Effect of On-Axis Incision on Surgically Induced Astigmatism Following Phacoemulsification Cataract Surgery

By Sadia Humayun, Amna Tariq, Samar Fatima, Muhammad Shahid

Affiliations

1. Department of Eye, Combined Military Hospital, Lahore, Pakistan

doi: 10.29271/jcpsp.2026.08.1023

ABSTRACT
Objective: To estimate surgically induced astigmatism (SIA) and the flattening effect produced by an on-axis incision in patients undergoing phacoemulsification cataract surgery.
Study Design: A quasi-experimental study.
Place and Duration of the Study: Department of Eye, Combined Military Hospital, Lahore, Pakistan, from May to September 2024.
Methodology: A total of 142 eyes underwent phacoemulsification with a 2.75-mm limbal incision placed on the steep axis. Keratometric (K) readings, including flat K (K1) and its axis, steep K (K2) and its axis, and astigmatism and its axis, were measured by the same technician before surgery and again 6 weeks postoperatively using an optical biometer (Tomey AL 100). Uncorrected visual acuity (UCVA) and corrected distance visual acuity (CDVA) were recorded for all patients preoperatively and postoperatively. SIA was calculated using the ASSORT online calculator, and changes in astigmatism, UCVA, and CDVA were analysed using the Wilcoxon signed-rank test (SPSS version 26), with statistical significance set at p < 0.05.
Results: The mean age of participants was 62.9 ± 6.9 years, with 60.6% males. Preoperative astigmatism averaged 0.88 ± 0.52 dioptres (D) and decreased to 0.65 ± 0.48 D postoperatively, representing a mean reduction of 0.23 ± 0.30 D (p < 0.001). The mean SIA was 0.58 ± 0.50 D, and target-induced astigmatism was 0.45 ± 0.45 D. Both UCVA and CDVA improved significantly (p <0.001). Nearly 70% of eyes gained three or more Snellen lines of corrected vision.
Conclusion: A 2.75-mm on-axis limbal incision during phacoemulsification produces low and predictable SIA, a statistically significant reduction in corneal astigmatism and substantial improvement in visual acuity. This approach provides a practical, cost-neutral option for astigmatism management in cataract surgery.

Key Words: Phacoemulsification, Astigmatism, Cataract extraction, Cornea/surgery, Visual acuity.

INTRODUCTION

Cataract has long been recognised as a leading cause of blindness, persisting as a significant public health concern on a global scale.¹ In developing nations, the prevalence of cataract tends to be higher, with onset occurring at earlier ages. Notably, Asia contributes to the majority of these cases.² Despite ongoing efforts in public health aimed at increasing cataract surgery rates, cataract remains the leading cause of visual impairment among older adults worldwide.³

With advancements in intraocular lens (IOL) calculations and the introduction of newer-generation formulas, approximately 94% of patients achieve a best-corrected visual acuity of 20/30 or better, while 72–80% achieve a postoperative refrac-tive  outcome within 0.5 dioptres (D) of emmetropia.⁴

Moreover, enhancements in cataract surgery techniques have minimised the level of surgical invasiveness. The width of the incision has been notably diminished, now typically measuring less than 3 mm. Consequently, there are fewer modifications to the corneal shape and an earlier attainment of stabilisation.⁵ Surgically induced astigmatism (SIA) is the change in astigmatism that occurs from before to after surgery. This astigmatism results from the healing and scar formation at the surgical incision site.^6,7

Research indicates that a substantial portion of patients undergoing cataract surgery already have pre-existing astig-matism. Corneal astigmatism can cause various visual dis- comforts, ultimately diminishing patient satisfaction follow-ing cataract surgery. Thus, mitigating pre-existing astigmatism and correcting corneal astigmatism are crucial principles in cataract surgery to enhance patient outcomes.⁸

Astigmatism significantly affects visual quality, and both the surgical technique and the surgeon’s proficiency are crucial determinants. The incidence of SIA ranges widely, from 7.50% to 75%, depending on the surgical approach and the skill level of the surgeon.

