Intraoperative and postoperative keratometric results. Group 1
Group 2
Operative cylinder
+2.15 D (0.19)
+2.06 D (0.12)
One day postoperative cylinder
+ 1.41 D (0.24)
+1.70 D (0.19)
0.74 D (0.22)
0.36 D (O.IS)
Cylinder "error"
P
value for " error"
.02
.11
Plus cylinder is with the rule; numbers in parentheses are standard errors of the mean. "Error" represents difference between operative and postoperative cylinder. 74
Fig. 10.
(Masket) Surgical applanation tonometer after Barraquer (courtesy Ocular Instruments, Bellevue, Washington).
J CATARACT REFRACT SURG-vOL 15, JANUARY 1989
Table 4. Postoperative astigmatic changes from presurgical astigmatism for three suture groups. Positive changes are with the rule; numbers in parentheses al'e standard errors of the mean. Results for the 10-0 nylon group are significantly different from the other groups only for the first two postoperative weeks. There is a marked similarity between the 9-0 nylon and 10-0 polyestel' groups at all stages after surgery.
(l\fasket) Combination surgical applanation tonometer and placido disc qualitative keratometer (courtesy HiLine Medical , El Toro , California).
greater tension as intraocular pressure normalizes after surgery; as a result of greater suture tension, a larger amount of transient with-the-rule cylinder will be encountered postoperatively. 4. In the early postoperative period, the wound tissue swells in response to surgery. As a result, th e sutures compress more tissue and, thereby, exert greater tension than induced at surgery. More rigid suture materials will create greater transient with-therule cylinder than will more elastic sutures. A controlled study22 compared the astigmatic characteristics of three suture materials for closure of a 6.5 mm scleral-pocket incision. All eyes were closed under tonometric and keratometric control. For one group of 35 eyes, a 10-0 nylon suture was used; in a second group of 25 eyes, a 9-0 nylon suture was used and for a third group of 25 eyes, 10-0 polyester (Mersilene TM) was used . During the first two weeks after surgery, the group closed with 10-0 nylon exhibited Significantly less (P < .02 for 9-0 nylon, P < .01 for 10-0 polyester) iatrogenic with-the-rule cylinder than the two other suture groups (Table 4). For the latter groups, the astigmatic results were remarkably similar throughout the four months of the study, and the data for all three suture groups were statistically and clinically indistinguishable after the first two weeks of surgery. However, during the early postsurgical phase, the most elastic of the three sutures, the 10-0 nylon, accommodated the edematous wound tissue more readily than the more rigid 9-0 nylon or 10-0 polyester. As a result , the more rigid sutures exhibited greater suture tension and a higher degree of suture-induced with-the-rule astigmatism during the early postoperative period.
9-0 Nylon
10-0 M ersilene ™
+ .91 (. 15) + .34 (. 12)
+ 1.48 (.21) + .84 (.20) + .07 (.16) -
6 weeks
- .15 (. 09)
+ 1.47 (. 27) + .81 (.23) + .03 (. 16)
4 months
-.40 (. 08)
-
1 week 2 weeks
Fig. 11.
10-0 Nylon
.19 (. 13)
.30 (. 11)
5. The ideal wound and suture combination exerts no change in corneal astigmatism, unless desired, at any time after surgery. Since approximately 90% of preoperative cataract patients exhibit less than 2.0 D of corneal astigmatism ,l an amount considered to be within an acceptable comfort range , the vast majority of patients would be best served by experiencing no iatrogenic astigmatic change. However, the two-phase surgically induced postoperative astigmatic decay curve may significantly retard functional vision return after surgery. Although foldable and expandable lens implants are evolving, current wound size requirements cannot eliminate astigmatic change. Fortunately, wound and closure options can significantly dampen postoperative astigmatic swings to provide an early and stable optical res ult. As an example, as reported in the literature,2-5 the traditional large limbal incision closed with interrupted sutures induces a net 6.0 D astigmatic swing during the postsurgical period. However, performing phacoemulsification through a scleral pocket incision and closing the wound with a deeply placed 10-0 nylon suture under tonometric and keratometric control allow a reduction of the two-phase astigmatic swing to less than 1.5 D during the healing period (Table 5). With this system , the initial with-the-rule change averages less than 1.0 D at one week after surgery, and Table 5. Range of net postoperative astigmatic swing from presurgical astigmatism. Large Limbal Wound
Scleral Pocket Incision
First week
+4.0 D
Fourth month
-2.0 D
< + 1.0 D < -0.5 D < 1.5 D
Postoperative Interval
Net astigmatic swing
6.0 D
Positive changes are with the rule; net swing is algebraic sum of initial and eventual postoperative astigmatic change.
