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Orthokeratology (Ortho-K) - PG Exam Notes

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Definition

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Mechanism of Action

• Central corneal epithelial thinning (flattening of central cornea)
• Mid-peripheral stromal and epithelial thickening (due to hydraulic forces redistributing epithelial cells from center to mid-periphery)
• The net result: central cornea flattens by ~6 µm per 1.00 D of myopia correction
• Creates peripheral myopic defocus on the retina - this is the mechanism behind myopia control (retinal signal inhibits axial elongation)
• Effect onset: observable after 1 night of wear; stable after 1-2 weeks
• Effect is fully reversible on cessation of lens wear

How much correction?

• 6 µm of corneal flattening = 1.00 D reduction in myopia
• Since cornea accounts for 60% of the eye's focusing power, even minor epithelial redistribution significantly alters refraction

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Lens Design (4-Zone / 5-Zone Design)

• CRT lenses (3-zone): Base Curve, Return Zone Depth (RZD), Landing Zone Angle (LZA) - sagittal height-based fitting
• VST lenses (4-zone): Use continuous transitional arcs

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Ideal Fluorescein Pattern ("Bull's-Eye" Pattern)

• Central zone: Touch / bearing (dark - no fluorescein)
• Reverse/mid-peripheral zone: Bright ring of fluorescein clearance (pooling)
• Alignment zone: Peripheral alignment/landing (thin, even band)
• Edge: Adequate tear exchange clearance

• Central flattened treatment zone (bull's-eye center)
• Surrounding steepened reverse curve zone - must be centered within the pupillary circumference
• Centered reverse zone = effective peripheral defocus = better myopia control

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Indications

• Low to moderate myopia (up to -6.00 D)
• With-the-rule astigmatism up to -1.75 D
• Children and adolescents (primary use for myopia progression control)
• Adults who want to avoid spectacles/surgery during the day
• Athletes, swimmers, active lifestyle individuals
• Patients not suitable for refractive surgery (age <18, thin cornea)
• Mildly symptomatic dry eye (better tolerated than soft lenses)

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Contraindications

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Fitting Process

1. Corneal topography - map cornea surface (essential; determines lens parameters)
2. Keratometry - measure corneal curvature (K readings)
3. Refraction - determine target correction
4. Trial lens fitting - assess fluorescein pattern
5. Lens dispense + training - insertion/removal, hygiene education
6. Follow-up at 1 day, 1 week, 1 month, then every 3-6 months

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Efficacy for Myopia Control

• Reduces axial elongation by ~40-50% compared to spectacles
• 0.1 mm reduction in annual axial growth ≈ 0.25-0.30 D less myopia per year
• A 40-50% reduction in myopia progression can reduce lifetime risk of myopic maculopathy by 30-40%
• Recent Cochrane meta-analysis (Lawrenson et al., 2025 - PMID 39945354) confirms ortho-K as one of the most effective myopia control interventions in children

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Complications

Infective

• Microbial keratitis - most serious complication
  • Most common organism: Pseudomonas aeruginosa (gram-negative, aggressive)
  • Also: Acanthamoeba keratitis (from tap water exposure)
  • Risk higher in children due to hygiene compliance issues
  • Strict lens hygiene is mandatory

Non-infective

• Epithelial staining (superficial punctate keratitis) - most common minor complication
• Corneal infiltrates (sterile)
• Lens binding / adherence on waking
• Halos and glare (especially at night) - from optical zone edge effects
• Ghost images / monocular diplopia
• Over-correction or under-correction
• Corneal decentration of treatment zone
• Giant papillary conjunctivitis (GPC) - mechanical hypersensitivity

Fluorescein Staining Grading (Chu & Xie):

• Grade 0: No/minimal staining
• Grade I: Scattered punctate staining
• Grade II: Dense punctate + mild discomfort
• Grade III: Localized epithelial defects + moderate irritation
• Grade IV: Extensive epithelial defects + severe symptoms

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Comparison with Other Myopia Control Methods

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Key Exam Points (High-Yield)

