Customized incision cataract surgery: Astigmatism modulation techniques in manual small-incision cataract surgery

Abstract

Advanced instrumentation and intraocular lenses (IOL) allow great refractive and visual outcome control to permit excellent correction of refractive aberrations. Residual astigmatism can be modified to provide depth of focus using an appropriate incision in the steepest meridian in manual small-incision cataract surgery (MSICS). The authors describe the nomogram for surgically correcting astigmatism (SCA). This technique can handle preoperative astigmatism of about 2.25 DCyl with the standard incisions—straight incision of 5, 6, and 7 mm in length, the minimally curved frown incision, the frown incision, the frown incision with an accentuated frown, and the U incision placed on the steep axis in the superior or the temporal quadrant depending on the axis of pre-operative astigmatism.

With advanced instrumentation and intraocular lenses (IOL), cataract surgery permits control over refractive and visual outcomes just like refractive surgery. Residual astigmatism post cataract surgery is an undesirable outcome in such a situation.

In the general population preoperatively 70% of patients have less than 1 D astigmatism. Various studies place the higher levels of astigmatism between 1 and 2 D at 20% and the remaining 10% at greater than 2 D.[1] The prevalence of preoperative astigmatism in cataracts was reported to be 86.6 with 19–22% having astigmatism in excess of 1.5 D. Based on the magnitude, a panel of cornea specialists agreed to classify this as low—0.25 to 1.5 D; medium—1.5 to 3 D; and high—more than 3 D.[2]

The first aim is to achieve surgery without complications and get the best-corrected visual acuity (BCVA) of logMAR score of 0.0/0.12 or less. The general target of emmetropia or leaving a refractive error for both sphere and cylinder of 0.50 D or less and cylinder of 1 D or less with the axis of 20 degrees or less is usually considered a success in the control of astigmatism. A less aggressive approach to the residual cylinder is based on the observation that residual myopic against the rule astigmatism improved the uncorrected near vision in monofocal IOL implant. Uncorrected myopic astigmatism more than 1 D degraded the distance acuity at no additional benefit to near acuity, whereas uncorrected hyperopic astigmatism lowered distance and near acuities of pseudophakic eyes.[3] Myopic astigmatism less than 1.0 D may be beneficial for near vision and is detrimental beyond that, whereas hyperopic astigmatism is undesirable at any level.[4]

To combat pre-existing astigmatism in cataract surgery on-axis corneal incision (OCI), limbal relaxing incisions (LRI), opposite clear corneal incisions (OCCI), toric IOLs, and bioptics are commonly advocated. These have been used in femto laser-assisted cataract surgery, phacoemulsification, and also in manual small-incision cataract surgery (MSICS).

Preoperative evaluation includes manifest refraction, slit-lamp examination tonometry, fundus examination, ocular biometry and computerized corneal topography where available. The corneal astigmatism is combatted using the incision-induced surgically induced astigmatism (SIA) to offset the pre-existing amount and meridian of corneal astigmatism. Therefore, the use of the term surgically corrected astigmatism (SCA) is used to differentiate this targeted modulation of astigmatism using the incision. Extra care is required if there is low pre-existing astigmatism.

Corneal astigmatism can be evaluated by manual keratometers, automated keratometers, placido-based corneal topographers, Scheimpflug image-based topographers, low-coherence reflectometers, and scanning-slit corneal topographers. Anterior corneal direct measurements can be taken from manual keratometers, automatic keratometers, and placido ring-based topographers. In the present case, the K readings were taken first on Topcon AutoK and several readings were taken. If the majority of readings are consistent, then the axes suggested by AutoK were explored using manual K (Appasamy), of which three readings were taken and averaged. These were then rounded off to the nearest 0.25 D step.

