Postkeratoplasty Contact Lens Fitting

22 

Postkeratoplasty Contact Lens Fitting VIJAY ANAND

CHAPTER CONTENTS

History of Keratoplasty,  423 Indications for Keratoplasty,  423 The Postkeratoplasty Cornea,  424

Patients who require contact lens fitting following keratoplasty (corneal graft) can be the greatest challenge to a contact lens practitioner. These patients have undergone a major surgical procedure and may be reluctant to revert to or start contact lens wear. The practitioner needs to consider both the practical challenges of fitting the postkeratoplasty cornea and the patient’s thoughts and attitude concerning contact lens wear.

History of Keratoplasty Keratoplasty has advanced significantly since its conception in 1886 by Von Hippel (1887). Zirm (1906), Magitot (1912), Elschnig (1930), Filatov (1935) and Castroviejo (1949,1950) were pioneers who contributed to the development of techniques for penetrating keratoplasty (PK) that became widely used from the 1940s. Deep anterior lamellar keratoplasty (DALK) was introduced in the 1980s (Archila 1983, Malbran, 1972), but technical difficulty prevented its use until 2002, when Anwar and Teichmann (2002a and b) described a technique to bare Descemet’s membrane by injecting air into the cornea to detach the membrane before carrying out an anterior lamellar keratectomy. This technique was faster, safer and easier to perform than previous methods. Since then, subsequent improvements in surgical techniques have been developed which prevent perforation and ensure better separation of the stromal tissue (Fournié 2007, Tan & Mehta 2007). In 2004, Melles described a technique for sutureless Descemet’s stripping automated endothelial keratoplasty (DSAEK) allowing for transplantation of posterior stroma, Descemet’s membrane and endothelium (Melles 2004). In 2006, Melles described the Descemet membrane endothelial keratoplasty (DMEK) technique, enhancing the DSAEK procedure (Melles 2006). The differences between transplant procedures is shown diagrammatically in Fig. 22.1 and with an OCT scan in Fig. 22.2. These advances have resulted in a shift in the type of transplant procedure undertaken. The Australian Corneal Graft Registry (ACGR; Williams et al. 2015) shows a 50%

Contact Lens Fitting,  430 Aftercare, 436 Conclusion, 437

reduction in penetrating keratoplasty over the past 10 years, while DSAEK and DALK have increased (Fig. 22.3). Keenan et al. (2010) also reported this trend in the UK. It is important to understand the different surgical techniques as these will influence postkeratoplasty corneal thickness, morphology, sensitivity and topography. While surgical techniques have been evolving, there has also been a steady increase in the variety of contact lenses available. This, in conjunction with preservative-free steroids, has allowed for earlier postgraft contact lens fitting.

Indications for Keratoplasty Advances in surgical techniques have resulted in changes in graft indications (Table 22.1). Fig. 22.4 breaks down the indications per graft type, PK, DALK and EK (ACGR Report 2015). Instead of PK, anterior stromal disease, including keratoconus, is now mostly treated with DALK, whereas endothelial disease is treated with DSAEK or DMEK. Fig. 22.5 shows the survival rates for graft surgical procedures over 30 years. The reader will note that survival rates are lower for newer techniques compared with traditional PK. Studies suggest that earlier DALK techniques caused early graft failures and complications (Fontana et al. 2007, Han et al. 2009), but more recent developments have greatly improved graft survival and visual outcomes, and there is less endothelial cell loss compared with PK. Borderie et al. (2009) analysed graft survival and showed that patients who had undergone DALK showed a graft survival of 97.2 ± 2.0% compared with 73.0 ± 2.0%, 5 years after surgery. They also showed a marked difference in the predicted graft survival indices. DALK survival was predicted to be 63.2 ± 6.0% at 20 years and 10.5 ± 4.0% at 40 years, whereas PK predicted graft survival was calculated as 23.9 ± 2.0% at 20 years and 1.2 ± 0.4% at 40 years. Despite the change in indications for keratoplasty, the intended benefit remains the same. The ACGR (2015) reports improving visual function is the primary intended benefit of corneal graft surgery for all graft types (Fig. 22.6). 423

424

SECTION 6  •  Specialist Lens Fitting Bowman’s membrane

Epithelium

A Descemet’s membrane

Penetrating Keratoplasty donor/host junction

Stroma

Endothelium

B

C

A Deep Anterior Lamellar Keratoplasty-donor/host junction

B DSAEK showing 0.11 mm thickness endothelial graft 0.11 mm

D 0.18 mm

C

E

Fig. 22.2  Anterior segment OCT scans highlighting the difference in graft/host junction for: (a) Penetrating keratoplasty; (b) Deep anterior lamellar keratoplasty; (c) Descemet’s stripping automated endothelial keratoplasty. 100% 90%

F

80% 70% 60% 50% 40%

Fig. 22.1  Diagram showing the different types of keratoplasties: (a) Five layers of the normal cornea: superficial multilayered epithelial cell layer, Bowman’s membrane; corneal stromal layer; Descemet’s membrane; endothelial cell monolayer; (b) Penetrating keratoplasty; (c) Anterior lamellar keratoplasty (ALK); (d) Deep lamellar endothelial keratoplasty (DALK); (e) Descemet’s stripping automated endothelial keratoplasty (DSAEK); (f) Descemet’s membrane endothelial keratoplasty (DMEK). (Reproduced from Tan, D.T., Dart, J.K., Holland, E.J., Kinoshita, S., 2012. Corneal transplantation. The Lancet 379, 1749–1761.)

30% 20% 10% 0%

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Penetrating

Tectonic Lamellar

DALK

DS(A)EK

DMEK

Fig. 22.3  Corneal graft surgery changes in practice as reported by the Australian Corneal Graft Registry from ACGR Report 2015.

The Postkeratoplasty Cornea CORNEAL THICKNESS Multiple studies have noted a large interpatient and intrapatient variability in corneal thickness following PK and DALK. Bourne (1983) showed that in a group of 231 PK eyes, average central corneal thickness (CCT) was 540 ± 60 µm with a range of 420–740 µm measured 2 months postoperatively. Kus et al. (1999) showed corneal thickness following PK after a follow-up of 22 years was 608 ± 75 µm. Sarnicola et al. (2010) showed in a retrospective evaluation of 236 DALK procedures between 2000 and 2006 that the

average central corneal thickness after surgery was 584 ± 49 µm. The observed changes relate to corneal swelling at the time of surgery, followed by reduction in thickness due to topical steroid use. Essentially, the corneal thickness is likely to be thicker than a typical cornea following PKs and DALK; therefore pachymetry should be measured and noted in view of possible endothelial changes.

