26.2 Corneal Transplantation: Penetrating Keratoplasty (PKP) & Lamellar Keratoplasty (DSAEK, DMEK, DALK)

Corneal Transplantation: Penetrating Keratoplasty (PKP) & Lamellar Keratoplasty (DSAEK, DMEK, DALK)

Core Clinical Mandate: Modern corneal surgery has transitioned from full-thickness penetrating keratoplasty (PKP) to targeted lamellar transplantation, replacing only the pathologically diseased anatomical layer while preserving healthy host tissue. Ophthalmic medical technologists must thoroughly understand donor trephination mechanics, suturing geometries, immunological graft rejection lines, and the microsurgical techniques of DALK, DSAEK, and DMEK.

Penetrating Keratoplasty (PKP): Full-Thickness Transplantation

Penetrating Keratoplasty (PKP) involves the full-thickness, 5-layer excision of a central button of diseased host cornea and its replacement with a full-thickness, clear donor corneal graft. It is indicated when pathology involves both anterior/stromal and posterior/endothelial layers (e.g., full-thickness corneal scars, advanced keratoconus with acute hydrops scars, severe infectious keratitis with perforation, combined stromal and endothelial dystrophies, or trauma).

Five Anatomical Layers Replaced in Full-Thickness PKP:
[1. Epithelium] ──► [2. Bowman's Layer] ──► [3. Stroma (90% thickness)]
                ──► [4. Descemet's Membrane] ──► [5. Endothelium (Monolayer)]

Trephination Sizing Principles: The Oversizing Rationale

The recipient host bed is cut using a circular suction trephine (e.g., Barron vacuum trephine, Hessburg-Barron, or Hanna trephine) or a femtosecond laser. The donor button is punched from the endothelial side using a guided donor punch.

• Standard Donor Oversizing (+0.25 mm to +0.50 mm):
  • In standard clinical practice, the donor graft punch is selected to be 0.25 mm to 0.50 mm larger in diameter than the recipient trephination aperture (e.g., cutting a 7.50 mm recipient bed and punching a 7.75 mm or 8.00 mm donor button).
  • Prevention of Shallow / Flat Anterior Chamber: An oversized graft provides additional tissue redundancy, facilitating watertight, tension-free closure. A same-sized graft placed under high suture tension tends to pull taut like a flat drumhead, shallowing the anterior chamber and causing peripheral anterior synechiae (PAS).
  • Prevention of Secondary Glaucoma: Flat anterior chambers and tight wounds crowd the iridocorneal drainage angle. Oversizing deepens the anterior chamber angle and preserves trabecular outflow, dramatically reducing post-operative intraocular pressure spikes.
  • Refractive Control (Prevention of Severe Hyperopia): An oversized donor button increases the anterior curvature (steepness) of the transplanted cornea. Corneas transplanted with same-size buttons flatten post-operatively, causing massive hyperopic refractive shifts (+5.00 to +10.00 D) that require thick spectacles or contact lens correction.
• Same-Size Trephination (0.0 mm Oversize):
  • Occasionally utilized in patients with extreme pre-existing axial myopia (> -8.00 D) to deliberately flatten the central cornea and decrease myopic refractive error. However, this carries a significantly higher risk of wound leak, flat anterior chamber, and angle closure.

Suture Geometries, Tensioning & Knot Burial

Corneal donor buttons are secured using 10-0 nylon (monofilament elastic suture with high tensile strength) mounted on sharp, spatulated side-cutting micro-needles. Suture bites must penetrate to approximately 90% of corneal stromal depth (just anterior to Descemet's membrane). Full-thickness penetration into the anterior chamber creates a conduit for aqueous leakage, microbial entry, and infectious endophthalmitis, while shallow bites (<75% depth) cause internal wound gaping, posterior wound sliding, and severe irregular astigmatism.

