Visual correction for sports

Visual Correction for Sports

BY HELEN VENTURATO AND NICOLA PEAPER

LEARNING OBJECTIVES

Understand how the wrap of a frame causes prism due to lens tilt, lens thickness, refractive index and base curve
Appreciate how wrap effects the position of the lens design
Realise why a corrected pupil distance is required with frames of this type
Understand how wrap effects the as worn powers
Appreciate why power compensation is necessary.

It is common practice to adapt our vision assessment and dispensing to specific occupational needs. With one in five Australians regularly participating in competitive sports, and one in two Australians regularly participating in fitness activities such as yoga, the gym and cycling, the demand for vision correction and eyewear tailored to sporting needs is significant.

Interestingly, the trend of ‘active wear’ becoming street wear is also driving an increase in demand for wrap eyewear and sunglasses. Whether for general wear or for specific sporting needs, the optical complexity of wrap eyewear requires careful dispensing attention. Understanding the visual demands, environment and risks of a sport underpins the selection of correction and eyewear. Understanding the optical properties of a lens and optical limitations enhances visual outcomes.

At the basic level, sports vision dispensing involves:

Identifying those who participate in sport and fitness activities;
Understanding the nature of the activity – particularly with respect to speed and predictability;
Understanding the environment and lighting in which the activity takes place;
Understanding and evaluating the relevant visual skills;
Providing advice on appropriate eyewear and protection.

When seeking to identify those who participate in sporting or fitness activities, it is important to question carefully. Rather than ask, “Do you play any sport?”, a broad question such as, “What fitness or leisure activities do you participate in?” will be more effective. Asking more broadly about fitness and leisure is more likely to uncover needs related to gym training, cycling, yoga, etc. Asking “What type of activities bother you with your spectacles/current correction?” is also useful to identify unmet needs. While you do not need to be an expert at every sport, it is useful to have some insight into the nature of a sporting activity. Even with a very basic understanding of squash and resistance training for example, it seems obvious that the visual demands of playing squash are higher than those required for resistance training in the gym. The size of a ‘target’, speed and predictability of an activity all have a bearing on visual demands and safety considerations.

Safety considerations should always be at the forefront with any sporting or leisure activity. Those activities involving a small object that moves fast and in an unpredictable manner pose a particular ocular safety risk, and suitable eye protection is critical. For example, a golf ball is small and moves very quickly once struck, but the movement is generally relatively predictable. However, a squash ball is small, moves very quickly and the pattern of movement is highly irregular and unpredictable. Target movement and dynamic environmental shifts such as rapid changes in posture and balance, broaden the visual skills needed for efficient decision-making, and may impact on dispensing decisions. Likewise, the distance over which the activity is undertaken and the duration of the activity should also be considered when contemplating correction and dispensing. Fatigue may adversely impact on visual skills, causing performance to deteriorate. In the ideal world, the vision testing environment would replicate the participation environment. Although, it is difficult to reproduce factors such as fatigue; background colour and movement; extraneous noise; and lighting changes in most vision testing environments, awareness of these factors will guide dispensing practice.

Any dispensing advice given should meet the mandatory requirements of the sport’s governing body. For instance, the world governing body for soccer, FIFA, states in Law 4: “Players are permitted to wear sports goggles, if in the opinion of the match official, they pose no danger to the player or other players”. This means that some sports goggle materials and designs may be deemed to place another player at risk in the event of a head clash, and subsequently disallowed.

Before a dispensing decision is made, there are a wide range of visual skills that may be considered as relevant to a particular sport or fitness activity. Monocular elements, such as refractive error; visual acuity; and contrast sensitivity; as well as skills such as pursuits and saccades; depth perception; and eye-hand–foot dominance and co-ordination may all contribute to the ability to perform within the sporting environment.

The importance of optimal visual acuity (VA) for sporting activities remains under debate. In 2013, Griffiths demonstrated that in tennis players and clay target shooters, uncorrected ametropia impacted significantly on performance and that the impact of blur was related to the pattern of eye and hand dominance. The study demonstrated that the performance of participants with a pattern of same side dominance of eye, hand and foot (eg. right dominant eye-hand-foot) was most affected by blur in the dominant eye. In 2017, Barrett et al conducted a study on elite and near-elite cricket and rugby league players and found that 20–25 per cent of participants had sub-optimal vision on the field. The study concluded that poor vision may not affect participation in the sport, and that the impact of correcting smaller refractive errors on performance remained unknown. Despite the lack of consensus about the benefits, there seems to be a general agreement that correcting refractive error and optimising visual acuity will not lead to poorer sporting performance. Uncorrected refractive error may lead to higher fatigue, and diminished performance over the course of activity, as well as reduced comfort and enjoyment.

