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Keywords:

  • corneal curvature;
  • corneal/scleral sagittal depth;
  • HVID;
  • optical coherence tomography

Abstract

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. GRANTS AND FINANCIAL SUPPORT
  7. REFERENCES

Purpose:  The purpose of the study was to obtain anterior segment biometry for 40 normal eyes and to measure variables that may be useful to design large diameter gas permeable contact lenses that sit outside the region normally viewed by corneal topographers. Also, the distribution of these variables in the normal eye and how well they correlated to each other were determined.

Methods:  This is a cross-sectional study, in which data were collected at a single study visit. Corneal topography and imaging of the anterior segment of the eye were performed using the Orbscan II and Visante OCT. The variables that were collected were horizontal K reading, central corneal/scleral sagittal depth at 15 mm chord, and nasal and temporal angles at the 15 mm chord using the built-in software measurement tools.

Results:  The central horizontal K readings for the 40 eyes were 43 ± 1.73 D (7.85 ± 0.31 mm), with ± 95% confidence interval (CI) of 38.7 (8.7 mm) and 46.6 D (7.24 mm). The mean corneal/scleral sagittal depth at the 15 mm chord was 3.74 ± 0.19 mm and the range was 3.14 to 4.04 mm. The average nasal angle (which was not different from the temporal angle) at the 15 mm chord was 39.32 ± 3.07 degrees and the ± 95%CI was 33.7 and 45.5 degrees. The correlation coefficient comparing the K reading and the corneal/scleral sagittal depth showed the best correlation (0.58, p < 0.001). The corneal/scleral sagittal depth at 15 mm correlated less with the nasal angle (0.44, p = 0.004) and the weakest correlation was for the nasal angle at 15 mm with the horizontal readings (0.32, p = 0.046).

Conclusion:  The Visante OCT is a valuable tool for imaging the anterior segment of the eye. The Visante OCT is especially effective in providing the biometry of the peripheral cornea and sclera and may help in fitting GP lenses with a higher percentage of initial lens success, when the corneal sag and lens sag are better matched.

Various imaging techniques have been used over the past few years to improve identification, characterisation and quantification of ophthalmic disorders. In recent studies, optical coherence tomography (OCT) has been presented predominantly as a microscopic imaging technique for in vivo examination of the posterior and the anterior segment.1–11 OCT has also been used to examine novel measurements such as the inferior tear meniscus12–14 and the tear film between the lens and the eye.15 Traditional uses in corneal pachymetry are extensive16–22 but some have added the ability to measure the corneal epithelium.6,23–26

Several imaging methods are capable of providing shape information of the anterior eye but not beyond the limbus.23,27 High resolution ultrasound (Artemis 2 from Arcscan, BC, Canada) and Scheimpflug camera systems (Pentacam by Oculus, Lynnwood, WA) can provide a topographic view of the cornea to the limbus and the high resolution optical coherence tomography (Visante OCT, Carl Zeiss, Dublin, CA) can visualise the anterior segment beyond the limbus.28 Pentacam and the Visante OCT are non-contact devices. All three instruments come with tools with which to measure the structures of the anterior eye.29,30

Visante OCT is a time domain device that uses optical coherence tomography to image the anterior segment. The Visante OCT Model 1000 can provide detailed in vivo examination of the anterior segment of the eye, including the iris, angle and anterior crystalline lens. The Visante OCT system provides unique images and measurements that may dramatically expand the potential for the fitting of customised contact lenses having the visualisation of the shape beyond the corneo-scleral junction that conventional topographers and other imaging techniques are unable to do.31 The repeatability of pachymetric measurements on healthy and keratoconic eyes is acceptable32 but there is little information on the accuracy of the anterior segment image of this OCT.

