Evolution of Biometric Formulas and Intraocular Lens Selection in Challenging Cases

Abstract

Various novel intraocular lens (IOL) power calculation formulas have been described to increase refractive precision following cataract surgery. These include the Barrett Universal II, Emmetropia Verifying Optical (EVO), Kane, Naeser 2, Olsen, Panacea, Pearl DGS, Radial Basis Function (RBF), T2, and VRF formulas. With a few notable exceptions, historical and regression formulas—first- and second-generation IOL formulas like Sanders, Retzlaff, Kraff (SRK), Binkhorst, Hoffer, and SRKII—are generally regarded as outdated. The effective lens position (ELP) is accounted for in third- and fourth-generation formulas which include more biometric data. A possible alternative that has shown to be remarkably accurate when used with the Olsen method is ray tracing. Artificial intelligence-derived IOL formulas are becoming increasingly common and may yield better lens power prediction accuracy. Despite improvements in surgical technique, biometry measurements, and IOL calculations, some clinical circumstances continue to challenge cataract surgeons to determine the appropriate IOL power. These unique situations include pediatric eyes, post-refractive eyes, and corneal ectasias. The obstacles to reliability include unrepeatable measurements and inaccurate biometry examinations. Researchers have tried to identify the most accurate IOL estimations for these challenging clinical scenarios to overcome these obstacles.

1. Introduction

The development of biometric criteria for choosing appropriate intraocular lens (IOL) power has been continuous. The power of the IOL that will be placed after cataract surgery is determined using biometric formulas. The objective is to provide the patient with the best visual result possible. Aiming for residual hyperopia, employing the Haigis formula for biometric calculation, or applying a customized surgeon’s A-constant are just a few of the ways previously recommended by authors to address the errors in IOL power assessment [1]. Artificial intelligence has been used in recent biometric formula innovations. Various generations of formulas have been developed to choose the IOL dioptric power as a function of its anticipated postoperative position. However, only a minority of formulas—usually one-dimensional—include information about crystalline lenses [2].

2. Evolution of formulas

2.1 Empiric formulas

The Sanders-Retzlaff-Kraff (SRK I, SRK II) formulas, the first to represent a statistical regression technique, were reported to produce better results than early theoretical formulas in the early years of IOL power computation. Any empirical approach has the benefit of being based on actual measurements, which reduces the need for assumptions. For instance, how to calculate corneal power, account for principal planes, correct axial length for retinal thickness, and make any clinical measurements work in the physical sense. A regression formula functions by generating a mean value and accounting for departures from the mean using regression coefficients. The mathematical mean errors of a regression equation should accumulate to zero in a representative patient sample if it is correctly derived [3].

The first formula of SRK I was a straightforward linear regression equation: $P=A-2.5AL-0.9K$ where AL = the axial length of the eye as measured by ultrasound, P0 = the power of the implant for emmetropia, K = the dioptric keratometry reading (using index 1.3375), A = the A-constant according to the kind of IOL, and the mean values of the K-readings and axial length readings.

Any empirical approach has the drawback that, in theory, its formula only applies to the dataset from which it was developed. For instance, the A-constant (and perhaps the regression coefficients) will change if the axial length is assessed using a different method in a different clinical situation. This would be the case when switching from ultrasound to partial coherence interferometry (PCI) (Carl Zeiss Meditec, Jena, Germany), which tends to provide longer readings than ultrasound. The formula, however, may also be susceptible to variations in surgical technique, such as whether the IOL is positioned inside or outside the capsular bag, which can change the IOL’s average location and refractive effect.

2.2 Theoretical formulas

They have a Gaussian optics foundation. This theoretical formula contains six variables, which are

1. Net corneal power (K)

2. Axial length (AL)

3. IOL power (IOLP)

4. Effective lens position (ELP)

5. Refraction desired (D Post Rx)

6. Vertex distance (V) [4]

2.2.1 First generation

The anterior chamber depth (ACD) was presumed to be constant for all eyes in the first generation of theoretical formulations for intraocular lenses, which included the Binkhorst I, Fyodorov, and Colenbrander formulas. These calculations were inaccurate for all eyes because they were built on the simplistic assumption that the ACD had a set value [5].

2.2.2 Second generation

They refer to specific theoretical or regression formulas. Intraocular lens formulas from the first generation were correct in eyes with standard lengths, but they struggled in eyes that were unusually long or short. As a result, the formula needed to be changed for the axial length to be factored correctly. The original SRK formula, P = A − 2.5 L − 0.9 K + C, was modified to create the SRK II or second-generation regression formula. Regression analysis of the postoperative refractive error in numerous eyes was used to calculate the C-value [4]. Regression investigation of the postoperative refractive error in many eyes yielded the C-value in the SRK II calculation. The C-value is 3 for an axial length of 10–20 mm. The C-value is 2.0 if the axial length is 20–21 mm. The C-value is 1 for 21–22 mm. It is 0 for 22–24.5 mm. The C-value is −0.5 if the axial length is more than 24.5. In conclusion, the SRK II and other second-generation intraocular lens formulas employed the C-value to modify the formula so that the axial length could be precisely factored. Other formulas used the observed axial length, such as the modified Binkhorst Formula and Hoffer’s ACD Adjustment [6].

2.2.3 Third generation

By accounting for unique differences in the position and shape of the crystalline lens, third-generation intraocular lens formulas have been created to increase the precision of IOL power estimation. The Holladay 1, Hoffer Q , and Sanders-Retzlaff-Kraff/Theoretical (i.e., SRK/T) formulas are examples of these formulas. Note that the SRK/T formula is the most recent version of SRK formulas. It is not a regression formula but a modified Binkhorst model with changed anterior chamber depth (ACD)-prediction algorithms. The third-generation intraocular lens formulas have been found to estimate IOL power more accurately than the first- and second-generation formulas [7, 8, 9].

The axial length, corneal power, and anterior chamber depth are used in the Holladay 1 formula to determine the IOL power. To increase the accuracy of IOL power estimation, the Holladay 2 formula adds the surgeon factor and lens thickness to the Holladay 1 method. The Hoffer Q formula, which calculates IOL power using the axial length, corneal power, and ACD, has been determined to be the most accurate method for doing so in eyes with axial lengths less than 22 mm. The best precise formula for calculating IOL power in eyes with axial lengths more than 22 mm is the SRK/T formula, which uses the axial length, corneal power, and ACD.

