Corneal Transplantation: Review

Dr. Priyal Taribagil

Academic Foundation Doctor Year 2
Royal Free Foundation Trust

Introduction

Corneal disease and pathology is the 5th leading cause of blindness worldwide (1). As the cornea possesses inert immune-privilege, corneal transplantation is a highly successful way of improving clinical outcomes. The history of corneal transplantation goes back decades with the first successful human allograft and penetrating keratoplasty (PK) being performed in 1905. Since then there has been a tremendous amount of development with introduction of newer surgical techniques and instrument devices.

Anatomy

The corneal layers are divided into the epithelium, Bowman’s layer, stroma, Descemet’s membrane and endothelium when described from anterior to posterior. Each layer has a distinctive role in the function of the eye and can easily be damaged in a number of pathological conditions (1).

Epithelium

  • Forms the overlying barrier to the eye
  • Provides an optical surface
  • Immunological support

Bowman’s layer

  • Composed of type I & V collagen
  • Contributing to the overall structure and shape of the cornea

Stroma

  • Arranged in regular structure with collagen fibres, glycosaminoglycans and keratocytes
  • Provides mechanical strength
  • Contributes to refractive power

Descemet’s Membrane

  • Composed of type IV collagen & laminin that is produced by the endothelial cells

Endothelium

  • Simple cuboidal monolayer of cells
  • Important in maintaining corneal transparency

Types of transplantation

There are two main subgroups of corneal transplantation: penetrating keratoplasty (PK) and lamellar keratoplasty. The latter includes numerous techniques to selectively remove the diseased layer, preserving the other corneal layers. This can be further categorised into anterior lamellar keratoplasty (deep or superficial) and posterior lamellar keratoplasty or endothelial keratoplasty (EK).

Penetrating keratoplasty (PK): This is a type of full thickness corneal transplantation. During the procedure, a trephine of the correct diameter is used to make a full thickness resection of the patient’s damaged cornea. A corresponding resection is made from the donor tissue. The new graft is sutured in place in a radial fashion using interrupted or running sutures with similar tension, in order to minimise the degree of induced astigmatism. The main disadvantages of this technique are the prolonged postoperative recovery and time taken to achieve the best-corrected visual acuity (2).

Superficial/Deep Anterior Lamellar Keratoplasty (SALK/DALK): This is a partial thickness corneal transplant procedure whereby the anterior layers of the cornea are removed and replaced. The endothelium and Descemet’s membrane are not affected. The wound has greater strength in comparison to PK and there is a reduced risk of endothelial rejection as the host endothelium remains in situ (3).

Posterior lamellar keratoplasty/Endothelial keratoplasty (EK): This is a partial thickness corneal transplantation. There are further sub-classifications within this including Descemet Stripping Automated Endothelial Keratoplasty (DSAEK) and Descemet Membrane Endothelial Keratoplasty (DMEK). The DSAEK procedure involves transplanting the posterior stromal end (100-150μm depth), Descemet’s membrane and the endothelium from the donor. An air bubble is often placed in the anterior chamber to help support adherence of the graft (4). The DMEK procedure involves transplantation of the Descemet and endothelium only (without additional stromal tissue). Similar to the other procedure, a gas bubble can be inserted to support graft adherence (5,6).

Indications for transplantation

Indications for penetrating keratoplasty include: keratitis, keratoconus and ectasias, corneal degeneration and dystrophies, corneal trauma, and congenital abnormalities.

Indications for endothelial keratoplasty include Fuch’s endothelial dystrophy, bullous keratopathy, and iridocorneal endothelial syndrome.

As corneal blindness is a major health problem worldwide, many studies have attempted to identify the most common cause for transplantation. Bullous keratopathy has been noted to be the most common indication in developed countries, with infective keratitis having the greatest prevalence in developing countries (7).

Complications

Graft rejection is a key complication post corneal transplantation. Although average rates of survival of the allograft are around 90%, this significantly reduces over time, with studies demonstrating graft survival of 74% (5 years) and 62% on average (10 years) (8). For some high risk patients, graft survival can be as low as 35% due to chronic unresolved ocular inflammation.

Other late complications include graft dislocation, corneal swelling, astigmatism and cataract formation. Short term complications include infection, bleeding and raised intraocular pressure post procedure (1).

For endothelial keratoplasty, there can be displacement of donor tissue (“refloating”) which requires repositioning. This is more commonly seen in DMEK. If there are folds within the tissues, this can compromise the overall quality of vision.

Future – Bioprosthesis

Keratoprosthesis implantation involves full thickness removal of cornea and replacement with an entirely artificial cornea. This technology is aimed at providing treatment for individuals who are unsuitable for corneal transplantation. An example of this is the ‘Boston KPro’ (more commonly used in USA). Multiple studies have demonstrated significantly improved visual acuity and clinical outcomes when compared to individuals treated with penetrating keratoplasty (9). Overall prognosis was better in eyes with chemical injuries (10).

