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   Table of Contents      
REVIEW ARTICLE
Year : 1990  |  Volume : 38  |  Issue : 3  |  Page : 114-123

Surgical correction of postoperative astigmatism


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Correspondence Address:
Richard L Lindstrom
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Source of Support: None, Conflict of Interest: None


PMID: 2272682

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  Abstract 

The photokeratoscope has increased the understanding of the aspheric nature of the cornea as well as a better understanding of normal corneal topography. This has significantly affected the development of newer and more predictable models of surgical astigmatic correction. Relaxing incisions effectively flatten the steeper meridian an equivalent amount as they steepen the flatter meridian. The net change in spherical equivalent is, therefore, negligible. Poor predictability is the major limitation of relaxing incisions. Wedge resection can correct large degrees of postkeratoplasty astigmatism, Resection of 0.10 mm of tissue results in approximately 2 diopters of astigmatic correction. Prolonged postoperative rehabilitation and induced irregular astigmatism are limitations of the procedure. Transverse incisions flatten the steeper meridian an equivalent amount as they steepen the flatter meridian. Semiradial incisions result in two times the amount of flattening in the meridian of the incision compared to the meridian 90 degrees away. Combination of transverse incisions with semiradial incisions describes the trapezoidal astigmatic keratotomy. This procedure may correct from 5.5 to 11.0 diopters dependent upon the age of the patient. The use of the surgical keratometer is helpful in assessing a proper endpoint during surgical correction of astigmatism.


How to cite this article:
Lindstrom RL. Surgical correction of postoperative astigmatism. Indian J Ophthalmol 1990;38:114-23

How to cite this URL:
Lindstrom RL. Surgical correction of postoperative astigmatism. Indian J Ophthalmol [serial online] 1990 [cited 2019 Nov 20];38:114-23. Available from: http://www.ijo.in/text.asp?1990/38/3/114/25523


  Introduction Top


The surgical correction of both idiopathic and post-sur­gical astigmatism has evolved from recent basic science and clinical studies. Basic principles of wound repair following traumatic corneal laceration which minimize astigmatism have been described by Eisner, [4] and Row­sey and Hayes. [19] Troutman [31],[33],[34] has described prin­ciples of suturing during penetrating keratoplasty while Cohen et a1 [3] have carefully documented the potential for induction of astigmatism when excessive tissue removal or inadequate tissue removal results. The use of thephotokeratoscope [3],[18],[20] has allowed a better under­standing of the aspheric nature of the cornea and an increased understanding of normal corneal topography. This has significantly affected the development of newer and more predictable models of surgical astigmatic cor­rection. Differential rates of wound healing, irregular collagen deposition and prolonged collagen remodeling all contribute to postoperative astigmatism. As our un­derstanding of wound healing increases, postoperative astigmatism may become more manageable. This dis­cussion will outline the current approach to planning and correcting postoperative astigmatism. This will include a discussion of relaxing incisions, compression sutures, wedge resection and astigmatic keratotomy, comprised by tangential incisions, semiradial incisions and the modified ruiz procedure.


  Relaxing incisions Top


Troutman [35] estimated that nearly 10 percent of all clear penetrating keratoplasties were complicated by high postoperative astigmatism. As a means of reducing post-keratoplasty astigmatism, relaxing incisions have become popular. [1][8][9][12][15][29][32][35] A cadaver study of the effect of relaxing incisions on corneal topography demonstrated a wide range of effect on corneal astig­matism ranging from 0.58 D for a single one clock-hour incision to 5.93 D for a unilateral three-clock hour in­cision. [16] Symmetrical relaxing incisions placed 180° apart produced 0.78 D of astigmatism change for one clock-hour incisions and 13.97 D of change for sym­metrical three clock-hour incisions. A marked disparity between the magnitude of change following symmetrical two and three clock-hour incisions in eye bank eyes indicated a narrow surgical "safe" zone. Clinically, marked undercorrections and overcorrections have been achieved in post-keratoplasty astigmatism using relaxing incisions. [9],[12],[15],[29 ] This relative unpredictability may relate to asymmetrical post-keratoplasty wound healing, as well as regression of the effect following relaxing incisions which makes the determination of a suitable endpoint very difficult.

