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CORNEAL TRANSPLANTATION

Whenever vision is reduced from corneal disorders like infections, corneal injuries and degenerative diseases, a corneal transplant can be an effective means of restoring vision. Our corneal transplant program was initiated in 1991 in SNEC and has been highly successful with 3,300 transplants being performed till date. SNEC performs 250-300 corneal transplants a year with an overall graft survival rate exceeding 90%, and which is close to 100% with new procedures like Endothelial Keratoplasty (Figure 1). The Singapore Eye Bank (SEB) provides high quality donor corneas each year for these transplants and SNEC works in close collaboration with SEB to ensure high quality and standards in donor corneal harvesting and retrieval. 

What is a Corneal Transplant?

When the cornea becomes cloudy, light rays are unable to pass through to reach the light-sensitive portion of the eye called the retina, leading to poor vision or reversible blindness. A corneal transplant is a surgical procedure that involves replacing cloudy/scarred cornea with a donor cornea in order to restore vision.

          
Corneal Scar                

Who will need a Corneal Transplant?

Most individuals with poor vision due to a diseased or cloudy cornea, whose nerve and retina at the back of the eye is still healthy, may benefit from a corneal transplant to see well. A cloudy or opaque cornea can occur from scarring of any cause (e.g., injuries or infections) or cloudy swelling of the cornea due to damage or ageing of the innermost layer of cells of the cornea (known as the corneal endothelial layer). This layer is not able to "grow back" or regenerate and can only be replaced by a donor cornea through transplantation.

Less commonly, corneal transplants can be done on an urgent basis to treat severe infections of the cornea, or to repair or "patch up" severe thinning, defects or perforations in the cornea or sclera (the sclera is the white coating of the eye). The latter may occur due to previous injury, inflammation or infections.

Unlike other forms of organ transplantation, corneal transplantation may be performed repeatedly several times if previous transplants have failed. However, the success rate of repeat transplants may be lower than a first time graft and often anti-rejection tablets may be useful in preventing problems of rejection in these cases. 

How is a Corneal Transplant performed?

There are several types of corneal transplant procedures that are performed in SNEC. The standard corneal transplantation technique is a Penetrating Keratoplasty (PK) or a full-thickness corneal grafting procedure. PK is a form of microsurgery in which the central 7 to 8mm portion of the damaged or cloudy cornea is removed and a clear and healthy donor cornea (Figure 2a) is sutured in its place with very fine microsurgical nylon sutures. 

                 Figure 2a : Forms of corneal transplantation – Penetrating Keratoplasty

Apart from PK, SNEC also performs newer and more advanced forms of corneal transplant called Lamellar Keratoplasty (LK).  In LK, only the diseased portions of the cornea are removed and replaced, preserving the healthy corneal tissues.

When only the anterior (front) layers of the cornea are replaced, the procedure is called Anterior Lamellar Keratoplasty (ALK).  DALK or Deep Anterior Lamellar Keratoplasty is a form of ALK in which most of the anterior layers of the cornea are removed sparing the posterior corneal layer (Descemet membrane).  Another form of ALK is Automated Lamellar Therapeutic Keratoplasty (ALTK), where we use a special motorized device (a microkeratome) to help perform the procedure. These are more surgically and technically challenging operations that preserve the deepest or posterior layer of the cornea (known as Descemets layer and the Endothelial Layer) (Figure 1b). About 40% of corneal transplants at SNEC are performed with ALK or DALK procedures. SNEC today is one of few transplant centres in the world that offers these new advanced LK techniques to corneal transplant patients.

            Figure 2b : Forms of corneal transplantation – Anterior Lamellar Keratoplasty 

When only the diseased posterior or back layers of the cornea are replaced, the procedure is called Endothelial Keratoplasty (EK).  EK today includes DSAEK – Descemets Stripping Automated Endothelial Keratoplasty) (Figure 2c) and DMEK - Descemet Membrane Endothelial Keratoplasty.

