The ultimate goal of modern-day refractive cataract surgery is to improve best corrected vision as well as maximize uncorrected distance, intermediate and near vision for each patient. However, there is no intraocular lens (IOL) on the market that perfectly mimics the natural non-presbyopic human lens. Standard monofocal IOLs, targeted for distance, require spectacle correction for intermediate and near tasks.1,2
The goal of multifocal IOL implantation in cataract surgery is for patients to get good vision at a variety of distances without the use of spectacles by splitting the incoming light into distance and near foci. Unfortunately, because of the imperfect optics of these lenses, many multifocal IOL patients suffer from certain postoperative visual disturbances such as reduced contrast sensitivity, halos around lights, night glare and “waxy” vision.3,4
Accommodating IOLs theoretically allow for improved near vision through ciliary muscle contraction. The incorporates hinges of accommodating IOLs allow the lens optic to move forward with ciliary muscle contraction.5 There is no diffractive or refractive optic design for accommodating monofocal IOLs, so low incidence of halo around light or night glare occurred after the implantation.
The TetraFlex KH-3500 accommodating IOL (Lenstec Inc., St. Petersburg, FL, Russia) is an accommodating posterior chamber IOL which is very flexible and 5 degrees anteriorly vaulted. This study aimed to research if such flexibility and vaulting allowed for anterior movement of the lens and could provide accommodation with near effort.
All eligible patients with visually significant cataracts who presented to the Beijing Tongren Hospital (Beijing, China) from August 2008 to May 2009 were enrolled in this prospective study of the safety and efficacy of the TetraFlex IOL implanted at the time of phacoemulsification after signing an informed consent. Exclusion criteria for the study included a history of any eye disease that might limit postoperative restoration of vision including glaucoma, corneal disease, retinal disease or iritis. Patients with more than 2.00 diopters (D) of corneal astigmatism were also excluded from the study. Patients who did not return for prescribed follow-ups or had any intraoperative complications were removed from the study. A total of 52 eyes of 44 patients were initially enrolled in the study. One eye was removed from the study because of posterior capsular rupture during surgery, and another eye removed from the study because of the development of severe systemic hypertension after the procedure.
Data from a total of 50 eyes of 42 patients were included in the study. The evaluated cohort included 24 eyes from 20 males and 26 eyes from 22 females. The average age of patients in the study was (58.21±9.36) years (range from 47 to 71 years).
Preoperatively, each patient underwent a complete eye examination including manifest refraction, keratometry, slit lamp examination, intra ocular pressure check, dilated retinal examination and ultrasonic axial length measurement for each eye. The mean refraction before surgery was (-1.47±0.51) D, and the postoperative target refraction power was projected at 0 to -0.5D.
All surgeries were performed by a single surgeon (Prof. WANG Ning-li). IOL power was calculated with the Holladay formula with a postoperative goal between plano and -0.50 D. Each sutureless procedure was performed under topical anesthesia. One drop of proparacaine was placed in the operative eye every 5 minutes starting 15 minutes before surgery. A 3.0 mm watertight clear corneal incision was made and a 5.0 to 6.0 mm continuous curvilinear capsulorhexis was performed and the nucleus was phacoemulsified in the capsular bag. The residual cortex was aspirated with the I/A handpiece. Viscoelastic was then used to inflate the capsular bag, and the TetraFlex was slowly injected into the bag. The lens was then moved back and forth along the long axis of the lens with a lens hook to verify that the leading and trailing haptics were properly positioned in the capsular bag. Viscoelastic was then removed and the wound verified for stability. Postoperatively, the patient was placed on tobramycin-dexamethasone drops four times per day for 1 to 2 weeks.
Each eye underwent postoperative examination at 1 day, 1 week, 1 month and 3 months after surgery. Postoperative examination included slit lamp examination, intraocular pressure measurement, manifest refraction and measurement of uncorrected visual acuities (UCVA), best corrected visual acuities (BCVA) and distance-corrected near visual acuities (DCNVA). The Snellen chart was used to measure the distance vision and the Jaeger chart was used to measure near vision at a distance of 40 cm under well illuminated conditions. Because all the patients were used to read paper at about 30 to 40 cm, so near visual acuity was examined at this distance range.
At the visit three months postoperative, subjective accommodation and pharmacologically induced IOL mobility were also measured. Near-point subjective accommodation as measured with the defocus method. This method involved placing a lens in front of the eye that created a +3.00 D add over the best distance refraction. The reading target (a Jaeger chart where the patient was asked to focus on the line just above the lowest discernable line) was placed 0.33 m away from the eye and -0.25 D spherical lenses were sequentially placed in front of the eye until the visual target blurred. The near-point accommodation power was the sum of the diopters of all minus lenses added until thetarget blurred. Pharmacologically induced IOL mobility was evaluated with the anterior segment optical coherence tomography (AS-OCT, Visante-1000, Carl Zeiss Meditec., Dublin, Germany) by measuring the distances from the anterior surface of the IOL to the corneal vertex both in the unaccommodated state as well as after induced accommodation with 2% pilocarpine drops (one drop placed every 5 minutes, totally 6 times prior to testing). The difference between these two distances was defined.
Statistical software SPSS 16.0 (SPSS Inc., USA) was used to analyze the linear correlation between IOL movement and pseudoaccommodation data, and the visual acuities at different time postoperative was compared with chi-square test. A P <0.05 was considered statistical significant.
