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Investigations > Biometry and pentacam > Flashcards

Biometry and pentacam Flashcards

1
Q

biometry

A

 To predict post-operative refraction for a particular IOL implant
 NB: the formulae only predict the post-operative spherical equivalent

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2
Q

partial coherence laser interferometry

A

superseding ultrasound biometry
 Greater accuracy
 Greater reproducibility
 Measures true foveal axial length

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3
Q

principles of zeiss IOL master

A

(partial coherence, non-contact biometry)
 Two coaxial laser beams that are partially coherent
 An interference pattern is produced
 This pattern is used to interpret measurements

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4
Q

axial length

A

 Significant disparity between the two eyes should be noted (ie. greater than
0.2mm difference)
 Aphakia, pseudophakia and silicone oil can all affect the readings generated
 Hoffer Q tends to be used for shorter eyes

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5
Q

average kertometry

A

the mean of the powers of the highest and lowest curvatures
 If astigmatism is very irregular, corneal topography must be studied
preoperatively
 Patients with previous keratorefractive surgery pose a challenge and biometry is
much less inaccurate. Haigis-L formula is used for such patients

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6
Q

a constant

A

 Correction factor applied to each specific lens type to achieve accuracy
 Reflects position within the eye ie. AC lenses have a different (lower) A
constant to PC lense

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7
Q

lens in PC rupture

A

the more posterior a lens is placed, the stronger it needs to be. Hence if
following PC rupture an AC lens is to be placed, it will need to be weaker
than the planned PC lens. A PC lens in the sulus can produce myopic shift if
it was intended to be within the bag since it will be slightly too strong for this
more anterior position

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8
Q

axial length <22

A

haigis, hoffer Q

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9
Q

axial length 22-26

A

SRK/T barrettsa

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10
Q

axial length >26

A

haigis, SRK/T

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11
Q

previous refractive surgery

A

haigis L

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12
Q

CL wear and biometry

A

soft - remove 1 week
rigid - 2-4 weeks

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13
Q

when to consider corneal topography

A

irregular cornea
significant astigmatism
previous corneal surgery

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14
Q

pentacam K1

A

3mm zone flattest meridian in diopters

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15
Q

pentacam k2

A

3mm zone steepest meridian in diopters.

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16
Q

pentacam kM

A

3mm mean power in diopters

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17
Q

pentacam rH

A

3mm zone horizontal curvature (in mm)

18
Q

pentacam Rv

A

3mm zone vertical curvature (in mm)

19
Q

pentacam Rm

A

: 3mm zone mean curvature (in mm)

20
Q

pentacam Qs

A

Quality specification. Must show OK. If not, then the quality of the scan is compromised

21
Q

pentacam astig

A

3mm zone anterior astigmatism

22
Q

pentacam axis

A

3mm axis of the corneal astigmatism

23
Q

pentacam k-max

A

This is the maximal K reading. Should be measured using the tangential map. Its location tells you the apex of the cone.

24
Q

pentacam q-val

A

Tells you about corneal shape (normally based on 6mm zone)
o Normal value between -1 and 0
o Positive > 0 = Oblate cornea
o Negative < -1 = Prolate (keratoconus)

25
Q

pentacam progression index

A

assess change in corneal thickness of the whole cornea.
o Useful for assessing ectatic disease
o > 1.1 suggests ectatic disease

26
Q

pentacam sagittal curvature maps

A
  • Most adults have with the rule (WTR) astigmatism (vertical axis is more steep than horizontal).
  • This can be seen on the sagittal curvature map as a vertical symmetric bow tie.
  • If the symmetric bow-tie is horizontal then it is against the rule astigmatism (ATR).
  • Astigmatism in either eye is usually a mirror image: enantiomorphism.
27
Q

pentacam sagittal curvature maps - superior steepening

A

Hotspot of steepening in the superior cornea

28
Q

pentacam sagittal curvature maps - inferior steepening

A

Hotspot of steepening on the inferior cornea

29
Q

pentacam sagittal curvature maps - irregular

A

No regular pattern

30
Q

pentacam sagittal curvature maps - SB/SRAX

A

symmetric bow tie but the lobes are angulated.

31
Q

pentacam sagittal curvature maps - asymmetric bow-tie / inferior steepening

A

The inferior lobe is bigger than the superior one. Considered significant if the inferior lobe is 1.5D steeper than superior lobe at the 5mm zone.

32
Q

pentacam sagittal curvature maps - asymmetric bow-tie / superior steepening

A

Superior lobe is bigger that inferior lobe. If the superior lobes is 2.5D steeper than the inferior lobe at the 5mm zone then this is significant.

33
Q

pentacam sagittal curvature maps - claw

A

Like the butterfly but inferior ends of the lobes are connected.

34
Q

pentacam sagittal curvature maps - butterfly

A

Horizonal bow-tie but the lobes are spread out like wings.

35
Q

pentacam - thickness maps

A
  • Important for ectasias i.e. keratoconus and PMD.
  • Useful for Fuch’s endothelial dystrophy
  • Helps determine the need for cross-linking
36
Q

pentacam thickness maps - diffuse display

A

Shows a range of corneal thicknesses across the cornea.

37
Q

pentacam thickness maps - five values display

A

One central thickness, and 4 at the 5mm circle

38
Q

pentacam thickness maps - main elements

A

central (based on pupil center), apex (based on the center of the cornea) and thinnest. The apex is the basis of the x and y co-ordinates. Therefore the central and thinnest positions are in relation to this.

39
Q

red flags for ectasia

A

All 5mm zone
o Superior and inferior thickness difference greater than 30 um.
o A difference in thinnest value between both eyes of more than 30um.
o A difference between the apex thickness and thinnest location of more than 10um.
o Posterior elevation map > 15um, or Anterior elevation > 12um

40
Q

features of early / occult keratoconus

A
  • Kmax =≥ 47D
  • Kmax difference between both eyes is > 2D
41
Q

Features of keratoconus progression (based on 1 year)

A

o Kmax change =≥1D
o Topographical astigmatism change =≥1D
o Corneal thickness change >30um
o Remember keratoconus can present in nearly all patterns

Investigations (8 decks)

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