Corneal topography: contact lens considerations.
Corneal topography provides an invaluable platform for the observation of corneal profile and ultimate choice of contact lens design. This article considers the benefits of increasing accuracy, patient comfort, and potential success when using 'virtual' contact lens fitting programmes.
Dispensing opticians **
Contact lens opticians **
The somewhat entertaining consignment of the keratometer to 'Room 101' at the British Contact Lens Association (BCLA) conference in June 2015, by the esteemed Professor Lyndon Jones, recognised the limitations of this instrument. The opinion was echoed by Professor James Wolffsohn in December 2017 who suggested that in an age of technology and professional excellence, the keratometer should perhaps be relegated to the clinical dark ages. (1)
The modern, computerised, corneal topographer provides infinitely greater levels of information, and importantly, adds a host of ancillary functions and additional dimensions to contact lens practice--in particular, the initial choice of any specific contact lens design, an exceptionally useful function which may assist in 'patient friendly' selection of successful contact lens options. Designing contact lenses from digital corneal maps has become commonplace, primarily with a 'system' approach to orthokeratology (ortho-k), alongside 'complex' rigid gas permeable (RGP) lenses in both corneal and mini-scleral formats. The concept of a 'virtual' fitting programme is undoubtedly popular and preferable from a patient's perspective, as this can help to avoid a potentially uncomfortable, time-consuming ordeal. Practitioners also appreciate avoiding the obstacles with hygiene or disinfection routines by using a virtual approach. However, there remains some controversy as to whether 'complex' contact lens fitting from a single topographical image can be expected to demonstrate an accurate 'on eye' fluorescein pattern and achieve an acceptable level of success at the first attempt.
Since its release in the 1950s, sodium fluorescein (NaFl) has been used as an indicator to visibly evaluate the relationship between rigid contact lenses and the ocular surface. (2) Essentially, fluorescein shows a two-dimensional representation of a three-dimensional shape, thereby presenting the practitioner with a contour map of tear film thickness under the lens. (3)
Typical NaFl patterns are well documented throughout the contact lens literature, and the fundamentals of three separate zones when fitting corneal RGPs, have been specified: (4) central corneal clearance; mid-peripheral bearing; and edge clearance, are familiar concepts at undergraduate level. Variance of NaFl patterns include toric fits, and practitioners will be familiar when observing a 'with-the-rule' astigmatic patient compared to 'against-the-rule' cases. It is essential to carefully review the position of bearing surfaces and any resultant displacement or decentration of the corneal RGP lens.
With-the-rule NaFl patterns will have bearing surfaces (touch), or at least NaFl thinning, in the mid periphery, at the 3 and 9 o'clock positions and this will determine the eventual vertical direction of movement of an RGP contact lens. Conversely, against-the-rule patterns typically demonstrate bearing at 6 and 12 o'clock, which may result in rapid displacement and decentration of the lens on blink; this can cause noticeable patient discomfort and reflex tearing, which will likely flush away any NaFl, making accurate assessment almost impossible, and deliver a suboptimal experience for the neophyte contact lens wearer.
NaFL is a suitable indicator for a 'normal' or regular contact lens design but shows frailty when evaluating a lens on an irregular, keratoconic, or post-surgical cornea; this is primarily due to the very thin apical tear layer these fittings often require and the relative visibility of NaFl under the lens. (5)
NaFl as a diagnostic tool becomes invisible to the human eye below clearance of ~20[micro]/m and gains maximum fluorescence at ~75[micro]m. (6) Hence, a visible 'pattern' is essentially between these two limits and will require sensitive and experienced observations in order to achieve the desired, precise clearances.
When evaluating corneal apex clearance with RGP lenses designed for reshaping the cornea, as with ortho-k, clearance may be as low as 3-4[micro]m, which is impossible to assess using the conventional method of NaFl alone. Ortho-k lenses should be fitted with nominal clearance to maintain corneal integrity; however, NaFl assessment has been shown to be insufficiently sensitive when fitting ortho-k lenses. (5) The classic 'doughnut' shaped mid peripheral clearance zone (reverse zone) of an ortho-k lens may be easy enough to visibly identify, but central clearance, or indeed the mid peripheral 'bearing' area is somewhat more challenging and experienced practitioners have debated and disagreed given identical 'patterns' to observe. (2) The topographer knows no such boundaries, and allows valid and reliable information, going boldly where once cautious rules applied.
