Optical coherence tomography (OCT), especially anterior segment OCT (AS-OCT), plays an important role in ophthalmology. With the technology evolving from time-domain to spectral-domain, more and more detailed ocular information has become available. Anterior segment OCT provides particularly useful information for cataract surgeons. This chapter focuses mainly on AS-OCT evaluation of eyes before and after cataract surgery. Four aspects including: (1) anterior lens capsule and lens epithelium evaluation using spectral-domain OCT (SD-OCT); (2) investigation of clear corneal incision in femtosecond laser assisted cataract surgery using SD-OCT; (3) capsular block syndrome evaluation before and after treatment using SD-OCT; (4) IOL power calculation in post-myopic excimer laser eyes using SD-OCT, will be discussed in this chapter.
- anterior lens capsule
- clear corneal incision
- capsular block syndrome
- Fuchs heterochromic cyclitis
- intraocular lens
Since the introduction of ophthalmic optical coherence tomography (OCT) in 1991, it has been widely used in ophthalmology over the years . The OCT interferometer detects and measures the echo delay time of light, which is projected from a super-luminescent diode and then reflected from different ocular structures. Compared with the 100-μm image resolution of ultrasonography, OCT can, in a non-invasive, non-contact procedure, provide detailed cross-sectional images of the eye on a 10-μm scale. Compared to conventional time-domain anterior segment OCT (AS-OCT) system (1300 nm), a shorter-wavelength infrared light beam (820 nm) is used for the spectral-domain AS-OCT system. This chapter will mainly focus on anterior segment time-domain and spectral-domain OCT applications before and after cataract surgery. Four aspects will be discussed in this chapter:
Anterior lens capsule and lens epithelium evaluation in senile cataract and Fuchs’ heterochromic cyclitis using spectral-domain anterior segment OCT (SD-OCT);
Investigation of clear corneal incision in manual phacoemulsification and femtosecond laser assisted cataract surgery using SD-OCT;
Capsular block syndrome evaluation before and after treatment using SD-OCT;
IOL power calculation (true net power measurement) in post-myopic excimer laser eyes using SD-OCT.
2. Anterior lens capsule complex measurement using SD-OCT
The lens, as a transparent structure, is a very important refractive element of the human eye. It consists of three major components: capsule, epithelium, and lens substance . The lens epithelial cells and fibers keep the balance of molecules moving into and out of the lens. Previous histological studies have demonstrated that the lens capsule thickness varies by location and that the posterior capsule is the thinnest. The lens epithelium, a single layer of cuboidal cells beneath the anterior capsule, extends to the equatorial lens bow. The lens substance consists of densely packed cells with little extracellular space. With the development of time-domain OCT, it became possible to image the entire anterior chamber and lens in vivo (Figure 1) . Due to the limited resolution (around 15-μm), it was not possible to measure the thickness of the lens capsule or the epithelium.
A previous study manually measured and constructed an anterior lens capsule thickness map across the entire field of view using a femtosecond laser OCT system, which has a central wave length of 780 nm with an axial resolution of 2.3 μm . In a study conducted by Szkulmowski et al., it was reported that the anterior lens capsule and epithelium could be clearly imaged using a SD-OCT system with a central wavelength of 820 nm and axial resolution of 4.2 μm with the help of the speckle contrast reduction technique . The commercial Avanti RTVue XR spectral-domain OCT system provides a high-resolution anterior segment scanning protocol with a scan length of 8 mm and an axial resolution of 5 μm. It can show a substantially clearer delineation after the two highly reflective layers, the anterior lens capsule and the subcapsular epithelium, are combined (Figure 2).