SNEC is the regional referral centre in Asia for DSAEK surgery and more than 40% of corneal transplants at SNEC are DSAEKs (Figure 2). DSAEK is probably the most significant advance in corneal transplantation today, because it is essentially a new form of sutureless, keyhole corneal transplant technique. In DSAEK only the inner layer of the donor cornea, about a 10th of a millimeter thick is transplanted onto the patient's own cornea  through a small (4-5mm) incision at the side of the cornea, which means that most of the patient’s cornea is no longer removed, no sutures on the cornea are now needed, and the eye is much stronger as compared to PK surgery. No sutures also means much faster visual recovery and EK patients also have better vision because they have much less astigmatism and other refractive errors. 

SNEC and SERI’s team of scientists have been working on continually improving the technique of DSAEK and were the first to pioneer a surgical device, the Tan EndoGlide, in 2009, to perform DSAEK. Licensed to a UK surgical company, the EndoGlide, is the first FDA approved device for DSAEK. This device enables donor insertion into the patient’s eye with ease and minimizes donor trauma that can occur while inserting into the eye through a small incision. It has been the most successful surgical device to implant this donor tissue through a keyhole incision. It has the best published safety record compared to other devices, and over 5000 corneal transplants using this device have been performed around the world to date. The Tan Endoglide 1 was first used in 2009 as part of the SNEC DSAEK Endoglide Clinical Trial. As of July 2012, more than 400 eyes have undergone DSAEK surgery using the Tan Endoglide. We have reported our results using this device and have achieved very good visual outcomes and lower rates of endothelial cell loss and reduced risks of complications. (Khor WB et al1). Similar good outcomes have also been reported from another centre that has successfully adapted this device for DSAEK surgery (Hollick E et al2).

DMEK is the latest innovation in the form of minimally-invasive, sutureless, keyhole corneal transplant surgery. Instead of implanting a 10th mm thick donor in DSAEK, this new complex procedure now just replaces the actual damaged corneal endothelial cell layer alone which is just 1/100th mm thick.  In selected patients this has the advantage over DSAEK in that patients now have the possibility of attaining 100% vision within a few weeks of surgery, i.e perfect 6/6 vision. Additionally it may be associated with a lowered risk of rejection as suggested by a recently published article (Anshu et al3).

Currently however, DMEK is suitable for milder forms of corneal edema and may not be suitable for all patients.  Our team at SNEC and SERI are developing a new surgical technique that is likely to be safer, and easier to perform. This method, which includes a new DMEK surgical insertion device is patented by SNEC/SERI. 

References:

1. Khor WB, Mehta JS, Tan DT. Descemet stripping automated endothelial keratoplasty with a graft insertion device: surgical technique and early clinical results. Am J Ophthalmol. 2011 Feb;151(2):223-32.

2. Gangwani V, Obi, A, Hollick EJ. A prospective study comparing Endoglide and Busin glide insertion techniques in descemet stripping endothelial keratoplasty. Am J Ophthalmol. 2012 Jan;153(1):38-43.

3. Anshu A, Price MO, Price FW. Risk of corneal transplant rejection significantly reduced with Descemet membrane endothelial keratoplasty. Ophthalmology 2011. Article in Press.

Figure 2c: Forms of corneal transplantation - Endothelial Keratoplasty  
 


We are also a training centre for complex forms of advanced corneal transplantation techniques including the ALK, DALK and EK procedures.

                  Figure 3 : Shifting trends in corneal transplantation techniques in SNEC


 

 

Where does the cornea come from and how long does it take to get one? Is there a waiting list in Singapore, and can non-Singaporeans get corneas from the Singapore Eye Bank (SEB)?

All corneas transplanted at our centre are of excellent quality and procured by the SEB. SEB gets corneas from local donors in Singapore, as well as from internationally accredited eye banks in the United States of America and other international eye banks around the world.

