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Corneal Transplant Techniques

Because of the relatively high cost, long recovery period and associated risk factors, a cornea transplant is usually considered a last resort to correct corneal pathology. However, when all else fails and it is indicated, a cornea transplant can very successfully restore good vision. Corneal transplants are known to be the most effective of all kinds of transplant surgery.

The various techniques available today include:

Lamellar Corneal Transplants

Lamellar Corneal Transplants

Lamellar corneal transplants are the latest additions to the field of corneal transplant surgery. With these techniques, only the affected layer of the corneaThe cornea is the clear, transparent 'front window' of the eye through which light enters the eye. It handles about two-thirds of the focusing power of the eye and is critical for good vision. See Info on Eyes – Anatomy. is removed and replaced with donor tissue. The recipient's own tissue is preserved as far as possible to minimize complications. Several transplant options are available, depending on the initial pathology.

A Superficial Anterior Lamellar Keratoplasty (SALKSALK is the acronym for Superficial Anterior Lamellar Keratoplasty. It is one of the lamellar techniques used for cornea transplants. See Cornea Centre – Techniques.) implies that only the superficial layers of the corneal stroma (100 to 200 microns or 0.1 to 0.2 mm) are removed and replaced by a similar donor section.

A Deep Anterior Lamellar Keratoplasty (DALKDALK is the acronym for Deep Anterior Lamellar Keratoplasty. It is one of the lamellar techniques used for cornea transplants. See Cornea Centre – Techniques.) is very similar to the SALK procedure, except that most of the corneal stroma is replaced. A number of variations to this technique are available, namely:

  • A 'Big bubble' DALK, which implies that Descemet's membrane is separated from the stroma, followed by excision of the entire central stroma to be replaced by a similar donor section.
  • A 'Mushroom' Lamellar Keratoplasty, the only among the lamellar procedures that comprise a full penetrating procedure.

An Endothelial Keratoplasty procedure is done for pure endothelial corneal disease. Here, two techniques are available, namely:

  • A Descemet Stripping Automated Endothelial Keratoplasty (DSAEKDSAEK is the acronym for Descemet's Stripping Automated Endothelial Keratoplasty. It is one of the modern lamellar techniques used to replace the corneal endothelium. See Cornea Centre – Techniques.) which replaces the corneal endothelium, Descemet's membrane and a thin layer of the deep internal stroma, which acts as a 'splint' to the delicate Descemet's membrane and endothelium.
  • A Descemet Membrane Endothelial Keratoplasty (DMEKDMEK is the acronym for Descemet Membrane Endothelial Keratoplasty. It is the most recent and elegant technique to replace the corneal endothelium. See Cornea Centre – Techniques.), an intricate and very delicate procedure which replaces only corneal endothelium and Descemet's membrane.

These procedures are significantly less invasive, in contrast to a penetrating transplant. Visual recovery is usually within a much shorter time span as opposed to a penetrating keratoplasty.

We are one of the few facilities in South Africa to offer the entire spectrum of these modern transplant techniques to our patients.

Superficial Anterior Lamellar Keratoplasty (SALK)

This technique implies that only the very superficial layers of the corneal stroma (100 to 200 microns) are removed and replaced by a similar donor section.

Advantages of this procedure:

  • As the surgery remains superficial, the host cornea's original profile is largely preserved along with the deep stroma and endothelial cell layer.
  • A smaller bulk of tissue is transplanted, which reduces the antigenic load and hence the chance for rejection.
  • Sutures are seldom required, since the donor lamella of cornea adheres to the recipient in the same way a LasikLasik is one of several types of laser treatments to permanently correct refractive errors of the eye. See Laser Vision – Introduction to Laser Treatment. flap remains intact.  This is due to the unique characteristics of the corneal physiology.

This procedure is indicated if only the superficial cornea is affected by pathology, thus negating the need for a penetrating corneal transplant.

Deep Anterior Lamellar Keratoplasty (DALK)

This technique is very similar to the SALK procedure, except that the majority of the corneal stroma is replaced. A number of variations of this technique are available, namely:

  • Manual DALK (manual dissection of the stroma).
  • Automated DALK (section of the stroma with the use of a microkeratomeA microkeratome is a high-precision motorised scalpel used to make the flap for a corrective lasik treatment. and artificial chamber).
  • Femtosecond laserA femtosecond laser is the latest high-tech laser device used to make extremely accurate corneal incisions. See About Us – Our Technology – femtosecond laser.-assisted DALK (excision of the stroma with the aid of a femtosecond laser).
  • 'Big Bubble' DALK (dissection of the entire stroma by means of the 'big bubble' technique, leaving only Descemet's membrane and endothelium intact).

At Optimed we have the ability to provide the entire spectrum of these modern techniques to our patients.

These procedures are indicated depending on the extent of stromal involvement. For example, if the whole corneal stroma is affected with only Descemet's membrane and the corneal endothelium remaining normal, a 'Big Bubble' DALK is probably the preferred technique.

Mushroom Lamellar Keratoplasty

This technique is the only lamellar procedure that comprises a full penetrating procedure.

It may be viewed as a combination of an automated DALK and DSAEK, where both the recipient and donor are dissected in two parts - the outer lamella having a large diameter (typically 9 mm), with the inner endothelial lamella much smaller (typically 5 mm), giving it the appearance of a 'mushroom' on cross section, hence its name.