The instrument uses low coherence interferometry, in which light is sent along two optical paths: the sample path (into the eye) and the reference path of the interferometer. The light source is a 1310 nm super-luminescent light emitting diode. Light returning from the sample and the reference paths is combined at the beam-splitter and the resulting interference signal is directed onto the photo-detector. Due to the short coherence length of the super-luminescent light-emitting diode, the strength of the combined return signal is a measure of the reflectance of a small volume of the tissue at each location of the scanning spot.33,34

The Visante OCT is capable of obtaining anterior segment views that can be used therapeutically or diagnostically for conditions such as post-LASIK flap placement, stromal scar depth, intra-stromal corneal rings (INTACS), anterior chamber depth, narrow angles and iris details.30,35–38 The Visante OCT is equipped with built-in callipers, angle and flap tools in the ‘anterior segment’ mode. The callipers measure to the tenth of a millimetre and the flap tool measures to the micron level. The tissue depth for each scan is six millimetres deep by 16 mm wide for anterior segment scans.34

The trend towards large diameter (semi-scleral) lens fitting has left clinicians with difficulty in approximating the sagittal depth of the cornea up to the scleral zone to correctly match the shape of the contact lens. Traditionally, to fit scleral lenses one would need to take a mould of the anterior eye (cornea and scleral zone) to define the shape of the peripheral contact zone of the scleral portion of the lens.39,40 One would expect, first, that having a tool to measure directly the corneal/scleral sagittal depth at the limit of the instrument's image that is, at 15 to 16 mm, where the complete corneal/scleral junction can be seen, would benefit the scleral lens fitter. The use of the 16 mm wide scan of the Visante OCT for contact lens fitting has not been established fully and is being suggested here for aiding in scleral lens fitting. Second, the measurement of the rate of flattening or change in radius of the limbal/scleral zone and visualisation of the shape of the sclera adjacent to the limbus are not possible with corneal topographers, as the radii of curvature cannot be measured in these areas. Thus, perhaps measuring the nasal and temporal angles at a particular chord length would be an indicator of the slope or shape of the sclera when the angle is measured tangentially to the sclera.

The two aims of this study were to collect anterior segment biometric data for 40 normal eyes and to look at the distribution of these variables in the normal eye and how well they correlate with each other.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. GRANTS AND FINANCIAL SUPPORT
  7. REFERENCES

Study design

This was a cross-sectional study that involved 40 participants. To be eligible for this study, all participants were at least 17 years of age and had low refractive error (less than -3.00 D) and average K readings from 39 to 46 D (8.65 to 7.34 mm), which were determined at the screening visit. Informed consent was obtained for all participants prior to their enrolment in the study, which was approved by the Office of Research Ethics at the university. If the participants wore contact lenses, they were asked to discontinue contact lens wear for the day of the visit (soft lens wear only and no overnight wear). Data and observations were collected at one study visit. Corneal abnormalities and degenerations were exclusion criteria, as was the wearing of any type of gas-permeable (GP) contact lenses. This study was conducted according to the Tenets of Helsinki.

Procedure

At the study visit, imaging of the anterior segment of the eye and corneal topography were performed in a randomised order, using the Visante OCT (Carl Zeiss, Dublin, CA) and the Orbscan II (B&L Surgical, Rochester NY), respectively. Three scans were taken of each randomly selected eye, for each participant using a standardised protocol. The data of the three repeated scans/images for each device were averaged.

For each participant, scans were completed at approximately the same time of the day (all in the afternoon from 1:00 pm to 2:00 pm to ensure no effects of corneal oedema) using the same equipment, which was calibrated daily and using a standardised protocol which included an adjustment of the fixation angle to ensure alignment of the OCT image with respect to the geometric axis of the anterior eye.

Image analysis

In the anterior segment mode (Figure 1), the Visante OCT instrument captures a 16 mm wide image. The transverse accuracy is 0.217 mm and the axial accuracy is 0.034 mm in the anterior segment mode.41

image

Figure 1. Visante OCT anterior segment image indicating the 15 mm chord, the measured sagittal depth and the temporal and nasal angles

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With the built-in callipers, it is possible to obtain sagittal depth and chord measurements at any point of the anterior segment. One can use these points in a standard conoid formula to define the peripheral corneal shape if the chord extends to the limbus.31 The relationship between the sagittal radius (r), the eccentricity value (p) and the sagittal depth (x) at any semi-chord (y) can be deduced using the equation:

  • image

which assumes that the cornea is a prolate ellipse. As this relationship relies on corneal shape or the p-value, which is based on radius of curvature measurements taken by a corneal topographer within a seven to nine millimetre zone, it cannot be extrapolated onto the scleral zone accurately, because a second elliptical shape extends from the limbus to the sclera. As a result, the nasal and temporal tangential angles were measured using the built-in software tools and used as a descriptor of scleral shape at 15 mm, a chord well outside of the limbus (in a zone where large diameter gas permeable lenses or soft lenses may lie) (Figure 1). These two angles can be extrapolated to determine the ‘cone angle’, which has been defined in early literature as a measure of the asphericity of the cornea, with which one would design an aspheric or conoid GP contact lens.42 Also, this tangential angle can be used to measure a peripheral curve radius perpendicular to the tangent line.

