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Clinical and Experimental Ophthalmology 2010; 38: 211–224 doi: 10.1111/j.1442-9071.2010.02231.x

Review

Keratoprostheses in clinical practice – a review

Ahmed Gomaa

FRCS PhD

Oliver Comyn

MRCOphth

and

Christopher Liu

FRCOphth

1,2

211..224

Sussex Eye Hospital, Brighton, and

Tongdean Eye Clinic, Hove, UK

BSTRACT

The search for a substitute for the natural cornea dates

back more than 200 years. Although several devices

have been developed and trialled, very few have had

successful long-term results and continue in regu-

lar clinical use. Keratoprosthesis (KPro) surgery is

complex and should be performed in centres with an

experienced multidisciplinary team. Currently avail-

able KPro devices range from the totally synthetic,

such as the Boston KPro, to the totally biological

tissue-engineered artiﬁcial cornea. The osteo-odonto

keratoprothesis combines a synthetic optic with a bio-

logical haptic. All keratoprostheses have signiﬁcant

limitations, although visual improvement is possible

with each of the devices in clinical use today. This

review discusses these devices with emphasis on their

indications, surgical techniques and results, before

brieﬂy exploring emerging devices and innovative

approaches for the future.

Key words:

AlphaCor, artiﬁcial cornea, Boston, kerato-

prosthesis, OOKP, Pintucci.

ISTORY

AND EVOLUTION OF KERATOPROSTHESES

Replacing an opaque cornea with a glass plate was

ﬁrst described by Pellier de Quengsy in France more

than 200 years ago.

He led the way by proposing a

device with a porous skirt, which could be colonized

by tissues, a concept that is adopted by most recent

devices and considered essential for success. In 1853,

Nussbaum described a collar-stud glass device con-

sisting of two plates sandwiching the cornea and

connected by an optical cylinder,

with trials in

rabbit eyes. Heusser in 1859 was possibly the ﬁrst to

implant a keratoprosthesis (KPro) in a human eye;

this was retained for 3 months.

Several other

attempts were made over the second half of the 19th

century (Von Hippel 1877, Dimmer 1889, Baker

1889, van Millingen 1895, Salzer 1895) but almost

all the implants failed and were extruded.

4–8

The interest in keratoprostheses declined follow-

ing the development of successful penetrating

keratoplasty (PKP) using human corneal allografts

which occurred in the ﬁrst decade of the 20th

century. Years later came the realization that trans-

planting a human cornea would not be successful in

all cases of corneal blindness, leading to a renewed

search for a corneal substitute, which could be suc-

cessfully implanted and retained in human eyes.

During the Second World War, the incidental discov-

ery of corneal tissue tolerance to plexi-glass frag-

ments from aeroplane canopies suggested a new

direction for future research.

Using plastic as a

corneal replacement, or inlay implant, was investi-

gated in rabbits by Herbert and Stone in the early

1950s. They found that lamellar implants were better

tolerated than full thickness ones, which were

extruded within 2 weeks.

Bock and Maumenée,

followed by Knowles, studied ﬂuid kinetics within

the corneal tissues and examined the effect of

implanting a plastic disc on the nutrition of the ante-

rior corneal layers. They showed a barrier effect from

these implants, which led to anterior corneal dehy-

dration and thinning. The results were similar for

implants made from poly(glycerylmethacrylate).

12–15

This research led to the concept of an ideal KPro.

This artiﬁcial implant would restore corneal clarity,

integrate with host tissues and withstand a hostile

ocular surface environment while leading to a

minimum of complications common to all kerato-

prostheses: infection, glaucoma, the formation of ret-

roprosthetic membranes and extrusion.

Further research and development inevitably led

to some trials which ended unsuccessfully with high

extrusion rates and infection occurring in nearly all

cases, as with the retrocorneal ﬁxation KPro by

Lacombe and the BioKPro by Leagais.

16,17

Advances

in microsurgical techniques, methods to control

Correspondence:

Dr Christopher Liu, Sussex Eye Hospital, Eastern Road, Brighton BN2 5BF, UK. Email: cscliu@aol.com

Received 3 October 2009; accepted 22 January 2010.