CORNEAL EPITHELIUM Borderie et al. (2006) showed the corneal epithelium in 1003 eyes had a re-epithelialisation time of 4.6 ± 13.2

22  •  Postkeratoplasty Contact Lens Fitting

425

Table 22.1  Keratoplasty Techniques – Indications and Selection (From Tan, D.T., Dart, J.K., Holland, E.J. and Kinoshita, S., 2012. Corneal transplantation. The Lancet 379, 1749–1761.) Selective LK

Ocular surface reconstruction

PK

ALK

Limbal epithelial transplantation

Effect of Replaces all five transplant on corneal layers the host cornea (figure 1) and requirements for donor preparation

Replaces only the Replaces only Replaces only the Replaces the epithelial Replaces all five epithelium and Descemet’s epithelial layer with layer with ex-vivo layers by a Perspex stroma with membrane and donor tissue which cultivated optical device donor the endothelium includes donor epithelium (usually fixated within a Bowman’s (removed by limbus (stroma and on a carrier of conventional membrane stripping) with epithelium) amniotic allogeneic donor and stroma; donor membrane) PK DALK decribes endothelium the removal of and Decemet’s almost all the membrane, with host stroma or without stromal carrier

Variations in technique

EK

The donor may Two variations in Two principal be cut DALK variations in manually or technique are technique are with a used: used: mechanical preDescemet’s trephine or Descemetic stripping EKS with a DALK† and and Descemet’s femtosecond Descemetic membrane EK¶ laser known as DALK‡ FLAK*

Cultivated mucosal epithelial transplantation

Boston type 1 keratoprosthesis

The donor limbus can The donor epithelium be a conjunctival can be of limbal limbal autograft; origin (CLET) or allografts can be mucosal origin from living related (COMET) donors (lr-CLAL) or from a cadaveric donor (KLAL)

Replaces only the epithelial layer; with donor tissue often with amniotic membrane transplantation as adjunctive treatment

Indications for corneal transplantation and for technique selection   Ocular surface disorders‖

NA

NA

NA

Yes

Yes

NA

 Corneal ectasias**

Yes

Yes

NA

NA

NA

Not as a primary procedure

  Primary and acquired stromal disorders††

Yes

Yes (unless the scarring is full thickness)

NA

Combined with PK or Combined with PK or In some cases of DALK when there is DALK when there is aniridia and limbal stem cell, limbal stem cell chemical injuries deficiency (eg, after deficiency (eg, after chemical burns or in chemical burns or in aniridia) aniridia)

 Endothelial disorders‡‡

Yes

NA

Yes

NA

NA

Not as a primary procedure

  Late endothelial Yes failureʃʃ

NA

Yes

NA

NA

Yes if associated with recurrent transplant rejection episodes

Yes (when central Yes and associated with corneal perforation)

NA

NA

NA

NA

NA

NA

NA

NA

 Immunological disorders¶¶

 Therapeutic Yes (usually carried out to treat infection)

Yes in some cases

None of these procedures are effective in severely dry eyes. PK = penetrating keratoplasty. LK = lamellar keratoplasty. ALK = anterior lamellar keratoplasty. EK = endothelial keratoplasty. DALK = deep anterior lamellar keratoplasty. FLAK = femtosecond laser- assisted keratoplasty. lrCLAL = living related conjunctival limbal allograft. KLAL = cadaver-donor keratolimbal allograft. CLET = cultivated limbal epithelial transplantation. COMET = cultivated oral mucosal epithelial transplantation. NA = not appropriate. *Femtosecond laser-assisted keratoplasty. †In pre-Descemetic DALK, more than 75% of the stroma is removed; in descemetic DALK, all the stroma is removed. ʃIn Descemet’s stripping EK, a manually prepared (Descemet’s stripping endothelial keratoplasty) or machine cut (Descemet’s stripping automated endothelial keratoplasty) posterior corneal layer is transplanted. ¶In Descemet’s membrane EK, the donor Descemet’s membrane and endothelium is used without the stroma using Descemet’s membrane endothelial keratoplasty, when the donor is prepared manually, or Descemet’s membrane automated endothelial keratoplasty, when the donor is prepared with the aid of a microkeratome. ‖Ocular surface disorders: such as chemical burns, severe inflammatory disease (eg, in some cases of ocular pemphigoid and Stevens Johnson Syndrome), and congenital disorders (eg, aniridia). **Corneal ectasias: commonly keratoconus, post-lasik corneal ectasia, keratoglobus, and pellucid marginal degeneration. ††Primary and acquired stromal disorders: corneal stromal dystrophies (eg, lattice, granular and macular), post-infectious scars (after herpes simplex virus, bacterial fungal, and Acanthamoeba keratitis), post-traumatic scars. ‡‡Endothelial disorders: commonly Fuch’s corneal dystrophy, pseudophakic and aphakic bullous keratopathy. ʃʃLate endothelial failure: follows acute or recurrent transplant rejection episodes or endothelial cell loss unrelated to rejection. ¶¶Immunological disorders: commonly associated with rheumatoid arthritis and Mooren’s ulcer.

426

SECTION 6  •  Specialist Lens Fitting Indications for PK

1.0

Penetrating Traditional lamellar DALK DS(A)EK DMEK

2% 2% 2%

3% 1% 2%

keratoconus failed previous graft

31%

10%

probability of graft survival

3%

bullous Keratopathy

1%

corneal dystrophy 15%

Fuchs Endothelial dystrophy herpetic eye disease

28%

corneal scar corneal ulcers trauma

4%

3%

4%

1% 2%

0.6 0.4 0.2 0.0

Indications for DALK

1% 2% 3% 3%

0.8

0

3

6

9 12 15 18 21 Trial time (years post graft)

27

30

keratoconus

1.0

herpetic eye disease

Penetrating Traditional lamellar DALK DS(A)EK DMEK

corneal degenerations

probability of graft survival

failed previous graft non-herpetic infections

77%

corneal scar corneal dystrophy trauma

Indications for DSAEK/DMEK 1%

24

1%

0.8 0.6 0.4 0.2

failed previous graft

19%

bullous Keratopathy

0.0

Fuchs Endothelial dystrophy

49% 30%

0

3

6 9 12 Trial time (years post graft)

trauma

15

Fig. 22.5  Comparison of overall graft survival for different types of keratoplasty: (a) over last 30 years; (b) over last 13 years (Courtesy of ACGR, 2015 with permission.)

other

100 90 80 70 60 50 40 30 20 10 0

VA

Im

ns tio ina Co mb er oth

St

ru ctu

an dT he

ra l

ra pe

re p

uti c

air

is es Co sm

ra pe Th e

ac isu al pr ov

ev

uti c

PK DALK DSEAK DMEK LK

uit y

Percentage

Fig. 22.4  Indications for different keratoplasty techniques adapted from the ACGR 2015 report: (a) PK over the last 30 years; (b) DALK over the last 14 years; (c) DMEK over the last 9 years.