J CATARACT REFRACT SLTRG -YOL 15. JANLTARY 1989
7.5
the eventual against-the-rule change is somewhat less than 0.5 D after wound healing. 20
INTENTIONAL ALTERATION OF PREEXISTING CORNEAL ASTIGMATISM As noted, the majority of cataract patients benefit from experiencing no astigmatic change after surgery. Alternatively, a number of individuals may be aided by reduction of presurgical corneal cylinder. However, prior to a consideration of astigmatic modification, the surgeon must clearly understand the indications and contraindications for such procedures to prevent asthenopia from astigmatic aniseikonia18.27.28 and must also have a firm knowledge of the astigmatic results of his or her routine wound closure. A variety of surgical options for the deliberate alteration of cylinder exists; all are in accord with the preceding principles. One simple modification is to place the incision in the steepest corneal meridian , anticipating ultimate meridional keratometric flattening and an overall reduction of corneal cylinder. 19.29.30 This form of astigmatic modification is particularly useful for secondary lens implantations, as has been demonstrated in a controlled study of ten patients haVing a temporally oriented scleral pocket incision for secondary lens implantation (Figure 12).19 In that investigation the mean preoperative cylinder was - 0.58 D or againstthe-rule; as a result of placing the incision in the steeper horizontal meridian, the mean overall astigmatic error was reduced to - 0.13 D because of an approximate 0.5 D with-the-rule shift.
Fig. 12.
76
(Masket) Schematic representation of a temporally oriented scleral pocket incision for secondary anterior chamber lens implantation. The superior surgical iridectomy was created at the initial surgery performed through a superior limbal incision.
Additional means for altering greater amounts of preexisting corneal astigmatism include graded recessions of the scleral flap/pocket in the steep meridian as has been suggested by Terry and Koch (Welsh Cataract Congress, Houston, September 1986). Osher and Maloney (Symposium on Cataract, IOL and Refractive Surgery, Orlando, April 1987) have reported on combined cataract extraction and corneal astigmatic keratotomy.
CONCLUSIONS The theoretical ideal wound and suture combination induces no change in corneal astigmatism at any time after surgery unless desired and carefully controlled; an incisional method of this nature would permit both instant and stable optical results of cataract surgery. While the ideal wound system is currently unattainable, progressive miniaturization of the incision will enable surgeons to reach that goal. In the interim, the cataract incision and its closure are powerful keratorefractive tools. Observing the behavior of ocular incisions and the principles of wound closure allows the surgeon to limit the iatrogenic astigmatic changes during the initial and long-term postoperative periods. Given present implant technology, the scleral pocket incision in combination with phacoemulsification seems to offer the best wound configuration; wound closure with adjustable suture tension under tonometric and keratometric control with deeply placed 10-0 monofilament nylon sutures appears to yield acceptable iatrogenic astigmatism. There appears to be no benefit to the induction of a large early postoperative with-the-rule astigmatic change since sutureinduced astigmatism is transient. REFERENCES 1. Masket S: Astigmatic analysis of the scleral pocket incision and closure technique for cataract surgery. CLAOJ 11:206-209, 1985 2. Swinger CA: Postoperative astigmatism . Surv Ophthalmol 31:219-248, 1987 3. Luntz MH. Livingston DG: Astigmatism in cataract surgery. Br ] Ophthalmol 61:360-365, 1977 4. Rowan PJ: Corneal astigmatism following cataract surgery. AIlIl Ophthalmol 10:231-234. 1978 5 . Thygesen J, Reersted P, Fleclelius H , Corydon L: Corneal astigmatism after cataract extraction . Acomparison of corneal and corneoscleral incisions. Acta OphthalmoI57:243-251. 1979 6. Jaffe NS, Clayman HM: The pathophysiology of corneal astigmatism after cataract extraction. Trans Am Acad Ophthalmol Otolaryngol 79:615-630, 1975 7. Jolson AS: The etiology and control of iatrogenic astigmatism in cataract incision closure. In : Ginsberg SP, ed. Cataract and Intraocular Lens Surgery; a Compendill1ll of Modem Theories and Techniqu es . Birmingham. Alabama, Aesculapius Publishing Co, 1984, vol 2, pp 537-569 8. Rowsey JJ: Ten caveats in keratorefractive surgery. Ophthalmology 90:148-155, 1983 9. Thrasher BH , Boe rner CF: Control of astigmatism by wound placement. Am Intra-Ocular Implant Soc] 10:176-179, 1984
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10. Thornton SP, Sanders DR: Graded nonintersecting transverse incisions for correction of idiopathic astigmatism. ] Cataract Refract SU/'g 13:27-31, 1987 11. Lindstrom RL, Destro MA: Effect of incision size and Terry keratometer usage on postoperative astigmatism. Am Intraocular Implant Soc] 11:469-473, 1985 12. Reading VM: Astigmatism following cataract surgery. Br ] Ophthalmol 68:97-104, 1984 13. Kratz RP, Johnson SH: Clinical results with the surgical keratometer. Int Ophthalmol Clin 23(4):87-99, 1983 14. Colvard DM, Mazzocco TR, Kratz RP, Davidson B: Clinical evaluation of the Terry surgical keratometer. Am Intra-Ocular Implant Soc] 6:249-251, 1980 15. Mazzocco TR: Newest trends in closure technique. Cataract 1:11-12, 1983 16. Girard LJ, Rodriguez J, Mailman ML: Reducing surgically induced astigmatism by using a scleral tunnel. Am] Ophthalmol 97:450-456, 1984 17. Masket S: Deep versus appositional suturing of the scleral pocket incision for astigmatic control in cataract surgery. ] Cataract Refract Surg 13:131-135, 1987 18. Masket S: Nonkeratometric control of postoperative astigmatism. Am Intra-Ocular Implant Soc] 11:134-137, 1985 19. Masket S: Temporal incision for astigmatic control in secondary implantation. ] Cataract Refract Surg 12:179-181, 1986 20. Masket S: Minimizing postoperative astigmatism after cataract surgery: The scleral pocket incision. Ophthalmic Pract 6:6-9, 26, 42-43, 1988 21. Masket S: Correlation between intraoperative and early post-
22. 23.
