In this guide, we will walk through the essential steps and tests involved in assessing the fit during a soft contact lens fitting. It is a crucial skill that ensures patients experience both comfort and clear vision, whilst keeping their eyes healthy. Understanding the full fitting process can help you provide the best care for your patients.

By mastering these techniques, you’ll be able to make accurate assessments and adjustments, leading to successful outcomes for your patients. This post is long, so please use the table of contents below to skip to the desired section. This guide covers soft contact lens fitting assessments of a single vision spherical contact lens. Future posts are planned for toric lenses and multifocal soft contact lens fittings.

1. Initial Parameters in Soft Contact Lens Fitting
2. Key Tests and Assessments in Spherical Soft Contact Lens Fitting
   1. Corneal Centration and Coverage
   2. Lens Lag (Horizontal Movement)
   3. Lens Sag (Vertical Movement on Upgaze)
   4. Lens Movement on Blink
   5. The Push-Up Test
3. Patient Subjective Response
4. Conclusion
5. Skill Activity

PLEASE NOTE: This guide is intended to be used to supplement the studies of student optometrists, contact lens opticians and ophthalmic medical practitioners. Whilst every effort goes into ensuring the accuracy of these guides, please remember your learning establishment may have their own methods of assessing contact lens fits, with their own guidance on acceptable values. Please follow the guidance of your education establishment in the first instance, with this guide being used as supporting material.

FOR EDUCATIONAL USE ONLY – IF YOU ARE A PATIENT WISHING TO TRY CONTACT LENSES, PLEASE CONSULT YOUR LOCAL OPTICAL PRACTICE FOR ADVICE.

Initial Parameters in Soft Contact Lens Fitting

Selecting the right contact lenses begins with understanding the unique features of your patient’s eyes. Soft contact lens fitting is a personalised process that ensures your patient receives clear vision and comfort, whilst minimising the risk to their eye health. By considering the following specific parameters, you can choose lenses that fit well and feel natural.

Keratometry Readings

Measuring the curvature of the cornea is much like measuring your feet for the right shoe size. Just as shoes need to match the shape of your feet to provide comfort and prevent blisters, contact lenses need to fit the unique curvature of your cornea. Keratometry readings give us an exact measurement of the central cornea’s curve, allowing us to select a lens with the appropriate base curve. This ensures the lens sits comfortably on the eye, providing clear, stable vision without causing irritation or movement issues. A precise fit is crucial to avoid discomfort and maintain optimal eye health, much like a well-fitted shoe supports and protects your feet.

Generally, we aim to choose a contact lens base curve (BC) of 0.7 to 1.0 mm flatter than the flattest meridian of the cornea, although a relatively recent study relating to the parameters of soft contact lenses has suggested that due to differences in modulus and water content of different lenses, this may not be as an effective predictor on soft lens fit. Nevertheless, it provides a good starting point.

Horizontal Visible Iris Diameter (HVID)

The size of the iris helps determine the appropriate lens diameter. Choosing a lens that fully covers the cornea is crucial. Proper coverage ensures the lens stays centred on the eye, providing comfort and stability during wear. A well-fitting lens that matches the HVID will move naturally with the eye, maintaining clear vision and reducing the risk of discomfort.

The average HVID is 11.8 mm and the goal is to ensure the total diameter (TD) of the lens is approximately 2 mm larger that this to ensure full coverage of the cornea and limbus, with approximately 1 mm overlap on to the conjunctiva to allow for the movement of the lens upon the eye. Most soft contact lenses have a total diameter of 14.0-14.5 mm, so often HVID does not limit the choice.

If the patient has an HVID much larger or much smaller than average, they may require custom lenses to be made to ensure an adequate fit.

Vertical Palpebral Aperture (VPA)

The vertical palpebral aperture (VPA) is the distance between the upper and lower eyelids. This measurement is crucial because it influences how the contact lens interacts with the eyelids during blinks.

For patients with smaller vertical palpebral apertures, fitting can be more challenging due to the limited space available for the lens. These patients may experience more difficulty inserting and removing the lens, and the lens may interact more with the eyelids, causing unstable vision as the lens moves up and down with the top eyelid.

By considering this measurement when soft contact lens fitting, you can select a lens that moves smoothly with each blink, ensuring better comfort and stability for the patient.