We defined the combination of anterior lens capsule and the subcapsular epithelium as the anterior lens capsular complex (ALCC) in a previous study . One hundred thirty-four normal eyes (age range: 5–86 years) were investigated using the Avanti RTVue XR OCT system. The results indicated that the manual measurement of ALCC was both reproducible and reliable. The mean thickness of the central ALCC was approximately 33 μm, which is approximately 15 μm thicker than the anterior lens capsule values calculated using time-domain OCT and other imaging modalities [4, 7]. Therefore, we hypothesized that the subcapsular epithelium was approximately 15 μm thick in vivo. Moreover, a positive correlation between age and ALCC was noted in our study. A 10-year increase in age resulted in a 0.74–1.4 μm increase in the ALCC thickness. From a certain point of view, these findings were consistent with previous findings regarding the positive correlation between age and lens capsule or total lens thickness [8, 9].
With the help of anterior segment SD-OCT, researchers also can obtain more detailed useful information regarding accommodation, presbyopia, Fuchs’ heterochromic iridocyclitis (Figure 3), posterior capsule opacification, and pseudoexfoliation syndrome [10, 11, 12, 13]. Moreover, with the OCT technology development, the respective measurement of anterior capsule, posterior capsule and epithelium layer thickness, even plotting corresponding topography in specific regions will be available in the future.
In summary, anterior segment SD-OCT can be used to visualize anterior and posterior lens capsule, lens epithelium, keratic precipitates, iris stroma atrophy. The use of anterior segment SD-OCT will enable us to better investigate changes in lens structure of patients with Fuchs’ heterochromic iridocyclitis or other lens related conditions.
3. Investigation of clear corneal incision using SD-OCT
In most industrialized countries, cataract surgery accounts for the largest proportion of surgical interventions in ophthalmology. The advent of minimal incision, phacoemulsification (introduced by Charles Kelman in the 1970s), and foldable intraocular lens, has made cataract surgery safer and more efficient . Moreover, modern cataract surgery combined with new generation intraocular lens types, which can be designated refractive surgery, aims to provide optimal visual quality and render the visual function better . As a significant part of the cataract surgery, the constructional architecture of proper clear corneal incisions (CCIs) is vital for a perfect outcome of phacoemulsification. Faulty CCI construction can lead to complications, such as wound leak, Descemet’s membrane detachment, and excessive surgically-induced astigmatism, and is a situation every cataract surgeon attempts to avoid. However, it is not easy to predictably construct perfect manual single-plane, two-plane, or three-plane clear corneal tunnels using steel or diamond blades . With the higher expectations of patients from modern cataract and refractive surgery and the advent of trifocal and accommodating IOLs, femtosecond laser was introduced into cataract surgery by Zoltan Nagy in 2009 . Previous studies demonstrated that a femtosecond laser-assisted cataract surgery technique could provide repeatable, predictable clear corneal incisions, perfect capsulotomies, and safe nuclear fragmentation [18, 19]. Therefore, CCIs created using femtosecond laser have the advantage of predictable configuration and dimensions.
Clinical manual CCI parameters (incision location, angle of incision, incision length) and features (posterior wound gape, Descemet’s membrane detachment, posterior wound retraction, loss of coaptation along the CCI tunnel) in the early postoperative period (up to 1 month) of standard cataract surgery have been reported [20, 21, 22, 23]. Based on the above-mentioned studies, we evaluated the quantitative CCI parameters and features in subjects who underwent femtosecond laser-assisted cataract surgery . Except for no sign of loss of coaptation along the CCI tunnel, all the other three CCI features were found in femtosecond laser CCI (Figure 4).
As expected, the femtosecond laser lens fragmentation ensures that the phacoemulsification energy (Femto group: 16 ± 13%; manual group: 20 ± 5%) and effective phacoemulsification time (Femto group: 17 ± 9 s; manual group: 32 ± 13 s) were dramatically lower in the Femto group than in the conventional manual group. Moreover, the femtosecond laser group had a lower incidence of Descemet’s membrane detachment and posterior wound gape at each follow-up time-point. However, a higher incidence of posterior wound retraction was found in the femtosecond laser group.