Clinically, Lavery and Lindstrom [15] found that the relax­ing incision technique effectively flattened the steeper meridian an equivalent amount as it steepened the flatter meridian. The change in spherical equivalent was, therefore, negligible. Other investigators have found small myopic shifts in the spherical equivalent to be the rule after relaxing incision [9],[29].However, Lundergren and Rowsey, [16] in their study of relaxing incisions in eye bank eyes, noted significant flattening in the meridian perpendicular to the corneal relaxing incisions with steepening 90° away in a ratio of 2 diopters of flattening to 1 diopter of steepening. Such a ratio of flattening to steepening would be expected to cause a hyperopic shift, but this has not been observed, clinically. The best explanation of such a disparity is that eye bank eyes have not undergone penetrating keratoplasty with the attendant variable healing.

The major complication of this procedure is poor predict­ability, such that the identical operation on one patient achieves a minimal effect with significant undercorrec­tion while another patient achieves a large effect with significant overcorrection. [9].[12],[15].[29] Inadvertent perfora­tions or wound dehiscence are other significant com­plications of relaxing incision. [12],[15],[29] Posterior wound gape at the graft-host margin can be difficult to ap­preciate preoperatively and perforations may occur easily with the manipulation of the graft-host interface. In the University of Minnesota experience, [12],[15] four of seventeen patients had microperforations, one of which required suturing; whereas in the University of Illinois series of 17 patients, [29] one patient had a perforation requiring suturing while another had wound dehiscence three days after the relaxing incision. Finally, after relax­ing incisions some patients develop prolonged instability of corneal topography with fluctuating keratometry as the graft appears to migrate while healing.


  Operative technique Top


Careful preoperative evaluation of refraction, kerat­ometry and corneoscopy is performed. As refractions in keratoplasty patients are often quite difficult and inac­curate, one has to rely primarily on keratometry and corneoscopy when planning procedures. Corneoscopy will often graphically depict areas of cicatricial wound healing with resultant corneal distortion. In such cases, a single relaxing incision on one side of the graft-host interface may be all that is necessary. However, in most cases, relaxing incisions must be performed on both sides of the graft-host interface along the axis of the steepest corneal meridian.

Relaxing incisions effect a negligible change in spherical equivalent. [15] For example, if a patient's preoperative error is -3.00 + 6.00 x 90(+3.00 - 6.00 x 180) with keratometry readings of 40.00/46.00 x 90, and the patient obtained a perfect result, the postoperative spherical equivalent would be piano with keratometry readings of 43.00/43.00. This procedure, therefore, does not correct any residual hyperopia or myopia.

An operating microscope with a quantitative surgical keratometer significantly increases the accuracy of the procedure because it helps the surgeon determine an appropriate endpoint. The patient is anesthetized with topical proparicaine hydrochloride. The periocular skin is prepped using a povidone/iodine solution. A sterile field is achieved with a plastic adhesive aperture drape and the lids are separated with a fine wire speculum. The patient is asked to fixate the operating microscope light and keratometry is performed. This axis of the steepest meridian is marked using a blade within the epithelium on each side of the graft-host interface [Fig­ure 1]. An appropriate incision length should never exceed three clock-hours or 90 0 of the circumference of the graft. The incision length and depth may vary, de­pendent upon the variability in wound healing as it contributes to astigmatism. As mentioned above, in­traoperative keratometry is especially helpful to deter­mine a proper endpoint of the relaxing incision. The length and depth of the incisions need to be adjusted during each case dependent upon the effect achieved during the surgical procedure.

The operative procedure involves careful dissection of the graft-host interface in a graded fashion utilizing a micro-sharp metal blade [Figure - 2]. A gradual and care­ful deepening of the incision utilizing a metal knife appears to be more controlled than when using a diamond knife. In some cases, blunt dissection using the noncutting edge of the knife may actually be helpful

Progressive deepening and lengthening of the incision is performed initially on one side of the graft-host inter­face while continuously observing the keratometer mire for correction of the astigmatism. Gradual deepening and lengthening of the incision is then made 180 0sub to the first incision until an overcorrection is achieved by use of the surgical keratometer. In general, approximately 1 /3 to 1 /2 of the effect of surgery is lost during the postoperative period and, therefore, an attempt is made to overcorrect the preoperative cylinder by 30-50%. For example, if the preoperative cylinder is approximately 6 D, a 2-3 D overcorrection is desirable.