Corneal topography to assess corneal clearance
The corneal topographer has the ability to make valuable assessment of corneal clearance at any given point where NaFl and a slit lamp have proven futile. The question is how accurate and effective can a corneal contact lens of minimal micron clearance, designed from a single topography map be? Should a contact lens be designed, manufactured and dispensed, from digital data from a single topographical capture? The answer would appear to lie with the accuracy of the corneal map being used, in combination with the practitioner's individual abilities and experience in capture and collation of digital imagery.
Corneal topography to assess progressive corneal ectasia
The corneal topographer has become the 'gold standard' when measuring progression in cases of corneal ectasia. Corneal collagen cross-linking (CXL) has become an established procedure in young keratoconic patients. It has been shown to be extremely effective in achieving long-term stabilisation of keratoconus, and therefore, reducing the potential need for corneal transplantation. (7) Consequently, an absolute level of accuracy and repeatability, combined with practitioner skill and interpretation of corneal maps may be considered an essential requirement.
Scheimpflug versus Placido
Scheimpflug topography utilises an optic section of the cornea. Directing a camera perpendicular to the slit beam creates the topography map; this method directly measures both the anterior and posterior elevations, and the data is converted into curvature in dioptres as well as providing a reading for corneal thickness. Placido topographers gather data by imaging sets of concentric rings onto the cornea; the curvature of the cornea is measured in dioptres. Placido topographers then use this data and a specialised algorithm to derive the elevation of the cornea; that is to say, it does not directly measure it. (8)
The Scheimpflug systems (Pentacam, Oculus, Germany; Orbscan, Bausch + Lomb, US) can analyse a vast amount of information, capturing approximately 25,000 data points in two seconds. However, even within a two-second window, potential margins of error based on tear film instability or early evaporation can arise.
As corneal topography typically measures the first ocular surface--that is to say, the tear film--some variance may be expected. The tear film is a medium that is constantly changing. The Placido-type capture is essentially a 'snapshot' at any specific moment and may give variable results based on a single image capture. Some suggest taking a minimum of four images per eye, selecting the most accurate. (9) Some debate arises between individual practitioners as to the most accurate, or 'best' image in the same way that they lack full agreement when observing NaFl patterns. The flattest, or lowest overall dioptric power is likely to be the most accurate measurement of corneal curvature. Repeatability, or quality functions are available on some instruments; however, anecdotal evidence would suggest that capable professionals are able to discern, trim, and save the ideal image for future reference.
Experienced practitioners will be acutely aware of topographical variance across a blink cycle, and some instruments now have a tear film sequence analysis, whereby tear film stability can be monitored, for example, over a 30-second period, observing and recording significant levels of dioptric inconsistency during this time interval.
Impact of lids
As well as the effect that tear film fluctuations can have on the accuracy of corneal topography, further consideration when capturing data needs to be given to the position of the upper eyelid. A fully retracted lid is assumed to have no compressive force on either the tear film, or the corneal surface. The upper eyelid in primary position is typically between 1mm and 2mm below the superior limbus and will have a potentially variable, compressive force on the cornea. Eyelid pressure has been shown to have greatest effect on corneal topography when in down gaze. (10) Others have considered the effect of a 'drooped' eyelid on corneal topography and interestingly the findings revealed a trend of subclinical keratoconus-like changes with the increase in ptosis severity. (11) Lid position can also influence the tear film and push it into a more restricted space, creating the likelihood of steeper, or more convex corneal maps.