Additionally, approximately 25–30% of eyes exhibit clinically significant astigmatism (>2 D), necessitating corrective measures.⁹ Strategies commonly employed to rectify pre- existing astigmatism during cataract surgery include on-axis incisions, limbal relaxing incisions, and the use of toric IOLs.¹⁰ Despite the predictable outcomes associated with toric IOL implantation, its universal adoption in developing nations is hindered by financial constraints. This study aimed to estimate the SIA and the flattening effect of an on-axis incision in patients undergoing phacoemulsification cataract surgery.

METHODOLOGY

This quasi-experimental study was conducted at the Department of Eye, Combined Military Hospital, Lahore, Pakistan, from May to September 2024. Written informed consent was obtained from all participants. Ethical approval was obtained from the hospital’s Ethics and Research Committee (No. 581/2024), which complied with the principles of the Declaration of Helsinki.

The sample size formula used for the Wilcoxon signed-rank test was:^11,12 n = (Zα/2+Zβ)² × σ² d / Δ², where n represented the required sample size (82 participants), Zα/2 was the critical value for the significance level (1.96 for α = 0.05), Zβ was the critical value for the desired power (0.84 for 80% power), σd was the standard deviation of the paired differences (0.58),¹³ and Δ was the minimum detectable (clinically significant) difference (0.18).¹³ Adjusting for an attrition rate of 20%, a minimum of 101 eyes was required for this study.

A simple consecutive sampling technique was used. All patients underwent the same surgical procedure performed by an experienced surgeon in the same theatre setting. The study included participants with visually significant cataracts who underwent uneventful cataract surgery and excluded those with a history of ocular trauma or previous ocular surgery, corneal disease or dystrophy, corneal ectasia, dry eye disease, glaucoma, diabetic retinopathy, or any intra- operative or postoperative complications.

Keratometric (K) readings, including flat K (K1) and its axis, steep K (K2) and its axis, and astigmatism and its axis, were measured by the same technician before surgery and again 6 weeks postoperatively using an optical biometer (Tomey AL 100). Uncorrected visual acuity (UCVA) and corrected distance visual acuity (CDVA) of all patients were recorded preoperatively and postoperatively. The 6-week postoperative measurements were chosen to ensure that SIA had stabilised by that time.⁹ Biometry was performed using an optical biometer (Tomey AL 100) to measure axial length and to calculate IOL power. Additionally, moxifloxacin and nepa-fenac 0.1% eye drops were administered every 6 hours starting 3 days before surgery.

On the day of surgery, the pupil was dilated with 1% tropicamide, and the 0° and 180° axes were marked at the slit lamp while the patient was placed in an upright position to minimise errors caused by cyclotorsion occurring when the patient assumes the supine position. The patient was positioned supine under the operating microscope, ensuring comfort and head stability throughout the procedure. The surgical eye was cleaned with a povidone-iodine solution (5%) to reduce microbial load. Sterile drapes were applied to cover the patient’s face, leaving the surgical field exposed. A sterile speculum was used to keep the eyelids open and immobilised. The steep axis was marked before starting the surgery. All surgeries were performed under topical anaesthesia. Under topical anaesthesia, a 2.75-mm single-plane limbal incision was created at the steep axis. A viscoelastic substance (hydroxypropyl methylcellulose) was injected into the anterior chamber to maintain its shape, protect the corneal endothelium, and facilitate surgical manipulation. A continuous curvilinear capsulorhexis, approximately 5-5.5 mm in diameter, was then created using a 26-gauge needle cystotome. This was followed by hydrodissection and hydrodelineation. The phaco probe was introduced into the anterior chamber through the main incision.

