9.2Control of Corneal Astigmatism and Aberrations

Jorge L. Alió and Bassam El Kady

Core Messages

ßSub-2 mm incision of MICS manipulates the tissues gently with minimal surgical trauma

and distortion of the surrounding tissues.

ßAmong the major advantages of MICS is the reduction of surgical trauma resulting in a reduc-

tion of surgically induced astigmatism (SIA).

ßMICS is superior to standard coaxial phaco with regard to corneal aberration induction.

ßExcellent incision quality in MICS demonstrated immediate postoperative visual recovery,

equivalent to that of microcoaxial 2.2 incision.

ßCorneal OCT is an excellent way to study the postoperative evolution of cataract surgery

incision.

9.2.1Introduction: Impacts of MICS Incision on the Outcomes of Cataract Surgery

An important aspect in the modern techniques of cataract surgery is to provide the patient with an immediate postoperative visual recovery with good quality of vision. To a great extent the optical quality of the cornea is an essential factor for such rapid and excellent visual and refractive outcomes, as it plays an important role in the recovery of visual function after cataract surgery. This idea is one of the basic concepts and principals in MICS through providing a recent and innovative surgical technique of cataract extraction with a refractive touch, thanks to a reduction of incision size down to the minimum possible size [1].

J. L. Alió ( )

Department of Ophthalmology, Miguel Hernández University, Alicante, Spain

e-mail: jlalio@vissum.com

The reason behind this is that the micro sub-2 mm incision of MICS manipulates the tissues gently with minimal surgical trauma and distortion of the surrounding tissues, leading to reduction of surgically induced astigmatism (SIA) and aberrations, with subsequent improvement of the corneal optical quality ending in excellent visual outcome and high patient satisfaction [1–5].

So, it is important to study the corneal incision as in MICS there is a bare phaco tip hitting the cornea with potential damage for the corneal lamellae. This might have an influence in the immediate outcome of cataract surgery, especially in filtration [1, 2].

9.2.2Objective Evaluation of Corneal Incision

Visual function after cataract surgery is determined by a combination of corneal and internal aberrations generated by the IOL and those induced by the surgery. These corneal refractive changes are attributed to the location and size of the corneal incision applying the fact that the smaller the incision, the lower the aberrations, the better the optical quality [1, 6–8].

Degraded optical quality of the cornea after incisional cataract surgery would limit the performance of the pseudophakic eye. So, it is important not to increase or to induce astigmatism and/or corneal aberrations after cataract surgery [9]. MICS could achieve reduction of the astigmatism and higher-order corneal aberrations [8].The need to study this with new technology evolved as clinical evaluation is insufficient to evaluate minimal changes in the incision configuration. Corneal topography, aberrometry and corneal optical coherence tomography (OCT) were the main tools for adequate and objective analysis of corneal incision and the best means to know whether this corneal trauma is affecting the quality of corneal optics [10, 11].

9.2.3Control of Corneal Aberration and Astigmatism with MICS

We have described the improved control on corneal SIA with MICS when compared to conventional 3 mm phacoemulsification. A great advantage of MICS is the

reduction of SIA and that the microincision does not produce an increase in astigmatism [1].The shorter the incision, the less the corneal astigmatism, as it was estimated that the magnitude of the SIA studied by vector analysis is around 0.44 and 0.88 D, rising as the size of the incision increases [12, 13].

Also, sutureless small-incision 3.5-mm surgery does not systematically degrade the optical quality of the anterior corneal surface. However, it introduces changes in some aberrations, especially in nonrotationally symmetric terms such as astigmatism, coma, and trefoil [14]; therefore, one has to expect better results and lesser changes with sub-2 mm incision (MICS).This is further supported by the finding that the corneal incision of sub-2 mm had no impact on corneal curvature [8, 15, 16].

