IOL Calculation Formulas

The earliest IOL calculation formula was proposed over 50 years ago by Fedorov et al. [27]. First generation formulas were based on a thin-lens paraxial approximation, omitting factors such as lens thickness, corneal thickness or IOL design. Second generation formulas (regression formulas) decoupled eye as an optical system and were purely based on statistical analysis of refractive outcomes. The main improvement of third generation formulas was based on the assumption that the effective lens position is not a constant, but a function of axial length and corneal power. The goal of fourth generation formulas was to develop a universal calculation method giving the best outcome in all eyes despite axial length. Fifth generation formulas consider even more input parameters including gender or race to achieve a higher level of customization [28].

Similarly to regression-based, artificial intelligence formulas use huge databases and a sophisticated statistical model to find relationship between not evident approaches; these include the Clarke neuronal network or Hill-RBF formula (radial basis function) [29, 30]. Another approach for improving the accuracy of IOL calculation is the application of ray-tracing analysis. Ray-tracing is method for calculation of every single ray passing through the optical system, and the refraction of rays at each optical surface is calculated using Snell’s law. A map of corneal power achieved in topography or tomography can be transformed into an array of individual measurements representing a polygonal shape. Ray-tracing is employed to analyze the optical properties of every element of the eye in order to establish the performance of the entire optical system. Particularly in eyes after refractive surgery it might help to solve the issue of higher-order aberrations of the cornea by selecting a particular design of an IOL. Software applied for IOL calculation include Okulix or Phacooptics (Olsen).

A notable percentage of patients undergoing PCS demonstrate corneal astigmatism. In a study by Hoffmann more than 36% of eyes had astigmatism 1 D or greater, while over 8% over 2 D or more [32]. For toric IOLs calculations manufacturers commonly provide an online calculator which employs the formerly presented formulae, but has incorporated IOL models and constants (e.g. Alcon, Bausch and Lomb, Johnson & Johnson Vision, Sifi). Also the Berdahl and Hardten calculator should be mentioned, as it allows to assess if the residual astigmatism after surgery is a result of toric IOL misalignment [33].

Benchmarking

Achieving an accurate refractive target is desired in contemporary PCS. Employing optical biometry and proper IOL calculation formulas allows achieving ≤0.5 D of the refractive target in 79.1% of eyes, while ≤1.0 D in 97.2% of cases [34]. Interestingly, the interocular axial length difference influences the refractive outcome. The odds ratio (OR) of having a refractive outcome >0.5 D is 1.4, 1.6 and 1.8, for interocular axial length difference of 0.2, 0.4 and 0.6 mm, respectively [34]. If the interocular axial length differences is 0.2 mm or more it would be advised to remeasure with the same device or, if possible, use another device to confirm the axial length [35]. Myopic patients have a lower chance of achieving the refractive target than non-myopic individuals (OR 1.9) [34].

Han and McGhee emphasized that in the current patient-focussed climate it becomes difficult to establish a clear definition of a complication in contemporary cataract surgery [36]. Thirty years ago a postoperative best correct visual acuity of 6/12 (20/40) might have been considered as a good outcome. Currently, in the era of postoperative visual acuity of 6/6 (20/20) and ±0.5 D of emmetropia, such a result could be considered a complication. With proper preoperative assessment and allocation of high-risk phacoemulsification procedures to experienced surgeons it is possible to reduce the rate of intraoperative complications by 40% and the rate of posterior capsule tear from 2.6 to 0.6% [37]. Within the Auckland Cataract Study the proposed stratification system risk factors for intraoperative complications included cataracts with no fundus view, pseudoexfoliation, phacodonesis, oral alpha-receptor antagonist intake, high ametropia, posterior capsule cataract or plaque or corneal scarring [37]. Moreover, it was proposed that patients after prior vitrectomy or with only one eye should be operated by experienced surgeons only.

IOL Types

IOLs were introduced by Sir Harold Ridley, and the first successful IOL implantation was performed in the 1950 [38]. IOLs can be divided based on the material they are made of. Silicone lenses composed of polyorganosiloxane materials, with high refractive indices, were the first available foldable IOLs. Acrylic lenses can be made of rigid polymethyl methacrylate (PMMA), foldable hydrophilic or hydrophobic acrylic materials [39]. Addition of a blue-light filtering chromofore or UV-protection is employed in some IOL models.

