Cone location and correction of keratoconus with rigid gas-permeable contact lenses

Article Outline

• Abstract
• 1. Introduction
• 2. Methods
• 3. Results
• 4. Discussion
• Acknowledgements
• References
• Copyright

Abstract 

Purpose

To evaluate the influence of cone location and corneal cylinder on RGP corrected visual acuities and residual astigmatism in patients with keratoconus.

Methods

In this prospective study, 156 eyes from 134 patients were enrolled. Complete ophthalmologic examination including manifest refraction, Best spectacle visual acuity (BSCVA), slit-lamp biomicroscopy was performed and corneal topography analysis was done. According to the cone location on the topographic map, the patients were divided into central and paracentral cone groups. Trial RGP lenses were selected based on the flat Sim K readings and a ‘three-point touch’ fitting approach was used. Over contact lens refraction was performed, residual astigmatism (RA) was measured and best-corrected RGP visual acuities (RGPVA) were recorded.

Results

The mean age (±SD) was 22.1±5.3 years. 76 eyes (48.6%) had central and 80 eyes (51.4%) had paracentral cone. Prior to RGP lenses fitting mean (±SD) subjective refraction spherical equivalent (SRSE), subjective refraction astigmatism (SRAST) and BSCVA (logMAR) were −5.04±2.27D, −3.51±1.68D and 0.34±0.14, respectively. There were statistically significant differences between central and paracentral cone groups in mean values of SRSE, SRAST, flat meridian (Sim K1), steep meridian (Sim K2), mean K and corneal cylinder (p-values<0.05). Comparison of BSCVA to RGPVA shows that vision has improved 0.3logMAR by RGP lenses (p<0.0001). Mean (±SD) RA was −0.72±0.39D. There were no statistically significant differences between RGPVAs and RAs of central and paracentral cone groups (p=0.22) and (p=0.42), respectively. Pearson's correlation analysis shows that there is a statistically significant relationship between corneal cylinder and BSCVA and RGPVA, However, the relationship between corneal cylinder and residual astigmatism was not significant.

Conclusions

Cone location has no effect on the RGP corrected visual acuities and residual astigmatism in patients with keratoconus. Corneal cylinder and Sim K values influence RGP-corrected visual acuities but do not influence residual astigmatism.

Keywords: Keratoconus, Rigid gas-permeable, Cone location, Visual acuity, Residual astigmatism

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1. Introduction 

Keratoconus is a non-inflammatory disorder characterized by progressive corneal thinning, steepening, anterior protrusion and ectasia of the cornea. These corneal changes may result in irregular astigmatism and corneal scarring, both of which reduce the best-corrected visual acuity (BCVA) of the patient [1], [2], [3]. Despite extensive research into the potential aetiology, the exact cause of keratoconus is not fully understood. Indeed, the aetiology of the disease is multifactorial and it has been putatively associated with genetic inheritance, atopic disease, eye rubbing, contact lens wear and Down syndrome [1], [2], [4], [5], [6].

In early stages of the disease, spectacles may improve vision to an acceptable level. As the disease progresses, myopia and irregular astigmatism are advanced and the vision corrected with spectacles or soft contact lenses is no longer acceptable to the patient. Rigid gas permeable (RGP) contact lenses are the optimal treatment for keratoconus as they provide good vision by forming a new, regular optical surface [2], [3], [7], [8], [9].

Computer-assisted videokeratoscopes, which generate color-coded maps and topographical indices, are currently the most sensitive and sophisticated devices for confirming the diagnosis of keratoconus [1]. Several numeric summaries of videokeratographic data exist to facilitate quantitative analysis but there is no generally accepted classification system for keratoconus. One of the proposed classifications is based on the cone apex conical degree which is classified as mild, moderate and advanced [10], [11]. Based on the shape of the cone, keratoconus has been classified as round or nipple, oval or sagging and globus. The nipple cone is smaller and more centralized; the oval cone is more inferior, whereas the globus cone is quite large in diameter [12], [13]. Based on the location of the cone, keratoconus has also been classified as central and paracentral cone [10], [14].

Due to the irregular shape of the cornea, fitting the keratoconic patient with RGP lenses can be very difficult and time consuming [9], [11], [14], [15]. Three widely used fitting techniques in keratoconus are: apical clearance, apical bearing and three-point touch. The three point-touch technique is one of the most accepted lens-to-cornea fitting relationship in clinical practice [9], [16].