24.
25. 26. 27. 28. 29. 30.
operative keratometry. ] Cataract Refract Surg 14:277-280, 1988 Masket S: Comparison of suture materials for closure of the scleral pocket incision. ] Cataract Refract Surg 14:548-551, 1988 Troutman RC, Kellv S, Kave D, Clahane AC: The use and preliminary results 'of the Troutman surgical keratometer in cataract and corneal surgery. Trans Am Acad Ophthalmol Otolaryngol 83:232-238, 1977 Terry CM: Introduction to a surgical keratometer. In: Emery JM, Jacobson AC, eds, Current Concepts in Cataract Surgery; Selected Proceedings of the Sixth Biennial Cataract Surgical Congress. St Louis, CV Mosby Co, 1980, pp 27-29 Samples JR, Binder PS: The value of the Terry keratometer in predicting postoperative astigmatism. Ophthalmology 91:280-284, 1984 Perl T, Binder PS, Earl K: Post-cataract astigmatism with and without the use of the Terry keratometer. Ophthalmology 91:489-493, 1984 Linksz A: Aniseikonia; with notes on the Jackson-Lancaster controversy. Trans Am Acad Ophthalmol Otolaryngol 63:0P117-140, 1959 Heaton JM: The pain in eyestrain. Am ] Ophthalmol 61: 104-112, 1966 Lindstrom RL, Harris WS, Lyle WA: Secondary and exchange posterior chamber lens implantation. Am Intra-Ocular Implant Soc] 8:353-356, 1982 Karickhoff JR: Plus meridian incision for secondary implantation. OphthalmiC Surg 18:658-660, 1987
J CATARACT REFRACT SURG-VOL 15, JANUARY 1989
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II
B TRA \I CUlT nt cataract inci 'ion induce tran ient and permanent chan e in corneal a tigmati. m, 'f picall a h o-pha. e a tigmalic I'e. ponse i. ob 'el' ed· an initial, ith-the-I'ule han e is folio, db an e entual and permanent again t-the-l'lIle hift from pI' per'ati e a tigmati m, The earliel'literatul'e uggests that the ma nitude of the net a tigmatic s' in appro imate. 6.0 diopter fOl' hu'ge limbal ,ound closed, ith intelTupted 'utUl'e, Ho' e er appl ing the t n ts of kenltorefracti e sur el' to the catal'act inci ion and it · clo 'ure allo\ , the ur eon to limit po top I'ati e iatro enic a. ti matic ., ing , The reduced phaco mul ification inci ion ize in combination , ith a cleral pocket clo ed , ith a continuou in Ie knott d 10-0 monofilament n Ion , uture under tonometr'ic and keratometric control ignificantly dampen ' the change. in corneal a tigmati 'm durin the earl and late po 'topel'ati e period ,~1 publi h d I'eport a aluated in the pr nt tud, I' al that the net a ti matic S' in ma be reduc d to les ' than 1.5 diopter. ther b affol'din rapid and table optical re ult ,
Ke
Word
against-th -rlll appo~itional Slit.!!!:.. c\oslIr " cataract utur' c\o. III' ~ intraop ' rath
Control of iatrogenic changes in postoperative astigmatism remains a challenge to the cataract surgeon . Since 90% of cataract patients have less than 2.0 diopters (D) of corneal astigmatism prior to surgery, which they tolerate well, the majority of them would benefit by experiencing no change in astigmatism after surgery. Unfortunately, however, all current cataract incisions induce transient and permanent changes in corneal astigmatism. 2 Typically, early after surgery, a with-the-rule astigmatic change is observed; this is followed by an against-the-rule astigmatic decay,l-6 The initial with-the-rule change appears to be caused by suture-induced wound compression, and the against-the-rule drift, by eventual subclinical wound
ectasia. 2 ,6,' Since there are two operant astigmatic changes, the challenge to the surgeon is to induce a minimal initial with-the-rule change while providing a stable wound environment that precludes a profound and prolonged against-the-rule astigmatic decay. The traditional large limbal wound (Figure 1) closed with interrupted sutures is known to exhibit large astigmatic changes postoperatively.3,4,5 Typically, an initial 4.0 D with-the-rule change from presurgical astigmatism is noted during the early postoperative phase; a subsequent astigmatic decay results in a 2.0 D against-the-rule change from the preoperative cylinder (Figure 2), the result of a net 6.0 D postsurgical astigmatic swing. 2 For modern cataract management,
Presented in part at the Symposium on Cataract , IOL and Refractive Surgery, Los Angeles, March 1988. Reprint requests to Samuel Masket , M.D ., Suite 204, 7230 Medical Center Drive, Canoga Park, California 91307-1957. 70
J CATARACT REFRACT SURG-VOL 15. JANUARY 1989
.6
WITH
RULE
+4
.2
..................... -2
AGAINST
WEEKS
8 ..............................