The IOL design basically mirrors the intended location of IOL implantation. Posterior chamber lenses can be placed in the capsular bag, in the ciliary sulcus, or fixated to the iris. Single-piece IOLs may be open loop or overall plate lenses (Fig. 3.2a, b). Three-piece IOLs have haptic components made of PMMA, polypropylene, polyimide or polyvinylidene fluoride (Fig. 3.2c). In general, IOLs placed in the sulcus should have rounded optic edge and thinner haptics to prevent irritating the posterior part of iris, larger size of more than 13–14 mm (depending on the size of the eye) and a three-piece design. Sulcus placement of standard one-piece IOLs should be avoided as it increases the risk of postoperative complications, including pigment dispersion, uveitis–glaucoma–hyphema syndrome, and recurrent vitreous hemorrhages. On the other hand, IOLs implanted in the capsular bag should have a squared, truncated posterior optic edge to prevent lens epithelial cell migration and posterior capsule opacification [40]. Anterior chamber IOLs can be placed in the anterior chamber angle (open-loop design is preferred) or fixated to the iris (iris-claw IOLs).

Based on the optical properties IOLs can be divided into monofocal IOLs or multifocal/accommodating IOLs . Both monofocal and multifocal IOLs might have a spherical power, or toric in order to compensate astigmatism. The positive spherical aberration of the cornea can be compensated by aspheric design IOLs, having negative or zero spherical aberration to improve the patient’s quality of vision. In order to achieve multifocality or an extended depth-of-field different optical principles are employed. A diffractive IOL generates multifocality making use of light interference and is independent of pupil size. It incorporates a pattern consisting of a series of annular concentric grooves less than one micron in depth, which are engraved around the optical axis on either the front or the back surface of a lens (the echelette technology). The refractive design allows to achieve depth of focus with light refraction on the IOL surfaces based on Snell’s law. The optical power decreases continuously from the center to the periphery of the lens creating an infinite number of focal points and is derived from the smooth hyperbolic shape of its optics. The performance of refractive design IOLs is dependent on pupil size and IOL centration. Other optical concepts such as the pinhole effect [41] or light sword optical element [42] might be employed. Accommodating IOLs may change their curvature, or have a fixed-power presenting axial shift in order to restore accommodation (Fig. 3.2d) [43].

Ametropia following cataract surgery can be treated by performing a corneal refractive enhancement. Secondary piggyback IOLs are designed for secondary implantation in the ciliary sulcus to correct pseudophakic ametropias or pseudophakic lack of accommodation. Another potential option for avoiding pseudophakic ametropia is implantation of Light Adjustable Lens (LAL). A technology developed by Calhoun Vision Inc. involves implantation of a three-piece light-adjustable silicone IOL with silicone macrometrs containing an ultraviolet (UV) light-activated photo initiator [44]. Postoperatively, the IOL power can be adjusted by exposure to a customizable UV light profile resulting in polymerization of the IOL macromers. Clinical studies confirmed the safety of this technology to corneal endothelial cells [45] and to the retina [46].

Glaucoma and Cataract Surgery

Cataract surgery alone results in a modest reduction of intraocular pressure [47, 48]. The reduction is proportional to preoperative IOP and ranges from 8.5 mmHg in eyes with preoperative IOP of 23–29 mmHg to 1.7 mmHg in eyes with IOP 5–14 mmHg [49]. The decrease in IOP is attributed to the increase in anterior chamber depth, flattening of the iris diaphragm, and subsequent extension of the trabecular meshwork. These changes are particularly advantageous in patients with angle-closure glaucoma. In open-angle glaucoma although IOP parameters improve after cataract surgery, glaucomatous visual field decay does not slow compared to rates measured during the progression of cataract [50]. Another option that should be considered in glaucoma patients is conjunction of PCS with endoscopic cyclophotocoagulation, trabecular micro-bypass stent, ab interno trabeculectomy, and canaloplasty [51]. These procedures are associated with a lower risk of surgical complications, however, are less effective than trabeculectomy.

Many antiglaucoma agents were reported to cause pseudophakic cystoid macular edema (PCME). Particularly prostaglandins—often the first line of treatment for IOP lowering—were believed to increase the risk of PCME [52, 53]. Miyake et al. presented findings suggesting that the preservatives used in these pharmaceutical cause increases synthesis of prostaglandins, intensifies postoperative inflammation and results PCME [54]. Nevertheless, more recently no association between prostaglandin analogue administration and PCME was reported [55]. It might be concluded that there is no evidence to discontinue antiglaucoma medications during cataract surgery and the risk associated with IOP elevation might be greater than disadvantages associated with their use.