The purpose of this study was to evaluate the influence of cone location and corneal cylinder on RGP-corrected visual acuities and residual astigmatism in patients with keratoconus.

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2. Methods 

In this prospective study, one hundred thirty four keratoconus patients (one hundred fifty six eyes) referring to Poostchi Corneal Clinic, affiliated with Shiraz University of Medical Sciences between 2006 and 2009 were selected for investigation. The patients were examined by two experienced clinicians and were diagnosed as keratoconus according to clinical findings (such as corneal thinning, cone-like shape deformity, Vogt's striae, Munson's sign, Fleischer's ring, Descemet's folds), refractive findings and corneal topography data. Current RGP lens wearers, subjects with corneal scars, other ectatic disorders such as pellucid marginal degeneration, pterygium and any other ocular abnormalities and systemic diseases that would interfere with vision or patients who could not be fitted with our trial lens set (based on assessment of centration, movement and fluorescein pattern) were excluded from the study.

This study was performed in accordance with the Declaration of Helsinki, and approved by our institution's ethics committee. Written informed consent was obtained from all the subjects before inclusion. All the patients underwent complete ophthalmologic examination including manifest refraction, best spectacle visual acuity (BSCVA), slit-lamp biomicroscopy, and corneal topography analysis (EyeSys Vision, Houston, TX). Visual acuities were recorded as the smallest Snellen line where at least half of the numbers of letters were read correctly.

According to the topographic map, cone location was classified as central (if the highest power was located in central 3mm) and paracentral (if the highest power was located out of central 3mm). Based on Mean K reading on corneal topography, the patients were classified into three stages: (1) K value less than 45D as mild, (2) K value between 45 and 52D as moderate and (3) K value more than 52D as advanced.

Trial lens was selected based on the flat Sim K readings on corneal topography. The lens fitting was evaluated with the slit lamp with white light initially for centration and movement, and then with cobalt blue illumination with fluorescein for assessment of the fit.

In our fitting protocol, we attempted to have a ‘three-point touch’ approach. If a three-point touch was not achievable, apical bearing pattern with a moderate amount of touch on the corneal apex was accepted. Maximum accepted touch at the apex of the cone was 2–3mm. Conflex KE rigid gas permeable lenses (Wohlk-Contact-Linsen GmbH) were used. These are multicurve spherical lenses with a secondary curve 1.5mm flatter than the base curve and aspheric edge shaping. The trial lens parameters ranged as follows: the base curves (BC) ranged from 6.40 to 7.70mm (in 0.1mm steps), the total diameter was 9.50 and optic zone was fixed (7.50mm) and did not vary with the base curve radius.

Over contact lens refraction was performed and residual astigmatism (RA) was measured using autorefractometer (Topcon RM8800, Topcon Corporation, Japan). Also; retinoscopic refinement and then subjective refraction were performed. RGP visual acuities (RGPVA) were recorded by using a Snellen chart at 6M distance. All the statistical analyses were performed using Statistical Package for Social Sciences software version 11.5 (SPSS Inc., Chicago, IL, USA). The results were analyzed using Independent 2 sample t test, paired sample t test, regression analysis and Pearson's correlation analysis. The p-value less than 0.05 was considered as statistically significant.

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3. Results 

One hundred fifty six eyes (82 right eyes and 74 left eyes) from 134 patients were enrolled in this study. Seventy-seven (57.5%) patients were male and 57 patients (42.5%) were female. The mean age (±SD) was 22.1±5.3 years. Ages ranged from 11 to 41 years. According to the cone location the patients were divided into central and paracentral cone groups. 76 eyes (48.6%) had central cone and 80 eyes (51.4%) had paracentral cone.

Based on the Mean K readings, 8 eyes (5%) had mild keratoconus (K value less than 45D), 95 eyes (61%) had moderate keratoconus (K value between 45 and 52D) and 53 eyes (34%) had advanced keratoconus (K value more than 52D). Prior to contact lens (RGP) fitting, subjective refraction was performed. Mean spherical equivalent was −5.04±2.27D, mean astigmatism was −3.51±1.68D and mean BSCVA (logMAR) was 0.34±0.14 (range: 0.05–1).

