Education · Fundamentals

Understanding
your vision

Before any treatment decision, understanding how your eye works is essential. This section explains the mechanisms of normal vision and the reasons it can deteriorate.

The eye: a precision
optical system

The eye performs two essential functions: it transmits and focuses light onto the retina, the light-sensitive membrane that converts this light into a nerve signal sent to the brain. Two structures contribute to this focusing: the cornea, at the front of the eye, and the crystalline lens, a naturally flexible internal lens.

When this system works perfectly, light converges exactly on the retina: this is called emmetropia. When there is a mismatch between the optical power of the eye and its length, focusing occurs in front of or behind the retina: this is a refractive error (ametropia).

Optical corrections — spectacles or contact lenses — compensate for the refractive error. In the same way, refractive surgery compensates for refractive errors by modifying the geometry of the cornea, by supplementing the lens, or by replacing it with an intraocular lens.

Refractive errors are anatomical or biomechanical variations of the eye. They are characterised by their type (myopia, hyperopia, astigmatism, presbyopia), their magnitude (expressed in dioptres) and the way they can combine in a single patient.

The mechanics of vision

Cornea, lens
and retina

The cornea provides about two-thirds of the eye's total refractive power. Transparent and avascular, it is in direct contact with the environment and is the main target of corneal laser surgery.

The crystalline lens, located behind the iris, completes the focusing and enables accommodation — that is, the dynamic adjustment from distance to near vision. This ability gradually erodes with age: this is presbyopia.

The retina receives the focused image; in particular the macula, the high-resolution central zone, provides fine vision. It is not affected by refractive surgery, but its health determines the benefit expected from any procedure.

Cornea Lens Vitreous body Macula Retina Sclera Optical axis Light rays Schematic Gullstrand eye — sagittal section
Refractive errors

The four refractive errors

Myopia

Myopia — “closed vision”

In a myopic eye, light rays converge in front of the retina. Distant objects appear blurred, while near vision remains naturally sharp.

This most often results from an eye that is too long relative to its optical power — known as axial myopia — or from a cornea that is too steeply curved.

Myopia is measured in negative dioptres (for example −3.00 D). It progresses mainly during childhood and adolescence, then usually stabilises between 18 and 25. Current studies suggest that environmental factors, notably a lack of exposure to natural light, play a role in its development.

Focal point in front of the retina

Hyperopia

Hyperopia — “vision beyond”

In a hyperopic eye, the rays converge behind the retina. The eye is too short for its optical power, or the cornea is too flatly curved.

Mild to moderate forms are long compensated for by the natural accommodation of the lens, which over-contracts to bring the focal point back onto the retina. This compensation has a cost: eye strain, headaches, discomfort during near tasks. With age, the accommodative reserve runs out and the discomfort becomes lasting.

Hyperopia is measured in positive dioptres (for example +2.50 D). It is often underestimated, as it does not always cause an obvious visual complaint before the age of 35–40.

Focal point behind the retina

Astigmatism

Astigmatismus — “without a single focal point”

An astigmatic eye has an asymmetry of curvature: the cornea, instead of being spherical like a ball, is slightly toric, resembling the surface of a rugby ball. Light rays do not converge at a single point but scatter along two perpendicular axes, producing a blurred image at all distances.

Astigmatism is described by its value in cylindrical dioptres and its axis in degrees. It is frequently associated with myopia or hyperopia, and can change following trauma or corneal surgery.

Optical corrections use cylindrical lenses. Surgical correction is matched to the axis of the astigmatism measured during the preoperative assessment.

Focal point scattered — two meridians

Presbyopia

Presbyopia — “vision of the elders”

Presbyopia is not strictly a refractive error: it results from the physiological ageing of the lens, which gradually loses its elasticity. The ability to accommodate declines inexorably from the forties onwards.

Everyone is affected, whatever their initial refractive status. Presbyopia is inevitable and bilateral. In most cases it cannot be corrected by a corneal laser alone: it requires either an optical compromise (monovision) or an approach addressing the lens itself.

Current surgical options can compensate partially or fully for presbyopia, depending on the characteristics of the eye and the needs of the patient.

Accommodation reduced with age
Three structures · three approaches

Cornea, phakic implant
or lens: where to act?

Corneal surgery

Reshaping
the optical surface

Corneal surgery acts on the curvature of the cornea by vaporising a thin layer of stromal tissue with an Excimer laser. It is intended for patients whose corneal anatomy — thickness, curvature, biomechanics — is compatible with the planned treatment.

The natural lens is kept intact. Corneal surgery by tissue reshaping is irreversible.

  • transPRK laser (surface)
  • LASIK laser (intrastromal)
  • KLEx — stromal lenticule

Phakic implantation

Supplementing
the cornea and lens

The phakic implant is an intraocular lens placed inside the eye, in front of the natural lens, without touching the cornea or removing the lens. It is intended for refractive errors too high for the laser, or for corneas whose biomechanics rule out reshaping.

This technique is reversible: the implant can be removed or exchanged. The natural lens and its ability to accommodate are preserved.

  • ICL (Implantable Collamer Lens)
  • IPCL (Implantable Phakic Contact Lens)

Lens surgery

Replacing
the internal lens

When the lens has become clouded (cataract) or too rigid to accommodate (advanced presbyopia), replacing this natural lens with a custom-calculated artificial implant makes it possible to correct the refractive error at the same time.

Modern implants offer a range of solutions according to the patient's anatomical characteristics, lifestyle and expectations.

  • Monofocal implants
  • EDOF implants (extended depth of focus)
  • Multifocal implants
  • Toric implants (astigmatism)

Which technique is right for you?

There is no objectively superior technique: there is the technique best suited to your eye, your needs and your lifestyle. The preoperative consultation — corneal topography, corneal biomechanics, ocular biometry — is the only setting in which a precise indication can be established. The indication always comes before the technique.

Deepen your
understanding

The following pages detail each surgical technique and the criteria that guide their selection. You can also consult the comparison of approaches or get in touch to ask your questions.

Explore the techniques → Compare the approaches