Intended benefit Fig. 22.6  Intended benefit of keratoplasty across all graft types adapted from the ACGR 2015 report.

days after surgery. Complete corneal epithelial healing was obtained in 1 day in 28.5% of patients, in 3 days in 65.8%, in 7 days in 93.6%, and in 14 days in 97.0%, with postoperative chronic epithelial defects occurring in 3.0% of eyes.

ENDOTHELIAL MORPHOLOGY Endothelial cell loss occurs at the time of surgery with traditional PK. Typically this occurs at the donor host junction, causing endothelial cells to migrate from the central cornea

22  •  Postkeratoplasty Contact Lens Fitting

427

to the periphery. Zadok et al. (2005) showed endothelial cell density, 10–17 years postoperatively, was 695 ± 113.6 cells/mm2 in corneas transplanted for keratoconus. Vasara et al. (1999) showed that the majority of endothelial cell loss occurred within the first 2 years after transplantation, approximately 48% cell loss within 2 years, and the same eyes only showing a 63% cell loss after 20 years. Tan et al. (2008) showed that endothelial rejection and late endothelial decompensation accounted for 50.51% of failures after 3 years. Fig. 22.7 shows the rapid drop of endothelial cell numbers by almost two-thirds in a patient after PK over a 3-year period. Although this may not directly result in corneal oedema, the post-PK cornea may be more prone to hypoxic stress, and prolonged contact lens wear may exacerbate the decline in corneal function. There is significantly less endothelial cell loss with DALK as the patient’s own endothelium is left intact. Endothelial keratoplasties were originally shown to have a slightly reduced endothelial cell density. Ang et al. (2012) showed the percentage of endothelial cell loss was lower in DSAEK compared with PK at 1 year (30% ± 22% vs 37% ± 25%; p = 0.045), 2 years (36% ± 23% vs 45% ± 33%; p = 0.018) and 3 years (39% ± 24% vs 47% ± 28%; p = 0.022) postoperatively. Newer surgical techniques that preserve the donor corneal endothelium have helped to reduce this cell loss to 15% in the first year (Khor et al. 2011, 2013). When undertaking contact lens fitting, it is important, therefore, to ascertain which type of surgery has been undertaken and consider the potential for corneal oedema and hypoxic stress induced by a contact lens. It is worth measuring a baseline CCT and endothelial cell density prior to contact lens fitting. A high-DK material should be used in post-PK fitting, especially during the first few years.

both PK and DALK, so that corneal sensitivity is significantly reduced. Rao et al. (1985) tested 145 PK eyes and found the central area of corneal transplants to be either completely anaesthetic or markedly hypo-aesthetic, even 32 years following corneal transplantation. Macalister et al.’s (1993) analysis of data from 66 subjects following PK revealed a clear trend towards a slow, but progressive, centripetal resensitisation. At 4 years after transplant, 68% had no central sensitivity, while only 9% had normal sensitivity, and 7 years after transplant, 39% were still without central sensitivity. Ceccuzzi et al. (2010) found similar changes in DALK eyes, with an average percentage of recovery of corneal sensitivity of 91% at 2 years. Therefore the patient may not experience pain associated with contact lens–related complications such as erosions after keratoplasty. This may be particularly important in DALK eyes that have been shown to be at greater risk of epithelial and stromal rejection compared with PK (Cheng et al. 2011). However, lid sensation in these patients remains at a normal level, and may in some cases be heightened, therefore lid sensation may still be a factor for lens wear dropout with rigid gas-permeable (RGP) lenses.

CORNEAL SENSITIVITY

There are a number of different topographical shapes that a practitioner may encounter, and they all generate their own fitting challenges:

The centre of the normal cornea is more sensitive than the periphery (see Chapter 3). Corneal nerves are severed during

CORNEAL TOPOGRAPHY (see Chapter 8) Understanding corneal topography is essential for achieving a suitable lens fitting. There are a variety of techniques to observe the topographical changes of a cornea, and all should be considered to attain a full picture of the corneal surface. These include: corneal profile evaluation (Fig. 22.8) corneal topography (Fig. 22.9) ■ slit-lamp assessment of central and graft margins (Fig. 22.10) ■ anterior segment OCT (Fig. 22.11). ■ ■

protrusive/proud grafts (Fig. 22.11 and 22.12) tilted/proud graft margin (Fig. 22.11) ■ protrusion at the graft/host interface (Fig. 22.13) ■ plateau-shaped grafts (Fig. 22.14) ■ eccentric grafts (Fig. 22.15) ■ regular postgraft astigmatism within the graft margin (Fig. 22.16). ■ ■

Endothelial Cell Loss Post Penetrating Keratoplasty Baseline 2527 cells/mm2

3,000 2,500 2,000 1,500 1,000 500 0

0

6 months 1553 cells/mm2

6

12

12 months 1200 cells/mm2

18

24

36 months 777 cells/mm2

30

36

Fig. 22.7  Endothelial cell loss following PK; the decline in endothelial cells can be seen from the specular microscopy images at baseline, 6, 12 and 36 months.

Fig. 22.8  Profile of corneal transplant achieved by manipulating the upper and lower lids. The steep junction between host and graft can be easily seen (Image courtesy of Ken Pullum.)

428

SECTION 6  •  Specialist Lens Fitting

Fig. 22.9  Corneal topography using the Oculus Pentacam: Sagittal curvature map showing steeping in the superior cornea (black arrow). Elevation map showing areas of elevation and depression (white arrows).

A

B Fig. 22.10  Graft margins: (a) Looking straight ahead, the corneal profile looks even and spherical. (b) With the patient looking down, the extent of the irregularity at the graft margin can more easily be seen.

Fig. 22.11  Anterior segment OCT showing proud superior graft margin.

22  •  Postkeratoplasty Contact Lens Fitting

429

Fig. 22.14  Plateau-shaped profile of corneal transplant. The donor button is much flatter than the peripheral host cornea. (Image courtesy of Ken Pullum.) Fig. 22.12  Anterior segment OCT showing thinning in the host tissue (white arrows) causing entire graft to sit proud.

Protrusion at graft/host interface

Fig. 22.13  Anterior segment OCT showing protrusion at graft/host interface (white arrow).

