is an associate professor of ophthalmology at the Cullen Eye Institute, Department of Ophthalmology, Baylor College of Medicine. Her areas of research include various aspects in cataract surgery, refractive surgery, diagnostic devices, optics, and wavefront technology and its use in refractive and cataract surgery.
is professor and the Allen, Mosbacher, and Law Chair in ophthalmology at the Cullen Eye Institute, Baylor College of Medicine, where he specializes in cataract and refractive surgery. His research interests include optics of cataract and refractive surgery, intraocular lens technology, anterior segment imaging, and surgical techniques in cataract and refractive surgery. He is Editor Emeritus of the Journal of Cataract and Refractive Surgery and past president of the American Ophthalmological Society, American Society of Cataract and Refractive Surgery, and International Intraocular Implant Club.
7.1 Introduction
Over the past decades, we have seen a constant decrease in sutureless cataract incision size, from more than 4 mm in the 1990s to less than 2 mm today. In January 1990, Dr. Michael McFarland introduced the concept of sutureless surgery. He performed a scleral tunnel incision, located 4 mm behind the surgical limbus, with a 4.5-mm internal width and a 3-mm external wound [1]. During spring 1992, Dr. Howard Fine developed a more efficient sutureless corneal incision. The incision was located temporally, with a rectangular shape architecture of 4 mm in width and 1.75 mm in length [2].
This evolution has been enabled by the improved surgical techniques and technology such as phacoemulsification and foldable lenses. As a result, cataract surgery has evolved from a simple opaque lens extraction surgery to a more refined refractive procedure, allowing for a fast visual rehabilitation and limited induced astigmatism, as envisioned by the father of modern cataract surgery, Charles Kelman.
However, even if the size of sutureless cataract incisions has decreased by more than half, from a 4-mm wound down to a standard 3 mm, then to “2.2-mm mini incision” and more recently “sub-2-mm microincision,” the subject is still of a compelling nature. One of the reasons is that a cataract incision is not only a simple entry port into the eye but also a potential source of surgically induced astigmatism (SIA) [3–5]. In fact, corneal incisions flatten the incised meridian, and the amount of SIA is primarily linked to its location, architecture, construction, and, last but not least, size [6–10].
7.2 Principles
An incision on the cornea induces flattening in the incised meridian and steepening in the meridian 90° away. This effect, known as SIA, has become popular particularly since the onset of toric intraocular lenses. The term SIA is defined as the amount and direction of corneal steepening occurring after the surgery when compared to the preoperative astigmatic state. The flattening effect corresponds to the flattening at the site of the incision, which is calculated by vector analysis, based on pre- and postoperative keratometry. It varies with the angular distance of the meridian of interest from the SIA (incision site), diminishing and reversing to steepening once the angle of separation exceeds 45°. The astigmatic changes are correlated with the incision size and location. The larger the incision, the higher the corneal astigmatic changes, and the more distant from the visual axis (sclera versus cornea, temporal versus nasal or superior), the less induced the astigmatism [3–10].
7.3 Measurement of Astigmatism
Thorough analysis of astigmatic data is crucial to understand the results of cataract surgery. Astigmatic analysis can be simple, but at the same time extraordinarily complex [11]. Astigmatism is characterized by both magnitude and axis. In terms of astigmatic analysis, the corneal changes inherent to a cataract incision can be measured either with a simple algebraic method or with a more sophisticated vector analysis. The algebraic method, which measures the magnitude of astigmatism and its meridional location, is clinically relevant because the actual amount of astigmatism impacts uncorrected visual acuity. However, the algebraic approach does not accurately quantify the surgically induced change that has occurred. Vector analysis allows for a more precise evaluation of the surgically induced astigmatic changes. Several methods have been developed to analyze surgically induced astigmatism. They calculate the astigmatic changes considering not only the magnitude but also the meridional shift of the astigmatism [12–15].
7.4 Astigmatic Effects of 3.2-, 2.2-, and 1.8-mm Superior Clear Corneal Incisions
In a randomized prospective study, we examined 190 consecutive eyes of 151 patients operated for cataract. All patients underwent superior clear corneal phacoemulsification (PEA) through a 3.2-mm CCI, 2.2-mm CCI, or 1.8-mm CCI. The 3.2-mm group comprised 61 eyes (47 patients); the 2.2-mm group comprised 66 eyes (52 patients); the 1.8-mm group comprised 63 eyes (52 patients).
