24 July 2021
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Original Citation:
Astigmatic equivalence of 2.2-mm and 1.8-mm superior clear corneal cataract incision.
Published version:
DOI:10.1007/s00417-014-2854-5
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Astigmatic equivalence of 2.2-mm and 1.8-mm superior clear corneal cataract incision
Jean-Luc Febbraro, Li Wang, Edmondo Borasio, Lorenzo Richiardi, Hamza N. Khan, Alain Saad, Damien Gatinel & Douglas D. Koch
Abstract
Purpose
To evaluate the astigmatic effects of 2.2-mm and 1.8-mm cataract incisions.
Methods
A randomized prospective study of 190 eyes of 151 patients undergoing superior clear corneal incision (SCCI) was divided into three groups: 61 eyes with a control 3.2-mm SCCI; 66 eyes with a 2.2-mm SCCI; and 63 eyes with a 1.8-mm SCCI. The corneal astigmatism was measured with an autokeratometer preoperatively and 1 month after surgery. The with-the-wound (WTW), against the-wound (ATW), and WTW-ATW changes were calculated using the Holladay-Cravy-Koch formula.
Results
The WTW, ATW, and WTW-ATW changes were significantly higher for the control 3.2-mm SCCI than for the 2.2- and 1.8-mm SCCI (all p < 0.001), and no difference was found between the 2.2- and 1.8-mm SCCI incision groups.
Conclusion
In our study, the astigmatic effects were the same for the 2.2 mm and 1.8 mm incisions and as expected, were significantly lower than the control 3.2 mm incision group.
Introduction
During the past decade, there has been ongoing work to reduce the phacoemulsification incision size. The advantages of incision size reduction are the improvement of ocular integrity during and after surgery, and the decrease in surgically induced astigmatism. Several studies have shown that astigmatism induced by a 3.2-mm CCI (clear corneal incision) is higher than that occurring after 2.6- or 2.2-mm incision surgery. Studies have demonstrated that incisions down to 2.5 mm still induce significant astigmatism, although the astigmatic effect is decreased [1–6]. Other studies have demonstrated that sub-2-mm micro-incision cataract surgery induces very limited astigmatism [7–9]. In refractive phacoemulsification the goal is to achieve an uncorrected vision better than 20/40. The less that induced astigmatism interferes with this goal, the better. We will show that 2.2-mm and 1.8-2.2-mm cataract incisions have a negligible impact on preexisting astigmatism.
Material and methods
Preoperative data
This randomized prospective study comprised a consecutive cohort of 190 eyes of 151 patients who underwent cataract surgery. All patients signed an informed consent and agreed to participate in the study. All patients underwent superior clear corneal
phacoemulsification through a 3.2-mm CCI, 2.2-mm CCI, or 1.8-mm CCI. The first 61 eyes received the control 3.2-mm incision, the next 66 eyes received the 2.2-mm incision, and the final 63 eyes received a 1.8-mm incision. The magnitude and axis of preoperative astigmatism were not considered in selecting the incision size. Exclusion criteria were eyes with associated corneal, vitreous, or retinal pathologies. Keratometric astigmatism was measured preoperatively and 1 month after surgery, using the same autokeratometer (Tonoref II, Nidek, Aichi, Japan). The Tonoref II measures the corneal curvature based on the projection onto the cornea of four near-infrared rays. The diameter of measured central corneal area was 3.3 mm, and the measurements are internally repeated three times to give the final result. The refractive index used was 1.3375, measured at steps of 0.25 D.
Surgical technique
All surgeries were performed by the same surgeon (JLF). Topical anesthesia was mostly used by topically instilling oxybuprocaine chlorhydrate and injecting 0.1 cc of intracameral preservative-free 1 % lidocaine through the stab incision. The incision was made with a pre-calibrated metal knife at the 11 o’clock position, starting with a partial thickness groove at the superior limbus, followed by an approximately 2-mm long stromal tunnel. The two-plane incision architecture resulted in an almost square shape incision in 2.2- and 1.8-mm incision subgroups and a more rectangular shape in the 3.2-mm subgroup. Capsulorhexis was completed with micro-forceps under viscoelastic protection. Phacoemulsification was performed after hydrodissection and hydrodelineation with a stop and chop or a divide and conquer technique. The aspiration of the residual cortex was conducted with the automated irrigation and aspiration tip. The Infiniti phaco platform, with 30-degree bevel Mini-Flared tips (Alcon Laboratories Inc., Ft Worth, TX, USA) was used for the control 3.2- and the 2.2-mm groups. Standard sleeves were used in the 3.2-mm group, whereas Ultra sleeves were used in the 2.2-mm group. The Stellaris (Bausch & Lomb, Rochester, NY, USA) phaco platform was used in the 2.2- and 1.8-mm group. In the 3.2-mm group, Adapt AO, (Bausch & Lomb, Rochester, NY, USA) intraocular lenses were implanted in the bag, whereas Acrysof Natural SN60WF (Alcon Laboratories Inc., Ft Worth, TX, USA) intraocular lenses were injected in the bag with Monarch II injectors and D cartridges in the 2.2-mm group, and MI 60 (Bausch & Lomb, Rochester, NY, USA) were implanted with a 1.8-mm Viscojet Medicel injector in the 1.8-mm group. No wound enlargement was necessary in any group, and a wound-assisted injection was performed in all cases of the 1.8-mm group. The viscoelastic was removed at the end of the surgery, and incision self-sealing was ensured by wound hydration at the end of the case.
