This is obviously the main issue when planning cataract surgery in keratoconic eyes. In a series of refractive lens exchange with toric IOL in keratoconus, the authors used Pentacam or Orbscan II keratometry and interferometry IOL calculation. The SRK II formula was used for the IOL power calculation [9]. Other authors also chose the K-values obtained with the Pentacam for the IOL calculation; however in this case, manual keratometry (Javal-Schiotz; Rodenstock, Dusseldorf, Germany) was used to obtain accurate determination of the axis of corneal astigmatism and the elected formula was the Sanders–Retzlaff–Kraff theoretical (SRK/T ) [2]. In the largest series of refractive lens exchange in keratoconus, the author calculated the IOL power by US biometry using the Allergan Humphrey 820 and Holladay 2 formula, targeting a residual myopic astigmatism. Keratometry readings were obtained by axial topographic maps. The dioptric power of the steepest meridian over the central 3 mm of the cornea was considered K ₁, and the dioptric power of the flattest meridian over the central 3 mm of the cornea was considered K ₂. Each meridian power was calculated by averaging its two semimeridian values, always considering the central 3 mm [13]. In a series of two cases of forme fruste keratoconus, the IOLMaster axial length, the Orbscan II keratometric readings, and the SRK II formula were used for the IOL calculation [14]. In contrast, other authors do not report enough information about how the IOL calculation was performed [8]. It is evident that there is big disparity among authors regarding the best method for IOL calculation in keratoconus patients. In any case, axial length measurement by optical interferometry and K readings from topographers are the preferred options. In our study [10], we had the impression that when the corneal shape is very altered, probably the deformity of the anterior and posterior cornea is similar, therefore the posterior cornea may not influence the calculation as much as we were thinking.

Regarding the use of different formulas for the calculation of the IOLs power, in one of the reported case series, the SRK/T formula seemed to provide better results compared with the Hoffer Q [10]. In contrast, in a different publication, the difference between the ideal IOL power and the calculated IOL power from SRK, SRK II, and SRK/T formulas was determined and the most accurate IOL power was found by using SRKII [1]. It is our opinion that the axial length and the rest of the factors that help in the effective lens position (ELP) prediction are very important. Formulas like Holladay II that take into consideration all these factors should be more accurate. We have also the hope that with the new ray tracing formulas we will be able to improve the predictability of these difficult cases in the future. In any case, greater series are needed to demonstrate which formula performs the best.

We advocate performing microincisional surgery (MICS ) in every cataract surgery but especially in those cases when astigmatism needs to be carefully managed or a toric intraocular lens is planned to be implanted. Incisions larger than 2.2 mm may induce variable amounts of surgically induced astigmatism (SIA ) leading to a more unpredictable outcome [15–17]. Other factors that may affect the amount of SIA are the amount of preoperative corneal astigmatism, suture use, and patient age [18, 19]. As in keratoconus eyes predictable results are more difficult to be achieved, it is extremely important to avoid all the possible sources of error and uncontrolled surgically induced astigmatism due to a too large incision may be an important one. In a recent paper from our research group [10], the results of MICS surgery in keratoconus eyes with cataract are shown. Surprisingly, in most of the papers published to date, the incisions used to perform the surgery are larger than 2.2 mm. Only in two of the series the incisions size were 2.2 mm or smaller [2, 10]. Other authors performed 2.7 and 3.2 mm sclera tunnel incisions [8, 12], and 2.8 and 3.2 mm corneal incisions [9, 14].

We need to emphasize that large corneal incisions must be avoided especially in keratoconus due to its weakening effect in an already weak cornea.

Regarding the toric IOL, in most of the series the implanted IOL was the AcrySof Toric (Alcon Laboratories Inc. Ft Worth, Texas) [2, 9, 10, 13, 14]. In one of the series the IOL implanted was the AT TORBI 709M (Carl Zeiss Meditec, Germany) [8]. This bitoric plate haptic IOL offers a wider range of astigmatism correction than the AcrySof extending up to 12 diopters and even higher levels can be manufactured to order. At this moment it is necessary to remind that some manufactured toric IOL calculators consider the relation between cylinder power at the IOL plane and at the corneal plane as a fixed amount [20, 21]. This is an important error because the effective cylinder power of the IOL at the corneal plane is a function of the effective lens position and the spheroequivalent power of the IOL. The IOL cylindrical and spherical powers must first be converted into the two principal lens powers, after which both lens powers are calculated to the corneal plane using a standard vertex formula. The difference between both lens powers at the corneal plane should be used to select the most appropriate IOL cylinder power [3].

Overall, outcomes after cataract surgery with toric IOL implantation in keratoconus are good. Nanavaty et al. [8] reported excellent outcomes with a UDVA of 20/40 or better in 75 % of the eyes and a reduction in the preoperative cylinder from 3.00 ± 1.00 D to 0.70 ± 0.80 D. Leccisotti [13] reported worse outcomes in his series of clear lens extraction and toric IOL implantation. IOL exchange surgery was needed in 11 cases (32 %) because the IOL power was not accurate due to the ultrasound biometry limitations. However, at 12 months, mean spherical equivalent (SE) was −1.31 ± 1.08 D and mean defocus equivalent was 1.94 ± 1.57 D. Twenty-two eyes (65 %) were within ±2 D of defocus equivalent, 16 eyes (47 %) were within ±1 D, and three eyes (9 %) were within ±0.5 D. The safety index was 1.38, and the efficacy index was 0.87.

Jaimes et al. [9], in their refractive lens exchange series in 19 keratoconus eyes showed that the mean preoperative sphere decreased from −5.25 ± 6.40 D to 0.22 ± 1.01 D postoperatively (p < 0.001). Mean preoperative cylinder and SE were 3.95 ± 1.30 and −7.10 ± 6.42, respectively, and improved to 1.36 ± 1.17 and −0.46 ± 1.12 after the surgery (p < 0.001). Preoperative mean UDVA was 1.35 ± 0.36 and changed to 0.29 ± 0.23 after the surgery (p < 0.001).

Thebpatiphat et al. [1] found a mean improvement in CDVA of four lines and a slight improvement in UDVA from 0.71 ± 0.41 to 0.63 ± 0.47; most of these patients were rigid contact lenses users before and after the surgery.

Our research group recently published the largest series of cataract surgery in keratoconic eyes [10]. Refractive outcomes before and after the surgery are presented in Table 29.1.