CORNEAL TOPOGRAPHY

HVID RANGE AND AVERAGE

10 - 14 mm average: 11.7 mm

VVID RANGE AND AVERAGE

average: 10.6 mm <HVID by 0.5 - 1.0 mm

CORNEAL THICKNESS Clinical studies and Gullstrands eye

Clinical Studies: 0.50 – 0.65 mm

Gullstrand’s Eye No. 1: 0.50 mm

Used to measure corneal thickness

pachometer

2 types of pachometers

• ultrasonic

• optical

Epithelium refractive index

1.401

Corneal Substance Refractive index

1.376

Anterior stroma RI

1.380

Posterior Stroma RI

1.373

During early life what kind of astigmatism does a person have

WTR

Limbal topography influences SCL fitting

TRUE

What is used to assess for limbal topography

• Slit lamp

• Placido disc

What is the deviation of peripheral curvature from the apical curvature

asphericity

The point of maximum curvature or shortest radius

CORNEAL APEX

Revolution of a conic about an axis of symmetry

CONICOID

Degree of peripheral asphericity

ECCENTRICITY

ECCENTRICITY VALUE OF THE HUMAN CORNEA Give the range and average

range: 4.41 - 0.58

average: 0.47

What are the regions of corneal asphericity

• central region or corneal cap

• mid-peripheral region

• peripheral region

identify the following letters

A. Apical zone

B. Transitional zone

C. Visual centre

D. Apical centre

E. Geometric centre

G. Keratometric limbus

F. Limiting margin

Region of greater flattening

Evidence of negative asphericity

Mid-peripheral region

Diameter of corneal cap

4mm approx

• 90% of semi-meridians had positive asphericity

• Remainder - zero or negative

• Nasal and superior-nasal asphericities were greater than the other semi- meridians

Peripheral regions

Give the method of measurement for corneal topography in optical

• reflection

• optical profiling

• interferometry/Mo

Give the method of measurement for corneal topography in contact methods

• casting and molding

• ultrasound

• trial contact lenses

This method of corneal topography measurement is

• Non-contact/non-invasive

• Rapid data collection

• Most convenient

• Output in readily used form

OPTICAL REFLECTION

Give example for optical reflection

keratometer

This method of corneal topography measurement is

• Non-contact/non-invasive

• Can be rapid data collection

• High accuracy possible

• Only one meridian per scan

• Still a technical challenge but could surpass keratometry

Optical profiling 2D

Give example for Optical Profiling

Laser tomographers

This method of corneal topography measurement is

• Some are non-contact

• Can be rapid data collection

• Very high accuracy possible

• 3D scan (single scan only)

• Some technical difficulties

OPTICAL PROFILING (3D)

Give example of optical profiling 3d

Laser interferometers

This method of measurement is:

• Contact may distort topography

• Time consuming

• On some methods only qualitative result possible

• Data can be difficult to interpret

contact method

Give example for contact method

• casting/moulding

• ultrasound

• trial contact lens

give instruments to measure corneal topography

• Placido disc

• Photokeratoscope

• Keratometer

• Computer-assisted Topographic analysis

This instrument has:

• Wide field of view

• More complete mire locus

• Qualitative

PHOTOKERATOSCOPE/ PLACIDO DISK

This instrument has the main function of measuring radius of curvature of the optic cap giving the information about front surface corneal radii and total corneal power

keratometer

Give the 4 types of doubling systems

• Fixed doubling

• Variable doubling

• Divided doubling

• Full doubling

In this doubling system there is image size and mire seperation as well as a fixed object height and doubling device distance

Fixed Doubling

In this doubling system there is a distance in doubling device, there is also a fixed mire seperation and image size

variable doubling

This doubling system has:

Full-aperture:

• beam-splitting device

Full doubling

Types of keratometer

• 2-position

• 1-position

This type of keratometer specifies in rotation about the axis to measure second meridian

2-POSITION KERATOMETERS

This type of keratometer specifies in simultaneous measurements of

both meridians

1-POSITION KERATOMETERS

Give the parametric descriptors for topographic analysis systems

hint: Sim K, SAI, SRI

• Simulated keratometry value (Sim K)

• Surface asymmetry index (SAI)

• Surface regularity index (SRI)

GIVE THE OCULAR PARAMETER MEASUREMENTS IN CONTACT LENS FITTING

• Corneal diameter

• Pupil diameter

• Palpebral aperture and lid position

• Keratometry

• Photokeratoscopy

This abnormal corneal topography has non-inflammatory thinning and

protrusion and cause disruption of mires

Keratoconus

This abnormal corneal topography is generalized enlargement of the cornea

KERATOGLOBUS

What changes can contact lens wear cause

• curvature/power

• refractive index

• thickness

PRINCIPAL RADII OF CURVATURE

What are the types of CONIC SURFACES hint* theres 2

• Sagittal radius

• Tangential radius

The normal perpendicular to the tangential radius extending from point P on the conic surface to its intersection with the optic axis

SAGITTAL RADIUS

The normal perpendicular to the sagittal radius extending from point P on the conic surface to the tangential point on the evolute

TANGENTIAL RADIUS