AMD and Cataract Surgery

The Beaver Dam Eye Study presented that having undergone PCS before baseline examination was associated with age-related maculopathy and the exudative age-related macular degeneration [56]. Some newer studies confirmed cataract surgery as being a risk factor for AMD [57–59], while other reported conflicting results [60–63]. The Cochrane Database for Systematic Reviews revealed that it is not possible to draw reliable conclusions from the available data as to whether cataract surgery is beneficial or harmful in people with AMD after 12 months [64]. It was concluded that in general cataract surgery provides short-term improvement in best corrected visual acuity in AMD patients compared to patients with no surgery. It is unclear whether the timing of surgery has an effect on long-term outcomes in AMD [64]. As in vivo studies demonstrated that blue light (430 nm wavelength) is harmful to retinal pigment epithelium cells, some authors believe that the protective effect of UV-blocking and blue-blocking IOLs might be greater than solely UV-blocking IOLs [65]. Nevertheless, the application of blue-blocking IOLs in not based on clinical evidence, as there are no studies truly confirming significant photoprotection [66].

Visual rehabilitation is necessary in patients with advanced AMD. Usually spectacles, magnifying glasses or electronic devices are used as visual aids. For several years specially designed intraocular implants have become an appealing alternative to extraocular aids. The possible options include implantable macular telescope, an IOL-VIP System, Lipshitz macular implant (for capsular bag or sulcus fixation), Fresnel Prism IOL, iolAMD or Scharioth Macula Lens [67]. However, the results so far were variable, and the available studies focused mainly on short-term outcomes [68].

Importantly, cataract surgery after previous intravitreal therapy is associated with a higher likelihood of posterior capsule rupture (PVR); 10 or more previous injections is associated with a 2.59 higher likelihood of PCR [69].

Combined Surgeries (Efficacy vs Safety)

The efficacy and safety of combined phacoemulsification and vitrectomy was reported in several surgical indications, including macular hole, macular pucker, and in diabetic patients. A combined procedure eliminates the need for two operations and enables faster visual rehabilitation. Another rationale for this approach is that vitrectomy itself is known to induce cataract. For example, in a study by Jackson et al. 51.8% of phakic patients after vitrectomy with gas tamponade developed cataract requiring PCS in the following 6 months [70]. Lens opacification is associated with increased retrolental oxygen levels, particularly in extended vitrectomy with surgical posterior vitreous detachment and anterior vitreous removal [71]. The increased risk of intraoperative complications during PCS in vitrectomized eyes is well known, and might be associated with lens touch during vitrectomy, zonular dehiscence or intraoperative miosis [72–75]. The absence of vitreous support might also result in increased anterior depth and a disparity between fluid inflow and outflow during phacoemulsification or irrigation/aspiration [76–78].

Combined phacoemulsification with intraocular lens implantation and keratoplasty is known as the triple procedure. It is a safe and effective approach in patients with coexisting cataract and corneal pathologies [79]. A significant problem in the triple procedure is an unacceptable postoperative refractive error. It is a concern both in penetrating keratoplasty (where some suggest PCS after suture removal due to the refractive change associated with their removal) and lamellar keratoplasties. In deep anterior lamellar keratoplasty a large proportion of the stroma is replaced giving uncertainty of postoperative refraction, while in endothelial keratoplasties the cornea is preoperatively swollen due to endothelial dysfunction altering its optical power.

In cases of visually significant cataract and co-existent glaucoma, combined surgery (phacotrabeculectomy) can be considered. Filtration surgery presents a high risk of intraoperative bleeding, in contrary to PCS alone [80]. With that, there is evidence that long-term IOP is lowered by combined glaucoma and cataract surgery, however, giving a smaller effect than trabeculectomy alone [81, 82]. This might be possibly due to inflammation related to phacoemulsification. On the other hand, some studies presented that PCS performed after trabeculectomy might reduce the function of a filtering bleb in some eyes [83, 84]. In patients with angle-closure glaucoma, PCS alone might be an effective treatment [85]. Interestingly, combined cataract surgery (with corneal, glaucoma or vitreoretinal procedures) has a higher incidence of acute postoperative endophthalmitis than stand-alone PCS (0.149% vs. 0.102%, respectively).

Settings (Ambulatory or Hospitalization, Operating Theater or in Office)

Currently, almost all cataract surgeries are performed in outpatient settings. These include a hospital-based outpatient departments or a standalone ambulatory surgical centers. The Cochrane Database for Systematic Reviews found cost savings associated with same-day discharge cataract surgery versus in-patient cataract surgery, however, the evidence regarding postoperative complications was inconclusive because the effect estimates were imprecise [86]. Some patients may still require an operative room or in-patient setting, intravenous sedation or general anaesthesia, and particularly complex cataract cases, or in individuals with severe comorbidities.