2.2.4 Fourth generation

These formulas have been utilized in numerous research and were created to increase the precision of IOL power calculations. The Holladay 2 technique is a formula that takes seven factors into account, including age, axial length (AL), keratometry, anterior chamber depth (ACD), white-to-white measurement, and lens thickness (LT). In short eyes (22 mm), the Holladay 2 formula is reliable for forecasting IOL power. Holladay 2 was more accurate than Holladay 1 and equal to Hoffer Q in short eyes <22.0  mm [10].

The Haigis formula was developed in 2000 to calculate the power of intraocular lenses (IOLs). It uses three independent constants called a0, a1, and a2 to predict IOL power. These constants, unique to the formula, are determined from regression analysis of clinical data. All three constants can be adjusted using linear regression to improve the function’s ability to predict outcomes accurately. In long eyes (>26 mm), the Haigis formula is reliable for forecasting IOL power [11].

2.2.5 New novel formulas

The Barrett Universal II formula is an IOL power calculation formula introduced in 2014 to modify the original Barrett Universal formula. It uses biometric parameters, including axial length, keratometry, anterior chamber depth, and lens thickness, to predict IOL power. The accuracy of the Barrett Universal II formula has been studied in various populations, including post-corneal refractive surgery eyes, highly myopic patients, and pediatric cataract patients with multifocal IOL implantation. The formula has produced better refractive outcomes than other formulas in some studies [12]. This formula has also been used in studies evaluating the efficacy of different IOLs, such as trifocal presbyopia-correcting IOLs and non-diffractive extended depth of focus or neutral aspheric monofocal IOLs [13].

The Barrett Universal II formula provides better predictability of IOL power calculation. It is less susceptible to the effect of the axial length and the corneal shape than the SRK/T formula in eyes requiring combined cataract surgery and trabeculectomy [14].

Emmetropia Verifying Optical (EVO) is a technology used in intraocular lens (IOL) power calculation to verify the accuracy of the selected IOL power. EVO uses a wavefront aberrometer to measure the patient’s postoperative refraction and compare it to the predicted refraction based on the desired IOL power. The desired IOL power is considered accurate if the measured refraction is within a specific range of the expected refraction. If the measured refraction exceeds this range, the EVO technology can suggest a different IOL power to achieve the desired refractive outcome. EVO can be used in both cataract surgery and refractive lens exchange procedures. Using EVO can improve the accuracy of IOL power calculation and reduce the need for postoperative refractive adjustments. However, EVO is not widely available and may only be necessary in some cases, particularly in patients with normal ocular anatomy and biometry. In conclusion, Emmetropia Verifying Optical (EVO) is a technology used in IOL power calculation to verify the accuracy of the selected IOL power. Its use can improve the accuracy of IOL power calculation and reduce the need for postoperative refractive adjustments, but it may not be necessary in all cases [15].

Based on theoretical optics, the new Kane formula (found at www.iolformula.com) also integrates regression and artificial intelligence to improve its predictions further. The Kane formula is a widely used IOL power calculation formula that accurately predicts IOL power in various populations. To anticipate the refractive result, it considers the patient’s gender, axial length, keratometry, anterior chamber depth, lens thickness, and central corneal thickness [16]. In several studies, the Kane formula had a higher percentage of eyes within ±0.25 D of the predicted refraction than other formulas, including the Hoffer Q , SRK/T, and Holladay 1 formulas. The Kane formula also had the highest percentage of eyes within ±0.25D and ±1.00D in a comparison study of 13 formulas [17]. The Kane keratoconus formula was the most accurate in a study of patients with keratoconus [18]. The Kane formula was also accurate in vitrectomized eyes and eyes with high axial myopia [19]. In a study of sharp eyes, the Kane formula had a statistically significantly lower mean absolute error compared to all other formulas except the EVO 2.0 [20].

The Naeser 2 formula was developed by Naeser in 1997. It is based on vergence calculation and lens design and uses two keratometry readings, axial length, and anterior chamber depth, to predict IOL power. The Naeser 2 formula has been studied in various populations and accurately predicts IOL power. In a study of patients older than 80 years with cataracts and corneal astigmatism, the Naeser-Savini formula achieved the lowest mean absolute error and had the highest percentages of eyes within an absolute error of 0.50 D and 1.00 D compared to other formulas [21]. However, the Naeser formula has not been widely studied compared to other formulas, and its accuracy has yet been proven from empirical data. Further research is needed to determine its efficacy as compared to other formulas.

The Olsen formula uses ray tracing technology to predict IOL power. It considers the individual optical properties of the cornea, lens, and eye axial length to calculate the optimal IOL power. The Olsen formula has been studied in various populations and accurately predicts IOL power [22]. In a study comparing the Olsen formula to other formulas, the Olsen formula had the lowest mean absolute error and the highest percentage of eyes within ±0.50 D of the predicted refraction [23]. The Olsen formula has also been found to be accurate in eyes with high myopia and eyes with previous corneal refractive surgery [24, 25].

The Panacea formula, created by David Flikier, is a vergence formula that includes the precise measurement of anterior and posterior corneal curvature data to establish and use total corneal astigmatism [26]. This formula can be accessed from the Panacea IOL and Toric calculator software (www.panaceaiolandtoriccalculator.com). The Panacea formula does not mathematically calculate the posterior corneal surface from the anterior surface. It takes into account the actual values of the anterior and posterior surfaces to provide the total corneal astigmatism, which generates more accurate calculations [27]. It is a thin-lens vergence formula with the unique possibility of including the anterior-to-posterior corneal curvature ratio and the asphericity (Q value at 6.0 mm) of the anterior corneal surface [28].

A more contemporary formula, the Pearl-DGS, was created by G. Debellemanière, D. Gatinel, et al. Optical and machine learning models were used to create this unpublished formula. AI-enhanced prediction and output linearization are used, and the A-constant from the SRK/T formula is necessary. This formula may be found at http://www.iolsolver.com [28].

2.2.5.1 Radial basis function 2.0

This AI-based algorithm calculates the IOL power using the radial basis function. It is available online at http://www.rbfcalculator.com [28]. Radial basis function methods are not limited to IOL power calculation. It has been used in various fields, including function interpolation, hyperspectral data classification, and artificial neural networks.

