Those of us who have been associated with the presbyopia correcting IOLs have at some point of time faced the dilemma, when the patient has not got an adequate near vision post operatively, despite a near perfect distance. It has been an embarrassment not only for the practice, but equally for the multifocal IOL comapany sales representative. 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.
Much attention has been given to choose the right IOL power (that would work for the distance) on the basis of a newer generation IOL calculation formula and optical biometry details. The earlier SRK generation formulae 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 ( for example the SRK T) is a potential risk. Newer generation IOL formulae like Barrett, Olsen, Hill-RBF have helped surgeons achieve the targeted refractive outcome in terms of distance vision with multifocal and monofocal IOLs.
However, one problem still exists. What do you do with patients having unusually long anterior segment size that would lead to a higher or shorter Effective Lens Position (ELP), beyond the average. Remember a bigger phakic ACD or anterior segment size, 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 ( Alcon SA6AD1) . What we overlook is that, this is an 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 spectacle plane add power of multifocal IOLs provided by manufacturers is calculated based on the average ELP of approximately 5 mm(1). Spectacle plane add power of multifocal IOLs can accurately predict postoperatve actual spectacle plane add power in eyes with average ELP, but the accuracy is poor in eyes with too small or too large ELP. A study by Savini et al, revealed that the ratio between toric surface of IOL plane and that of corneal plane depends on ELP(2). Therefore multifocal IOLs with the same near add power exhibit varying degrees of actual spectacle lens plane add power on different ELP eyes(1).
Since ELP effects the add power the patient will get on the spectacle plane with a multifocal IOL, the surgeon should carefully plan a multifocal IOl based on the add power available with different multifocal lenses. Some common multifocal IOLs and their add power on the IOL and spectacle plane are given below which are based on average ELP.
Influence of Keratometry readings and Axial Length on multifocal lens add power on spectacle plane:
In their study(3) Savini, Hoffer et al, showed that for a multifocal IOL with the same near add power the near focal distance change as a function of axial length and keratometry values. With the same add power the near focal distance was closer in hyperopic eyes and farther away for myopic eyes. The near focal distance was closer in flat corneas and further in eyes with steep corneas (see figure 3). Savini, Hoffer et al (3) says that surgeons should warn myopic patients that a longer distance is expected to get the best near focus. In addition, they point out that longer reading (near) distances could decrease the near viusal acuity in myopic patients
This study of Savini, Hoffer et al (3) confirms a previous study by Petermeir et al who found reading distance with ReSTOR +4.00D add at 34.6 cm for myopes compared to 29.5 cm for hyperopes
With the same add power the near focal distance was closer in hyperopic eyes and farther away for myopic eyes
In their study Youngsub Eom et al (4) found that the predicted spectacle plane add power of MIOLs using the ELP of the Haigis formula was more accurate than that using the value presented by the manufacturer. The spectacle plane add power of MIOLs decreased as ELP increased. The predicted spectacle plane add power of the MIOL was calculated by
adjusting the predicted corneal plane add power of the MIOL for a back vertex distance of 12.0 mm. The predicted corneal plane add power of the MIOL according
to ELP was calculated using the refractive vergence formula.
The authors presented their own Multifocal IOL Add Power calculator that they published in Graffe's Clinical Ophthalmology(5). Using the author's calculator I calculated the real add power (multifocal IOL lens plane add power 3.5D as claimed by the company) for a patient with average K readings and Axial length. Below are the patient details.
And here are the estimated IOL add power in the spectacle plane using the Refractive Vergenge formula and ELP calculated by the calculator with Haigis formula
In the above image the multifocal IOL add power on the IOL Plane is shown as quoted by the company (3.50D). The expected real add power as per Haigis in the spectacle plane is 2.48 D with a K reading of 45D, axial length 23.50 mm and ACD of 3.20 mm.
To explain more lucidly the relation between corneal readings and how it effects the add power of a multifocal IOL and consequently the near vision of patients, consider the below figure. A flatter cornea (42 dioptre) leads to a more effective add power and a steeper cornea leads to an lesser effective add power in the spectacle plane per Haigis formula when all other biometry input remains the same.
In Table 2 (below ) you can see how the effective add power of Alcon PanOptix decreases with an increase in steepness of cornea.
Table 3 helps us to understand the add power at the spectacle plane of the manufacturing company's whose add power in IOL plan and near focal distance are quoted in Table 1. For calculation of the add power of respective multifocal IOLs, the constants are sourced from manufacturer provided constants in the iolcon.org. The calculations are based on an average axial length of 23.50, anterior corneal power of 43.50 D, and phakic ACD of 3.2mm. As you can notice from the below calculation of effective add power on spectacle plane by the Haigis ELP method, the add power in the spectacle plane are lower than the manufacturer provided add power in spectacle plane for the above quoted axial length, corneal reading and ACD dimensions.
At present most multifocal IOL companies come with one version of add power. Therefore one of the considerations that a clinician can make is to choose different multifocal IOLs based on their add power by referring to the patient's K readings, axial length, or expected ELP (or looking at the anterior segment size pre operatively).
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 !!
The add power of multifocal IOLs in the spectacle plane can be lower than the manufacturer quoted add power in spectacle plane as shown in Table 2 for even average eyes as per Haigis method
Consider a stronger add power multifocal IOL for patients with longer axial length and steeper cornea, and a lower add power multifocal IOL for patients with shorter axial length and flatter cornea.
1.Influence of ocular biometric parameters such as effective lens position.....Pin Wu, Vol 48, issue 11, JCRS
2. Influence of axial length and corneal power on the astigmatic power of toric intraocular lenses, JCRS 2013;39; 1900-1903
3. Influence of the effective lens position, as predicted by axial length and keratometry, on the near add power of multifocal intraocular lenses Giacomo Savini, MD, Kenneth J. Hoffer, et al, J Cataract Refract Surg 2016; 42:44–49.
4. Spectacle plane add power of multifocal intraocular lenses according to effective lens position Youngsub Eom, MD, et al, CAN J OPHTHALMOL—VOL. NO. 2016
5. Development of a program for toric intraocular lens calculation considering posterior corneal astigmatism, incision-induced posterior corneal astigmatism, and effective lens position Youngsub Eom, Dongok Ryu, et al, Graefe's Archive for Clinical and Experimental Ophthalmology volume 254, pages1977–1986 (2016)