Those of us who have been associated with the presbyopia correcting IOLs have at some point of time faced the dilemma. It has been an embarrassment not only for the practice, but equally for the vendor. Not to speak of the disappointment of the patient. In this article I will try to analyze a possible cause of such an incident, and what can be done to avoid a repetition of an avoidable situation.
While much attention has been given to choose the right IOL power on the basis of the right formula and optical biometry, we sometimes forget that not all eyes are same. The earlier SRK generation formulas worked on simple assumption that a bigger axial length would be having an equivalently bigger anterior chamber depth (ACD). Jack Holladay in the mid nineties challenged this concept showing us that a bigger axial length may often come with an average or a less than average ACD.
Consequently, calculating an IOL power based on no measured ACD values ( the SRK and Hoffer Q works on average ACD with no place to input values of patients ) is a potential risk. I would strongly recommend using a fourth generation formula which allows for patient specific ACD ( Holladay II/Barrett/Olsen).
However, one problem still exists. What do you do with patients having unusually long phakic ACD ? Remember a bigger phakic ACD, outside of an average value, runs the risk of a bigger postoperative ELP. With a higher post operative ELP than normal, the effcetive power on the corneal plane would be less. For example, a presbyopia correcting multifocal IOL with +3 Add power on the IOL plane would typically have a +2.5 add power on the corneal plane. What we overlook is that, this is a assumed value based on an average phakic ACD. For a patient with a large phakic ACD ( and hence a large post operative ELP ) the effective power in the corneal plane drops.
The choice of a multifocal IOL should be based on many considerations. A brief description of the considerations are available in another presentation in my page. But it is common to overlook the ACD values preoperatively, and get sub-optimal near results post operatively.
The choice of presbyopia correction IOL may not be based on overlooking this very important aspect. All the best !!