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Acute rigid gas permeable contact lens intolerance.
A J Jackson1, C Wolsley, J L Briggs
1Department of Ophthalmology, The Royal Victoria Hospital, Belfast.
Summary
Contact lens inversion can cause acute intolerance upon insertion. This study measured the force needed to invert rigid gas permeable and PMMA lenses, finding minus-powered lenses easier to invert.
Area of Science:
- Ophthalmology
- Materials Science
Background:
- Rigid gas permeable (RGP) and polymethylmethacrylate (PMMA) contact lenses can cause acute intolerance.
- Contact lens inversion is a potential, though often overlooked, cause of this intolerance.
Purpose of the Study:
- To investigate contact lens inversion as a cause of acute intolerance.
- To quantify the force required to invert RGP and PMMA lenses of varying powers and materials.
Main Methods:
- A custom-built calibrated strain gauge was used to measure inversion forces in Newtons (N).
- RGP lenses (Dk 30-90) and PMMA lenses with powers of +/-3.00 D and 9.00 D were tested.
- Forces to invert and revert lenses were recorded.
Main Results:
- Minus-powered lenses required significantly less force (17.5 +/- 4.8 N) to invert than plus-powered lenses (31.7 +/- 7.4 N).
- PMMA lenses required more force to invert than RGP lenses.
- High-powered PMMA lenses were more prone to fracture upon inversion.
Conclusions:
- Contact lens inversion should be considered by practitioners for unexplained acute intolerance.
- The ease with which inverted lenses (non-fractured) return to their original shape contributes to inversion being overlooked.