Phakic Intraocular Lenses and Clear Lens Extraction

PHAKIC INTRAOCULAR LENSES AND CLEAR LENS EXTRACTION

Excimer laser photorefractive keratectomy or PRK has been demonstrated to be effective for low degrees of refractive error with an increasing incidence of postoperative haze and regression at higher levels of refractive error. Laser in situ Keratomileusis or LASIK describes the use of the excimer laser in combination with the elevation of a corneal flap with a microkeratome. LASIK extends the range that refractive error can be corrected predictably with good quality of vision beyond that achievable with PRK.

There is a limit however, to the amount of corneal tissue that can be removed by the excimer laser in reshaping the corneal stroma, and therefore at high levels of refractive error the optical zone of the treated cornea has to be reduced. This may result in undesirable visual side effects such as glare, and halos, especially when driving at night, as well as a higher prevalence of loss of best corrected visual acuity, and poor predictability.

An alternative strategy to keratorefractive surgery for patients with higher refractive error is the use of one of several combinations of intraocular lens insertion and crystalline lens removal to modify the refraction of the eye.

Clear Lens Extraction

Clear lens extraction is the removal and replacement of the crystalline lens with an intraocular lens implant. This surgery has been developed over many years to a high level of precision for patients who have cataracts and undergo cataract extraction and intraocular lens implantation. Selection of the appropriate lens power can reduce the patients dependency on spectacles following surgery with a greater degree of precision than corneal refractive surgery at corresponding levels of myopia and hyperopia. Younger patients undergoing this form of surgery will lose the ability to alter their focus for near vision or accommodation. The major concerns, however relate to the associated risks of an intraocular procedure such as endophthalmitis, which are not present in keratorefractive procedure. Clear lens extraction has the same associated risks as those of cataract extraction and intraocular lens implantation but with the added concern that in patients with high refractive error it may significantly increase the prevalence of retinal detachment.

Phakic Intraocular Lenses

The introduction of an intraocular lens in a phakic eye is also an intraocular procedure with many of the same risks associated with modern cataract surgery.

The power of accommodation is retained and the risk of retinal detachment may be less than with clear lens extraction. Phakic intraocular lenses however, have only recently been introduced and the long-term effect of these implants on the eye is unknown. The design and material of these lenses are still being developed and the technology is still in a process of refinement compared to implants used in association with cataract extraction. Phakic intraocular lenses can be fixated in the anterior chamber, on the iris or within the posterior chamber, and it is unclear at this stage which is the preferred location. The physical location of these lenses may be in the anterior chamber or posterior chamber. Several studies are underway to clarify these issues and it is recommended that phakic intraocular lenses remain investigational until further data becomes available.

Summary

The Royal Australian and New Zealand College of Ophthalmologists considers that clear lens extraction and phakic intraocular lenses are surgical procedures for the correction of refractive errors where existing techniques would not provide optimum results in patients with high levels of refractive error who find the correction of their refractive status with spectacles or contact lenses unacceptable. Like all refractive and surgical procedures appropriate counselling of patients as to the associated risks, complications, and expected outcomes, as well as alternative available options, is considered essential.