1. Lens type: RGP (Rigid Gas-Permeable) lenses worn overnight - NOT soft lenses
2. Mechanism: Central epithelial thinning + mid-peripheral thickening via hydraulic forces
3. Fluorescein pattern: "Bull's-eye" = central touch + mid-peripheral clearance + peripheral alignment
4. Myopia limit: Effective up to -6.00 D; astigmatism up to -1.75 D
5. Most dangerous complication: Microbial keratitis (Pseudomonas aeruginosa)
6. Myopia control mechanism: Peripheral myopic defocus - mid-peripheral corneal steepening shifts peripheral retinal image focus anterior, reducing stimulus for axial elongation
7. Effect is reversible - distinguishes it from LASIK/PRK
8. Tool for fitting: Corneal topography (mandatory)
9. Stability: Achieved in 1-2 weeks; retainer lenses required to maintain effect
10. Primary target population: Children 6-18 years with progressing myopia
11. CRT (Paragon) and VST (BE Retainer) are the two main FDA-approved systems

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Recent Evidence

• Sartor et al. 2024 (PMID 38904856) - Systematic review + meta-analysis confirming benefits and quantifying risks of ortho-K
• Lawrenson et al. 2025 (PMID 39945354) - Cochrane living systematic review confirming ortho-K among top myopia control interventions
• Schmidt et al. 2025 (PMID 40219611) - Network meta-analysis comparing all myopia interventions in children

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Summary Mnemonic for Complications: "MICAH-G"

• M - Microbial keratitis (Pseudomonas, Acanthamoeba)
• I - Infiltrates (sterile)
• C - Corneal staining / epithelial SPK
• A - Adherence / binding of lens
• H - Halos and glare
• G - Giant papillary conjunctivitis

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Osteo-Odonto Keratoprosthesis (OOKP) - PG Exam Notes

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Definition

Simple exam definition: "A tooth-and-bone lamina used as a biological haptic to support a PMMA optical cylinder implanted into a blind eye, covered by buccal mucosa."

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Historical Background (High Yield)

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Why a Tooth?

• Biologically autologous - no rejection
• Dentin is biocompatible and resistant to resorption (compared to synthetic materials)
• Provides rigid structural support for the PMMA optic
• Natural cement/periodontal tissue integrates with surrounding ocular tissue
• Bioceramics are inferior: at low pH (infection/inflammation), they degrade faster than natural tooth/bone

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Indications

1. Stevens-Johnson Syndrome (SJS) / Toxic Epidermal Necrolysis (TEN) - most common indication
2. Ocular Cicatricial Pemphigoid (OCP)
3. Severe chemical burns (acid or alkali) - second most common
4. Trachoma (end-stage cicatricial)
5. Graft-vs-Host Disease (GVHD) affecting ocular surface
6. Multiple failed conventional corneal grafts
7. Bullous keratopathy post-glaucoma surgery
8. Severe dry eye with failed ocular surface reconstruction
9. Aniridia-related keratopathy

Key rule: Bilateral disease only - not indicated for unilateral disease

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Contraindications

Absolute Contraindications

• Age < 17 years
• No perception of light (NPL) - surgery will only harm
• Advanced glaucoma (already discussed separately in assessment)
• Irreparable retinal detachment
• Phthisis bulbi (high risk of losing residual perception of light)
• Active TB (systemic)
• Smoking and betel nut chewing (some centers)

Relative Contraindications

• Mentally unstable patients
• Defective light perception (may indicate advanced glaucoma)
• Unrealistic expectations (cosmetic or visual)
• Unable to commit to lifelong follow-up
• Systemic diseases adversely affecting wound healing

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Patient Assessment (Pre-operative)

1. Ophthalmological Assessment

• Full ocular history and examination
• Retinal function assessment - ERG, ultrasound B-scan (to confirm intact retina and optic nerve)
• Perception of light (PL) - must be present
• Intraocular pressure (IOP) - rule out advanced glaucoma (most common cause of MOOKP failure)
• Visual field (if possible)
• Fornix depth and lid status

2. Dental / Maxillofacial Assessment

• Tooth selection: mono-radicular tooth, preferably canine (longest root, single root)
  • Upper canine preferred
  • If no canine: premolar or central incisor
• Imaging: Orthopantomography (OPG), X-ray, cone-beam CT to evaluate root structure and alveolar bone
• If edentulous (no teeth): allograft considered (but worse outcomes due to HLA mismatch and laminar resorption)