212

Gomaa

et al.

Figure 1.

Boston Keratoprosthe-

ses types 1 (left) and 2 (right).

infection and manage glaucoma contributed to the

development of the devices available and in contin-

ued clinical use today. Although limited in number,

these devices solve the problem of the artiﬁcial

cornea in a number of ways, from the totally syn-

thetic Boston KPro, to the combined synthetic optic

and biological frame of the osteo-odonto keratopro-

thesis (OOKP) and the entirely biological tissue-

engineered cornea. Although all keratoprostheses

have limitations, varying degrees of visual improve-

ment can be achieved with all of the devices in

current clinical use, which will be discussed later.

EVICES IN CURRENT USE

The Boston Keratoprosthesis (previously

known as the Dohlman-Doane

Keratoprosthesis)

The Boston KPro has been under development since

the 1960s but did not receive FDA approval for mar-

keting in the USA until 1992. Two types are available

and in current clinical use, types 1 and 2 (Fig. 1);

both have optical components made from medical-

grade poly(methyl methacrylate) (PMMA). The type

1 device consists of a PMMA optical front plate and

stem, which is inserted through and hence sand-

wiches a corneal graft button which has been cen-

trally trephined, locking into a larger back plate.

Originally, the back plate was either threaded onto

or glued to the stem. In 2003, a locking ring made of

titanium was added to ensure that the back plate is

secured safely in place. In 2007, the design of the

Boston KPro evolved to feature a threadless stem.

Since then it has been modiﬁed to include a titanium

back plate with larger holes. The new threadless

design is expected to produce less corneal graft

damage during assembly, be easier for use by inex-

perienced surgeons and make inexpensive manufac-

ture of the device possible by moulding rather than

machining. Optically, the power of this device can be

adjusted to match the patient’s refractive power. In

practice, a pseudophakic eye with an intraocular lens

in situ

can be corrected with a KPro of a standard,

single power, leaving the eye emmetropic. The

typical anterior surface power of this device is 43–44

dioptres. Aphakic eyes require a variety of powers

depending on the axial length and it is suggested by

the developers that a typical bank of available KPros

might cover a range of axial lengths from 15 mm to

30 mm. The full technical speciﬁcation of the Boston

KPro type 1 is shown in Table 1.

18–22

The type 2

device is of a similar design, with an added anterior

cylinder that protrudes through a permanently

closed upper eyelid, and is used in end-stage dry eye

(Fig. 1).

Indications

The Boston KPro is suitable for use in cases of refrac-

tory corneal blindness with repeated failed corneal

grafts where further corneal grafting is deemed very

high risk. Patients should be monocular, or bilater-

ally blind according to the WHO criteria. Ideally,

there should be no evidence of retinal or optic nerve

disease with a minimum vision of light perception

with good projection in the four quadrants.

In prac-

tice, however, many patients who require a KPro do

Review – Keratoprostheses

Table 1.

Design and dimensions of the type 1 Boston KPro

Diameter (front plate): 5.0, 6.0 mm

Stem: three segments and groove (1st next to front plate)

1st segment: height 0.62 mm, diameter 3.35 mm

2nd segment: height 0.84 mm, diameter 3.0 mm

Groove: height 0.33 mm, diameter 2.74 mm

3rd segment: variable height

213

Front Part

Back Plate

Locking Ring

Diameter: 8.5 mm

Central hole diameter: 3.0 mm

Plate thickness next to the hole is 0.8 mm and at the periphery 0.6 mm

16 holes of 1.17 mm diameter

Smaller back plate (children): diameter of 7.0 mm, with 8 holes of 1.3 mm diameter each

Outer diameter: 3.6 mm

Inner diameter: 2.8 mm

Thickness: 0.23 mm

have some degree of glaucoma and may have previ-

ously undiagnosed macular disease. Patients should

have good lid anatomy and blink to preserve a

healthy ocular surface and help to retain the protec-

tive contact lens which is used postoperatively. They

should have good access to the hospital carrying out

the surgery and be committed to a regular follow-up

schedule. Autoimmune cases such as Stevens–

Johnson syndrome (SJS) or ocular cicatricial pem-

phigoid (OCP) are usually excluded. Although the

device is usually used in adults, Aquavella

et al.

reported good success using the Boston type 1 KPro

in children with corneal opacity due to primary con-

genital disease and/or previous failed keratoplasty.

