Understanding Defocus Curve of multifocal IOLs
- Subhabrata Bhattacharya
- Jan 25, 2023
- 9 min read
Updated: Mar 9
When it comes to the field of Ophthalmology, understanding the science behind the product is as critical, as remembering the features, benefits and advantages of the product. Explaining the science, helps to create a ground for the surgeon to understand the value of the product in his/her practice. This science, may often be the road to create or address the ‘need’ that the product would address.
One such topic, is the science of Defocus Curve, and how the understanding of the science would help us to comfortably understand what the patients and clinicians can expect from the multifocal presbyopia correcting intra ocular lens(IOL). Thus we will look into the process of creating a defocus curve, its primary objective, and how one multifocal IOL can be differentiated from the other in terms of where will be the peak near focus, where will be the peak intermediate focus, and what will be the range of near and intermediate vision.
What we are trying to see ? - Objective of doing a defocus curve
At the outset, please remember, that the very objective of creating a defocus curve, is not to understand the vision that the patient is getting post operatively. The objective is to understand, what a multifocal lens is capable of giving to the patient, provided individual biometry, keratometry, and other variables are removed. In other words, we are looking to understand the lens function or lens capability of giving peak near and intermediate and its range of vision, than evaluating the patient post-surgery. Thus a Defocus Curve provides an opportunity to analyse the performance of presbyopia correcting IOLs by eliminating other influencing factors, and is not only helpful for clinicians but from a regulatory perspective also. Hence, the start of a defocus curve is to select a binocularly implanted multifocal patient, without any refractive surprise (6/6). If there is a pre existing refractive error, you should first correct the refractive error and make the patient 6/6 (BCDVA should be 6/6 as starting point of defocus curve). Ensuring the patient is 6/6 either uncorrected or best corrected removes biometry inaccuracies that may influence our understanding of what the multifocal presbyopia correcting IOL is capable to provide in terms of range of vision.
Understanding the basics:
Before we analyze the defocus curve in the figure 2 below, some basic concepts should be clear.
First, a convex lens converges rays of light. A convex lens is a plus glass that is being dialled in the phoropter. It is used to correct hyperopia to make rays of light fall on retina, as in hyperopia rays of light converge behind the retina.
