1. Intra corneal ring segments (ICRS)

ICRS allow the correction of low myopia and astigmatism. Half ring segments of Perspex are inserted into channels created in the corneal stroma, causing the central cornea to flatten. They possess a number of advantages over PRK and LASIK in that they do not involve the central cornea and are positioned outside the pupil margin. They also maintain the prolate shape of the cornea and are relatively reversible. Only low degrees of myopia can be treated (<-4.50D) but they are also useful for treating some cases of keratoconus and LASIK-induced ectasia where the cornea has become abnormally thin. The technique has been proven safe and predictability is good (Asbell and Ucakhan, 2001; Asbell and Ucakhan, 2001; Bisantis et al., 1996) however the market-leader (INTACS), is no longer available and the popularity of the technique among UK surgeons is falling.

 

2. Holmium Laser ThermoKeratoplasty (LTK) and Diode ThermoKeratoplasty (DTK)

Both LTK and DTK techniques employ an infrared laser to coagulate the cornea. Spots are arranged in a ring between 6 and 9mm from the centre of the cornea, and as scar tissue forms, steepening of the central cornea results, causing a reduction in hypermetropia. The central cornea generally remains clear but the recovery time is 6 months or more and the stability of the refractive outcome can be poor (Charpentier et al., 1995). Visual performance has not been examined in detail but the regularity of the central cornea once the result has stabilised suggests that such techniques may provide a better outcome than hyperopic PRK or LASIK in the future.

 

3. Intraocular Refractive Surgery

Intraocular surgery is always more invasive than corneal surgery but bypassing the cornea avoids inducing aberrations and the outcome is more predictable. It also allows much larger refractive errors to be corrected without being limited by the corneal thickness. Since these procedures tend to be reserved for the treatment of high refractive errors, applicants presenting with a history of intraocular refractive surgery are likely to be considered unsuitable for visually demanding professions on the basis of their increased risk of myopic retinal problems.

 

1.  
Phakic intraocular lenses (PIOL)

An additional lens made of Perspex or Silicon Acrylate material is place inside the eye to alter its refractive power. Phakic IOL’s can be used to treat a wide range of refractive errors but are often reserved for high refractive errors where corneal surgery is not an option. The additional lens can be placed in the anterior chamber in front of the iris, clipped to the iris (iris claw lenses), or placed in the posterior chamber between the iris and the crystalline lens, or in the capsular bag itself. Anterior chamber and iris claw lenses such as the Artisan lens, tend to be associated with a relatively high incidence of intraocular inflammation, endothelial damage and an increased risk of secondary glaucoma. However, 5 year follow-up data for a group of 26 eyes treated for around –16D, indicates a good refractive outcome with 58% of eyes within ± 1.0D of emmetropia and 73% achieving 6/12 or better unaided(de Souza et al., 2001). Posterior chamber lenses, (sometimes referred to as implantable contact lenses) show promising refractive results and are increasing in popularity but they are also not without risk. Intraocular inflammation is not uncommon and sight-threatening endophthalmitis has an incidence of 0.1%. There is an increased risk of secondary glaucoma (Perez-Santonja et al., 1997; Marinho et al., 1997; Jimenez-Alfaro et al., 2001). It is not unusual for young patients (who tend to have larger pupils) to complain of glare and poor quality vision at night. This is related to the small optic zone, which is often only 4-5mm in diameter. Any patient whose pupil dilates to become larger than the optic under low illumination will suffer from reduced visual performance.

 

2.  
Refractive lens exchange

This procedure is identical to cataract extraction and differs only in that the crystalline lens is removed despite being clear and not cataractous. The crystalline lens is replaced with an intraocular lens calculated to give the required refractive outcome. The potential complications are identical to those found post-cataract extraction – retinal detachment, intraocular inflammation and cystoid macular oedema.

 

4. Presbyopia treatments

Surgery for myopia and low to moderate levels of astigmatism is now well established with a vast data-pool. LASIK for low hypermetropia and refractive lens exchange for higher refractive errors in presbyopic individuals is becoming the accepted technique, but presbyopia surgery is still in its infancy. Although presbyopia is the only universal refractive error, it is frequently the "forgotten refractive error" and successful safe treatment algorithms remain the final frontier for refractive surgeons. At present, refractive surgery for presbyopia must be considered investigational.

If one ignores the compromise concept of monovision (one eye corrected for distance and one for near) then refractive surgery for presbyopia can be subdivided into corneal, scleral and lenticular:

Corneal surgery for presbyopia consists of either creating a multifocal cornea under a LASIK flap by creating an inferior steepening, or implanting a multifocal intra-corneal inlay. The former has some protagonists but the latter is still at a developmental stage because of the problems with corneal physiology in the presence of an inlay.

Scleral surgery is based on the highly contentious theory of presbyopia proposed by Schacher. Scleral surgery can be incisional (with or without inlay segments) or laser. Short-term data shows some effect on reading acuity, but little or no improvement in measurable accommodative power. It is possible that this surgery has an effect by creating corneal multifocality. The long-term stability is contested, as is the aesthetic result.

Lens surgery is now termed PRELEX, an acronym for PREsbyopic Lens EXchange, and was first described in 1997. Essentially, a standard cataract procedure is undertaken but using either a multifocal (producing pseudo-accommodation) or so-called "accommodative" IOL. Although the optics of the multifocal IOL is conventional and well understood, so-called "accommodative" IOLs depend on novel physiological assumptions demanding movement of the vitreous gel behind the lens. The data on these IOLs is promising, and although it is too soon to recommend them to patients, although the concept of a true accommodating IOL remains an aspirational as well as an inspirational target.

© British Society for Refractive Surgery and Catharine Chisholm