Citation: Li PP, Huang YM, Cai Q, Huang LL, Song Y, Guan HJ. Effects of steep-axis incision on corneal curvature in one-handed phacoemulsification. almol 2019;12(8):1277-1282. DOI:10.18240/ijo.2019.08.07
·Clinical Research·
Effects of steep-axis incision on corneal curvature in one-handed phacoemulsification
Pan-Pan Li1, Ye-Meng Huang2, Qi Cai1, Li-Li Huang1, Yu Song1, Huai-Jin Guan2
1Department of Ophthalmology, Nantong City No.1 People’s Hospital and Second Affiliated Hospital of Nantong University, Nantong 226000, Jiangsu Province, China
2Department of Ophthalmology, Affiliated Hospital of Nantong University, Nantong 226000, Jiangsu Province, China
Correspondence to: Yu Song. Department of Ophthalmology, the First People’s Hospital of Nantong, Second Affiliated Hospital of Nantong University, Nantong 226000, Jiangsu Province China. songyuleye@126.com; Huai-Jin Guan. Department of Ophthalmology, Affiliated Hospital of Nantong University, Nantong 226000, Jiangsu Province, China. guanhuaijinleye@126.com
Received:
Abstract
AIM: To examine the effects of one-handed phacoemulsification with steep-axis incision on corneal curvature and analyze surgically induced astigmatism (SIA) on the true net power, anterior and posterior corneal surfaces.
METHODS: Patients with cataracts underwent one-handed phacoemulsification with a 2.4-mm steep-axis of clear corneal incision (CCI) based on true net power. CCI was created under the guidance of Verion. Central corneal thickness (CCT), keratometry readings of the true net power and anterior and posterior corneal surface were obtained using Pentacam. Biometry, such as axial length, anterior chamber depth (ACD) and white-to-white (WTW) were performed using Lenstar pre- and 3mo post-operatively.
RESULTS: The study evaluated 68 eyes of 65 patients. The mean
age was 65.93±9.40y; CCT was 529.21±37.40 μm; WTW was 11.59±
CONCLUSION: One-handed phacoemulsification with steep-axis incision can effectively decrease astigmatism of true net power and anterior corneal astigmatism. In the same surgery, the difference in personal SIA potentially originated from a difference in personal corneal thickness and diameter, both CCT and WTW distance should always be measured preoperatively when planning steep-axis phacoemulsification.
KEYWORDS: steep-axis; one-handed phacoemulsification; true net power; surgically induced astigmatism
DOI:10.18240/ijo.2019.08.07
Citation: Li PP, Huang YM, Cai Q, Huang LL, Song Y, Guan HJ. Effects of steep-axis incision on corneal curvature in one-handed phacoemulsification. almol 2019;12(8):1277-1282
INTRODUCTION
Currently, cataract surgery has entered the era of refractive surgery, and the intraoperative correction of corneal astigmatism is a necessary requirement for refractive cataract surgery (RCS). Many methods are available to correct preexisting corneal astigmatism, such as single or paired opposite clear corneal incision (CCI) at the steepest meridian[1-2], limbal relaxing incision[3], and Toric intraocular lens (IOL) implantation[4]. Precise measurement of corneal astigmatism is crucial for RCS and other refractive surgical corrections. Conventional devices, such as keratometry, calculate corneal astigmatism [known as keratometric astigmatism (KA); the keratometric refractive index is typically 1.3375] only based on the anterior corneal planes[5]. Current Scheimpflug tomographers are based on the Scheimpflug principle and measure both the anterior and posterior corneal surfaces. The device calculates the true net power of the total cornea by adding sagittal curvature values of the anterior and posterior corneal surface and measures the total corneal astigmatism (TCA) by a ray tracing technique[6].
Previous studies have analyzed surgically induced astigmatism (SIA) based on steep-axis CCI of anterior corneal astigmatism (ACA) or KA in two-handed phacoemulsification[1]. The main purpose of our study was to examine the effects of steep-axis CCI (based on the steep meridian of true net power) on corneal curvature in one-handed phacoemulsification and to analyze the SIA on the true net power and anterior and posterior cornea surfaces.
SUBJECTS AND METHODS
Ethical Approval This study was performed in accordance with the ethical principles of the Declaration of Helsinki and was approved by the Affiliated Hospital of Nantong University’s Ethics Committee. All patients provided written informed consent after receiving a full explanation of the study.
