Contact Lens and Corneal Anatomy Review

Flashcards covering corneal anatomy, tear film physiology, contact lens types, fitting mechanics, and clinical evaluation rules.

Lattice Theory

A theory explaining corneal transparency as being explicitly due to the orderly arrangement of fibers.

Unlattice Theory

Characterizes the opaque structure of the sclera.

HVID (Horizontal Visible Iris Diameter)

Average measurement of $11.7\,mm$ (dimensions are generally $0.1\,mm$ less in females).

VVID (Vertical Visible Iris Diameter)

Average measurement of $10.6\,mm$.

Corneal Sagittal Depth

Measures $2.6\,mm$ from the steepest to the flattest part of the cornea.

Corneal Index of Refraction

The average index is $1.376$.

Corneal Composition

Consists of $78\%$ water and $22\%$ protein ($15\%$ Collagen, $5\%$ other proteins, $1\%$ GAGs, and $1\%$ salts).

Microvilli and Microplicae

Finger-like projections that extend into the tear film mucin to aid in tear film retention and stability.

Anterior Epithelium Turnover

The cellular turnover of cells from the basal layer to the surface takes $7$ days.

Bowman's Membrane

Also known as the Anterior Limiting Membrane; it provides resistance against infection and physical injury but does not regenerate once damaged.

Stroma (Substantia Propria)

The forward continuation of the sclera accounting for $90\%$ of total corneal thickness, consisting of approximately $200$ layers of collagen fibrils.

Dua Layer

A newly discovered ($2013$) fine layer in the cornea.

Descemet's Membrane

The Posterior Limiting Membrane, which is a structureless membrane produced by endothelial secretion and is capable of regeneration.

Hassall-Henle Warts

Represent a periodic thickening of the Descemet's membrane.

Polymegethism

An age-related or contact lens-induced change manifested by disorderly shaped endothelial cells.

Deturgescence

The state of partial dehydration of the cornea that directly preserves transparency.

Maurice Lattice Theory ($1960$)

States corneal transparency is explicitly due to the orderly arrangement of fibers.

Corneal Innervation

Supplied by the Trigeminal Nerve, with the apex having the greatest sensitivity.

Placido Disc

A qualitative instrument consisting of equally spaced, alternating black and white concentric rings used to assess regular or irregular corneal shape.

Axial or Sagittal Map

A topography format that measures the power of the cornea across the central $1$ to $2\,mm$.

Sebaceous Gland of Zeis

Secretes sebum to prevent dryness and brittleness of the eyelashes.

Meibomian Glands

Long sebaceous glands ($25\text{--}40$ in the upper lid, $20\text{--}30$ in the lower) responsible for the oily layer of tears and airtight eyelid closure.

Glands of Krause & Wolfring

Accessory glands located in the stroma of the conjunctival fornix that contribute to the aqueous layer of tears.

Blepharospasm

An involuntary, abnormal spacing or twitching condition causing repetitive closure of the eyelids.

Tear Film Lipid Layer

The outermost layer composed of waxy and cholesterol esters; source includes Meibomian glands, glands of Moll, and Zeis.

Tear Film Aqueous Layer

The middle layer, also called the "soldier of tears," containing inorganic salts, proteins, and antibacterial substances.

Tear Film Mucin Layer

The innermost layer derived from conjunctival Goblet cells; it is hydrophilic and aids in the adhesion of the aqueous layer to the hydrophobic ocular surface.

Cotton Thread Test

Also known as the Phenol Red Thread Test; evaluates aqueous tear deficiency over $15$ seconds with results less than $10\,mm$ indicating Dry Eye.

Tear Break-Up Time (TBUT)

A qualitative assessment measuring the stability of the tear film using fluorescein dye.

Tear Meniscus Height (TMH)

Estimates remaining tear volume; normal clinical range is $0.20\,mm$ to $0.35\,mm$.

Prokera Lens

A biological bandage containing a piece of cryopreserved amniotic membrane secured inside a flexible polycarbonate ring.

Optic Zone Diameter (OZD)

The diameter of the central posterior region of a contact lens containing the primary refractive prescription.

LARS Rule

Adjustment rule for toric lens rotation: Left = Add, Right = Subtract (relative to the examiner's perspective).

Wettability

A surface property describing the tendency of a liquid to spread onto a solid surface.

Oxygen Permeability (Dk)

The degree to which oxygen can pass through a material; Daily Wear requires $24.1\,Dk$ and Extended Wear $87.1\,Dk$.

Silicone Hydrogel Lenses

Often called "breathable lenses"; they allow significantly more oxygen to pass through than regular hydrogel contacts.

FDA Group IV Lenses

Lenses characterized by High-water content ($>50\%$) and Ionic polymers.

Bowl Test

A method to verify soft contact lens orientation where a correct lens takes the shape of a perfect bowl and an inverted lens flares outward.

Lid Push-Up Test

A test used to evaluate soft contact lens movement (e.g., fast movement indicates a loose fit).

Prism Ballast

A toric lens stabilization technique involving the incorporation of $1$ to $1.5$ base-down prism at the inferior portion of the lens.

Dynamic Stabilization

Also known as double slab-off or thin zones; stabilization achieved by placing thin zones at the top and bottom of the lens.

Front Surface Toric (FST)

A lens design where the cylinder power is located on the front surface; indicated when refractive astigmatism is higher than corneal astigmatism.

Thimerosal

A mercurial-base antimicrobial agent effective against both bacteria and fungus.

BAK (Benzalkonium Chloride)

An antimicrobial agent restricted for use in hard contact lens solutions only.

Jelly Bumps (Lens Calculi)

Deposits composed of calcium, lipids, and mucoprotein, appearing as mulberry-like bumps most common on FDA Group 4 lenses.

Monovision CL

A presbyopic correction method using a dominant eye-distance prescription and a non-dominant eye-near prescription.

Radiuscope

An instrument based on Drysdale's principle used to verify the curvature (base curve) of a contact lens.

Myoflange

A lenticular flange modification with increased edge thickness to help lift low minus or most plus lenses that drop inferiorly.

SAM-FOS Rule

Clinical tear lens matching rules: Steep Add Minus (SAM) and Flat Add Plus (FOS).

BVP of a Fluid Lens ($BVP_{fl}$)

Calculated as $F_1 + F_2$, where $F_1 = \frac{1000(n - 1)}{r_2}$ and $F_2 = \frac{1000(n - 1)}{-r_c}$.