Basics of fitting Contact Lenses
All Trial Sets
Keep in mind that your trial set is in fact a diagnostic set and needs the same care you advise your patients to take of their prescription lenses. Keep in mind also that the trial lenses are going to be used in many eyes and therefore require rigorous cleaning and maintenance and frequent change of storage solution.
If, perchance, the lenses get mixed up, then the prescription lens you order is going to be wrong. When using the lenses ensure that they are returned to the correct container and avoid taking out too many lenses at the same time. Hard trial lenses are engraved for ready identification, however soft lenses are not, and if a number of lenses are removed at the same time, there can be confusion in returning them to the correct container.
Hard and GP Lenses
After carrying out your fitting, if you intend to order a prescription lens of a diameter different to that of the best fitting trial set lens, for whatever reason, then remember to go flatter if going larger or steeper if going smaller.
All soft lenses, due to their very nature, require time to settle and are affected by tonicity and pH of the solution they find themselves in. To understand this fully it has to be remembered that:
“All hydrophilic polymers consist of a three dimensional array of hydrophilic and hydrophobic monomer units, though the geometry and method of the interconnection vary from one material to another. In all cases, hydration involves moisture being present in the structure in two ways, as “BOUND” moisture, more or less firmly tied to the hydrophilic sites by chemical bonds and “FREE” moisture which occupies the pores or spaces between the molecules – these become enlarged as moisture is absorbed into the structure and the material swells during hydration. “FREE” moisture is relatively mobile and is lost from the structure by changes of temperature, osmotic conditions and pH. Usually 30% to 40% of the liquid in the hydrated material is bound, the rest being “FREE” and it is mainly by gain or loss of pore moisture that a hydrophilic material comes to hydraulic equilibrium with its environment. It has been found that take-up and loss of moisture is a continuous process with hydrophilic materials.”
• Evaluate the fit of a soft lens trial lens only after the lens has been in the eye long enough, to loose the solution it has been stored in, and arrive at equilibrium in the environment of the patient’s tear fluid. Trial by low water lens (38%) to fit a high water lens (70%+) or use of a low power trial lens to prescribe a high power lens will not work.