Subjects This study consisted of patients with age-related cataract (ARC) who underwent one-handed phacoemulsification at the Department of Ophthalmology, Affiliated Hospital of Nantong University, Jiangsu, China, from January 2017 to September 2017. The Lens Opacities Classification System III (LOCS III) was used for cataract classification. Inclusion criteria included the following: 1) patients with ARC and regular astigmatism; 2) nuclear opalescence ≤3; 3) normal anterior segment and fundus examination; 4) no history of intraocular surgery or injury. Exclusion criteria included the following: 1) cataract other than ARC; 2) corneal scars, corneal opacities and other ocular disease that might affect visual outcomes.
Measurements All patients underwent a complete ophthalmologic measurement. Biometry, such as white-to-white (WTW), anterior chamber depth (ACD) and axial length, were performed using a Lenstar non-contact optical low-coherence reflectometer (LS900; Haag-Streit, Koniz, Switzerland). Central corneal thickness (CCT), keratometry readings of the true net power, anterior and posterior corneal surface were obtained using a Pentacam (70700, Oculus, Wetzlar, Germany). Anatomic landmarks of the eye were obtained using Verion (Alcon Laboratories, Inc., Fort Worth, TX, USA). The image was then transferred onto a USB drive inserted into the Verion Digital Marker monitor screen, which was used to guide the position of steep-axis CCI. The 2.4-mm CCI was selected for the steep axis, which was chosen according to the steepest corneal meridian of the true net power. The triangle representing the target incision appeared on both the microscope and monitor (Figure 1).
Figure 1 The steep-axis clear corneal incision was created at 30° under the guidance of Verion.
Surgical
Technique All operations were performed by an
experienced surgeon (Guan HJ). A 2.4-mm single-plane CCI was created at the
steep meridian of true net power under the guidance of Verion. Central
continuous curvilinear capsulorhexis (CCCC) was performed under the guidance of
Version. After CCCC, hydrodissection and hydrodelineation were performed, and
nuclear emulsification was performed using the one-handed technique[7]. All phacoemulsification was performed using the
Centurion Vision System (Alcon, Fort Worth, America) in burst mode. The
ultrasound and fluidic settings were as follows: bottle height,
Data Analysis The true net power of the cornea and ACA were classified into three types according to the meridian of the maximal convergent power: within 90°±30° [with the rule (WTR)], within 0±30° [against the rule (ATR)], and within 30°-60° or 120°-150° [oblique (OBL)][8]. Given that the dioptric power of the posterior corneal surface was negative, we classified posterior corneal astigmatism (PCA) as WTR when the steep meridian was within 0±30° and as ATR when the steep meridian was within 90°±30°. We classified the remaining astigmatism as oblique astigmatism[9].
Vector
Analysis SIA was analyzed by vector analysis
according to the Alpins method[10]. SIA= [A12+A22
Statistical Analysis Statistical analysis was performed using SPSS for Windows software (version 22, SPSS, Inc., America). The Chi-square test was used for comparison of the type of corneal astigmatism. The paired-samples t-test was used for the comparison of preoperative and postoperative data. The level of statistical significance was P<0.05.
RESULTS
Our study
evaluated 68 eyes (37 right and 31 left) from 65 patients (42 females and 23
males). The mean age was 65.93±9.40y (range: 50 to 85y, 95%CI for the mean:
63.65 to 68.20y). The axial length was 24.09±
Table 1 presents the types of corneal astigmatism preoperatively and at 3mo postoperatively, and no statistically significant difference was noted in the composition of astigmatism type on true net power and the anterior and posterior corneal surface preoperatively versus postoperatively (all P>0.05).
Table 1 Types of corneal astigmatism preoperatively and 3mo postoperatively n (%)
Astigmatism |
Preoperative |
Postoperative |
P |
True net power |
0.221 |
||
With the rule |
12 (26.00) |
15 (22.06) |
|
Against the rule |
40 (52.00) |
30 (44.12) |
|
Oblique |
16 (2.00) |
23 (33.82) |
|
Anterior corneal surface |
0.248 |
||
With the rule |
15 (22.06) |
23 (33.82) |
|
Against the rule |
33 (48.53) |
25 (36.76) |
|
Oblique |
20 (29.41) |
20 (29.41) |
|
Posterior corneal surface |
0.804 |
||
With the rule |
7 (10.29) |
7 (0.29) |
|
Against the rule |
57 (83.82) |
55 (80.88) |
|
Oblique |
4 (5.88) |
6 (8.82) |
Tables 2-4 present the keratometric data [keratometric value at the flattest corneal meridian for the 3-mm central zone (Ks), keratometric value at the steepest corneal meridian for the 3-mm central zone (Kf), astigmatism, power vectors were computed at axis 0 according to the Jackson coefficient orthogonal coordinate system (J0), power vectors were computed at axis 45 according to the Jackson coefficient orthogonal coordinate system (J45)] and SIA of the true net power, anterior and posterior corneal surface preoperatively and at 3mo postoperatively.