Commonly in practice, VA is measured on a static high contrast chart, and 6/6 is considered ‘normal’. In sports such as sailing, golf, cycling and shooting, where the activity involves a small object, is fast moving, or involves significant distance, VA should be corrected to the maximum achievable for the individual and consideration should be given to correcting small amounts of ametropia to maximise the VA. While dynamic vision testing may be a realistic representation of the nature of sports, it requires specialised equipment, includes a large number of variables and may be difficult to relate to activity performance.

An assessment of VA should also include contrast sensitivity. In golf, the ability to ‘read a green’ - to see the grain of the grass and subtle topographical changes – is heavily reliant on contrast sensitivity. A low contrast chart provides a reliable, repeatable and quantifiable measure. Reduced contrast sensitivity may require the use of filters, anti-reflection coatings or a lens designed to reduce higher-order aberrations.

When considering the binocular demands of sports, attention should be paid to eye dominance; stereopsis; phorias and fixation disparity; accommodation and vergence; eye movements such as pursuits and saccades. It is important to ensure that the visual skill assessed is relevant to the activity. When assessing these skills, consider the posture, the field of view, and the main plane of gaze required during the activity. For instance, a target shooter may perform the activity in the standing, kneeling or prone positions and it is appropriate to attach more significance to the binocular skills in the superior field of gaze. Although vision training to enhance already good visual skills remains controversial, it is important to ensure that the visual skill will meet the demands of the activity. Poor binocular vision skills may be related to slower, inefficient visual processing, leading to poorer hand eye co-ordination, slower reaction times, slower decision making and increased levels of fatigue.

It is critical that any dispensing solution offered does not interfere with visual skills. The dispensing solution must address:

Safety issues
UV protection, glare and light control
Field of view
Maintaining VA and efficient binocularity.

Contact lenses are frequently considered the ideal dispensing choice for sports and fitness participants as they provide stability of vision for activities involving sudden postural changes and rapid movement, and they are unlikely to fog. Contact lenses provide a wide, uninterrupted field of view and reduce the impact on retinal image size. By moving with the eyes, off-axis errors are reduced. Single use lenses are the ideal choice for sports – a clean lens for each wear gives better VA and tear stability. Where the activity is outdoors and UV protection is unlikely to be worn (for example any football code), the lens of choice should include UV protection. However, contact lenses are not suitable for swimming and water sports and due to safety issues eye protection may still be required.

Although contact lenses have distinct optical advantages for sports, there are many reasons why a spectacle correction may be chosen. In this situation, in an effort to maximise field of view, comfort and protection, a wrap frame is usually chosen. The wrap aspect creates potential optical concerns: significant amounts of prism may be unwittingly induced affecting binocularity, and the position of the lens design with respect to line of vision may be altered, which in turn may induce aberration and adversely impact on visual acuity. These problems may impact the comfort and clarity of the ‘everyday’ wrap wearer, and they may have a profound impact on performance in the sporting arena. As such, they are worthy of closer consideration.

Figure 1. Prism induced by lens curvature and compensation.

Figure 2. A corrected PD is necessary to compensate for the wrap of the frame.

Aspects of Centration in High Wrap Frames

A lens placed into a wrapped frame will have an amount of prism caused by the curvature of the lens (Figure 1) described by the formula:

Δ = 100tanθ t/n F1

Where:

Δ = the prism induced

θ = the tilt angle

t = thickness of the lens at the reference point in meters

n = refractive index

F1 = the front (base) curve of the lens

A plano lens produced on a 6.5 base, with thickness of 2.0mm, tilt of 20° and refractive index of 1.5 will have induced prism of 0.3Δ BI. To avoid vergence problems and associated binocular problems lenses produced for high wrap frames should be produced with prism to compensate for this induced prism (Figure 1).

When ordering a PD for a high wrap frame it is important to consider what happens when a measurement taken horizontally with no wrap, e.g. with a pupilometer, is then ‘bent’ to take into account of wrap. If the patient’s PD is less than the frame PD (Figure 2) then a corrected PD, which is less than the patient’s PD, needs to be used to fit the lens. If the patient’s PD is greater than the frame PD, the corrected PD will be larger than that ordered. The corrected PD should be given by the lens manufacturer for fitting and checking purposes.

If a corrected PD is not used, then the lens design will not sit in the correct position in relation to the visual axis. In the case of single vision aspheric lenses this will impact upon the amount of aberration the patient will experience. With progressive lenses the patient will not be able to access the centre of the corridor without a head turn.

