W5 Rigid Lenses Fitting and Measurement

What is the process of rigid CL fitting?

• Px selection

• Preliminary measurements

• Diagnostic CL’s

• Diagnostic CL’s selection

• Fitting assessment

• CL ordering

What rule should be followed when choosing TD?

• TD should be 2mm smaller than HVID

• Usually in range 8.50-10.50mm

What size is BOZD?

• 1mm or larger than pupil in average illumination

• Narrow peripheral curve width hard to measure

What do ill-fitting corneal lens cause?

• Alters corneal integrity

• Changes corneal physiology

• Distorts corneal surface

• Alters corneal curvature

• Changes eye refractive status

What are optimum characteristics for a patient?

• Highly motivated- will adapt to CL quicker

• Moderate to high rx- desire for an alternative to glasses is higher

• Corneal toricity- px with significant against the rule corneal toricity have less success with spherical CL due to CL decentration and residual astigmatism

• Myopic control- GP CL have been shown to stabilise myopic rx

What preliminary measurements are taken?

• Keratometry reading

• Corneal diameter (HVID)

• Lid characteristics

• Pupil size

• Spectacle rx

• Ocular health

What’s the TD and HVID?

CL total diameter will be 2.3mm-2.5mm less than horizontal visible iris diameter

How do we assess lid characteristics?

• Position of upper and lower lids

• Positions where the lid margins cross the upper & lower limbus

• Inter-palpebral aperture

How do we measure pupil size?

• Size of pupil in both bright and dim illumination should be measured

• Dilated pupil size is important for selection of appropriate BOZD

• If optical zone is too small, visual disturbances can occur

Which factors dictate the suitability of the chosen BOZR?

• Corneal toricity

• BOZD

• Dynamic fitting characteristics

Do we use a smaller or larger BOZD?

Smaller BOZD for steeper corneas

Larger BOZD for flatter corneas

Whats the shape of plus power CL’s?

0.5mm larger than average to help maintain good centration- CL must be produced with minimum thickness where possible to reduce unnecessary weight

What happens to a CL with a high BVP?

• > +8.00DS CL’s are made larger than average to allow for adequate lenticulation of the front peripheral design

Whats the rule with BOZR regarding cyl power?

• Less than 0.50DC- 0.05mm steeper than flattest K

• 0.50 to 1.00DC- 0.05mm to 0.1mm steeper than flattest K

• 1.00DC to 2.00DC- 1/3 difference steeper than flattest K

• Bigger than 2.00DC- toroidal

What is lens settling time?

Time taken for watering to subside- perhaps 10 mins

How do we grade comfort?

1 is incredibly uncomfortable, 10 is very comfortable

How is coverage graded?

• From -2 to +2

• If lens is 1mm smaller than HVID then coverage is 0

• If lens is between 0.5mm smaller than HVID then coverage is +1

• If lens is the size of the limbus then coverage is +2

• If lens is 1.5mm smaller than HVID then coverage is -1

• Is lens is 2mm smaller than or greater than HVID then coverage is -2

What is centration?

• If lens edges are contained within the limbus and neither edge is touching limbus ,this is C

• If lens crosses the pupil in dim light conditions, this is P

• If lens crosses the limbus, this is L

What falls under dynamic fit?

Position and movement

How do we assess movement?

• Reduce slit size (2-12mm)

• Observe lens movement when px blinks

What is the movement inter-blink?

• If lens moves more than 2mm, this is in excess and we write +2

• If lens moves 1.6mm-2mm, this is +1

• If lens moves 1mm-1.5mm, this is optimal and we write 0

• If lens moves 0.5mm-1mm, we write -1

• If lens moves less than 0.5mm, we write -2

What do central zone gradings mean?

• -2: no fluorescein bearing, no tear film, very dark zone and background tissue very visible

• -1: faint fluorescein pooling

• 0: optimal- medium fluorescein pooling

• +1: dark fluorescein pooling

• +2: significant fluorescein pooling, background tissue not visible

What is the grading for edge zone or clearance?

• +2: band is bigger than 2mm

• +1: roughly 1.5mm band

• 0: roughly 1mm band

• -1: 0.5mm band (optimal?)

• -2: no visible band

Why might alterations to the edge profile be done?

• Producing an edge with a well-rounded, centrally, or posteriorly located apex for maximum comfort

• A thin edge causes less CL-lid interaction, increasing comfort

What happens if the CL is made thicker?

• CL removal is more difficult as there’s reduced tear meniscus as TD reduces

Whats the value of BOZR?

If cyl is more than 2 it is toroidal. If cyl is less than 2.00DC it’s a 1/3 difference steeper than flattest K