Ultrasonic energy was used to break the lens nucleus into fragments (commonly via the divide-and-conquer technique). Lens fragments were aspirated using the probe while maintaining anterior chamber stability with irrigation. Residual cortical material was removed using a coaxial irrigation/aspiration (I/A) device. After these steps, a suitable IOL was inserted into the capsular bag. The viscoelastic material behind the IOL was removed. Finally, the incision was sealed with a balanced salt solution.

All subjects attended routine postoperative follow-up and received prednisolone and moxifloxacin eye drops every 4 hours for a week, after which the dosage was tapered to every 8 hours. Moxifloxacin eye drops were stopped after two weeks postoperatively, while prednisolone eye drops were continued every 8 hours for the next two weeks. In addition, nepafenac 0.1% eye drops were continued every 8 hours for a week. All patients were followed up on postoperative day 1 and at 1, 3, and 6 weeks postoperatively. At 6 weeks postoperative, UCVA and CDVA were assessed in all patients and biometry was repeated to measure K1 and its axis, K2 and its axis, and astigmatism and its axis for final analysis.

SIA was calculated using an online ASSORT SIA calculator, and Residual astigmatism was calculated by subtracting the postoperative astigmatism from the preoperative astigmatism measured in D. All data were entered into a workup Pro forma for analysis. Descriptive statistics were performed using the Statistical Package for the Social Sciences (SPSS 26.0) for Windows. Given the small sample size, normality of the data was assessed using the Shapiro–Wilk test, which showed that the data were not normally distributed. Therefore, the Wilcoxon signed-rank test was performed to compare UCVA, CDVA, and corneal astigmatism preoperatively and postoperatively. Continuous data were described in terms of mean ± standard deviation (SD), while categorical data were depicted in frequencies (percentages) for each group. A p-value of <0.05 was considered statistically significant.

RESULTS

A total of 142 eyes of 142 patients were analysed. The mean age of patients was 62.97 ± 6.98 years (range 31–78 years). Of these, 86 patients (60.6%) were males, and 56 (39.4%) were females.

The mean preoperative UCVA was 0.76 ± 0.46 logarithm of the minimum angle of resolution (logMAR), which improved significantly to 0.14 ± 0.15 logMAR at 6 weeks postoperatively (p <0.001). CDVA also showed marked improvement from 0.58 ± 0.46 logMAR before surgery to 0.03 ± 0.07 logMAR postoperatively (p <0.001, Table I).

Table I: Baseline characteristics and visual outcomes (n = 142).

Variables	Minimum	Maximum	Mean	SD
Age (years)	31	78	62.97	6.98
Preoperative UCVA (logMAR)	0.10	1.70	0.76	0.46
Postoperative UCVA (logMAR)	0.00	0.60	0.14	0.15
Preoperative CDVA (logMAR)	0.00	1.70	0.58	0.46
Postoperative CDVA (logMAR)	0.00	0.30	0.03	0.07
Preoperative Astigmatism (D)	0.15	2.95	0.88	0.52
Postoperative Astigmatism (D)	0.00	3.00	0.65	0.48
UCVA: Uncorrected visual acuity; CDVA: Corrected distance visual acuity; D: Dioptre.

Table II: SIA and related indices (n = 142).

Variable	Minimum	Maximum	Mean	SD
TIA (D)	0.000	2.45	0.45	0.45
SIA (D)	0.06	4.86	0.58	0.50
Reduction in astigmatism (D)	–0.44	1.64	0.23	0.30
TIA: Target induced astigmatism; SIA: Surgically induced astigmatism; D: Dioptre.

Figure 1: Change in CDVA in 142 eyes, expressed as change in Snellen lines.

The mean preoperative corneal astigmatism was 0.88 ± 0.52 D, which reduced to 0.65 ± 0.48 D at 6 weeks, with a mean reduction of 0.23 ± 0.30 D (p <0.001). Of 142 patients, more than 1.5 D of corneal astigmatism was observed in 14 patients. The mean SIA was 0.58 ± 0.50 D, while the mean target induced astigmatism (TIA) was 0.45 ± 0.45 D (Table II).

