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Scleral lens glossary

tomarnoldSAG  – sagittal height.  Used to measure overall height of the cornea from a given plane (chord diameter).  Also used to describe the depth of the well of a scleral contact.  Units usually expressed in microns (um) or millimeters (mm).    e.g., 4800 um = 4.800 mm 

Not the same as the base curve (radius of curvature) of the lens.   A lens may be manufactured with various sagittal depths without changing the base curve.   As in Zenlens.   If you do not change the base curve, there is no need to alter the prescription or power of the lens.

BC – base curve (millimeters)

OAD – overall diameter of the lens

HVID – horizontal visible iris diameter.   Used to determine optimal lens diameter

Limbal/ intermediate curves (may have 3-4).  Curvatures of different radii from the BC in order to align the lens with the peripheral cornea. 

Smart curve (Zenlens) – an intermediate curve that is used to adjust the limbal curves and the base curve independently from one another

APS – advanced peripheral system (Zenlens). Proprietary term for the Zenlens extra- wide landing zone.

Landing zone – the most peripheral curve on the scleral lens that actually rests on the sclera (conjunctiva)

Toeing – when the edge of the lens (landing) digs into the conjunctiva

Heeling – when the edge of the lens lifts up from the conjunctiva because the edge is too flat

Conjunctival prolapse – when the limbal conjunctiva is drawn up under the lens over the cornea. Usually the result of excessive limbal clearance of the lens.   May be red (hyperemic) and lead to long term complications as a result of sticking to the cornea.

“Burned” lens – surface of the lens will not wet as the result of the manufacturing process.  Very hydrophobic (“water repellent”).  Either the lens was polished at too high a speed (higher temperature) or an abrasive polishing compound.  Lens must be replaced.

Midday fogging – may occur in up to 50 % of wearers.  Actual mechanism unknown.  The PLTL becomes milky/foggy with tear debris. Studies point to excessive proteins, mucus and goblet cells in tears.   Do scleral lenses stimulate this?  More studies underway.

Addipaks – 0.9% sodium equivalent sterile saline used in nebulisers (C-PAP) as a humidifier.   Very commonly used in scleral lenses as it has no preservative or chemicals of any kind.

LacriPure – same thing as Addipaks but now approved by FDA.  Made by Menicon.

Progent – biweekly or monthly deep cleaner for sclerals. Thirty-minute cleaning cycle.  Must be followed by overnight soaking in disinfecting solution.

Elevation map (topography) – compares corneal curvature to a perfect sphere of a known radius.  Areas lying above the sphere shown in red; below the sphere shown in blue.  Useful in lens design.

KCN – keratoconus

PMD – pellucid marginal degeneration

PKP (PK) – penetrating keratoplasty.   Full thickness corneal graft from donor cornea.  Used in very advanced KCN, PMD and Fuch’s dystrophy (degeneration of the endothelium of the cornea)

CXL – corneal cross-linking.  Riboflavin applied to cornea and then irradiated with UV light for a number of minutes.  Forms bonds within the cornea to strengthen and slow or halt the progression of keratoconus.

DSEK – Descement’s Stripping Endothelial Keratoplasty.   Removal and replacement of the endothelial (inner ) layer of the cornea as another treatment option for Fuch’s dystrophy

Toric (toricity) – two curves, 90 degrees apart, that are unequal in radius

Haptic – another term for the landing zone or most peripheral curve of the lens.  From a Greek word meaning “to grasp”.

MF – multifocal

“Centre- near” – MF lens with the reading portion in the centre of the lens

“Centre-distance” –  distance portion in the centre of the lens

Aspheric – not defined by a single curvature but a continuous arc of curves.  Think ski slope or water slide.  The cornea is aspheric as it continually flattens from the center to the limbus.

Literally, “without a sphere”.   “a” = without

AT – artificial tears

Surfactant – “soap”  – used to describe the cleaning solutions applied to the lens.

OD – right eye

OS – left eye

OU – both eyes

TX – treatment

SX – surgery

Phaco – phacoemulsification – “cataract surgery”

IOL – intra ocular lens.  Replaces the natural lens from cataract