Misleading Orientation Marks on Soft Toric Contact Lenses
Misleading Orientation Marks on Soft Toric Contact Lenses
BY GRAEME YOUNG, MPHIL, PHD, FCOPTOM, DCLP, FAAO, & ROBERTA McILRAITH, BOPTOM (HONS)
Assessing orientation is an essential aspect of toric soft contact lens fitting. Thankfully this is a simple process — or is it? We assume that an orientation marking that's 10 degrees off the vertical indicates that the toric lens has rotated because of lid forces and that we need to modify the contact lens prescription to compensate. By the same token, a vertical orientation mark indicates no lens rotation and that no compensation is required.
These assumptions are usually valid, but in a significant proportion of cases, perhaps in one-third, this is not necessarily correct.
Parallax and Asymmetry Effects
Toric soft contact lenses with markings at 3 o'clock and 9 o'clock have demonstrated that lens decentration can distort the angle of the orientation marks. For instance, in lenses showing inferior decentration, the orientation marks no longer align but appear to slope down in a ‘V‘ shape (Figure 1). This relates to parallax and the fact that the contact lens is tilted slightly when decentered. With an inferiorly decentered lens, the top edge (that nearest to the limbus) is generally closer to the observer while the bottom edge sits further away from the observer. As a result, the lens is viewed from an off-axis position and the markings appear to be rotated.
Even when the lens is well centered, a similar apparent twisting of the orientation marks can occur because of asymmetry of the cornea and, more importantly, of the sclera. The pioneers of scleral lens fitting were well aware that the nasal sclera is invariably flatter than the temporal sclera. From plaster casts of the eye, Marriott (1966) found that the temporal sclera is curved with an average radius of about 16mm whereas the nasal sclera was usually too flat to be measured. Because of this asymmetry, the nasal sclera is often positioned anterior to the temporal sclera, and a soft contact lens placed on this type of eye will, again, adopt a tilted position. However, without paired markings (such as at 6 o'clock and 12 o'clock), it's difficult to assess which soft toric lens fittings are affected.
Figure 1. Effect of lens decentration on orientation markings. The 3 o'clock and 9 o'clock markings are horizontally aligned, but because of parallax the nasal mark appears rotated counterclockwise.
Corneal asymmetry may exacerbate this effect. Most corneal topographies are relatively symmetrical about a vertical axis, but approximately one-third show some asymmetry, perhaps secondary to the scleral asymmetry. This can manifest as a difference in asphericity or corneal astigmatism between the nasal and temporal cornea (Lui, Huang and Pflugfelder, 1999; Reddy, Szczotka, Roberts 2000).
In Figure 2, the orientation marks have been extended across the center of the lens to better demonstrate the lens tilt effect. Because of parallax, the inferior part of the orientation mark appears rotated counterclockwise by about 10 degrees. However, assessing the marking as a whole indicates that there is no rotation, but merely that the marking looks slightly off-axis due to effective tilting of the lens. The horizontal marking provides further confirmation of no rotation, although its slight bend suggests one or a combination of three possibilities: slight inferior decentration; that the superior sclera is positioned anterior to the inferior sclera; and/or slight down gaze.
Consider Using Paired Markings
As with decentered lenses, this misleading appearance can go undetected unless there is a corresponding orientation mark on the opposite side of the lens. Only a few toric lenses incorporate paired orientation markings; these include Acuvue Advance for Astigmatism, Acuvue Oasys for Astigmatism (both Vistakon) and Air Optix for Astigmatism (CIBA Vision).
Because of this parallax effect, orientation marks can erroneously appear to be rotated 10 or more degrees. With a cylinder as low as 1.25D, this rotation would produce a resultant error of nearly 0.50D of oblique cylinder. The deceptive appearance of orientation marks may explain some instances in which apparently well-fitting toric soft lenses provide poor vision.
Modern molded toric soft lenses are manufactured with impressive accuracy and are thin enough to prevent any complicating tear lens. Normally, therefore, the optimum toric lens prescription should be calculable using the spectacle prescription and any observed lens rotation. However, in the event of poor vision, having rechecked the refraction and verified the accuracy of the lens itself (Young, Hickson-Curran, 2005), the next step should be to consider whether the orientation has been misread because of distortion of the orientation mark. Is the lens decentered? Does the eye appear to be naso-temporally asymmetrical? One clue in checking for asymmetry is whether you can focus the temporal and nasal limbal blood vessels by slit lamp simultaneously or whether some back and forth adjustment is needed to focus from one to the other.
Figure 2. Centered toric lens on a tilted cornea resulting in apparent counterclockwise rotation of the inferior orientation mark; this is mirrored by the opposite mark, confirming the absence of rotation. Note that nasal limbal capillaries are in focus while the temporal vessels are out of focus, indicating a difference in the heights of the temporal and nasal sclera.
Improving Soft Toric Success
Nowadays, most toric soft lens fittings are relatively straightforward. However, this observation suggests some additional pointers for successful toric fitting:
1. During the pre-fitting assessment, note any significant asymmetry in ocular topography or any other abnormalities (such as pingueculae) that might distort lens fit.
2. If the lens is decentered, be aware that this may have twisted the orientation mark. Generally, this extra rotation will be in the infero-nasal direction.
3. Where possible, note whether paired orientation marks are aligned and, if not, make the appropriate adjustment to your assessment of rotation.
4. In the event of unexpected vision results, consider the possibility of twisted orientation markings. In the case of 6 o'clock markings, these will tend to over-estimate infero-nasal rotation.
In conclusion, improvements in the quality and accuracy of toric soft lenses enable us to re-examine some of the finer points of soft toric fitting. This hitherto overlooked feature of misleading orientation marks helps to explain some instances of unexpected failure and provides some pointers towards better assessment of toric lens performance. CLS
Acknowledgements: These observations arose out of work sponsored by Vistakon Inc.
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #151.
Dr. Young is director of Visioncare Research Ltd., a United Kingdom-based company specializing in eyecare clinical research. He's a past president of the British Contact Lens Association and a member of the International Society for Contact Lens Research. Ms. McIlraith is a New Zealand qualified optometrist currently working at Visioncare Research as a research assistant.
Contact Lens Spectrum, Issue: June 2008