Pupil Sizes

Understanding pupil size in different lighting conditions is crucial for ensuring clear vision throughout the day and night. Pupils dilate in darker conditions, and if the optic zone of the soft contact lens is smaller than the patient’s maximum pupil size, they may experience flare or halos around the edges of their vision.

This measurement is traditionally more important in rigid gas permeable lens fitting, as these lenses are smaller than the iris and more likely to interfere with the pupil. However, it’s still important for soft lens patients too. High prescriptions often focus the corrective power in the central region of the lens to minimise thickness. Additionally, multifocal lenses and lenses used for myopia control have specific zones that require pupils of certain sizes for them to work as intended, so don’t neglect to consider the patient’s pupil size!

Prescription

An accurate prescription is fundamental for clear and stable vision, so please ensure you have an up to date refraction for your patient. When soft contact lens fitting for those with higher prescriptions, it’s important to adjust for back vertex distance – as the effective power of the prescription can be affected significantly when moving from the spectacle plane to the cornea. This adjustment ensures that the lens power is correct when placed on the eye, maintaining the intended visual acuity.

For patients with astigmatism of 0.75 DC or more, toric lenses should be considered. These lenses have the cylinder power and axis to correct the irregular curvature of the cornea. Toric lenses are now available in a variety of powers, axes, modalities and materials, so they can be discussed with your patients. That said, due to the increased number of potential prescription combinations that adding a cylindrical correction to a lens, some compromise may be required when choosing the most appropriate power specification for a given prescription.

Modality

Whilst our biometric findings may be what we feel are most important factors when in the process of soft contact lens fitting, we also need to make sure the lenses we choose are right from the patient’s point of view too!

Choosing the right modality for contact lenses – whether daily, bi-weekly, or monthly – depends on several factors tailored to the patient’s lifestyle and eye health. Daily disposable lenses are convenient and hygienic, ideal for patients who prefer minimal maintenance or have allergies. They’re great for those who wear lenses infrequently or engage in sports.

Bi-weekly and monthly lenses require more care but are often more cost-effective for regular wearers. These lenses are suitable for patients who need a broader range of prescriptions, including those with astigmatism or presbyopia. The availability of specific prescriptions can influence the choice of modality, ensuring the patient receives the correct visual correction.

Material suitability also plays a crucial role. Some materials offer higher oxygen permeability, which is beneficial for patients with dry eyes or longer wearing times. Ensuring the chosen modality aligns with the patient’s daily routines and eye health needs is essential for a successful and comfortable lens-wearing experience.

Taking the time to consider all aspects of soft contact lens fitting prior to selecting the initial lens can increase the chances of selecting optimal lenses from the start, minimising the amount of chair time for both you and the patient and increasing patient satisfaction in your skills. We will now look the key tests and assessments during the soft contact lens fitting process.

Key Tests and Assessments in Soft Contact Lens Fitting

We will now look at some of the key tests and assessments that you should perform when assessing the contact the lens whilst soft contact lens fitting. Please note, the contact lens will need time to settle on the eye and for the patient’s eyes to adapt to having a contact lens in place. Ideally, allow 30 minutes or so to pass after inserting the lens to assess the fit, but whilst clinical time is short and chair time limited, this may not always be possible. That said, please allow at least 5 minutes before assessing and recording your findings.

Corneal Centration and Coverage

Importance of Centration and Coverage

Getting the corneal coverage and centration right is a cornerstone of soft contact lens fitting. The lens should fully cover the cornea, not just when looking straight ahead, but in all directions of gaze. Ideally, there should be about 1 mm of the lens overlapping the iris and limbus. This overlap ensures the cornea stays protected, reducing the risk of irritation and damage.

Assessing Centration and Coverage

To assess corneal coverage and centration, take a close look with a slit lamp. Observe the lens position when the patient is looking straight ahead and when they move their eyes in the 8 positions of gaze. Check for any signs of the lens edge crossing the limbus or the lens sitting off-centre. Remember, a well-centred lens that fully covers the cornea is what you want to observe.

