Purpose
To investigate the changes in higher-order aberrations (HOAs) induced by implantation of Implantable Collamer Lenses (STAAR Surgical) and to explain them in relation to the surgical incision and optical properties.
Design
Prospective, observational case study.
Methods
This study included 56 eyes undergoing Implantable Collamer Lens insertion for myopic correction. The corneal incision size was determined according to the amount of astigmatism. HOAs were measured before surgery and 3 months after surgery in 25 eyes having small superior incision (<3.2 mm) surgery and in 31 eyes with large superior incision (3.2 to 4.5 mm) surgery. Changes in spherical aberration, coma, trefoil, and total HOAs (third to sixth order) were analyzed. Laboratory measurements of aberration profiles of Implantable Collamer Lenses with different optical powers were performed to validate clinical measurements.
Results
In the small-incision group, trefoil (Z 3 −3 ) and spherical aberration changed significantly ( P = .004). In the large-incision group, in addition to trefoil and spherical aberration, total HOA changed significantly (mean change, 0.13 ± 0.17; P = .001). Significant correlations were observed among the incision size, the astigmatism induced, and the trefoil induced. Induced trefoil showed a predominant pattern at the orientation of the incision meridian. Optical measurement of aberrations of the Implantable Collamer Lenses confirmed the postoperative negative spherical aberration.
Conclusions
HOA changes after Implantable Collamer Lens insertion were increased trefoil and induced negative spherical aberration. These changes may be explained by the effect of the corneal incision and the negative spherical aberration in the Implantable Collamer Lens, respectively.
Wavefront-guided or wavefront-optimized laser vision corrections recently were introduced in an effort to suppress the increase in higher-order aberrations (HOAs). Although wavefront-driven laser ablation technology offers good results, many factors such as the coupling effect of preoperative HOAs on postoperative lower-order aberrations, cyclotorsional misalignment, and pupil centroid shift should be taken into account. It seems that postoperative aberrations cannot be predicted accurately in practice.
In contrast to laser refractive correction, phakic intraocular lens insertion, which preserves the prolate shape of the cornea, has relatively little risk of HOA induction in the cornea. It may induce HOA by the innate optical properties of the phakic intraocular lens or as a consequence of the surgical procedure. To the best of our knowledge, only a few studies have reported on changes in HOAs after posterior chamber phakic intraocular lens insertion, and the sources of HOA changes have not been fully elucidated. This study aimed to evaluate the changes in HOAs induced by insertion of the Implantable Collamer Lens (ICL; STAAR Surgical, Monrovia, California, USA), a posterior chamber phakic intraocular lens, and to correlate them with regard to the surgical procedure and optical properties.
In our preliminary study, we noted that the induction of trefoil and the reduction of spherical aberration were consistent findings after ICL insertion. We hypothesized that the corneal incision and optical properties of intraocular lenses have important roles in these changes. Thus, to study the effect of the corneal incision on the change in wavefront aberrations, we compared the HOA changes between the small-incision group and the large-incision group. Furthermore, we examined the HOAs induced by the phakic intraocular lens optic on the optical bench system to elucidate the optic effect.
Methods
Patients
This prospective, observational comparison study included 56 eyes from 30 patients who underwent ICL (ICM version 4) insertion performed by a single surgeon (H.Y.) for the correction of myopia. Subjects were assigned to two groups depending on the amount of preoperative corneal astigmatism. Subjects with less than 1.0 diopter (D) of astigmatism underwent standard incision surgery and those with more than 1.0 D of astigmatism underwent large-incision surgery. As such, 25 eyes were assigned to the small-incision group (<3.2 mm) and 31 eyes were assigned to the large-incision group (3.2 to 4.5 mm).
The width of the corneal incision in the large-incision group was determined by the surgeon’s own nomogram with regard to the amount of corneal astigmatism. Inclusion criteria for surgery were as follows: stable refraction during the previous 2 years; anterior chamber depth of 3.0 mm or more; normal pupil and iris configuration; endothelial cell density of 2500 cells/mm 2 or more; no history of glaucoma; and no history of preexisting corneal, lenticular, or retinal pathologic features likely to affect vision.
Surgical Techniques
Two peripheral laser iridotomies were performed one week before surgery. All surgeries were performed by a single surgeon (H.Y.) under topical anesthesia. The phakic intraocular lens power was calculated using software provided by STAAR Surgical AG. The main incision was made along the steep meridian of corneal astigmatism. As a result, all incisions were made in the superior position at angles ranging from 70 to 110 degrees. The width of the incision (mean, 3.42 ± 0.49 mm; range, 3.00 to 4.50 mm) was adjusted according to the amount of preoperative astigmatism (mean, −1.07 ± 0.70 D; range, 0 to −2.75 D). Briefly, the width of the incision was enlarged by 0.25 mm from 3.0 mm with an increase 0.25 D in the astigmatism from 1.0 D; 3.0 mm for less than 1.0 D, 3.5 mm for 1.5 D, and 4.0 mm for 2.0 D. A personalized, surgically induced astigmatism for a 3.0-mm clear corneal incision was 0.25 D. Thus, with this nomogram, we intended to have a postoperative astigmatism that remained within 1 D. For 7 eyes with a more than 4.0-mm incision, 1 vertical suture was performed on the wound, and was then removed within 1 week after surgery. All surgeries were uneventful and all patients received routine postoperative medications without complications.
Clinical Evaluation
Preoperative examinations included uncorrected visual acuity, best spectacle-corrected visual acuity (BSCVA), subjective and objective refraction, topography, pupil size, and wavefront HOAs. Wavefront HOA measurements were obtained and recorded using a Hartmann-Shack aberrometer (WASCA; Carl Zeiss Meditec, Jena, Germany) with 6-mm pupils after instillation of Mydrin-P (Santen, Osaka, Japan). All Zernike coefficients were expressed consistently with the Optical Society of America’s standard notation. Follow-up was at 1 day, 7 days, 1 month, and 3 months after surgery. Uncorrected visual acuity, BSCVA, manifested refraction, and slit-lamp examination results were evaluated routinely at all visits, and topographic astigmatism and HOAs were recorded at 3 months.
Comparison with preoperative values was made for refractive astigmatism, root mean square (RMS) of total HOA (third to sixth order), trefoil (Z 3 −3 , Z 3 3 ), coma (Z 3 −1 , Z 3 1 ), and spherical aberration (Z 4 0 ). Differences in each Zernike term should not be interpreted as simply reductions in negative signed values or inductions in positive signed values. Instead, a shift to a more negative or more positive aberration is implied in each case. For these reasons, RMS values also were used in comparisons of trefoil and coma when computing aberrations.
To investigate the effect of corneal incision on astigmatism and HOA changes, we obtained both the magnitude and orientation of the induced astigmatism and trefoil. Thus, manifest refractions in conventional notation were converted to Cartesian values using the following formulas, and calculated differences between post- and preoperative values were reconverted to the polar notation:
X = Cylinder × cos ( 2 × Axis ) Y = Cylinder × sin ( 2 × Axis ) Δ X = X post − X pre , Δ Y = Y post − Y pre
Induced astigmatism = Δ X 2 + Δ Y 2 A n g l e = 1 / 2 arctan ( Δ Y / Δ X ) , i f Δ X a n d Δ Y > 0 t h e n A x i s = A n g l e i f Δ X < 0 , t h e n A x i s = A n g l e + 90 i f Δ X > 0 a n d Δ Y < 0 , t h e n A x i s = A n g l e + 180
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