The use of preset diamond knives in the graft-host interface has resulted in a significantly greater number of perforations. [12],[15] Because the procedure is per­formed in the graft-host interface in keratoplasty patients, it is not unusual to enter areas of poor wound healing or posterior wound gape. If perforation does occur with significant loss of chamber volume, suturing is necessary. Unfortunately, suturing negates most of the effect of the procedure. At the completion of surgery, relaxing incisions are gently irrigated with a blunt can­nula to remove any blood or debris. The cornea is irrigated with topical gentamicin sulfate. In most cases, a single drop of 0.25% scopolamine hydrochloride is applied to the eye for cyclopegia. The eye is patched and a shield is taped in place. Postoperatively, the patient is treated with topical antibiotics 3-4 times daily for one week. Topical steroids are appropriately utilized for maintenance of graft clarity.


  Compression sutures Top


If the relaxing incisions do not attain an adequate cor­rection of astigmatism, compression sutures may be placed on each side of the graft-host interface 90° away from the relaxing incisions [Figure - 3]. Suture depth should be approximately 75%. These sutures are tied with a slip knot and adjusted under keratometric control until an overcorrection of 33 to 50% is achieved [Figure - 4]. The knots are then cut short and buried. The use of mersilene suture is preferable because, if perfect surgi­cal correction of astigmatism is achieved, it may be desirable to leave the suture in place for extended periods of time. Whereas nylon will hydrolyze in 1-2 years, mersilene suture will last greater than 7-10 years.

If overcorrection persists at 8 weeks after placement of compression sutures, one of the sutures is removed. If overcorrection still persists, the second compression suture is removed at 12 weeks. If an inadequate surgical correction is achieved, the operation may be repeated, or an alternative procedure such as wedge resection or astigmatic keratotomy may be considered.


  Wedge resection Top


The wedge resection technique is reserved for correc­tion of large degrees of postkeratoplasty astigma­tism. [1],[2],[9],[12],[[15],[32] In general, resection of 0.10 mm of tissue results in approximately 2 diopters of astigmatic correction. This operation is reserved for patients with greater than 10 D of astigmatism. This technique is readily capable of correcting up to 20 D of astigmatism. Wedge resection, however, requires a prolonged pos­toperative rehabilitation. The long visual rehabilitation is necessitated by the placement of multiple sutures which induce significant irregular astigmatism and adequate wound healing must be allowed before selective suture removal can be undertaken to reduce the irregular astig­matism prior to eventual visual rehabilitation. Neverthe­less, the alternative to wedge resection in such patients is repeat penetrating keratoplasty and, therefore, an attempt at wedge resection appears preferable to imme­diate repeat keratoplasty.

The overall effect of wedge resection is to steepen the flatter meridian approximately twice as much as it flat­tens the steeper meridian. The net effect is an increase in myopia or decrease in hyperopia following this proce­dure. For example, if the preoperative refraction was -6.00+ 12.00 x 90(+6.00-12.00 x 180) and keratometry was 40.00/52.00 x 90, a perfect result should give postoperative keratometry measuring 48.00/48.00 with a refraction of -2 D [Figure - 5]. This would have resulted in a 2 D myopic shift.


  Operative technique Top


The patient is evaluated preoperatively as for relaxing incisions. A wedge resection requires retrobulbar anes­thesia because the dissection and placement of multiple sutures demands excellent akinesia. Initially, the axis of the flattest meridian is marked, preferably after confir­mation of the axis using a surgical keratometer. When performing wedge resection, a diamond knife is helpful. Intraoperative ultrasonic pachymetry readings are per­formed adjacent to the graft-host interface in the proper axis where the wedge resection is to be performed. The diamond blade is set at 100% of the thinnest pachymetry reading, and the blade setting is then checked with a coin gauge or similar device. A front cutting diamond blade is preferred as one can then better visualize the tissue as it is excised using free-hand dissection. A 90° or 3 clock-hour section of the keratoplasty wound is incised nearly to the level of Descemet's membrane, and the wedge of tissue is then removed [Figure - 6].

