Biometry and pentacam Flashcards
biometry
To predict post-operative refraction for a particular IOL implant
NB: the formulae only predict the post-operative spherical equivalent
partial coherence laser interferometry
superseding ultrasound biometry
Greater accuracy
Greater reproducibility
Measures true foveal axial length
principles of zeiss IOL master
(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
axial length
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
average kertometry
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
a constant
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
lens in PC rupture
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
axial length <22
haigis, hoffer Q
axial length 22-26
SRK/T barrettsa
axial length >26
haigis, SRK/T
previous refractive surgery
haigis L
CL wear and biometry
soft - remove 1 week
rigid - 2-4 weeks
when to consider corneal topography
irregular cornea
significant astigmatism
previous corneal surgery
pentacam K1
3mm zone flattest meridian in diopters
pentacam k2
3mm zone steepest meridian in diopters.
pentacam kM
3mm mean power in diopters
pentacam rH
3mm zone horizontal curvature (in mm)
pentacam Rv
3mm zone vertical curvature (in mm)
pentacam Rm
: 3mm zone mean curvature (in mm)
pentacam Qs
Quality specification. Must show OK. If not, then the quality of the scan is compromised
pentacam astig
3mm zone anterior astigmatism
pentacam axis
3mm axis of the corneal astigmatism
pentacam k-max
This is the maximal K reading. Should be measured using the tangential map. Its location tells you the apex of the cone.
pentacam q-val
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)
pentacam progression index
assess change in corneal thickness of the whole cornea.
o Useful for assessing ectatic disease
o > 1.1 suggests ectatic disease
pentacam sagittal curvature maps
- 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.
pentacam sagittal curvature maps - superior steepening
Hotspot of steepening in the superior cornea
pentacam sagittal curvature maps - inferior steepening
Hotspot of steepening on the inferior cornea
pentacam sagittal curvature maps - irregular
No regular pattern
pentacam sagittal curvature maps - SB/SRAX
symmetric bow tie but the lobes are angulated.
pentacam sagittal curvature maps - asymmetric bow-tie / inferior steepening
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.
pentacam sagittal curvature maps - asymmetric bow-tie / superior steepening
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.
pentacam sagittal curvature maps - claw
Like the butterfly but inferior ends of the lobes are connected.
pentacam sagittal curvature maps - butterfly
Horizonal bow-tie but the lobes are spread out like wings.
pentacam - thickness maps
- Important for ectasias i.e. keratoconus and PMD.
- Useful for Fuch’s endothelial dystrophy
- Helps determine the need for cross-linking
pentacam thickness maps - diffuse display
Shows a range of corneal thicknesses across the cornea.
pentacam thickness maps - five values display
One central thickness, and 4 at the 5mm circle
pentacam thickness maps - main elements
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.
red flags for ectasia
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
features of early / occult keratoconus
- Kmax =≥ 47D
- Kmax difference between both eyes is > 2D
Features of keratoconus progression (based on 1 year)
o Kmax change =≥1D
o Topographical astigmatism change =≥1D
o Corneal thickness change >30um
o Remember keratoconus can present in nearly all patterns