Recently, safety and effectiveness of PCS performed in an office-based minor procedure room in a series of 21,501 eyes was presented [87]; it was efficient for surgeon, as well as comfortable for the patient [85]. Another study conferred the safety of outpatient cataract surgery without presence or a dedicated access to anaesthetic service [88]. The monitoring was limited to blood pressure and plethysmography pre- and intraoperatively. Although office-based surgery is presently not reimbursed by Medicare, such relocation of the procedure might occur in future as it is cost-effective.

Anesthesia (Topical vs Peribulbar vs Retrobulbar vs General)

The majority of cataract surgeries are performed under local anesthesia. General anesthesia should be considered in pediatric patients and for adults having difficulties with cooperation during surgery due to head tremor, deafness, mental retardation, neck or back problems, claustrophobia.

Retrobulbar anesthesia is performed by injecting the anesthetic drug to the intraconal space behind the eye [89]. In peribulbar anesthesia the drug is injected to the extraconal space, and diffuses between the intra- and extracone space to achieve anesthetic effect. Peribulbar and retrobulbar anaesthesia for cataract surgery show similar akinesia and pain control in cataract surgery [90]. However, the need for additional injections of local anaesthetic is greater with peribulbar anaesthesia. Another option is sub-Tenon anesthesia which involves creating a conjunctival buttonhole, blunt dissection of the Tenon’s space and introduction of anesthetic to the subtenonian space.

Topical anesthesia may be preferred in phacoemulsification due to lower complication rates. Nevertheless, PCS under topical anesthesia may not be a completely painless procedure [91]. In these cases another option is additional intracameral anesthetic administration during the surgery.

Difficult Cases: Increased Risk of Complications

Preoperative definition which eyes poses a potential risk of complications is critical, as in these eyes proper planning of the surgery is the key to success. Anatomic difficulties including deep set eyes or a protruding brow might impede surgical manipulations and proper instrument fitting it the operative field. Another group of challenges are high ametropias. Small hyperopic eyes make it difficult to perform intraocular manipulations, have increased chance of irid injury, cyclodialysis or fluid misdirection syndrome [92]. In deep myopic eyes the nucleus can be very large and the anterior chamber very deep. These patients also have a lower risk of accurate IOL power prediction [34]. Corneal opacities or scarring impede proper visualization of the operative field, which is particularly important during the capsulotomy [93]. Hard, dense or hypermature cataracts might be difficult to remove with phacoemulsification, and in these cases extracapsular extraction might be required. Eyes with pseudoexfoliation, with subluxated or dislocated lens (due to very mature lens, trauma, of Marfan’s syndrome) have an increased chance of zonular dialysis, and might require supplementary means to fixate the lens. Eyes with small pupils, bound down pupil, floppy iris syndrome or a posterior pole cataract present an increased risk for intraoperative complications. General health problems including chronic obstructive pulmonary disease or dementia might impede proper patient positioning and cooperation [92].

The National Institute for Health and Care Excellence recommends that one IOL should be in the theater, an additional identical one should be in stock, and an alternative IOL (anterior chamber (AC)-IOL, sulcus IOL or iris-claw IOL) in case the lens needs to be changed if there are complications during surgery [20]. Additional accessories such as pupil expansion rings, iris hooks or capsular tension rings should be in stock.

Bilateral Operation

The principle of immediate sequential bilateral cataract surgery (ISBCS) is to treat each eye as an individual and autonomous surgery during the same session in the operating theater. Each eye requires a change of draping, gloves, gowns and instruments [94]. Some authors recommend that instruments should come from disparate sterilization cycles, while viscoelastics or irrigation fluids from different companies or, at least, have different lots [95]. With these precautions employed, the greatest potential risk of ISBCS—bilateral endophthalmitis—has never been reported.

ISBCS is comparable with delayed sequential bilateral cataract surgery (when the second eye is operated on days to weeks later) in long-term patient satisfaction, visual acuity and complication rates [96]. A significant advantage of ISBCS is faster visual rehabilitation [97]. This approach is also cost-effective: it requires fewer hospital visits, allows faster return to work and only one pair of glasses. ISBCS is an ideal solution for patients requiring general anaesthesia, in order to eliminate the risks associated with a second intervention [98].

An argument commonly picked up by its opponents is the problem of anisometropia. In delayed sequential cataract surgery it is possible to adjust the IOL power of the second eye based on the postoperative results of the surgery. Nevertheless, with the advances of optical biometry, and due to the fact after simultaneous surgery, the errors are minor and symmetrical, ISBCS does not cause anisometropia.