Table 2 Keratometric data and SIA of the true net power preoperatively and 3mo postoperatively
Parameters |
Preoperative |
Postoperative |
P |
||
Mean±SD |
95%CI |
Mean±SD |
95%CI |
||
Ks |
43.97±1.58 |
43.59 to 44.36 |
43.76±1.33 |
43.43 to 44.08 |
0.031 |
Kf |
42.72±1.54 |
42.72 to 1.54 |
42.73±1.38 |
42.40 to 43.07 |
0.825 |
Astigmatism |
1.21±0.56 |
1.08 to 1.35 |
1.02±0.58 |
0.88 to 1.16 |
0.021 |
J0 |
-0.29±0.51 |
-0.41 to -0.17 |
-0.14±0.41 |
-0.24 to -0.44 |
0.005 |
J45 |
-0.19±0.26 |
0.13 to 0.26 |
-0.12±0.38 |
-0.21 to -0.03 |
0.000 |
SIA |
- |
- |
1.26±0.63 |
1.11 to 1.41 |
- |
Ks: Keratometric value at the flattest corneal meridian for the 3-mm central zone; Kf: Keratometric value at the steepest corneal meridian for the 3-mm central zone; J0: Power vectors were computed at axis 0 according to the Jackson coefficient orthogonal coordinate system; J45: Power vectors were computed at axis 45 according to the Jackson coefficient orthogonal coordinate system; SIA: Surgically induced astigmatism.
Table 3 Keratometric data and SIA of the anterior corneal surface preoperatively and 3mo postoperatively
Parameters |
Preoperative |
Postoperative |
P |
||
Mean±SD |
95%CI |
Mean±SD |
95%CI |
||
Ks |
45.15±1.51 |
44.79 to 45.52 |
43.04±1.33 |
44.72 to 45.36 |
0.095 |
Kf |
44.07±1.47 |
43.72 to 44.43 |
44.17±1.37 |
43.88 to 44.50 |
0.203 |
Astigmatism |
1.08±0.51 |
0.96 to 1.20 |
0.87±0.46 |
0.76 to 0.98 |
0.002 |
J0 |
-0.16±0.49 |
-0.28 to -0.09 |
0.03±0.37 |
-0.11 to -0.06 |
0.011 |
J45 |
0.18±0.24 |
0.12 to 0.24 |
-0.04±0.32 |
-0.12 to 0.03 |
0.000 |
SIA |
- |
- |
1.05±0.54 |
0.92 to 1.19 |
- |
Ks: Keratometric value at the flattest corneal meridian for the 3-mm central zone; Kf: Keratometric value at the steepest corneal meridian for the 3-mm central zone; J0: Power vectors were computed at axis 0 according to the Jackson coefficient orthogonal coordinate system; J45: Power vectors were computed at axis 45 according to the Jackson coefficient orthogonal coordinate system; SIA: Surgically induced astigmatism.
Table 4 Keratometric data and SIA of the posterior corneal surface preoperatively and 3mo postoperatively
Parameters |
Preoperative |
Postoperative |
P |
||
Mean±SD |
95%CI |
Mean±SD |
95%CI |
||
Ks |
-6.55±0.27 |
-6.61 to -6.65 |
-6.65±0.28 |
-6.72 to -6.58 |
0.000 |
Kf |
-6.59±0.30 |
-6.48 to -5.71 |
-6.63±0.23 |
-6.39 to -6.72 |
0.000 |
Astigmatism |
-0.25±0.14 |
-0.29 to -0.22 |
-0.31±0.18 |
-0.35 to -0.26 |
0.008 |
J0 |
-0.09±0.10 |
-0.06 to -0.11 |
-0.10±0.12 |
-0.13 to -0.08 |
0.180 |
J45 |
-0.01±0.06 |
-0.02 to 0.01 |
-0.04±0.07 |
-0.05 to -0.01 |
0.012 |
SIA |
- |
- |
0.21±0.17 |
0.17 to 0.25 |
- |
Ks: Keratometric value at the flattest corneal meridian for the 3-mm central zone; Kf: Keratometric value at the steepest corneal meridian for the 3-mm central zone; J0: Power vectors were computed at axis 0 according to the Jackson coefficient orthogonal coordinate system; J45: Power vectors were computed at axis 45 according to the Jackson coefficient orthogonal coordinate system; SIA: Surgically induced astigmatism.