Wilcoxon signed-rank test analysis confirmed statistically significant differences for UCVA, CDVA, and corneal astigmatism between preoperative and postoperative values (all p <0.001).

Changes in Snellen lines for CDVA were documented. Only 0.7% of eyes lost one line, and 2.8% showed no change, whereas 10.6% gained one line, 16.9% gained two lines, and the majority (69%) gained three or more lines (Figure 1).

DISCUSSION

This single-surgeon series indicates that placing a 2.75-mm limbal incision on the steep meridian during phacoemulsification produces low and predictable SIA, with a small but statistically meaningful reduction in corneal astigmatism and clear gains in UCVA and CDVA at 6 weeks. Although the average vector change of ~0.2–0.3 D may appear modest, it is clinically important for the large proportion of cataract patients who present with low pre-existing astigmatism, especially in contexts where access to toric IOLs is limited by cost or availability.

The findings of this study are consistent with recent work showing that steep-axis (on-axis) incisions can effectively reduce low-to-moderate corneal astigmatism during routine phacoemulsification by flattening the incision site, thereby improving postoperative refractive outcomes.¹⁴ A recent community-based study from 2024 demonstrated favourable refractive outcomes with steep-axis phacoemulsification, highlighting its value as a simple, inexpensive approach.¹⁴ Incision site also influences optical quality: a randomised trial found that superior clear-corneal incisions (CCIs) induced more coma and trefoil, while temporal incisions left higher-order aberrations largely unchanged, reinforcing the benefits of small, axis-aligned wounds in mini-mising visual distortion.¹⁵

Direct comparisons between temporal and superior clear- corneal entries consistently show lower SIA and quicker stabilisation with temporal approaches. A long-term evaluation confirmed that temporal wounds induced smaller shifts in SIA and higher-order aberrations than superior inci-sions,¹⁶ while anterior surface mapping has shown pre- dictable axis-related tendencies depending on incision site.¹⁷ The results of this study fall comfortably within the range reported for 2.2–2.8 mm phaco wounds, where smaller incision size has been consistently linked to predictably low SIA.^18,19

Emerging evidence also points to biomechanics as a deter- minant of astigmatic change. A study using dynamic cor-neal response parameters reported a significant relationship between baseline biomechanical indices and the magnitude of induced astigmatism, suggesting that such preoperative metrics could help identify eyes most likely to benefit from on-axis incisions.²⁰ Furthermore, recent reviews have emphasised the value of standardised vector analysis, as distinguishing the flattening effect from torque provides a clearer understanding of corneal changes and improves comparability across studies.²¹

In the Pakistani setting, where for the majority of patients self- finance their surgery and toric IOLs are often out of reach, the ability to modestly reduce astigmatism with a no-cost adjustment, such as on-axis incision, has important practical implications. Incorporating this technique into routine cataract practice can improve visual outcomes without adding to financial burden, making it particularly useful in resource-constrained healthcare systems.

From a clinical standpoint, data from this study support an on-axis 2.75-mm limbal corneal incision as a pragmatic option for patients with ≤1.0 D of regular astigmatism, where even small reductions in cylinder can enhance uncorrected vision. However, for eyes with >1.0–1.5 D of astigmatism, on-axis incisions alone are insufficient to reach refractive targets. In such cases, adjunctive methods—limbal relaxing incisions, femtosecond laser arcuate keratotomy, or toric IOL implantation—remain necessary. Comparative evidence continues to show minimal SIA with small CCIs, moderate correction with paired arcuate or opposite incisions, and the most predictable outcomes with toric IOLs when available.²²

The strengths of this series include the consistency of a single standardised technique and assessment at 6 weeks, when keratometric stability is usually reached. Limitations include the absence of a comparator incision group, a lack of detailed vector decomposition, and no stratification by baseline cylinder or biomechanical profile factors repeatedly highlighted in recent literature.^16,20,21 Future studies with randomised incision placement, detailed vector analysis, and extended follow-up would provide more robust evidence.

