Optical correction to improve vision has been around for centuries. In more recent times, medical scientists realised that vision could be improved without the use of glasses that break, fall off, or fog up. Though contact lenses were a big step forward in terms of convenience and freedom of vision, they also come with their own set of disadvantages, such as the risk of eye infection and the ongoing cost of maintaining a supply.
The excimer laser was first described in the 1970s and in 1987 the first laser eye surgery was performed with a technique still used to today, photorefractive keratectomy (PRK). Over the last three decades, laser eye surgery has continued to advance and evolve, with millions of people and their eyes benefiting from this technology.
How Does Laser Eye Surgery Work: the Principles
To understand how laser eye surgery works, we must first understand a bit about refractive error and eye anatomy.
Refractive error refers to a mismatch of the focusing power of the eyeball to its axial length. A simple way to think of it is that the refractive components of the eye, which include the tear film (the very first surface that light hits as it enters the eye), the cornea (the dome of transparent tissue at the front of the eye), and the crystalline lens (sitting behind the coloured iris inside the eye), bend and refract light rays entering the eye in such a way that they come together at a single point.
In the case of ametropia, the medical term describing the state of an eye with refractive error, the point at which these light rays come together falls either too short or too far of the sensory retina at the back of the eyeball. The array of light rays that do end up hitting the retina is therefore not contained as a single clear point but as a defocused set of rays, which we interpret as blurry vision.
There are several categories of refractive error:
- Myopia (short-sightedness) refers to light focusing to a point too soon, i.e. before the plane of the retina
- Hyperopia (long-sightedness) refers to light focusing to a point too late, i.e. after the plane of the retina
- Astigmatism refers to light focusing at more than one point within the eye, depending on the angle at which the rays enter
- Presbyopia is a normal age-related deterioration of the near focusing ability of the eye
Laser eye surgery is based on the principle of reshaping the cornea to adjust the angle at which light rays pass through the eye such that they come to that clear focal point right on the sensory retina. Though there have been numerous techniques devised to achieve this, including PRK, LASIK (laser assisted in situ keratomileusis), and SMILE® (small incision lenticule extraction), they all involve the application of a laser to correct the eye for ametropia.
There are five layers of the cornea. From the surface to the innermost layer, these are the epithelium, Bowman’s membrane, the stroma, Descemet’s membrane, and the endothelium. The corneal stroma makes up the bulk of the thickness of the cornea and this is the tissue targeted for re-sculpting by the laser. Although the cornea does not contribute the majority of the focusing power of the human eye, changing its shape through laser surgery can have significant effects.
How Does Laser Eye Surgery Work: the Specifics
Prior to undergoing any surgery, a comprehensive eye examination at the surgeon’s clinic is carried out to ensure you are suitable for the procedure. This includes a discussion regarding your lifestyle, vocation and hobbies, a careful eye health exam to exclude eye diseases such as cataract or glaucoma, and various tests for precise measurements of your refractive error and eye anatomy.
The PRK treatment requires debridement of the corneal epithelium to allow access to the stroma for the laser. This is typically performed by softening the adhesions of the epithelium to the underlying Bowman’s membrane with a chemical solution, which is then gently scraped away.
As all laser eye surgery procedures are performed under topical anaesthetic, there is often very minimal discomfort experienced during the process. A specialised excimer laser is then used to vaporise calculated areas of stromal tissue to reshape the cornea and adjust its refractive power in a process known as photoablation. The epithelial layers then later regenerate during the post-operative healing process.
LASIK circumvents the need for epithelial debridement and its subsequent prolonged recovery time by employing the use of either another laser known as a femtosecond laser, or a handheld tool called a microkeratome, to create a flap of superficial corneal tissue that can later be replaced after the excimer laser has been applied to the stroma during photoablation.
SMILE® is a newer technique which boasts only a keyhole incision and minimal disruption to the corneal tissues; this aids in maintaining the structural integrity of the cornea and further improves recovery time. A femtosecond laser is used to create a small disc-shaped sliver of corneal stroma, which is then removed via a tiny incision in the surface of the cornea. The disc of tissue, also called a lenticule, is calculated to leave behind the appropriate corneal shape to correct for the refractive error of the eye once it is removed.
Though no surgery is without risks, with access to the technology we have, all these surgical treatments have proven excellent safety and efficacy results and have been a popular way of reducing dependency on prescription glasses and contacts.
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