Knot Burial Protocol: Every suture knot must be rotated and buried completely into either the host stroma or the donor stroma (away from the limbus). Exposed nylon knots attract mucus, induce giant papillary conjunctivitis (GPC), promote corneal neovascularization, and serve as a nidus for microbial colonization and devastating infectious stitch abscesses.

Immunology of Corneal Graft Rejection & The Khodadoust Line

The normal cornea is an immunologically privileged site due to several physiological factors: absence of vascular and lymphatic vessels (afferent and efferent immune limb blockade), expression of immunosuppressive molecules (Fas ligand, CD95L, TGF-beta), and anterior chamber-associated immune deviation (ACAID). However, when host-donor human leukocyte antigens (HLA class I and II) are recognized by recipient CD4+ and CD8+ T-lymphocytes, an immune-mediated allograft rejection episode is triggered.

Corneal graft rejection is strictly categorized by the anatomical layer under immunological attack:

1. Epithelial Rejection

• Clinical Manifestation: Presents as an elevated, irregular, migratory linear ridge of degenerating donor epithelial cells that stains brightly with fluorescein. The line originates near a vascularized graft border and sweeps across the donor surface over several days, leaving normal host epithelium behind.
• Prognosis: Mildest form of rejection. Readily reversible with topical corticosteroids. Because host epithelium eventually replaces donor epithelium, epithelial rejection does not cause permanent graft failure unless accompanied by endothelial rejection.

2. Subepithelial Rejection (Krachmer Spots)

• Clinical Manifestation: Presents as discrete, round, subepithelial infiltrates confined exclusively to the donor button, identical in morphology to the nummular infiltrates seen in adenoviral epidemic keratoconjunctivitis (EKC). These lesions are termed Krachmer spots and represent focal lymphocytic inflammatory aggregates situated immediately beneath Bowman's layer.

3. Stromal Rejection

• Clinical Manifestation: Presents with full-thickness or mid-stromal haze, circumscribed stromal edema, and deep stromal vascularization extending across the graft-host junction. It is frequently accompanied by conjunctival ciliary injection and anterior chamber cell and flare.

4. Endothelial Rejection: The Khodadoust Line

• Clinical Significance: The most common, critical, and visually devastating form of rejection. Unlike epithelium, human corneal endothelial cells have virtually zero mitotic regenerative capacity ($G_1$ cell cycle arrest). If endothelial cells are destroyed by cytotoxic T-cells, permanent corneal decompensation, bullous keratopathy, and irreversible graft failure ensue.
• Pathognomonic Sign: The Khodadoust Line:
  • Named after Iranian-American ophthalmologist Ali Khodadoust.
  • Appears on slit lamp examination as an irregular, elevated, white line composed of inflammatory mononuclear leukocytes and keratic precipitates (KPs) marching along the posterior surface of the donor endothelium.
  • The Khodadoust line almost always originates at a peripheral vascularized graft segment and advances centrally across the donor endothelium.
  • The Danger Zone: Ahead of the advancing line, the donor cornea remains relatively compact and clear. Behind the advancing Khodadoust line, the endothelium has been destroyed, leaving severe full-thickness stromal and epithelial edema.
• Emergency Therapeutic Intervention: Requires immediate, high-frequency topical pulse corticosteroids: prednisolone acetate 1% or difluprednate 0.05% every 1 hour, supplemented with subconjunctival steroid injections or high-dose intravenous methylprednisolone pulse therapy (500 mg to 1 g) for fulminant rejections.

Deep Anterior Lamellar Keratoplasty (DALK): Preserving Endothelium

Deep Anterior Lamellar Keratoplasty (DALK) is an advanced partial-thickness corneal procedure that removes the recipient's diseased corneal epithelium, Bowman's layer, and 95% to 100% of the stroma down to the pre-Descemet layer (Dua's layer) and Descemet's membrane, while preserving the patient's native, healthy Descemet's membrane and endothelium.

