Table 4.

Practical comparison of major surgical approaches for keratoconus-related astigmatism.

Surgical approach	Surgical principle	Practical indications/selection criteria	Key limitations	Supporting references
Corneal collagen cross-linking (CXL)	Riboflavin saturation followed by ultraviolet-A irradiation to increase corneal biomechanical stability and halt ectatic progression.	Documented progression on tomography, keratometry, refraction, or clinical worsening, usually over 6–12 months; feasible when corneal thickness permits safe treatment.	Limited refractive predictability; does not directly neutralize astigmatism or HOAs; safety depends on corneal thickness and treatment protocol.	(3, 7–9)
Customized PRK + CXL	Tissue-sparing topography- or wavefront-guided surface ablation combined with simultaneous or sequential CXL to regularize the anterior corneal surface.	Mild-to-moderate KC with HOA/coma-dominant visual limitation and useful visual potential; typically CDVA ∼0.6–0.9, TCT ≥450 μm, planned ablation ≤50 μm, and post-ablation/CXL safety thickness ≥400 μm.	Requires adequate stromal reserve and high-quality topographic/wavefront data; refractive predictability is moderate; unsuitable for marked thinning, poor visual potential, or visually significant central scarring.	(1, 8, 15–29)
Intracorneal ring segments (ICRS)	Synthetic arc-shaped stromal implants inserted into intrastromal tunnels to flatten the cone, redistribute corneal stress, and reduce corneal asymmetry.	Contact lens intolerance, poor spectacle correction, or need for corneal regularization when ablation is not feasible; clear cornea; suitable cone morphology/location; tunnel-site thickness usually ≥400 μm or according to manufacturer criteria; CDVA <0.6 may favor indication, whereas CDVA ≥0.6 requires caution.	Nomogram-dependent; moderate refractive predictability; outcomes depend on cone morphology, segment selection, centration, tunnel depth, and baseline CDVA.	(10, 30–45)
Corneal allogeneic intrastromal ring segments (CAIRS)	Allogeneic corneal tissue segments implanted into stromal channels to provide biological corneal remodeling and stromal volume support.	Clear, regularizable KC; possible option in thinner corneas or when avoidance of synthetic material is preferred; may be considered when ICRS-like regularization is desired but biological stromal support is advantageous.	No universal nomogram; evidence remains less standardized than synthetic ICRS; long-term durability and optimal timing with CXL remain uncertain; caution is required in extremely ectatic corneas.	(11, 46–72)
Toric phakic intraocular lenses (pIOLs)	Anterior or posterior chamber toric phakic lens implantation to correct residual regular myopia and astigmatism without modifying corneal shape.	Stable or post-CXL KC; good CDVA; predominantly regular residual cylinder; reproducible manifest refraction and axis; adequate anterior segment anatomy; ACD generally ≥2.8–3.0 mm, with adequate endothelial reserve and appropriate sizing/vault.	Does not correct irregular astigmatism, coma, or corneal HOAs; requires stable refraction, reliable axis, adequate anatomy, and rotational stability.	(12, 73–106)
Cataract/lens-based surgery	Cataract extraction or refractive lens exchange with monofocal or toric IOL implantation; secondary piggyback toric correction may be considered in selected cases.	Cataract or lens-related visual limitation; stable KC; toric in-the-bag IOL only when astigmatism is sufficiently regular; conservative monofocal IOL preferred in advanced irregularity.	IOL power calculation is challenging; toric predictability decreases in advanced or highly irregular KC; residual spectacles, contact lenses, or secondary correction may still be needed.	(13, 107–110)

The practical indications and selection criteria summarized in this table integrate the cited evidence with expert clinical interpretation and should be individualized. Evidence levels and recommendation strengths are presented separately in Table 1.

ACD, anterior chamber depth; CAIRS, corneal allogeneic intrastromal ring segments; CDVA, corrected distance visual acuity; CXL, corneal collagen cross-linking; HOA, higher-order aberration; HOAs, higher-order aberrations; ICRS, intracorneal ring segments; IOL, intraocular lens; KC, keratoconus; pIOL, phakic intraocular lens; PRK, photorefractive keratectomy; TCT, thinnest corneal thickness.