CONCLUSION

Placing an on-axis 2.75-mm limbal incision during phacoemulsification produces low and predictable SIA, consistent and modest reduction of corneal astigmatism and significant improvement in visual acuity within six weeks of surgery. This approach offers surgeons a simple and cost-effective method of enhancing postoperative outcomes in eyes with low pre-existing astigmatism. In contrast, patients with higher cylinders may still require additional corrective strategies.

ETHICAL APPROVAL:
Ethical approval was obtained from the Ethics and Research Committee of Combined Military Hospital, Lahore, Pakistan (No. 581/2024).

PATIENTS’ CONSENT:
Written informed consent was obtained from all patients for the collection, analysis, and publication of the data for this study.

COMPETING INTEREST:
The authors declared no conflict of interest.

AUTHORS’ CONTRIBUTIONS:
SH: Contributed to the conception and design of the study, interpreted the data, and gave final approval of the manuscript.
AT: Drafted the manuscript and performed the data analysis.
SF: Contributed to data acquisition.
MS: Critically revised the manuscript for important intellectual content.
All authors approved the final version of the manuscript to be published.

REFERENCES

1. Reis T, Lansingh V, Ramke J, Silva JC, Resnikoff S, Furtado JM. Cataract as a cause of blindness and vision impairment in Latin America: Progress made and challenges beyond 2020. Am J Ophthalmol 2021; 225:1-10. doi: 10.1016/j.ajo. 2020.12.022.
2. Xu T, Wang B, Liu H, Wang H, Yin P, Dong W, et al. Prevalence and causes of vision loss in China from 1990 to 2019: Findings from the Global Burden of Disease Study 2019. Lancet Public Health 2020; 5(12):e682-91. doi: 10.1016/ S2468-2667(20)30254-1.
3. GBD 2019 Blindness and Vision Impairment Collaborators. Causes of blindness and vision impairment in 2020 and trends over 30 years, and prevalence of avoidable blindness in relation to VISION 2020: The right to sight: An ana- lysis for the Global Burden of Disease Study. Lancet Glob Health 2021; 9(2):e144-60. doi: 10.1016/S2214-109X(20) 30489-7.
4. Todd T, Schmitz J. Toric and multifocal lens utilisation independent of patient cost and physician remuneration at a single institution. Am J Ophthalmol Case Rep 2019; 15: 100500. doi: 10.1016/j.ajoc.2019.100500.
5. Holladay JT, Pettit G. Improving toric intraocular lens calculations using total surgically induced astigmatism for a 2.5 mm temporal incision. J Cataract Refract Surg 2019; 45(3): 272-83. doi: 10.1016/j.jcrs.2018.09.028.
6. Fernandez J, Rodriguez-Vallejo M, Martinez J, Tauste A, Pinero DP. Prediction of surgically induced astigmatism in manual and femtosecond laser-assisted clear corneal incisions. Eur J Ophthalmol 2018; 28(4):398-405. doi: 10. 1177/112067 2117747017.
7. Mujawar RS, Padvi UI, Vasavada A, Shah M, Chaudhari A. A comparative study of surgically induced astigmatism from temporal and steep-axis clear corneal incisions in phacoemulsification cataract surgery. Cureus 2025; 17(9): e91714. doi: 10.7759/cureus.91714.
8. Xiao YH, Liu YQ, Chen ZG. Changes in corneal curvature and astigmatism in senile cataract patients after phacoemulsification. Front Med (Lausanne) 2024; 11:1481285. doi: 10.3389/fmed.2024.1481285.
    