3. Psychological Assessment

• Past years of poor sight - risk of psychopathology
• Realistic expectations regarding vision and cosmesis
• Ability to commit to lifelong follow-up
• Financial and emotional preparedness

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Surgical Procedure - Two-Stage Operation

Stage 1a - OOKP Lamina Preparation and Subcutaneous Implantation

1. Tooth extraction (canine + surrounding alveolar bone)
2. Lamina fabrication - tooth root trimmed to a lamina (disc-shaped plate) of dentin and bone
3. A hole is drilled in the center of the lamina
4. PMMA optical cylinder (the artificial optic) is fitted and cemented into the hole
5. The assembled tooth-PMMA complex is then implanted subcutaneously under the skin of the lower eyelid or infraorbital region (contralateral side or cheek)
   • Purpose: to allow vascularization of the lamina over 3-4 months
   • This is the "incubation" period

Stage 1b - Ocular Surface Preparation (can be concurrent or sequential)

1. Removal of diseased ocular surface: conjunctiva, corneal epithelium, Bowman's membrane
2. Full-thickness buccal mucosal graft (~3 cm diameter, muscle-free) harvested from the cheek
3. Mucosal graft sutured over the anterior ocular surface to create a stable mucosal barrier
4. Time allowed for graft healing (~3-4 months)

Stage 2 - Implantation (~4 months after Stage 1)

1. Retrieval of the vascularized OOKP complex from the subcutaneous site
2. Central trephination of the eye (removing central cornea)
3. Mucosal flap elevated (the buccal mucosa placed in Stage 1b)
4. OOKP complex inserted into the central corneal defect and sutured in place
5. Mucosal flap re-draped over the OOKP lamina (covers the tooth-bone-PMMA complex)
6. Only the PMMA optical cylinder protrudes through the center of the mucosa - this is the functional "window"

Key point for exams: The tooth-bone lamina is NOT placed directly; it is first incubated subcutaneously for vascularization - this is what makes OOKP survive long-term without rejection.

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Structure of the OOKP (What It Looks Like)

[Buccal Mucosa covering] → [Dentin-Bone Lamina (tooth)] → [PMMA Optical Cylinder protruding]
                                       ↕
                              [Eye / Corneal opening]

• The PMMA cylinder acts as the clear window/cornea
• The tooth lamina acts as the haptic (structural scaffold)
• The oral mucosa acts as the biological covering (replaces conjunctiva/cornea)

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Outcomes

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Complications

Maxillofacial / Surgical Complications

• Mandibular fracture
• Failed tooth extraction
• Fistula formation
• Maxillary sinus perforation
• Exposure of adjacent tooth root
• Oral mucosal flap perforation
• Paresthesia at implantation site
• Sinusitis / graft site infection

Mucosal Complications

• Mucosal graft defect
• Mucosal overgrowth (grows over optical cylinder - reduces vision)
• Mucosal necrosis / melting (most serious surface complication)
• Submucosal scarring

Ocular Complications

Exam high-yield: Glaucoma = most common cause of vision loss post-OOKP. Laminar resorption = most common cause of OOKP failure (structural).

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Comparison: MOOKP vs Boston KPro

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Multidisciplinary Team Required

• Ophthalmologist (corneal specialist)
• Maxillofacial / Oral surgeon (tooth extraction and lamina preparation)
• Radiologist (OPG, cone-beam CT)
• Psychologist (pre-operative counseling)
• Anesthesiologist

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Monitoring Post-Operatively

• Vision testing and visual fields
• CT scanning - standardized radiological CT monitoring of laminar integrity (longitudinal)
• Slit-lamp examination for mucosal health
• IOP monitoring (glaucoma surveillance)
• Watch for signs of laminar resorption, retinal complications

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Key Exam Points (High-Yield Summary)

