Why does the Pentacam® not include Placido topography?
Placido topography is a technology based on the reflection of Placido discs on the tear film of the eye. By definition it therefore requires a stable tear film. Furthermore Placido topography requires a central camera which automatically eliminates central Placido discs to be projected on the cornea, resulting in the central (most important area for measurement) to be extrapolated and not actually measured. Placido topography can only give information about the anterior cornea.
Scheimpflug technology is essentially: Scheimpflug photography – accurately taking images of the anterior section of the eye. This is not being influenced by the tear film, central data is measured and not extrapolated, and data from anterior and posterior cornea calculated. The inclusion of Placido technology is not needed to enhance the Pentacam® measurement. Clinical studies have proven this concept.
Is there any information explaining measured data on the different displays of the Pentacam®?
Yes, the Pentacam® manual is summarized in the software. In every display yellow triangles or lines can be found next to all measured data. These are called the "Help topics". By clicking on these yellow triangles or lines a summary of the measurement and also normal values (if available) will be given.
Is the Pentacam® an image device as well?
It's often overviewed that the Pentacam® can be used as an imaging device. Each Scheimpflug image taken during the measurement procedure can be viewed separately. The software allows for the operator to adjust the image manually. Conditions like Fuchs' Dystrophy, Pelucid Marginal Degeneration (PMD), Keratoconus, Pterygium progression, nuclear lens changes to name only a few can be detected inspecting the different Scheimpflug images.
When new software displays are released can my current Pentacam® display these?
Yes, always make sure your device is uploaded with the latest software. This will enable you to view the latest displays according to the purchased options of your device.
Does the Pentacam® compensate for eye movement during fixation?
A Pentacam® scan takes 2 seconds to complete. However, eye movement can still occur during the 2 seconds while fixating. Apart from the Scheimpflug camera a second camera called the pupil camera is included. The function of the latter camera is to detect eye movement during the measuring process. The software recognizes this and any eye movement is taken into consideration when calculating the 3D image of the anterior chamber.
What should I do if an examination has a poor Quality Specification (QS-value)?
The measurement should be deleted and repeated. Storing bad examination may lead to wrong interpretations.
When should a Pentacam® examination be performed?
Unless for specific reasons, a Pentacam® exam should be done first when performing diagnostic tests. No pupil dilation (except for certain lens examinations), no eye drops or any other ointments should be instilled prior to a Pentacam® examination.
C. Power Calculation
Does Pentacam® measure SimKs?
Yes, SimK values are derived from the sagittal map.
What are the differences between all the power maps displayed by the Pentacam®?
Different corneal power maps are displayed by the Pentacam®:
- Sagittal and Tangential Power map
- Refractive Power map
- True Net Power map (TNP)
- Total Corneal Refractive Power map (TCRP)
Pentacam® measures height (elevation) data. These are converted into curvature (mm) and then converted into power values (diopter).
Different factors are taken into consideration to calculate the different power maps of the Pentacam®:
- The refractive effect (Snell’s law of refraction)
- Inclusion of anterior/posterior surfaces
- Correct refractive indices used
- Location of principle planes (Ray tracing)
|Inclusion anterior/posterior surface
|Correct refractive indices used
|Refractive power map
Above table shows the factors used to calculate the different power maps.
Why is it necessary to have various different power maps?
Accurate corneal power is important specifically when calculating intraocular lens (IOL) power. Different IOL power calculating formulas are available today using different corneal power. The more traditional formulas still use power values derived from sagittal power maps, but during the calculation process the formula corrects for the use of the assumed refractive index. Newer generation formulas and ray traced formulas are more advanced and incorporate Ks from power maps such as TNP and TCRP.
Furthermore in the case of toric IOL implants, as well as post-refractive patients, newer generation formulas generate better post-op outcomes. Better Ks, better formulas lead to better results.
What are the differences between the references in the Power Distribution display to calculate K values?