-4
RULE
Fig, 2.
Fig. l.
(Masket) Schematic representation of a large superior limbal wound for extracapsular cataract extraction ; note wound closure by inte rrupted sutures.
the prolonged visual recovery time for this wound method in addition to the large eventual change from presurgical astigmatism seems unsatisfactory. Fortunately, however, the advent of keratorefractive surgery has added to the understanding of the astigmatic behavior of planned ocular incisions and has afforded the cataract surgeon the opportunity to apply the principles of radial and astigmatic keratotomy to cataract wound incision and closure, since the latter are potentially powerful keratorefractive tools. The work of Rowsey8 is a guideline for inte rpreting the astigmatic relationships of corneal incisions. One may extrapolate from it to understand the astigmatic behavior of cataract incisions.
BASIC INCISION CONCEPTS 1. The keratometric effect of partial or full thickness corneal, limbal, or scleral incisions is an eventual flattening of the surgical meridian .2 , i ,8 2. Transverse or arcuate incisions placed close to the corneal apex induce greater meridional flattening than do more peripheral incisionsY As an example, in creating T cuts for astigmatic keratotomy, the smaller the central clear zone the greater the effect.l° 3. The longer a given transverse or arcuate incision, the greater the keratometric flattening in the surgical meridian. ll , 12 With long incisions, an additional "coupling" effect or steepening of the opposite meridian is observed. 8 4. Radial incisions induce less keratometric flattening of the surgical meridian than do transverse or circumferential incisions . From these principles, one may assume that the astigmatic effect of the cataract wound is a keratometric J CATARACT REFRACT
(Masket) Schematic postoperative astigmatic decay curve commonly observed for large limbal wounds with interrupted suture closure. Time course for astigmatic decay from with-the-rule (positive change) to against-the-rule (negative change) varies with suture material used andlor the cutting of sutures .
flattening of the surgical meridian, that more peripheral and shorter incisions induce less astigmatic change, and that radially oriented incisions cause less change in corneal cylinder than circumferential incisions. Unfortunately, however, current technology does not permit radial entries for cataract surgery. (Hirschman [Welsh Cataract Congress, Houston , September 1986] has suggested that a triangular trab eculectomy flap incision for cataract surgery offers a radial wound approach; however, because the chamber entrance in such a wound is circumferential at the limbus, the trabeculectomy flap is only half scleral thickness.) Therefore , it appears that the short phacoemulsification incision combined with a scleral pocket or tunnel incision (Figure 3) as originally proposed by Kratz and co-workers l .'3.14 and modified by others lS - li offers the most physically stable wound concept to reduce the incisional component, or againstthe-rule aspect, of iatrogenic astigmatism.
ASTIGMATIC EFFECTS OF SUTURE CLOSURE While the cataract wound causes an eventual keratometric flattening of the surgical meridian, or an against-the-rule change in corneal cylinder in the common case of a superiorly placed incision, suture closure of the wound induces a transient keratometric steepening or with-the-rule change for superior incisions, which is the initial aspect of the two-phase postsurgical iatrogenic astigmatic swing. The degree of suture-induced transient meridional steepening varies with suture orientation , suture tension, and suture material. Suture-induced steepening of the surgical me ridian is due to wound tissue compression , the "calling card" effece (Figure 4). The greater the suture-induced wound compression, the more the initial transient with-the-rule iatrogenic cylinde r. The more centrally the suture is placed and the tighter the SURe-VOL 1.5, JANVARY 1988
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Fig. 4. Fig. 3.