1. "Tooth-in-eye" surgery - uses patient's own canine tooth + alveolar bone as scaffold
2. Invented by Strampelli (1963); modified by Falcinelli (MOOKP); standardized as Rome-Vienna Protocol (2001-2002)
3. Indication: bilateral end-stage ocular surface disease (SJS most common)
4. Optic material: PMMA (polymethyl methacrylate) cylinder
5. Tooth chosen: Canine (mono-radicular, longest root) - preferably upper canine
6. Stage 1a: OOKP assembled and incubated subcutaneously (cheek/lower eyelid) for ~4 months for vascularization
7. Buccal mucosa used: ~3 cm, full-thickness, muscle-free graft
8. Most common complication of vision loss: Glaucoma
9. Most common cause of structural failure: Laminar resorption (especially in SJS patients; associated with S. epidermidis)
10. Anatomic success at 18 years: ~85%
11. Gold standard KPro for end-stage disease: MOOKP > Boston KPro
12. If no tooth available: allograft used (worse outcomes - more laminar resorption due to HLA mismatch)
13. Contraindicated if no PL, advanced glaucoma, phthisis, age < 17 years
14. Vision improvement expected: 78% achieve 20/400 or better

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Mnemonic for Indications: "SJS-COT-GABS"

• S - Stevens-Johnson Syndrome
• J - (and TEN)
• S - Severe chemical burns
• C - Cicatricial pemphigoid (OCP)
• O - Other autoimmune (GVHD)
• T - Trachoma (end-stage)
• G - Grafts (multiple failed corneal grafts)
• A - Aniridia keratopathy
• B - Bullous keratopathy
• S - Severe dry eye

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Lamellar Keratoplasty - PG Exam Notes

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Definition

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Corneal Anatomy (Quick Revision - Essential Context)

1. Epithelium (~50 µm)
2. Bowman's layer (~12-15 µm) - acellular
3. Stroma (~500 µm) - 90% of corneal thickness
4. Pre-Descemet's Layer (PDL) / Dua's layer (~10 µm) - described 2013
5. Descemet's Membrane (DM) (~10-15 µm) - basement membrane of endothelium
6. Endothelium (~5 µm) - single cell layer

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Classification of Lamellar Keratoplasty

LAMELLAR KERATOPLASTY
│
├── ANTERIOR LAMELLAR KERATOPLASTY (ALK)
│   ├── 1. Bowman's Layer Transplant (BLT / BMT)
│   ├── 2. Superficial Anterior Lamellar Keratoplasty (SALK)
│   └── 3. Deep Anterior Lamellar Keratoplasty (DALK)  ← Gold Standard ALK
│
└── POSTERIOR LAMELLAR KERATOPLASTY (PLK) / Endothelial Keratoplasty (EK)
    ├── 4. DSEK (Descemet's Stripping Endothelial Keratoplasty)
    ├── 5. DSAEK (Descemet's Stripping Automated EK)  ← Most widely performed
    ├── 6. DMEK (Descemet's Membrane EK)  ← Best visual outcomes
    ├── 7. PDEK (Pre-Descemet's Endothelial Keratoplasty)
    └── 8. DMAEK (Descemet's Membrane Automated EK)

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ANTERIOR LAMELLAR KERATOPLASTY (ALK)

1. Bowman's Layer Transplant (BLT / BMT)

• Isolated Bowman's layer (~12-15 µm) is harvested from donor cornea and transplanted onto a prepared recipient bed
• Does NOT involve any biological cellular tissue (Bowman's is acellular) - hence NO RISK OF ALLOGRAFT REJECTION
• Originally described for post-PRK subepithelial scarring

• Keratoconus - to flatten the cornea and improve contact lens fit/retention, slowing progression
• Arrest of keratoconus progression - delaying or avoiding need for DALK/PK
• Post-refractive surgery subepithelial haze (post-PRK)

• Bowman's layer peeled from donor corneoscleral disc
• Recipient corneal surface prepared (epithelium removed)
• Donor Bowman's layer laid on recipient surface

• No rejection risk (acellular)
• Does not restore vision dramatically - improves corneal biomechanics and CL fit
• Experience is limited compared to DALK
• Prepared with microkeratome or femtosecond laser

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2. Superficial Anterior Lamellar Keratoplasty (SALK)

