PK, DALK and endothelial keratoplasty

Corneal transplantation has evolved from a non-selective full-thickness tissue exchange into targeted lamellar microsurgery. Because each anatomical layer of the cornea—epithelium, stroma, and endothelium—exhibits distinct physiological properties and immunological vulnerability, modern corneal surgery adheres to a foundational principle: replace exclusively the diseased tissue layer while preserving all healthy host architecture.

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1. Penetrating Keratoplasty (PK)

Penetrating Keratoplasty involves full-thickness circular excision of the host cornea and replacement with a full-thickness donor corneal button.

Primary Indications

1. Full-thickness corneal scarring (dense post-infectious scars, traumatic lacerations).
2. Advanced keratoconus with deep stromal scarring secondary to resolved corneal hydrops.
3. Severe active microbial keratitis unresponsive to maximal antimicrobial therapy, presenting with impending or actual perforation.
4. Combined stromal and endothelial disease (e.g., failed graft with stromal scarring, severe bullous keratopathy with dense subepithelial fibrosis).

Trephination & Donor Sizing Mechanics

• Trephination Instruments: Vacuum trephines (Hessburg-Barron, Hanna) or femtosecond laser trephination (enabling custom shaped interlocking incisions such as top-hat, mushroom, or zig-zag patterns to enhance surface area and biomechanical stability).
• The Donor Oversizing Principle: The donor button is routinely punched from the endothelial side $0.25\text{ mm}$ to $0.50\text{ mm}$ larger than the recipient host trephination opening (e.g., an $8.0\text{ mm}$ donor punch into a $7.5\text{ mm}$ or $7.75\text{ mm}$ host bed):
  • Watertight Closure: Facilitates tension-free edge-to-edge wound apposition.
  • Anterior Chamber Depth: Deepens the anterior chamber, preventing iris-corneal touch.
  • Glaucoma Prevention: Reduces the risk of peripheral anterior synechiae (PAS) and angle-closure glaucoma resulting from trabecular meshwork compression.
  • Refractive planning: Donor–recipient sizing affects curvature and anatomy. Select sizing for the indication, globe and surgeon technique; same-size grafting is not reserved exclusively for aphakia or high myopia.

Open-Sky Vulnerability & Suprachoroidal Haemorrhage

Trephination creates an "open-sky" state where intraocular pressure abruptly drops to atmospheric pressure, leaving intraocular structures unsupported.

• Suprachoroidal (Expulsive) Haemorrhage: The most catastrophic complication in open-sky intraocular surgery. Sudden decompression causes rupture of the posterior ciliary arteries within the suprachoroidal space. Signs: sudden elevation of intraocular contents, acute shallowing of anterior chamber, dark choroidal mass visible in the pupil, extrusion of lens, vitreous, and retina. Emergency protocol: immediate digital occlusion of the wound, rapid placement of full-thickness Cardinal sutures, and urgent vitreoretinal consultation after closing the wound. Drainage timing is individualised; immediate routine sclerotomy can worsen bleeding.

Suturing Techniques & Astigmatism Management

• Interrupted sutures allow selective adjustment and removal and are useful in inflamed, vascularised, paediatric or asymmetric beds. Number, depth and pattern depend on graft geometry and surgeon technique; running sutures are another option in suitable beds.

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2. Deep Anterior Lamellar Keratoplasty (DALK)

DALK selectively excises the corneal epithelium, Bowman layer, and stroma down to Descemet membrane, preserving the recipient's healthy, native endothelium.

Indications

Any condition with stromal opacification or ectasia but intact, functional endothelium: keratoconus (without hydrops scars), stromal dystrophies (granular, lattice, macular), and post-infectious or traumatic anterior stromal scars.

Anwar "Big-Bubble" Technique

1. Partial trephination of host cornea to 60% to 80% stromal depth.
2. A 27-gauge or 30-gauge bent needle is advanced into the deep paracentral stroma parallel to Descemet membrane.
3. Air is forcefully injected. Air dissects along the path of least resistance, cleaving stroma from the underlying membrane to create a glistening "Big Bubble".
   • Type 1 Bubble: Forms between the pre-Descemet layer (Dua layer) and deep stroma. It is large (up to $8.5\text{ mm}$), tough, circular, and resistant to rupture.
   • Type 2 Bubble: Forms between Dua layer and Descemet membrane proper. It is thin, transparent, fragile, and begins in the periphery.
4. Anterior stroma is debulked, the bubble is punctured and filled with viscoelastic, and deep stroma is divided into four quadrants and excised to the bubble interface; a type 1 bubble retains the pre-Descemet layer, while type 2 exposes Descemet membrane.
5. Manual dissection (Melles technique) is employed if the big bubble fails to form.