Ks of true net power was significantly decreased at 3mo postoperatively (P=0.031). Kf of true net power was increased at 3mo postoperatively, but no significant difference was noted (P=0.825). Astigmatism of true net power was 1.21±0.56 D preoperatively and significantly decreased to 1.02±0.58 D at 3mo postoperatively (P=0.021). J0 and J45 of true net power exhibited significant differences preoperatively versus postoperatively (all P<0.05).
No significant difference in Ks and Kf of the anterior corneal surface was noted preoperatively and postoperatively (all P>0.05). ACA was 1.08±0.51 D preoperatively and significantly decreased to 0.87±0.46 D at 3mo postoperatively (P=0.002). J0 and J45 of the anterior corneal surface exhibited significant differences between preoperative and 3-month postoperative values (all P<0.005).
Ks and Kf of the posterior corneal surface were significantly increased at 3mo postoperatively (all P<0.05), and PCA was also increased (P=0.008). J45 of the posterior corneal surface exhibited significant differences between preoperative and postoperative values (P=0.012).
The SIA values of true net power and the anterior and posterior corneal surfaces at 3mo postoperatively were 1.26±0.63 D (range: 0.11 to 2.80 D, 95%CI for the mean: 1.11 to 1.41 D), 1.05±0.54 D (range: 0.23 to 2.40 D, 95%CI for the mean: 0.92 to 1.19 D), and 0.21±0.17 D (range: 0.01 to 0.07 D, 95%CI for the mean: 0.17 to 0.25 D), respectively. SIA values of true net power and anterior and posterior corneal surfaces greater than 0.5 D were noted in 60 eyes (88.23%), 55 eyes (80.88%) and 4 eyes (5.88%), respectively.
DISCUSSION
Correcting corneal astigmatism is one of the main goals during modern RCS. In previous practice, cataract surgeons typically paid more attention to ACA or KA because instruments to measure PCA or TCA were not commercially available. In recent years, people have become increasingly aware of the importance of total corneal curvature or true net power in RCS. The main purpose of our study was to examine the effects of one-handed phacoemulsification on corneal curvature and analyze the SIA of the true net power and the anterior and posterior cornea surfaces.
Improvements in ophthalmic instruments and microsurgical techniques have propelled CCI into the current trend of phacoemulsification[11]. In RCS, CCI at the steepest meridian is the simplest and most commonly used surgical procedure for correcting preexisting astigmatism of the cornea. Previous studies hypothesized that CCI at the steep axis could steepen the flat meridian[12]. In our study, Ks of true net power significantly decreased at 3mo postoperatively (P=0.031). Ks of the anterior corneal surface was decreased at 3mo postoperatively, but no significant difference was observed (P=0.095). This result is likely attributed to the fact that the steep median of the anterior corneal surface is close to the true net power but not completely identical, and our steep-axis CCI was based on the steep meridian of true net power. Borasion et al[13] reported that corneal astigmatism was 1.18±0.67 D preoperatively and 0.97±0.54 D postoperatively in the steep-axis CCI group, and the difference was statistically significant (P=0.03). In our study, astigmatism of true net power and ACA were significantly decreased at 3mo postoperatively (P<0.005). But PCA was significantly increased at 3mo postoperatively (P=0.008). This result is attributed to the fact that the steep median of the posterior corneal surface is different to the true net power in our study.
