COURSE: OPTO 354 – Contact lens I
Q1. Answer the following questions
1- The major breakthrough into a useful contact lens occurred
in 1947? explain
When Kevin Tuohy began making contact lenses out of plastic rather than glass. The
initial lenses caused corneal edema after hours of wear because the diameter were very large, ranging from 10.8 to 12.5 mm. In reality these were scleral lenses.
2- The ideal corneal lens material should have?
- Oxygen permeability.
- Surface wettability.
- Low surface reactivity.
Flexure resistance and recovery. -
- Surface hardness.
- Machining and polishing capability.
- Fracture resistance.
3- what are the disadvantages of soft contact lens?
- Poor or difficult correction of astigmatism.
- Tendency to deposit formation on the lens surfaces.
- Complex and expensive maintenance.
- Relatively short lens life.
- Corneal vascularization in some cases.
- Reactions to solutions and cleaners.
- Palpebral lid reactions, e.g. giant papillary conjunctivitis (GPC).
4) Suppose the initial lens for OD is loose and that for OS is
a- What are the signs to diagnose each case?
Characteristic of steep fitting
There are characteristics of steep fitting for soft contact lens:
- Little or no movement either on blinking or as the eye change fixation.
- Tight fit is quite comfortable, sometimes more so than a correct fit, because a complete immobile lens produces the minimum of lid sensation.
- Usually good centration
- The slit lamp may show irritation of the conjunctival or limbal vessels and, with very light lenses an annular ring of conjunctival compression may be seen this often visible when lens removed.
- Vision unstable and poor because momentary pressure on the eye during blinking occurred with steep fitting vault the corneal apex.
- Subjective refraction is difficult with no clearly defined end point, and more negative power than predicated may be required because of a positive liquid lens.
- Retinoscopy and keratometer mires both show irregular distortions these mires improve with blinking.
Characteristic of loss fitting
There are characteristics of loss fitting for soft contact lens:
- Easily to diagnose because of poor centration, greater lens mobility on blinking and excessive lag on lateral eye movements.
- Very uncomfortable especially on looking upwards, lower lid sensation experienced if the lens drops
- Vision and over refraction are variable, but nevertheless may still give satisfactory results.
- The retinoscopy reflex may be clear centrally but with peripheral distorted.
- The keratometry mires change according to lens movement.