• Partial-thickness excision of corneal epithelium + superficial stroma
• Endothelium, Descemet's membrane, and deep stroma are retained
• Donor tissue: corresponding superficial partial-thickness graft

• Opacification of the superficial one-third of corneal stroma NOT caused by potentially recurrent disease
• Marginal corneal thinning or infiltration: recurrent pterygium, Terrien marginal degeneration, limbal dermoid, other tumours
• Localized thinning or descemetocoele formation

• No risk of endothelial rejection
• Less invasive than DALK or PK
• Endothelial quality in donor irrelevant

• Depth of dissection limited - cannot reach deep opacities
• Interface haze may compromise vision

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3. Deep Anterior Lamellar Keratoplasty (DALK)

• Corneal tissue removed almost to the level of Descemet's membrane
• Endothelium and DM are left in situ (not transplanted)
• Because endothelium is the major target for rejection, no risk of endothelial rejection
• Gold standard for anterior lamellar keratoplasty

• Disease involving the anterior 95% of corneal thickness with normal endothelium
• Absence of breaks or scars in Descemet's membrane
• Keratoconus (without history of acute hydrops) - most common indication
• Superficial trauma with corneal opacification
• Chronic inflammatory disease (e.g., atopic keratoconjunctivitis) - increased rejection risk with PK
• Stromal dystrophies (Granular, Lattice, Macular dystrophy)
• Post-infectious corneal scars (healed bacterial/viral keratitis)
• Chemical injury with intact endothelium (see Fig. 8.8, Kanski's)

• Air injected via 27G needle into deep stroma at ~90% depth
• Type 1 bubble: cleavage at pre-Descemet's / PDL level
• Type 2 bubble: cleavage within Descemet's membrane itself - risk of perforation
• Ideal: Type 1 bubble giving maximum depth with intact DM

• No risk of endothelial rejection (epithelial/stromal rejection may still occur)
• Less astigmatism than PK
• Structurally stronger globe than PK
• Increased donor availability - endothelial quality irrelevant
• Can be converted to PK if DM perforates

• Difficult and time-consuming with high risk of perforation
• Interface haze may limit final best corrected visual acuity
• Visual outcome not quite equal to PK in all cases
• Significant surgeon learning curve

• Intraoperative DM perforation (→ convert to PK)
• Interface haze
• Stromal/epithelial rejection (not endothelial)
• Double anterior chamber (if DM perforates)

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POSTERIOR LAMELLAR KERATOPLASTY (PLK) / Endothelial Keratoplasty (EK)

• Fuchs' endothelial corneal dystrophy (most common)
• Pseudophakic/Aphakic bullous keratopathy (post-cataract surgery endothelial failure)
• Failed previous corneal graft
• Posterior polymorphous dystrophy
• Iridocorneal endothelial (ICE) syndrome

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4. DSEK (Descemet's Stripping Endothelial Keratoplasty)

• Host DM and endothelium stripped out using a Sinskey hook or specialized stripper
• Donor tissue: posterior stroma + DM + endothelium, manually cut
• Inserted as a "taco fold" through 5 mm incision
• Held in place by air bubble in anterior chamber

• Small incision - less astigmatism
• No sutures on cornea
• Faster visual recovery
• Lower rejection risk

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5. DSAEK (Descemet's Stripping Automated Endothelial Keratoplasty)

• Same as DSEK but donor tissue prepared with automated microkeratome for uniform thickness
• Graft inserted as taco-fold or scroll through ~5 mm incision
• Air bubble in AC holds graft against host stroma
• Patient lies supine post-op to allow bubble to tamponade graft

• More uniform graft thickness
• Better reproducibility
• Eye bank can pre-cut tissue

• Small incision (~5 mm vs full trephination)
• No or minimal sutures
• Faster visual recovery (1-3 months)
• Lower rejection rates
• More rapid rehabilitation

• Significant learning curve
• Specialized equipment required
• Visual outcome suboptimal vs DMEK due to stromal interface
  • Causes: graft thickness variation, graft irregularities, high-order aberrations, donor-recipient interface fibrosis
• Endothelial rejection can still occur
• Graft dislocation in early post-op period
• Requires patient to remain supine post-op

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6. DMEK (Descemet's Membrane Endothelial Keratoplasty)

