Corneal implants for keratoconus (interventional procedures consultation)


Interventional Procedure Consultation Document

Corneal implants for keratoconus

Keratoconus is a disease of the cornea which affects the shape of the eyeball and causes refractive errors, some of which cannot be corrected by spectacles or contact lenses. The insertion of clear plastic implants into the cornea is an interventional procedure aiming to restore eyesight in patients with this condition.

The National Institute for Health and Clinical Excellence is examining corneal implants for keratoconus and will publish guidance on its safety and efficacy to the NHS in England, Wales, Scotland and Northern Ireland. The Institute's Interventional Procedures Advisory Committee has considered the available evidence and the views of Specialist Advisers, who are consultants with knowledge of the procedure. The Advisory Committee has made provisional recommendations about corneal implants for keratoconus.

This document summarises the procedure and sets out the provisional recommendations made by the Advisory Committee. It has been prepared for public consultation. The Advisory Committee particularly welcomes:

  • comments on the preliminary recommendations
  • the identification of factual inaccuracies
  • additional relevant evidence.

Note that this document is not the Institute's formal guidance on this procedure. The recommendations are provisional and may change after consultation.

The process that the Institute will follow after the consultation period ends is as follows.

  • The Advisory Committee will meet again to consider the original evidence and its provisional recommendations in the light of the comments received during consultation.
  • The Advisory Committee will then prepare draft guidance which will be the basis for the Institute's guidance on the use of the procedure in the NHS in England, Wales, Scotland and Northern Ireland.

For further details, see the Interventional Procedures Programme manual, which is available from the Institute's website (

Closing date for comments: 24 April 2007
Target date for publication of guidance: July 2007

Note that this document is not the Institute's guidance on this procedure. The recommendations are provisional and may change after consultation.

1 Provisional recommendations
1.1 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.

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2 The procedure
2.1 Indications

Keratoconus is a progressive disease in which the normally round corneal surface becomes thinner and begins to bulge into a cone like shape. This changes the normal physical properties of the cornea and affects refraction. Keratoconus is often associated with astigmatism. Patients should have clear central corneas in order for them to be suitable candidates for the insertion of corneal implants.

2.1.2 This procedure can also be used for pellucid marginal degeneration: a non-inflammatory, peripheral corneal thinning disorder characterised by thinning of the peripheral band of the inferior cornea. It is not known whether pellucid marginal degeneration and keratoconus are distinct diseases or different manifestations of the same disorder. The cornea within and adjacent to the thinned area is ectatic.

In mild keratoconus, spectacles or soft contact lenses may help. In more severe disease, other treatments may include collagen cross-linking riboflavin eye drops. Invasive procedures include penetrating keratoplasty to modify the shape of the cornea. Ultimately a corneal transplant may be required in some patients.

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2.2 Outline of the procedure
2.2.1 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.
2.2.2 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.

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2.3 Efficacy

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 segments; 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=""><>


A second case series reported that UCVA had improved by 2 lines or more in 72% (53/74) of eyes, and BSCVA had improved by 2 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 2 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).

2.3.3 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).="">
2.3.4 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. For more details, refer to the sources of evidence (see Appendix).
2.3.5 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 which would be the inevitable next step. However, there is some unreliability of effect from patient to patient and in advanced cases of keratoconus the effect on refraction may be too small to be useful.

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2.4 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 successfully rechannelled and fitted.

2.4.2 In four studies, implant segment extrusion occurred in 0%, 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.
2.4.3 A feeling of discomfort occurred 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.
2.4.4 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 (see Appendix).
2.4.5 The Specialist Advisers noted that theoretical adverse events include occasional ring erosion and inflammation around the ring segments, as well as intraoperative damage to the retina/optic nerve due to increased intraocular pressure, and a loss of effect over time.
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3 Further information
3.1 The Institute has published interventional procedures guidance on photorefractive (laser) surgery for the correction of refractive errors (

Bruce Campbell
Chairman, Interventional Procedures Advisory Committee
March 2007

Appendix: 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
Available from:

This page was last updated: 30 March 2010