A

Fig. 22.15  Eccentric inferior displaced corneal graft performed following corneal perforation secondary to trauma. (Image courtesy of Ken Pullum.)

B

Fig. 22.16  (a) Topography showing regular astigmatism of the left eye. The steep areas of the cornea are within the 8 mm zone with the peripheral cornea showing a more regular spherical shape. (b) RGP contact lens on the same eye. Note the areas of corneal touch, which are within the graft margins with adequate edge clearance except in the upper temporal quadrant. (Images courtesy of Ken Pullum.)

The differences in corneal topography frequently relate to the suturing technique used resulting in postoperative astigmatism, the commonest cause for contact lens wear. Despite modifications in suturing techniques, excessive suture tension and poor suture apposition still commonly occur and result in irregular healing at the graft margin (Javadi et al. 2006). Single interrupted sutures (Fig. 22.17a) generate the most astigmatism due to the uneven distribution of corneal tension around the wound circumference when compared with a single continuous suture (see Fig. 22.17b)

(Filatov et al. 1993) or double continuous sutures (Tan & Mehta 2007). With newer surgical techniques, the depth of the suture placement is vital to ensure reduced astigmatism postoperatively, and the surgeon’s experience is key to the surgical outcome.

VISUAL OUTCOME Despite evolving techniques, both PK and DALK still produce large degrees of irregular astigmatism (Reinhart et al. 2011).

430

SECTION 6  •  Specialist Lens Fitting

A

B Fig. 22.17  (a) Interrupted suture following DALK; (b) Continuous suture following DALK.

70 ACGR 2004 PK ACGR 2015 PK ACGR 2015 DALK ACGR 2015 DSAEK

60 Percentage

50 40 30 20 10

IO

L

OL

&

dI Sp

ec

s,

cl

an

OL

CL

dI

ec

tac

les

an

an

dC

L

L Sp

Sp

ec

tac

les

len

so

IO

nly

ly on

ct nta

Co

Sp

ec

tac

les

No

ne

0

Mode of correction Fig. 22.18  Mode of correction used after keratoplasty. (Adapted from Williams, K.A., Keane, M.C., Galettis, R.A., et al., 2015. The Australian Corneal Graft Registry 2015 Report. All rights reserved. Adelaide, Australia.)

However, Fig. 22.18 shows that only a small proportion of patients require contact lenses for visual rehabilitation following PK and DALK. Price et al. (2005) and Ousley et al. (2005) showed that endothelial keratoplasty provided stable refractions. The absence of corneal sutures avoids the refractive changes seen in other types of corneal graft surgery. Patients undergoing endothelial keratoplasty often have phacoemulsification followed by an endothelial graft procedure, and therefore a large number of these patients are visually corrected with an intraocular lens. There are a number of techniques that can reduce the amount of astigmatism after keratoplasty: arcuate keratotomy (Koffler et al. 1996) LASEK (Rashad 2000) ■ topography supported customised laser ablation (Hjortdal & Ehlers 2001) ■ femtosecond-assisted astigmatic keratotomy (Kymionis et al. 2009). ■ ■

As these techniques improve, the need for postgraft contact lens fitting may reduce, but where irregular topography is present, lens fitting will still be necessary for visual rehabilitation.

Contact Lens Fitting There is no exact science when it comes to lens fitting, and there is no single lens that will provide comfort and vision for every patient, so a wide range of trial lens designs are needed to achieve the best outcome. Contact lenses should only be fitted following advice from the surgeon, normally once the corneal sutures have been removed as there is a correlation with graft survival and loose sutures (Kirkness et al. 1990). Most corneal sutures are left in situ for 12–24 months and there is a reduction in graft survival when sutures are removed less than 6 months postoperatively (ACGR Report 2015). However, some ophthalmologists are happy for the patient to undergo contact lens fitting sooner if early visual rehabilitation is required. This is generally undertaken after 3–6 months when the topical steroid use has reduced as this can be a further complication to contact lens fitting. The following should be undertaken before lens fitting: full refraction with best corrected visual acuity of both eyes; these patients may have tolerated high degrees of ametropia and anisometropia preoperatively, so spectacles alone may achieve a reasonable visual outcome ■ full slit-lamp examination recording all clinically relevant details, including: ■ suture presence or absence and whether loose, tight, deep or superficial, and whether there is staining around the sutures ■ neovascularisation Fig. 22.19a – measure the length of the vessels, and note whether they cross the graft margin (often best viewed with a red-free filter; Fig. 22.19b) ■ corneal staining secondary to poor tear film or epithelial erosions ■ tarsal conjunctival inflammation or scarring. ■ topography ■ corneal pachymetry ■ specular microscopy when possible. ■

SOFT CONTACT LENSES Soft contact lenses should only be used once the sutures have been removed to avoid infection. Those patients

22  •  Postkeratoplasty Contact Lens Fitting

431

A

achieving reasonable visual acuity with a low degree of astigmatism may manage with standard disposable soft toric lenses (see Chapter 11). There are a number of lenses available, with an extended toric range, that may be suitable, e.g. Proclear® toric XR, Clariti® toric XR (both Coopervision), Saphir RX, Gentle 80 (both Mark’Ennovy). Hydrogel lenses may drape better over the graft/host margin but have a lower Dk, so require careful monitoring. Silicone hydrogel lenses have greater oxygen permeability, but possibly a higher modulus and therefore may not provide an optimal fit. Where good spectacle corrected acuity is achieved, but a poor fitting from a standard soft disposable lens, a lathecut soft lens can allow customisation of central, peripheral and sectorial curves, e.g. Novacone™ toric (Alden Optical), Soflex Contact lenses, Acuity K soft, Rose K2 Soft™ (Menicon), Kerasoft® (Ultravision). Fig. 22.20 shows an example of a patient with a steep inferior margin at the graft/host junction who requires a quadrantspecific Kerasoft SMC lens (sector management control lens – Ultravision) in order to achieve an optimal fit (see KThin lens fitting details available at: https://expertconsult .inkling.com/). The lens has a standard superior quadrant (30°–150°), while the inferior quadrant (220°–320°) is two steps steeper (see Fig. 22.23 for a Quadrant-Specific Rigid Lens).

B

HYBRID CONTACT LENSES

Fig. 22.19  Corneal neovascularisation a) in white light and b) using a red-free filter. Note the vessels crossing the graft margin, seen more easily with the red-free filter.