The authors describe the technique to modify astigmatism to achieve the most favorable results from astigmatic modification using MSICS. This has been used for almost two decades, and the audit of the performance of the incision on a steep axis tailored to counter preoperative astigmatism leads to the description of the standard incisions used and their modifications described here in the form of the Boramani algorithm for customized incision cataract surgery (CICS). The algorithm is based on the results of over 320 independent surgeries performed by a single surgeon (Boramani J), and the analysis was conducted by separate analysts blinded to group allocation to eliminate bias.

Type of incision: The authors have studied the results of over seven different incisions—the straight incisions of 5, 6, and 7 mm length, the minimally curved frown incision, the frown incision, the frown incision with an accentuated frown, and the U incision. The incisions were placed in the superior and temporal quadrants to counter the polar description of astigmatism. CICS with astigmatic modulation (AM) can be used in almost 9 out of 10 cases of cataracts to correct pre-existing astigmatism to leave a minimal myopic astigmatic error. The technique uses instruments and techniques from toric IOL implantations. Reference marks are made with a bubble marker in the sitting position as shown in the photograph and the accompanying video. The steep axis was marked with a degree gauge and axis marker with the patient lying on the table as shown in the photograph. The steep axis is used for the incision.

Surgical Technique

The surgery is conducted under topical anesthesia without any superior rectus suture. A conjunctival peritomy is done in the steepest meridian and a fornix-based conjunctival flap is raised. AC maintainer is inserted and connected to BSS bottle kept at about 60 cm above eye level. It is the first thing to go in and the last one to come out. The irrigation is continuous in this positive pressure surgery. Viscoelastic is used in complicated cases. The episcleral tissue is meticulously cleaned, and wet field cautery is used to control the bleeding. Usually, a sterile Johnson bud and gentle pressure can easily provide a clean surgical field. The customized incision is already decided on the basis of the nomogram developed by the authors. The target is minimal myopic astigmatism to be left behind as residual astigmatism. For larger corrections, a straight incision is used, whereas for lower targets of surgical astigmatism the authors use a curved incision, which may be minimally curved, frown, an accentuated frown, or even U shaped incision with back cuts. The incision is marked on the steepest meridian. The surgeon prefers to start dissecting the sclera with a crescent knife without initial groove marking. However, a groove can also be used made with a diamond blade or a Bard–Parker knife or even the crescent to the depth of about 200 microns with the same results. If it is shallower, then the depth can be increased by a 45-degree curved-crescent blade as we extend this incision forward at a depth of about half scleral thickness into the cornea. The crescent, seen through the sclera, acts as a guide to depth. If the crescent is seen too clearly through the sclera, then the depth is shallow and there is a need to go deeper. If the surgeon is not able to see the crescent, then the depth of the crescent is too deep and can result in perforation or premature entry. The dissection is carried out into the cornea for about 1–1.5 millimeters. Side pockets with convexity outward are created from the external to the internal incision. The internal lip is a curved incision made using a keratome after the side-port has been made using an angled MVR blade and capsulorhexis has been completed. Cortical cleavage hydro dissection with a bent cannula creates a freely rotating nucleus. The nucleus prolapsed by nudging at the equator with a dialler. The nucleus is prolapsed into the anterior chamber and then engaged into the internal lip. The delivery is with Blumenthal’s Mininuc technique using Sheets’ glide or iris repositor. In the case of a smaller incision, this is assisted by “Endoexpression” by Sinsky hook passed through side port with or without fracture of the nucleus in the scleral tunnel. The cortical clean-up is achieved using the aspiration cannula (from the set of duet cannulae), attached to a 5 mL syringe, through a silicon tube. Hydroimplantation of the IOL is followed by hydration of ports and closure of the case.

A special axis marker was used to mark the steep axis under topical anesthesia. The incision was customized, depending on the need to modulate the amount of astigmatism. For lower amounts of astigmatism correction, a frown incision was used but for a higher amount of change, the straight incision was preferred. The type of incision was influenced by the hardness of the nucleus through all incisions, permitting nucleus management of all types of cataracts.