The nominal central keratometric (K) readings were measured using the Orbscan II corneal topographer.

Data analysis

Data were analysed using Statistica 7. A student t-test was performed to compare nasal angles, temporal angles and horizontal K readings. Distribution plots were created for all data. Parametric statistics were used as all data were normally distributed. Pearson's correlation (1.0 representing perfect concordance) was performed for correlations among horizontal K, nasal angle, temporal angle and corneal/scleral sagittal depth at 15 mm.

RESULTS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. GRANTS AND FINANCIAL SUPPORT
  7. REFERENCES

There were 27 female and 13 male study participants and the test eyes were selected randomly. They had a mean age of 32 years and a range of 22 to 42 years. Half of the group were previous soft lens wearers and the other 20 were non-wearers. Comparisons between the wearers and non-wearers were made including refractive error and central K readings. Despite the fact that there was a difference in refractive error between wearers and non-wearers, (-2.37 ± 1.53 D and +0.67 ± 0.81 D, respectively) (p = 0.001); there were no significant differences in K readings, (43.85 ± 1.55 D [7.7 ± 0.26 mm] and 42.83 ± 1.78 D [7.88 ± 0.33 mm], respectively) (p = 0.062) or in either corneal sagittal depth or nasal and temporal angles, (p = 0.266 and 0.362, respectively) comparing these two groups. For the remainder of the results, the 40 eyes that were measured were considered together.

The mean horizontal K reading for the 40 eyes was 43 ± 1.73 D (7.85 ± 0.31 mm) with ± 95% CI of 38.7 D (8.7 mm) and 46.6 D (7.24 mm). The distribution of these data is seen in Figure 2. The mean corneal/scleral sagittal depth at a 15 mm chord was 3.74 ± 0.19 mm and the range was 3.14 to 4.04 mm (Figure 3). There was no difference between the nasal and temporal angles (p = 0.282). The mean nasal angle at a 15 mm chord was 39.32 ± 3.07° and the ± 95% CI were 33.7° and 45.5°, while the mean temporal angle was 39.94 ± 3.31°with the ± 95% CI of 31° and 46.9° (Figures 4 and 5). In summary, 65 per cent of the group had K readings between 42 and 45 D (8.04 and 7.5 mm), a sagittal depth at 15 mm between 3.6 to 3.9 mm and a nasal angle between 37 and 41 degrees.

image

Figure 2. Distribution of horizontal K readings (D)

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image

Figure 3. Distribution of sagittal depth (mm) at a 15 mm chord length

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image

Figure 4. Distribution of nasal angle (degrees) at a 15 mm chord length

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image

Figure 5. Distribution of temporal angle (degrees) at a 15 mm chord length

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Linear correlations were performed and the results indicated that the correlation between the horizontal K reading and the corneal/scleral sagittal depth at 15 mm, was r = 0.577 but significant (p < 0.001) (Figure 6). The corneal/scleral sagittal depth at 15 mm was less correlated with the nasal angle, r = 0.444 and also significant (p = 0.004) (Figure 7). There was a weak correlation between the nasal angle and the horizontal K readings r = 0.317, and barely significant (p = 0.046) (Figure 8).

image

Figure 6. Correlation between horizontal K reading and sagittal depth

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image

Figure 7. Correlation between nasal angle and sagittal depth

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image

Figure 8. Correlation between horizontal K reading and nasal angle

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DISCUSSION

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. GRANTS AND FINANCIAL SUPPORT
  7. REFERENCES

The clinical fitting performance of large diameter gas permeable contact lenses is related to how effectively they fit on the peripheral cornea and sclera. Parameters such as sagittal depth, corneal curvature and peripheral corneal/scleral shape affect the lens fit. Although several studies have developed theoretical models to explore the effect of soft contact lens design on clinical performance,43–45 little work has been done in regards to semi-scleral or large diameter gas permeable contact lenses without the use of corneal/scleral moulding as has been done in the past.39,46