(Masket) Clinical photograph of a completed scleral pocket incision with continuous, double running, single knotted monofilament suture closure. The suture is oriented to place the knot within the scleral pocket.
sutures are pulled, the greater the change. The scleralpocket incision method assures that the sutures will be peripherally oriented, and a continuous, double running, single knotted suture assures that the suture tension will be equally distributed along the full length of the incision. An analysis of my studies1,17-22 for the key factors of suture-induced astigmatism with closure of a standard 6.5 mm scleral pocket incision showed the following: 1. Suture-induced with-the-rule astigmatism is transient and will decay with time and/or cutting of sutures .1,8,18 Figure 5 demonstrates the mean induced postoperative change in corneal cylinder for 50 eyes that had phacoemulsification with posterior chamber lens implantation through a 6.5 mm scleral pocket incision and closed with a running 10-0 nylon suture without the aid of operative keratometry.18 Note that the with-the-rule change abates with time. In one eye from that series (Figure 6), a 3.0 D induced vertical steepening abated fully by four months after surgery. Another study 1 of induced with-the-rule change after surgery demonstrates that the greater the suture-induced with-therule change, the greater the eventual degradation of induced astigmatism (Table 1). Accordingly, with regard to the scleral pocket method, the surgeon cannot expect to induce a permanent keratometric steepening 72
(!'vlasket) The "calling card" effect of suture-induced wound compression. The schematic cross-sectional figure of the unsutured cornea is on the left. The figure to the right is a schematic cross-sectional view of the cornea after suture-induced compression superiorly. Note that the central corneal curvature is increased or steepened as a result of peripheral compression.
of the surgical meridian. Therefore, to provide an early useful optical result of cataract surgery, the surgeon must induce little with-the-rule cylinder; it appears that an intraoperative 1.5 D with-the-rule change from presurgical astigmatism is appropriate .14,21 It is also essential to create a physically sound wound to prevent
Fig. 5.
(Masket) Mean induced astigmatic change over time for 50 eyes that had phacoemulsification with posterior chamber lens implantation performed through a scleral pocket incision closed with a running appositional suture. Note that positive changes are with the rule, whereas negative changes are against the rule.
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Table 1. Final astigmatic change related to induced astigmatic change within first postoperative week. The two groups demonstrate similar final change, indicating that large initial with-the-rule changes abate with time. Induced Astigmatic Change
One Week
Four Months
> 2.0 D
+3.40 D .31 SEM
+ .20 D .15 SEM
::; 2.0 D
+
- .38 D .09 SEM
.ilO D .15 SEM
From Masket l
Fig. 6.
(Masket) Astigmatic decay curve of one eye. Note that the initial with-the-rule change of 3.0 D degrades almost completely by four months after surgery.
a prolonged against-the-rule drift. The scleral-pocket incision method offers a large surface area, which provides for wound stability. 2. A deeply placed suture limits initial with-the-rule astigmatic change and creates a physically stable wound. A controlled study l7 comparing two methods of continuous suturing of the scleral pocket incision determined that when the deep layer of the scleral bed was included with the suture (Figure 7), the initial
with-the-rule cylinder change was significantly less than that exhibited for an appositional method of suture closure (Figure 8). The data of that investigationl7 (Figure 9) indicated that the deep suture method induced approximately 1.0 D of iatrogenic with-therule change at one week after surgery compared to approximately 2.0 D observed for the appositional group. Incorporation of the deep layer of the bed prevents overtightening of the suture and unintentional sliding of the outer layer of the scleral pocket. At four months after surgery, the healed astigmatic change was clinically similar for the two groups, thereby discounting any need to induce a large degree of initial with-the-rule cylinder to preserve a final low astigmatic change from presurgical corneal astigmatism. 2o Moreover, the range of induced astigmatic errors observed at one week after surgery for the deep suture group was markedly lower than that for the appositional group, as noted in Table 2.20 An additional
DEEP
Fig. 7.
(Masket) Schematic cross-sectional representation of the scleral pocket incision closed in "deep" fashion to include the deeper layer of the scleral bed. Care is taken to avoid penetration of the ciliary body.
APPOSITIONAL
Fig. 8.
(Masket) Schematic cross-sectional representation of the scleral pocket incision closed in appositional fashion. The suture apposes the layers of the scleral pocket but does not include the deep scleral layer.
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+20
MEAN ASTIGMATIC CHANGE WITH TIME
+10
ot-------~~----~~~~--------------------
- 1L-__- .__- .____. -__- .____. -__. -__- .____. -__- . 2 4 6 8 10 12 14 16 18
WEEKS
Fig. 9.
