Advances in Corneal Transplantation Surgery

Dr Leonard Ang Pek Kiang, MBBS, Doct Medicine, FRCS(Ed), MRCOphth(Lond), M.Med(Ophth), Assistant Professor and Consultant, Corneal and Refractive Surgery Service, Singapore National Eye Centre

Recent advances in corneal transplantation surgery have improved our ability to treat many corneal diseases more effectively. Penetrating keratoplasty (full thickness corneal transplantation) has had good success over the last few decades but graft rejection remains a persistent problem in some cases. In lamellar keratoplasty a partial-thickness donor corneal graft is placed over the recipient corneal bed and advances in this surgical technique have led to better visual outcomes and longer graft survival following corneal transplantation. Hence there has been renewed interest in lamellar keratoplasty in recent years. Lamellar keratoplasty can be divided into anterior and posterior lamellar keratoplasty.

LAMELLAR KERATOPLASTY 

Anterior lamellar keratoplasty (ALKP)

Anterior lamellar keratoplasty (ALKP) is performed for patients with diseases involving the anterior cornea such as 1) anterior scars, 2) anterior dystrophies, 3) keratoconus and 4) infectious keratitis. The advantages of ALKP are that it is essentially an extraocular  procedure, which results in a stronger wound, produces less astigmatism, and has a significantly lower risk of allograft rejection and graft failure as compared to conventional penetrating keratoplasty.

Traditionally, lamellar keratoplasty requires the manual preparation of the recipient corneal bed and donor button. This is technically more difficult to perform compared to penetrating keratoplasty, and despite meticulous surgery, often result in suboptimal visual outcomes because of interface problems. Various improvements in surgical techniques have resulted in the ability to achieve deeper stromal dissection planes (deep lamellar keratoplasty). These modalities include the use of intrastromal air, fluid (hydrodelamination) or viscoelastic material injections to help create the lamellar dissection planes. With deeper dissections, we are now able to remove the entire corneal stroma leaving only Descemet’s membrane behind, thereby achieving visual results that are equal to or superior to that achieved with conventional penetrating keratoplasty.  
  
Semi-automated microkeratomes, originally designed for refractive surgical procedures,  have also been recently introduced to further overcome some of the problems related to manual dissection (automated lamellar therapeutic keratoplasty - ALTK) (Fig. 1). The advantages of ALTK are that it offers smoother lamellar dissection, as well as more precise control of the depth and thickness of both recipient and donor corneas, which in turn leads to better visual outcomes  (Fig. 2) 

FEMTOSECOND LASER-ASSISTED KERATOPLASTY 

Femtosecond laser-assisted lamellar keratoplasty

The femtosecond laser is a near-infrared laser that has been used successfully in creating precise lamellar flaps in LASIK surgery. The femtosecond laser can now also be used for lamellar keratoplasty and penetrating keratoplasty. The laser can be focused to precise intrastromal locations with submicron accuracy; combined with the tight spot placement, this allows us to create smoother stromal beds in lamellar dissections. 

Femtosecond laser-assisted penetrating keratoplasty

Conventional corneal trephination results in the classic vertically aligned wound edge, which requires relatively tight sutures to ensure a tight seal. The femtosecond laser can create a wide variety of corneal “cuts”, which allows surgeons to specifically create side cuts of various configurations in the donor and recipient corneas, so as to enhance wound stability and reduce corneal irregularity and astigmatism. This can potentially reduce the number of sutures needed, require less suture tension, allow earlier suture removal and therefore reduce astigmatism. Preliminary results using the femtosecond laser to assist in the  creation and modification of various shaped wound edges look promising and more work is currently underway.

DOHLMAN-DOANE KERATOPROSTHESIS

Despite these advances in corneal transplantation surgery, there are still many severe cornea and ocular surface diseases (e.g. Stevens-Johnson syndrome, chemical burns or ocular cicatricial pemphigoid) where conventional keratoplasty is associated with a high risk of graft failure. The Dohlman-Doane keratoprosthesis has been effectively used to restore vision in patients with severe blinding corneal diseases and in high risk, repeated failed corneal grafts, where further keratoplasty would be futile (Fig. 3). The Dohlman-Doane keratoprosthesis consists of a collar button shaped device consisting of a PMMA optic that clamps within it a donor ring of corneal tissue. Recent modifications to its design and postoperative management have led to significantly better clinical outcomes, with better graft retention and reduced complications, such as infection and melting. In severe corneal blinding diseases where further keratoplasty is associated with dismal results,  keratoprosthesis implantation may prove to be the best option for restoring long-term vision in these patients.
                                                                                                 
In summary, recent advances in lamellar and penetrating keratoplasty surgery have improved our ability to treat many corneal diseases more effectively. The use of refractive surgical devices, such as the precision microkeratomes and femtosecond lasers has significantly expanded the armamentarium of corneal surgeons for performing corneal transplantation. For repeated, failed grafts or severe corneal and ocular surface diseases where further corneal transplantation surgery appears futile, the Dohlman-Doane keratoprosthesis has demonstrated promising results in restoring long-term vision in these patients.