A

C

There are now a number of hybrid lenses available (see Chapter 20). These should be used cautiously after graft surgery because despite significant improvements in

B

D

E

Fig. 22.20  (a) Scheimpflug image from Pentacam showing inferior steepening at graft margin; (b) axial/sagittal map showing inferior corneal steepness; (c) slit-lamp cross-section on up-gaze showing the host/graft margin; (d) Kerasoft IC® standard periphery lens in situ showing marked fluting at lens edge due to steep nature of inferior cornea; (e) Kerasoft IC® 8.60/14.50 (A1-30 and A2-150 standard periphery; A3-220 and A4- 320 two-step steep periphery – see KThin Lens Fitting Details available at: https://expertconsult.inkling.com/) showing alignment of the toric marker and resolved fluting at lens edge.

432

SECTION 6  •  Specialist Lens Fitting

oxygen permeability compared with earlier versions there is still a risk of hypoxia. Unacceptable lens tightening and inadequate lens movement may be a triggering factor for graft rejection in an already compromised cornea (see Fig. 22.29).

RIGID GAS-PERMEABLE LENSES RGP lenses are still the most common lens type fitted postkeratoplasty as the curves can be manipulated to give adequate fit and acuity. Where refraction and topographical evaluation of the cornea shows a regular appearance, RGP lens fitting can follow conventional methods (see Chapter 9). However, for those that cannot be fitted with conventional lenses, many other designs are available including reverse geometry lenses whereby the first peripheral curve is 0.40–0.80 mm steeper than the BOZR and quadrant-specific designs (see below). As the donor corneal button is typically 7.75–9.5 mm in diameter, a larger diameter lens may be preferable to ensure better centration and stability. This incorporates a larger BOZD and a larger front optic zone, thereby improving the quality of vision, especially when the lens decentres (see below). It also makes the sag greater so the lens fits more steeply, and therefore a flatter BOZR is required to prevent excessive central clearance. Where there is a significant difference between the flat and steep graft meridians, a BOZR 0.20–0.30 mm steeper than the flattest meridian is a good starting point for fitting. Many postgraft corneas are flatter centrally and steeper in the midperiphery (see Fig. 22.14). This combination can cause the lens to decentre towards the steepest part of the cornea, due to the nature of the host/donor graft margin. Corneal topography will show the steepest areas of the cornea and can help to better understand the fluorescein fit.

A

F

B

G

The aim of lens fitting is to achieve, as near as possible, alignment of the central cornea. Lens bearing should occur just inside or just outside the graft margin, thereby clearing the margins. The edge of the lens should rest on the host cornea and vault the irregular donor/host junction. Often lenses are seen to decentre, particularly onto the superior conjunctiva. This, together with support from the upper lid, good movement and a relatively flat fluorescein pattern can aid lens comfort (Eggink et al. 2001). However, if it results in significant blanching of the conjunctival vessels and conjunctival indentation on lens removal, it can lead to neovascularisation (see ‘Perilimbal or Corneoscleral’ below). During primary gaze, upward gaze and between blinks, the upper edge of the lens should be retained under the upper lid. The inferior edge of the lens should be above the lower lid but with the edge of the optical zone below the inferior pupillary margin in primary gaze. A tight lens will probably lead to lens adherence and compromise the cornea, while a lens that is too flat may cause mechanical injury and possibly corneal scarring. Fig. 22.21 shows a series of different RGP designs fitted to the same cornea. All the lenses chosen were of a similar BOZR, showing that despite a wide variety of diameters and lens designs, overall fitting of the lenses was equivocal, and the practitioner may have to rely on the patient to ascertain the most comfortable lens option. Most of the lenses ride slightly high and tuck under the upper lid. Despite the irregular topography of the cornea, a spherical contact lens often fits adequately as the irregularity tends to be confined to the donor button (see Fig. 22.16), so it is not always necessary for these eyes to be fitted with a full toric lens. However, lenses incorporating a toric periphery may be required to improve the edge clearance of the lens. Fig. 22.22 shows a spherical RGP lens on a grafted cornea with minimal edge clearance. It is not possible to increase

C

H

D

I

E

J

Fig. 22.21  (a) post-PK optic section; (b) Aspheric lens 7.70 : 9.80; (c) Bi-curve lens 7.70 : 11.20; (d) Rose K2 post-graft (Menicon) 7.70 : 10.40; (e) Profile (Jack Allen, UK) Post graft PG4 786 : 10.60; (f) Offset 2 7.70 : 10.00; (g) Ultravision (UK) Xtralens 7.80 : 10.50; (h) Dyna-Intra Limbal (Lens Dynamics Inc.) 7.70 : 11.20; (i) Reverse geometry 7.60 : 11.50 second curve 0.4 steeper than BOZR; (j) Reverse geometry 7.80 : 11.50 second curve 0.8 steeper than BOZR.

22  •  Postkeratoplasty Contact Lens Fitting

the edge clearance by further flattening the lens as there is already a contact zone present within the donor area. A toric periphery on this lens can alleviate the poor edge clearance. In addition to providing toric peripheries, as with soft lenses, RGP lenses are available with quadrant-specific lens peripheries which allow steepening or flattening in one quadrant while the rest of the periphery remains the same, e.g. Dyna Intra Limbal (Lens Dynamics Inc.) and Rose K (Menicon). Fig. 22.23 shows a case where a quadrant-specific rigid lens design improves the fitting (see Fig. 22.20 for a quadrantspecific soft lens).

PIGGYBACKING Piggybacking, the use of a soft lens underneath a rigid lens to improve comfort, should be approached with caution in

433

postgraft eyes, particularly those that have undergone PK, due to the endothelial cell loss described previously. However, silicone hydrogel daily disposable lenses may be safe for piggybacking in order to avoid areas of touch that may result in erosions. Piggybacking with a soft lens can relatively improve the shape of an irregular corneal surface where an RGP alone has failed to achieve a suitable fit. Fig. 22.24a shows topography after a PK where there is a large degree of steepening superiorly. The anterior keratometry shows a difference of 0.65 mm between steep and flat meridians, and the elevation map shows significant areas of elevation with a depression centrally. Fig. 22.24b shows the same cornea with a positivepowered soft daily disposable lens in situ and a significantly more regular (albeit steeper) corneal shape, with the difference between the steep and flat meridians reduced to 0.29 mm. The elevation map is also more regular with the absence of any significant elevation or depression. This will make fitting a rigid lens easier, and the fluorescein pattern should be much more regular.

EC Minimal

PERILIMBAL OR CORNEOSCLERAL LENSES EC good

Contact Zone

Fig. 22.22  Spherical RGP on astigmatic graft. Note minimal edge clearance (EC) between 1 and 3 o’clock and 8 and 10 o’clock and contact zone within the graft area.