The magnitude of preoperative astigmatism was not considered in selecting the incision size. 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 at four points onto the cornea of four near-infrared rays. The diameter of measured central corneal area was 3.3 mm, and the measurements were internally repeated three times to give the final result. The steps of the measurements were set at 0.25 D.
7.4.1 Surgical Technique
All surgeries were performed by the same surgeon (JLF). The incision was performed with a pre-calibrated metal knife at 11 o’clock, 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.
No wound enlargement was necessary in any group, and a wound-assisted injection was performed in all cases of the 1.8-mm group.
7.4.2 Results
7.4.2.1 Vector Analysis
We used a vector analysis, based on 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.
7.4.2.2 With-the-Wound Change
The WTW mean changes were −0.38 + 0.47 D, −0.05 + 0.45 D, and −0.04 + 0.39 D, in the 3.2-mm, 2.2-mm, and 1.8-mm group, respectively (Table 7.1). The WTW changes were significantly greater in the 3.2-mm group compared to both the 2.2- and the 1.8-mm group (both P < 0.001), whereas the comparison between the two smaller incisions showed no difference (P = 0.90).
Table 7.1
With-the-wound (WTW), against-the-wound (ATW), and WTW-ATW changes in three groups (mean ± SD, range)
WTW (D) | ATW (D) | WTW-ATW (D) | |
|---|---|---|---|
3.2-mm group | −0.38 ± 0.47 | +0.38 ± 0.40 | −0.76 ± 0.60 |
−1.43 to +1.05 | −0.39 to +1.45 | −1.95 to +0.53 | |
2.2-mm group | −0.05 ± 0.45 | +0.16 ± 0.34 | −0.20 ± 0.56 |
−1.49 to +1.05 | −0.76 to +0.94 | −1.70 to +1.35 | |
1.8-mm group | −0.04 ± 0.39 | +0.16 ± 0.29 | −0.20 ± 0.45 |
−0.93 to +1.21 | −0.50 to +1.19 | −1.39 to +1.12 |
7.4.2.3 Against-the-Wound Change
The ATW mean changes were 0.38 + 0.40 D, 0.16 + 0.34 D, and 0.16 + 0.29 D in the 3.2-mm, 2.2-mm, and 1.8-mm group, respectively (Table 7.1). The ATW changes were statistically significantly higher in the 3.2-mm group compared to both the 2.2- and the 1.8-mm group (both P < 0.001), whereas it did not differ between the two smaller incision groups (P = 0.93).
7.4.2.4 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, 2.2-mm, and 1.8-mm group, respectively (Table 7.1). The change was statistically significantly higher in the 3.2-mm group compared to both the 2.2- and the 1.8-mm group (both P < 0.001), but showed no difference among the two smaller incision groups (P = 0.96).
7.5 Astigmatic Effects of 2.2-mm Superior and Temporal Clear Corneal Incisions
We conducted a randomized prospective study of 121 consecutive eyes of 98 patients operated for cataract. All patients underwent clear corneal PEA through a 2.2-mm CCI located superiorly or temporally. The superior group comprised 66 eyes (52 patients) and the temporal group 55 eyes (46 patients).
The magnitude of preoperative astigmatism was not considered in selecting the incision size. Keratometric astigmatism was measured preoperatively and 1 month after surgery, using the same autokeratometer (Tonoref II, Nidek, Aichi, Japan). The steps of the measurements were set at 0.25 D.
7.5.1 Surgical Technique
The same surgeon performed all the surgeries (JLF). The incision was created with a pre-calibrated metal knife at 11 o’clock, starting with a partial thickness groove (approximately 300 μm depth) 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. No wound enlargement was necessary in either group.
7.5.2 Results
7.5.2.1 Vector Analysis
We used a vector analysis, based on the Holladay-Cravy-Koch formula detailed in the previous study.
7.5.2.2 With-the-Wound Change
The WTW mean changes were −0.05 ± 0.45 D in the 2.2-mm superior CCI group and −0.14 ± 0.24 D in the 2.2-mm temporal (Table 7.2). The comparison between the two incisions showed no difference.
Table 7.2
With-the-wound (WTW), against-the-wound (ATW), and WTW-ATW changes in two groups (mean ± SD, range)
WTW (D) | ATW (D) | WTW-ATW (D) | |
|---|---|---|---|
Temporal 2.2-mm group | −0.06 ± 0.24 | +0.17 ± 0.29 | −0.29 ± 0.45 |
−0.74 to +0.46 | −0.50 to +1.00 | −1.50 to +0.66 | |
Superior 2.2-mm group
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