4.2.1 Effective Lens Plane

One of the most important variables in any IOL power calculation formula is the predicted position of the IOL after surgery. In fact, this is the only true “prediction” that needs to be made by the formula, and the actual method by which the different IOL formulas perform this task makes them differ in performance. The rest is “just” optics.

Some confusion may arise about the exact meaning of the ELP. The thin-lens formula (equation 6) requires an ELP holding all the refractive power of the IOL. The ELP may, however, be different from the actual position of the IOL. The reason for this is the way such formulas were developed and how they were made accurate. Usually the procedure of optimization proceeds as follows: From a large database of known refractive outcome the exact ELP required with the formula to produce that outcome from the preoperative measurements and the IOL power is calculated in retrospect for each case. Next, the dependence of the ELP on preoperative measures, such as the axial length and the K-reading, is analyzed, often by regression analysis. Such analysis will most probably show that the ELP depends on the axial length (because the IOL tends to locate deeper in a long eye than in a short eye); therefore an improvement of the formula could be to implement a function of the axial length in the prediction of the ELP. The same procedure might be done for other variables, giving rise to two-variable, three-variable, or multiple-variable formulas of increasing complexity.

A formula developed in this way may be accurate for the sole reason that it is based on actual clinical data. Even though it may be theoretically wrong, adjustments to the virtual, “hidden” variable, called the ELP, can make it work in practice. However, if the formula uses the K-reading based on the index of 1.3375 (most thin-lens formulas do) the corneal power may be approximately 1 diopter (D) too high, as will be described later in this chapter. Consequently, the IOL power will be underestimated by almost the same amount. The formula may, however, compensate for this error by assuming a deeper position of the IOL than is really the case, thereby increasing the estimated IOL power. This is the main reason why the ELP of most thin lens formulas is deeper than the real IOL position.

4.3.1 Modern Conditions for IOL Power Calculation

The clinical environment for IOL power calculation has taken a quantum leap since the early days of IOL implantation, improving the condition for accurate IOL power calculation in several ways:

1. 
   

   Thanks to small incision techniques—whether done by hand or with laser—the corneal anatomy is almost undisturbed by the surgery with minimally induced astigmatism if not deliberately aimed for. This gives less variability of the corneal power in IOL power calculation and makes it feasible to target the astigmatism by selecting the appropriate toric power of the implant.

   
   
2. 
   

   Thanks to the standardized opening of the lens capsule—whether done by hand or with laser—there is less variability in the position of the implant after surgery. Most surgeons prefer to use the continuous curvilinear capsulorhexis (CCC) technique ¹¹ to create a circular opening of the capsule, providing a predictable environment for in-the-bag placement of the IOL. The size of the rhexis is usually selected so that the anterior edge of the capsule will symmetrically overlap the peripheral parts of the optic. As the capsule eventually contracts it will fixate the IOL not only in the axial position but also in the lateral position with minimal tilt and rotation.

   
   
3. 
   

   Thanks to laser biometry for the measurement of the axial length ¹² we can now measure this important variable with unsurpassed accuracy. According to the manufacturers the accuracy of optical biometers is often stated to be within 0.020 mm between repeated measurements. Recalling that 1 mm axial length error is the equivalent of a 2.5 D error in the spectacle plane you will find that the laser error accounts for only 0.05 D error in the IOL refractive prediction error.

   
   
4. 
   

   Thanks to the development of diagnostic equipment for corneal refractive surgery we now have sophisticated tools for the analysis of corneal shape and function. Although the familiar K-reading still forms a cornerstone in IOL power calculation (to be discussed), there is abundant evidence that the corneal refraction is only partly measured by recording the curvature of the front surface in a paracentral ring area of ~ 3 mm. When it comes to post–laser in situ keratomileusis (post-LASIK) cases, nonregular astigmatism, and keratoconus, a standard K-reading is simply not adequate for an accurate analysis of the true, effective corneal power. For a complete description the posterior surface as well as the higher-order aberrations of the cornea should be taken into account.

   
   
5. 
   

   Although there is little documentation in the scientific literature we must assume that the optical quality of modern IOLs has improved considerably over the years due to improvement in manufacturing standards. More than 20 years ago the author of this chapter found significant deviations between the labeled and the true power of the IOL in many cases. ¹³ The power checks were performed using a keratometer-like setting with measurement of the curvature of both IOL surfaces in two meridians, and errors were found in the region 0.5 to 1 D in terms of both spherical and cylindrical power. Considering the low error of modern IOL power calculation with mean absolute prediction errors well below 0.5 D there seems to be less room for major IOL labeling errors in present manufacturing.

   
   
6. 
   

   Finally, the art of IOL power calculation per se has improved. ¹⁴ However, the concept of “garbage in … garbage out” also works for the ideal formula. Therefore, to have the best results with the best formula we must understand the principles behind its use and avoid the pitfalls.