The formula has been studied in various populations and accurately predicts IOL power. In a study comparing the Hill-Radial Basis Function 2.0 formula to the Barrett Universal II formula and the SRK/T formula, the Hill-Radial Basis Function 2.0 formula had similar accuracy to the other formulas [29]. In another study comparing to other artificial intelligence-based formulas, including the Kane and PEARL-DGS formulas, the Hill-Radial Basis Function 2.0 formula had similar or better accuracy than the other formulas [30]. The Hill-Radial Basis Function 2.0 formula has been accurate in eyes undergoing manual or femtosecond laser-assisted cataract surgery.

The T2 formula was created to enhance the SRK/T formula’s original design. Excel was programmed with the actual data in mind. For postoperative prediction of the anterior eye portion, it uses the same optical A-constant of the SRK/T and an upgraded regression algorithm [28]. However, further research is needed to determine its efficacy compared to other formulas in different populations.

2.2.5.2 VRF

This vergence-based thin-lens formula uses the optical CACD constant and four variables to determine the IOL power: AL, K, ACD, and the horizontal CD. The author programmed it into Excel [28]. In a study comparing the VRF-G formula to other formulas, including the Kane, Hoffer QST, and Barrett Universal II formulas, the VRF-G formula was more accurate than older formulas. The VRF-G formula had a standard deviation of ±0.387 D, which was lower than the standard deviations of the other formulas [31]. The VRF formula is unrelated to the other references, which discuss formulas and mathematical calculations in various fields, including medicine, engineering, and linguistics.

The Castrop intraocular lens (IOL) calculation formula is a recent development to evaluate the accuracy of IOL power calculation in patients with short axial eye length who are strong hyperopes. The Castrop formula is based on a pseudophakic model eye and has shown slightly superior performance to classical formulas such as the SRKT, Hoffer-Q , Holladay1, or Haigis formulas. The Castrop formula has not been published before but has been disclosed in a ready-to-use Excel sheet as an addendum to a research paper. The formula was evaluated in a single-center study in Germany that involved patients who underwent uneventful cataract surgery and were implanted with either spherical or aspheric IOLs. While the Castrop formula showed slightly superior performance compared to the classical formulas, future studies are needed to evaluate the reliability and accuracy of the formula. Further optimization of formula constants and consideration of variables such as keratometer or corneal refractive index in different IOL formulas can improve overall performance and accuracy in IOL power calculations [32, 33, 34, 35].

At this juncture, it is necessary to briefly discuss the effect of anterior segment depth (ASD) on residual refraction after cataract surgery. The ASD, i.e., the sum of the anterior chamber depth and lens thickness, can affect the accuracy of IOL calculation formulas. Kesim et al. found that different ASD measurements affect the accuracy of seven different IOL calculation formulas, with larger ASD leading to higher mean absolute error values [36]. Positive correlations were found between ASD and the predictive errors of the SRK/T, Holladay I, Hoffer Q , Barrett II, Hill-RBF, and Haigis formulas. In cases with mean K greater than 42,0 D, ASD was similarly correlated with PE, except for the Olsen OLCR formula. In eyes with an AL between 22.5 and 24.5 mm, the predictions of lens formulas were substantially hyperopic in cases with greater ASD, according to their findings. Overall, the accuracy of IOL power calculations depends on many factors, including ASD, axial length, keratometry, and individual patient characteristics, which should be considered when selecting the appropriate IOL formula.

3. The approach in challenging cataract cases

3.1 Pediatric eyes

Pediatric cataract is a leading cause of childhood blindness, and their management can be challenging due to the growing size of the affected eyes and the risk of amblyopia. The etiology of pediatric cataracts varies and can be classified according to their time of onset, morphology, and underlying cause. The most common etiology of pediatric cataracts is idiopathic, meaning the reason is unknown. However, genetic factors play a role in the development of congenital cataracts, and genetic counseling and molecular testing should be undertaken in cases of hereditary cataracts. Other etiologies of pediatric cataracts include trauma, infections, metabolic disorders, and syndromes. Early diagnosis and treatment of pediatric cataracts are essential for good visual outcomes, and identifying the etiology of cataracts is necessary for counseling and preventive public programs.

A thorough ocular examination is essential in evaluating pediatric cataracts, including the onset, duration, and morphology of the cataract. Ocular malformations such as microphthalmos/microcornea are frequently associated with pediatric cataracts. The management of pediatric cataracts is a team effort involving ophthalmologists, pediatricians, anesthetists, and parents. It should be customized depending on the age of onset, laterality, the morphology of the cataract, and other associated ocular and systemic comorbidities. Newborns should be examined for ocular structural abnormalities, such as cataracts, corneal opacity, and ptosis, which are known to result in visual problems. Congenital cataracts and blunt ocular trauma are the most frequently observed congenital ocular disease and causes of ocular trauma among children. The frequency of strabismus and chronological, etiological, and morphological features should also be evaluated in patients with pediatric cataracts.

The decision to operate on pediatric cataracts depends on various factors, including the patient’s age, the cataract’s type and severity, and associated ocular and systemic comorbidities. Advances in surgical techniques and methods of optical rehabilitation have substantially improved the functional and anatomic outcomes of pediatric cataract surgeries in recent years. However, good visual outcomes require occlusion therapy and optical correction.

Pediatric cataract surgery is different from adult cataract surgery. It presents several intraoperative challenges, including low scleral rigidity, which makes it difficult to construct incisions and close wounds; the smaller size of the eyeball, shallow anterior chamber depth, and small pupil size, which reduces maneuverability; and an elastic capsule, which increases the risk of vitreous loss and expulsion of intraocular contents. The benefits of primary IOL implantation include quick postoperative refractive correction, minor to no optical aberration, an entire visual field, a reduced risk of amblyopia onset and progression, and little reliance on patient compliance. IOL implantation in children under the age of two is still debatable. The primary reasons for the limited use of IOL implantation in children under two years of age include the lack of long-term data to predict the success rate, linked additional ocular problems with cataracts, systematic development of deprivation amblyopia, and increased postoperative morbidity.

