SoftSpecialEdition is a quarterly newsletter that provides independent updates from the international literature on soft specialty lens-related topics. View as Webpage
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The Impact of Contact Lens Dropouts on the Market
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An interesting mental game to play on a quiet evening during lock-down is to consider the changes to contact lenses that might have the greatest positive impact on the overall market. Dropouts - patients who are prescribed and fitted with contact lenses but who later discontinue - are a key issue across the contact lens industry. By definition, such an eventuality is a failure for the wearers. They have expressed a desire to use contact lenses for their vision correction, but some aspect of the process has fallen short of their needs or expectations. Two outstanding recent papers looked at the dropout rates in hundreds of patients who were fitted with contact lenses for the first time and determined annualised dropout rates of 22% and 26% depending on the methodology used, whereas the rate across all contact lens wearers (i.e., a typical mix of new and established wearers) has been estimated at 17%. The impact of better management here is profound. If the current estimate of 17% dropouts per year is reduced only slightly to 15% per year, the projected number of wearers over 20 years increases to 6700 from the expected 4600 - almost 50% more. In other words, careful consideration of contact lens dropouts and their management by eye care practitioners can lead to a significant and sustainable improvement in the clinical success of lens wearers and to a dramatic impact on the overall contact lens market. To read more on this as well as about the key question 'what is the cause of these dropouts', see link below.
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Soft Lens Fit Evaluation with OCT
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Conjunctival Imprint of Soft Lens Fit
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Alexandra Jandl et al investigated the relationship between soft contact lens imprint into the conjunctival tissue, observed by optical coherence tomography (OCT), to conjunctival staining and to contact lens wearing comfort. Participants were fitted with three different base curves of the same silicone hydrogel custom lens type in a randomised order. One lens was optimally fitted according to the manufacturer’s recommendation, one lens was fitted 0.4 mm flatter and one lens was fitted 0.4 mm steeper. After 4 hours of lens wear, the contact lens edge in the area of the conjunctiva was imaged nasally and temporally. To correct the artefact due to optical distortion with OCT, the imprint of all worn lenses was measured afterward on a glass plate. The mean conjunctival imprint of all contact lens edges was 32.0±8.1 μm before and 7.3±6.5 μm after distortion correction of the OCT images. The distortion-corrected conjunctival imprint with the 0.4 mm steeper lens (11.5±6.2 μm) was statistically significantly greater compared to the optimally fitted lens (6.5±5.9 μm) and was greater compared to the 0.4 mm flatter lens (3.9±5.3 μm). There was no statistically significant difference between the optimally fitted lens and the 0.4 mm flatter lens. The nasally measured imprint (11.4±9.0 μm) was significantly greater than the temporally measured imprint (3.3±7.6 μm). There was no statistically significant correlation between the amount of conjunctival imprint and the graded conjunctival staining or the wearer’s comfort. In conclusion, steeper lenses cause greater conjunctival imprint, but this was not related to clinically significant staining or to changes in comfort. The observed differences between nasal and temporal imprint are likely to be caused by variations of conjunctival thickness and the shape of the underlying sclera.
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Understanding Soft Lenses & Regulations
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Joe Barr reviews soft lens affairs from a scientific and regulatory angle in Contact Lens Spectrum. Among many other things, he states that, based on the literature, sagittal or sag depth (the total depth of the back of the lens determined by the posterior curvature and diameter) is the most important lens dimension controlling the fit of the lens to the eye. If it is not replicated when switching from one lens to another, the likelihood of a misfit or an improper fit increases. Two lenses may appear similar from base curve and diameter numbers but may differ in sag depth due to the factors listed above. He quotes two frequently used lenses that have the same labeled parameters - 8.6 mm base curve and 14.0 mm diameter - but have a 125-micron difference in sag depth on the back surface. This difference in depth can be a clinically significant. And, although both lenses have the same labeled parameters, beyond the sag depth difference, they also have different elastic modulus (stiffness) and may have different thickness profiles, both of which can affect lens flexure on the eye. Barr continues that comfort can change dramatically from one lens brand to another due to a number of factors, including material, peripheral lens design, thickness, and the shape of the edge and that lens comfort is the major reason for patients’ discontinuation of lens wear. Surface wettability and lubricity vary from material to material, and these differences can significantly affect lens comfort depending on the patient’s tear film chemistry and ocular surface condition. Another important - and often overlooked - aspect of comfort with soft lens wear is the company’s manufacturing process resulting in a different edge shape, which can interact differently with the eye and affect the interaction of the lens with the conjunctiva. See link below for the full story and to take the 'Barr exam' on understanding soft lens fitting. Photo by Nguyen Dang Hoang Nhu
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Soft Lens Induced Corneal Deformation
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A publication from Japan in Contact Lens & Anterior Eye looked at soft contact lens-induced corneal deformation in the posterior corneal surface. In previous studies, corneal deformation induced by wearing contact lenses was evaluated by keratometry or corneal topography. Using AS−OCT (anterior segment optical coherence tomography), the investigators found, in a case report series of two patients, the deformation of the posterior as well as the anterior surface of the cornea due to soft lens wear. Both cases showed a ring-shaped deformation, which they attributed to hypoxic conditions. Corneal deformation may alter the results of the preoperative examination, such as changes in refractive error, decrease in corrected visual acuity, and steepening or flattening of the corneal curvature, and may subsequently affect the outcome of refractive surgery. Practitioners should monitor the changes in the shape of the posterior and anterior surfaces of the cornea after prescribing soft lenses, the authors state.
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Toric Fitting Practices Not Meeting Patients Needs
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The inclusion of full astigmatic correction in spectacles is common practice. Karen Walsh and Anna Sulley look into this from a contact lens angle. Currently, toric soft lens fits account for 28% of all soft lens fits, while the estimated global prevalence of astigmatism of at least 0.75 D is 40%. Table 1 in this open-access paper provides a list with tips to capitalise on soft toric opportunities. They state that there are good reasons to check and ensure that soft toric prescribing is not overlooked: patients’ visual demands are high, and suboptimal vison performance leads to contact lens dropout. Neophytes, lapsed wearers, low astigmats, monocular astigmats and astigmatic presbyopes all present significant opportunities to recommend toric lenses and to ensure that patients’ visual needs in their contact lenses are being met. Quality of life scores have been shown to improve with a soft toric compared to a spherical lens for low to moderate astigmats.
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In previous editions of softspecialedition.com, the importance of pupil centration - and line of sight - has been covered regularly. This especially becomes relevant for specialty lens fitting such as multifocal lenses for presbyopes as well as other lenses with more sophisticated optics. An interesting image from Massimo Ferrari from Italy as posted on LinkedIn is shown below. In the photo, we have an eye with a particular anatomical feature: an obvious decentralized pupil. Characteristic was that for the patient's vision, it has little relevance when his ametropia is compensated with glasses but becomes highly relevant when deciding to compensate it with multifocal contact lenses. The ability to have a millimeter eyepiece allowed us to have precise data, and better predict the outcome, the author states.
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