I find aligning the slit beam on the nasal edge of the lens and adjusting the beam width to match the gap between the limbus and the lens edge. I then compare this to the gap on temporal edge. If both of these slits are the same width, then the lens is centred horizontally. If it is thicker nasally, then the lens is decentred nasally (and vice versa if wider on the temporal side – as shown in the video below).

For vertical alignment, you can see if there is more lens within the superior or inferior part of the beam, enabling you to assess if the lens is positioned low in the eye (decentred inferiorly) or higher in the eye (displaced superiorly).

You can then check for coverage in the 8 cardinal gazes and check to see the lens edge doesn’t cross the limbus in any of those positions.

Interpretation of Centration and Coverage

In a perfect fit, the lens sits centred over the cornea. It doesn’t drift off to one side, and it moves naturally with the eye. This central placement helps maintain comfort and keeps vision sharp. The lens should stay put during blinks and eye movements, without slipping or shifting noticeably.

An acceptable fit might have the lens slightly off-centre, but still covering the entire cornea in all directions. As long as the lens isn’t rubbing against the limbus or causing discomfort, it can be considered adequate. However, it’s important to keep an eye on such fits, as even slight decentration can lead to disruption to vision or other issues over time.

A poor fit shows signs like the lens not covering the cornea completely, especially when the eyes move. The lens edge might cross the limbus, which can lead to mechanical stress on the peripheral cornea and the limbus. This can cause irritation or even damage if left unchecked. Significant decentration can also degrade the vision, particularly if the optic zone isn’t aligned with the pupil.

Lens Lag (Horizontal Movement)

Importance of Measuring Lag

Lag is a useful measurement in assessing how well a contact lens fits. It refers to the movement of the lens when the eye looks left or right (it follows and “lags” behind the movement of the eye). Proper lag ensures that the lens moves with the eye, allowing for appropriate tear exchange and also ensuring the lens remains fairly central when looking away from primary gaze.

Assessing Lag

In a similar way to assessing the centration of the soft lens, have the patient look straight ahead in primary gaze. Create the slit beam on the nasal edge of the lens and then extending it to the nasal edge of the limbus to get the nasal centration in primary gaze.

The patient should then look in temporal gaze. Follow the gap between lens edge and limbus and align the beam with the nasal edge of the limbus. If there is lag, the gap between the limbus and the lens edge should be larger. You can either measure the size of this gap or work out the percentage increase of the gap compared to the original width of the beam. This measurement is the lag.

This should be repeated on the temporal side of the lens and then have the patient look nasally.

Interpretation of Lag

I have often noted there are a range of conflicting information regarding what is an acceptable amount of lag, with anything between 0.2 mm to 1.5 mm of lag being described! With the viewpoint that we would expect the average soft lens diameter to be about 2 mm larger than the iris, this leaves a ring of overlap of a width of 1 mm. Therefore, any values above 1 mm would likely mean the other side of the lens would be crossing the limbus, deeming the lens fit unacceptable.

Whilst soft contact lens fitting involves taking the fit of the lens as a whole, it is therefore utilise common sense when looking at the lens and assessing whether the lens fit it adequate or not.

Ideal Fit: The lens should exhibit a lag of around 0.25mm to 0.75mm during horizontal eye movements. Alternatively, this can be measured as a percentage increase in overlap, with the lens moving about 25-75% more onto the sclera from its central position. This moderate movement is crucial for facilitating tear exchange without causing discomfort.

Too Steep or Tight Fit: If the lens shows minimal lag, less than 0.2mm, or less than a 20% increase in overlap, it is likely too steep or tight. This can lead to discomfort due to reduced tear circulation and excessive pressure on the surrounding conjunctiva and sclera.

Too Flat or Loose Fit: Excessive lag, more than 0.75mm, or more than a 75% increase in overlap, indicates a flat or loose lens. The lens moves too much with eye movements, causing unstable vision, poor vision off axis and potential irritation as the lens edges rub across the limbus.

Lens Sag (Vertical Movement on Upgaze)

Importance of Lens Sag

The sag of the lens on patient upgaze is essentially the same parameter as lag is on horizontal gaze shifts, with the added force of gravity affecting the movement of the lens. The reasons this is assessed is to ensure stable vision and adequate tear exchange under the lens. Similarly, it is also generally considered a poor predictor on how well a lens fits, but does aid in assessing the fit of the lens as a whole when considering other measurements.