Complications & Conversion

• Microperforation ($< 1\text{ mm}$): Can typically be completed by decompressing the anterior chamber, continuing gentle manual dissection, and placing an air bubble in the anterior chamber at closure.
• Macroperforation ($> 1–2\text{ mm}$): May require conversion to full-thickness Penetrating Keratoplasty (PK).

Supreme Advantages of DALK over PK

• No donor endothelial rejection: DALK retains the recipient endothelium, although epithelial/stromal rejection, interface problems and later recipient endothelial disease remain possible.
• Preserved Endothelial Reserve: Long-term endothelial cell attrition matches the normal aging population rather than the accelerated decay seen in PK.
• Closed-Globe System: Avoids the open-sky phase and reduces the risk of expulsive suprachoroidal haemorrhage.
• Superior Tectonic Strength: Preserved recipient margin and basement membrane resist traumatic rupture.
• Steroid requirement: Epithelial and stromal rejection remain possible; taper and monitor steroids according to graft and surface status.

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3. Endothelial Keratoplasty: DSAEK versus DMEK

Endothelial Keratoplasty (EK) has replaced PK as the gold standard for treating corneal endothelial failure (Fuchs endothelial corneal dystrophy, pseudophakic bullous keratopathy, and ICE syndrome).

Descemet Stripping Automated Endothelial Keratoplasty (DSAEK)

• Donor Lenticule: Prepared from donor corneoscleral tissue using an automated microkeratome. Consists of donor posterior stroma ($70–130\,\mu\text{m}$; ultra-thin $< 100\,\mu\text{m}$), Descemet membrane, and endothelium.
• Surgical Technique: Host Descemet membrane is scored and stripped (descemetorhexis) over an 8 mm central zone under air or viscoelastic. The donor lenticule is loaded into a glide (Busin glide) or injector and introduced into the anterior chamber through a $3.2–5.0\text{ mm}$ clear corneal or scleral tunnel incision. An air bubble is injected into the anterior chamber to push the graft against the stripped host stroma.
• Optical Outcomes: Visual recovery occurs within 1 to 3 months, with BCVA typically plateauing at 20/25 to 20/40. Induces a mild hyperopic shift ($+0.75\text{ D}$ to $+1.50\text{ D}$) because the donor lenticule is thicker in the periphery than centrally, acting as an optical minus lens. Subtle higher-order aberrations occur across the stromal-stromal interface.

Descemet Membrane Endothelial Keratoplasty (DMEK)

• Donor Lenticule: Pure anatomical replacement consisting strictly of donor Descemet membrane and endothelium ($\sim 10–15\,\mu\text{m}$) without any donor stroma.
• Donor Preparation: Peeling donor Descemet membrane underwater using fine forceps ("no-touch" technique). Due to intrinsic elastic forces, the membrane spontaneously rolls into a scroll with the endothelium facing outwards and Descemet membrane facing inwards.
• Orientation Marker: An asymmetrical "S-stamp" or "F-stamp" is applied to the graft edge to confirm correct orientation inside the anterior chamber (preventing inverted insertion, which causes complete primary graft failure).
• Delivery & Unscrolling: Injected through a $2.4–2.8\text{ mm}$ incision using a glass cartridge. The graft is unscrolled using non-contact fluid taps and gentle surface strokes on the external cornea (Dapena, Dirisamer, or Moutsouris technique).
• Tamponade and positioning: Air or an appropriate non-expansile gas supports attachment. Follow the surgeon’s positioning and review protocol and assess pupillary block; duration depends on technique and the eye.
• Rebubbling: Required if graft detachment involves $> 1/3$ of the graft surface or threatens the visual axis (performed in-office via air/gas re-injection).

Landmark Superiority of DMEK over DSAEK

1. Visual recovery is often faster with DMEK and stromal interface scattering is lower, but final acuity depends on comorbidity and surgical course.
2. Refraction: DMEK can cause a modest hyperopic shift; absence of donor stroma does not guarantee refractive neutrality.
3. Rejection generally occurs less often after DMEK than DSAEK or PK, but remains possible.

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4. Comprehensive Keratoplasty Comparison

Choose the procedure by the diseased layer, not a fixed acuity promise. A clear anterior stroma with endothelial failure favours endothelial replacement; a healthy endothelium beneath a stromal scar favours DALK where technically feasible. Scarring after hydrops may complicate lamellar dissection but does not automatically rule it out. Combined cataract surgery requires biometry and discussion of the expected refractive shift.

After endothelial keratoplasty, examine bubble size, pupil/iridotomy patency, pressure, graft attachment and the epithelial surface. Symptoms of pain, headache or nausea after a gas-filled anterior chamber require urgent pressure assessment. Positioning and rebubbling depend on technique and detachment extent. Long-term review includes endothelial function, steroid pressure response and rejection symptoms.