Fig. 13.1
In this eye with epithelial edema due to Fuchs endothelial dystrophy (a), irregularity of the corneal surface impeded analysis of the anterior curvature using Placido disk-based topography (b). Keratometric readings for intraocular lens calculation were taken using Pentacam HR rotating Scheimpflug camera (Oculus, Wetzlar, Germany) (c)
To calculate IOL power in candidates to the new triple procedure, the hyperopic shift induced by the endothelial lamellar graft must be taken into account. A number of studies have shown that microkeratome-prepared posterior lamellae change the postoperative sphero-equivalent of manifest refraction by between +0.62 and +1.26 diopters (D) [1–6]. The postoperative hyperopic shift is caused by the endothelial lamellar graft’s decreasing the cornea power, by flattening the anterior cornea and steepening the posterior cornea [3, 4, 6–10]. The more significant changes occur on the posterior cornea. Microkeratome-prepared lamellae have a minus lens shape, which alters the corneal thickness profile and increases posterior curvature. In 23 consecutive patients who had undergone DSAEK, the microkeratome-prepared lamella graft was found to have decreased the anterior corneal power by −0.24 ± 0.61 diopters (D), on average, and increased the negative posterior corneal power by −0.96 ± 0.42 D [10].
A number of different methods for IOL power calculation have been proposed to compensate for this reduction in corneal power induced by the posterior lamellar graft. These methods include:
Selection of an IOL power with predicted refraction more myopic than desired
Adjustment of the keratometric (K) readings used in the IOL calculation formula
Optimization of the IOL A constant.
The first method was used by Covert and Koenig [11], who selected IOL power with predicted refraction ranging from −0.50 to −1.15 D, and by Terry et al. [12], who selected implants with predicted refraction ranging from −0.80 to −1.25 D. This method led to accurate IOL power calculations. After surgery, 62–74 % of eyes were within 1.00 D of emmetropia (Table 13.1). However this method requires complex calculation. The change in corneal power induced by the lamellar graft modifies both the IOL power calculation and the predicted refraction, depending on the biometric characteristics of the eye. Thus, to optimize the use of this method, the degree of myopia in the predicted refraction must be calculated taking corneal curvature, anterior chamber depth, and axial length of each eye into consideration [15].
Table 13.1
Refractive error after cataract surgery combined with Descemet’s stripping automated endothelial keratoplasty
Author | Number of eyes | Follow-up (months) | Absolute prediction error (D) | Proportion of eyes ±1.00/± 2.00 D of target refraction |
|---|---|---|---|---|
Covert and Koenig, Ophthalmology [11] | 21 | 6 | NA | 62 %/100 %a |
Terry et al., Ophthalmology [12] | 135 | 6 | NA | 74 %/97 %a |
de Sanctis et al., Am J Ophthalmol [13] | 39 | 6 | 0.59 ± 0.42 | 83 %/100 % |
Bonfadini et al., Ophthalmology [14] | 30 | 18.4 ± 9.8 | 0.61 ± 0.40 | 83 %/NA |
The adjustment of keratometric (K) readings used for IOL calculation should take into account the average expected reduction of corneal power induced by the endothelial lamellar graft [10]. This method was used in 39 consecutive patients operated for cataract and Fuchs endothelial dystrophy and gave predictable postoperative refractive results (Table 13.1). Six months after surgery, the absolute prediction error (absolute difference between predicted and achieved refraction) was 0.59 ± 0.42 D (range +0.05 to −1.52 D). The achieved refraction fell within ±0.50 D, ±1.00 D, and ±2.00 D of the predicted refraction in 55.5 %, 83.3 %, and 100 % of cases, respectively.
An optimized IOL A constant was used by Bonfadini et al. in 30 eyes undergoing the new triple procedure using pre-sectioned lamellar endothelial graft [14]. This approach significantly decreased the mean absolute error (from 1.09 ± 0.63 D to 0.61 ± 0.40 D; p = 0.004) and increased the proportion of eyes falling within ±0.50 (43 % versus 20 %) and within ±1.00 D (83 % versus 50 %) of the target refraction (Table 13.1).
The results of the above studies [11–14] highlight the fact that the refractive outcome of the new triple procedure is highly predictable, provided that the IOL power is calculated taking into account the postoperative refractive shift induced by the lamellar graft. The absolute prediction error is just slightly higher than that normally observed after phacoemulsification with posterior chamber IOL implantation. Seven highly experienced senior surgeons found a mean absolute prediction error of 0.25 D after simple phacoemulsification with posterior chamber IOL implantation [16], a result that is considered a benchmark of excellence for cataract surgery. In other studies, the mean absolute prediction error after phacoemulsification with IOL implantation was comparable to that obtained after the new triple procedure, varying between 0.32 and 0.71 D [17–21].
The accuracy of IOL power calculation, and thus the postoperative refractive outcome of the triple procedure, might further be improved by combining cataract surgery with DMEK. DMEK grafts, which contain only donor Descemet’s membrane and endothelium, should induce very slight changes in corneal power. The minus lens effect cannot occur, because the grafts do not contain donor stroma. However, a postoperative hyperopic shift has also been reported using this technique [22, 23]. Ham et al. [22] analyzed corneal power by Scheimpflug imaging and showed that the negative power of the posterior cornea increased on average by +1.00 D after DMEK. The study authors attributed this change to the postoperative de-swelling of the posterior stroma, which leads to a steepening of the posterior corneal curvature. Lasser et al. also reported a postoperative hyperopic shift in 61 eyes that underwent DMEK combined with phacoemulsification and IOL implantation [23]; they suggested selecting IOL power with a predicted refraction of −0.75, to optimize postoperative results. Using this approach, 54.5 % of eyes were within 1 D of emmetropia and 77.3 % were within 2 D of emmetropia, 6 months after surgery.