Assessing Lens Sag

The lens sag is assessed in a similar way to assessing the lag. The slit beam should be rotated horizontally and aligned over the lens overlap in the inferior region of the eye. Unfortunately, in many patients, the lower lid is occluding the view of this overlap, so you may need to carefully lower the inferior eyelid and ask the patient to look towards your forehead to ensure an adequate view of the inferior edge of the lens. You then compare the height of the beam with the overlap resulting when you ask the patient to look up.

Interpretation of Lens Lag

As we have discussed, various resources state different measurements for what is considered ideal, tight and loose and therefore it is again useful to consider what you are seeing as a whole instead of judging the fit purely on this element alone.

Ideal Fit: The lens should exhibit a lag of around 0.25mm to 0.75mm during upgaze. Alternatively, this can be measured as a percentage increase in overlap, with the lens moving about 25-75% more onto the sclera from its central position

Too Steep or Tight Fit: If the movement on upgaze is less than 0.25 mm/25% increase in overlap, it can be considered a tight fit.

Too Flat or Loose Fit: If the movement on upgaze is more than 0.75 mm/75% increase in overlap, it can be considered a loose fit. Remember, if there is 100% increase, it is likely that the edge of the lens is crossing the superior limbus.

Lens Movement on Blink

Importance of Assessing the Lens Movement on Blink

Assessing lens movement on blink is a crucial part of evaluating the fit of soft contact lenses. During each blink, the lens should exhibit a slight, controlled movement. This movement allows for tear exchange underneath the lens, which is essential for maintaining eye health and comfort.

However, too much movement will shift the lens and result in unstable vision as the lens may not immediately settle back centrally. Furthermore, if the lens consistently moves too much on blinking, the patient will notice their vision blurring for a few moments after each blink.

Assessing Lens Movement on Blink

To accurately assess lens movement on blink during a soft contact lens fitting, I ask my patients to look at my forehead. This subtle elevation of the eye allows me to observe the inferior edge of the lens without significantly altering the overall primary gaze position. By doing so, I can effectively evaluate the lens movement while keeping the patient’s gaze close to its natural position. It is important to avoid touching the eyelid during this assessment, as any interference can impact the lens’s natural movement.

I then set my slit height to 0.3 mm (most slitlamps have this as a pre-set – but you may need to check the model that you are using), align the bottom of the slit beam with the bottom edge of the inferior edge of the lens. I then ask my patient to blink and observe the movement of the lens in comparison to the height of my slit. If the movement is the same height as the slit height, then the movement is 0.3 mm, if it is less, calculate the percentage of the movement within the beam and if beyond the height of the slit, estimate by how much and calculate the equivalent distance.

Interpreting Lens Movement on Blink

As we have discussed in the previous section on lag and sag, this information is best used when considering the other measurements taken to assess the fit of the lens as a whole.

Ideal Fit: Movement of 0.2 mm to 0.4 mm would be considered adequate lens movement on blink, allowing for sufficient tear exchange but not moving to excess and causing visual disturbance. It is generally easy to make this assessment given this is roughly the height of the 0.3 mm beam.

Too Steep or Tight Fit: Any movement of less than 0.2 mm would be considered too tight and indicate that there is poor tear exchange. It is likely that the patient finds their vision as stable and may describe the fit as comfortable, due to lack of movement of the lens.

Too Flat or Loose Fit: Any movement in excess of 0.4 mm would be considered too loose and you are likely to see excessive movement of the lens on the eye. The patient is likely to state their vision is unstable when blinking and/or the lens is uncomfortable.

The Push-Up Test

The Importance of the Push-Up Test

The push-up test is a valuable method for assessing the fit and movement of soft contact lenses. This test helps determine how easily the lens can be moved and how well it returns to its proper position on the cornea. Out of all of the tests used in soft contact lens fitting, this one is regarded as one of the most valuable.

It will allow you to see how easy it is to displace the lens, understand how it moves upon the eye and also how well it recovers from being dislodged, either through a blink or a patient rubbing their eyes. It will also give you understanding and context for the other tests you will have performed prior.

