Rigid gas permeable lens tolerance limits and their possible effect on mode of fit.
BACKGROUND
Rigid gas permeable corneal lenses should be manufactured according to the tolerance limits specified in the International Standard ISO 18369-2: 2006. The purpose of this study was to model and quantify the central and edge fit provided by lenses with manufacturing errors that are at the limits of these dimensional tolerances.


METHODS
A spreadsheet was used to design a tri-curve lens that provided specified values for apical tear layer thickness and axial edge clearance on corneas with three different apical radii and six different p values. The lens radii and diameters were then varied by amounts that were at the limit of the corresponding tolerances to ascertain the resultant central and edge fit of the lens.


RESULTS
Rigid gas permeable lenses made with manufacturing errors at the limits of tolerances in either the back optic zone radius (BOZR) or back optic zone diameter (BOZD) are unlikely to provide an acceptable apical tear layer thickness. When the BOZR is 0.05 mm steep and the BOZD is 0.20 mm large, a lens intended to have an apical tear layer thickness (ATLT) of 5 microm will provide an actual value between 12.3 and 19.4 microm, depending on corneal radius and shape. When the BOZR is 0.05 mm flat and the BOZD is 0.20 mm small, the value of ATLT will be zero. Lenses in which all radii and diameters have been made with such errors may allow a clinically acceptable edge clearance in most cases.


CONCLUSIONS
Current tolerance limits for RGP lenses may allow the ATLT to differ significantly from that intended but the edge fit may be acceptable. While the full specifications of a RGP lens should be verified prior to its supply to a patient, it is particularly important to measure the BOZR and desirable to measure the BOZD, to ensure that resultant ATLT does not differ significantly from that intended. Minimally blended transitions, especially on trial lenses, should allow the back surface dimensions to be measured in accordance with International Standard ISO 18369-3: 2006. Ideally, the practitioner should be aware of any differences between nominal and actual specifications of both trial lenses and those supplied to patients.
Pearson
Richard M
RM
Tonbridge, Kent, United Kingdom. rpearson@contacts.fsnet.co.uk
D003261:Contact Lenses; D003315:Cornea / Q000033:anatomy & histology; D006801:Humans