Despite advances in soft and RGP lens technology, some grafts will require a lens that completely vaults the corneal structures to obtain a suitable fit. There has been an increase in the number of corneoscleral lenses (14.0–16.0 mm) over the past few years, e.g. SoClear (Dakota Sciences), ICD (Paragon Vision Sciences), Rose K2 XL (Menicon), Profile 14, Profile 16 (Jack Allen, UK), Zen Lens (Alden), Scotlens CorneoScleral and Maxim (Bausch & Lomb). Although these lenses can vault the cornea, they often bear on the limbal area and may cause peripheral corneal

A

B

C Fig. 22.23  OCT (left) and slit-lamp image (right) (a) showing proud inferior graft margin; (b) standard RGP lens in situ. Excessive clearance can be seen inferiorly; (c) quadrant-specific RGP lens in situ showing improved edge clearance and better fit.

434

SECTION 6  •  Specialist Lens Fitting

A

B Fig. 22.24  (a) Topography of a cornea after PK showing superior steepening and significant areas of elevation and depression; (b) a positive-powered soft daily disposable lens in situ on the same cornea normalises the shape of the cornea. A reduction in the areas of elevation and depression can then be seen.

A

B

D

C

E

Fig. 22.25  (a) 16 mm corneo-scleral lens on a post DSAEK eye; (b) area of blanching on superior conjunctiva (red oval); (c) area of blanching viewed with red-free filter; (d) optic section of conjunctiva showing indentation following lens removal (red arrow); (e) area of indentation that fills with fluorescein after lens removal.

neovascularisation. Little is known of the long-term effect on limbal stem cells but Nixon et al. (2017) found peripheral corneal staining and epithelial bullae after as little as 6 hours of wearing corneoscleral lenses of diameter 14.6 mm. They expressed concern about the proximity of these sequelae to the limbal stem cells. Frequent aftercare is essential to prevent complications from developing and, at each appointment, note should be made of any blanching of the conjunctival blood vessels and corneal neovascularisation. Large-diameter lenses tend to be thicker than standard RGP lenses and may have minimal tear exchange. This can have a detrimental effect on the endothelial function of the graft, especially PKs. Fig. 22.25a shows a 16 mm TD corneoscleral lens on a postDSAEK eye with limbal blood vessel blanching between the 8 and 11 o’clock area of the conjunctiva. This is exacerbated

when the patient looks down (see Fig. 22.25b). A red-free filter enhances the appearance of the blood vessels (see Fig. 22.25c). Fig. 22.25d shows the conjunctival indentation following lens removal and the same area pooling with fluorescein (see Fig. 22.25e).

SCLERAL AND MINISCLERAL LENSES (see Chapter 14) Scleral lenses vault the entire cornea, regardless of topographical profile. The large diameter of these lenses, typically 16 mm or greater in size, means that they rest on the conjunctival surface, posing less risk to the limbal stem cell zone. There is still a risk of corneal neovascularisation and minimal tear exchange as with perilimbal lenses, but where other

22  •  Postkeratoplasty Contact Lens Fitting

A

C

435

B

D

E

Fig. 22.26  (a) Corneal topography showing a steep area in the superior aspect of the cornea; (b) The slit-lamp cross-section of the eye showing a steep graft/host junction; (c) The fluorescein fit of a 13.5 mm diameter RGP lens showing bubbles forming at the 2 and 8 o’clock positions; (d) Flattening the base curve of the lens causes increased spot contact on the graft without alleviating the bubbles; (e) A scleral lens in situ vaults the graft/host margin with no bubbles forming.

lens fits are not suitable, a scleral lens option may be the only means of correcting the patient’s visual acuity. There are an increasing number of these lenses available now, e.g. Bausch & Lomb Scleral (23 mm) and mini-scleral (18 mm), Scotlens EasyScleral. Fig. 22.26 shows a typical problem encountered with a postkeratoplasty RGP lens fitting due to the graft/host junction steepening in the superior aspect of the cornea shown topographically in Fig. 22.26a and in slit-lamp paralleliped in Fig. 22.26b. Excessive pooling with an RGP lens leads to mobile trapped bubbles at the graft/host junction (guttering). A flatter BOZR causes increased contact zones within the graft without alleviating the trapped bubbles (Fig. 22.26c and d). A scleral lens vaults over the irregular graft/host shape without causing trapped bubbles (Fig. 22.26e). In addition to problem solving, scleral lenses that vault the cornea are ideal for early rehabilitation while sutures are in situ. Fig. 22.27 shows a DALK with interrupted sutures. A corneal lens that rests on the sutures can cause the sutures to loosen and/or result in infection (Fig. 22.27b and c). A 23  mm scleral lens on the same cornea shows no contact with the corneal sutures (Fig. 22.27d and e).

THERAPEUTIC LENSES (see Chapter 26) Therapeutic contact lenses (TCLs) are used after keratoplasty, especially in the early postoperative phase for the following: persistent epithelial defect beyond 6–7 days protruding sutures that cannot be removed

■ ■

protection of the corneal surface from an abrasive tarsal conjunctiva ■ perforation or aqueous leak secondary to wound dehiscence from incomplete graft/host adhesion or suturing. ■

The use of TCLs has been shown to aid corneal healing (Beekhuis et al. 1991, Lim et al. 2001). Fitting a TCL must be carried out under direction from the corneal surgeon. The postkeratoplasty cornea is immunosuppressed and completely denervated, and therefore at particular risk of extended-wear complications. Rapid epithelialisation of a keratoplasty is important to re-establish a barrier to infection and to prevent subepithelial scarring. Typically large flat-fitting lenses (TD >15.0 mm, BOZR >9.0 mm) are required to provide adequate corneal coverage, although occasionally conventional hydrogel and silicone hydrogel lenses can be used if there is good draping over the irregular graft/host margin and adequate lens movement.

BOSTON KERATOPROSTHESIS The Boston Keratoprosthesis (KPro) (Fig. 22.28a) was developed for patients in whom a traditional full-thickness corneal transplant would likely fail. It provides a clear visual axis without astigmatism and with rapid visual recovery postoperatively. It consists of a PMMA optic and back plate with donor corneal tissue clamped in between. The assembled donor and KPro unit is sutured into a trephined host as in a traditional PK (Doane et al. 1996). Patients undergoing this procedure require lifelong TCLs. The goal of the TCL

436

SECTION 6  •  Specialist Lens Fitting

A

B

D

C

E

Fig. 22.27  (a) DALK with interrupted sutures; (b and c) corneal RGP lens diameter 10.80 sitting low on graft and resting on superior corneal sutures; (d and e) 23 mm scleral lens in situ showing no corneal contact.