3.2 Surgical technique

3.2.1 Anterior capsule management

Pediatric cataract therapy relies heavily on anterior curvilinear capsulorhexis (CCC) since it chooses the surgical approach and location of the IOL fixation. Since the anterior capsule in children is relatively elastic, a controlled manual CCC may be challenging. The rhexis margin is still the gold standard in strength, however. Therefore, a manual anterior continuous curvilinear capsulorhexis should be performed wherever possible. The size and shape of the anterior capsulotomy are crucial for the IOL to remain centered for a long time. Staining the anterior capsule with trypan blue 0.1% has been shown to allow recognition of capsule flaps and facilitate the creation of complete anterior CCC and posterior PCCC in pediatric cataract surgery. Indocyanine green (ICG) staining has been evaluated to enhance the visualization of the anterior lens capsule in dense pediatric, mature cataracts. Femtosecond laser-assisted capsulorhexis, vitrectorhexis, radiofrequency diathermy, and Fugo plasma blade-assisted rhexis are options for manual CCC.

3.2.2 Management of the posterior capsule and anterior vitreous face

After juvenile cataract surgery, visual axis opacification (VAO) is the most prevalent and severe issue. Significant VAO in children hinders recovery and development of the visual system. Amblyopia may also result from it. A primary posterior capsulotomy is therefore regarded as a “routine surgical step,” particularly in young children up to the age of eight years (with or without anterior vitrectomy). The posterior capsulotomy procedure can be carried out using various techniques, including radiofrequency diathermy, the Fugo plasma blade, femtosecond laser-aided capsulorhexis, vitrectorhexis, and manual posterior CCC. In contrast, if a pars plana vitrectorhexis is performed, most surgeons prefer to execute it after IOL implantation. Most surgeons prefer to perform manual posterior CCC before IOL implantation. Nevertheless, it entirely depends on the surgeon’s judgment and preferences [37].

Although it can be postponed, posterior CCC cannot prevent visual axis obscuration alone. The proliferating lens epithelial cells might use the anterior vitreous face (AVF) as a scaffold. Additionally, due to the severity of the inflammatory reaction in young children, fibrous membranes may develop on the intact AVF, leading to VAO. Therefore, a posterior capsulotomy and anterior vitrectomy are recommended for newborns and young children. In infants under the age of four, an anterior-posterior capsulotomy with anterior vitrectomy is preferred, whereas, in patients aged four to eight, only posterior capsulotomy is performed without anterior vitrectomy. However, some surgeons favor performing anterior vitrectomy on patients as young as six to seven.

3.2.3 Primary IOL implantation

Several studies have shown that primary IOL implantation can be a safe and effective option for treating pediatric cataracts, with outcomes comparable to contact lens correction or secondary IOL implantation. One study compared the visual outcomes of primary IOL implantation versus contact lens correction in children with unilateral congenital cataracts. The study found that both groups had similar visual outcomes in the first year of age [38]. Another study showed that at a 4.5-year follow-up, infants who underwent primary IOL implantation or were treated with contact lenses had similar visual outcomes. However, pseudophakic children had a greater incidence of complications and second interventions than the contact lens group [39].

On the other hand, Infant Aphakia Treatment Study also revealed the complications of primary IOL implantation. It is a clinical trial designed to compare the visual outcomes of patients who received either contact or intraocular lenses after cataract surgery during infancy. The study included 114 infants with unilateral congenital cataracts at 12 sites [40]. The study reported that after 4.8 years of surgery, the incidence of glaucoma and glaucoma plus glaucoma suspect in operated eyes for children up to age five years were 17 and 31%, respectively. However, neither the contact lens nor the IOL group had a significant difference in either outcome: glaucoma (hazard ratio HR, 0.8; 95% CI, 0.3–2.0; P = .62) and glaucoma + glaucoma suspect (HR, 1.3; 95% CI, 0.6–2.5; P = .58) [41]. Another study evaluated the outcomes of bilateral cataract surgery in infants aged one to seven months with Infant Aphakia Treatment Study (IATS) investigators and reported that 24% of children received primary IOL implantation, with a median visual acuity of 0.35 logarithm of the minimum angle of resolution in the better-seeing eye at the final study visit closest to five years of age [42]. The study also found that strabismus was detected in 81% of infants by age five [43]. Comparably, after 12 months of the follow-up, a secondary outcome analysis in a prospective, randomized clinical trial that included 114 infants with a unilateral congenital cataract, IOL, or contact lens replacement of the lens, observed a proportion of patients that developed strabismus [44]. However, the study found no significant difference in grating visual acuity between the IOL and contact lens groups [45]. The study observed patients up to five years old and found that both received similar visual outcomes [46].

In summary, the Infant Aphakia Treatment Study reported the frequency of different ocular conditions in infants after unilateral cataract surgery and the impact of different optical correction methods on visual outcomes. The study findings indicated that IOL implantation or contact lenses are suitable options for Infant Aphakia, with both alternatives having their risks and benefits.

Various techniques have been described for primary IOL implantation in pediatric cataracts, including posterior optic buttonholing, which involves implanting the IOL through the posterior capsulorhexis margin in cases of anterior capsulorhexis extension. This technique is feasible for children who experienced anterior capsulorhexis extension during pediatric cataract surgery, resulting in satisfactory surgical outcomes and few ocular complications [47].

In conclusion, primary IOL implantation may be a safe and effective option for treating pediatric cataracts, with outcomes comparable to contact lens correction or secondary IOL implantation in selected eyes. Various techniques and types of IOLs have been evaluated in different age groups and populations, and further studies are needed to determine the optimal approach for primary IOL implantation in pediatric cataracts.

3.2.4 Secondary IOL implantation

This technique involves the implantation of an IOL in the posterior chamber of the eye, either in the capsular bag or the ciliary sulcus. Several studies have evaluated the long-term visual outcomes and factors affecting visual results in children undergoing secondary IOL implantation following primary congenital cataract extraction. One study found that secondary IOL implantation resulted in good long-term visual outcomes in children with congenital cataracts [48]. Other studies have identified factors that may increase the risk of complications, such as secondary glaucoma, including high insertion of the iris and IOL implantation in the ciliary sulcus [49]. To reduce the risk of complications, researchers have modified the cataract extraction technique and secondary IOL implantation in pediatric aphakic eyes to achieve secondary in-the-bag IOL implantation [50]. However, the optimal size of the anterior capsulorhexis that should be used to obtain superior capsular outcomes for secondary IOL implantation in primary pediatric cataract surgery has yet to be reported [51].