Surgical procedure and postoperative

management

The Boston type 1 KPro can be ordered from the

Massachusetts Eye and Ear Inﬁrmary (MEEI)

(Boston, MA, USA). Surgeons should be experienced

at performing conventional full-thickness PKP and

need to have knowledge of the device and its

assembly. Surgery begins with the assembly of the

device. A donor corneal button (usual size 8.5–9.

0 mm) is prepared and a central 3 mm hole is

trephined. The front plate is ﬁxed to the adhesive

surface supplied with the device. The donor button

is then placed over the stem of the front plate and the

back plate is slid into place on top of this without

screwing or turning. A titanium locking ring is then

pushed onto the remaining exposed stem until an

audible ‘snap’ is heard (Fig. 2). The assembled KPro

is examined under the operating microscope for

correct assembly before proceeding with host tre-

phination. The recipient cornea is then trephined

as for conventional PKP (trephine diameter 0.5 mm

less than donor trephine size). If simultaneous cata-

ract extraction is performed, it is advisable to leave

the posterior capsule of lens intact if possible, other-

wise an anterior vitrectomy should be performed.

The donor graft with the KPro is then sutured in

place with interrupted 10–0 nylon, using the same

technique as a standard PKP. Surgery usually con-

cludes with the intracameral injection of 0.4 mg dex-

amethasone and the application of a soft contact

lens (Kontur Contact Lens, Richmond, CA, USA)

(Fig. 3).

18,23,25

Assembly of the type 2 KPro is similar to the type

1 and surgery features the same initial steps of sutur-

ing the donor graft and KPro assembly in place. The

device is then covered with lid skin or in some cases

a mucous membrane graft. A pericardium graft can

also be used to increase overall thickness by ﬁxation

to the front plate followed by closure of the lid skin

on top of this. Boston type 2 KPro surgery is techni-

cally challenging and has only rarely been performed

outside the MEEI. It usually necessitates meticulous

dissection of the lids from the underlying ocular

surface with division of synechiae, which often leads

to profuse bleeding.

Postoperatively, all patients are prescribed topical

steroids, starting four times daily for the ﬁrst week

and tapering over 6 weeks to a maintenance dose of

at least once daily. Similarly, topical antibiotics

should be administered four times daily for the ﬁrst

month, followed by maintenance therapy according

to the recommended protocol and the availability of

214

Gomaa

et al.

Figure 2.

Assembly

Boston keratoprosthesis.

the

Results

Several studies have recently been published to

report the results of the Boston KPro. To date, more

than 3000 Boston KPros have been implanted

worldwide. The majority of procedures are now

being carried out by surgeons in units outside the

MEEI in Boston. Numbers have increased signiﬁ-

cantly since 2006, with the demand for the device

continuing to increase.

In 2005, Aquavella reported

the results of using the Boston KPro type 1 in a case

series of 25 patients. The age of patients ranged from

5 to 87 years, with a follow up of between 2 and

12 months. All devices used in this retrospective case

series were retained with no extrusion. No cases of

endophthalmitis or surface infections were reported

during the study. Retroprosthetic membranes were

observed in three patients, all of which were treated

successfully with the YAG laser. The majority of

patients (74%) achieved a VA better than 6/120 with

about half (48%) achieving a visual level of 6/60–6/

7.5. Patients achieved their best acuity in an average

of 60 days (range 1–180 days). Cases of SJS and OCP

were excluded from this series.

Results of the ﬁrst multicentre study on the Boston

type 1 KPro by Zerbe were published in 2006. They

analysed 141 Boston type 1 KPro surgical procedures,

from 17 surgical sites. Preoperative diagnoses

included graft rejection (54%), chemical injury

(15%), bullous keratopathy (14%) and herpes

simplex virus keratitis (7%). Preoperative glaucoma

was reported in 82 eyes (60%). Preoperative best-

corrected visual acuity (BCVA) ranged from 6/30 to

light perception. At an average follow up of

8.5 months, postoperative vision improved to 6/60 in

57%. Among eyes followed-up for at least 1 year after

Figure 3.