Cataract Surgery Combined with Penetrating Keratoplasty
Cataract surgery combined with PKP is routinely performed for the simultaneous surgical treatment of cataract and corneal stromal diseases, such as ectasia, postinfectious scars, traumatic leukomas, and dystrophies. In eyes scheduled for cataract surgery combined with PKP, IOL power calculation is truly challenging: the postoperative refractive power of the corneal graft is extremely variable, the eye’s axial length may change after the procedure, and the reliability of theoretical formulas that calculate the effective lens position from the preoperative corneal curvature and axial length is reduced [24].
The great variability of postoperative corneal power is caused by the full-thickness trephination of the recipient cornea and the suturing of the donor tissue. After suture removal, the corneal power may be below 40 D or above 48 D. Since the postoperative corneal power is highly unpredictable, Katz and Foster have suggested using the keratometric readings of the fellow eye to calculate IOL power [25]. However, this approach is only suitable for patients with unilateral diseases and leads to unpredictable refractive results. Today, many surgeons use the average postoperative keratometric readings obtained from a previous series of corneal grafts; for this purpose, the series should comprise grafts performed using a surgical technique that is standardized in terms of trephination method, donor-recipient disparity, and suture technique. However, the resulting predictability of postoperative refractive outcome is only moderate. Davis et al. [26] report on a series that included 106 eyes; they found postoperative sphero-equivalent values in the range of – 6.00 D to +4.00 D and differences from the target refraction of ≥2.00 D in 48 % of cases. Javadi et al. reported similar results [27]; in a series of 76 interventions, the postoperative sphero-equivalent values ranged from −6.55 to +3.78 D and the difference from target refraction was ≥2.00 D in 54 % of cases.
The refractive results would be better if phacoemulsification with IOL implantation were performed as a secondary procedure, after the corneal graft [28]. However, the surgical trauma due to cataract extraction increases postoperative endothelial cell loss, and this two-step approach delays postoperative visual recovery, since cataract surgery is not usually performed until 12–24 months after PKP, when all sutures have been removed.
IOL Calculation in Patients with Prior Corneal Graft
Cataract extraction is the most frequent clinical situation that requires IOL power calculation in eyes with prior corneal graft; other special circumstances include piggyback IOL and phakic IOL implantation. In these clinical situations, IOL power calculation should be planned 2–3 months after suture removal, when serial topographical analysis demonstrates corneal curvature to be stable.
Cataract Surgery in Eyes with Previous Corneal Graft
Cataract occurs quite commonly in eyes with prior corneal graft, because of preoperative and postoperative intraocular inflammation, surgical trauma, and prolonged use of corticosteroids. Cataract is frequently associated with clinically significant corneal astigmatism. The multicenter Corneal Transplant Follow-up Study showed that corneal astigmatism after keratoplasty was ≥4 D in 43 % of eyes and ≥6 D in 20 % of eyes [29]. In eyes with marked corneal astigmatism, postoperative visual recovery is generally only modest after phacoemulsification with monofocal IOL implantation. After surgery, anisometropia makes this refractive error difficult to correct fully by means of spectacles. Contact lenses are difficult to fit and frequently not tolerated and carry the risk of severe complications. Moreover, keratorefractive procedures to correct severe astigmatism on corneal grafts have moderate predictability and high complication rates.
Phacoemulsification with toric IOL implantation provides an opportunity to correct both corneal astigmatism and cataract with a single procedure. The first toric IOL for correcting post-PKP astigmatism was implanted during cataract surgery in 1999 [30]. It was made of PMMA and required a 6 mm incision. Since then, many toric IOL models, made of different materials and with different designs, have become available (Table 13.2). The surface adhesiveness of acrylic materials and the new designs that have been introduced have increased toric IOL stability in the capsular bag and decreased the risk of postoperative rotation [31].
Table 13.2
Toric IOLs available in Europe
Toric IOL model | Material | Haptic | Diameter (mm) | Power (diopters) | Incision size (mm) | |
|---|---|---|---|---|---|---|
Sphere | Cylinder (steps) | |||||
AcrySof (Alcon) | Hydrophobic acrylic | Loop | 13.0 | +6.0/+30.0 | 1.5/6.0 (0.75) | 2.2 |
AF-1 toric (Hoya) | Hydrophobic acrylic PMMA haptic tips | Loop | 12.5 | +6.0/+30.0 | 1.5/3.0 (0.75) | 2.0 |
Acri.Comfort/ AT Torbia (Zeiss Meditec) | Hydrophobic acrylic with hydrophobic surface | Plate | 11.0 | +10.0/+32.0 | 1.0/12.0 (0.50) | <2.0 |
Fil 611 T (Soleko) | Hydrophilic acrylic | Plate | 11.8 | +5.00/+30.0 | 1.0/6.0 (0.50) | 2.0 |
Lentis Tplus (Oculentis) | Hydrophobic acrylic with hydrophobic surface | Loop/plate | 12.0/11.0 | 0/+30.0 | 0.25/12.0 (0.75) | 2.6 |
LAL (Calhoun Vision) | Silicone with PMMA haptics | Loop | 13.0 | +17.0/+24.0 | 0.75/2.0 | 3.0 |
MicroSil/Toricaa (HumanOptics) | Silicone with PMMA haptics | Loop | 11.6 | −3.5/+31.0 | 2.0/12.0 (1.0) | 3.4 |
Morcher 89A (Morcher GmbH) | Hydrophilic acrylic | Bag in lens | 7.5 | +10.0/+30.0 | 0.5/8.0 (0.25) | 2.5 |
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