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OPT 355, Contact lens II Med- Term Exam

Q1- Answer the following questions


1- What are the advantages of gas permeable material over hard PMMA lenses?


The advantage of gas permeable material Over hard PMMA lenses: -


- Fewer physiological problems such as central corneal clouding (oedema).

- Quicker adaptation.

 Few over-wear syndromes. -

- Less spectacle blur/post-wear refractive changes.

- Lenses may be fitted larger because anoxia due to corneal coverage by the lens is no longer of great significance.

Extended wear becomes possible as transmissibility increases. -

- Alternation of daily and extended wear becomes feasible.

- Less like hood of lens loss because RGP lenses can be fitted with minimal edge clearance  

2-compare between the Silicone acrylate and Styrene material?



- It is combination of Silicone and PMMA to enhance the hardness and stability of the material. To insure adequate wettability, hydrophilic monomers such as methacrylic acid are also incorporated into the polymer. The ratio of PMMA to silicone is generally 55% PMMA to 35% silicone.

- The greater the proportion of silicone the higher the oxygen permeability, but the poorer the wettability, stability and strength.

- The range of Dk values between the materials commonly available is fairly limited (generally 9-16 units)


- The silicone-acrylates are approximately twice as good thermal conductors as PMMA and CAB and this becomes extremely useful for patients living or working in warm or hot environments. 

- The oxygen permeability of the silicone- acrylate group ranges from 50% to several hundred per cent greater than CAB.

- The material is less susceptible to warpage and hydration-dehydration changes than CAB.


- Lenses will still scratch more easily than PMMA lenses and flex on astigmatic corneas.

- The hydrophobic nature of silicone allows the attachment of both lipid and protein deposits to the lens surface.

- Increased permeability is often traded for poor optical quality, poor wettaility etc. Manufacturers have attempted to rate proprietary ingredients to work against these problems.


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