Table 1 summarizes minimum, maximum, mean and standard deviation values of the flat meridian, steep meridian, mean K and corneal cylinder according to the measurements obtained from the corneal topography. Table 2 summarizes refractive status and topographic data obtained from both central and paracentral cone groups.

Table 1. Corneal topographic Sim Ks data in all patients.

	Minimum (D)	Maximum (D)	Mean±SD
Flat meridian (Sim K1)	41.92	56.53	48.14±3.25
Steep meridian (Sim K2)	44.23	66.96	53.30±4.70
Mean K	43.50	61.75	50.72±3.85
Corneal cyl.	−0.52	−11.14	−5.17±2.44

D, diopter; SD, standard deviation.

Table 2. Refractive status and topographic data for central and paracentral cone groups.

	Cone position	Mean±SD
SRSE (D)	Central	−5.46±2.59
	Para central	−4.64±2.10
SRAST (D)	Central	−3.85±1.82
	Para central	−3.20±1.50
BSCVA (LogMAR)	Central	0.36±0.21
	Para central	0.31±0.16
Sim K1 (D)	Central	49.00±3.56
	Para central	47.31±2.70
Sim K2 (D)	Central	54.69±5.16
	Para central	51.98±3.80
Mean K (D)	Central	51.85±4.24
	Para central	49.56±3.10
Corneal cylinder (D)	Central	−5.69±2.59
	Para central	−4.67±2.19

SRSE, subjective refraction spherical equivalent; SRAST, subjective refraction astigmatism; BSCVA, best spectacle corrected visual acuity; D, diopter.

There were statistically significant differences between the central cone and paracentral cone groups in values of subjective refraction spherical equivalent, subjective refraction astigmatism, flat meridian (Sim K1), steep meridian (Sim K2), mean K and corneal cylinder (p-values<0.05). There was no statistically significant difference between groups in values of best spectacle corrected visual acuity (p=0.075).

RGP corrected visual acuity (logMAR) was 0.036±0.04. Comparison of BSCVA to RGP corrected visual acuity shows that vision has improved 0.3logMAR by RGP lenses. In terms of Snellen visual acuity, vision improved from 20/42 to 20/22 on average. The number of the eyes with 20/40 or better corrected vision increased from 88 (56.4%) with spectacles to 153 (98.1%) with RGP fitting. Paired sample t-test showed that there is a statistically significant difference between BSCVA and RGP corrected visual acuity (p<0.0001). Mean residual astigmatism (±SD) was −0.72±0.39D (range 0.00 to −2.00). Mean (±SD) RGP-corrected visual acuities of central cone and paracentral cone groups were 0.032±0.035 and 0.040±0.044, respectively. There was no statistically significant difference between RGP corrected visual acuities of the central cone and paracentral cone groups (p=0.22). Mean (±SD) residual astigmatism of the central cone and paracentral cone groups was −0.70±0.38 and −0.75±0.39, respectively. There was no statistically significant difference between residual astigmatism of central cone and paracentral cone groups (p=0.42).

In this study, there was also an attempt to find a simple rule for determining the base curve of this type of RGP lens. The linear regression between the Sim K1 (flat meridian) reading and the base curve (mm) of the RGP lenses in all the patients (both central and paracentral groups) was:

RGP BC=10.59−0.071 Sim K1

(R=0.819, R²=0.671, p<0.0001)

The linear regression between the Sim K1 (flat meridian) reading and the base curve (mm) of the RGP lenses in the central cone group was:

RGP BC=10.38−0.067 Sim K1

(R=0.827, R²=0.684, p<0.0001)

The linear regression between the Sim K1 (flat meridian) reading and the base curve (mm) of the RGP lenses in the paracentral cone group was:

RGP BC=11.255−0.086 Sim K1

(R=0.833, R²=0.693, p<0.0001)

The results of Pearson's correlation coefficients between Sim Ks (Sim K1, Sim K2 and mean K) and BSCVA (LogMAR), RGPVA (LogMAR) and RA (D) are summarized in Table 3. The results show that in both the central and paracentral cone groups there is a statistically significant relationship between Sim Ks (Sim K1, Sim K2 and mean K) and BSCVA (LogMAR). There is also a statistically significant relationship between Sim Ks (Sim K1, Sim K2 and mean K) and RGPVA (LogMAR). However, there is no statistically significant relationship between Sim Ks (Sim K1, Sim K2 and mean K) and residual astigmatism. Table 4 shows the results of Pearson's correlation coefficients between corneal cylinder (D) and BSCVA (LogMAR), RGPVA (LogMAR), and RA (D). The results show that in both the central and paracentral cone groups there is a statistically significant relationship between the corneal cylinder and BSCVA (LogMAR). There is also statistically significant relationship between corneal cylinder and RGPVA (LogMAR). However, there is no statistically significant relationship between corneal cylinder and residual astigmatism.