Automated keratometry generates K values on the 3 mm ring centred on the pupil centre. The Pentacam® gives the clinician the option to have K values calculated for different maps using either the pupil centre or the apex as the centre. Once the option for the centre reference has been chosen, the area to be calculated can be chosen either as a ring around the centre, or in a zone (average of measurements in the area in the ring).
The Fast Screening Report summarizes data for each patient displaying normal and pathological results. What are these results based on?
The normative data of this display are based on published studies. These studies can be found by clicking the “Literature button” next to the results.
What is the difference between a Best-fit-Sphere (BFS) and an Enhanced Best-fit-Sphere (EBFS) as used in the Belin/Ambrósio Enhanced Ectasia Display (BAD)?
In the BAD all the curvature data for a specific diameter are used to calculate the radius of a reference surface (BFS) to evaluate height data – the goal is to detect suspicious elevation in the anterior and the posterior cornea. The EBFS uses curvature data for the same diameter as above, however it is excluding an area around the thinnest point (TP) of the cornea. In other words, the TP is detected; an area (usually 3.5 mm) around the TP is excluded in calculating the radius for the new reference surface. In a normal cornea the visual appearance of the maps does not differ between the BFS and the EBFS, but in an ectatic cornea where elevation (different curvature) will occur around the TP, excluding an area around the TP flattens the reference surface making it easier to visually detect early elevation.
What is ARTmax (Ambrósio Relational Thickness)?
The formula to calculate this parameter is as follows:
ARTmax = TP (thinnest point) / PPImax
The TP is detected by the software. Furthermore the progression from the TP towards the periphery of the cornea is calculated. The half meridian where the progression occurs the fastest from TP to periphery is PPImax.
Are all the parameters equally important in the BAD?
No, the most important parameter for early ectasia screening is the “Final D”. The other 5 parameters contribute to the “Final D” value, but none of the 5 should be used on its own as a screening parameter.
Why does BAD show a yellow “Final D” more often than the general population incidence of keratoconus?
It is important to note that BAD is an early ectasia risk screening tool often used to screen possible refractive candidates.
The incidence of yellow “Final D” is therefore higher than the occurrence of keratoconus.
When should the Hyperope option be activated in the BAD?
If the patient’s refraction is hyperopic - regardless of amount - the Hyperope option should be activated. The reason for this is that normative data for posterior elevation differs between myopes and hyperopes.
What is ABCD staging?
It is a staging (grading) display for keratoconus. In the 1950s the Amsler-Krumeich staging system for keratoconus was developed and is still frequently used today. However the Amsler-Krumeich only takes anterior curvature and central corneal thickness into consideration. The ABCD Keratoconus Staging display is a newly developed display, approved by the four major ophthalmology societies in the world. It looks at A - anterior curvature, B - back (posterior) curvature, C - corneal thickness and D - distance corrected visual acuity. Keratoconus is a progressive disease; this display detects progression easily aiding the clinician to treat patients more successfully.
How accurate is anterior chamber angle measurement with Pentacam® and can it be used as a screening tool for angle closure glaucoma?
Three screening parameters for glaucoma are automatically displayed in the General Overview display of the Pentacam®: Anterior chamber angle, anterior chamber depth and anterior chamber volume. Angles measurement data will be displayed for all the images, however sometimes and eyelid for example may obstruct the angle being displayed, resulting in the specific image to have an extrapolated rather than measured angle. But as mentioned above the Pentacam® measures three parameters aiding in glaucoma screening and in a study by Grewal et al. it was shown that anterior chamber volume is a more sensitive parameter than chamber angles.
Comparison of scheimpflug imaging and spectral domain anterior segment optical coherence tomography for detection of narrow anterior chamber angles; DS Grewal, GS Brar, R Jain and SPS Grewal; Eye (2011), 1–9 & 2011 Macmillan Publishers Limited All rights reserved 0950-222X/11
Apart from anterior chamber parameters helpful for glaucoma screening, are there any other functions Pentacam® performs which are helpful for glaucoma screening/glaucoma patients?