Descemet’s membrane detachment may potentially hinder the local endothelium pump mechanism and affect corneal wound healing. The routine procedure for CCI by femtosecond laser is from the anterior chamber (inner cornea) to the epithelium (outer cornea), which makes the Descemet’s membrane detachment an unusual complication for this group. However, phacoemulsification probe manipulation through the CCI, the blunt dissection of the tunnel, and incision hydration probably contributed to the occurrence of Descemet’s membrane detachment in the femtosecond group. For the difference in posterior wound gape incidence, we are inclined to implicate the difference in tunnel incision geometry between the two groups. Compared to 1-plane manual CCI, we mainly used a 2-plane CCI in femtosecond group. As previous studies have mentioned, a 2-plane CCI with a partial lamellar cut positioned parallel to the collagen lamellae may improve the shearing force effects of the stromal collagen lamellae across the entire depth of the cornea [25, 26, 27]. Posterior wound retraction, defined as an abrupt step-off or recession of the central edge of the posterior wound surface, was not observed in manual group in this study. This may be attributed to the relatively high incidence of posterior wound gape, in which the posterior wound margins were still separated. A high incidence of posterior wound retraction in the femtosecond group may indicate remodeling of the CCI resulting from endothelial cell necrosis, molecular dissociation, and biomechanical and thermal changes from the femtosecond laser . A previous study, using transmission electron microscopy confirmed the difference between femtosecond laser corneal flap formation (necrotic keratocytes) and microkeratome corneal flap formation (keratocyte apoptosis), which may potentially explain the different incidence of posterior wound retraction in this study . Moreover, the above-mentioned three CCI features may cause changes to posterior corneal curvature, corneal astigmatism, and total corneal power; these should be evaluated in future studies.
Anterior segment SD-OCT can highlight CCIs findings that are not as obvious by UBM, slit lamp. The ability to detect detailed postoperative corneal incision changes of standard or femtosecond laser-assisted cataract surgery is clinically important as it allows for the evaluation and comparison of surgical induced astigmatism and corneal remodeling by different surgical technology. In the future, clear corneal incision length, inner and outer corneal incision thickness, even the lens capsulotomy margin may be evaluated in traditional and femtosecond laser-assisted cataract surgery using OCT.
4. Capsular block syndrome evaluation before and after treatment using SD-OCT
Capsular block syndrome (CBS), as an uncommon complication of phacoemulsification, is characterized by the accumulation of liquid between the IOL and posterior capsule . Davison and Holtz first described this syndrome in 1990 and 1992, respectively [31, 32]. In 1993, Masket first used the term CBS to refer to this condition . Due to a continuous capsulorhexis and adhesion of anterior capsule to the IOL, the exchange of aqueous between the anterior chamber and inner capsular bag gets blocked and fluid is retained behind the IOL . Generally, CBS is related to IOL implantation in the capsular bag, but it has also be reported in cases with IOL implantation in the sulcus [35, 36]. Various manifestations of CBS, such as decreased visual acuity, ocular hypertension, closed angle glaucoma, and posterior capsule rupture have been reported. The two main reasons for vision loss include: (1) the effective position of IOL moves forward (myopic shift); (2) a white liquid accumulates behind the IOL (lacteocrumenasia) . Based on time to development, CBS can be divided into three types: intraoperative capsular block, early postoperative CBS, and late postoperative CBS [37, 38]. Most intraoperative CBS occurs due to hydrodissection maneuvers in posterior polar or white cataract surgeries. Due to the block of capsulorhexis borders, liquid accumulates between the posterior nucleus and the posterior capsule. Increasing pressure in the capsular bag may cause posterior capsular rupture and even lens luxation into the vitreous . Early postoperative CBS occurs 1–15 days after cataract surgery. The two main causes are as follows: (1) viscoelastic accumulation behind the IOL; (2) osmotic gradient behind the IOL induced by viscoelastic materials or trapped lens fragments [38, 39]. On average, late postoperative CBS occurs approximately 3.8 years after cataract surgery and deposition of a white fluid (proliferation and pseudo-metaplasia of the lens epithelium) is found between the IOL and posterior capsular bag [34, 40].