(Masket) Graph illustrating the mean induced astigmatic decay curves for the appositional (upper tracing) and the deep (lower tracing) suture groups. The deep suture group demonstrates significantly less initial with-the-rule astigmatic change, whereas the final healed results are clinically similar for the two groups.
benefit of the deep suture method is a reduced tendency for unintentional filtering bleb formation, noted in three of 50 eyes closed appositionally and in none of 35 cases closed with the deep method. 17 3. Quantitative intraoperative keratometry, when properly performed, allows suture tension adjustment Table 2. Range of induced astigmatic change at one week after surgery. Appositional Suture Group
Deep Suture Group
N=50 -0.50 to +6.12 Net range 6.62
N=35 -1.00 to + 2.S7 Net range 3.S7
to prevent large, unexpected and undesired astigmatic changes in the early postoperative period. 21 It is essential to establish physiologic levels of intraocular pressure prior to keratometry and/or suture tension adjustmen t. Surgical keratometry has long been proposed to aid the surgeon in control of astigmatism after cataract and corneal surgery.13,14,23,24 However, because of the variables in performing operative keratome try, evidence of its benefits has been inconclusive and the published information from Lindstrom l l and Binder25 ,26 and their coworkers failed to establish statistically valid results for quantitative (Terry) surgical keratometry with cataract surgery. A more recent controlled study21 demonstrated the positive value of intraoperative keratometry in preventing large early undesired postoperative astigmatic changes. In that study,21 as noted in Table 3, it was determined that intraoperative quantitative (Terry) keratometry was statistically similar to one day postsurgical routine office keratometry when necessary variables, including intraocular pressure, were strictly controlled at surgery; when lOP was not controlled, postoperative and intraoperative keratometric readings were statistically dissimilar. As in that study, operative intraocular pressure readings may be determined with a modified Barraquer surgical applanation tonometer (Figure 10) or with a combined applanation tonometer and qualitative placido disc keratometer (Figure 11). Although that study evaluated only a scleral pocket incision method, it is essential to establish intraocular pressure at physiologic levels prior to final tying of sutures with any wound method. Should eye pressure be too low when the sutures are tied, the suture will act with
Values represent change in diopters of corneal cylinder from preoperative readings; positive values are with the rule; negative values are against the rule. From Masket20
Table 3. Intraoperative and postoperative keratometric results. Group 1
Group 2
Operative cylinder
+2.15 D (0.19)
+2.06 D (0.12)
One day postoperative cylinder
+ 1.41 D (0.24)
+1.70 D (0.19)
0.74 D (0.22)
0.36 D (O.IS)
Cylinder "error"
P
value for " error"
.02
.11
Plus cylinder is with the rule; numbers in parentheses are standard errors of the mean. "Error" represents difference between operative and postoperative cylinder. 74
Fig. 10.
(Masket) Surgical applanation tonometer after Barraquer (courtesy Ocular Instruments, Bellevue, Washington).
J CATARACT REFRACT SURG-vOL 15, JANUARY 1989
Table 4. Postoperative astigmatic changes from presurgical astigmatism for three suture groups. Positive changes are with the rule; numbers in parentheses al'e standard errors of the mean. Results for the 10-0 nylon group are significantly different from the other groups only for the first two postoperative weeks. There is a marked similarity between the 9-0 nylon and 10-0 polyestel' groups at all stages after surgery.
(l\fasket) Combination surgical applanation tonometer and placido disc qualitative keratometer (courtesy HiLine Medical , El Toro , California).
greater tension as intraocular pressure normalizes after surgery; as a result of greater suture tension, a larger amount of transient with-the-rule cylinder will be encountered postoperatively. 4. In the early postoperative period, the wound tissue swells in response to surgery. As a result, th e sutures compress more tissue and, thereby, exert greater tension than induced at surgery. More rigid suture materials will create greater transient with-therule cylinder than will more elastic sutures. A controlled study22 compared the astigmatic characteristics of three suture materials for closure of a 6.5 mm scleral-pocket incision. All eyes were closed under tonometric and keratometric control. For one group of 35 eyes, a 10-0 nylon suture was used; in a second group of 25 eyes, a 9-0 nylon suture was used and for a third group of 25 eyes, 10-0 polyester (Mersilene TM) was used . During the first two weeks after surgery, the group closed with 10-0 nylon exhibited Significantly less (P < .02 for 9-0 nylon, P < .01 for 10-0 polyester) iatrogenic with-the-rule cylinder than the two other suture groups (Table 4). For the latter groups, the astigmatic results were remarkably similar throughout the four months of the study, and the data for all three suture groups were statistically and clinically indistinguishable after the first two weeks of surgery. However, during the early postsurgical phase, the most elastic of the three sutures, the 10-0 nylon, accommodated the edematous wound tissue more readily than the more rigid 9-0 nylon or 10-0 polyester. As a result , the more rigid sutures exhibited greater suture tension and a higher degree of suture-induced with-the-rule astigmatism during the early postoperative period.
