Progression of Myopia

• Younger age at onset (earlier onset = more years of progression, generally worse outcome)
• Genetic predisposition/family history
• Environmental factors: intense near work, limited time outdoors
• Under-correction of refractive error (historically thought protective, but evidence now suggests it may actually accelerate progression rather than retard it) - Kanski's Clinical Ophthalmology, p. 4590s (When to prescribe section)

1. Optical correction - full correction is generally preferred; older children should have correction of even low myopia for clear distance vision. Under-correction and bifocals were once used to try to retard progression, but this approach is under investigation and not well supported - Kanski's Clinical Ophthalmology.
2. Low-dose atropine - Atropine 0.01% instilled at bedtime can significantly slow progression of myopia in children. A reasonable approach is to offer this to progressive myopes, though there is no universal consensus on concentration or duration - Kanski's Clinical Ophthalmology, p. 629-630. Goodman & Gilman's notes doses from 0.01% to 0.5% can slow and even partially reverse progression, though the exact mechanism remains unclear - Goodman & Gilman's Pharmacological Basis of Therapeutics.
3. Orthokeratology (overnight corneal reshaping contact lenses) - shown in meta-analyses to reduce axial elongation and myopia progression in children.
4. Specially designed spectacle lenses and soft contact lenses (peripheral defocus-modifying designs) - reduce progression compared to standard single-vision correction.
5. Behavioural/lifestyle measures - increased outdoor time and reduced continuous near work are recommended as adjuncts, with reasonable evidence for outdoor time reducing myopia onset/progression risk.

Bioengineered Cornea — Short Note (PG Ophthalmology)

Introduction

1. Keratoprostheses (KPro) - synthetic/biosynthetic optical devices implanted into a damaged cornea
2. Tissue-engineered corneal substitutes - scaffold- or cell-based constructs aiming to regenerate a biological cornea (epithelium, stroma, endothelium)

1. Keratoprostheses

Classification

• Rigid (biosynthetic composite): Boston Type I and II KPro, Osteo-Odonto-Keratoprosthesis (OOKP), Modified OOKP (MOOKP), LV Prasad Eye Institute (LVPEI) KPro
• Soft/flexible synthetic: AlphaCor, CorNeat KPro, EndoArt

Osteo-odonto-keratoprosthesis (OOKP) - classic exam favourite

• Uses the patient's own tooth root and alveolar bone to support a central PMMA optical cylinder, covered by a buccal mucous membrane graft
• Two-stage surgery, 2-4 months apart; technically demanding
• Retention rate ~85% at 18 months in experienced hands; ~80% of patients achieve vision between counting fingers and 6/12 - Kanski's Clinical Ophthalmology, p. 297-298
• Indications: bilateral blindness from severe but inactive anterior segment disease unsuitable for conventional keratoplasty (Stevens-Johnson syndrome, OCP, chemical burns, trachoma); vision counting fingers or worse in the better eye; intact optic nerve/retinal function without significant glaucomatous damage
• Complications: glaucoma (up to 40%, often needing aggressive medical therapy plus drainage device), retroprosthetic membrane formation (~25%, managed with YAG membranectomy), tilting/extrusion, retinal detachment, endophthalmitis - Kanski's Clinical Ophthalmology, p. 298

Newer flexible/soft designs (recent literature)

• CorNeat KPro: full-thickness PMMA optic integrated with a polyurethane fiber "skirt" that bio-integrates with host conjunctiva rather than cornea - a novel approach to enhance biointegration and reduce extrusion
• AlphaCor and EndoArt: flexible synthetic devices reported to have lower rates of glaucoma than rigid keratoprostheses, currently in clinical trials
• These aim to reduce the surgical invasiveness and complication profile of rigid devices - Review of Ophthalmology, "What is New in Artificial Corneas" (2024-2025)

2. Tissue-engineered corneal substitutes ("true" bioengineered corneas)

Scaffold/biomaterial strategies studied in animal and early human models

• Decellularized corneal matrices (porcine, human) - retain native collagen architecture, reduce immunogenicity
• Collagen-based hydrogels/vitrigels (e.g., recombinant human collagen constructs, MBPC/HBPC collagen scaffolds) - among the best-performing in comparative animal studies for transparency retention and low fibrosis
• Fibrin-based and GelMA (gelatin methacryloyl) constructs
• Corneal stromal stem cell (CSSC) sheets - cell-only implants without material scaffold
• 3D bioprinting - emerging strategy to fabricate layer-specific corneal constructs (epithelium, stroma, endothelium) using bioinks such as collagen and primary human keratocytes - "3D Printing Strategies for Bioengineering Human Cornea," Adv Healthc Mater 2025 (Yuan et al.)
• A 2025-2026 systematic review comparing bioengineered grafts in animal models found extracellular-matrix-derived grafts (decellularized/recellularized human cornea, GelMA composites, aligned collagen sheets) performed best, while acellular porcine stroma and 3D-bioprinted decellularized constructs were less successful, showing opacity, abnormal thickness, or fibrotic/immune reactions - Advances in Corneal Tissue Engineering (Anitua et al., Medicina 2025, PMID 41597366).

Adjunct/supportive tissue-based techniques (already in clinical use, feed into bioengineering approaches)

• Amniotic membrane transplantation - anti-inflammatory, anti-scarring basement membrane scaffold used for surface reconstruction and promoting epithelialization
• Limbal stem cell (LSC) grafting (autologous/allogeneic, or ex vivo expanded) - replenishes the limbal stem cell pool in limbal stem cell deficiency (LSCD); signs of LSCD include conjunctivalization with goblet cells, corneal vascularization, fibrovascular pannus, persistent epithelial defects - Kanski's Clinical Ophthalmology, p. 296-297
  • Advantages: regeneration of corneal epithelium, improved comfort/vision
  • Disadvantages: conjunctivalization, filamentary keratitis, donor-eye scarring (autologous); infection and rejection (allogeneic)

Newer adjunct technologies mentioned in current reviews

• Femtosecond laser-assisted keratoplasty (FLAK), DMEK, Bowman layer transplantation - improve precision and reduce rejection in conventional grafting, complementing bioengineered approaches
• CRISPR-Cas9 gene editing and bioadhesives - being explored to enhance graft integration/survival
• Artificial intelligence - proposed for surgical planning and outcome prediction, though bias/access concerns remain

Advantages vs Limitations of bioengineered corneas (exam summary table)

Key take-home points for exam

• Keratoprosthesis is indicated only when conventional keratoplasty has a poor prognosis (bilateral end-stage disease, intact posterior segment).
• OOKP remains the classic high-yield topic: tooth-root autograft + PMMA optic, glaucoma is the most common and most difficult complication.
• True tissue-engineered corneas (decellularized matrices, collagen scaffolds, 3D bioprinting, stem-cell sheets) are still largely investigational/animal-model stage, with ECM-derived and collagen-based grafts currently showing the most promise.
• Newer flexible synthetic KPro designs (CorNeat, AlphaCor, EndoArt) are in clinical trials and show fewer complications (lower glaucoma rates) than rigid devices.