In general, paracentesis is unnecessary prior to resutur­ing the wound. Five to seven deep and evenly spaced interrupted 10-0 mersilene sutures are placed [Figure - 7]. Mersilene suture is preferred as this allows the sutures to be left in place indefinitely if a desired effect is achieved. Each suture is placed with a slipknot and the sutures are then tightened under keratometric con­trol until an overcorrection of 1 /3 to 1 /2 the preoperative cylinder is achieved [Figure - 8]. Sutures are then tied with an additional square knot, cut short and buried. For example, if the preoperative astigmatism is 12 D, su­tures are tightened until a 4-6 D overcorrection is achieved in the axis opposite to the preoperative cylinder.

The immediate postoperative care involves the use of topical antibiotics for one week and topical steroids as required to maintain the graft clarity. Sutures are left in place for a minimum of 8 weeks. Thereafter, one or two sutures may be removed selectively every three to four weeks in the axis of the steepest residual astigmatism. Once a satisfactory result is achieved, the remaining sutures may be left indefinitely [Figure - 9].


  Astigmatic keratotomy Top


Astigmatic keratotomy incisions include transverse in­cisions, semiradial incisions or the combination of the two which describes the trapezoidal astigmatic keratotomy popularized by Ruiz.

The axis of plus cylinder measured refractively or the steeper meridian measured keratometrically should be identified in order to direct the placement of semiradial or transverse incisions. Transverse incisions are placed perpendicular to the steeper meridian, and are indicated for the correction of low degrees of astigmatism. These incisions flatten the steeper meridian an equivalent amount as they steepen the flatter meridian. Therefore, the net effect is no change in spherical equivalent [10],[11],[14],[15],[21][Figure - 10].

A single transverse incision 3 mm in length placed coincident with a 5 mm optical zone mark corrects approximately 1.25 diopters of astigmatism in that meridian [Figure - 11]. Such an incision may be placed under the upper lid when only a small amount of astig­matism needs to be corrected. A pair of transverse incisions is useful if a greater degree of correction is desired. In cadaver eye studies, a single pair of transverse incisions placed 5 mm apart and 2.5 - 3.0 mm in length predicted 1.54 diopters of astigmatic correction in a 30 year-old patient." Clinical results from 124 patients from several surgeons have yielded a mean astigmatic correction of 1.58 diopters from a single pair of transverse incisions placed from 5 mm- 8mm apart. [10]

Studies of radial keratotomy have shown that the closer radial incisions get to the optical axis, the greater the effect on corneal flattening. [26],[36] Although it was logical to presume that transverse incisions made closer to the optical axis would yield more effect, human cadaver eye studies, as well as clinical studies, have demonstrated that the maximal effect of trap verse incisions is achieved when placed coincident with a 5 mm optical zone mark. [13],[14],[15],[30] Transverse incisions placed coin­cident with a 3 mm optical zone mark actually decreased the net effect when added to previously placed transverse incisions 5 mm apart. Therefore, transverse incisions placed more central than a 5 mm optical zone are to be avoided as they are less effective, induce more glare and carry a greater risk of inadvertently violating the visual axis. Transverse incisions placed further that 5 mm apart also have less effect than those placed exactly 5 Mm apart.

Transverse incisions placed at 7 mm have less effect than those placed at 5 mm. A single transverse incision placed 7 mm apart corrects approximately 0.75 diopters of astigmatism, whereas a pair of transverse incisions placed 7 mm apart corrects 1.0 diopter[Figure - 12].

Thornton [27] has suggested that longer transverse incisions at larger optical zone sizes result in a similar amount of astigmatic correction as shorter tangential incisions at smaller optical zone sizes. He contends that transverse incisions which span a full 45 degree arc, whether placed 5 mm apart of 8 mm yield a similar result because the length of the incisions vary depending on their distance from the optical center. Therefore, the desired effect may be varied by either the optical zone placement or the length of transverse incisions. The appropriate optical zones range from 5-8 mm and transverse incision length should range form 2.5-4 mm.

