Introduction

Corneal opacity is a leading cause of monocular blindness, and follows cataracts as a leading cause of blindness worldwide. The World Health Organisation has estimated that ocular trauma and corneal ulceration may be responsible for 1.5–2.0 million new cases of monocular blindness every year.1 The leading cause of blindness in children is corneal opacification caused by a combination of measles, xerophthalmia, and the use of traditional eye medicine.2 Many of the corneal scars resulting from these diseases involve the full stromal thickness. Treatment options are the use of contact lenses or surgery using one of the several keratoplasty techniques.

Rigid contact lenses

Rigid contact lenses can provide good acuity, with minimal risk, for patients with scars involving the pupillary zone by eliminating the effect of corneal irregular astigmatism on vision. However, the quality of vision, including contrast sensitivity and glare, may not be improved. If a contact lens service is available, this technique should be tried first as results can be good,3, 4 and many patients will require contact lenses for optimal vision after keratoplasty anyway.

Keratoplasty techniques

Keratoplasty techniques include deep anterior lamellar allokeratoplasty, penetrating allokeratoplasty, penetrating bilateral autokeratoplasty, and ipsilateral rotational autokeratoplasty (IRA). Indications for these different techniques vary and may be influenced by the availability of donor material and a contact lens service.

Anterior allogeneic lamellar keratoplasty

Anterior allogeneic lamellar keratoplasty may be automated, manual, or deep manual. Anterior lamellar keratoplasty almost completely avoids the problems of corneal endothelial rejection and late endothelial failure that are associated with penetrating corneal allografts and, as a result, this is the technique of choice for many surgeons for partial thickness scars retaining clear deep stromal tissue. However, the depth of the scar and the opacity of the posterior layers are often difficult to assess clinically before surgery. This is important because deep scars result in the failure of some deep lamellar techniques (eg, big bubble) resulting in the need to convert to a penetrating graft, or increased technical difficulty (eg, Melles technique) together with the inability to determine the presence and opacity of residual deep stromal scar tissue until after the dissection had been completed at which point a decision could be made about whether to proceed with a penetrating or lamellar graft. However, anterior segment ocular coherence tomography (OCT) permits imaging through corneal scar tissue allowing assessment of the thickness of residual clear stroma before surgery, so that these problems can be anticipated.

Penetrating allogenic keratoplasty

Penetrating allogenic keratoplasty has been the procedure of choice for full-thickness corneal opacity. However, endothelial rejection rates of 15–20% are common,5, 6 and are increased in non-compliant patients and vascularised corneas.5, 7, 8, 9, 10 In addition to rejection, late corneal failure after a successful penetrating allogenic transplant, is anticipated due to the continued loss of donor corneal endothelial cells with time.11

Both lamellar and penetrating allogeneic keratoplasty techniques require organ donation. The supply of donor allogenic corneal transplants is limited in countries without an eye banking infrastructure or where cultural considerations prohibit the postmortem removal of any organ from the body.12 Alternative techniques to allogenic keratoplasty are bilateral autokeratoplasty and IRA, which eliminate the requirement for both corneal donation and allograft. Both of these techniques minimise the loss of donor endothelial cells resulting from the tissue handling and storage necessitated by eye banking procedures.13 Nevertheless, both techniques may result in corneal decompensation.14

Bilateral autokeratoplasty

Bilateral autokeratoplasty is indicated when there is a healthy cornea in an eye with retinal blindness and an opaque cornea in the contralateral eye, which has visual potential. The clear cornea from the eye with retinal blindness is switched with the opaque cornea from the eye with visual potential.15

Ipsilateral autokeratoplasty

Ipsilateral autokeratoplasty is indicated when the extent of the scarring in the affected cornea, and any associated endothelial damage, is limited so that a central opacity can be exchanged for peripheral clear cornea. In the 1960s, techniques were described for rectangular16 and ‘Figure 8’ grafts17 that minimised the size of the corneal incision to achieve transposition of clear peripheral with opaque central corneal tissue. However, these techniques have been rarely reported, probably because the wound apposition is more difficult with rectangular grafts, and modified instruments are required for ‘Figure 8’ grafts. In the subsequent decades, the necessary transposition has been achieved by eccentric trephination—the technique of IRA.18 This review will describe the aims, indications, surgical techniques, and outcomes for this procedure. Because the procedure is relatively infrequently performed, the data available are based on a few reports of case series12, 13, 14, 18, 19, 20, 21, 22, 23 with 2714 or fewer cases, often with incomplete data.

