Comparison of pre-chop technique using a reverse chopper and classic stop-and-chop technique in the treatment of high myopia associated with nuclear cataract

Background To evaluate the safety and efficacy of the pre-chop technique using a novel reverse chopper vs. the classic stop-and-chop technique in phacoemulsification for patients with high myopia and associated grade III–IV nuclear cataracts. Methods In this prospective cohort study, a total of 44 consecutive patients (44 eyes) with grade III–IV nuclear cataracts who were admitted to our hospital for cataract surgery between March 2018 and September 2018 were enrolled. All patients had ocular axial length > 27 mm and myopic refraction more than -10 diopters. Patients were randomly divided into a pre-chop group and stop-and-chop group using a randomization table. Nucleus splitting was performed surgically in both groups using either the pre-chop technique with reverse chopper or the classic stop-and-chop technique. Results Postoperative visual acuity was significantly improved in both groups compared with preoperative values. Significantly better visual acuity, lower degree of corneal edema and lower rates of corneal endothelial cell loss were observed in the pre-chop group compared to those in the classic stop-and-chop group. No complications were reported in either group. Conclusions In treating patients with high myopia associated with grade III–IV cataracts, the pre-chop technique using a reverse chopper reduces damage to corneal endothelial cells and improves visual acuity better than the classic stop-and-chop technique. Supplementary Information The online version contains supplementary material available at 10.1186/s12893-022-01658-0.


Background
The prevalence of myopia and high myopia is increasing globally, especially in Eastern and Southern Asia [1]. The pathological conditions associated with high myopia, including retinal damage, cataracts, and glaucoma, significantly increase the risk of visual impairment [2]. High myopia is defined as myopia with ≥ 27.00 mm of ocular axial length and more than − 10.00 diopters of refraction [3]. Rates of myopia with ≥ 27.0 mm of the ocular axial Open Access *Correspondence: 183547004@qq.com 5 Mei'ermu Hospital, Beijing, China Full list of author information is available at the end of the article length among elderly Chinese are reported to be significantly greater than 1.0%, compared to the rate of 0.1% reported in European and American populations [4][5][6]. Patients with high myopia usually have associated hard nuclear cataracts, and therefore will usually undergo early cataract surgery [7].
The nucleus-splitting technique is one of the key aspects of cataract surgery and requires surgeons to control ultrasound and negative pressure, as well as handfoot coordination, together. These increase the technical difficulty of the procedure. When compared with conventional ultrasound phacoemulsification, it reduces required ultrasound energy, surgery time, extent of tissue damage, and occurrence of complications associated with energy release [8]. Consequently, stop-and-chop phacoemulsification is widely used by many surgeons [9]. In patients with high myopia and associated cataracts, the anterior chamber is deep and the nucleus of the lens is relatively hard; thus, the phacoemulsification procedure often induces breakage of the posterior capsule [10]. Therefore, despite wide use among surgeons, phacoemulsification surgery is classified as a highly difficult surgery [11].
The manual pre-chop technique reduces the energy and time required for intraoperative phacoemulsification and reduces damage to the intraocular tissue [12], which is shown to be useful in both normal and small pupil cataract surgery [13]. However, most pre-chop methods require downward pressure on the lens, which may result in a certain amount of traction on the suspensory ligaments [14]. For this reason, these methods provide only poor management of the hard nucleus [15]. These shortcomings limit the application of new techniques in the treatment of myopia associated with cataracts. Further, femtosecond laser nuclear pre-chopping also manages the hard nucleus poorly and requires expensive equipment, limiting its wider use [16].
Given the above-mentioned limitations, we developed a novel pre-chop technique using a reverse chopper before phacoemulsification of the lens nucleus, which achieves efficient in situ nucleus-splitting without using ultrasonic energy. Our new technique exhibits advantages such as reducing time and energy during phacoemulsification, as well as reducing surgical complications. We have applied this technique in surgeries for high myopia [17] and complex cataracts [18]. The present study was conducted to compare clinical outcomes between the new pre-chop technique and the classic stop-and-chop technique in patients with myopia and associated cataracts.