9. Arthur E, Sadik AA, Kumah DB, Osae EA, Mireku FA, Asiedu FY, et al. Postoperative corneal and surgically induced astigmatism following superior approach manual small incision cataract surgery in patients with preoperative against-the-rule astigmatism. J Ophthalmol 2016; 2016:9489036. doi: 10. 1155/2016/9489036.
10. Melles RB, Holladay JT, Chang WJ. Accuracy of intraocular lens calculation formulas. Ophthalmology 2018; 125(2):169-78.
11. Eslami Y, Mirmohammadsadeghi A. Comparison of surgically induced astigmatism between horizontal and X-pattern sutures in the scleral tunnel incisions for manual small incision cataract surgery. Indian J Ophthalmol 2015; 63(7): 606-10. doi: 10.4103/0301-4738.167113.
12. Chow SC, Shao J, Wang H. Sample size calculations in clinical research. 2^nd ed. Boca Raton: Chapman & Hall/CRC; 2008. p. 50. Available from: httpss://www.mobt3ath.com/uplode/ book/book-43792.pdf.
13. Rosner B. Fundamentals of biostatistics. In: Cost Analysis of Diffusion Tensor Imaging and MR Tractography of the Brain. 4^th ed. Duxbury Press, Belmont, 141-190. Available from: httpss:// www.scirp.org/reference/referencespapers?referenceid=1284027.
14. Raihan MH, Kabir FH, Khan KH, Kajmina N, Hasan AKMR, Hasan MM. Study on the outcome of phacoemulsification cataract surgery with clear-corneal steep-axis incision. Community Based Med J 2024; 13(1):41-5. doi: 10.3329/ cbmj.v13i1.71073.
15. Akbas YB, Buyuktortop G, Gur Gungor S, Kaya G, Mirza E, Ozerturk Y. Changes in corneal higher order aberrations following cataract surgery with different incision sites: A prospective, randomised study. J Refract Surg 2024; 40(11): 792-6.
     
16. Cao X, Shao J, Zhang Y, Zheng L, Zhang J. Long-term evaluation of surgically induced astigmatism and corneal higher-order aberrations after 2.2 mm clear corneal incisions in femtosecond laser-assisted cataract surgery: Temporal versus superior approach. Clin Ophthalmol 2024; 18:1067-82. doi: 10.2147/OPTH.S456110.
17. Li PP, Huang YM, Cai Q, Huang LL, Song Y, Guan HJ. Effects of steep-axis incision on corneal curvature in one-handed phacoemulsification. Int J Ophthalmol 2019; 12(8):1277-82. doi: 10.18240/ijo.2019.08.07.
18. Laliwala F, Patel S, Prajapati V, Patel L, Wanjari MB, Singhal D. Comparative evaluation of astigmatic changes induced by superior and temporal corneal incisions in sutureless phacoemulsification surgery: A case series. Cureus 2023; 15(10): e48084. doi: 10.7759/cureus.48084.
19. Sheoran K, Arya SK, Bansal RK, Jinagal J, Jha UP. Surgically induced astigmatism and posterior corneal curvature changes following phacoemulsification. Indian J Ophthalmol 2022; 70 (2):406-12. doi: 10.4103/ijo.IJO_882_21.
20. Yin Y, Li Y, Liu G, Su J, Lu J, Jiang Y, et al. The influence of corneal biomechanical factors on surgically induced corneal astigmatism. Sci Rep 2025; 15(1):26660. doi: 10.1038/ s41598-025-11991-y.
21. Zhang J, Xu L, Shan M, Yuan L, Dong Y, Wang Y. Astigmatism research and application of vector method of the last half century: A bibliometric and visualised analysis. Front Med (Lausanne) 2025; 12:1519487. doi: 10.3389/fmed.2025. 1519487.
22. Faal NB, Ojaghi H, Ahari SS. Paired opposite 4 mm clear corneal incisions on the steep meridian during phacoemulsification. J Curr Ophthalmol 2021; 33(4):400-407. doi: 10.4103/ joco.joco_205_20.