Advantages of mushroom PKMushroom PK is an abbreviation for 'Mushroom Lamellar Keratoplasty'. It is one of the lamellar techniques used for cornea transplants. See Cornea Centre – Techniques. over conventional PKPPKP is the acronym for Penetrating Keratoplasty. It is one of the techniques used for cornea transplants. See Cornea Centre – Techniques.

  • Less surgically induced astigmatismAstigmatism is a condition caused by a cornea that has more than one curvature, similar to a rugby ball. The same image, therefore, focuses on multiple points in front of, on and/or behind the cornea, creating multiple and ghost images. Astigmatism may also be found in association with either myopia or hyperopia, which complicates the condition substantially. See Info on Eyes, Optics and Refractive Errors - Astigmatism..
  • Smaller bulk of donor endothelium (only approximately 25% of the endothelium is replaced).
  • Stronger wound configuration.

Endothelial Keratoplasty

  • Descemet's Stripping Automated Endothelial Keratoplasty (DSAEK)
  • Descemet Membrane Endothelial Keratoplasty (DMEK)

It is now possible to replace the corneal endothelium along with Descemet's membrane and a thin layer of the deep internal stroma (DSAEK). Dr Gerrit Melles from the Netherlands Institute of Innovative Ocular Surgery in Rotterdam recently developed a refinement of this surgery with the advent of DMEK. Here, only Descemet's membrane and the endothelium layers are replaced.

These procedures are significantly less invasive when compared to a penetrating transplant and the recovery is usually within a short time span. Their proper execution, however, demand a significant amount of surgical skill. These methods are indicated in patients with corneal endothelial problems such as Fuch's endothelial dystrophy, post-surgical corneal oedema and corneal regrafts following primary endothelial failure or rejection.

Dr Potgieter introduced the DSAEK technique into South Africa in 2008.

Mushroom lamellar keratoplasty

MUSHROOM LAMELLAR KERATOPLASTY

Mushroom lamellar keratoplasty is the term used for the transplant of donor corneal tissue that combines a larger diameter section towards the surface of the cornea with a smaller diameter section deeper down. The term 'mushroom' comes from the shape of the graft, which resembles a flat mushroom.

The sections from the diseased and donor corneas have to be identical and, to ensure that they match precisely, both sections are cut from the corneas with the aid of either precision purpose-made instruments, or a femtosecond laser.

This transplant technique is used in cases of:

  • Keratoconus
  • Steep corneas with a thin periphery in the recipient cornea
  • Full-thickness opacities (but with a healthy endothelium)
  • Stromal dystrophies, and
  • Deep central scars

This procedure offers several inherent benefits:

Astigmatism and distortion: Transplantation of a larger superficial diameter graft results in a smoother corneal curvature and thus decreased astigmatism.

Refractive: Although penetrating keratoplasty in keratoconus generally has good results, it can be complicated by high or irregular astigmatism, resulting in a delayed or poor visual recovery. The larger superficial diameter graft results in a higher degree of topographic regularity and a higher quality of vision with better refractive results afterwards.

Endothelium: The small diameter of the deeper section of the graft may prevent the need for additional surgery, even if the host immune system rejects the donor endothelial cells. This is true because only about 25% of the recipient endothelium is replaced by donor endothelium. The result is that in many eyes that would develop endothelial rejection, the corneal clarity is restored over time due to the host endothelium migrating over the area of rejected cells, thereby repopulating the area. This is only possible because the majority of the recipient endothelium is preserved, even though a penetrating transplant is done.

Healing: Another advantage is fast healing after the procedure. Fast healing and optical superiority of large-diameter penetrating keratoplasty is therefore combined with the reduced distortion and minimal endothelial transplantation. Mushroom keratoplasty also ensures quicker recovery than a standard penetrating graft. Finally, because of the step-wise wound configuration of the 'mushroom' graft, a larger surface area is available for stronger wound adhesion and thus a more stable result compared to a conventional transplant procedure.

Penetrating Keratoplasty

Penetrating Keratoplasty

Fig 1: Actual conventional penetrating corneal transplant

Fig 1: Actual conventional penetrating corneal transplant

This technique is the most established of all and has been available for many years.  This implies the removal of the entire full-thickness central corneaThe cornea is the clear, transparent 'front window' of the eye through which light enters the eye. It handles about two-thirds of the focusing power of the eye and is critical for good vision. See Info on Eyes – Anatomy., with the replacement thereof by donor tissue.  This procedure is indicated in advanced cases of keratoconusKeratoconus is an eye condition that causes the cornea to spontaneously bulge outward, like the tip of a rugby ball. It causes very high levels of myopia and astigmatism. See Eye Diseases – Keratoconus., or when scarring or other corneal disease processes involve the entire thickness of the cornea.

With the introduction of femtosecond laser technology, a number of variants of the classic penetrating procedure became available.  All these femtosecond laser assisted transplant procedures aim at producing a stronger wound adhesion while minimizing the post-operative irregularity of the profile.  Due to the versatility of the femtosecond laser, new incision patterns are available, which were not possible with conventional surgery.  These include, but are not limited to the following incision configurations, namely:

  • 'Top Hat'.
  • 'Mushroom'.
  • 'Zig-Zag'.