Factors affecting astigmatism in cataract surgery

1. Preoperative factors

• Magnitude and axis of corneal astigmatism: Regular astigmatism is more predictable than irregular astigmatism.
• Anterior and posterior corneal curvature: Both must be considered. Ignoring posterior corneal astigmatism may result in overcorrection or undercorrection, especially in with-the-rule and against-the-rule cases.
• Corneal surface disease: Dry eye, blepharitis, epithelial irregularity, scars, pterygium, keratoconus, and previous corneal refractive surgery can make keratometry and IOL calculations unreliable.
• Age-related change: Corneal astigmatism often shifts from with-the-rule to against-the-rule with age.
• Accuracy of measurements: Keratometry, corneal topography or tomography, biometry, and repeated consistent readings are essential. ESCRS recommends topography or tomography when refractive astigmatism is present. ESCRS recommendations

2. Incision-related factors

• Site of incision: An incision placed on the steep corneal meridian tends to flatten that meridian and reduce astigmatism. An incision on the flat meridian may increase it.
• Incision size: Larger incisions produce greater SIA. Modern small clear-corneal incisions generally induce less astigmatism than large ECCE or MSICS wounds.
• Incision architecture: Length, tunnel configuration, depth, location relative to the limbus, and wound stability influence corneal flattening.
• Number of incisions: A main incision plus side-port incisions can alter the final corneal shape.
• Sutures: Tight, loose, unequal, or prematurely removed sutures can cause significant astigmatism. Kanski notes that suturing of cataract incisions may induce considerable corneal astigmatism. Kanski's Clinical Ophthalmology: A Systematic Approach, 10th ed., section “Manual cataract surgery.”

3. Intraoperative factors

• Surgical technique and manipulation: Excessive wound stretching, cautery, thermal injury, or difficult nucleus delivery may increase SIA.
• Location and construction of corneal relaxing incisions: Their effect depends on depth, arc length, optical zone, corneal thickness, age, and nomogram used.
• Toric IOL selection: Correct cylinder power and axis calculation, including anticipated SIA and posterior corneal astigmatism, are required.
• Toric IOL alignment: Marking errors, cyclotorsion, inaccurate placement, and postoperative rotation reduce correction. Rotation away from the intended axis leaves residual astigmatism.
• Capsular bag and zonular stability: These affect centration and rotational stability of the IOL.
• Intraoperative aberrometry, where available, may help refine IOL power and axis selection. AAO guidance

4. Postoperative factors

• Wound healing and remodeling: Early postoperative astigmatism can change as the incision heals.
• Suture-related effects: Loosening, breakage, vascularization, selective removal, or unequal tension can alter the axis and amount of astigmatism.
• Corneal edema, inflammation, infection, or wound leak: These can temporarily or permanently change corneal curvature.
• Toric IOL rotation or tilt: Causes residual or recurrent astigmatism.
• Posterior capsular opacification and refractive changes: May affect perceived visual quality, though they do not usually directly induce corneal astigmatism.