Table 3. Pearson's correlation coefficient between Sim Ks and BSCVA, RGPVA and RA.

BSCVA, best spectacle corrected visual acuity; RGPVA, RGP corrected visual acuity; RA, residual astigmatism, D, diopter.

Table 4. Pearson's correlation coefficient between corneal cylinder and BSCVA, RGPVA and RA.

BSCVA, best spectacle corrected visual acuity; RGPVA, RGP corrected visual acuity; RA, residual astigmatism; D, diopter.

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4. Discussion 

Due to the irregular shape of the cornea, fitting the keratoconic patient can be very difficult and time consuming. As a result, many different lens types and designs have been developed. At present, many designs of contact lenses such as rigid gas permeable (RGP), hybrid designs such as SynergEyes, piggy-back, semi-scleral, and scleral contact lens types are used [1], [2], [3], [7], [8], [9], [10], [16], [17], [18], [19], [20], [21]. In addition to the advantages and disadvantages that are attributed to each of keratoconic lenses, the use of these specially designed lenses is difficult, experience-dependent and time-consuming. Furthermore, these lenses are not readily available everywhere and thus many practitioners are not familiar with them. So, non-specially designed RGP lens could be considered as the treatment of choice concerning the optical correction of keratoconus due to its unique characteristics and advantages. Previously in several studies, keratoconus patients have been corrected successfully with nonkeratoconic RGP lenses [10], [11], [22]. Our results showed that RGP lenses improved visual acuity about 0.3logMAR compared with BSCVA, In terms of Snellen's visual acuity, vision improved from 20/42 (0.34logMAR) to 20/22 (0.036logMAR) on average. The number of eyes with 20/40 or better corrected vision increased from 88 (56.4%) with spectacles to153 (98.1%) with RGP fitting. These results are somewhat better than those of the study performed by Lin et al. on 25 patients (48 eyes) with keratoconus corrected with RGP contact lenses. In their study, compared to BSCVA, RGP lenses corrected visual acuity 0.22logMAR. In terms of snellen visual acuity (logMAR), vision improved from 10/20 (0.32logMAR) to 16/20 (0.11logMAR) on average. The number of eyes with 20/40 or better corrected vision increased from 70.83% with spectacles to 97.92% with RGP fitting [10]. In the study of Zadnik et al. on 1579 keratoconus cases, the number of eyes with 20/40 or better corrected vision increased from 58% with spectacles to 88% with RGP fitting [22].

Because the central aspherical area of the cone cannot be paralleled by a spherical back optic zone of the contact lens, three fitting relationships are possible: these three widely debated philosophies are apical clearance, apical bearing and divided support (three-point-touch) [9]. Although other philosophies such as apical bearing and apical clearance have advantages, divided support or three-point-touch is a delicate balance between the apical clearance and apical bearing techniques. This method lightly touches the apex with peripheral alignment and provides a well-distributed pressure between the cone apex and the relatively normal peripheral cornea. This delicate balance between a steep and a flat-fitting method reduces the shortcomings of the two techniques. In this method, the weight of the lens is distributed over a larger area of the cornea and minimizes the corneal scarring compared to apical bearing lenses and is easier to achieve than apical clearance method [9]. The ‘three-point-touch’ approach is one of the most widely accepted corneal lens fitting techniques in clinical practice and has been found to be successful for long-term comfort and increased wearing time [9]. This cornea-lens fitting philosophy technique was the method of choice in our study. We used RGP lenses with a large diameter (9.50mm) in order for the lens to touch the peripheral cornea, as this part of the cornea is thicker than the central part and can facilitate better alignment of the lens.