At all postoperative visits, none of the following complications were noted: IOL decentration or malposition, increased intraocular pressure, infection or cystoid macular edema.
At one month postoperation, distance UCVA and BCVA was 20/40 or better in 80% (40/50) and 90% (45/50) of eyes, respectively. At three months postoperation, distance UCVA and BCVA was 20/40 or better in 82% (41/50) and 92% (46/50) of eyes, respectively (Figure 1).
Monocular DCNVA (Figure 1) were J4 or better in 24% (12/50), 52% (26/50), and 66% (33/50) of eyes at 1 week, 1 month and 3 months poseoperative, respectively. The mean positive spherical lens required to achieve a DCNVA of J2 or better in each eye were (2.01±1.22) D, (1.76±1.01) D, and (1.62±0.97) D at 1 week, 1 month and 3 months postoperation, respectively.
Three months after TetraFlex implantation, the average subjective accommodation was (0.94±0.61) D (range from 0.50 to 1.50 D). The average pilocarpine-induced IOL mobility was (337±124) μm (range from 121 to 501 μm). There was significant correlation (Figure 2) between subjective accommodation and IOL mobility (r2=0.901, P <0.01).
Typical results of cataract surgery with monofocal IOL implantation is excellent uncorrected distance vision but poor near vision due to presbyopia. The optics of diffractive and refractive multifocal IOLs can split the incoming light into distance and near foci, resulting in improved uncorrected reading ability without glasses. However, these multifocal optics frequently result in optical issues including reduced contrast sensitivity, halos around lights and night glare.6–8 An ideal presbyopic IOL would overcome these visual side effects. The aim of accommodating monofocal IOL's design was to improve uncorrected near vision after cataract surgery with low induction of these problems.
Huber et al9 and other researchers believe that some degree of accommodative effect may be generated by corneal astigmatism. Additionally, the pupillary constriction caused by the near reflex can produce an accommodative effect of a certain degree by increasing depth of field.10 However, pupillary constriction still occurs after implantation of standard monofocal IOLs without significant near effect. The non-multifocal mechanism of inducing accommodative effect after cataract surgery that has gained the most attention in recent years, however, is anterior movement of accommodating IOLs during the near effort. In addition, it is postulated that an increase in certain higher order aberrations with the near effort may also be a factor.
During phacoemulsification, creation of a continuous circular capsulorhexis has multiple benefits including reducing the potential for damage to the remaining lens capsule through a peripheral radialization of the capsular tear and maintaining tension in the capsule. The even margin of a well constructed capsulorhexis symmetrically covers the margin of an accommodating IOL optic, which ensures that IOL will be driven forward by an even force when the ciliary muscle contracts. The specially designed haptic loops and anteriorly vaulted optic of the TetraFlex has excellent pliability, allowing it to shift forward when the contraction effect of the ciliary muscle increases vitreous pressure.
Although measurements such as DCNVA and reading speed are typically used to evaluate near vision after presbyopic IOL implantation, we know that near vision may be influenced by higher order aberrations of the cornea and lens, residual myopic astigmatism, anterior chamber depth, pupil diameter, and anterior displacement of the IOL.9–11 Because of this, we also evaluated subjective accommodation by the defocus method and objective pilocarpine-induced anterior displacement of the IOL in this study.
Baikoff et al12 applied a defocus technique combined with SL-OCT to measure the forward movement of crystalline lens during near effort and found that with 1.0 D of accommodation, the anterior pole moved forward by a mean of 30 microm, the radius of curvature decreased 0.3 mm. Marchini et al13 measured the forward shift of the Crystalen with accommodative effort with an ultrasonic biological microscope (UBM) and found that the forward shift could reach 330 μm 6 months after the implantation. In this study, the forward mobility of TetraFlex measured with SL-OCT was (337±124) μm, similar to the Crystalens.
In this study, we found that the mean forward mobility of the TetraFlex of (337±124) μm correlated strongly with a mean subjective accommodation of (0.94±0.61) D. However, (0.94±0.61) D alone was not enough for near vision, but as stated previously, subjective accommodation may also be affected by induced higher order aberrations in the IOL, corneal astigmatism, anterior chamber depth and pupil diameter. One or more of these factors increases the subjective accommodation which made the patients see near targets easily. This mobility result, however, is not consistent with Nawa and colleagues study14 which demonstrated that 1 mm displacement equates to 0.8 D change in power in an eye of axial length of 27 mm, keratometry 7.7 mm, and IOL power 11. Whereas a 1 mm displacement equates to 2.3 D of change in power in an eye with axial length of 21 mm, IOL power 30 and the same keratometry. We think that this difference in results has two explanations. First, as a miotic was used in our study to simulate the accommodative state (static accommodation), the forward shift of the TetraFlex was hampered by the increased tensile force in the iris created by the miotic. Second, in our research, the keratometry was similar with the one in Nawa's study, but the patients’ axial length range from 22 to 25 mm, this might cause the mobility difference.
Monocular DCNVA improved as time passed as we measured separately 1 week, 1 month and 3 months postoperative. This phenomenon might be caused by the recovery of the eye which received surgery, and also might because of the strengthen of ciliary muscle as reading practice proceeded.
In summary, implantation of the TetraFlex IOL is safe and effective at providing vision for both distance and near. The average patient in the study achieved about 1 D of subjective accommodation, which is at least partially accounted for by anterior movement of the IOL during the accommodative effort.
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