Aims

To achieve an adequate clear corneal pupillary zone, with minimal astigmatism, and an improved cosmetic result (by placing the scar under the upper lid where possible), without precipitating corneal decompensation.

Indications

IRA has been associated with higher corneal astigmatism and lower visual acuity when compared with homologous penetrating keratoplasty.20 However, appropriate patient selection may minimise these problems. The following criteria are suggested:

Type of opacity

Non-progressive corneal scars following blunt and penetrating corneal trauma, postinfectious keratitis scars, congenital opacities, and chemical injuries12, 14, 21 in progressive cases of idiopathic or postherpetic (zoster and simplex) lipid keratopathy.23

Size of opacity

Assuming a normal mean corneal diameter of 11 mm (vertical)–11.5 mm (horizontal), there should be at least 421, 23 or 5 mm14 of clear cornea in the peripheral cornea (although one author has suggested a minimum of 3 mm12). This clear corneal diameter is required to obtain a clear entrance pupil of 3 mm, free of suture track scars, when employed with maximum decentration of the trephine to the limbus. The optimal clear pupillary zone is probably 5 mm to minimise glare,18 but this can only be achieved with scars that occupy less than half of the mean corneal diameter. This is because an additional 1.5 mm of clear cornea is needed to allow for a suture bite length of 0.75 mm, and scars of such a small size may not require surgery.

Endothelial cell count

This technique may fail when endothelial cell counts are below 1000 mm2. However, in practical terms, meaningful endothelial cell counts are difficult or impossible to obtain, so that the reliance is often placed on the quality of the specular reflex and the presence of normal pachymetry in the peripheral clear cornea.

Patients with high risk of rejection

Densely vascularised corneal leukomas23, or surgery in children because of their higher risk of rejection.14

Reduced availability of donor material

The technique is useful in countries where donor material is scarce and expensive, or donation is forbidden.12

Surgical technique

The technique is identical in principle to that of conventional penetrating keratoplasty with the exception that host cornea is eccentrically cut and then rotated, instead of being replaced with a donor cornea, before suturing. Any additional anterior segment procedures (separation of anterior synechiae, pupilloplasty, cataract extraction, and primary or secondary lens implantation) may be carried out.12, 14, 20 The details of cutting the cornea, suturing, and ancillary techniques will not be described here other than to say that interrupted sutures are recommended because of the more rapid loosening of sutures that occurs with sutures placed into the anterior sclera; this may lead to premature loosening of a continuous suture.

The size and position of the host corneal trephine and position of the rotated scar are critical in achieving the key aims of the procedure (a clear central optical zone with minimum astigmatism and a good cosmetic result). A number of different methods are described to help predict the feasibility of the procedure and optimise the results.

Determining the size and location of the trephination

Several papers describe the results without identifying a technique other than surgeon's judgment.12, 14, 19 However, several methods have been described to aid judgment, and these can be summarised as surgical guidelines, mathematical formulas of varying simplicity, and the use of digital image manipulation.

Surgical guidelines

One publication recommends that a minimum of 3 mm of clear peripheral cornea and trephine size of 6–7 mm will normally be adequate—these guidelines could result in placement of the edge of the clear cornea in the geometric centre of the cornea leaving the patient little better off it at all.12

The other publication recommends:21

  1. 1)

    Choosing cases having a minimum diameter of clear cornea of 4 mm and utilising a trephine size of 8.00–9.00 mm

  2. 2)

    Decentration of the trephine so that the peripheral edge of the corneal button is very close to the limbus.