Vector analysis
We used the Holladay-Cravy-Koch formula to calculate the with-the-wound (WTW) change, located at the meridian of the incision, the against-the-wound (ATW) change, located 90° away from the surgical meridian, and the WTW-ATW change, which
represents the overall change induced by the incision [10]. Simple scalar comparisons were also made by comparing the preoperative and postoperative keratometric astigmatism magnitude (arithmetic astigmatism difference).
We carried out multivariable linear regression analysis adjusting each outcome variable for age and gender, and we allowed for intra-patient correlation using the cluster option in the software STATA. As the original dataset included both unilateral and bilateral cases, we reported for each comparison the estimates obtained considering all evaluated eyes (unilateral and bilateral cases) and then repeated the analysis after restricting the dataset
to one eye per patient by choosing one eye randomly (via a computer-generated algorithm) in bilateral cases (data not shown).
The results for each variable were displayed graphically by means of box plots. For each value, comparisons were made between the 3.2-mm control group and the 2.2-mm and 1.8-mm superior incision groups. First, we carried out a Wald test for an overall difference among the three groups and, if a difference was found, we conducted pair-wise
comparisons using multivariable linear regression.
Results
Demographics
The analysis comprised 190 eyes of 151 patients. The control 3.2-mm group comprised 61 eyes (47 patients; 14 bilateral cases); the 2.2-mm group comprised 66 eyes (52 patients; 14 bilateral cases); the 1.8-mm group comprised 63 eyes (52 patients; 11 bilateral cases). The overall mean age was 75.1 ± 9.2 years with no significant differences between the groups (74.7 ± 11.3 in the 3.2-mm control group, 73.9 ± 8.4 in the 2.2-mm group, 76.6 ± 7.6 in the 1.8-mm group) (p = 0.20). There was an overall higher prevalence of women (65 %) compared to men, and the proportion differed among groups: female patients were 45.6 % of all patients in the 3.2-mm control group, 71.7 % in the 2.2-mm group, 67.9 % in the 1.8-mm group (p < 0.005).
Scalar comparison of astigmatism
The scalar magnitude of the preoperative astigmatism was similar in the three groups, but the scalar magnitude of the postoperative astigmatism was higher in the 3.2-mm control group (0.94 ± 0.68 D) compared to the 2.2 mm group (0.78 ± 0.42 D) and the 1.8-mm group (0.67 ± 0.49 D). The difference between the two smaller incision groups was not significant (p = 0.146). (Table 1, Fig. 1)
Against-the-wound change
The ATW mean changes were 0.38 ± 0.40 D in the 3.2 mm control group, 0.16 ± 0.34 D in the 2.2 mm group, and 0.16 ± 0.29 D in the 1.8 mm group
(Table 2). The ATW changes were statistically significantly higher in the 3.2-mm control group compared to both the 2.2-mm and the 1.8-mm groups (both p < 0.001), whereas it did not differ between the two smaller incision groups (p = 0.93).
WTW-ATW change
The mean WTW-ATW changes were −0.76 ± 0.60 D, −0.20 ± 0.56 D, and −0.20 ± 0.45 D, in the 3.2-mm control group, 2.2-mm group, and 1.8-mm group, respectively (Table 2). The change was statistically significant in the 3.2-mm control group compared to both the 2.2-mm and the 1.8-mm groups (both p < 0.001), but showed no difference between the two smaller incisions groups (p = 0.96).