A second pair of transverse incisions may increase the effect achieved from an initial pair of incisions 5 mm apart. The addition of a second pair of transverse in­cisions 7 mm apart when added to previously placed transverse incisions 5 mm apart increased the amount of corrected astigmatism to approximately 2.25 diop­ter[Figure - 11]. In human cadaver eye studies, the addi­tion of a pair of transverse incisions 9 mm apart to previously placed transverse incisions 5 mm apart resulted in a statistically insignificant change in cor­rected astigmatism. [21] Use of greater than two pairs of tangential incisions is unwarranted. [14]

When transverse incisions are placed in conjuction with radial keratotomy incisions, they may be placed be­tween radial incisions or they may straddle radial in­cisions [Figure - 11]. Four-incision radial keratotomy cases, if planned properly, allow for placement of transverse incisions on axis, but between radial in­cisions. In 8-incision radial keratotomy cases, the transverse incisions may either straddle the radial in­cisions(jump T-cuts) or be placed between radial in­cisions. Experience has shown that intersecting transverse incisions with radial or semiradial incisions is to be avoided. [14],[15] Previous clinical studies have shown that when transverse incisions cross radial or semiradial incisions, entire facets of cornea have extruded. [15]sub When two incisions intersect, the point of intersection under­goes poor wound healing that often leads to epithelial inclusion cysts and/or exaggerated scar formation [7][Fig­ure 13].


  Semiradial incisions Top


Semiradial incisions have been described as lines placed halfway between a true radial line drawn from the optical zone and a line drawn perpendicular to the transverse incisions. [37] The effect of semiradial incisions alone has been studied in a laboratory setting in an attempt to better understand astigmatic keratotomy. [14] Paired semiradial incisions induce overall corneal flat­tening resulting in a less myopic spherical equivalent. However, the effect is approximately two times the amount of flattening in the meridian of the incision compared to the meridian 90 0 away [Figure - 14]. There­fore, semiradial incisions alone may correct moderate amounts of myopia while also correcting modest amounts of astigmatism. In cadaver eye studies, semi­radial incisions extended to a 3 mm optical zone cor­rected 1.4 diopters of astigmatism and 1.6 diopters of myopia [14][Figure - 15]


  Modified ruiz procedure Top


Semiradial incisions combined with transverse incisions describe the trapezoidal astigmatic keratotomy popularized by Ruiz[Figure - 16]. This procedure was initially designed to include 2 sets of semiradial incisions and 5 sets of equally spaced transverse incisions. Several laboratory and clinical studies have now con­clusively demonstrated that the maximal correction of astigmatism may be attained with only a single set of transverse incisions placed 5 mm apart between two sets of semiradial incisions made from a 3mm opticalzone. [13],[14],[30] Mild additional effect may be achieved in post-keratoplasty cases by adding a second pair of transverse incisions coincident with a 7 mm or 9 mm optical zone, between the semiradial incisions. Cadaver eye studies have shown that semiradial incisions were equally effective in reducing astigmatism when added to previously placed pairs of transverse incisions at either 5 and 7 mm apart or 5 and 9 mm apart. [21] These studies have shown that the multiple pairs of transverse incisions initially described by Ruiz are unnecessary [Figure - 16]

Predicting the effect of trapezoidal astigmatic kerat­otomy, may be thought of as the addition of semiradial incisions to transverse incisions. The net effect is an approximate threefold flattening in the steeper meridian with no change in the meridian 90 0 away [Figure - 17][Figure - 18]. As a guide, the trapezoidal astigmatic keratotomy can be expected to correct 5.5 diopters of astigmatism in a 30 year-old patient and up to 11.0 diopters in an 80 year-old patient[Figure - 19].

Cadaver eye studies have been shown to have an excellent correlation to clinical cases in both radial keratotomy studies as well as in astigmatism surgery. [5],[6],[17],[22],[23],[24],[25],[28] They, therefore, can serve as an accurate predictor for the clinical situation. A cadaver eye is assumed to approximate an 80 year-old patient's eye, whereas, a 30 year-old patient achieves ap­proximately 50% of the effect of the cadaver eye.


  Operative technique Top


Anesthesia can be achieved in the great majority of cases with topical 0.5% proparacaine drops. Oc­casionally a facial nerve block is necessary, but the use of retrobulbar or peribulbar injections are quite infre­quent. Perhaps the most important consideration in astigmatic keratotomy surgery is correctly identifying and marking the steeper meridian with a marking pen, which may often be simply done at the slit lamp. The visual axis is marked utilizing the operating microscope filament. A front cutting diamond knife is preferred for placement of transverse incisions, because the endpoint of the incisions can be better visualized when using such a knife. The appropriate knife setting is based on paracentral ultrasonic pachymetry readings. The knife is usually set at 100% of the paracentral pachymetry in order to achieve an 85% depth cut. Following placement of a single pair of transverse in­cisions 5 mm apart, the surgical keratometer is utilized to assess the effect. If more correction is desired, a second set of transverse incisions placed 7 or 9 mm apart may be added using the same technique employed with the first incisions. Placement at 9 mm may be preferred in post-keratoplasty patients to avoid the graft-host interface.