  3. 3)

    Placing the central edge of the corneal button at least 3 mm from the visual axis (how this is ascertained is not stated; for an 11.5 mm cornea, this will require a 9.0-mm trephine if the peripheral edge is at the limbus).

  4. 4)

    If possible rotate the opacity under the upper lid (to optimise the cosmetic result).

These guidelines have been formalised by Rao and Lam24 who suggest some simple intraoperative measurements to aid the sizing and centration of the autograft.

Mathematical formulas

The two most useful are described in Table 1 . The most frequently quoted method13, 14, 23 is that of Bourne and Brubaker,18 which utilises a simple geometric method for determining the maximum area of clear postoperative central cornea and the trephine size to achieve this result. Placement of the trephine is at the limbus. Examples of this in use are shown in their study.

Table 1 Formulas for calculating trephine size in ipsilateral rotational autografting
Full size table

When the site of the pupil cannot be identified, this method is a good guide to the location of the trephine. However, Harris25 implies that substituting the centre of the pupil, when this can be located, instead of the geometric centre of the cornea in Bourne and Brubaker's18 calculations will give a better estimate of the centre of the entrance pupil of the eye and improve the visual outcome.

The other easy to use formula by Jonas et al,20 also shown in Table 1, utilises a trephine size of 0.75 of the overall corneal diameter, when the scar lies at the geometric centre of the cornea, and adjusts this according to the extent of the scar. Both the Jonas et al20and Bourne and Brubaker18 formulas give similar results with the Jonas et al20formula giving a slightly larger trephine size. When these sizes are rounded up to the nearest 0.25 mm, there is often no difference between the estimates given by both formulas. Figure 1 shows a case in which the trephine sizes differ by 0.25 mm for each formula; in this instance, the Bourne and Brubacker18 formula gives the optimal estimate based on the simulation by digital image manipulation (see below).

Figure 1
figure 1

Examples of digital simulations of a trauma case for planning the feasibility of ipsilateral rotational autokeratoplasty (IRA). This example has only 3.0 mm of clear cornea and is not a good candidate for IRA because of the limited postoperative clear pupillary zone. The best centration of the clear cornea over the geometric axis of the cornea is using a 7.25-mm trephine for this case. (a) Digital image of the cornea adjusted to 11.5 mm overall diameter. (b) Simulation of a trephine size of 7.25 mm. This is closest to the result of the Bourne and Brubacker formula (see Table 1) where: Dt (diameter of the trephine)=(1.5 × 3.0)+1.7=7.2 mm. (c) Simulation of a trephine size of 7.75 mm. This is closest to the result of the Jonas formula (see Table 1) where: Dt (diameter of the trephine)=(0.75 × 11.5)+(0.5 × 1.7)=7.8 mm. (d) Simulation of a trephine size of 8.0 mm.

Full size image

A third formula is designed to optimise the rotation of a linear corneal scar away from the entrance pupil.26 It makes assumptions that are arguable25 has limited applications, and it is probably redundant.

Digital image manipulation

An early publication describes the use a digitised corneal image and a computer programme to optimise the centre of trephination and the optimal trephine diameter, and seems to have no advantages over the previously described methods.27 A more recent publication describes the use of widely available digital image manipulation software (Photoshop 5.0, Adobe) to plan the size, location, and rotation of the graft to provide optimal centration of the clear cornea over the pupil, and the scar position to optimise the cosmetic result. Appendix 1 describes a method for this in more detail using Adobe Photoshop 7.0, adopting the use of the digital ruler, and resizing the image to facilitate the measurement of the diameters of the cornea, the scar, and the maximum clear zone. Figure 1 shows the results of digital manipulation in one case using a selection of simulated trephine sizes two of which have been calculated by the formulas described above.

