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OV2006 contact lenses > principles of rigid CL fitting > Flashcards

principles of rigid CL fitting Flashcards

1
Q

what do rigid CLs provide ?

A

. provide excellent visual acuity and contrast sensitivity
. more effective in correction of high corneal astigmatism
. better for patient with dry eyes
. lower number of serious CLs complication

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2
Q

why are only 10% of CL wearers fitted with rigid CLs?

A

. due to the initial discomfort and adaptation

. due to the time required to successfully fit and manage rigid CLs

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3
Q

what are the aims of RGP fitting ?

A

. good visual performance
. minimal ocular response
. optimised comfort

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4
Q

how is good visual performance of RGP fitting achieved?

A

. achieved by alignment of back surface of the CL lens with the cornea
. the curvature of the back of the CL has approximately the same curvature as the cornea

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5
Q

how is minimal ocular response of RGP fitting achieved?

A

. achieved by tears interchange helped by lens movement
. tears interchange= the amount of tears that sit between the lens and the cornea - with every time the px blinks is exchanged with new tears
-helped with lens movement

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6
Q

how is optimised comfort of RGP fitting achieved?

A

. achieved by weight distribution

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7
Q

what are we trying to achieve with rigid lenses?

A

. reasonable centration- the same distance between the limbus and the lens edge

. adequate movement- when px moves , the lens moves up and back down

. alignment or minimal clearance centrally- space between the back surface of the lens and the cornea

. acceptable edge clearance- where there are a lot of tears- EDGE ALWAYS STANDS OFF - edge width

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8
Q

what are the forces involved to keep the rigid CLs on the eye ?

A

. eye lid force
. frictional forces - the viscosity of the tear film maintains the lens in a stationary position
. capillary attraction - the ability of a liquid to flow in narrow spaces without the assistance of and in opposition to external forces like gravity
- the closer the lens matches the shape of the cornea, the greater capillary attraction

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9
Q

which lenses give better centration?

A

minus lenses and very steep lenses give better centration

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10
Q

what is the behaviour of lenses dependent on ?

A

. depends on their specific gravity

. lenses with increased specific gravity have more weight

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11
Q

how to improve centration of a lens that drops?

A

. to improve centration of a lens that drops because gravity is greater than fluid forces, choose lower SG for second trial lens

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12
Q

what are the materials of RGP

A

. low Dk<50
. medium Dk:50-100
. high Dk: > 100

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13
Q

what does high DK/t mean ?

A

. allows more oxygen to eye- improves health of eye , see less corneal staining over time
. behave less stable on the cornea
. lens material breaks often

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14
Q

what are the different designs of RGP?

A

. bicurve C2, Tricurve C3, multicurve

. aspherical

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15
Q

what are the advantages of multicurve lenses?

A

. you can change any curve to change the fit of lens
. allows you a much greater control in designing the lens
. e.g. you can change the optical area of the lens because the pupils are too big
. you can change edge lift

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16
Q

what can you change is aspherical lenses ?

A

. you can only change the backup zone radius(BOZR) or total diameter of lens(TD)

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17
Q

what do aspheric lenses show ?

A

. aspheric lenses show a closer alignment fitting relationship between back surface of lens and cornea

  • which is what we want
  • however if too close alignment- there is hardly any room for tears therefore causes the lens to bind and makes it impossible for the lens to be removed from the eye
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18
Q

what fitting information is required for RGP fitting?

A

. refraction ( spectacle Rx/BVD)
. corneal integrity ( slit lamp/fluorescein )
. keratometry
. horizontal visible iris diameter (HVID)
. vertical palpebral aperture (VPA)
. pupil size
. corneal staining with fluorescein - draw where you can see corneal staining

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19
Q

what to fill up in patient record in the preliminary investigation ?

A

. name, date and eye
. spectacle Rx(< 1 year old) and VA
. convert to ocular Rx using BVD
.Ks from keratometry and calculate CA (corneal astigmatism )
. Lids ( draw position of lid ) - when px looks straight ahead

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20
Q

what is horizontal visible iris diameter ?

