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Tear Film Substitutes
Cornea
1. Clinical Analogy - The Window Wiper Model
Think of the ocular surface as a glass window and the tear film as the thin water layer left after a wiper stroke. For the window to be optically clear, the water must:
- Spread uniformly (mucin anchors it to the epithelium)
- Remain stable long enough between blinks (lipid prevents evaporation)
- Wash debris with appropriate volume (aqueous provides bulk)
- A dry eye state is like a smeared, incomplete wiper pass - patches of dryness, refraction distorted.
- Tear film substitutes try to replicate one or more components or functional aspects of the tear film, each targeting a different failure point in the “wiper system.”
2. Normal Tear Film - Anatomy & Physiology
- The tear film is approximately 2–6 µm thick at the precorneal surface.
- The Tear Film & Ocular Surface Society Dry Eye Workshop II (TFOS DEWS II) report (2017) supports a two-phase model, with a lipid layer overlying a mucoaqueous gel phase; the traditional three-layer model is a simplification of reality.
| Layer | Thickness | Source | Composition | Function |
|---|---|---|---|---|
| Lipid (outer) | • Approximately 0.04 µm (about 42 nm). • Thickness varies with measurement method; reported interferometric values range approximately 20–160 nm. | • Meibomian glands are the principal source of tear-film lipids. • Specific attribution of polar lipids to meibomian glands and nonpolar lipids to the glands of Zeis and Moll is not established. | • Phospholipids • Wax esters • Cholesterol esters • Triglycerides | • Thought to retard tear evaporation, although the magnitude and mechanism remain debated. • Lowers tear-film surface tension, helping maintain stability and resist collapse. • Spreads over the mucoaqueous tear film during blinking. |
| Aqueous (middle) | • Traditionally reported as approximately 7–8 µm in the classical three-layer model. • Thickness is model- and measurement-dependent; Tear Film and Ocular Surface Society Dry Eye Workshop II (TFOS DEWS II) favors a lipid layer overlying a mucoaqueous phase and reports the entire precorneal tear film as approximately 2–5.5 µm thick. | • Produced mainly by the main lacrimal gland, with contributions from the accessory glands of Krause and Wolfring. • Basal and reflex secretion are neurally regulated; a strict division assigning basal secretion only to accessory glands and reflex secretion only to the main lacrimal gland is debated. | • Water (approximately 98%) • Electrolytes • Proteins, including lysozyme, lactoferrin, secretory immunoglobulin A (IgA), and lipocalin • Growth factors | • Contributes to optical clarity by helping maintain a smooth, stable refracting surface. • Provides antimicrobial protection. • Contributes oxygen to the exposed ocular surface. • Helps nourish the ocular surface. |
| Mucin (inner) | • Do not assign a fixed separate mucin-layer thickness. • The mucoaqueous layer forms most of the approximately 2–6 µm tear film. • Transmembrane mucins extend approximately 0.2–0.5 µm above the epithelial surface. | • Goblet cells produce mucin 5AC (MUC5AC). • Stratified squamous cells express mucin 1 (MUC1), mucin 4 (MUC4), and mucin 16 (MUC16). | • Glycoproteins • Secreted mucins • Transmembrane mucins | • The mucin-rich glycocalyx contributes to conversion of the hydrophobic epithelial surface into a hydrophilic, wettable interface. • Mucins help retain and stabilize the tear film at the epithelial surface through the mucoaqueous gel and epithelial glycocalyx. • Secreted mucins contribute to hydration and lubrication; transmembrane mucins and their glycocalyx provide boundary lubrication during blinking. • The mucoaqueous/mucin component traps debris, which is cleared by blinking and tear drainage. |
| Layer | Thickness | Source | Composition | Function |
|---|---|---|---|---|
| Lipid (outer) | • Approximately 0.04 µm (about 42 nm). • Thickness varies with measurement method; reported interferometric values range approximately 20–160 nm. | • Meibomian glands are the principal source of tear-film lipids. • Specific attribution of polar lipids to meibomian glands and nonpolar lipids to the glands of Zeis and Moll is not established. | • Phospholipids • Wax esters • Cholesterol esters • Triglycerides | • Thought to retard tear evaporation, although the magnitude and mechanism remain debated. • Lowers tear-film surface tension, helping maintain stability and resist collapse. • Spreads over the mucoaqueous tear film during blinking. |
| Aqueous (middle) | • Traditionally reported as approximately 7–8 µm in the classical three-layer model. • Thickness is model- and measurement-dependent; Tear Film and Ocular Surface Society Dry Eye Workshop II (TFOS DEWS II) favors a lipid layer overlying a mucoaqueous phase and reports the entire precorneal tear film as approximately 2–5.5 µm thick. | • Produced mainly by the main lacrimal gland, with contributions from the accessory glands of Krause and Wolfring. • Basal and reflex secretion are neurally regulated; a strict division assigning basal secretion only to accessory glands and reflex secretion only to the main lacrimal gland is debated. | • Water (approximately 98%) • Electrolytes • Proteins, including lysozyme, lactoferrin, secretory immunoglobulin A (IgA), and lipocalin • Growth factors | • Contributes to optical clarity by helping maintain a smooth, stable refracting surface. • Provides antimicrobial protection. • Contributes oxygen to the exposed ocular surface. • Helps nourish the ocular surface. |
| Mucin (inner) | • Do not assign a fixed separate mucin-layer thickness. • The mucoaqueous layer forms most of the approximately 2–6 µm tear film. • Transmembrane mucins extend approximately 0.2–0.5 µm above the epithelial surface. | • Goblet cells produce mucin 5AC (MUC5AC). • Stratified squamous cells express mucin 1 (MUC1), mucin 4 (MUC4), and mucin 16 (MUC16). | • Glycoproteins • Secreted mucins • Transmembrane mucins | • The mucin-rich glycocalyx contributes to conversion of the hydrophobic epithelial surface into a hydrophilic, wettable interface. • Mucins help retain and stabilize the tear film at the epithelial surface through the mucoaqueous gel and epithelial glycocalyx. • Secreted mucins contribute to hydration and lubrication; transmembrane mucins and their glycocalyx provide boundary lubrication during blinking. • The mucoaqueous/mucin component traps debris, which is cleared by blinking and tear drainage. |
- Key concept - Fluorescein tear break-up time (TBUT): Fluorescein tear break-up time abnormality cutoff is method-dependent: <10 s, or <5 s only with controlled low fluorescein volumes; non-invasive tear break-up time (NIBUT) is a preferred non-invasive measure for tear-film stability assessment..
- Lipid layer thickness correlates strongly with fluorescein tear break-up time - meibomian dysfunction is the most common cause of evaporative dry eye.
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Keep reading Tear Film Substitutes
12 more sections are included with the Clinical plan.
- DEWS II Classification of Dry Eye Disease (2017)
- Classification of Tear Film Substitutes
- Individual Agents - Pharmacology & Mechanisms
- Preservatives in Tear Substitutes - A Critical Issue
- Clinical Decision-Making Framework - Choosing the Right Drop
- Key Investigations for Dry Eye & Monitoring Therapy