Indications include history of multiple failed grafts, severe keratitis, Steven Johnson syndrome, cicatricial pemphigoid and aniridia. There are unfortunately a number of long term risks associated with this including ongoing risk of infection or extrusion of the device directly (11). There is also an increased prevalence of glaucoma post operatively (12). Larger randomised controlled studies are required to fully assess the long term efficacy and safety of the prosthesis.

Conclusion

There have been significant developments in the field of corneal transplantation over the last few decades, with an exponential increase in the number of endothelial keratoplasty procedures over the traditional PK technique (7). The shift towards these partial thickness grafts has helped to reduce overall recovery times and minimise risk of graft rejection. Other avenues such as keratoprosthesis insertion warrant more large-scaled clinical studies as they may provide another treatment option for individuals who are not suitable for traditional transplantation.

References

1. Maghsoudlou P, Sood G, Akhondi H. Cornea Transplantation [Internet]. StatPearls. StatPearls Publishing; 2022 [cited 2022 Jan 28]. Available from: http://www.ncbi.nlm.nih.gov/pubmed/30969512

2. Penetrating Keratoplasty (PK) [Internet]. [cited 2022 Jan 29]. Available from: https://webeye.ophth.uiowa.edu/eyeforum/tutorials/cornea-transplant-intro/2-PK.htm

3. Deep Anterior Lamellar Keratoplasty (DALK) [Internet]. [cited 2022 Jan 29]. Available from: https://webeye.ophth.uiowa.edu/eyeforum/tutorials/cornea-transplant-intro/3-DALK.htm

4. Descemet Stripping Automated Endothelial Keratoplasty (DSAEK) [Internet]. [cited 2022 Jan 29]. Available from: https://webeye.ophth.uiowa.edu/eyeforum/tutorials/cornea-transplant-intro/4-DSAEK.htm

5. Espandar L, Carlson AN. Lamellar Keratoplasty: A Literature Review. J Ophthalmol [Internet]. 2013 [cited 2022 Jan 28];2013:1–8. Available from: http://www.hindawi.com/journals/joph/2013/894319/

6. Descemet Membrane Endothelial Keratoplasty (DMEK) [Internet]. [cited 2022 Jan 29]. Available from: https://webeye.ophth.uiowa.edu/eyeforum/tutorials/cornea-transplant-intro/5-DMEK.htm

7. Singh R, Gupta N, Vanathi M, Tandon R. Corneal transplantation in the modern era. Indian J Med Res [Internet]. 2019 [cited 2022 Jan 28];150(1):7–22. Available from: http://www.ncbi.nlm.nih.gov/pubmed/31571625

8. Waldock A, Cook SD. Corneal transplantation: how successful are we? Br J Ophthalmol [Internet]. 2000 Aug 1 [cited 2022 Jan 28];84(8):813–5. Available from: http://www.ncbi.nlm.nih.gov/pubmed/10906082

9. Akpek EK, Cassard SD, Dunlap K, Hahn S, Ramulu PY. Donor Corneal Transplantation vs Boston Type 1 Keratoprosthesis in Patients with Previous Graft Failures: A Retrospective Single Center Study (An American Ophthalmological Society Thesis). Trans Am Ophthalmol Soc [Internet]. 2015 [cited 2022 Jan 28];113:T3. Available from: http://www.ncbi.nlm.nih.gov/pubmed/26538773

10. Rudnisky CJ, Belin MW, Guo R, Ciolino JB, Boston Type 1 Keratoprosthesis Study Group. Visual Acuity Outcomes of the Boston Keratoprosthesis Type 1: Multicenter Study Results. Am J Ophthalmol [Internet]. 2016 Feb [cited 2022 Jan 28];162:89-98.e1. Available from: http://www.ncbi.nlm.nih.gov/pubmed/26550696

11. Jassim SH, Sivaraman KR, Jimenez JC, Jaboori AHJ, Federle MJ, de la Cruz J, et al. Bacteria Colonizing the Ocular Surface in Eyes With Boston Type 1 Keratoprosthesis: Analysis of Biofilm-Forming Capability and Vancomycin Tolerance. Investig Opthalmology Vis Sci [Internet]. 2015 Jul 23 [cited 2022 Jan 28];56(8):4689. Available from: http://iovs.arvojournals.org/article.aspx?doi=10.1167/iovs.15-17101

12. Keratoprosthesis [Internet]. [cited 2022 Jan 28]. Available from: https://webeye.ophth.uiowa.edu/eyeforum/tutorials/cornea-transplant-intro/6-kprosth.htm

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