A 
. needed for : total diameter CL
. true corneal diameter is much larger
. measure with PD rule 
. measure from temporal side to nasal side 
. expected average = 11.6mm
- range between 10.2 and 13.0 mm
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21
Q

how to take an automated measurement of horizontal visible iris diameter ?

A

. use a oculus topographer or medmont topographer

. measure corneal diameter along the 45 and 135 meridian from black to black limbal margins

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22
Q

what is vertical palpebral aperture >

A

. distance from superior eyelid all the way to the inferior eyelid
. needed for :
- total diameter CL
- explain behaviour of CL
. narrow palpebral aperture - increase difficulty with insertion
. large palpebral aperture - associated with incomplete blinking and signs of corneal desiccation
. average 9.0-10.5mm

23
Q

when to measure pupil size ?

A

measure under

  1. average illumination (ambient cubicle white light)
  2. low illumination ( dim light /burton lamp blue light )
24
Q

what to expect when measuring pupil size ?

A

. photopic between 2-5mm

. mesopic between 4.5-9mm

25
Q

what to do when recording corneal staining ?

A

. grade of staining

. depth of staining

26
Q

what is a property of spherical rigid corneal lens?

A

. normally the radii flattens towards the edge of the lens ( like the cornea)

  • 7.65mm centre
  • 8.40mm mid-periphery
  • 9.50mm periphery
27
Q

explain the radii C3 design?

A

. BOZR- back optic zone radius

. BPR1 - 1st back peripheral radius

. BPR2-2nd back peripheral radius

. FOZR- front optic zone radius

. FPR1- 1st front peripheral radius

28
Q

what happens if a patients has pupils that are bigger than the back optic zone diameter ?

A

. complain of visual problems , as they are looking through a peripheral curve which doesn’t have the correct curvature to support the prescription

29
Q

what are the 2 parameters that you order with the manufacturer?

A

. BOZR - back optic zone radius

. total diameter

30
Q

what are the specification for spherical RGP lens ?

A

BOZR:BZOD/
. BOZD is the diameter that goes with the radius in central 2/3 of CLs

BPR1:BPD1/
. 1st peripheral curve and its diameter

BPR2:TD
. 2nd peripheral curve and its diameter

31
Q

what is the geometry of spherical RGP lens ?

A

. BPR1 ( first curve ) is at least 0.60mm flatter than BOZR
. for C3, last curve is usually 10.50mm with a width of 0.50 to 1.00m
. aids removal of lens
. encourages tear exchange

32
Q

what is the geometry of aspherical rigid corneal lens ?

A

. have closer alignment fitting
. aspheric posterior design incorporates
. A (a)spherical optic zone
. An innovative progressively flattening aspheric periphery
. A uniform edge thickness design

33
Q

what is the specification of aspheric contact lenses ?

A

. BOZR

. TD

34
Q

how do we start rigid lens fitting ?

A

. we need to choose the BOZR
. trial lens based on keratometry readings
. fit on the flattest K

35
Q

what is the rule of thumb for BOZR choice ?

A

. fit on flattest k
1- if corneal astigmatism <1.00DC - fit on flattest k
2- if corneal astigmatism 1.00- 2.50DC - pick BOZR that sits between two meridians , so go steeper than the flattest K
0.05-0.1mm steeper than flattest
3->2.50DC - use toric lens

36
Q

how is the fluorescein pattern linked to the BOZR that you picked ?

A

. the bigger the difference between the keratometry readings , the more obvious fluorescein pattern differ in one direction compared to other

37
Q

which corneal K to order if

  1. 80 along 180
  2. 75 along 90
A

. order 7.80mm (flattest)

38
Q

when should you describe fluorescein pattern?

A
  • you should describe fluorescein pattern when the lens sits in the centre of the cornea
39
Q

what to expect if you order a lens of 7.70 mm which is steeper than corneal Ks
.7.80 along 180
. 7.75 along 90?

A
  • the fluorescein pattern we expect to find is steep
  • fluorescein in the centre will be bright and you have band around it where the mid peripheral touch
  • the edge is normally thinner
40
Q

why do we decide to order flatter lens ?