All of these factors were the driving forces for us to carry out an objective study to investigate if MICS sub 2 mm incision effectively decreases the induction or changes in corneal higher-order aberrations (HOA) during cataract surgery, evaluating its effect on the corneal optical quality [10]. We studied 25 eyes of 25 patients with nuclear or corticonuclear cataracts graded from +2 to +4 (lens opacities classification system III) [17]. All of them underwent MICS through a 1.6–1.8 mm clear corneal incision, placed on the axis of the positive corneal meridian, and using low ultrasound power. An Acri.Smart 48S lens (Acri.Tec, Germany) was implanted in all eyes. Seidel aberration root mean square (RMS) values were obtained for a 6-mm pupil using the CSO topographer (CSO, Firenze, Italy) preoperatively, 1 and 3 months after surgery [10].

9.2.4Role of Corneal Aberrometry in Evaluating MICS Incision

Owing to its role in providing the most important metrics for evaluating the visual function, we used the corneal aberrometry to evaluate corneal optical quality, as about 80% of all aberrations of the human eye occur in the corneal surface [18]. The topography system we used (CSO topographer) analyzes up to 6,144 corneal points within the corneal zone between 0.33 and 10mm with respect to corneal vertex, with more expressive analysis and assessment for up to the seventh order aberration by using the Zernike polynomials. In addition, it avoids superimposing of the light spots associated to

different parts of the wavefront produced by highly aberrated eye. Finally, with some kind of global aberrometry, it is assumed that the slope of the wavefront in each analyzed portion is locally flat; this could induce significant errors in the calculated final results [18].

Corneal aberrations were derived from the data of the anterior surface of the cornea obtained from this topography system, the software of this topography system, the EyeTop2005 (CSO), makes the conversion of the corneal elevation profile into corneal wavefront data using the Zernike polynomials with an expansion up to the seventh order. The RMS of the wave aberration (Seidel aberrations) was studied at 6-mm pupil diameter for: total, coma (Z3 ± 1), spherical (Z40), reported with its sign, astigmatism (Z2 ± 2) and HOA. Measurements were performed preoperatively, as well as 1 and 3 months postoperatively [10].

9.2.5Role of OCT in Evaluating MICS Incision

The OCT system Visante proved its usefulness as a very helpful tool in assessing corneal incision quality for different types of cataract surgery. It allows dynamic evaluation of incision behaviour and its effect on the corneal tissue as a sensitive indicator of corneal function and incision quality. Additionally, it provides a new non-contact device which is able to accurately evaluate incision quality and to detect even mild signs undetectable by slit-lamp [11]

OCT, being a non-contact method, is able to study and evaluate all the parameters of the incision and so any degree of incision deformability is attributable to ocular dynamics without external forces being applied to the wound. This confirms the role of OCT as an accurate quantitative tool assessing the incision effect and quality [11].

9.2.6Our Experience in Corneal Aberrations and Astigmatism After MICS

The comparison between corneal topography and aberrometry maps preoperative vs. 1 and 3 months after

Fig 9.2.17 Top: Corneal topography map (axial map) preoperative (left) vs. 1 month (center) and 3 months (right) after MICS surgery for one of the 25 eyes of the study. Bottom: Corneal

aberrometry map (Seidel panel) preoperative (left) vs. 1 month (center) and 3 months (right) after MICS surgery for one of the 25 eyes of the study

Corneal Power (D)

44.6

44.4

44.2

44.0

43.8

43.6

43.4

43.2

Corneal Astigmatism (D)

Fig. 9.2.18 Corneal power in diopters (D). Preoperative vs. 1, 3 months after surgery

Fig. 9.2.19 Corneal astigmatism in diopters (D). Preoperative vs. 1, 3 months after surgery

MICS surgery is shown in Fig. 9.2.17, in order to illustrate the slight change induced by the surgery. The corneal power slightly changed after surgery (Fig. 9.2.18): preoperative, 44.08 ± 1.21 D; 1 month, 43.97 ± 1.05 D, and 3 months, 44.17 ± 1.18 D. There were no statistical differences between the different follow-up visits (Bonferroni, p > 0.05), even when comparing the

corneal power at 1 and 3 months after surgery (p = 1.00). Also, the corneal astigmatism (Fig. 9.2.19) did not show statistically significant changes (pre: −0.80 ± 0.76 D; 1 month post: −0.61 ± 0.57; 3 months post −0.63 ± 0.62) (Bonferroni, p > 0.05 for all comparisons). The mean change in corneal astigmatism preoperatively to 3 months was −0.19 ± 0.40 D.