Other studies have evaluated the safety and efficacy of different types of IOLs and techniques for secondary IOL implantation in pediatric cataracts. For example, posterior iris-fixated IOL implantation is an excellent alternative to other IOLs in pediatric traumatic cataracts without adequate capsular support [52]. Secondary PC-IOL implantation has also been effective in pediatric cataract eyes with microcornea and microphthalmos [53]. A recent multicenter, single-blinded, randomized controlled trial evaluated the safety and efficacy of in-the-bag versus sulcus fixation for secondary IOL implantation in pediatric cataract patients who have undergone primary cataract extraction. The study found that both techniques were safe and effective, with no significant differences in visual outcomes or complications between the two groups [54].

Various factors, such as the location of IOL implantation and the size of the anterior capsulorhexis, may affect the risk of complications. Further studies are needed to determine the optimal approach for secondary IOL implantation in pediatric cataracts.

3.2.5 IOL Power calculation in children

IOL power calculation is a crucial step in pediatric cataract surgery, as it determines the refractive outcome of the procedure. However, IOL power calculation in pediatric cataracts presents unique challenges due to the differences in ocular biometry and growth patterns compared to adults. Several studies have evaluated the accuracy of different IOL power calculation formulas and techniques in pediatric cataracts. New technologies for biometric measurements and keratometry in pediatric eyes have been developed to improve the accuracy of IOL power calculation.

Controversies in pediatric cataract management regarding the timing of surgery, IOL power calculation, and the choice of IOL still exist. Currently, the majority of surgeons prefer to use the SRK-T formula. One study compared the SRK-T, Holladay 1, Holladay 2, and Hoffer Q formulas and showed that SRK-T and Holladay 2 formulas have the lowest prediction error [55]. Young children develop myopia over time due to significant retina growth and corneal curvature changes. Therefore, many surgeons plan initial under-correction and provide refractive correction with contact lenses or spectacles [56, 57]. In cases of unilateral cataracts, fellow eye’s refractive status, dense amblyopia, the likelihood of poor compliance, socioeconomic considerations for contact lens use, and individual surgeon practice patterns, however, surgeons tend to make the eye less hypermetropic. Further research is needed to improve the accuracy of IOL power calculation in pediatric cataracts.

3.3 High myopia

Cataract surgery in patients with severe pathologic myopia and high axial length is well documented. These patients often have a higher risk of complications such as retinal detachment, cystoid macular edema, and posterior capsular opacification. Patients face severe pathologic myopia and high axial length challenges during cataract surgery. Multiple studies have been conducted to evaluate the outcomes of cataract surgery in these patients.

According to the Beaver Dam Eye Study and the Blue Mountains Eye Study, myopia and nuclear cataract are associated. The Blue Mountains Eye Study also found that moderate and high myopia, particularly when it begins before age 20, is related to the formation of posterior subcapsular cataracts [58]. The Singapore Malay Eye Study also found that patients with severe myopia have a three to fivefold increased risk of nuclear cataracts and a 30% increased risk of posterior subcapsular cataracts [59].

Patients with severe pathologic myopia and high axial length are at risk of zonulopathy, which can be secondary to many pathologies, including mature cataracts, prior ocular trauma, or prior ocular surgery. Venkateswaran and Henderson reported that loose zonules could make cataract surgery more challenging in these patients [60]. Capsular or iris hooks, capsular tension rings, and capsular tension segments (CTS) are all viable options for capsular bag support in zonulopathy. Numerous surgical techniques for inserting these devices can be tailored to the surgeon’s preference and the patient’s eye morphology.

One of the significant risks associated with cataract surgery in high myopia patients is the presence of chorioretinal degenerations, which can lead to poor visual outcomes [61]. High myopia patients also have a higher risk of complications such as retinal detachment, cystoid macular edema, and posterior capsular opacification. These complications can lead to permanent vision loss and require additional treatment. Suprachoroidal hemorrhage (SCH) is another risk associated with cataract surgery in high myopia patients. Bozkurt and Miller reported a case of a patient with high myopia who developed an SCH at the time of cataract surgery following three femtosecond laser docking attempts [62]. Other risks associated with cataract surgery in high myopia patients include preexisting maculopathy or posterior staphyloma and early postoperative BCVA recovery risk factors [63]. Highly myopic cataract eyes are also at risk of low vision, and risk factors for low vision include younger age, longer axial length, and preexisting maculopathy.

Another risk associated with cataract surgery in high myopia patients is the variability of axial length, anterior chamber depth, and lens thickness [64]. This variability can make selecting the appropriate IOL power challenging, leading to postoperative refractive errors. Postoperative refractive errors can cause poor visual outcomes and require additional treatment.

Finally, high myopia patients undergoing cataract surgery may be at a higher risk of IOL dislocation, especially in younger patients [65]. Immediate sequential bilateral cataract surgery is another technique that may be associated with increased risks in high myopia patients [66].

In summary, high myopia patients undergoing cataract surgery are at a higher risk of complications and poor visual outcomes than emmetropic eyes. Therefore, careful monitoring and precautions should be taken to minimize the risks and improve postoperative outcomes in high myopia patients undergoing cataract surgery.

3.3.1 IOL calculations in high myopia

One of the main difficulties in IOL calculation in high myopia cataract patients is the accurate measurement of axial length (AL) and corneal curvature. High myopia patients often have longer AL, leading to errors in IOL power calculation if not adequately accounted for [67]. High myopia patients may have corneal astigmatism, further complicating IOL calculation [67]. Therefore, accurate AL and corneal curvature measurement is crucial for successful IOL calculation in high myopia cataract patients.

Another difficulty in IOL calculation in high myopia cataract patients is selecting the appropriate IOL design and power. Aspheric IOLs with low negative or zero primary spherical aberration are recommended for cataract patients with high myopia [68]. Negative power IOLs have also been used successfully in patients with extremely high myopia [69]. However, selecting the appropriate IOL design and power can be challenging due to the variability of axial length, anterior chamber depth, and lens thickness in high myopia eyes.

