Inventory sets typically contain a large range of over 100 lenses that have different base curves and sag heights. The commonality across these designs is that changing the base curve alters the amount of refractive change made while changing sag height alters the fit. Make the base curve flatter to increase the myopic correction change. Increase the sag height and the apex of the lens will be further from the center of the cornea, and conversely decreasing sag height will cause the lens to move closer to the corneal apex. But decrease the sag height too far and the lens will rest too hard on the corneal apex and will likely decenter. If this doesn't already sound complex changes to base curve also influencing sag height - flattening the base curve will reduce the lens sag and steepening the base curve will increase the lens sag.
While all this may sound complex it really isn't as the manufacturers provide systems to choose the lens that is likely to fit best, either in the form of a look-up table, slide rule or computer program. At follow up their system will also help guide you through the steps to take to troubleshoot common problems. The advantage is that your patient doesn’t have to wait for the lab to send lenses, as is the case for empirical fitting, they can instead take and wear the lenses from the inventory set. The disadvantage is that you are less likely to have someone to hold your hand as you get started. Local labs also tend to have less overall expertise in fitting these lenses as, unlike empirical systems where they gain experience from handling fits for their many customers, labs supplying inventory-based lenses can be detached from the fitting process.
Inventory designs offer a lot of flexibility that allows you to speedily deal with fit situations that you confront you while your patient is still in the chair. On the flipside, this makes them more difficult to use if you are less experienced. That said if you are starting out and have one of these sets in your practice don't be discouraged. The fitting systems the manufacturers have created to support them are usually easy to follow. Just take a little time to familiarise yourself with how they work before attempting your first patient fits.
- Lens fit and refractive effect can both be modified by using different lenses within the inventory
- Total control over lens fitting process
- Inventory lenses can be dispensed as final fit – patient achieves a more consistent OK correction from trial lens stage through to dispensing due to less interruption of wear
- CRT (Paragon Vision Sciences, USA)
- Contex (Contex, USA)