Assessing the Lens by the Push-Up Test

The setup for the push-up test closely resembles that for assessing movement on blink, and for efficiency, I usually perform the push-up test immediately afterward. Start by having the patient seated at the slit lamp, directing their gaze towards your forehead. Inform the patient that you will be applying gentle pressure to their lower eyelid.

Adjust the beam width to create a shallow horizontal beam. Then, carefully push the lower lid upward to displace the contact lens upwards. Observe how the lens moves and recovers. Repeat this process several times to gather consistent observations and then make your assessment.

Interpretation of the Push-Up Test

Performing this test allows you a qualitative assessment of the contact lens on the eye – allowing you to assess movement and recovery. Below are some interpretations of the result.

Ideal Fit: For a well-fitting lens, the push-up test should show that the lens moves easily and smoothly when nudged upward with the lower eyelid. It should return to its central position promptly and without hesitation. This indicates that the lens fits well, allowing for appropriate tear exchange and maintaining stable vision and comfort.

Too Steep or Tight Fit: If the lens is difficult to move and shows minimal displacement during the push-up test, it indicates a too-steep or tight fit. The lens may move but returns to its position with a jerky recovery. This restricted movement can cause discomfort due to inadequate tear circulation and excessive pressure on the ocular surface. Patients might also find it challenging to remove the lens, as a tight lens can be difficult to dislodge.

Too Flat or Loose Fit: Excessive movement during the push-up test suggests a flat or loose lens. The lens moves too easily and may take longer to return to its central position due to it lodging under the top eyelid. This excessive movement can lead to unstable vision and potential irritation as the lens edges rub against the limbus and eyelids. A loose lens may not stay properly centred, affecting both visual stability and comfort.

Patient Subjective Response

Assessing the patient’s subjective response is a crucial aspect of determining the success of a soft contact lens fitting. Two primary factors to evaluate are comfort and vision stability. It is also useful to remember that if the patient’s subjective response to a soft contact lens fitting is poor – they aren’t going to want to proceed with a trial – even if the fit looks perfect in your eyes.

Comfort

The lens should feel indiscernible on the eye, providing a sensation similar to not wearing any lens at all. Patients often describe well-fitting lenses as “barely noticeable” or “like wearing nothing.” Any sensation of dryness, irritation, or foreign body feeling indicates that the fit may not be optimal. This could be due to incorrect lens parameters, inadequate moisture retention, or lens material incompatibility. Encourage patients to wear the lenses for a few hours before the assessment to get an accurate sense of long-term comfort.

Vision

The patient’s vision should be stable and clear, without any fluctuations throughout the day. Vision stability means that the lens stays properly centered and does not shift during eye movements or blinks. Any complaint of blurry or fluctuating vision suggests that the lens may be moving excessively or not providing the correct prescription. For patients with astigmatism or multifocal lenses, precise alignment of the optical zones is critical for clear vision.

During the fitting process, ask the patient to perform various activities, such as reading, looking at distant objects, and moving their eyes in different directions. This helps identify any vision stability issues in different scenarios.

Patient Feedback

It’s also essential to gather comprehensive feedback from the patient. Ask open-ended questions about their overall experience, any discomfort they feel, and the clarity of their vision. Patient feedback provides valuable insights that can guide adjustments in lens parameters or materials to achieve an optimal fit.

By thoroughly assessing and addressing the patient’s subjective response, you ensure that the contact lenses provide both comfort and stable vision, leading to higher satisfaction and successful lens wear.

Conclusion

In conclusion, achieving the perfect fit for soft contact lenses is essential for ensuring both comfort and vision stability for patients. By thoroughly assessing corneal coverage, centration, lag, sag, and lens movement on blink, practitioners can identify any issues that may affect the wearer’s experience. Utilising the push-up test and closely observing patient subjective response provides valuable insights into the fit and comfort of the lenses.

These tests help ensure that the lenses move naturally on the eye, allowing for tear exchange and minimising discomfort. Ultimately, a well-fitted lens should feel indiscernible to the patient, providing stable and clear vision throughout the day.

By prioritising these key aspects of soft contact lens fitting, practitioners can enhance the overall satisfaction and ocular health of their patients, ensuring a successful and comfortable lens-wearing experience. The key to successful soft contact lens fitting lies in attention to detail and the ability to respond effectively to patient feedback.

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Skill Activity

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