Because the SEB is very successful in procuring corneal tissue today, one usually has to wait only one or two weeks to receive a donor cornea.

SEB also provides corneas for non-Singaporeans undergoing corneal transplants in Singapore. Singapore is a leading transplantation centre in Asia, with many international patients successfully undergoing transplantation surgery here. 

OUTCOME RESULTS AND SUCCESS RATES OF CORNEAL TRANSPLANTS     
PERFORMED IN SNEC

Corneal Surgery in General

  The success rate for routine uncomplicated corneal transplants is about 91% in
the first year. This high rate of corneal graft survival matches the best results in other  
centres in the West. Success rates for ALK and EK now range closer to 100% as less risk
of rejection occurs in these new procedures.

However, complications can occur following a transplant. The more common complications  
that can occur include raised eye pressure, which can cause damage to the eye nerve
(glaucoma), and corneal graft rejection. Most of the complications occur in the first
year after transplantation, but most can still be treated successfully if detected early.

The table in Figure 3 describes the short- and long-term outcomes of corneal grafts in
several centres from around the world. There are very few countries that have long-term
follow up, and our Singapore Corneal Transplant Study (SCTS), which spans more than 20 years, is one of the largest transplant databases worldwide. Our results in Singapore
show that the results in Asian Eyes in our centre are equivalent to that of 
Caucasian eyes in the West, and ours is the only major database monitoring success
in Asian eyes. 

  Lamellar Keratoplasty (LK)

  In SNEC, nearly 75 cases of Lamellar Keratoplasty are performed yearly.

  Because ALK retains the innermost corneal layer, it greatly reduces the risk of corneal
graft rejection to almost zero, a significant cause of corneal transplant failure.

Compared with the 20% overall risk of rejection after PK, rejection risk following ALK is
less than 1%1.

There is better long-term graft survival following ALK. Our SCTS results show that the
overall 1-year graft survival for ALK in SNEC is 94%.

In terms of recovery of vision, with the newer surgical techniques, we have been able
to achieve equal if not better visual outcomes following ALK as compared with PK
performed for similar indications2.

  Endothelial Keratoplasty (EK or DSAEK)

  In SNEC, more than 100 cases of Endothelial Keratoplasty are performed yearly, and
SNEC leads the field in EK surgery in Asia.

EK offers several advantages over a full-thickness procedure like PK in patients who
have selective damage to the inner layers of the cornea (endothelium):

  1. No suture-related problems: Because EK does not involve any donor suturing onto 
the patients cornea it does not have the risk of suture-related problems like suture-
related abscess. There is also reduced astigmatism, resulting in better vision when
compared with PK.

  2. Lower risk of graft rejection: Early results have shown that the risk of rejection is
much lower with EK. Compared with the 20% overall risk of rejection after PK,
rejection risk following EK is just 7.6% in the first year2. This is probably because far less
donor tissue is transplanted, compared to conventional penetrating keratoplasty, where
the whole cornea is transplanted. 

In SNEC we have performed over 500 EK procedures and have had only 3 eyes with
graft rejection (2%).

  3. Very good graft survival: Graft survival appears to be excellent in our SNEC series,
with very few failures occurring to date. Graft survival appears to be better than
conventional PK surgery. This may be related to the lower risk of graft rejection.

  4. Faster visual recovery: The visual recovery is much faster following an EK
procedure with reduced astigmatism compared with PK.

  REFERENCES

  1 Tan et al. Penetrating keratoplasty in Asian eyes - The Singapore Corneal Transplant
Study. Ophthalmol 2008;115:975-82.

  2 Tan and Anshu et al. Visual acuity outcomes after deep anterior lamellar keratoplasty
- A case control study. British J of Ophthalmol 2010. In press.

  3 Tan et al. Comparison of donor insertion techniques for descemet stripping automated
endothelial keratoplasty. Arch Ophthalmol 2008:126(10):1383-8.