9-0 Nylon
10-0 M ersilene ™
+ .91 (. 15) + .34 (. 12)
+ 1.48 (.21) + .84 (.20) + .07 (.16) -
6 weeks
- .15 (. 09)
+ 1.47 (. 27) + .81 (.23) + .03 (. 16)
4 months
-.40 (. 08)
-
1 week 2 weeks
Fig. 11.
10-0 Nylon
.19 (. 13)
.30 (. 11)
5. The ideal wound and suture combination exerts no change in corneal astigmatism, unless desired, at any time after surgery. Since approximately 90% of preoperative cataract patients exhibit less than 2.0 D of corneal astigmatism ,l an amount considered to be within an acceptable comfort range , the vast majority of patients would be best served by experiencing no iatrogenic astigmatic change. However, the two-phase surgically induced postoperative astigmatic decay curve may significantly retard functional vision return after surgery. Although foldable and expandable lens implants are evolving, current wound size requirements cannot eliminate astigmatic change. Fortunately, wound and closure options can significantly dampen postoperative astigmatic swings to provide an early and stable optical res ult. As an example, as reported in the literature,2-5 the traditional large limbal incision closed with interrupted sutures induces a net 6.0 D astigmatic swing during the postsurgical period. However, performing phacoemulsification through a scleral pocket incision and closing the wound with a deeply placed 10-0 nylon suture under tonometric and keratometric control allow a reduction of the two-phase astigmatic swing to less than 1.5 D during the healing period (Table 5). With this system , the initial with-the-rule change averages less than 1.0 D at one week after surgery, and Table 5. Range of net postoperative astigmatic swing from presurgical astigmatism. Large Limbal Wound
Scleral Pocket Incision
First week
+4.0 D
Fourth month
-2.0 D
< + 1.0 D < -0.5 D < 1.5 D
Postoperative Interval
Net astigmatic swing
6.0 D
Positive changes are with the rule; net swing is algebraic sum of initial and eventual postoperative astigmatic change.
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the eventual against-the-rule change is somewhat less than 0.5 D after wound healing. 20
INTENTIONAL ALTERATION OF PREEXISTING CORNEAL ASTIGMATISM As noted, the majority of cataract patients benefit from experiencing no astigmatic change after surgery. Alternatively, a number of individuals may be aided by reduction of presurgical corneal cylinder. However, prior to a consideration of astigmatic modification, the surgeon must clearly understand the indications and contraindications for such procedures to prevent asthenopia from astigmatic aniseikonia18.27.28 and must also have a firm knowledge of the astigmatic results of his or her routine wound closure. A variety of surgical options for the deliberate alteration of cylinder exists; all are in accord with the preceding principles. One simple modification is to place the incision in the steepest corneal meridian , anticipating ultimate meridional keratometric flattening and an overall reduction of corneal cylinder. 19.29.30 This form of astigmatic modification is particularly useful for secondary lens implantations, as has been demonstrated in a controlled study of ten patients haVing a temporally oriented scleral pocket incision for secondary lens implantation (Figure 12).19 In that investigation the mean preoperative cylinder was - 0.58 D or againstthe-rule; as a result of placing the incision in the steeper horizontal meridian, the mean overall astigmatic error was reduced to - 0.13 D because of an approximate 0.5 D with-the-rule shift.
Fig. 12.
76
(Masket) Schematic representation of a temporally oriented scleral pocket incision for secondary anterior chamber lens implantation. The superior surgical iridectomy was created at the initial surgery performed through a superior limbal incision.
Additional means for altering greater amounts of preexisting corneal astigmatism include graded recessions of the scleral flap/pocket in the steep meridian as has been suggested by Terry and Koch (Welsh Cataract Congress, Houston, September 1986). Osher and Maloney (Symposium on Cataract, IOL and Refractive Surgery, Orlando, April 1987) have reported on combined cataract extraction and corneal astigmatic keratotomy.