Major Clinical Indications

DALK is indicated for any corneal pathology where the endothelium is completely healthy:

• Keratoconus (without prior history of acute corneal hydrops, which tears Descemet's membrane).
• Stromal Corneal Dystrophies (Lattice, Granular, Macular, Avellino dystrophies).
• Superficial and Mid-Stromal Scars (post-traumatic, post-infectious, healed bacterial or fungal ulcers).
• Corneal Ectasia following LASIK or PRK.

The "Big-Bubble" Technique (Anwar Technique)

The critical surgical challenge in DALK is cleanly separating the deep posterior stroma from Descemet's membrane without perforating the gossamer-thin (10–12 µm) Descemet membrane:

Anwar Big-Bubble Cleavage Sequence:
[Partial Trephination ~80% Depth] ──► [Insert 27G/30G Bent Needle into Deep Stroma]
──► [Air Injection] ──► [Cleavage: Pre-Descemet Plane Forms 'Big Bubble']
──► [Stromal Debulking into 4 Quadrants] ──► [Suture Donor Stroma to Bare Host DM]

1. Partial Trephination: The host cornea is trephined to approximately 75% to 80% stromal depth.
2. Needle Insertion: A 27-gauge or 30-gauge bent needle (or specialized blunt DALK cannula) is advanced through the paracentral stroma into the deepest pre-Descemet posterior stroma.
3. Pneumodissection ("The Big Bubble"): Air is forcefully injected. The pressurized air creates a pneumatic cleavage plane that strips Descemet's membrane away from the posterior stroma, forming a circular, glistening, dome-shaped air bubble (type 1 bubble separating Dua's layer, or type 2 bubble separating bare Descemet's membrane).
4. Stromal Debulking: The anterior and posterior stroma overlying the big bubble is divided into four quadrants with micro-scissors and excised, exposing a pristine, crystal-clear host Descemet's membrane.
5. Graft Suture: A donor button (stripped completely of its own donor Descemet's membrane and endothelium) is placed into the host bed and secured with 10-0 nylon sutures.

Unmatched Clinical Advantages of DALK over PKP

• Zero Risk of Endothelial Rejection: Because the patient retains their native endothelium, true endothelial graft rejection (Khodadoust line and endothelial cell destruction) is biologically impossible. Patients can only experience mild, easily treatable epithelial or stromal rejection.
• Closed-System Tectonic Safety: Because the anterior chamber is never opened (in non-perforated cases), the devastating "open-sky" risks of PKP—specifically expulsive suprachoroidal hemorrhage and vitreous prolapse—are completely eliminated.
• Shorter Steroid Regimens: Topical corticosteroids can be tapered and discontinued much earlier than in PKP, drastically reducing the incidence of steroid-induced secondary open-angle glaucoma and posterior subcapsular cataracts.
• Superior Globe Tectonic Strength: Retaining the native Descemet's membrane and peripheral stroma preserves structural ocular integrity against blunt traumatic rupture.

Endothelial Lamellar Keratoplasty: DSAEK vs. DMEK

When corneal pathology is restricted exclusively to the endothelial monolayer and Descemet's membrane (e.g., Fuchs Endothelial Corneal Dystrophy [FECD], Pseudophakic Bullous Keratopathy [PBK], posterior polymorphous corneal dystrophy, or failed prior graft endothelium), replacing the entire cornea with PKP is obsolete. Instead, Endothelial Keratoplasty (EK) selectively replaces the diseased endothelium and Descemet's membrane through small, self-sealing clear corneal incisions (3.0 to 5.0 mm).