Outcomes and complications

These are summarised in Table 1 for seven case series,12, 13, 14, 19, 20, 21, 23 which include 160 cases in whom the visual acuity outcomes have been reported for 79 cases and the astigmatism outcomes for 48 cases. These case series vary in the quality of the data they present, but they provide a substantial number of cases for the assessment of the value of the procedure.

Astigmatism

The study by Jonas et al20 is the only one to have compared visual outcomes with that of a non-randomised control group of penetrating allografts, and found significantly higher astigmatism in the nine patients having autografts. However, the reported astigmatism is higher in this study than in any of the other reported series, which have reported astigmatism levels in the same order as those for penetrating keratoplasty. Therefore, it is possible, but not proven, that astigmatism outcomes are higher in this group. Potential reasons for the increased astigmatism, if this truly occurs, are the eccentric trephination, disparity of corneal thickness between the peripheral clear cornea and the central scarred cornea into which it is sutured, and the proximity of one edge of the trephination to the corneal pupillary zone.20

Visual acuity

Visual acuity results are reported in Table 1. From the five studies in which the results of individual cases are reported,12, 14, 19, 21, 23 31/64 (48%) were 6/12 or better, although it is not clear whether this was unaided with spectacles or with contact lenses.

Transplant survival and endothelial cell loss

In Table 1, there were 4/160 decompensated corneas, one of which followed uncontrolled glaucoma. These corneal decompensations are likely to have been related to loss of endothelium, over and above the loss resulting from the disease causing the opacity that were related to the surgery. Long-term results are unknown, but Bertelmann et al13 has shown that mean endothelial cell loss was 15% in rotational autografts compared with 40% in homografts at 1 year, such that the survival of autografts can be expected to compare well with the failure rates of 20% after 10 years due to the continued attrition of endothelial cells and the high rate of transplant rejection in penetrating corneal homografts.11, 28

Other complications

These are summarised in Table 2 and are similar to those expected in any series of penetrating corneal surgery.

Table 2 Summary of literature giving outcomes for ipsilateral rotational autokeratoplasty
Full size table

Conclusions

IRA probably does not provide as good best-corrected visual acuity as penetrating keratoplasty and may result in higher astigmatism. However, there are three benefits, which outweigh these disadvantages for many patients, particularly when the risk of allograft rejection may be higher than normal as in paediatric keratoplasty and when the cornea is vascularised as in many patients following trauma:

  1. 1

    The retention of host endothelium therefore avoiding endothelial rejection.

  2. 2

    The stability of the endothelial cell population in autografts unlike that of allografts, which is expected to lead to transplant failure after 20–30 years.11

  3. 3

    The reduced need for compliance with the use of postoperative steroid drops to prevent rejection and the associated complications of glaucoma, cataract, and infection.29, 30, 31

A limited visual outcome may well outweigh the potential short-term benefits of a penetrating allograft in patients who are suitable for ipsilateral rotational autografts.

In this context, an autograft should probably be attempted when some improvement in vision can be expected, particularly in patients whose second eye is normal and for whom some improvement in vision, at lower risk, is acceptable.

Unfortunately, few patients are suitable for this procedure. Only patients with a minimum diameter of uninvolved cornea of 4–5 mm are suitable (those with only 2–3 mm may get minimal improvement, but this will be limited by the likelihood of suboptimal centration of the clear cornea with the line of sight,32 in addition to the effect of the additional scarring, resulting from suture track scars on the clarity of the remaining clear cornea). In such patients with extensive scars, the failure rate may be higher due to endothelial cell loss resulting from the original trauma and further loss as a complication of necessary additional procedures required at the time of surgery; the effective endothelial cell count is usually impossible to assess with precision before surgery because of poor visualisation of endothelium through scar tissue and difficulty obtaining peripheral endothelial cell counts in residual clear cornea.

For those patients in whom the procedure is practicable, planning of the procedure is by manipulating digital images, as described in Appendix 1, using an estimate of the trephine size and postoperative pupillary clear zone derived from the techniques of Bourne and Brubaker18 or Jonas et al.20