Relative to inventory systems, trial lens-based designs make a compromise in only containing a range of lenses with different sag heights to reduce the number of lenses to only around 25 that need to be stocked. In a sense this makes them synonymous with many rigid contact lens trial sets in that they provide just enough lenses for you to assess the lens fit before you order the first lenses that your patient will wear over the longer term. In rigid lens fitting, you would select a trial lens based on some formula derived by the lens manufacturer. Put this lens on the eye, instill fluorescein, assess whether the lens fit is too flat or steep and if so select a different lens, repeating the process until the best fit is achieved at which point you would over refract the lens to calculate the back vertex power needed.
A similar process is followed when using OrthoK trial sets, except the fitting assessments are done overnight. The first trial lens fit is usually determined by computer software and based purely on corneal shape. The accuracy of lens fit is determined by examining changes to corneal topography after overnight wear, and feeding this information back into the computer software, either to determine the next lens that should be trialed. Once a good fit is achieved the refractive power change made by the lens is assessed and used to establish the order parameters of the lens that will be dispensed.
- Trial lens can be issued at the first fitting visit
- No prior knowledge of rigid lens fitting required
- Less setup cost than full inventory fitting set
- Trial lens will only partially correct refractive error
- Typically requires more chair time than empirical or inventory fitting
- BE (Katt/Capricornia, Australia)
- Z Night (Menicon, Japan)