A

Fig. 22.29  Graft failure in penetrating keratoplasty with a continuous suture. These sutures may be left in situ when a contact lens is fitted. The lens is likely to increase the risk of rejection, especially in a graft with a reduced endothelial cell count. (Courtesy of A.J. Phillips.)

surrounding ocular tissue is irregular, a large flat mid to high water-content hydrogel lens is ideal, e.g. 9.80 mm base curve, 16.00 mm diameter. The appearance of the eye can be greatly improved by adding a tint to the therapeutic lens (see Fig. 22.28b). B Fig. 22.28  (a) A KPro in situ in an eye that had previously suffered an alkali burn. Note the flat profile of the central graft and the sutures in the irregular surrounding tissue. (b) The same eye fitted with a cosmetic tinted contact lens to improve the appearance.

is to maintain hydration and to protect the corneal tissue that surrounds the anterior plate of the keratoprosthesis, which is vulnerable to desiccation, epithelial breakdown, dellen formation, and corneal melt (Thomas et al. 2015). As the profile of the KPro is flat (see Fig. 22.28a), and the

Aftercare (see Chapter 16) Regular aftercare with all postkeratoplasty contact lens patients is essential, and this must be emphasised at the fitting stage. The risk of inflammation from an ill-fitting lens, inadequate oxygen supply, poor retro-lens tear flow, or host and graft neovascularisation are all present. Although greatest in the first few years after surgery, the risk of graft rejection (Fig. 22.29) is always present, and there should be ongoing dialogue with the corneal surgeon to prevent unnecessary complications.

22  •  Postkeratoplasty Contact Lens Fitting

As well as standard aftercare, follow-up should include: slit-lamp examination as with initial lens fitting (see ‘Contact Lens Fitting’ above): ■ contact lens fit ■ sutures ■ neovascularisation – any signs of new vessels in the transplant can be serious as it can increase the risk of graft rejection ■ corneal staining ■ tarsal conjunctiva. ■ corneal topography to note changes following suture removal, which may have affected the lens fitting ■ intraocular pressure (especially if still on topical steroids) ■ corneal pachymetry (this may be a feature of the topographer, e.g. Pentacam) to evaluate potential endothelial dysfunction post–lens wear ■ specular microscopy when possible if endothelial dysfunction suspected. ■

Conclusion Postkeratoplasty contact lens fitting can be one of the greatest challenges to the contact lens practitioner, who must consider the physiology and topography of the cornea and the potential for complications. Ongoing developments in keratoplasty surgery will have an effect on the types of postkeratoplasty corneas that will exist in years to come. Phacoemulsification combined with DSAEK/DMEK has resulted in a reduction of PKs, meaning the number of patients requiring postkeratoplasty fitting has reduced. Corneal cross-linking for keratoconus is beginning to show a reduction in the number of corneal transplants that are being undertaken for keratoconus (Sandvik et al. 2015). Advances in laser-assisted surgical techniques and postoperative correction of irregular corneas will also reduce the future need for postkeratoplasty contact lenses. Enhanced lens designs, using advanced lathe technologies increases the range of patients able to achieve the best visual potential. Practitioners must always be cautious about the risk of infection, neovascularisation and graft rejection, and remember to consider the patient not just the eye.

References Ang, M., Mehta, J.S., Lim, F., et al., 2012. Endothelial cell loss and graft survival after Descemet’s stripping automated endothelial keratoplasty and penetrating keratoplasty. Ophthalmology 119, 2239–2244. Anwar, M., Teichmann, K.D., 2002a. Deep lamellar keratoplasty: surgical techniques for anterior lamellar keratoplasty with and without baring of Descemet’s membrane. Cornea 21, 374–383. Anwar, M., Teichmann, K.D., 2002b. Big-bubble technique to bare Descemet’s membrane in anterior lamellar keratoplasty. J. Cataract Refract. Surg. 28, 398–403. Archila, E.A., 1983. Deep lamellar keratoplasty dissection of host tissue with intrastromal air injection. Cornea 3, 217–218. Beekhuis, W.H., Van Rij, G., Eggink, F.A., et al., 1991. Contact lenses following keratoplasty. CLAO J. 17, 27–29. Borderie, V.M., Boëlle, P.Y., Touzeau, O., et al., 2009. Predicted long-term outcome of corneal transplantation. Ophthalmology 116, 2354–2360. Borderie, V.M., Touzeau, O., Bourcier, T., et al., 2006. Graft reepithelialisation after penetrating keratoplasty using organ-cultured donor tissue. Ophthalmology 113, 2181–2186.