To summarise, if a patient is dispensed a lens into a wrap frame that does not have a compensating prism to take into account the tilt angle and is not fitted with a corrected PD then the following may happen:

There will be visual discomfort and asthenopia due to constant vergence. This may break down to diplopia.
The vision will not be clear and in progressive lenses, the corridor will not be accessible without a head tilt.

Aspects of Power in High Wrap Frames

If central vision i.e. directly ahead, is the most important aspect of vision for the sport, such as for a stationary shooter, then an aspheric lens compensated for position of wear is the least that we should consider recommending to our patient. If the sport involves tracking a target, as in racquet sports, then an individualised multi aspheric/atoric design, correcting off axis performance far into the periphery of the lens, should be considered. Taking into account the large number of presbyopes engaging in sporting activities, similar progressive lens solutions are now available.

Figure 3. Tilt angle – the effect of face form angle and pupillary distance.

Figure 4. Aberration pattern of spherical vs aspheric lens

For simplicity we will consider the effect of Wrap or Face Form Angle (FFA) only.

For compensation it is not the FFA, i.e. the angle between the plane of the eye shape and the plane of the frame that needs to be considered, but rather the angle of tilt of the lens in front of the eye. The way these two can differ from one another is shown in Figure 3. For a small Pupil Distance (PD), the main ray is almost perpendicular when it is incident on the lens in contrast to a larger PD, where the angle of tilt almost corresponds to the FFA. Normally the angle of tilt is smaller than the FFA, but rarely close to 0° as in the extreme example shown.

The magnitude of the angle of tilt depends on:

• The FFA

• The frame data and the fitting data, including PD

• The front surface curvature of the lens.

Let us compare what happens when a refracted script of -4.00DS, measured in a frame or phoropter head with no FFA is then produced in a frame with a fairly standard FFA of 5° and a high FFA of 25° (resulting in a tilt angle of 20°), common with sports corrections. The power experienced at ocular centre (OC) due to the FFA can be calculated by:

FSPH = F (1 + sin² θ/2n) and FCYL = FSPH tan ² θ

where

FSPH =the as worn spherical power

F = the refracted power

θ = the tilt angle

n = the refractive index of the lens material

FCYL = the as worn cyl power.

The as worn power of the lens in the 5°FFA frame will be:

-4.01/-0.03 x 90.

This small deviation may not be perceptible to the patient. However, if this lens is produced in a spherical form then as the eye rotates away from OC, aberrations such as oblique astigmatism and distortion will quickly come into play to effect vision. An aspheric lens will reduce these aberrations, improving off axis performance to noticeably improve mid to peripheral VA.

The as worn power of the lens in the 20° tilt (FFA 25°) frame will be:

-4.16/-0.55 x 90.

This clearly may impact VA and produce a swim sensation for the wearer. A compensated power lens of -3.85/+0.55 x 90 is necessary to supply the best correction and visual experience. Again, this will be better if produced in an aspheric form (Figure 4).

If we now consider a progressive lens in the 5°FFA frame, the position of wear will correspond to that forming the basis of calculation. As such large useful vision zones are produced (Figure 5) and the corridor is symmetrical and central to convergence (black dotted line). (N.B. All aberration patterns illustrated are based on a script of +3.00/-1.00 x 180 Add +2.00). When the lens is fitted into a frame with a high degree of wrap the design completely fails, with reduced vision zones and inaccessible corridor (Figure 5).

Figure 5. Visual acuity representation of monocular aberration of a progressive lens in a non-tilted frame and a high wrap frame.

Figure 6. Visual acuity representation of monocular aberration with a progressive lens with power compensated at the ocular centre.

Figure 7. Visual acuity representation of binocular aberration experienced when a conventional progressive lens is fitted to a wrap frame.

If the compensation principles as described are applied, both for power at OC and prism, then symmetry will be better. However, the useful width of the distance zone will still be reduced by oblique astigmatism and other aberrations such as distortion, making the lens unsuitable for tracking objects (Figure 6).

Since the actual image is made up by the visual impressions of both eyes, the asymmetry brought about by fitting a conventional progressive lens in a high wrap frame means that the images are too different to merge, and the resultant useful fields of vision will be dramatically reduced (Figure 7).

To summarise, if a patient is dispensed with a single vision spherical lens in a wrap frame you can expect the patient to complain of central blur and swim, which will increase with lens power.

If a patient is dispensed with a standard progressive lens designed for a normal frame, in to a frame with a high degree of wrap, you can expect the patient to complain of small areas of clear vision for distance and little to no useable intermediate and near vision. This can be improved by dispensing a progressive lens that has been compensated for position of wear, but peripheral vision may still be poor.