Results

The mean surgically corrected astigmatism of the entire dataset was 0.60 (standard deviation [SD] =0.46) D. These data were then analyzed for the effect of the location of the incision in different quadrants and finally based on the type of incision employed. The results are given in Tables 1 and 2 as well as shown in Figs. 1 and 2.

Table 1.

Surgically corrected astigmatism with straight incisions with Boramani algorithm

Site	Shape	Size	Astigmatism observed
Superior	Straight	7.0 mm	1.69 (SD=0.19) D
Superior	Straight	6.0 mm	1.24 (SD=0.24) D
Superior	Straight	5.0 mm	0.82 (SD=0.19) D
Temporal	Straight	7.0 mm	1.08 (SD=0.28) D
Temporal	Straight	6.0 mm	0.53 (SD=0.19) D
Temporal	Straight	5.0 mm	0.44 (SD=0.18) D

Table 2.

Surgically corrected astigmatism in Boramani algorithm

Site	Shape	Astigmatism observed
Superior	Minimal curve	0.58 (SD=0.20) D
Superior	Frown	0.54 (SD=0.19) D
Superior	More curved frown	0.41 (SD=0.07) D
Superior	U shaped frown	0.29 (SD=0.16) D
Temporal	Minimal curve	0.26 (SD=0.14) D
Temporal	Frown	0.21 (SD=0.06) D
Temporal	Straight - 5.0 mm	0.44 (SD=0.18) D

Figure 1.

Straight incisions

Figure 2.

Curved incisions

Discussion

The surgically corrected astigmatism in the present series for the pooled data was 0.60 (SD = 0.46) D. However, this cannot be compared with surgically induced astigmatism in other studies as the incision modification was used to target some residual astigmatism, which was universally achieved. In 4–7% of cases, complete emmetropia may be achieved but overcorrection into hyperopic astigmatism was avoided as it can degrade the quality of vision. In 2005, Gogate et al.[5] reported a mean SIA of 1.2 (SD = 1.1) D in 187 patients of MSICS, whereas George et al.[6] reported a mean of 1.5 (SD = 0.77) D in 53 subjects. In 2012, Goel et al.[7] in their study of 30 subjects showed an SIA of 0.95 (SD = 0.48) D. Gogate et al.[8] in 2010 in their report of 75 subjects reported a mean of 0.95 (SD = 0.7) D. In 2009, Singh et al.[9] in a study of 89 patients found a mean SIA of 0.09 (SD = 0.82) D. Venkatesh et al.,[10] in 2010, reported SIA to be 1.2 (SD = 0.36) in 117 eyes. However, in all these studies, the aims were not astigmatism modulation by incision. The target with steep axis incision is to eliminate the spherical residual error by optical or ultrasonic biometry so that only astigmatic error is left to be dealt with, to leave behind either emmetropia or a very low myopic astigmatism. This provides a better depth of focus and spectacle freedom for near work. A review of the existing literature supports the view that a small degree of astigmatism is induced surgically in MSICS. However, no other study is available, which has studied the effects of customizing the incision to modulate the surgically induced/corrected astigmatism. The graphs for surgically corrected astigmatism for straight and curved incisions have been given in the Figs. 1 and 2 for further interpolation studies. The location of the incision also had a bearing on the SCA, with the superior incisions being more punishing than the temporal ones. The rough guide for incision selection is given in Fig. 3.

Figure 3.

Boramani algorithm with incision architecture

Conclusion

The authors advocate the use of incision on the steep axis, with the external incision configuration being used to modulate residual astigmatism. The surgical technique has been reliably used for corrections up to 1.75 D. Thus pre-existing errors up to 2.25 D can be tackled with this technique. For errors beyond this, other techniques have been used. The incision size used has been from 5.0 mm to 7.0 mm or beyond depending upon the intended astigmatism modulation. This customized small-incision cataract surgery uses surgically corrected astigmatic modulation to target residual astigmatism to the benefit of the patient in all cases of cataract surgery.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.