The SIA is an integral component of refractive surgery, and personalized SIA power has been advocated in the planning of cataract surgery. SIA is influenced by the incision numbers, size (width and length), configuration (1-step, 2-step, and 3-step) and location (clear corneal on-axis and temporal incisions)[11,14-18]. Cavallini et al[19] reported a mean SIA value of 0.72 D after coaxial phacoemulsification with a 2.2-mm CCI. Kawahara et al[14] reported that the mean SIA was 0.40±0.28 D and 0.39±0.25 D respectively in the one-handed and two-handed technique with a 2.4-mm transconjunctival single-plane sclerocorneal incision. Koç et al[20] reported a mean SIA value of 0.85±0.42 D with a 2.8-mm CCI and two 1-mm paracentesis 90° from the main port. Nemeth et al[1] reported that the SIA on the anterior surface was 0.49±0.29 D and SIA on the posteriorsurfac surface was 0.32±0.29 D after phacoemulsification with a 2.8-mm steep-axis CCI, and a 1.2-mm side port incisions was made 60° left of the main incision. Xu et al[21] reported that the mean SIA was 0.60±0.24 D in the temporal CCI group and 0.57±0.27 D in the steep-axis CCI group (P=0.84) with a 3.2-mm CCI. In our previous study, 3mo after one-handed phacoemulsification, the SIA on the anterior surface was 0.75±0.44 D. The SIA on the posterior surface was 0.22±0.14 D with a 2.4-mm single-plane CCI created at 135° position[11]. In this study, the SIA values of the true net power and anterior and posterior corneal surfaces at 3mo postoperatively were 1.26±0.63, 1.05±0.54, and 0.21±0.17 D, respectively. SIA of the anterior corneal surface was slightly increased compared with one-handed phacoemulsification with the same CCI created at 135°. SIA values of true net power and anterior and posterior corneal surfaces greater than 0.5 D were noted in 60 eyes (88.23%), 55 eyes (80.88%) and 4 eyes (5.88%), respectively. In our study, SIA was slightly higher than in previous studies.
The corneal
thickness and radius also affect corneal astigmatism. Woo and Lee[22] reported that CCT was negatively correlated with the
value of SIA. In his study, CCI was 537±30 μm, and SIA was approximately 0.6 D
with a 2.7-mm CCI. Theodoulidou et al[23] reported
that SIA was 0.77±0.43 D in group A (WTW≤
(range: 449
to 619 μm). The WTW was 11.59±
Toric IOLs are another preferred solution to correct the preoperative corneal astigmatism during cataract surgery. We need to input steep K, steep axis, flat K, flat axis, SIA, incision location and other parameters using an online calculator. Personal CCT and WTW would influence his SIA thus influence calculating the type and axes of Toric IOLs.
We often need to input the position of the main incision and ignore the position of the side incision when calculating the type and axes of Toric IOLs using the online calculator. Kawahara et al[14] found that there was no significant difference in SIA between the one-handed and two-handed group, but the corneal side port had a rotating effect on the axis of astigmatism which means corneal side incision would influence the calculation of axes of Toric IOLs. In addition, Xu et al[21] reported that compared with the temporal CCI, steep-axis CCI could improve the accuracy of Toric lens implantation and the ability to correct astigmatism. Combined with the above Kawahara et al[14] and Xu et al[21] two points, when we want to correct corneal astigmatism with Toric IOLs, we may be more inclined to performed steep-axis one-handed phacoemulsification without corneal side incision. Due to the limitation of the instrument (Pentacam, 70700, Oculus, Wetzlar, Germany), we must replace the total corneal curvature with true net power. Recently, some instruments obtained total corneal curvature by ray tracing calculation. Given that the anterior and posterior surfaces’ principal planes are slightly different, total corneal curvature measured by ray tracing technique is more realistic than true net power[24].
In conclusion, our results indicate that one-handed phacoemulsification with steep-axis incision can effectively decrease the astigmatism of true net power and ACA. The CCT and WTW distance should always be measured preoperatively when planning steep-axis CCI or Toric IOL implantation. Further studies are needed to evaluate the effectiveness and accuracy of the one-handed technique combined with Toric lens implantation.
ACKNOWLEDGEMENTS
Authors’ contributions: Study concept and design (Li PP, Song Y, Guan HJ); data collection (Huang YM, Huang LL, Cai Q); analysis and interpretation of data (Li PP, Huang YM, Huang LL); writing of the manuscript (Li PP); critical revision of the manuscript (Song Y, Guan HJ); administrative, technical, or material support (Song Y, Guan HJ); supervision (Song Y, Guan HJ).
Foundation: Supported by the Jiangsu Province Science and Technology Department of Social Development Major Projects-Key Diseases Standardization Diagnosis and Treatment Projects (No.BE2016699).
Conflicts of Interest: Li PP, None; Huang YM, None; Cai Q, None; Huang LL, None; Song Y, None; Guan HJ, None.
REFERENCES