Posterior capsule opacification (PCO) remains the most significant complication of cataract extraction even when performed using modern techniques.1 In many patients, PCO leads to a considerable reduction in visual acuity and problems with glare.2,3 The treatment of PCO with a neodymium:YAG (Nd:YAG) laser capsulotomy is relatively easy but is costly in time and money for both the health care system and patients.4 In addition, an Nd:YAG laser capsulotomy carries a risk of complications; some of them (eg, retinal detachment, macular edema) are serious.5,6
The low availability of Nd:YAG lasers is an obstacle to conversion to extracapsular cataract extraction (ECCE) with intraocular lens (IOL) implantation in many less developed areas of the world.4 Also, PCO formation is the greatest remaining obstacle to the development of an accommodating IOL injected through a minicapsulorhexis.7 Thus, it is important to elucidate the factors in PCO formation and find methods to diminish or prevent it.
The formation of PCO is probably dependent on many factors, some of which are known and others which remain hypothetical or undiscovered. These include factors related to epidemiology (eg, age, sex, comorbidity),8–18 surgical technique (eg, ECCE versus phacoemulsification),19 IOL design/material,4,20 the surgeon (eg, skill, amount of hydrodissection),21,22 and cataract type.11,23 Unknown inherent factors such as proliferative potential may also play a role.
In the past few years, there have been several sponsored and independent reports of a low incidence of PCO after implantation of the AcrySof® IOL. Most studies were controlled and prospective and of patients without ocular comorbidity.24–28 The present study evaluated whether the reported low PCO incidence with AcrySof IOLs exists in everyday clinical practice with a mixed population of patients. The study was performed without the knowledge or financial support of any of IOL manufacturer.
Patients and methods
This retrospective comparative study was performed at the Department of Ophthalmology, Kärnsjukhuset, a medium-sized hospital in Skövde, Sweden, where all cataract surgery for 250 000 inhabitants is performed. The only Nd:YAG laser available for the area's population is at this eye clinic, so it is probable that all patients included in this study who needed capsulotomy had it performed at the hospital. The situation for ophthalmic care in the area was stable during the study period; that is, availability for cataract surgery and laser capsulotomy remained unchanged.
Patients were divided into 2 groups based on the type of IOL implanted: AcrySof (study group, n = 145) or poly(methyl methacrylate) (PMMA) (comparison group, n = 153). Table 1 shows the characteristics of the patients in both groups.
All patients had phacoemulsification by 1 of 2 experienced surgeons (A.Ö., A.L.). The surgeons began using the AcrySof as their standard IOL during the last months of 1995 (Figure 1). The study group had surgery in 1996. The comparison group had surgery during the 13 months (September 1994 to September 1995) before the surgeons began using AcrySof IOLs (Figure 1). Four PMMA IOL designs were used in the comparison group; 9 patients received a heparinized PMMA IOL.
In both groups, patients were eligible for study participation if they had an intact posterior capsule at the end of surgery and were alive at the end of follow-up. Eighty patients in the study group and 87 in the comparison group were not eligible because they did not have a posterior capsule or an IOL (10% and 11%, respectively), died during the follow-up (8% and 6%), or had a different surgical technique (3% and 19%). Thirty-one of the 80 patients having surgery during 1996 were not eligible for the study group because they received an IOL other than an AcrySof even though they had cataract extraction by phacoemulsification.
The performance of an Nd:YAG laser capsulotomy was used as the end point for clinically significant PCO. The information on capsulotomies was collected retrospectively from the patients' medical records. The indication for capsulotomy remained the same throughout the study; that is, it was performed when a patient sought consultation for decreased visual acuity or glare and the treating physician found a relationship between the PCO and the symptoms. The surgeon performing the capsulotomy was not always the surgeon who performed the cataract surgery.
The follow-up ranged from 2 years 6 months to 3 years 6 months in the study group and from 2 years 6 months to 3 years 7 months in the comparison group (Table 1).
Statistical analysis was by the Mann-Whitney U test (patient age) and the chi-square test (all other variables). Confidence intervals (CIs) for the same variables were calculated. The relative risk with CIs was also calculated.
During the 3-year follow-up, 6.2% in the study group and 22.2% in the comparison group required an Nd:YAG laser capsulotomy The difference was significant at the 95% CI level (chi square = 15.5; P < .001), with the study group having a significantly lower incidence of capsulotomy. The relative risk ratio (PMMA versus AcrySof) was 3.6 (95% CI 1.8-7.2).
There was no statistically significant between-group difference at the 95% CI level in any patient characteristic (Table 1). In addition, the differences between noneligible patients with an AcrySof IOL and noneligible patients with a PMMA IOL were not significant. The number of patients with uveitis was too small for statistical analysis in any group.
The results of this retrospective study confirm previous reports of a low PCO incidence with the AcrySof IOL. Clinically significant PCO developed in 6.2% of eyes in the AcrySof group and 22.2% in the PMMA group during a 3-year follow-up. Other studies of AcrySof and PMMA IOLs report PCO incidences from 2.7% to 11.0% and from 24.7% to 56.0%, respectively. The methods of investigation and follow-up were different among reports.24–26,28 It is reassuring to know that the good results are applicable to an unselected group of patients in everyday cataract practice.
It has been proposed that the mechanism by which AcrySof lowers the PCO rate is a combination of its sharp, rectangular optic edge and its sticky surface, which adheres to the lens capsule.20,29,30
In studies of PCO, systematic biases must be considered. A main concern is whether the compared groups belong to the same population. We tried to adjust for this by using patients from the population in the same region of Sweden who had surgery at the same clinic by 1 of 2 experienced surgeons using the same surgical technique. All Nd:YAG laser capsulotomies were performed at the same hospital with the same criteria as there was no other Nd:YAG laser available in the area. The follow-up was equal and the patients' characteristics such as sex, age, and coexisting pathology (eg, diabetes mellitus, pseudoexfoliation, uveitis) were the same in the study and comparison groups. The choice of characteristics to be analyzed was based on reports of factors that do or might influence the amount of PCO production.8–18 In addition, the same statistical analyses were performed for patients excluded from the study and patients in the comparison group to determine whether the background population from which the included patients were taken or the selection criteria had changed. No evidence of systematic bias was found.
There are few studies of the influence of surgeon-related factors (eg, skill) on PCO. Apple et al.21 propose that hydrodissection is an important factor in PCO prevention; however, we do not know whether the amount of hydrodissection increases or decreases with improved surgeon skill. In the present study, the study group had surgery the year after the comparison group. It is reasonable to assume that the surgeons' skills had improved. However, the proportion of patients in the study group having phacoemulsification rather than ECCE was higher than in the comparison group (89% versus 71%). This indicates that there were more complicated cases in the study group having phacoemulsification. If skill is an important factor in PCO formation, we believe that it is reasonable to assume that it is outweighed by the change in surgical technique.
The use of the performance of Nd:YAG laser capsulotomy as an end point for PCO has been criticized. However, the incidence of Nd:YAG laser capsulotomy is probably a good reflection of the number of patients with clinically significant PCO. In addition, comparing the capsulotomy rate addresses the benefits to the patient of PCO prevention strategies. In an earlier study,13 we reported a 9% incidence of “hidden” PCO during a 5-year follow-up. However, the number of such cases was probably equal in the study and comparison groups and thus did not alter the proportions.
In conclusion, the risk of PCO formation in the AcrySof group was significantly less than in the PMMA group during a 3-year follow-up. Our findings indicate the importance of selecting an IOL type that reduces PCO formation in a mixed group of cataract patients.
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