Intraoperative Considerations

Wound Construction

Currently cataract surgery employs CCI or scleral incisions, and their classification is presented in Table 3.4. Superiorly placed scleral tunnel incisions are used mainly in MSICS and by beginning cataract surgeons. Scleral incisions induce significantly less astigmatic change compared to CCI, which constitutes a significant advantage of this approach [124]. Creating a scleral incision is more challenging and time consuming compared to a CCI, and peribulbar or retrobulbar anesthesia is required. Proper tunnel incision architecture, at least 1–2 mm into the clear cornea, has self-sealing wound properties. Disadvantages of scleral incisions is occasional requirement of cautery, as the conjunctiva is highly vascular. If the initial groove is too superficial, a thin scleral flap will be prone to tearing or lacerations. If the initial groove is too deep, the anterior chamber might be penetrated too early. When the corneal part of the tunnel is performed too anteriorly, the visualisation will be hampered due to striae when the phaco tip is tilted down. Ballooning of the conjunctiva might impede access to the tunnell and require an additional cut.

Table 3.4

Classification of scleral and corneal tunnel incisions

Scleral tunnel incisions
Conjunctival flap architecture	Limbal-based flap
	Fornix-based flap
Based on scleral groove shape	Smile incision	Following the limbus
	Straight incision	Straight line
	Frown incision	Curve opposite to the limbus
	Blumental side cut	Straight line with sides receding from limbus
	Chevron ‘v’ incision	V-incision, sides receding from the limbus
Corneal tunnel incisions
Based on external incision location	Clear corneal incision	Entry anterior to conjunctival insertion
	Limbal corneal incision	Entry through the conjunctiva and limbus
	Scleral corneal incision	Entry posterior to the limbus
Based on architecture	Single plane	No groove
	Shallow groove	Below 400 μm
	Deep groove	Over 400 μm

Primarily CCIs were criticized because of presumed increased risk of endophthalmitis due to uncertain sealability and poor wound healing (Fig. 3.3). Thus CCIs were indicated only in patients with pre-existing filtering blebs, taking anticoagulants [125], with blood coagulation disorders or cicatrizing diseases i.e., ocular cicatricial pemphigoid or Stevens-Johnson syndrome. Afterwards, as CCI can be performed under topical anesthesia, they became more and more commonly used. In general, regional blocks presented increased risk of complications compared to topical anesthesia. Other advantages of CCIs include ease of approach to the incision site, preservation of options for future filtering surgeries, increased stability in refractive results (neutralization of the forces from lid blink and gravity), no need for bridle sutures and the location of the lateral canthal angle under the incision which facilitates drainage. A disadvantage of CCI is the induction of astigmatism. In general, the main incision should be located in the steep meridian of the cornea, particularly if the corneal astigmatism is higher than 0.50 cylindrical diopter. Interestingly, the CCI-induced astigmatism significantly differences among surgeons. In a study conducted by Ernest et al. all of the surgeons have performed CCIs in the same manner and using identical surgical tools, the surgically induced astigmatism differed double-fold (mean induced astigmatism from 0.38 to 0.88 D depending on surgeon) [126]. Furthermore, the location of the incision influences the size of astigmatism. Incisions performed in the nasal quadrant induce significantly higher astigmatism than the temporal ones [127, 128]. Although clear corneal and scleral incisions might engender complications by nature of their architecture and location, some complications are unique to CCI. If the conjunctiva is unintentionally incised at the time of creating a CCI, ballooning of the conjunctiva can develop which may compromise visualization of anterior structures. If this develops, the use of a suction catheter is usually required by the assistant to aid in visualization. Early entry into the anterior chamber might result in an incision of insufficient length to be self sealing, increased tendency for iris prolapse, and requires placing a suture at the conclusion of the procedure. Late entry may result in a long corneal tunnel, so that the phacoemulsification tip would create striae in the cornea and hamper visualization of the anterior chamber. Manipulation of the phacoemulsification handpiece intraoperatively may result in tearing of the roof of the CCI, particularly at the edges, compromising the ability to self-seal, or occasionally resulting in minor detachment or scrolling of Descemet’s membrane. Incisional burns similarly compromise self-sealability, result in corneal edema and severe induced astigmatism [129]. In addition, manipulations in the proximity of the wound can cause epithelial abrasion.

Postoperative Complications

Postoperative Follow Ups

The timing of postoperative examinations should be adjusted to ensure the expeditious recognition and management of complications in order to optimize the final outcome of surgery. The frequency of postoperative complications in recent studies is presented in Table 3.6.

Table 3.6

Prevalence of postoperative complications in selected cataract surgery studies

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