DSAEK (Descemet Stripping Automated Endothelial Keratoplasty)

• Donor Tissue Architecture: Donor tissue is prepared in an eye bank or operating room using an automated microkeratome. The resulting lenticule consists of donor posterior stroma, Descemet's membrane, and endothelium. Standard DSAEK lenticule thickness ranges between 100 µm and 150 µm (Ultra-Thin DSAEK [UT-DSAEK] is refined to <100 µm, typically 70–90 µm).
• Surgical Execution:
  1. Host Descemetorhexis: Under air or viscoelastic, the host's diseased central Descemet's membrane and dysfunctional endothelium (with guttae) are scored and stripped across an 8.0 mm zone (descemetorhexis) using a reverse-bent hook (Sinskey or Price hook) and removed from the anterior chamber.
  2. Lenticule Insertion: The donor lenticule is loaded onto a glide system (e.g., Busin glide, Tan EndoGlide) or taco-folded (60/40 endothelial fold) with cohesive viscoelastic and pulled/injected into the anterior chamber through a 4.0 mm corneal or scleral incision.
  3. Unfolding & Tamponade: The donor lenticule unfolds easily due to the structural stiffness provided by the donor stromal carrier layer. A full anterior chamber air bubble is injected to press the lenticule firmly against the stripped host stroma.

DMEK (Descemet Membrane Endothelial Keratoplasty)

• Donor Tissue Architecture: Represents true anatomical replacement. The donor tissue consists exclusively of bare Descemet's membrane and endothelium (~10 to 15 µm thick) with zero donor stroma.
• Donor Peeling & Spontaneous Scrolling: Bare Descemet's membrane is stripped manually from donor rim stroma using fine forceps. Because of the elastic physical properties of Descemet's membrane, once stripped of stroma, it spontaneously rolls up like a tight scroll (the DMEK roll), with the endothelial monolayer oriented on the outside of the scroll.
• Orientation Marking: To prevent inverted ("upside-down") insertion—which results in immediate primary graft failure—the stromal side of the membrane is stamped with an asymmetric vital dye mark (e.g., an "S-stamp" or peripheral triangle notches). Correct orientation confirms the stamp reads correctly from the anterior approach.
• Insertion & Non-Touch Unscrolling: The tightly curled DMEK roll is aspirated into a specialized glass cartridge (e.g., Straiko or Jones tube) and injected into the anterior chamber through a tiny 2.4 to 2.8 mm clear corneal incision. Unscrolling is achieved using gentle, indirect fluidic waves and external corneal tapping (the "DAP" [Dirisamer-Aquavella-Parker] technique) without ever touching the fragile donor endothelium directly.

DSAEK vs. DMEK: Comparative Clinical Metrics

Tamponade Bubbles, Post-Operative Supine Positioning & Rebubbling

Because lamellar endothelial grafts do not utilize sutures, initial graft adherence depends entirely on the mechanical buoyant force of an anterior chamber gas or air bubble:

• Tamponade Agents: The anterior chamber is filled with sterile air or 20% sulfur hexafluoride (SF₆) gas (which lasts 7 to 10 days, providing longer tamponade for DMEK).
• Mandatory Strict Supine Positioning: The patient must maintain a strict, flat supine (face-up) posture for 24 to 48 hours post-operatively (lying flat in bed looking straight at the ceiling). The floating bubble presses the endothelial lenticule upward against the host stroma, allowing endothelial cellular pump function to dehydrate the interface and establish permanent biological adherence.
• Pupillary Block Prevention: A full 100% gas fill can wedge the iris forward against the trabecular meshwork, causing acute pupillary block glaucoma. To prevent this, the surgeon routinely performs an inferior peripheral iridotomy/iridectomy (PI) at 6 o'clock before gas injection, and dilates the pupil post-operatively, leaving a partial fluid meniscus inferiorly.
• Rebubbling Protocol: If slit-lamp biomicroscopy or anterior segment OCT (AS-OCT) reveals graft detachment exceeding one-third (33%) of the graft area or any detachment crossing the central visual axis, the patient must undergo prompt rebubbling. Under topical anesthesia at the slit lamp or in the operating room, an anterior chamber paracentesis is tapped to release fluid, and sterile air or 20% SF₆ is reinjected to re-appose the detached lenticule.