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OP1.4 | Refractive Surgery: Principles and Indications — SDL Guide

Learning Objectives

• Explain the optical principle by which corneal laser surgery corrects refractive errors
• Describe the procedure and indications for LASIK, PRK and SMILE
• Describe the rationale and indications for phakic IOL and refractive lens exchange (RLE)
• Enumerate the pre-operative assessment criteria and absolute contraindications for refractive surgery
• Identify the common complications of laser refractive surgery and their management principles

INSTRUCTIONS

Refractive surgery has transformed the management of myopia, hypermetropia, and astigmatism, offering millions of patients spectacle-independence. As a physician, you will routinely encounter patients who have undergone or are considering refractive surgery — and who ask questions about risks, suitability, and outcomes. Understanding the principles and indications helps you counsel patients, identify unsuitable candidates (preventing the catastrophic complication of post-LASIK ectasia), and co-manage post-surgical patients appropriately.

References

• Khurana AK. Comprehensive Ophthalmology, 7th ed. New Age International, 2019. Ch 4, Refractive Surgery. (textbook)
• Parsons J, Duke-Elder S. Diseases of the Eye, 22nd ed. Elsevier, 2011. Ch 6, Surgical Correction of Refractive Errors. (textbook)

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Version 2.0 | NMC CBUC 2024

Clinical Indication and Relevance: Who Seeks Refractive Surgery and Why

Refractive surgery presents to the clinician in two ways: as a planned elective procedure sought by a patient who wishes to be free of spectacles or contact lenses, and as a past surgical history that now complicates subsequent ophthalmic management. Both presentations require the physician to understand the principles, indications, and potential consequences of the surgery.

The patient seeking refractive surgery is almost always motivated by a desire to reduce dependence on optical correction — a quality-of-life goal that is entirely legitimate and, in well-selected patients, reliably achieved. Common presenting stories include the young professional who finds spectacles impractical in an active occupation or sport; the contact lens wearer with recurrent lens-related infections or discomfort who wants a permanent solution; the military or aviation candidate whose service requires unaided visual acuity that they currently cannot meet; and the patient with high myopia whose thick spectacle lenses produce magnification and cosmetic distortion that contact lenses alone do not fully resolve.

The physician's role in this initial encounter is not to perform the surgery — but to counsel accurately about candidacy, to understand the pre-operative workup that distinguishes suitable from unsuitable patients, and to recognise the absolute contraindications that must be screened before any surgical evaluation. The single most important of these — and the one responsible for the most devastating surgical outcomes — is the presence of keratoconus or subclinical keratoconus, a corneal ectatic disease that is invisible to clinical inspection but detectable on corneal topography and tomography. Knowing this — and knowing that any physician who refers a patient for refractive surgery should enquire about topographic screening — is the clinical knowledge this module is designed to provide.

The Optical Principle: How Laser Surgery Reshapes the Cornea

All corneal refractive procedures exploit a single unifying principle: modifying the anterior curvature of the cornea to adjust its refracting power, thereby changing the focal point of the eye. This is possible because the cornea is responsible for approximately 43 D of the eye's total ~60 D refracting power — even a small geometric change in corneal curvature produces a clinically significant refractive change.

The key instrument in laser corneal surgery is the excimer laser (argon fluoride, wavelength 193 nm — ultraviolet). The 193 nm wavelength is precisely absorbed by the peptide bonds of corneal stromal collagen without generating heat — this is photoablation (also called 'cold' laser ablation). Each laser pulse removes approximately 0.25 micrometres of corneal tissue. A computer-controlled scanning laser ablates tissue in a spatially controlled pattern to reshape the corneal surface:

• To correct myopia: tissue is removed centrally (central ablation), flattening the corneal curvature and reducing the central refracting power. The ablation zone is larger centrally than peripherally.

• To correct hypermetropia: tissue is removed in a ring pattern peripherally (annular ablation), causing relative steepening of the central cornea and increasing central refracting power.

• To correct astigmatism: a cylindrical ablation pattern selectively removes tissue in the steeper meridian to equalise the two principal meridians.

The total depth of ablation is proportional to the degree of refractive error — higher corrections require deeper ablation and therefore more corneal tissue removal. This is why a minimum residual stromal bed (RSB) of ≥250 micrometres must be maintained after ablation to prevent post-operative corneal ectasia (biomechanical failure leading to progressive corneal bulging).

Excimer Laser Corneal Reshaping in Myopia and Hypermetropia

Corneal Laser Procedures: LASIK, PRK and SMILE

Three corneal laser procedures dominate current refractive surgery practice. They differ primarily in how the corneal surface is managed before excimer laser ablation — each choice represents a trade-off between visual recovery speed, biomechanical impact, and complication profile.

Provided image

LASIK (Laser-Assisted In Situ Keratomileusis) is the most widely performed refractive procedure worldwide. A thin hinged corneal flap (90–120 µm thick) is first created using either a microkeratome (mechanical oscillating blade) or, more commonly in modern practice, a femtosecond laser (which creates a more precise, predictable flap with lower flap complication rates). The flap is folded back, the excimer laser ablates the stromal bed beneath, and the flap is repositioned (it adheres by suction — no sutures required). Advantages: rapid visual recovery (24–48 hours), minimal pain, excellent predictability. Disadvantages: flap complications (dislodgement, striae, epithelial ingrowth), flap interface is a permanent weakness — contact sports or trauma risk, thin corneas may not have sufficient stroma for a safe RSB.

PRK (Photorefractive Keratectomy) predates LASIK and involves surface ablation — the epithelium is removed mechanically (a 'hockey stick' instrument), chemically (20% ethanol), or by laser (transepithelial PRK, TransPRK) before excimer laser ablation of Bowman's layer and anterior stroma. There is no flap; the epithelium regenerates over 4–7 days. Advantages: no flap risk, greater biomechanical strength retained (RSB threshold is met more easily because no flap depth is subtracted), suitable for patients with thinner corneas, active occupations/contact sports. Disadvantages: slower visual recovery (1–2 weeks), more post-operative pain (nerve endings exposed), slightly higher haze risk (mitigated by intraoperative mitomycin C 0.02%).

SMILE (Small Incision Lenticule Extraction) uses a single femtosecond laser to create an intrastromal lenticule (a disc of corneal stroma of the appropriate refractive shape) and a small 2–4 mm access incision. The surgeon manually dissects and removes the lenticule through the small incision — no flap, no excimer laser. Advantages: flapless (greater biomechanical integrity), minimal surface disruption, less dry eye (fewer corneal nerves cut), no excimer laser. Disadvantages: technically more demanding, less versatile (approved for myopia and myopic astigmatism; limited data for hypermetropia), cannot easily re-treat if under-correction occurs.