CONCLUSIONS The theoretical ideal wound and suture combination induces no change in corneal astigmatism at any time after surgery unless desired and carefully controlled; an incisional method of this nature would permit both instant and stable optical results of cataract surgery. While the ideal wound system is currently unattainable, progressive miniaturization of the incision will enable surgeons to reach that goal. In the interim, the cataract incision and its closure are powerful keratorefractive tools. Observing the behavior of ocular incisions and the principles of wound closure allows the surgeon to limit the iatrogenic astigmatic changes during the initial and long-term postoperative periods. Given present implant technology, the scleral pocket incision in combination with phacoemulsification seems to offer the best wound configuration; wound closure with adjustable suture tension under tonometric and keratometric control with deeply placed 10-0 monofilament nylon sutures appears to yield acceptable iatrogenic astigmatism. There appears to be no benefit to the induction of a large early postoperative with-the-rule astigmatic change since sutureinduced astigmatism is transient. REFERENCES 1. Masket S: Astigmatic analysis of the scleral pocket incision and closure technique for cataract surgery. CLAOJ 11:206-209, 1985 2. Swinger CA: Postoperative astigmatism . Surv Ophthalmol 31:219-248, 1987 3. Luntz MH. Livingston DG: Astigmatism in cataract surgery. Br ] Ophthalmol 61:360-365, 1977 4. Rowan PJ: Corneal astigmatism following cataract surgery. AIlIl Ophthalmol 10:231-234. 1978 5 . Thygesen J, Reersted P, Fleclelius H , Corydon L: Corneal astigmatism after cataract extraction . Acomparison of corneal and corneoscleral incisions. Acta OphthalmoI57:243-251. 1979 6. Jaffe NS, Clayman HM: The pathophysiology of corneal astigmatism after cataract extraction. Trans Am Acad Ophthalmol Otolaryngol 79:615-630, 1975 7. Jolson AS: The etiology and control of iatrogenic astigmatism in cataract incision closure. In : Ginsberg SP, ed. Cataract and Intraocular Lens Surgery; a Compendill1ll of Modem Theories and Techniqu es . Birmingham. Alabama, Aesculapius Publishing Co, 1984, vol 2, pp 537-569 8. Rowsey JJ: Ten caveats in keratorefractive surgery. Ophthalmology 90:148-155, 1983 9. Thrasher BH , Boe rner CF: Control of astigmatism by wound placement. Am Intra-Ocular Implant Soc] 10:176-179, 1984
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10. Thornton SP, Sanders DR: Graded nonintersecting transverse incisions for correction of idiopathic astigmatism. ] Cataract Refract SU/'g 13:27-31, 1987 11. Lindstrom RL, Destro MA: Effect of incision size and Terry keratometer usage on postoperative astigmatism. Am Intraocular Implant Soc] 11:469-473, 1985 12. Reading VM: Astigmatism following cataract surgery. Br ] Ophthalmol 68:97-104, 1984 13. Kratz RP, Johnson SH: Clinical results with the surgical keratometer. Int Ophthalmol Clin 23(4):87-99, 1983 14. Colvard DM, Mazzocco TR, Kratz RP, Davidson B: Clinical evaluation of the Terry surgical keratometer. Am Intra-Ocular Implant Soc] 6:249-251, 1980 15. Mazzocco TR: Newest trends in closure technique. Cataract 1:11-12, 1983 16. Girard LJ, Rodriguez J, Mailman ML: Reducing surgically induced astigmatism by using a scleral tunnel. Am] Ophthalmol 97:450-456, 1984 17. Masket S: Deep versus appositional suturing of the scleral pocket incision for astigmatic control in cataract surgery. ] Cataract Refract Surg 13:131-135, 1987 18. Masket S: Nonkeratometric control of postoperative astigmatism. Am Intra-Ocular Implant Soc] 11:134-137, 1985 19. Masket S: Temporal incision for astigmatic control in secondary implantation. ] Cataract Refract Surg 12:179-181, 1986 20. Masket S: Minimizing postoperative astigmatism after cataract surgery: The scleral pocket incision. Ophthalmic Pract 6:6-9, 26, 42-43, 1988 21. Masket S: Correlation between intraoperative and early post-
22. 23.
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25. 26. 27. 28. 29. 30.
operative keratometry. ] Cataract Refract Surg 14:277-280, 1988 Masket S: Comparison of suture materials for closure of the scleral pocket incision. ] Cataract Refract Surg 14:548-551, 1988 Troutman RC, Kellv S, Kave D, Clahane AC: The use and preliminary results 'of the Troutman surgical keratometer in cataract and corneal surgery. Trans Am Acad Ophthalmol Otolaryngol 83:232-238, 1977 Terry CM: Introduction to a surgical keratometer. In: Emery JM, Jacobson AC, eds, Current Concepts in Cataract Surgery; Selected Proceedings of the Sixth Biennial Cataract Surgical Congress. St Louis, CV Mosby Co, 1980, pp 27-29 Samples JR, Binder PS: The value of the Terry keratometer in predicting postoperative astigmatism. Ophthalmology 91:280-284, 1984 Perl T, Binder PS, Earl K: Post-cataract astigmatism with and without the use of the Terry keratometer. Ophthalmology 91:489-493, 1984 Linksz A: Aniseikonia; with notes on the Jackson-Lancaster controversy. Trans Am Acad Ophthalmol Otolaryngol 63:0P117-140, 1959 Heaton JM: The pain in eyestrain. Am ] Ophthalmol 61: 104-112, 1966 Lindstrom RL, Harris WS, Lyle WA: Secondary and exchange posterior chamber lens implantation. Am Intra-Ocular Implant Soc] 8:353-356, 1982 Karickhoff JR: Plus meridian incision for secondary implantation. OphthalmiC Surg 18:658-660, 1987
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