Refractive surgery refers to eye surgery performed to correct focusing errors (refractive errors) of the eye. It is performed on those who wish to reduce their dependence on glasses or contact lenses.
To achieve clear vision without glasses or contact lenses, the eye needs to focus the light travelling through it. In those with myopia (“short-sightedness”), hyperopia (“long sightedness”) and astigmatism, light is poorly focused by the eye resulting in blurred vision. Myopia, hyperopia and astigmatism can be treated with glasses or contact lenses and clear vision achieved.
Presbyopia is a condition in which the eye, with age, loses its ability to focus on near objects. Presbyopia occurs in the mid 40’s and affects everyone. Initially it is apparent as the need to hold near objects further away from the eye to obtain a clear image. Eventually, those with presbyopia lose their ability to read and require reading glasses.
There are 2 main categories of refractive surgery. The first type is laser refractive surgery such as LASIK or Photorefractive Keratectomy (also known as “surface LASIK”). In laser refractive surgery, a highly precise laser known as an excimer laser is applied to the cornea (the clear focusing window at the front of the eye) to improve the way light is focused by the eye, resulting in improved vision without glasses or contact lenses.
The second main type of refractive surgery is lens-based refractive surgery. In lens-based refractive surgery, the eyes’ natural lens can be removed and replaced with a new artificial lens which improves the focusing ability of the eye. Alternatively, the eye’s natural lens is left in place and a new artificial lens is planted on top of the natural lens to correct the eye’s refractive error.
LASIK is a type of laser refractive surgery used to treat refractive errors such as myopia (short- sightedness), hyperopia (long-sightedness), astigmatism and presbyopia.
LASIK acts in a similar way to glasses or contact lenses but is a permanent treatment. In LASIK a flap is created in the cornea, the flap is lifted, and the excimer laser is applied to the deeper layers of the cornea. The laser changes the shape of the cornea and alters the way it focuses light, resulting in much clearer vision.
Photorefractive keratectomy (PRK), like LASIK, uses an excimer laser to change the shape of the cornea. In PRK, the laser is applied to the surface of the cornea and, unlike LASIK, no flap in the cornea is created.
A recent large review of international literature comparing PRK and LASIK was performed by the Cochrane Library, considered to provide highest quality of scientific evidence. This study showed that there was no significant difference in the final vision and accuracy between LASIK and PRK.
The benefits of LASIK include the fact that the procedure has less pain immediately after surgery and that the vision improves rapidly. The disadvantages of LASIK are that, because a flap of tissue is created in the cornea, the eye will always have some structural weakness. This means that the flap can dislodge if there is future trauma to the eye and there is a small chance the shape of the cornea can become irregular with time (post-LASIK ectasia), both of which can result in deterioration of vision.
The benefits of PRK include the fact that structurally the cornea is stronger as there is no flap created. This means that there is no flap that can be dislodged if there is ever trauma to the eye and that there in essentially no risk of the cornea becoming irregular in the future. The disadvantages include the fact that there is some pain after the operation (that can be controlled with pain-killers) and that the vision takes several days to become clear
In a refractive lens exchange, the natural lens of the eye is removed and replaced with an artificial lens that is permanently implanted into the eye to correct any focusing error. Refractive lens exchange is most commonly performed for presbyopia. There are 2 ways in which the need for glasses or contact lenses can be reduced in refractive lens exchange.
Multifocal/trifocal intraocular lenses work by splitting the light that travels through the eye into 3 different focal points: distance, intermediate and near. The result is very good vision for objects at all distances and significantly reduced spectacles dependence or even spectacles independence. However, spectacles may still be required for fine near work in low light conditions.
In blended vision or monovision, intraocular lenses with a single focal point are used (monofocal intraocular lenses). In one eye (usually the dominant eye), a lens focused for distance is implanted to allow for excellent distance vision. In the other eye (usually the non-dominant eye), an intraocular lens implant focused for near is inserted which provides good near vision. With both eyes open, one is able to see clearly for distance and near. With monovision, it can take several months for the brain to adapt to having each eye set for a different distance.
So why aren’t multifocal/trifocal intraocular lenses (IOL’s) used in everyone? Multifocal/trifocal IOL’s have some potential downsides which are unique to these lenses which don’t occur with monofocal IOL’s. Because these lenses work by splitting the light travelling through the eye into 3, the quality of vision for distance, for example, is not quite as sharp as is it would be if the lens focused all light for distance vision. However, the great benefit is these lenses is that they give very good vision at all distances.
The second issue is that glare and haloes around lights are more commonly experienced by patients than in monofocal lenses. This usually does not cause any significant problem and, after several months, these are much less noticeable. However, in around 1 in 200 cases, glare and haloes can be very troubling to the point where these lenses need to be removed.
The third is that multifocal/trifocal lenses require that the distance focal point be very precise. Highly sophisticated scans are taken of your eye before surgery to determine the correct strength of lens required to achieve this correct focus. These scans are not perfect, however, and the focal point for distance after surgery may not be exactly what the scans predicted. In monofocal lenses, this is rarely an issue. In eyes with multifocal IOLs, further surgery to “fine tune” the focus of the eyes with either laser or a second intraocular lens may be required.
The ICL is an artificial lens implant that is inserted into the eye and placed in front of the natural lens. It is akin to a permanent contact lens. Each ICL is custom made for the individual eye.
ICLs have traditionally been used in eyes with varying degrees of refractive error where PRK and LASIK may be less likely to have an excellent outcome. With recent advances in the designs of these lenses and increased safety, the ICL is being increasingly used in eyes with lower refractive errors as an alternative to PRK and LASIK.
There are many benefits of the ICL. These include the fact the procedure is reversible (in the very unlikely event of an unsatisfactory outcome, the ICL is simple to remove), whereas PRK and LASIK are not. Also, an ICL doesn’t change the shape of the cornea like PRK and LASIK do. In the future, most people will require cataract surgery, where the natural lens that has become clouded (ie a cataract) is removed. In eyes that have had prior PRK or LASIK, determining the correct strength of lens to implant into the eye at the time of cataract surgery can be very challenging. There is increased risk of the focus not being correct in the eye. In an eye with a prior ICL, this is not an issue and cataract surgery can be performed with confidence that the lens implant that will be inserted into the eye will be correct.
Other benefits include the fact that there are none of the possible complications that can occur in LASIK due to the presence of a corneal flap, such as dislocation from trauma, dry eye and changing corneal shape with time (post-LASIK ectasia).