Discussion
Several methods have been developed to analyze surgically induced astigmatism [10–15]. Many studies have demonstrated that incision size, location, and meridian all affect the surgically induced astigmatism (SIA) [1–9]. With regards to follow-up, Masket has shown that a 1-month postoperative evaluation is sufficient to estimate the surgically induced astigmatism with sub-3-mm temporal corneal incision [16]. Corneal incisions of any sort flatten the meridian of application and steepen 90 degrees away through coupling. The advantage of the Holladay-Cravy-Koch method is to calculate the incision-induced changes at both the surgical meridian (WTW) and also 90° away (ATW). In our study, no WTW, ATW, and WTW-ATW changes were found between the two smaller incision groups while, as expected, statistically significant greater changes were found in the 3.2-mm control group. The mean values found in the 1.8-mm and 2.2-mm groups were clinically small: less than 0.25 D. These values are near to zero and close to the limit of accuracy of refraction and keratometry measurements. These results are consistent with previously published results by Alio et al. [17].
This past decade, microincision cataract surgery has been adopted by more and more surgeons worldwide, and the trend toward smaller incisions seems to be unstoppable. The main advantage of decreasing incision size is reduced induced astigmatism. In practical terms, this advantage can be counteracted by several disadvantages. In fact,
phacoemulsification through sub-2-mm incisions can be considered technically more challenging, require compatible phaco platforms, with efficient fluidics, and well-calibrated phaco and I/A probes. In addition, this incision size can only accommodate hydrophilic intraocular lens implantation, and with a wound assisted injection technique. On the contrary, a slightly larger incision, such as a 2.2-mm one, may offer more flexibility for the surgeon in terms of intraoperative maneuvers, lens implantation technique, and intraocular lens options. In addition, these incisions become even more appealing to the surgeon if the induced astigmatism is equivalent to sub-2-mm ones.
The limitations of this study are: 1. 1.
Our study included in several cases the two eyes of the same patient, which may alter the results due to problems of correlation within-patients. When our analyses were repeated using only one eye per patient, we found consistent findings,
although based on fewer observations and thus associated with larger standard errors.
2. 2.
We studied only superior incisions. It would be valuable to see analogous data comparing the effect of incisions of these sizes on corneas receiving temporal clear corneal incisions.
3. 3.
The incision size may be modified during phacoemulsification due to intraoperative manoeuvers with instruments and probes and intraocular injection. As a result, a
slight enlargement of the incision may occur, but this difference of size between the beginning and the end of the surgery was not studied in all of our patients
4. 4.
We have not considered the optical zone radius of each incision size. In fact, proximity to visual axis is an important factor which, along with incision width, plays an important role and affects the induced astigmatism. The distance of incision from the visual axis varies for each incision and could be part of the explanation for the higher cylinder variance in the control group. Further studies could be valuable to determine the impact of optical zone radius on induced astigmatism with various cataract incision sizes.
We have proven that micro-incision (sub-2.2-mm) cataract surgery is, as expected, superior to the older standard 3.2-mm incision in terms of astigmatic changes, even superiorly. Superior incisions heal better because they are under the lid, but even this advantage does not significantly help the 3.2-mm incision. The surprising result is that 2.2-mm and 1.8-2.2-mm incisions are equivalent in terms of induced astigmatism, perhaps due in part to the improvement in wound healing under the lid. Unexpected changes are less likely to occur after 2.2-mm and 1.8-mm CCI, and therefore, the preoperative corneal cylinder is more likely to remain unchanged after the surgery.
If the aim of cataract surgery is astigmatic neutrality, micro-incisions smaller than or equal to 2.2-mm appear to be sufficient. Even though this study shows statistical equivalence of the 1.8-mm and 2.2-mm incision in terms of the mean, further developments should be pursued to reduce the standard deviation of the results and reduce outliers.
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Financial disclosure
All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge, or beliefs) in the subject matter or materials discussed in this manuscript.
Precis
Our study has shown no statistically significant difference in terms of astigmatic changes between 2.2-mm and 1.8-mm superior clear corneal cataract incisions.
Author information
Affiliations
1. Rothschild Foundation, 25 rue Manin, 75019, Paris, France
o Jean-Luc Febbraro o , Alain Saad
o & Damien Gatinel
2. Cullen Eye Institute, Baylor College of Medicine, 6565 Fannin, NC 205, Houston,
TX, 77030, USA o Li Wang
o & Douglas D. Koch
3. Moorfields Eye Hospital Dubai, Dubai Healthcare City, POBOX 505054, Dubai,
United Arab Emirates o Edmondo Borasio
4. Unit of Cancer Epidemiology, CeRMS and CPO Piemonte, University of Turin,
Turin, Italy
o Lorenzo Richiardi
5. University of British Columbia, Vancouver, BC Canada, 1120 Yates Street, Suite
400, Victoria, BC, V8V 3M9, Canada o Hamza N. Khan
6. Center for Expertise and Research in Optics for Clinicians, (CEROC), Paris, France
o & Damien Gatinel
7. 1 Bld de Beauséjour, 75016, Paris, France
o Jean-Luc Febbraro