Fig. 9.2.20 Corneal aberrations (Seidel coefficients) at 6-mm pupil diameter before, and 1 and 3 months after MICS

RMS (microns)

The evolution of the corneal aberrations after surgery is shown in Fig. 9.2.20. The RMS value of the total corneal aberrations slightly decreased on average after MICS (pre, 2.15 ± 2.51 μm; 1 month post, 1.87 ± 1.87 μm; 3 months post, 1.96 ± 2.01 μm), but the difference were no statistically significant (Bonferroni, p = 1.00 for both comparisons). The analysis of some individual Zernike terms, such as astigmatism, spherical, and coma, showed: Astigmatism pre, 0.85 ± 0.74 μm; 1 month post, 0.65 ± 0.44 μm, and 3 months post, 0.69 ± 0.46 μm; Spherical: pre,−0.11 ± 0.25 μm, 1 month post, −0.09 ± 0.25 μm, and 3 months post, −0.19 ± 0.13 μm. Coma aberration slightly decreased on average: pre, 0.45 ± 0.40 μm, 1 month post, 0.39 ± 0.36 μm, and 3 months post, 0.42 ± 0.44 μm, with no statistically significant differences between the follow-up visits (Bonferroni, p = 1.00 for both comparisons). Also, no statistically significant changes were found for higher order aberration (HOA): pre, 0.47 ± 0.26 μm, 1 month post, 0.59 ± 0.32 μm and 3 months post 0.54 ± 0.25 μm (Bonferroni, p > 0.47 for both comparisons).

Although the corneal astigmatism didn’t show statistically significant changes, there were slight differences between preoperative, 1 and 3 month postoperative values (Bonferroni, p > 0.05 for all comparisons). The mean change in corneal astigmatism preoperatively to 3 months was −0.19 ± 0.40 D, with little difference between 1 and 3 month measures, indicating that MICS provides completely stable incision for up to 3 months postoperatively. This results supports the previously published studies [1, 8, 9, 13], augmenting the advantage of MICS over traditional small incision cataract surgery, which induces low levels of astigmatism [9].

Now we can conclude that MICS provides a neutral incision in terms of astigmatism.

All the values of individual aberration, except HOA, slightly decreased on average with no statistically significant differences between the follow-up visits. The whole measures were stable with no changes for up to 3 months, indicating the successful golden role of MICS in avoiding induction of higher order aberrations and creating surgically neutral and stable procedure, thus returning the patients back after cataract surgery to high standards of visual life with good satisfaction like that of healthy subjects of a similar age.

Before drawing a conclusion, we have reported that such results have been recently confirmed, referring to our series of publication in that aspect, which also proved good behaviour of MICS in terms of neutral incision with stabilization of corneal optics post surgery [19]. Even more, MICS incision is not only optically perfect, but additionally it respects more corneal prolateness with less corneal oedema in the short-term outcomes and less induction of corneal aberrations in the longterm results, with an excellent quality both optically and morphologically (Figs. 9.2.21 and 9.2.22) [11], enabling outstanding postoperative wound dynamics and facilitating faster visual recovery and early postoperative patient ambulation [11].

9.2.7 Conclusion

Finally, we can conclude that MICS sub 2 mm incision surgery is effective in stabilizing the cornea after surgery, effectively decreasing the induction of corneal HOA. It does not degrade the corneal optical quality

nor induces modification of the corneal astigmatism, even in the axis. This demonstrates that a maximum reduction to sub-2 mm in incision size improves the control of the optical performance of the human eye.

Stable corneal optics with MICS frames this technique as a refractive procedure through controlling and even decreasing astigmatism and HOA, so minimized incision maximized the outcomes.