- C Oxygen Permeability Is A Key Measure In Clinical Contact Lens Perfor Mance Where Decreased Oxygen At The Cornea Ma 1 (72.36 KiB) Viewed 8 times
- C Oxygen Permeability Is A Key Measure In Clinical Contact Lens Perfor Mance Where Decreased Oxygen At The Cornea Ma 2 (140.79 KiB) Viewed 8 times
(c) Oxygen permeability is a key measure in clinical contact lens perfor- mance, where decreased oxygen at the cornea may lead to oedema and neovascularisation. Manufacturers provide standard data of their products, as shown in the table below. 1-DAY DAILIES Aqua Proclear 1 day Soflens Daily Disposable AIR OPTIX Aqua ACUVUE OASYS ACUVUE Comfort MOIST Plus Johnson & CooperVision Bausch & Lomb CIBA Vision Manufacturer CIBA Vision Johnson Johnson & Johnson Vision Care Vision Care nelfilcon A Il 2 etafilcon A IV 2 hilafilcon B II 2 Polymer omafilcon All 2 lotrafilcon B15 senofilcon A15 69% 58% 60% 59% 33% 38% Water content 26 28 25 22 110 Oxygen permeability (Dkx10-11) 103
i. Commmercially available hydrogel lenses vary in their water con- tent. It is generally said that a higher level of hydration would lead to an increased capacity for oxygen transmission. Does the data support this? Briefly discuss. ii. The oxygen permeability is defined as DH, where D is the diffusion coefficient. Using the appropriate form of Fick's law and by making simplifying assumptions, show what role D plays in determining the steady state oxygen profile across the lens. Clearly state your conclusion. iii. The oxygen transport to the eye is dependent on the concentration of oxygen at the lens surface, i.e. the boundary condition. The process by which oxygen dissolves into the lens at the air-lens in- terface is described via Henry's law C = Hp, where C denotes the dissolved oxygen concentration, and p the partial pressure of oxy- gen in air. H is known as the Henry solubility (measured in), and is the second parameter in the oxygen permeability. Derive the units for the oxygen permeability and thus define what it measures as clearly as you can. iv. Although currently there is no technology available to directly mea- sure the cornea oxygen concentration under the lens, it is known that the oxygen uptake rate by the cornea can vary depending on the lens worn, further complicating the product comparison. Can you suggest a more universal indicator for comparing oxygen per- formance between lens types? [30 Marks] [2 marks] [4 marks] [3 marks] [3 marks]