New methods for stabilising toric lenses have greatly improved their performance and patients’ comfort, but some design features may cause prism in the optical zone.
Keeping toric soft contact lenses rotationally stable on the eye has always been a challenge for lens designers. Traditionally achieved by differential weighting or prism-ballasting, stabilisation now relies on better ballasting and stabilisation zones that leverage the power of the blink, thereby improving the lenses’ performance and patients’ comfort. Owing to these changes in design, my colleagues and I can analyse the quality of the lenses’ optics instead of merely counting successful versus unsuccessful fits. We recently found, for example, that the degree of vertical prism present in toric soft contact lenses varies considerably, up to 1.00∆ of vertical prism or more, depending on the design.1 In this article, I discuss why, whether it matters, and how to elicit more information about quality of vision from patients who wear toric contact lenses.
Design Approaches
An estimated 47 per cent of prospective contact lens wearers have some degree of clinically significant astigmatism,2 but historically, for a variety of reasons (chiefly discomfort and unstable vision), many of them have avoided wearing toric contact lenses. Current generation toric lenses have caused this paradigm to shift, and the correction of even small amounts of astigmatism (0.75 D) with toric contact lenses is becoming commonplace. The change is due primarily to innovations in lens design, manufacturing, and materials. The first soft toric contact lens designs were prism-ballasted designs. Originally, these lenses were truncated, leading to decreased comfort and increased lid interaction.
Later designs eliminated the truncation and added thickness throughout the lens, specifically in the bottom to allow gravity to orient the lens. There were a number of problems with this approach.2 The thicker portion of the lens, particularly with the older hydrogel materials, was less oxygen permeable, increasing the likelihood of neovascularisation and localised corneal oedema. Unpredictable interaction between the lower lid and the prism-ballasted area was also problematic. Toric contact lens wearers could feel the thicker portion of the lens, contributing to a foreign body sensation and discomfort. The increased lower lid interaction may also have caused the lens to rotate unpredictably, reducing the stability the ballasting was supposed to create. It may also have sometimes led to transient blur and visual fluctuations that made toric soft contact lens wear initially unacceptable to many patients and eye care providers.
lenses that do not use a base-down prism design for stabilisation were found to have no vertical prism in the optic zone
Dual-thin-zone designs were one attempt to resolve the problems of prism-ballasting. Lenses of this type had thinner areas on both their superior and inferior portions. The pressure applied by the lids on the thin zones rotated the lens into the correct position. The thicker areas in between the lids, however, resulted in lid interaction and an unpredictable fit.3 Not surprisingly, many practitioners simply recommended against contact lenses for patients with astigmatism, and/or continued masking lower levels of cylinder with a spherical lens. Two newer approaches have been more successful. The peri-ballast approach attempts to minimise the prism ballast to increase comfort and reduce lower lid interaction. Peri-ballast designs are more compatible physiologically, offer more predictable rotation, and have better visual performance than the traditional prism ballast lenses. The gravity-dependent mechanism utilised by some of these lenses is not optimal with some head movements (such as tilting the head sideways) or extreme positions of gaze, making it difficult for the lens to remain correctly positioned.4
Since 2005, the approach Johnson and Johnson Vision Care has taken with Acuvue for astigmatism is called ‘accelerated stabilisation design’ (ASD). Based on years of research on the mechanism of the blink – including studying lid movement and mechanics as well as lens-lid interaction – ASD uses four ‘active’ zones placed in specific locations on the lens’ surface. If the lens becomes misaligned, the natural power of the blink will rotate the lens back into position. When correctly aligned, the lens interacts very little with the eyelids, regardless of eye movement or head position, so it remains essentially stable during the blink. Even when the lens is misaligned (e.g., during nsertion), interaction between the upper and lower lids on its active zones quickly orients the lens and helps keep it stable and in place.5
Table 1. Vertical Prism in Soft Toric Contact Lenses The amount of vertical prism varies considerably in commercially available toric soft contact lenses, ranging from 0.00 in ASD and dual-thin-zone lenses to more than 1.15∆ BD in prism-ballasted lenses.
Vertical Prism in Contact Lenses
We visually observed that designs using a prism or peri-ballast design had residual prism (typically base down) in the optical zone. This is an optical artefact from the prism. We therefore evaluated vertical prism in currently commercially available soft toric lenses using an objective in vitro measurement technique. We tested several different toric soft contact lenses with eight parameter combinations per product covering -6.00 D sphere to +3.00D sphere with -1.25D cylinder at both the 90 and 180 axes.1 Thickness maps were obtained using scanning transmission microscopy over the full surface of the lenses (figure 1). We used more than 650,000 thickness points in the central 6-mm optical zone of the lens to determine the average change in thickness and to compute prism via least-squares-fit of a linear equation. As expected, lenses that do not use a base-down prism design for stabilisation were found to have no vertical prism in the optic zone (table). Most of the other lenses measured had a mean vertical prism ranging from 0.50Δ to 1.20Δ.
Clinical Relevance
Prism in the optic zone produces a slight shift of the image being viewed. As long as the displacement due to vertical prism is the same in both eyes, there is unlikely to be any visual consequence. Nearly half of people with visually significant (≥ 0.75 D) astigmatism, however, have astigmatism in only one eye.2 In a patient with a toric contact lens in one eye and a spherical or multifocal contact lens in the other, prism-ballasted lenses will therefore create vertical disparity between the two eyes, which may cause symptoms of vertigo, double vision, tension in the forehead muscles, or eye fatigue/strain.6,7 This problem is of particular concern in individuals with a fragile binocular system.
It can be challenging to predict who might struggle with binocularity or have weaker fusional ability. Many individuals likely have anatomical imperfections that cause minor exophoria, esophoria, or vertical phoria for which they simply compensate in order to achieve fusion. Clinical studies have not been conducted to fully characterise the clinical effects of differences in base-down prism among different contact lenses. The effects are likely to be subtle and may be most noticeable with prolonged visual effort, such as at the end of a long day at the computer. The greater the disparity in the amount of vertical prism and the lower an individual’s fusional reserves, the more likely he or she is to experience the optical effects of unilateral vertical prism.
An artificially induced vertical phoria, possibly caused by prism, is one factor to consider when fitting toric contact lenses for monocular astigmats or those requiring a mix of toric soft contact lens designs. Future clinical work could further explain the impact of vertical prism and other optical qualities of toric contact lens designs.
Giovanna E. Olivares, is a Doctor of Optometry and the director, global platform research and development, for Johnson and Johnson Vision Care in Florida USA.
This article is published with permission from Bryn Mawr Communications and was originally published in the June 2014 issue of Advanced Ocular Care.
References
1. Olivares G, et al. Are toric soft contact lenses prism free in the optic zone? Poster presented at: BCLA May 2014 Birmingham, UK
2. Eghbali F et al. Oxygen Transmissibility at Various Locations in Hydrogel Toric Prism-Ballasted Contact Lenses. Optometry & Vision Science. 1996; 73(3):164-8
3. Gillies, E. Soft Torics, see what could be. mivision
March 2012
4. Zikos GA et al. Rotational Stability of Toric Soft Contact Lenses During Natural Viewing Conditions. Optometry & Vision Science. 2007; 84 (11): 1039–45
5. JJVC Data on file 2008
6. Jackson DN, Bedell HE. Vertical heterophoria and susceptibility to visually induced motion sickness. Strabismus. 2012;20(1):17-23.
7. Bennett ES, Weissman BA. Clinical Contact Lens Practice. 2005, 496-7.
