Corneal implants for keratoconus

# Guidance
Current evidence on the safety and efficacy of corneal implants for keratoconus appears adequate to support the use of this procedure provided that normal arrangements are in place for consent, audit and clinical governance.# The procedure

# Indications
Keratoconus is a progressive disease in which the normal cornea becomes more irregular in shape over time resulting in astigmatism, and can progress to a stage where the cornea becomes thinner and begins to bulge into a cone-like shape.
This procedure can also be used for pellucid marginal degeneration: a non-inflammatory, peripheral corneal thinning disorder characterised by the erosion of the peripheral band of the inferior cornea.
In mild to moderate keratoconus, spectacles or a range of contact lenses may help as well as treatment with riboflavin eye drops. In more severe disease, penetrating or deep lamellar keratoplasty corneal grafting (transplantation) to restore the normal corneal shape may be required.

# Outline of the procedure
Corneal implants are flexible, crescent-shaped rings of polymethyl methacrylate that are placed in the periphery of the cornea. They affect refraction in the eye by physically changing the shape of the cornea, flattening the front of the eye, and so correcting the irregular corneal shape.
The procedure is undertaken under local or general anaesthesia. An incision is made in the cornea and channels are created in it by rotating a lamellar dissector or by using a femtosecond laser. One corneal implant segment is introduced to each channel. Various implants with a range of implant thicknesses are available for different degrees of correction.
If required, the implant can be removed at a later date.

# Efficacy
Most efficacy data outcomes reported in the literature were up to 12 months' follow-up.
One case series (n = 34 eyes) reported that best spectacle-corrected visual acuity (BSCVA) improved significantly from baseline to 6 months after insertion of corneal ring implants: 62% of eyes gained two to eight lines, 32% had no change and 6% lost two or more lines (p < 0.001). An uncorrected visual acuity (UCVA) score of 20/40 or more was recorded in 24% (8/34) of eyes at 12-month follow-up, compared with 4% (2/53) of eyes at baseline (p < 0.001).
A second case series reported that UCVA had improved by two lines or more in 72% (53/74) of eyes, and BSCVA had improved by two lines or more in 45% (33/74) of eyes at 9-month follow-up (p values not reported). A third case series of 31 eyes reported that BSCVA had improved by two lines or more in 87% (27/31) of eyes and UCVA had improved by the same amount in 81% (25/31) of eyes at 12-month follow-up (p values not reported).
In one case series of 51 eyes, the mean refractive astigmatism decreased from 3.69  2.20 D (dioptres) at baseline to 2.21  1.96 D after surgery (p < 0.01) (duration of follow-up not stated). A second case series of 13 eyes treated with corneal ring implants reported that mean corneal curvature improved from 48.46  3.72 D at baseline to 45.32  3.01 D at 6-month follow-up, although this was not sustained at 3-year follow-up (47.00  3.57 D). A third case series of 100 eyes reported that mean corneal curvature improved from 50.1  5.6 D at baseline to 46.6  5.3 D at 1 year and 46.8  4.9 at 2 years (p < 0.001 for both).
In one case series of 13 eyes with 3-year follow-up, all patients who were contact lens intolerant at baseline were able to wear a contact lens after surgery as a result of the change in corneal shape. For more details, refer to the 'Sources of evidence' section.
The Specialist Advisers considered that the procedure aims to reduce astigmatism in keratoconus and reduce the need for corneal transplant, with a rapid recovery time and little ocular morbidity. They noted that it is performed in an attempt to delay corneal transplantation. However, there is some variation of effect from patient to patient and in advanced cases of keratoconus the effect on refraction may be too small to be useful.

# Safety
One case series of 57 eyes reported that there were no intraoperative complications or clinically significant postoperative complications. In another case series, creation of a superficial channel perforated the Bowman's layer in 1% (1/74) of eyes, although the implant was able to be successfully refitted.
In four studies, implant segment extrusion occurred in 0% (0/58), 1% (1/74), 14% (5/36) and 20% (10/51) of eyes. Bacterial infection following corneal implant procedures occurred in 0%, 0%, 3% and 2% of eyes, respectively.
A feeling of discomfort persisted in 2% (1/57) of eyes in one study, and chronic foreign body sensation requiring removal of the implants occurred in 4% (3/74) of eyes in another study. Corneal channel deposits were found in 31% (4/13) of eyes in a third study although these did not affect visual outcome.
The most commonly reported visual disturbances were halos or glare which occurred in between 3% (2/74) and 5% (3/57) of eyes. For more details, refer to the 'Sources of evidence' section.
The Specialist Advisers noted that theoretical adverse events include occasional ring erosion and inflammation around the ring segments, intraoperative damage to the retina or optic nerve due to increased intraocular pressure, and a loss of effect over time.

# Other comments
The Committee noted that a previous implant is unlikely to have an impact on the success of subsequent corneal implants.# Further information
The Institute has published interventional procedures guidance on photorefractive (laser) surgery for the correction of refractive errors.
Andrew DillonChief ExecutiveJuly 2007

# Sources of evidence
The following document, which summarises the evidence, was considered by the Interventional Procedures Advisory Committee when making its provisional recommendations.
'Interventional procedure overview of corneal implants for keratoconus', January 2007.

# Information for patients
NICE has produced information describing its guidance on this procedure for patients and their carers ('Understanding NICE guidance'). It explains the nature of the procedure and the decision made, and has been written with patient consent in mind.# About this guidance
NICE interventional procedure guidance makes recommendations on the safety and efficacy of the procedure. It does not cover whether or not the NHS should fund a procedure. Funding decisions are taken by local NHS bodies after considering the clinical effectiveness of the procedure and whether it represents value for money for the NHS. It is for healthcare professionals and people using the NHS in England, Wales, Scotland and Northern Ireland, and is endorsed by Healthcare Improvement Scotland for implementation by NHSScotland.
This guidance was developed using the NICE interventional procedure guidance process.
We have produced a summary of this guidance for patients and carers. Information about the evidence it is based on is also available.
Changes since publication
January 2012: minor maintenance.
Your responsibility
This guidance represents the views of NICE and was arrived at after careful consideration of the available evidence. Healthcare professionals are expected to take it fully into account when exercising their clinical judgement. This guidance does not, however, override the individual responsibility of healthcare professionals to make appropriate decisions in the circumstances of the individual patient, in consultation with the patient and/or guardian or carer.
Implementation of this guidance is the responsibility of local commissioners and/or providers. Commissioners and providers are reminded that it is their responsibility to implement the guidance, in their local context, in light of their duties to avoid unlawful discrimination and to have regard to promoting equality of opportunity. Nothing in this guidance should be interpreted in a way which would be inconsistent with compliance with those duties.
Copyright
 National Institute for Health and Clinical Excellence 2007. All rights reserved. NICE copyright material can be downloaded for private research and study, and may be reproduced for educational and not-for-profit purposes. No reproduction by or for commercial organisations, or for commercial purposes, is allowed without the written permission of NICE.
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