What is the role of the Pentacam® in corneal crosslinking (CXL)?
The inclusion of the Belin/Ambrósio Enhanced Ectasia Display and the Belin ABCD Keratoconus Staging and Progression Display makes Pentacam® an excellent tool to detect and monitor keratoconus progression. The ABCD Staging Display is the only display able to detect changes even in the posterior cornea. It is important to treat patients as soon as progression occurs to optimally preserve vision. The Pentacam® can be automatically linked to CXL software (Avedro) to transfer necessary data for treatment. Comparison display allows for pre- and post-treatment investigation.
Does the Pentacam® measure higher order aberrations?
Yes, the Pentacam® measures wavefront aberrations of not only the anterior and posterior cornea, but also total wavefront aberrations of the whole cornea. These data are displayed and compared to a normalized database.
Does the Pentacam® measure angle Kappa?
No, but it measures chord μ or chord distance. This is the distance between the pupil centre and vertex normal (first Purkinje image). Chord μ is related to angle Kappa, the latter being the angle between the visual axis and the pupil centre.
How does the Pentacam® aid in premium IOL selection?
The Cataract Pre-OP Display was developed to aid in premium IOL selection. It consists of four easy steps to help the surgeon determine if a specific eye is suitable for premium IOLs. The four steps are:
Step 1: Evaluation of corneal irregular astigmatism and chord distance (Is a cornea suitable for MF IOL)
Step 2: Detection of abnormal corneal shape (Aids in formula choice)
Step 3: Evaluation of corneal spherical aberration (Is a cornea suitable for aspheric IOL)
Step 4: Evaluation of corneal cylinder (Compare anterior astigmatism to total astigmatism)
Is it really important to take into consideration posterior astigmatism for toric IOL calculations?
Yes, when comparing corneal cylinder between anterior and total corneal astigmatism in certain cases not only does the amount of astigmatism differ, but also the orientation of the axis can differ when considering the posterior cornea. If not considered, this will influence post-op outcomes for toric patients.
What should I do in case my patient has a difference in anterior and total corneal astigmatism?
In the Pentacam® IOL Calculator various formulas does consider the posterior cornea for toric IOL calculation. Examples are the Savini toric calculator specifically developed using Total Corneal Refractive power Ks, PhacoOptics which is a ray-traced formula and also the Barret formula predicting posterior astigmatism.
Can the Pentacam® HR calculate IOL power?
Yes, for the Pentacam® HR the IOL calculator can be added. This IOL calculator includes various IOL formulas and more than 350 IOL geometries. Automatic transfer of correct K measurements eliminates transcription errors and helps to save time. The axial length can be typed in manually.
If the device does not have the IOL calculator, links to various external IOL calculators can be set up with Pentacam® and Pentacam® HR.
What makes the Pentacam® different to other topography devices?
The Pentacam® is a tomographer, generating a 3D image of the anterior segment of the eye and it is not tear film dependant. In only 2 seconds information regarding the anterior segment is derived aiding in contact lens fitting, glaucoma screening, corneal imaging, early keratoconus (ectasia) detection, keratoconus staging and various other functions.
How does the Pentacam® compliment my contact lens fitting?
The Pentacam® offers a contact lens fitting software suggesting lens geometries not only taking central Ks but the curvature of the entire measured cornea into consideration. Manual changes for lens parameters can be done. Fluo images are generated for the suggested lenses. Clearance in micron between lens back surface and anterior cornea is not only given for the main meridians, but displayed for the whole lens in the fluo image.
How is it a time-saving tool in my practice?
The measurement is very fast, but also all the important parameters are summarized on one display – Fast Screening Report – comparing each patient’s results to a normal and pathological database. This is a very good way to detect abnormal results quickly.