Boston keratoprosthesis type 1

in situ.

antimicrobial agents locally (further details on the

recommended regimen and combination can be

found in Appendix I). Five per cent povidone–iodine

drops are instilled once per month at follow-up visits

to patients deemed to be at high risk of infection, for

example those with autoimmune disease or patients

living in developing countries with humid climates.

Following discharge, follow-up appointments are

scheduled at 1, 2 and 4 weeks for the ﬁrst month

followed by monthly follow-up appointments.

The soft contact lens should be replaced every

3–4 months, and it is recommended that the removed

lens be cultured in the above high-risk patients. At

each follow-up visit, the visual acuity (VA) is

assessed, followed by slit-lamp examination and

intraocular pressure measurement with careful docu-

mentation of ﬁndings.

18,23,25,26

Review – Keratoprostheses

Figure 4.

Schematic of the

osteo-odonto

keratoprosthesis

in situ.

215

the operation (62 eyes), vision was 6/60 in 56% and

6/12 in 23%. At an average follow up of 8.5 months,

graft retention was 95%. Excellent outcomes were

achieved in chemical burns and non-cicatrizing graft

failure (94% and 90%, respectively, of eyes achieving

at least 6/60 vision maintained it at ﬁnal follow up

[average 8.5 months]). Failure of VA to improve fol-

lowing Boston type 1 KPro was usually secondary to

coexisting eye pathology such as advanced glaucoma,

macular degeneration or retinal detachment. Retro-

prosthetic membrane formation was the most com-

mon postoperative complication observed (n

35),

with YAG laser membranectomy being the most

common postoperative surgical procedure performed

26), although four required surgical excision.

The second most common complication was raised

intraocular pressure (n

21), with the second most

common postoperative surgical intervention being

placement of a tube shunt (n

11). Other less fre-

quent complications included vitritis, vitreous haem-

orrhage, retinal detachment, choroidal detachment/

haemorrhage and posterior capsular opaciﬁcation.

In the reported studies, successful outcome appeared

to depend upon careful case selection, with exclusion

of cases with SJS or OCP. It has been known since

Yaghouti’s review in 2001 that cases with a preopera-

tive diagnosis of SJS had the worst prognosis follow-

ing Boston KPro type 1 surgery, with no eyes having

a VA better than 6/60 at 5 years.

Very recently, Aldave

et al.

reported results of

using the Boston type 1 KPro in a case series of 50

eyes. They had a very good retention rate of 84%,

rising to 100% in eyes with no previous history of

keratoplasty. Retroprosthetic membrane formation

and persistent epithelial defects were the most

common postoperative complications seen in this

case series (22 and 19 eyes, respectively).

The osteo-odonto-keratoprosthesis

(OOKP)

The OOKP was ﬁrst described by Strampelli in

1963.

It uses the patient’s own tooth root and sur-

rounding alveolar bone to support a centrally

cemented optical cylinder. This lamina is implanted

onto an eye that has undergone corneal trephination,

total iridodialysis, cryo-extraction of the lens and

anterior vitrectomy, under cover of a full-thickness

buccal mucous membrane graft (Fig. 4). Other bio-

logical materials used for supporting a synthetic

optical cylinder have been cartilage (Casey)

and

tibial bone (Temprano).

The latter continues to be

used when no tooth from the patient or suitable

allograft is available. The main theory behind all

these devices was to have a biological skirt which

could easily be integrated into the surrounding

tissues (sclera and mucous membrane) and derive its

own blood supply with subsequent longer survival,

and hence a lower extrusion rate. The main strength

of the OOKP lies in the fact that it can withstand a

very hostile ocular surface environment in patients

with corneal blindness and a severely dry eye. The

innovative, stepwise approach to improving surgery

by Falcinelli resulted in signiﬁcant modiﬁcations to

Strampelli’s original technique with a resultant

higher retention rate and better long-term visual

outcome.

Recent modiﬁcation of the optical cylin-

der design by Hull

et al.

has resulted in a wider

visual ﬁeld postoperatively with a better functional

outcome.

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