Study design and sample
A prospective cohort study design was adopted. Consecutive patients with high myopia and associated cataracts who were admitted to the Cataract Center of Beijing Tongren Hospital between March and September 2018 were enrolled. The inclusion criteria were: (1) patients with grade III-IV nuclear cataract based on Emery's classification [19]; (2) an ocular axial length > 27 mm, and myopic refraction more than − 10 diopters; (3) no obvious or only mild lens tremor on mydriatic examination, and no history of glaucoma or iritis; and (4) patients had shown good compliance, knew that new surgical equipment would be used in the surgery, were aware of the risks of surgery, agreed to participate in the study, provided signed informed consent, and agreed to be followed regularly. The exclusion criteria were: (1) patients with medical history of diabetic retinopathy, macular edema, pseudoexfoliation, or optic neuropathy; (2) patients who had undergone a specific ocular surgery within the preceding 60 months; and (3) patients with a history of eye trauma.
Patients were randomly divided into a pre-chop group and a stop-and-chop group using a randomization table. Before surgery, slit-lamp microscopic examination was performed for all patients and showed a clear cornea, a deep anterior chamber, and normal iris texture without iris atrophy or depigmentation. After mydriasis, the lens was observed to be in a normal location, the nucleus became turbid, the color of the nucleus was dark yellow, brown or amber, and the intraocular pressure was ≤ 21 mm Hg (noncontact tonometer TX-20; Canon).

Ethical considerations
The study protocol was reviewed and approved by the Institutional Review Board of Beijing Tongren Hospital (TRECKY2017-028). All included patients provided signed informed consent to participate in the study.

Surgical procedure
A specially made metal reverse chopper is used during the surgery (Fig. 1A). This chopper is shaped like an inverted circular arc with a round and blunt tip and an inner blade on the circular arc. The distance between the 2 tips of the arc is about 3 mm. The distal curvature is slightly larger than the proximal curvature (Chinese National Patent Number 201520394994X) [20]. All surgeries in this study were performed by the same experienced surgeon (Y. Zhao).

Pre-chop phacoemulsification and intraocular lens implantation
The procedure followed our previous report with some modifications [20]. In detail, phacoemulsification and intraocular lens (IOL) implantation are carried out using an ultrasound emulsification device (INFINITI Vision System; US Alcon, Fort Worth, TX, USA). The intraoperative phacoemulsification parameters are: ultrasound energy 55%, negative pressure 380 mm Hg (1 mm Hg = 0.133 kPa), and a flow rate of 40 mL/min. A standard 3.0-mm incision is made at the 10 o'clock position of the transparent cornea. Ocular viscoelastic solution (EYEFILL ® S.C, Bausch & Lomb, Montpellier, France) is injected into the anterior chamber. A standard puncture opening is made at the 2 o'clock position using a 15° puncture scalpel. Continuous curvilinear capsulorhexis is performed for the anterior capsule of the lens using capsulorhexis forceps. The diameter of the capsule opening is 5-6 mm.
The reverse chopper is inserted into the anterior chamber via the incision made at the 10 o'clock position of the cornea. Although it is a standard 3.0-mm incision, the operatable incision can be as small as 2.2 mm in width. The distal end of the chopper is then gently pressed downward to contact the nucleus of the lens. The reverse chopper is then tilted laterally and slid into the capsule along the nucleus of the lens. The chopper is then raised gradually to bury its arcuate part into the cortical shell located between the nucleus of the lens and the capsule.
Its arc-shaped inner blade is placed perpendicular to the nucleus equator and fixed.
A Nagahara chopper is inserted into the anterior chamber via the lateral incision made at the 2 o'clock position. The Nagahara chopper is slid into the space beneath the cortex and the capsule membrane so that its blade is perpendicular to the equator of the nucleus. The surgeon holds the Nagahara chopper in the left hand, and the reverse chopper in the right hand, which is placed on the radial line of the lens (Fig. 1B, C). The two devices are pushed toward the center of the lens, and the surgeon ensures that both devices move in the horizontal direction. The nucleus of the lens is split, and the Nagahara chopper meets the reverse chopper at the center of the lens. The two devices are then gently separated horizontally to divide the nucleus completely into 2 semiellipsoids (Additional file 1: Fig. S1). Then the reverse chopper remains near the right half of the nucleus while the Nagahara chopper is slid again into the bottom of the capsule at the 8 o'clock position. It is then pulled to the center to divide the right half of the nucleus into 2 parts (Additional file 2: Fig. S2).
Next, a conventional phacoemulsification procedure is carried out. The phacoemulsification needle is inserted, and the nucleus is suctioned to the iris plane. The nucleus is removed after emulsification. The cortex of the lens is removed, and an IOL (AcrySof IQ Aspheric Natural IOL SN60WF, Alcon, Vernier-Geneva, Switzerland) is implanted in the capsule. In some patients, a capsular tension ring is also implanted. After surgery, tobramycin dexamethasone eye ointment is applied to the conjunctival sac.