In a simple experiment by the authors at two separate clinics, Scheimpflug and Placido images were captured and evaluated for consistency, accuracy and repeatability (unpublished data). Two sets of measurements were taken using two different topographical instruments. A constant capture timing, one second post-blink, with upper lid fully retracted was then repeated with lid position approximately lrnm below superior limbus (see Figures 1A and 1B). Flattest, simulated keratometry readings were recorded and considered for choice of RGP base curve. All results were subsequently repeated, with full upper lid retraction, four times.
Image capture at one second post-blink was considered by the authors to have good stability, without excessive or unwanted aberrations from tear film evaporation. A total of 50 patients including a cross section of keratoconic, post-graft and postrefractive ectasia patients were assessed, with a view to designing RGP lenses.
The Placido instrument appeared to have greater sensitivity to tear film anomalies, and variation in results would appear to be due to tears condensed into a more restricted space as opposed to direct corneal compression. Scheimpflug images demonstrated notable corneal compression in the superior quadrant, and while the flattest central, simulated keratometry readings show little difference between maps, the easily identifiable peripheral corneal curvature changes are significant factors when designing the mid periphery and edge profiles of RGP contact lenses.
Contact lens considerations from corneal topography
Design features of 'virtual' NaFl displays allow changes to base curves, peripheral curves, diameters, and practitioner interpretation and input may be altered to show the presumed effective change this will induce at centre, mid-periphery and edge of the lens/cornea relationship. (4) Contact lens fitting from corneal maps can potentially save vast amounts of time, effort (and money) when choosing a contact lens type, or design.
Figure 2 provides an example of a computer-generated lens design based upon the corneal map. The corneal map reveals a moderate level of keratoconus and an interiorly displaced cone. The computer software has defaulted to a four-curve design with an ability for the practitioner to change radius and/or diameter of any one, or all four zones of the lens. Corneal clearance in microns can be evaluated at any given point and the graph at lower right of the image demonstrates clearance at the lowest and highest 'central' points, both of which are highly unacceptable at around 2[micro]m and 200[micro]m, respectively. The 2[micro]m clearance at the apex will inevitably result in impact on blink, and ocular surface insult. Central clearance on a corneal contact lens in excess of 40[micro]m will induce corneal distortion and result in spectacle blur. (9) Inferior edge clearance exceeding 200[micro]/m would allow ingress of air and probability of lens 'stand-off', subsequent displacement (and likely loss) after two or three blinks.
In reality, this contact lens would be unlikely to stay on the eye long enough to allow NaFl observation or recording. The resultant epiphora would create excessive front surface NaFl and clouding thereby making it difficult to interpret the NaFl pattern. The image in Figure 2 and a virtual fitting programme was used extensively at BCLA conferences in Birmingham, London and Singapore in 2018 as a demonstration of the importance and relevance of lens choice prior to application. The combined efforts of over 100 experienced contact lens professionals were unable to achieve satisfactory fitting with a corneal contact lens on this specific topography, with the net, unanimous conclusion that a hybrid, or more likely a scleral contact lens would be a certain requirement for this patient.
Contact lens fitting from corneal topography
Most manufacturers are happy to load their individual software for any one specific contact lens design. Design programmes have many quirks and nuances but are nonetheless user-friendly allowing for considerable 'offeye' efforts to achieve a potentially satisfactory fitting. A study at Moorfields Eye Hospital showed that a satisfactory lens design for patients with keratoconus can be achieved using a topography-based system to design a lens when compared to conventionally fitted lenses. (12) Others have shown that, when using topography-based systems to design contact lenses, the skill of the practitioner is required to account for eyelid position and tear meniscus heights to determine the final lens specification. (13)
Toric, and importantly, quadrant specific corneal lens designs, and also scleral lens designs are available on some instruments. The ability to measure ocular surface curvature to the limbus and beyond is now within the reach of certain instruments and provides accurate data of corneal, limbal and scleral curves, allowing for irregularities and height differences (typically between nasal and temporal sclera), required as an evaluation for scleral and mini-scleral contact lenses.
Contact lens designs
With little practice, designing contact lenses using topography is a speedy and reliable method.