Using the Visante OCT, Gemoules47 converted corneal sagittal depth measurements taken at various chord diameters to corneal radius measurements assuming that the rate of flattening of the cornea continued onto the sclera. This assumption was based on the fact that the Visante OCT appears to smooth out the corneal/scleral junction so that the change of curvature or slope of the sclera relative to the cornea is not apparent, probably due to its lack of ultra-high resolution. He transmitted the data to a GP manufacturing laboratory to aid in the construction of semi-scleral lenses. He reports on three cases, in which he successfully fitted patients using this empirical fitting method.47

The nasal and temporal angles measured tangential to the sclera, as in this study, have not been reported previously. Tangential corneal ‘cone’ angles (that is, 180° - [nasal + temporal]°) have been calculated by Thomas42 up to a limit of a 7.25 mm chord and a central radius from 6.0 to 8.6 mm. For example, using his calculations, the steepest tangential angle for a cornea with a central radius of 7.8 mm at a 7.25 mm chord would have nasal/temporal angles of 58.9°.42 Beyond this chord, the nasal and temporal angles will no longer be tangential to the periphery of the cornea and cannot be measured. In this study, nasal and temporal angles at a single chord were measured tangentially to the sclera to describe the flatter scleral shape, which has not been described in these terms before and supports the notion that the sclera may have a more tangential shape as opposed to an actual curvature at that specified location due to the perfect alignment of the angle measuring tool of the Visante OCT in this location.

It is widely accepted that the cornea is steepest centrally and flattens towards the periphery, as would a prolate ellipse.48,49 Little information is available to allow us to compare the shape measured in this study with normal predictors used in contact lens fitting (that is, horizontal visible iris diameter and central corneal curvature) but they appear to have a weak but significant correlation. It appears that being able to view the corneal/scleral area and measure its shape with high resolution instruments, such as the Visante OCT, is essential if one is to consider the fitting of lenses that rest on this area.

The mean corneal/scleral sagittal depth was 3.74 ± 0.19 mm when measured with the Visante OCT at a 15 mm chord. Due to individual changes in shape beyond the corneal/scleral junction, there was a poor, though significant, association between corneal/scleral sagittal depth and central horizontal radius of curvature of the cornea and the nasal/temporal angle for this group of participants, when measured at 15 mm. Neither the central radius of the cornea nor the corneal/scleral sagittal depth was highly predictive of the scleral shape (that is, the nasal/temporal angle). As the cornea steepened, the corneal/scleral sagittal depth increased and the nasal/temporal angles became larger, as one would expect but individual differences in the scleral shape and small sample size resulted in low correlations between these parameters.

Radius of curvature is fundamental to selecting contact lens parameters for lens fitting. Mandell49 reports the mean radius of curvature of the cornea to be 7.8 ± 0.25 mm (43.25 D) for the normal population with a range of 6.75 (36.00 D) to 9.25 mm (50.00 D). Our results indicated similar findings for the mean horizontal K reading, which is in agreement with previous work on the normal population and confirms that our group of participants is an equivalent sample.48,49

The results also suggest that the measurement of corneal curvature is of little help in predicting the sagittal depth of the anterior eye, to a chord of 15 mm. This indicates the importance of measuring sagittal depth directly, combining this information with the nasal and temporal angles when fitting very large diameter contact lenses.31 The measurement of both sagittal depth and nasal/temporal angles may prove to be vital parameters that the Visante OCT can supply.

Although other techniques can be used for the assessment of contact lens fit, the ones listed here provide limited information about the space between the surface of the eye at the corneal/scleral area and the posterior surface of the contact lens. Visualisation of this space with ultra-high resolution OCT may aid us in the future. High resolution anterior segment imaging is a valuable adjunct to conventional topography in fitting larger RGP lenses, such as scleral lenses. Visante OCT anterior segment imaging is effective in providing biometry of the peripheral cornea and sclera. Large diameter lenses may become the preferred method of fitting difficult corneal topographies in the hope of producing more comfortable, better fitting lenses, thus reducing lens re-orders and having a higher percentage of initial lens success.40,50–53 The anterior segment Visante OCT can help visualise the corneal/scleral anatomy and possibly the interface between contact lens and eye.

GRANTS AND FINANCIAL SUPPORT

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. GRANTS AND FINANCIAL SUPPORT
  7. REFERENCES

This investigation was supported by a grant from Bausch & Lomb.

REFERENCES

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. GRANTS AND FINANCIAL SUPPORT
  7. REFERENCES
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