In addition, preexisting conditions such as posterior staphyloma, maculopathy, and photorefractive keratectomy (PRK) can further complicate IOL calculation in high myopia cataract patients. For example, PRK can alter the corneal curvature and affect the accuracy of IOL calculation [70]. Therefore, carefully considering preexisting conditions is necessary for successful IOL calculation in high myopia cataract patients. Furthermore, intraoperative aberrometry can help improve IOL calculation accuracy in high myopia cataract patients. Intraoperative aberrometry measures the eye’s refractive error during surgery and can be used to adjust the IOL power accordingly [71]. However, intraoperative aberrometry may not be feasible in all cases and may add to the cost of the procedure.

In conclusion, IOL calculation in high myopia cataract patients can be challenging due to the unique anatomical and refractive characteristics of these eyes. Accurate measurement of axial length and corneal curvature, selection of the appropriate IOL design and power, consideration of preexisting conditions, and intraoperative aberrometry can contribute to successful IOL calculation in high myopia cataract patients. Therefore, careful planning and care of these factors are necessary for optimal outcomes in high myopia cataract surgery.

3.4 Hyperopia

The definition of a small eye is based on assessing ocular axial length (AL), anterior chamber depth (ACD), corneal diameter, and concomitant anatomical malformations. Simple microphthalmos is an eye with an AL shorter than the age-adjusted mean by two standard deviations, with a normal ACD, average scleral thickness, and without anatomical malformations. Nanophthalmos is a rare condition characterized by short AL with a shallow anterior chamber and thickened choroid and sclera but with no other anatomical malformations. Preoperative evaluation is essential for adequate surgical planning, predicting possible complications, and determining visual prognosis. Comparing the best corrected visual acuity, refractive status (hyperopia), and the degree of cataract bilaterally is essential for surgical planning.

Preoperative evaluation is an essential aspect of hypermetropia cataract patients due to the unique anatomical and refractive characteristics of these eyes. A critical aspect of preoperative evaluation in hypermetropia cataract patients is the measurement of endothelial cell density and corneal thickness. Stănilă et al. have shown that preexisting hypermetropia can modify the evolution of intraoperative and postoperative cataract surgery, leading to a loss of endothelial cells [72]. Therefore, measuring endothelial cell density and corneal thickness before and after surgery can help to identify any changes and ensure optimal outcomes. Another important aspect of preoperative evaluation in hypermetropia cataract patients is excluding certain conditions, such as glaucoma. Noted that patients with known open-angle or angle-closure glaucoma should be excluded from refractive cataract surgery, as both conditions are associated with specific refractive errors [73]. Therefore, thoroughly evaluating the patient’s ocular health must ensure that preexisting conditions are appropriately managed before cataract surgery. In addition, preoperative visual acuity is essential in hypermetropia cataract patients. A meta-analysis found that the outcome of cataract surgery, evaluated as objective and subjective visual improvement, was independent of preoperative visual acuity [74]. However, preoperative patient expectations should also be considered, as cataract surgery has become well-recognized as a refractive procedure, and patient satisfaction is related to preoperative expectations [75]. In conclusion, preoperative evaluation is essential for cataract surgery in hypermetropia patients. Measuring endothelial cell density and corneal thickness, excluding certain conditions, evaluating preoperative visual acuity and expectations, examining the macula, considering subjective experiences and objective functional visual outcomes, and using a risk-based approach to medical evaluation can all contribute to successful outcomes in hypermetropia cataract surgery.

However, selecting the appropriate IOL in eyes with short axial length (AL) can be challenging. Traditional IOL measurement formulas in eyes with short AL have reduced reliability. Several technical and surgical strategies have been proposed to optimize the visual outcome and decrease the rate of surgical complications. It is essential to understand their applications in these cases [76]. To compare the accuracy of a new IOL power formula (Kane formula) with existing formulas using IOLMaster, predominantly model 3, biometry, and optimized lens constants, it found that the Kane formula was more accurate than existing formulas. Röggla et al. found that the Haigis formula showed the highest percentage of cases with ≤0.5 D in eyes with a short AL [77]. In eyes with short AL, the Haigis formula helps calculate IOL power length and estimate the postoperative effective lens position (ELP) using preoperative anterior chamber depth and axial length [78].

3.4.1 Cataract surgery on small eyes

As expected, cataract surgery in small eyes has inherent anatomical challenges that must be addressed to prevent complications. The most commonly reported complications are posterior capsule rupture, zonular dehiscence, iris prolapse, corneal endothelial/Descemet membrane trauma, transient severe corneal edema, cystoid macular edema (CME), severe anterior uveitis, uveal effusion, angle-closure glaucoma, retinal detachment, and aqueous misdirection [79]. These eyes also have a higher risk of severe complications during cataract surgery, including uveal effusion or suprachoroidal hemorrhage. Pressure fluctuations during the surgery should be limited to reduce the risk of these complications. Reported that using a soft-shell technique with viscoelastic agents can help maintain a stable anterior chamber and reduce the risk of complications. Iris expansion devices are commonly used to visualize the surgical field better during cataract surgery. However, these devices can cause iris trauma and increase the risk of complications in small eyes. The iris hooks are better than most iris expansion devices for small eyes. Preoperative intravenous mannitol can dehydrate the vitreous, reducing its volume and the likelihood of significant posterior pressure during cataract surgery in small eyes. It reported that intravenous mannitol can be used safely and effectively in small eyes to reduce the risk of complications. Sometimes, a vitreous tap may be necessary to reduce the risk of complications during cataract surgery in small eyes. A vitreous tap can be performed using a trocar system, which may limit complications from pars plana vitrectomy in small eyes.

Postoperative refractive error is another concern in hypermetropia cataract patients. It reported that preoperative IOL power calculation using A-scan and biometry could help achieve the desired postoperative refractive status. In addition, postoperative management should include regular follow-up visits to monitor visual acuity and potential complications. In conclusion, hypermetropic cataract patients may experience difficulties during and after surgery, including posterior capsular thickening, uveal effusion or suprachoroidal hemorrhage, inflammation, and refractive error. To manage these complications, a personalized approach should be used, taking into account the individual characteristics of each patient. Regular follow-up visits are also essential to monitor visual acuity and potential complications.