In cases where still further correction is desired, the trapezoidal astigmatic keratotomy is completed. Semi­radial incisions are, therefore, placed on either side of the tangential incisions so that they extend from the border of a 3 mm optical zone mark to a point ap­proximating but not crossing the limbus. A back cutting knife is preferred for these incisions. In cases where transverse incisions are combined with radial keratotomy, the radial incisions are generally placed first.

The results of the trapezoidal astigmatic keratotomy in post-keratoplasty patients may vary considerably from the predicted values because of the marked variability in wound healing in these patients. We have found that the surgical keratometer is extremely helpful in these patients. Our approach to the correction of astigmatism in post-keratoplasty patients is to begin with a single pair of transverse incisions 5 mm apart. We then measure the effect with a quantitative surgical keratometer. If adequate effect is not achieved, a second pair of transverse incisions is added 9 mm apart. Another ad­vantage from a second pair of transverse incisions that two pairs of transverse incisions tend to reduce the acute angle of the transition zone between central and peripheral cornea [Figure - 20]. Because most keratoplasty incisions are close to 7 mm in diameter, transverse incisions 7 mm apart are avoided. This ob­viates operating in the graft-host interface with the in­herent complication of perforation. Transverse incisions placed outside of the graft-host interface tend to counter the effect of disparate wound healing. Because transverse incisions themselves do not change the spherical equivalent, these incisions alone are desirable in cases where further hyperopic shift should be avoided. In myopic eyes with high degrees of astigma­tism, the addition of semiradial incisions to transverse incisions is warranted. To plan the procedure, the refrac­tion should be converted to a minus cylinder form. If a perfect result is achieved, one should be left with only the spherical component of the refraction [Figure - 18]. Although patient follow-up is still underway, we have observed occasional progression of the effect of transverse incisions and trapezoidal astigmatic kerat­otomy with time. It appears wise, therefore, to titrate the response, minimize the number of incisions and con­clude the procedure before overcorrection is achieved.

Postoperatively the effect can be enhanced a small amount by the use of topical corticosteroids four times daily for one month, followed by twice daily for one month and then once daily for an additional month. As has been mentioned previously, our impression is that progression rather than regression of effect is the rule over the course of the ensuing four to six months pos­toperatively. Undercorrections, therefore, seem more desirable than overcorrections at the present time, espe­cially in congenital cases where the patient has adapted to a cylinder axis over many years.

Persistent overcorrection following astigmatic kerat­otomy may be managed by placement of an interrupted suture across transverse or even semiradial incisions. Under topical anesthesia, previously healed wounds need to be opened and freed of epithelial plugs such that a stable scar might be achieved after placement of interrupted sutures. This may be accomplished by ir­rigating the depth of the wound or by curetting the wound with a fine chalazion curette. Interrupted 10-0 mersilene sutures are placed at approximately 75% depth and tightened with slip knot. Using the Terry keratometer, appropriate adjustment can be made with the slipknot to achieve a desired endpoint. Some degree of regression is expected with time, so an exaggerated effect should be obtained at the time of surgical correction. An addi­tional square knot is then tied on the slip knot and the knots are buried. The use of mersilene suture which may be left in the cornea for years has obvious advantages. The use of interrupted sutures allows for selective suture removal in cases of differential healing or post-suturing astigmatism.

The correction of postoperative astigmatism is an excit­ing area of refractive surgery where the understanding of the principles and the potential for improved predict­ability continues to evolve rapidly.

 
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    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12], [Figure - 13], [Figure - 14], [Figure - 15], [Figure - 16], [Figure - 17], [Figure - 18], [Figure - 19], [Figure - 20]



 

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  In this article
Abstract
Introduction
Relaxing incisions
Operative technique
Compression sutures
Wedge resection
Operative technique
Astigmatic kerat...
Semiradial incisions
Modified ruiz pr...
Operative technique
References
Article Figures

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