A simple, clinical examination can diagnose most CBS cases. With the development of ophthalmic imaging technology t, a definitive diagnosis can be achieved using Scheimpflug-based photography, AS-OCT, or anterior segment UBM. Compared with Scheimpflug-based photography, spectral domain AS-OCT fails to provide complete anterior chamber information but it can provide more details about the IOL and any fluid between the IOL and posterior capsular bag (Figure 5).
In some CBS cases with relatively transparent fluid behind the IOL and a large distance between IOL and posterior capsule, UBM, Scheimpflug-based photography or AS-OCT can just demonstrate a partial cross-sectional image (Figure 6).
Compared to surgical management, we may also choose anterior or posterior capsulotomy with a Nd:YAG laser to treat CBS, especially for early postoperative and non-cellular late postoperative cases . Due to the relatively lower success rate and higher recurrence rate for anterior capsulotomy, some doctors may choose posterior capsulotomy . In some cases, posterior capsulotomy may cause unexpected irregular posterior capsule rupture, which may cause the IOL to drop into the vitreous cavity (Figure 7). Therefore, surgical management is a better choice for late CBS, especially incases with dense white liquid, which makes aiming the Nd:YAG laser accurately almost impossible, and the capsulotomy may cause IOP increase or vitreous inflammation . As an important imaging technology, with the imaging depth enhanced, more and more details about CBS and other posterior capsular related disease can be investigated using OCT.
While varying in cause and course, these CBS cases all consistently demonstrated the ability of anterior segment SD-OCT to clearly diagnose and delineate with impressive detail the onset and treatment follow-up. Anterior segment SD-OCT offers extraordinary anatomic details, the more accurate diagnosis, and the potential of quantitative measurement to improve the assessment of treatment efficacy.
5. IOL power calculation in post-myopic excimer laser eyes using SD-OCT
It is well known that correct IOL power calculation in patients undergoing cataract surgery depends mainly on the accurate measurement of corneal power, axial length, corneal keratometry, and the effective lens position after surgery [44, 45]. Due to index of refraction error, instrument error, and IOL formula error, precise prediction of IOL power is always a big challenge for cataract surgery in patients with previous corneal refractive surgery (PRK, LASIK, RK) [46, 47, 48, 49, 50, 51]. For post corneal refractive surgery cataract patients, both, patient expectations of spectacle independence and reduction in the accuracy of conventional IOL power calculation formulas due to previous corneal surgery should be considered. To deal with these issues, many innovative post-corneal refractive surgery IOL power calculation formulas have been developed [52, 53, 54, 55, 56, 57, 58]. Compared to traditional keratometry, RTVue spectral-domain AS-OCT can measure anterior and posterior corneal curvature and then calculate the net corneal power, which in conjunction with IOLMaster biometry data (AL, ACD) provides the IOL power . Previous studies have demonstrated that Fourier-domain AS-OCT can provide highly repeatable corneal power measurement and spectral-domain OCT-based IOL formulas have also provided promising results in post-refractive surgery IOL calculation in eyes undergoing cataract surgery; this is especially meaningful for patients for whom prior data are not available [59, 60, 61]. To avoid significant IOL power fluctuation between different formulas, surgeons can also use ASCRS online post-refractive IOL calculator (link:
Anterior segment SD-OCT can noncontactly provide true net corneal power, which is essential for IOL power calculation after excimer laser corneal surgery. The SD-OCT-based IOL formulas enable to provide promising results in post-refractive surgery IOL calculation, especially meaningful for eyes whose prior data unavailable.
In conclusion, OCT, as a noninvasive and high-resolution imaging technology, can provide the ophthalmologist with clinically useful findings not only about the retina, but also for the cornea, lens, and anterior chamber. Anterior segment high-speed SD-OCT system offers efficient information about lens capsule evaluation, clear corneal incision investigation, capsular block syndrome management, and post-refractive surgery IOL power calculation. In the future, anterior segment SD-OCT may be a useful tool for detecting and monitoring more ocular disease progression and treatment response in clinic.
This work was supported by the National Natural Science Foundation of China under Grant No. 81501544.
The authors have neither a conflict of interest nor commercial interest in the equipment mentioned in this paper.