3.5 Astigmatism

Astigmatism is a standard refractive error that can cause blurred vision and decreased visual acuity. Cataract surgery allows one to correct preexisting astigmatism, improving visual outcomes, and reducing the need for glasses or contact lenses. There are several techniques for correcting preexisting astigmatism during cataract surgery. One option is corneal incisions, such as limbal relaxing incisions (LRI) or femtosecond laser-assisted astigmatic keratotomy (FSAK). These incisions are made on the steep axis of the cornea to reduce astigmatism. Reported that FSAK and toric intraocular lens (IOL) implantation were both effective in correcting astigmatism in cataract surgery patients with corneal astigmatism ranging between 0.5 D and 4.5 D. Another option for correcting astigmatism during cataract surgery is toric IOL implantation. Toric IOLs have a specific orientation that can correct astigmatism. Reported that toric IOL implantation has higher predictability than incisional methods because it is independent of corneal wound healing. The choice of technique for astigmatism correction depends on several factors, including the severity and axis of astigmatism, the patient’s age and comorbidities, and the surgeon’s experience and preference. They are reported that astigmatism correction during or even after cataract surgery is a safe and effective method to improve visual outcomes. It is important to note that not all astigmatism needs to be corrected. Correcting pre-surgical astigmatism should be considered separately depending on whether a patient has residual accommodation and the type of refractive surgery under consideration. In addition, accurate alignment of toric IOLs is critical for successful astigmatism correction. They reported that a three-random-point marking method using the iTrace Aberrometer could improve the accuracy of toric IOL alignment (Figure 1).

Figure 1.

Toric IOL implantation after pterygium recurrence after four different excision surgeries.

In conclusion, astigmatism correction during cataract surgery can improve visual outcomes and reduce the need for glasses or contact lenses. The choice of technique depends on several factors, and accurate alignment of toric IOLs is critical for successful astigmatism correction. It is essential to consider each patient’s characteristics and preferences when deciding on the best approach for astigmatism correction during cataract surgery [80].

3.6 Post-refractive surgery eyes

Corneal refractive surgery, such as LASIK, PRK, or RK, can alter the corneal curvature, making IOL power calculation challenging in cataract surgery patients. Accurate IOL power calculation is crucial for achieving the desired refractive outcome and reducing the need for additional corrective procedures. One of the main challenges in IOL power calculation after corneal refractive surgery is the altered ratio between the anterior and posterior corneal surface, which makes the keratometric index invalid. Reported that the effective lens position is erroneously predicted if such a prediction is based on the post-refractive surgery corneal curvature. Therefore, alternative methods have been proposed for measuring corneal power, such as total corneal power measurement using high-speed optical coherence tomography. Another challenge is the measurement of corneal curvature radius outside the optical zone, which can lead to an underestimation of the surgically induced refractive change. Reported that a new method, the Iida-Shimizu-Shoji (ISS) method, combines the anterior-posterior ratio of the corneal radius of curvature after LASIK to improve IOL power calculation accuracy. Several IOL power calculation formulas have been developed to address the challenges of IOL power calculation after corneal refractive surgery. The Holladay 2 formula and the American Society of Cataract and Refractive Surgery (ASCRS) Post-Refractive IOL Calculator have commonly used formulas that have been shown to provide accurate results in some studies (Figure 2). However, other studies have reported that IOL power calculation for eyes that have previously undergone refractive surgery is less accurate than that for virgin eyes.

Figure 2.

Post-refractive surgery IOL power calculation.

The Zhang & Zheng (ZZ) formula is a newly-developed intraocular lens (IOL) calculation formula that has shown promising results in clinical accuracy analysis for post-corneal refractive surgery eyes [81]. They compared the precision of the ZZ, Haigis-L, Shammas, Barrett True-K (no history), and ray tracing (C.S.O Sirius) IOL power assessments in eyes that had undergone corneal refractive surgery. No data from before the patient’s refractive procedure was used in the analysis. Compared to ray tracing, the ZZ IOL formula produced a smaller arithmetic IOL prediction error (PE; P = 0.04), whereas all the other formulas produced similar results (P > 0.05). When compared to Shammas (P 0.01), Haigis-L (P = 0.02), Barrett true K (P = 0.03), and ray tracing (P 0.01), ZZ IOL produced a lower absolute IOL PE. They theorized that the ZZ IOL formula could provide better results for calculating the power of an IOL in eyes that have undergone corneal refractive surgery but do not have any preexisting refractive data.

In addition, the type of multifocal IOL used can also affect the accuracy of IOL power calculation after corneal refractive surgery. It was reported that factors such as the type of multifocal IOL, the degree of preoperative refractive error, and the amount of corneal astigmatism could affect the prediction error after cataract surgery with implantation of various multifocal IOLs in patients with previous refractive laser surgery. In conclusion, IOL power calculation after corneal refractive surgery is challenging due to the altered corneal curvature and the invalidity of the keratometric index. Alternative methods for measuring corneal power and new IOL power calculation formulas have been developed to improve accuracy. Considering each patient’s characteristics and the type of IOL used when calculating IOL power after corneal refractive surgery is essential [82].

3.7 Eyes with corneal ectasias

Cataract surgery is a standard procedure that can improve visual acuity in cataract patients. However, cataract surgery in patients with keratoconus poses particular challenges due to the altered corneal shape and thickness. Preoperative workup for cataract surgery in keratoconus patients is crucial to ensure accurate IOL power calculation and identify potential complications. Ton et al. investigated the visual and refractive outcomes in patients with keratoconus having cataract surgery with a toric IOL. They compared the IOL power calculation accuracy of conventional formulas and keratoconus-specific formulas [83]. They found that the Barrett Universal II, Holladay 2, and SRK/T were the most accurate IOL power calculation formulas in patients with keratoconus undergoing cataract surgery. Additionally, a study reported that a planned combination of primary and piggyback IOLs implantation in keratoconus at the time of cataract surgery could correct complex refractive defects associated with the disease [84]. Toric IOL implantation is a viable option for correcting astigmatism in keratoconus patients undergoing cataract surgery. Allard et al. l. reported a case of successful toric IOL implantation in a patient with keratoconus and previous penetrating keratoplasty (PKP) in one eye [85]. Additionally, some reports have been published about a two-stage surgical intervention in patients with keratoconus and cataracts to correct ametropia and prevent disease progression effectively. They initially implanted Ferrara 150–350 μm intrastromal corneal ring segments (ICRS). In Stage 2, to rectify the remaining refractive error, the hazy lens was removed and replaced with a toric posterior chamber intraocular lens (TIOL) in five to seven months [86]. The conclusion was that two-stage surgery could correct ametropia in individuals with keratoconus and cataracts.