Stop-and-chop phacoemulsification and intraocular lens implantation
The preoperative preparation, equipment, surgical incision, circular capsulorhexis, and postoperative procedures for the stop-and-chop method are similar to those previously reported for the new procedure. Hydrodissection is performed after continuous curvilinear capsulorhexis. The phacoemulsification needle is then used to suction the nucleus to its tip. The suctioned nucleus is then split using the chopper, and phacoemulsification is performed. The nucleus is gradually broken into pieces and removed. Next, the cortex of the lens is removed, and an IOL is implanted into the capsule. In some patients, a capsular tension ring was implanted.

Observation indicators and evaluation standard
The actual ultrasound energy (%) and valid ultrasound time (s) required for the phacoemulsification of the lens nucleus were compared between the two groups in this study. Best corrected visual acuity (BCVA) was recorded before surgery and then at postoperative days 1 and 7. A corneal endothelial cell count was carried out before surgery and at 1 month after surgery using a specular microscope (SP-3000P; Topcon Corp, Tokyo, Japan). The status of corneal edema and postoperative complications were compared between groups at postoperative days 1 and 7. Corneal edema and opacity were classified using Dickey's standard 5: Grade 0: the cornea is transparent; grade 1: the cornea is slightly opaque (hazy); grade 2: the cornea is opaque, but the structures of the anterior chamber are clearly visible; grade 3: the corneal opacity is aggravated, and observation of the anterior chamber is difficult; grade 4: corneal opacity is severe, the iris structure cannot be observed, and the anterior chamber is not visible.

Statistical analysis
All data were analyzed using SPSS version 19.0 (IBM Corp., Armonk, NY, USA). Based on results normality testing, normally distributed data are represented as x ± s (mean ± standard deviation, SD), skewed data are represented as M (Q1, Q3), and frequency data are represented as the number of eyes. A single-factor, completely randomized design was used for grouping. An independent 2-sample t-test was used to compare the ultrasound energy and valid ultrasound time between the pre-chop group and the stop-and-chop group. The Chisquare test was used to compare visual acuity and corneal edema between the pre-chop group and the stop-andchop group. An independent 2-sample t-test was used to compare the postoperative corneal endothelial cells and the loss rate between the pre-chop group and the stopand-chop group. All tests were 2-tailed and a significance level of 0.05 was adopted.