According to our results, in patients with central cone, mean values of subjective refraction spherical equivalent, subjective refraction astigmatism, flat meridian (Sim K1), steep meridian (Sim K2), mean K and corneal cylinder were higher compared with patients with paracentral cone. These finding can be explained on the basis of the influence of the cone position on the visual axis. A centrally placed cone impinges upon and covers the visual axis and therefore influence of irregular central cornea results in higher values of spherical equivalent, astigmatism, flat meridian (Sim K1), steep meridian (Sim K2), mean K and corneal cylinder compared with patients with paracentral cone. In patients with paracentral cone, the cone location is relatively outside the visual axis and has less effect on spherical and astigmatism refraction and also on K readings.

However, there was no difference between the central cone group and paracentral cone group in RGP corrected visual acuities (p=0.22). In the study performed by Zadnik et al. on 6 patients (10 eyes) with keratoconus, the eyes were divided according to placement of the cornea's apex to central cone (6 eyes) and inferiorly displaced cone (4 eyes). In their study, regardless of the lens type or base curve, the patients with central cone had poorer visual acuity than those with inferiorly displaced cone [14]. Their study included only 10 eyes but our study on 156 eyes (76 eyes in the central cone and 80 eyes in the paracentral cone groups) showed that the position of the cone cannot affect the RGP-corrected visual acuities of patients with keratoconus. The finding that RGP corrected visual acuity is independent of the location of cone position can be explained by the fact that regardless of the corneal topography, RGP contact lenses form a new, regular optical surface over the surface of the cornea and virtually neutralize the regular and irregular corneal astigmatism. However, measurement of visual acuities in our study was high-contrast measurements. Low-contrast visual acuity testing might have revealed more differences between the central and paracentral cone groups.

According to our results, there was no significant difference between residual astigmatisms of central cone and paracentral cone groups (p=0.42). This indicates that RGP lenses can equally correct different types of cone with different amounts of irregular astigmatism.

Our results showed that, in both the central and paracentral cone groups there is a relationship between Sim Ks (Sim K1, Sim K2 and mean K) and BSCVA (LogMAR) and RGPVA (LogMAR). This finding can be explained by the fact that as the keratoconus progresses, Sim K values increase. So, high Sim K values indicate an advanced disease and consequently worse BSCVA (LogMAR) and RGPVA (LogMAR).

However, after correction with RGP lenses, there was no significant relationship between Sim Ks (Sim K1, Sim K2 and mean K) and residual astigmatism. This is because RGP contact lenses neutralize the regular and irregular corneal astigmatism and lenticular astigmatism persists.

Based on the results, in both the central and paracentral cone groups there was a relationship between corneal cylinder and BSCVA (LogMAR). There was also a significant relationship between corneal cylinder and RGPVA (LogMAR). These findings can be explained by the natural history of the keratoconus. As the disease progresses and the corneal shape deformity increases, regular and irregular astigmatism become greater. So, higher values of corneal cylinder indicate a more severe stage of the disease and consequently worse visual acuities. However, as it can be predicted, after correction with RGP lenses and conversion of the surface of the cornea to a spherical shape, there was no significant relationship between corneal cylinder and residual astigmatism.

Finding a simple rule for determining the initial diagnostic RGP lens will reduce chair time in fitting keratoconus patients and can result in a more efficient contact lens fitting process. In this study we tried to achieve this purpose. Because the optic zone diameter directly determines the sagittal height of the RGP lens, the initial diagnostic lens formula that we derived from the linear regression that was calculated in our patients (RGP BC≈10.6−0.07 Sim K1) may be useful in selecting an initial diagnostic lens for this type of lens and other keratoconic GP lenses with similar optic zone sizes.

In conclusion, the results of our study performed on patients with clinically diagnosed keratoconus clearly demonstrate that RGP lenses, due to their unique advantages, could be considered as the treatment of choice in the correction of keratoconus. The linear regression that was calculated in our patients (RGP BC≈10.6−0.07 Sim K1) can introduce a good start point for Conflex KE lenses and other keratoconus RGP lenses with similar optic zone sizes.

According to our results, cone location has no effect on the RGP corrected visual acuities and residual astigmatism in patients with keratoconus. Corneal cylinder and Sim K values influence RGP-corrected visual acuities but do not influence residual astigmatism.

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Acknowledgements 

The authors have no commercial or proprietary interests in the materials discussed in this manuscript.

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