Figure 3 shows a patient with 2.50DC against-the-rule astigmatism fitted with an RGP corneal lens with a complex toric periphery in a logical and sequential series of six 'virtual' steps.
Figure 4 shows a mini-scleral lens fitted to an early stage keratoconic patient, intolerant to corneal RGPs. Acceptable 'virtual' fitting was achieved in seven, sequential 'virtual' steps. Please note final NaFL clearance of just fewer than 300[micro]m at the centre, full limbal clearance achieved in all positions and an accurate prediction of the landing position at 3 and 9 o'clock, beyond limbus--all achieved prior to placing a lens on eye.
Corneal topography is an invaluable tool for the design of RGP lenses, regardless of the experience of the practitioner. The speed and efficiency of reaching the required end point with both the choice and individual design of appropriate contact lenses can have a highly beneficial impact on comfort, cost and patient satisfaction.
The growing popularity of ortho-k has aptly demonstrated the accomplished professional's ability to design and manufacture RGP lenses to crucial micron accuracy, accelerating understanding and confidence when using 'complex' RGP lenses at all levels. Careful consideration of lid position, image repeatability, and especially tear film stability, are essential factors when assessing corneal integrity and designing RGP lenses from digital corneal maps. Accurate corneal mapping, particularly with a diseased, dry, or compromised cornea is vital, and can result in precise, sustainable and repeatable RGP successes.
Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 21 June 2019. You will be unable to submit exams after this date. Please note that when taking an exam, the MCQs may require practitioners to apply additional knowledge that has not been covered in the related CET article.
CET points will be uploaded to the GOC within 10 working days. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.
Course code: C-70427 Deadline: 21 June 2019
* Be able to obtain accurate information using a corneal topographer (Group 3.1.1)
* Be able to fit a range of contact lenses using corneal topography (Group 5.1.3)
* Be able to understand how accurate information is obtained using a corneal topographer (Group 3.1.1)
* Be able to understand how corneal topography is used for contact lens design (Group 5.1.3)
* Be able to obtain accurate information using a corneal topographer (Group 3.2.1)
* Be able to fit a range of contact lenses using corneal topography (Group 5.1.2)
Visit www.optometry.co.uk, and click on the 'Related CET article' title to view the article and accompanying 'references' in full.
Nick Howard FBDO Hons CL, FBCLA and Emily Searle BSc (Hons), MCOptom
Nick Howard completed advanced contact lens training and qualification in 1984. Having been involved in a broad variety of clinical settings and training programmes, his specialty contact lens work is currently divided between two Lancashire NHS Hospitals, fitting an extensive range of complex contact lenses for the more challenging ocular conditions. In addition, Nick continues to work in private community practice, developing and implementing innovative projects including dry eye management, ortho-k, and myopia management.
Emily Searle qualified as a dispensing optician in 2015 and immediately continued her education to become an optometrist, completing her pre-registration year at Moorfields Eye Hospital in 2018. Now working in the refraction, low vision, and specialty contact lens clinics at Moorfields, Emily continues to work part-time in her family-owned independent practice in Essex where her career started as a 15-year-old laboratory technician.
Caption: Figure 1a Placido topography map with fully retracted upper lid (left) and repeated with lid 1mm below limbus (right). Note the central dioptric differences (measured on flattest simulated 'K'). Figure IB Scheimpflug topography map with fully retracted upper lid (left) and repeated with lid 1mm below limbus (right). Note the nominal central dioptric difference (measured on flattest simulated 'K'); however, notable superior quadrant difference which will have significant impact on virtual RGP designs
Caption: Figure 2 Computer-generated corneal lens design for a keratoconic eye
Caption: Figure 3 Corneal lens design for a patient with 'against the rule' astigmatism, showing (top to bottom): baseline topography; computer default design; virtual design
Caption: Figure 4 Scleral lens design on a keratoconic (low apex) eye showing (top to bottom): baseline topography; corneal clearance computer default; corneal clearance after three virtual adjustments; corneal clearance after seven virtual adjustments
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|Author:||Howard, Nick; Searle, Emily|
|Date:||May 1, 2019|
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