Patients' baseline characteristics
A total of 44 patients (44 eyes) with high myopia and associated cataracts were enrolled, and 22 patients (22 eyes) were randomly placed into each of two groups: a pre-chop group and a stop-and-chop group. Patients' ages ranged overall from 37 to 68 years. In the pre-chop group, 11 eyes were in males and 11 in females, mean age was 57.41 years (range 38-68 years), and the mean with standard deviation (mean ± SD) preoperative BCVA was 0.14 ± 0.09. In the stop-and-chop group, 10 eyes were in males and 12 in females, the mean age was 56.36 years (range 37-67 years), and the mean preoperative BCVA was 0.15 ± 0.08. No significant differences were noted between the 2 groups in sex, age and BCVA (Table 1). Surgeries were performed successfully in both groups, and no serious intraoperative complications occurred in either group.

Comparison of intraoperative ultrasound energy and time between groups
The mean actual ultrasound energy used in the pre-chop group (55.00 ± 0.00) was the same as in the stop-andchop group (55.00 ± 0.00). The mean ultrasound time in the pre-chop group was significantly shorter than that in the stop-and-chop group (48.55 ± 11.62 vs. 56.68 ± 14.23, P = 0.04).

Comparison between preoperative and postoperative BCVA
The postoperative BCVA for visual acuity was significantly improved in both groups compared with the preoperative values ( Table 2). No significant differences in preoperative BCVA were found between the two groups (P = 0.888). The BCVA was significantly improved on postoperative day 1 in the pre-chop group (0.32 ± 0.13 from 0.14 ± 0.09; P < 0.001), and in the stop-and-chop group (0.22 ± 0.13 from 0.15 ± 0.08; P = 0.004). On postoperative day 7, the mean BCVA in the pre-chop and stop-and-chop groups had improved to 0.58 ± 0.14 (P < 0.001) and to 0.49 ± 0.15 (P < 0.001), respectively. Postoperative mean BCVA was significantly higher in the pre-chop group than that in the stop-and-chop group (P = 0.011 on postoperative day 1, P = 0.032 on day 7).

Comparison of postoperative corneal edema scores
Varying degrees of corneal edema were observed one day after surgery and resolved gradually with treatment (medication: tobramycin dexamethasone eyedrops were used 6 times/day for 2 weeks). The degree of corneal edema in the pre-chop group was significantly lower than that in the stop-and-chop group on postoperative days one (P = 0.020) and 7 (P = 0.013) ( Table 3).

Comparison of corneal endothelial cell status between groups
Prior to surgery, no significant differences were noted in the number of corneal endothelial cells between the two groups (P = 0.174). One month after surgery, the number of corneal endothelial cells in the pre-chop group was slightly higher than that in the stop-and-chop group, but without significant differences (P = 0.341). However, the rate of corneal endothelial cell loss in the pre-chop group was significantly lower than that in the stop-and-chop group one month after the surgery (P = 0.001) ( Table 4).