  4 Khor WB, Mehta JS, Tan DT. Descemet stripping automated endothelial keratoplasty with a graft insertion device: surgical technique and early clinical results.  Am J Ophthalmol. 2011 Feb;151(2):223-32. 

Figure 3: One-year corneal graft survival outcomes in several centres from around the world

Name of corneal graft registry / study

1-year graft survival for PK

Singapore Corneal Transplant Study (Singapore)

91%

Australian Corneal Graft Registry (Australia)

90%

Corneal Graft Outcome Study (Canada)

No available data at 1 year

Indian Corneal Transplant Study (India)

79% 

The Asia Cornea Society

SNEC hosts the secretariat of the Asia Cornea Society (ACS), a professional society of corneal surgeons from many Asian countries including Singapore, Japan, India, China, Taiwan, Philippines, Korea and Thailand. The Asia Cornea Society (ACS) was recently formed by the current President, Professor Donald Tan, to promote the corneal development in Asia, with the objective to develop new initiatives in alleviating corneal blindness. ACS currently has over 400 members and formal affiliations with national corneal societies in Thailand, Philippines, Korea and Israel. To learn more about the Asia Cornea Society, please visit www.asiacorneasociety.org.

Singapore Eye Bank

In 1993, the Singapore Eye Bank (SEB) was set up within the premises of
Singapore National Eye Centre with money raised by the Rotary Club of
Singapore West and the Inner Wheel Club of Singapore West with
matching funds from the Ministry of Health.

The mission of the Singapore Eye Bank is to assist in the restoration of sight to those blind from corneal disease through the procurement of high quality corneas for corneal transplantation. Corneas are sourced through local eye donation programmes as well as through acquisition of overseas tissue from certified international eye banks. SEB provides families of potential donors with a mechanism to donate their loved one’s eyes and is committed to promote public awareness of corneal donations among all Singaporeans.

The eye bank is staffed and fully equipped to carry out procurement, evaluation, storage and distribution of ocular tissue for transplantation. It also adheres to the highest international standards set by the Eye Bank Association of America (EBAA) and undergoes regular quality assurance reviews.

In its first few years, SEB functioned mainly to import and distribute corneas from overseas eye banks. In November 1995, Professor Donald Tan took over the medical directorship of SEB and instituted a Hospital Eye Donation Programme (HEDP) in SGH to boost local donor rates. This was a collaborative effort to counsel bereaved relatives of potential hospital donors about the benefits of eye donation and to obtain consent. SEB counselors were sited in SGH to identify and counsel potential donor families within SGH wards and by the end of 1996, there was a 240% increase in local donor rates. The highly successful programme was adopted in Tan Tock Seng Hospital in 2002, further increasing the number of corneas donated locally. In 2010, a further HEDP programme in Changi General Hospital is being established.

To date, SEB remains highly successful in procuring corneas for corneal transplantation in Singapore. In 2008, a total of 347 corneas were procured for corneal transplantation throughout Singapore and our corneal grafts continue to have high success rates exceeding 91% for one year graft survival. Our local donor rate for local transplants currently stands at 43%. In addition, SEB continues to source foreign corneas for some local patients who urgently need corneas for emergency transplants when no local tissue is available, and for our foreign patients who fly in for corneal transplantation which makes up 36% of all corneal transplants performed locally.

The SEB is also the epicenter of the Association of Eye Banks of Asia (AEBA), which is headquartered in SNEC. AEBA is a professional organization of eye banks in Asia dedicated to promoting eye donation and improving standards of eye banking in Asia.

Please visit the following relevant links:

Outcome Results on Corneal & External Eye Disease Procedures
Corneal & External Eye Disease Clinical Staff
I Want an Eye Examination / an Appointment
Care After Corneal Transplant
SNEC Smartphone Apps for Patients
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Read more on related Eye Conditions and Treatments:

Corneal Infections
Corneal Oedema
Dry Eye
Eye Infections
Pterygium

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