• Developed by Gerrit Melles (2006)
• Only DM and endothelium transplanted - ultra-thin graft
• Host DM + endothelium stripped; donor DM + endothelium rolled into a scroll and injected via small incision (~3 mm)
• Graft unscrolled in AC using tapping/air maneuvers
• Air bubble (20% SF₆ gas) used to support graft

• Best visual outcomes of all EK procedures - clearest interface
• Fastest visual recovery (1-2 weeks for initial clarity)
• Lowest rejection rates of all corneal transplants (graft is almost acellular except endothelium)
• No donor stromal tissue = less interface irregularity, less HOA
• Better refractive predictability

• Most technically demanding EK procedure - steep learning curve
• Graft is only ~10-15 µm - extremely fragile, difficult to handle
• Higher risk of rebubbling (graft detachment requiring repeat air injection)
• Higher risk of primary graft failure (graft destroyed during preparation/insertion)
• Tissue wasted if preparation fails in OR

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7. PDEK (Pre-Descemet's Endothelial Keratoplasty)

• Developed by Agarwal (2013-2014) - based on discovery of Dua's layer
• Donor tissue: PDL + DM + endothelium harvested using Type 1 bubble technique (air injected into stroma to create cleavage at PDL level)
• PDL provides structural support to the otherwise fragile DMEK scroll
• Easier to handle than DMEK due to the added PDL scaffolding

• Easier to prepare and handle (less fragile than pure DMEK)
• Similar optical clarity to DMEK (minimal stromal content)
• Lower rebubbling rates than DMEK
• Can use younger donor tissue (easier to scroll/unscroll)
• Interface same quality as DMEK

• Thinner graft → better visual outcomes (approaching DMEK quality)
• Less interface haze

• Relatively newer - less long-term data
• Requires specialized PDEK clamp
• Type 1 bubble creation can be technically difficult

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8. DMAEK (Descemet's Membrane Automated Endothelial Keratoplasty)

• A modification of DMEK (described 2009) where a peripheral rim of stroma is retained at the edge of the donor tissue
• Provides a handling rim → easier to manipulate than pure DMEK scroll
• Combines DMEK optical quality with DSAEK ease of handling

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Comparison Table: All EK Procedures

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Comparison: ALK vs EK vs PK

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Key Exam High-Yield Points

1. Lamellar = partial thickness (vs Penetrating = full thickness)
2. DALK = Gold standard for anterior LK - uses Big Bubble (Anwar) technique
3. DALK indication: Keratoconus with normal endothelium and no DM breaks - most common
4. DALK advantage: No endothelial rejection risk; more donor tissue available (endothelial quality irrelevant)
5. DSAEK = most widely performed EK worldwide; automated microkeratome for donor prep
6. DMEK = best visual outcomes + lowest rejection; but most technically difficult
7. PDEK = PDL + DM + Endo; easier than DMEK; introduced by Agarwal
8. Bowman's transplant = no rejection risk (acellular tissue); used in keratoconus
9. Air bubble used post-operatively in ALL EK procedures to hold graft - patient must remain supine
10. SF₆ (20%) gas used in DMEK for graft tamponade
11. Endothelial rejection can still occur in DSAEK/DMEK but much lower than PK
12. DSAEK disadvantage: Interface haze due to donor stroma → suboptimal vision vs DMEK
13. DMEK rebubbling (graft detachment + re-injection of air) = most common post-op intervention
14. Fuchs' dystrophy = most common indication for EK procedures
15. In DSEK/DSAEK: graft interface = donor stroma → host pre-DM or host DM In DMEK: graft interface = donor DM → host pre-DM (cleanest interface = best vision) In PDEK: graft interface = donor PDL → host PDL (always PDL-to-PDL)

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Mnemonic: "Layers Replaced" (Posterior to Anterior)

• DMEK = D+E only (DM + Endothelium) - thinnest, best vision
• PDEK = P+D+E (PDL + DM + Endo) - slightly thicker, easier
• DSAEK = S+D+E (Stroma + DM + Endo) - thickest EK, most common
• DALK = Everything down to DM (but NOT DM itself) - anterior disease

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