It is important to note that cataract surgery in keratoconus patients may carry a risk of corneal ectasia following surgery. A rapidly progressive corneal ectasia was reported in a patient with keratoconus following uncomplicated phacoemulsification surgery for cataract removal [87]. Therefore, careful preoperative evaluation and postoperative monitoring are essential to minimize the risk of complications.

In conclusion, cataract surgery in keratoconus patients requires careful preoperative evaluation and planning to ensure accurate IOL power calculation and identify potential complications. Toric IOL implantation and a two-stage surgical intervention may be necessary to correct ametropia and astigmatism effectively. It is essential to consider each patient’s characteristics and the potential risks and benefits when deciding on the best approach for cataract surgery in keratoconus patients.

3.8 Eyes with sequential/simultaneous vitreoretinal surgery

Cataract surgery with sequential or simultaneous vitreoretinal surgery is a complex procedure that requires careful planning and execution. The literature suggests that combined surgery is recommended for selected patients with simultaneous vitreoretinal pathological changes and cataracts [88, 89, 90]. However, the optimal timing and approach for combined surgery remain controversial. A comparative analysis of two-stage or simultaneous vitreoretinal surgery results with phacoemulsification in patients with advanced proliferative diabetic retinopathy and complicated incipient cataract showed that both approaches could be practical [91, 92]. The choice of approach may depend on the severity of the retinopathy and the degree of cataract. Patients with a history of trauma, pseudoexfoliation syndrome, degenerative myopia, uveitis, retinitis pigmentosa, and previous vitreoretinal surgery are at increased risk for complications during combined surgery [93]. Therefore, careful preoperative evaluation and planning are essential to minimize the risk of complications. Most British and Eire Association of Vitreoretinal Surgeons members would use the opposite eye biometry in a patient with a cataract and macula-off rhegmatogenous retinal detachment undergoing combined phaco-vitrectomy surgery. In contrast, most Punjab surgeons would leave the patient aphakic [94]. This highlights the importance of considering each patient’s characteristics and the potential risks and benefits when deciding on the best approach for combined surgery. The level of proinflammatory cytokines in tear of patients with advanced proliferative diabetic retinopathy and complicated primary cataract after phacoemulsification surgery and IOL implantation with vitreoretinal surgery accomplished at once in comparison with vitreoretinal surgery only has been studied [95]. The results showed that combined surgery did not significantly increase the level of proinflammatory cytokines in tears. Clear corneal phacoemulsification combined with 25-gauge transconjunctival sutureless vitrectomy and standard 20-gauge vitrectomy for patients with cataracts and vitreoretinal diseases has been compared [96]. The study showed that both approaches were safe and effective, with no significant differences in visual outcomes or complication rates.

In conclusion, cataract surgery with sequential or simultaneous vitreoretinal surgery is a complex procedure that requires careful planning and execution. Combined surgery is recommended for selected patients with simultaneous vitreoretinal pathological changes and cataracts. However, the optimal timing and approach for combined surgery remain controversial, and careful preoperative evaluation and planning are essential to minimize the risk of complications.

3.9 Cataract surgery after vitrectomy

Cataract surgery after vitrectomy or phaco-vitrectomy is standard due to the high incidence of cataract formation after vitreoretinal surgery. According to Lahey et al., 75% of patients will develop visually significant cataracts within one year and 95% within two years after vitrectomy [97]. Therefore, cataract surgery is often necessary to restore visual function in these patients. Factors influencing refractive outcomes after combined phacoemulsification and pars plana vitrectomy have been studied by Jeoung et al. [98]. They found that the presence of silicone oil tamponade, preoperative axial length, and intraoperative complications were significant factors affecting refractive outcomes. In a study by Do et al., the effectiveness and safety of surgery for post-vitrectomy cataracts were evaluated [99]. The study aimed to evaluate visual acuity, quality of life, and other outcomes. The results showed that surgery for post-vitrectomy cataracts was effective and safe in improving visual acuity and quality of life. Fernandez has also studied cataract formation following pars plana vitrectomy in the pediatric population [100]. The study aimed to analyze post-vitrectomy cataract formation in the pediatric population and the perioperative factors affecting cataract development in these patients. The conclusion was that pediatric eye care providers should know the significant risk of cataract formation following phakic PPV.

In conclusion, cataract surgery after vitrectomy is expected due to the high incidence of cataract formation after vitreoretinal surgery. Factors such as silicone oil tamponade, preoperative axial length, and intraoperative complications can affect refractive outcomes. Surgery for post-vitrectomy cataracts effectively and safely improves visual acuity and quality of life.

4. Conclusions

In summary, the evolution of biometric intraocular lens calculation formulas and IOL selection in challenging cases are advancing rapidly. The development of optical biometers, surgical techniques, types of IOLs, and accurate formulas for their adequate selection has significantly improved cataract surgery outcomes over the years. The current trend in IOL selection aims to achieve optimal refractive outcomes by utilizing accurate corneal and biometric measurements, advanced formulas, and improved IOL designs. With these advancements, potential complications arise, that include incorrect lens power, optical aberrations, and IOL dislocation, which can lead to explantation. However, with the use of advanced techniques, patient selection, and appropriate biometric parameters, cataract surgery outcomes continue to improve, providing greater visual outcomes and a higher quality of life for patients.

Intraocular lens selection in challenging cases requires careful consideration of various factors. The choice of IOL will depend on the patient’s individual anatomy, pathology, and visual needs. Patients with irregular corneas, such as keratoconus, require special consideration to achieve optimal visual outcomes. However, the implantation of multifocal IOLs in these patients should be avoided. Various ophthalmic pathologies and systemic comorbidities can also exacerbate complications related to IOL opacification. To achieve the best results in IOL selection, clinicians should perform a thorough preoperative evaluation of the patient and appropriately select the IOL. Different IOLs and IOL power formulas have been developed, making choosing the most appropriate lens for the patient challenging. Further research is needed to refine IOL selection in challenging cases.