Notes on new perspectives in contact lenses.
He says: 'The relationship between the modulus of a lens and its thickness also has important implications with respect to the amount of oxygen reaching the cornea'. There is no such relationship; modulus is an intrinsic property of the material. Modulus is not synonymous with stiffness which is related to both modulus and thickness.
For example, consider the case of the Bausch & Lomb lenses Purevision and Prevision 2 HD. Looking at the lens details provided in the ACLM 2011 Yearbook, both lenses are manufactured from Balafilcon A; a Purevision lens having a power of-3.00D is stated to have a centre thickness of 0.09mm while a Prevision 2 HD lens of the same power is stated to have a centre thickness of 0.07mm. The 2 HD product will therefore presumably feel less stiff, both in handling and during wear, due to its reduced thickness but both have a modulus of 1.1 MPa.
Dr Hudson also says that there is a relationship between modulus and lens thickness profile and cites the unreferenced, unsupported and partially labelled set of profiles in Figure 1 as evidence that 'material modulus can provide practitioners with a useful indicator of the change in Dk/A expected over the lens surface [sic] for any given lens type! It cannot. If you added profiles for, perhaps, Acuvue 1-Day Moist and Focus Dailies to the collection it would be evident that this statement is incorrect.
The feature does make the very good point that fitters need to be provided with 'better visibility of the highest, lowest and average Dk/t for a given lens type' to promote a clearer understanding of how this varies with lens power. It would be very helpful if all manufacturers would do this, soon.
Tony Hough, Nelson Cottage,
Upper Dean, Huntingdon
In response to the letter received from Mr Hough, with respect to the lens centre of a -3.00D lens, I agree with Mr Hough's comments; however, this is not the point made in the article. There is a direct relationship between lens thickness and the amount of oxygen reaching the cornea. There is also a relationship between the material modulus and its thickness profile'(and not centre thickness). This is clearly stated in the article. Whilst this relationship may ultimately be due to specific lens design used by the given manufacturer, it appears from the data that manufacturers choose to create lens designs with greater peripheral thickness for materials with low modulus. Thus, the effect of modulus on oxygen transmission is implied by its relationship with lens thickness profile. This is the reason for describing them as having 'implications' on oxygen transmission rather than a 'direct' effect on oxygen transmission.
I agree with Mr Hough's comment that the material modulus is not a measure of lens stiffness.
It would be of genuine interest to see Mr Hough's data which differs from that of our own regarding lens thickness profile and modulus as there are no reports currently available in the literature. Our findings are also supported by oxygen map profiling of lenses (across their entire surface) with known differences in modulus and DK/t (not included in the original article). It is pleasing that the need for better visibility of oxygen 'performance' of commercially available contact lenses is a view common to us both.
Dr Cameron Hudson, GBA Vision (UK),
head ofprofessional affairs
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|Title Annotation:||LETTERS 2|
|Author:||Hough, Tony; Hudson, Cameron|
|Article Type:||Letter to the editor|
|Date:||Sep 2, 2011|
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