Misleading orientation marks

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Assessing orientation is an essential aspect of toric soft lens fitting. Thankfully, this is a simple process - or is it? We assume that an orientation marking 10 degrees off the vertical indicates that the toric lens has rotated due to lid forces and that a modification to the lens prescription will be needed in order 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 a third, this is not necessarily correct.

Parallax and asymmetry

Toric soft lenses with makings at 3 and 9 o'clock such as Vistakon's Acuvue Toric have shown 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 the lens is tilted slightly when decentered. With an inferiorly decentered lens, the top edge (ie that nearest 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 centred, a similar apparent twisting of the orientation marks can occur due to asymmetry of the cornea and, more importantly, the sclera (Figure 2). 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, Marriott1 found the temporal sclera to be 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 lens placed on this type of eye, again, adopts a tilted position. However, without paired markings (eg 6 and 12 o'clock), it is difficult to assess which lens fittings are affected.

Corneal asymmetry may exacerbate this effect. Most corneal topographies are relatively symmetrical about a vertical axis however, 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.2,3

In Figure 2, the orientation marks have been extended across the centre of the lens to better demonstrate the lens tilt effect. Because of parallax, the inferior part of the orientation mark appears rotated counter clockwise by about 10°. However, observation of the marking as a whole indicates that there is no rotation, but merely that the marking is viewed slightly off-axis due to effective tilting of the lens. The horizontal marking gives 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 set anterior to the inferior sclera
• Slight down gaze.

Assessing orientation

As with decentred lenses, this misleading appearance can easily 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 degrees or more. With a cylinder as low as 1.25D, this rotation would produce a resultant error of nearly half a dioptre of oblique cylinder. Misleading orientation marks may explain some instances where apparently well fitting toric soft lenses give poor vision.

Improving soft toric success

Modern moulded toric soft lenses are manufactured with impressive accuracy and are thin enough to avoid any complicating tear lens. Normally, therefore, the optimum toric lens prescription should be calculable from the spectacle prescription and any observed of lens rotation. However, in the event of poor vision, having rechecked the refraction and verified the accuracy of the lens itself,4 the next step should be to consider whether the orientation has been misread due to distortion of the orientation mark. Is the lens decentred? Does the eye appear to be naso-temporally asymmetrical? One clue is whether the temporal and nasal limbal blood vessels can be focused by slit lamp simultaneously or whether some back and forth adjustment is need to focus from one to the other.

Nowadays, most toric soft lens fittings are relatively straightforward however, this observation suggests some additional pointers for successful toric fitting:

• During the pre-fitting assessment note any significant asymmetry in ocular topography or other abnormalities (eg pinguecula) that might distort lens fit
• If the lens is decentered, be aware that this may have twisted the orientation marking. Generally this extra rotation will be in the inferio-nasal direction
• Where possible, note whether the paired orientation marks are aligned and, if not, make the appropriate adjustment to the assessment of rotation
• 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 overestimate inferio-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 toric soft lens fitting. This hitherto overlooked feature of misleading orientation marks helps to explain some instances of unexpected failure and gives some pointers towards better assessment of toric lens performance.

Acknowledgements

These observations arose out of work sponsored by Vistakon. Thanks to Chris Wildsmith and Eric Papas for some useful pointers.

References

1. Marriott PJ. An analysis of the global contours and haptic contact lens fitting. Brit J Physiol Opt, 1966 23:1 40.
2. Lui Z, Huang AJ, Pflugfelder SC. Evaluation of corneal thickness and topography in normal eyes using the Orbscan corneal topography system. Br J Ophthalmol, 1999 83:774-778.
3. Reddy T, Szczotka LB, Roberts C. Peripheral corneal contour measured by topography influences soft toric contact lens fitting success. CLAO J, 2000 26:180-185.
4. Young G, Hickson-Curran S. Reassessing toric soft lens fitting. CL Spectrum, 2005 20(1):42-45.

? Dr Graeme Young is managing director of Visioncare Research. Roberta Mcllraith is a research optometrist formerly with Visioncare Research