437

Bourne, W.M., 1983. Morphologic and functional evaluation of the endothelium of transplanted human corneas. Trans. Am. Ophthalmol. Soc. 81, 403–450. Castroviejo, R., 1949. Trephines for keratoplasty with micrometric regulation. Transactions –American Academy of Ophthalmology and Otolaryngology. Am. Acad. Ophthalmol. Otolaryngol. 54, 373–374. Castroviejo, R., 1950. Lamellar keratoplasty: technique and results: comparative study with penetrating keratoplasties and keratectomies. Am. J. Ophthalmol. 33, 1851–1862. Ceccuzzi, R., Zanardi, A., Fiorentino, A., et al., 2010. Corneal sensitivity in keratoconus after penetrating and deep anterior lamellar keratoplasty. Ophthalmologica 224, 247–250. Cheng, Y.Y., Visser, N., Schouten, J.S., et al., 2011. Endothelial cell loss and visual outcome of deep anterior lamellar keratoplasty versus penetrating keratoplasty: a randomized multicenter clinical trial. Ophthalmology 118, 302–309. Doane, M.G., Dohlman, C.H., Bearse, G., 1996. Fabrication of a keratoprosthesis. Cornea 15, 179–184. Eggink, F.A., Nuijts, R.M., 2001. A new technique for rigid gas permeable contact lens fitting following penetrating keratoplasty. Acta Ophthalmol. Scand. 79, 245–250. Elschnig, A., Vorisek, E.A., 1930. Keratoplasty. Arch. Ophthalmol. 4, 165–173. Filatov, V.P., Sitchevska, O., 1935. Transplantation of the cornea. Arch. Ophthalmol. 13, 321–347. Filatov, V., Steinert, R.F., Talamo, J.H., 1993. Postkeratoplasty astigmatism with single running suture or interrupted sutures. Am. J. Ophthalmol. 115, 715–721. Fontana, L., Parente, G., Tassinari, G., 2007. Clinical outcomes after deep anterior lamellar keratoplasty using the big-bubble technique in patients with keratoconus. Am. J. Ophthalmol. 143, 117–124. Fournié, P., Malecaze, F., Coullet, J., et al., 2007. Variant of the big bubble technique in deep anterior lamellar keratoplasty. J. Cataract Refract. Surg. 33, 371–375. Han, D.C., Mehta, J.S., Por, Y.M., et al., 2009. Comparison of outcomes of lamellar keratoplasty and penetrating keratoplasty in keratoconus. Am. J. Ophthalmol. 148, 744–751. Hjortdal, J.Ø., Ehlers, N., 2001. Treatment of post-keratoplasty astigmatism by topography supported customized laser ablation. Acta Ophthalmol. Scand. 79, 376–380. Javadi, M.A., Naderi, M., Zare, M., et al., 2006. Comparison of the effect of three suturing techniques on postkeratoplasty astigmatism in keratoconus. Cornea 25, 1029–1033. Keenan, T.D., Carley, F., Yeates, D., et al., 2010. Trends in corneal graft surgery in the UK. Br. J. Ophthalmol. 95, 468–472. Khor, W.B., Han, S.B., Mehta, J.S., et al., 2013. Descemet stripping automated endothelial keratoplasty with a donor insertion device: clinical results and complications in 100 eyes. Am. J. Ophthalmol. 156, 773–779. Khor, W.B., Mehta, J.S., Tan, D.T.H., 2011. Descemet stripping automated endothelial keratoplasty with a graft insertion device: surgical technique and early clinical results. Am. J. Ophthalmol. 151, 223–232. Kirkness, C.M., Ficker, L.A., Steele, A.D.M., et al., 1990. The success of penetrating keratoplasty for keratoconus. Eye (Lond.) 4, 673–688. Koffler, B.H., Smith, V.M., 1996. Corneal topography, arcuate keratotomy, and compression sutures for astigmatism after penetrating keratoplasty. J. Refract. Surg. 12, S306–S309. Kus, M.M., Seitz, B., Langenbucher, A., et al., 1999. Endothelium and pachymetry of clear corneal grafts 15 to 33 years after penetrating keratoplasty. Am. J. Ophthalmol. 127, 600–602. Kymionis, G.D., Yoo, S.H., Ide, T., et al., 2009. Femtosecond-assisted astigmatic keratotomy for post-keratoplasty irregular astigmatism. J. Cataract Refract. Surg. 35, 11–13. Lim, L., Tan, D.T.H., Chan, W.K., 2001. Therapeutic use of Bausch & Lomb PureVision Contact Lenses. CLAO J. 27, 179–185. Macalister, G.O., Woodward, E.G., Buckley, R.J., 1993. The return of corneal sensitivity following transplantation. J. Br. Cont. Lens Assoc. 16, 99–104. Magitot, A., 1912. Transplantation of the human cornea previously preserved in an antiseptic fluid. JAMA 59, 18–21. Malbran, E., Stefani, C., 1972. Lamellar keratoplasty in corneal ectasias (Part 2 of 2). Ophthalmologica 164, 59–70. Melles, G.R., Ong, T.S., Ververs, B., et al., 2006. Descemet membrane endothelial keratoplasty (DMEK). Cornea 25, 987–990. Melles, G.R., Wijdh, R.H., Nieuwendaal, C.P., 2004. A technique to excise the Descemet membrane from a recipient cornea (descemetorhexis). Cornea 23, 286–288.

438

SECTION 6  •  Specialist Lens Fitting

Nixon, A.D., Barr, J.T., VanNasdale, D.A., 2017. Corneal epithelial bullae after short-term wear of small diameter scleral lenses. Cont. Lens Anterior Eye 40, 116–126. Noble, B.A., Agrawal, A., Collins, C., et al., 2007. Deep anterior lamellar keratoplasty (DALK): visual outcome and complications for a heterogeneous group of corneal pathologies. Cornea 26, 59–64. Ousley, P.J., Terry, M.A., 2005. Stability of vision, topography, and endothelial cell density from 1 year to 2 years after deep lamellar endothelial keratoplasty surgery. Ophthalmology 112, 50–57. Price, F.W., Price, M.O., 2005. Descemet’s stripping with endothelial keratoplasty in 50 eyes: a refractive neutral corneal transplant. J. Refract. Surg. 21, 339–345. Rao, G.N., John, T., Ishida, N., et al., 1985. Recovery of corneal sensitivity in grafts following penetrating keratoplasty. Ophthalmology 92, 1408–1411. Rashad, K.M., 2000. Laser in situ keratomileusis for correction of high astigmatism after penetrating keratoplasty. J. Refract. Surg. 16, 701–710. Reinhart, W.J., Musch, D.C., Jacobs, D.S., et al., 2011. Deep anterior lamellar keratoplasty as an alternative to penetrating keratoplasty: a report by the American Academy of Ophthalmology. Ophthalmology 118, 209–218. Sandvik, G.F., Thorsrud, A., Råen, M., et al., 2015. Does corneal collagen cross-linking reduce the need for keratoplasties in patients with keratoconus? Cornea 34, 991–995. Sarnicola, V., Toro, P., Gentile, D., et al., 2010. Descemetic DALK and predescemetic DALK: outcomes in 236 cases of keratoconus. Cornea 29, 53–59.

Tan, D.T., Dart, J.K., Holland, E.J., et al., 2012. Corneal transplantation. Lancet 379, 1749–1761. Tan, D.T., Janardhanan, P., Zhou, H., et al., 2008. Penetrating keratoplasty in Asian eyes: the Singapore corneal transplant study. Ophthalmology 115, 975–982. Tan, D.T., Mehta, J.S., 2007. Future directions in lamellar corneal transplantation. Cornea 26, S21–S28. Thomas, M., Shorter, E., Joslin, C.E., et al., 2015. Contact lens use in patients with Boston keratoprosthesis type 1: fitting, management, and complications. Eye Contact Lens 41, 334–340. Vasara, K., Setälä, K., Ruusuvaara, P., 1999. Follow-up study of human corneal endothelial cells, photographed in vivo before enucleation and 20 years later in grafts. Acta Ophthalmol. Scand. 77, 273–276. von Hippel, A., 1877. Über die operative Behandlung totaler stationärer Hornhaut-Trübungen. Graefes Arch. Clin. Exp. Ophthalmol. 23, 79–160. Williams, K.A., Keane, M.C., Galettis, R.A., et al., 2015. The Australian Corneal Graft Registry. South Australian Health and Medical Research Institute, 409 p. Zadok, D., Schwarts, S., Marcovich, A., et al., 2005. Penetrating keratoplasty for keratoconus: long-term results. Cornea 24, 959–961. Zirm, E., 1906. Eine erfolgreiche totale Keratoplastik. Graefes Arch. Clin. Exp. Ophthalmol. 64, 580–593.