When tracking of an object is necessary in a sport, head and eye movements need to be quick and accurate. Research into driving shows that single vision, bifocal and progressive lens corrections all increase the path length of both head movements and eye movements (saccades) when viewing distant dynamic stimuli compared to contact lens corrections.6 For stereopsis to occur, the right and left images must be similar and aberration may have an impact on this. It would therefore make sense to advise the best possible quality lens available to either minimise the aberration encountered or to match the aberration that the right and left eyes are experiencing.

As the eye scans across the lens, so the tilt angle and Corneal Vertex Distance (CVD) will change. This means that the compensation calculations will need to change away from the OC (Figure 8). In these cases, the best solution will be an individualised design based on the script and wearing parameters, including PD, CVD and FFA.

Another aspect that can be considered is that the eye rotates as it moves towards the periphery of the lens as governed by Listings Law. To take this into account, the cyl axis would need to be changed progressively away from the OC of the lens. This compensation is available in a spectacle lens.

Finally, on clarity of monocular vision, Higher Order Aberrations (HOA) are known to reduce contrast. Many lens manufacturers include HOA compensations into their lens designs.

If all these things are taken into account then:

A single vision lens in a wrap frame would have clear vision virtually up to the rim. Binocularly, there would be little aberration to stop the ability to merge the right and left images, giving wide useful vision areas and good binocularity in all directions of gaze (see Figure 4, aspheric/atoric design).
A progressive lens would have a similar aberration pattern to the conventional lens in a non-tilted frame. The symmetry would allow for easy merging of right and left images, giving the widest possible vison zones (Figure 9).

To summarise, the best spectacle lens solution for a sports player wearing a wrap frame is a lens of individualised design based on the script and wearing parameters, including PD, CVD and FFA, and a compensation for HOA and eye rotation. This will give wide, symmetrical fields of view, which allow for good binocular vision.

Figure 8. Corneal vertex distance and tilt alter with position of gaze.

Figure 9. Visual acuity representation of binocular aberration experienced when a fully compensated lens is fitted to a wrap frame.

While much can be achieved with lens design and digital production there are limits – the laws of physics cannot be suspended with tilted lens systems. To this end, sports lenses will always have a reduced power range over which optimal monocular and binocular vision will be obtained.

Environmental Considerations

Sports players, in common with all patients, require protection from glare and possible light-induced ocular damage, while maintaining contrast and colour perception. This aspect is out of the scope of this article, however information on this subject is covered in a previous article, Controlling Light – Transmission, Reflection and Absorption of Spectacle Lenses.7 Suffice to say, there are numerous materials and lens treatments designed to reFigure 9. Visual acuity representation of binocular aberration experienced when a fully compensated lens is fitted to a wrap frame.duce glare and enhance the contrast between various colours for sports vision.

Finally, frame materials need to be taken into consideration when dispensing for sport and fitness activities. Many of the cheaper, ‘fashion’ wrap frames are made of brittle materials that will snap under pressure, resulting in sharp and possibly damaging edges that could easily perforate the eye. Safer, more flexible materials should be recommended.

No matter what level of sports the patient competes in, good vision can enable better performance and so increase enjoyment. Many sports solutions include high wrap frames to provide good protection from glare, UV, wind and impact. When a lens is fitted to a wrap frame the following issues need to be addressed:

The base curve necessary will be a 6-8D curve. This is not suitable for many scripts and, if produced in a simple spherical form, the off-axis performance will be poor. The patient will encounter high levels of oblique astigmatism only millimetres away for the OC, giving rise to poor vision and swim. Optimisation can reduce this effect.
The act of tilting the lens in front of the eye will mean that the power experienced by the patient at OC will not be as refracted and ordered. Again, this will result in poor vision and swim. A compensated power is necessary to resolve this issue.
As the eye scans across the lens surface, the tilt with respect to the visual axis and CVD will alter. The compensations that produce good vision at OC will not work peripherally and a lens that is individualised, based on the script and wearing parameters, including PD, CVD and FFA is necessary for good peripheral vision.
Prism will be induced based on lens thickness, refractive index, the angle of tilt and base curve. A compensating prism is necessary to enable comfortable binocular vision.
The PD will need to be compensated as the wrap moves the OC away from pupil.
Binocular vision depends upon the eyes receiving similar images – aberration caused by all of the points above may impact upon this.

Giving consideration to all of these issues, it becomes apparent that a spherical ‘grind’ lens is unlikely to result in satisfactory vision or comfort for general purpose wear, and will not meet the visual demands of sports participants. Most lens manufacturers have a range of tailored sports lenses to help resolve these issues.

Credits: https://www.mieducation.com/pages/visual-correction-for-sports