A
  • we don’t want a steep lens because steep lenses don’t tend to move very much unless the patient has a lot of tears in the eye
  • you don’t get enough tear exchange behind the contact lens
  • pool of tears doesn’t get refreshed with every blink
41
Q

how to make a change to a steep lens ?

A
  • smallest change to make clinically significant impact:0.1 mm. if you see air bubble in centre:0.3mm flatter
42
Q

what happens if we order a lens of 7.90mm which is flatter than corneal Ks

  • 7.80 along 180
  • 7.75 along 90?
A

. fluorescein pattern shows flat
. the centre is now touching the cornea and edge is standing off
. less fluorescein in the middle

43
Q

how to make a change to a flat lens ?

A
  • order steeper

- smallest change to make clinically significant impact: 0.1 mm. if you see air bubble in periphery: 0.3mm steeper

44
Q

what to do we need to know about the BOZD ( back optic zone diameter )?

A
  • we don’t need to tell the manufacturer at the beginning
  • should be 1.50mm larger than pupil size
  • small: <7.30 mm
  • medium: 7.30-7.90mm
  • large: >7.90 mm
  • large BOZD minimises flare/haloes
45
Q

what happens when you change BOZD?

A

. change in BOZD alters the fluorescein pattern

46
Q

what is the relationship between BOZR and BOZD?

A
  • 0.5mm change in BOZD needs a 0.05mm change in BOZR to keep the same fluorescein pattern
  • this means that if you are not happy with BOZD and BOZR, you need to change the BOZR twice
- example 
. trial lens BOZR=8.0mm and BOZD=7.50mm
. need BOZD of 8.00mm 
.  increasing the BOZD by 0.50 mm, you create a 'steeper' fit centrally (i.e. more pooling) so you need to order the new lens flatter by 0.05mm to keep the same fluor pattern 
. order 8.05 BOZR ( flatter)
47
Q

how to order BOZR and BOZD ?

A

. BOZD 1 step (0.50 mm) larger = BOZR 1 step (0.05 mm) flatter

. BOZD 1 step (0.50mm) smaller= BOZR 1 step (0.05 mm) steeper

48
Q

what is total diameter (TD) dependent on ?

A 
. dependent on lid position and HVID
. smaller TD causes more movement 
. choose 1.5 to 2mm smaller than the HVID
- small <9.20mm
- medium 9.20-9.80
- large> 9.80 mm
49
Q

what is the relationship between BOZR AND TD?

A
  1. lens BOZR 7.80mm and TD 9.30mm
  2. increasing the TD by 0.50mm, you create a ‘steeper’ fit centrally (i.e. more pooling) so you need to order the lens flatter by 0.05mm to keep the same fluorescein pattern
  3. order: 7.85mm TD 9.80mm
50
Q

how to order BOZR and TD?

A
  1. make TD 1 step bigger (0.50 mm) = BOZR 1 step (0.05 mm ) flatter
  2. make TD 1 step smaller (0.50 mm ) = BOZR 1 step ( 0.05 mm ) steeper
51
Q

what is the BOZR ?

A
  • back optic zone radius - the curvature of the back of the contact lens
  • based on flattest K reading
  • unless large corneal astigmatism ( toric lens )
52
Q

why do we change BOZR?

A

. BOZR is changed to create an aligned ( or ever so slightly steep ) fluorescein pattern

  • too steep: little/no movement : not enough tear exchange behind the lens: increase BOZR (flatter)
  • too flat: lens moves too much: reduce BOZR ( steeper)
53
Q

how is the BOZR related to the BOZD ?

A

. if px experiences haloes, the BOZD may be too small. increase BOZD causes a steeper fit
. management : increase BOZR to keep same FP

54
Q

how is BOZR related to TD?

A

. if the lens is moving outside limbus on versions, the lens may be too large. Reduced TD causes a flatter fit
. management: if the TD is too large, reduce BOZR to keep same FP

OV2006 contact lenses (11 decks)

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