Discussion
The present study has shown that the new pre-chop technique using a reverse chopper is a safe and efficient technique in treating patients with high myopia associated with grade III-IV cataracts. Compared with patients in the classic stop-and-chop group, those in the pre-chop group had significantly less corneal edema, lower corneal endothelial cell loss rate and better BVCA postoperatively. Our new pre-chop technique displayed better recovery of visual acuity and reduced damage to corneal endothelial cells compared to the classic stop-and-chop approach, suggesting that it can be used safely, effectively and quickly in treating patients with high myopia and associated hard nuclear cataracts, while greatly reducing the application of phacoemulsification energy and decreasing the overall difficulty of the surgery. Various techniques have been developed for cataract surgery in order to reduce the damages to the capsule. Technique like miloop does not require concurrent ultrasound [21,22], which seems to be a very good option. But there was a trend towards lower endothelial cell loss in the miLOOP eyes [21], and miloop technique needs specialized tools and longer time for operation and familiar with. Thus, we prefer pre-chop with lower ultrasound usage than miloop.
Recent study has suggested that directing ultrasound at the capsule is more suitable for this type of cataract surgery than directing ultrasound inside the capsule, primarily because of the significantly deeper anterior chambers in patients with high myopia [20]. However, although the ultrasound energy required in the two types of surgery is similar, the pre-chop technique significantly reduces intraoperative complications such as capsule breakage, suspensory ligament rupture, and others. Recently, several new manual pre-chop techniques, including the cystotome-assisted pre-chop technique, the pinpoint pre-chop technique, and the use of a reverse chopper with the pre-chop technique have been developed specifically to help reduce the energy and time requirements of intraoperative phacoemulsification, including decreasing damage to eye tissue, reducing the loss of corneal endothelium, and accelerating the speed of postoperative recovery [17]. The effectiveness of these techniques has been demonstrated in routine cataract surgery [18]. In fact, our eye surgery group is planning to broaden the application of the pre-chop technique using a reverse chopper to treat patients with other types of complicated cataracts.
In the present study, we found that the force applied when using the reverse chopper in the right hand during the pre-chop procedure, and the force applied by the Nagahara chopper in the left hand must be on the diagonal line in the horizontal direction to carry out in situ nucleus splitting, and no downward pressing force is applied to the lens nucleus. This technique is shown to reduce traction on the suspensory ligament, avoid breakage of the suspensory ligaments, and increase the surgical success rate [23], making it especially suitable for patients with high myopia associated with cataracts. Importantly, because the pre-chop technique reduces both the ultrasound energy and time required, the overall quality of cataract surgery is improved. Recently, we developed a novel pre-chop procedure using a reverse chopper that can be embedded in the equator of the lens so that the force can be applied on the horizontal direction of the lens to carry out in situ pre-chopping of the nucleus that only rarely pulls on the suspensory ligaments; this prechop technique significantly reduces the intraoperative use of ultrasound energy, in turn reducing the loss of corneal endothelium and injuries to other intraocular structures [24].
In the present study, we applied the pre-chop technique using a reverse chopper together with the Nagahara chopper, placing them at the 5 o'clock position and 11 o'clock position in the capsule; then, we held and fixed the lens nucleus, applying force to the center of the nucleus on a diagonal line in the horizontal direction, and divided the nucleus into two parts as if using scissors. Because the suspensory ligament is loose in myopia patients, the nucleus is not rotated 90° after being divided into two parts. The reverse chopper remains close to the right half of the lens nucleus while the Nagahara chopper is slid into the 8 o'clock position of the equator, and then the right half of the nucleus is split into two parts using the reverse chopper. Finally, the entire nucleus is divided into three free pieces. During the whole process of nucleus splitting, no ultrasound energy is released, which greatly reduces the valid ultrasound time, and provides satisfactory outcomes. Both the reverse chopper and the Nagahara chopper have inner blades that fix the nucleus of the lens tightly during nucleus splitting, in which the relatively hard lens nuclei can be split easily. Moreover, the round and blunt design of the tip effectively protects the membrane of the posterior capsule during nucleus splitting and rotation. The most difficult step of the surgery is nucleus splitting. The nucleus should be split safely and quickly into 2-4 pieces for removal from the capsule. Such safe and quick nucleus splitting is the most important step for a successful surgery. However, during the surgery in patients with high myopia with cataracts, the anterior chamber is abnormally deepened, and the suspensory ligament is loose. Therefore, whether the surgeon uses a manual pre-chop technique or a stop-andchop technique, the operating space is limited when the downward force is applied to the lens nucleus, and the downward force must be gently controlled to avoid suspensory ligament rupture due to excessive traction.
In conclusion, the reverse chopper can be used safely and effectively during the pre-chop procedure for patients with high myopia and associated hard nuclear cataracts, greatly reducing the application of phacoemulsification energy and decreasing the difficulty of surgery. Applying the present pre-chop technique using a reverse chopper displays better recovery of visual acuity and greater reduction of damage to corneal endothelial cells compared to the classic stop-and-chop approach. We have applied the newly developed technique to the treatment of difficult cataracts and obtained favorable outcomes, including treating patients with cataract grade IV hard nucleus and small pupil cataracts [17,18]. Given the instrument's simplicity, and its efficient use of time and short learning curve, the reverse chopper has the potential to be used widely.