Patients who visited the B & VIIT Eye Clinic, Seoul, Korea were recruited for this study. Thirty-seven patients who underwent implantation of an Artisan PIOL by the same surgeon (JKK) at B & VIIT Eye Clinic from December 2006 to June 2007 were enrolled. All procedures conformed to the tenets of the Declaration of Helsinki. During the study periods, there was no institutional review board in our hospital, so informed consent was obtained from the participants after explanation of the study. Exclusion criteria were those with manifest tropia, a history of strabismus surgery, absent binocular vision, and anterior segment pathologic condition.

Preoperative examination included uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA) with Snellen chart, slit-lamp biomicroscopy, intraocular pressure, corneal keratometry, pupil diameter, iris configuration, and indirect ophthalmoscopic examination. Complementary evaluations included: corneal topography (Orbscan II; Bausch & Lomb, Rochester, NY, USA), axial length, anterior chamber depth with ultrasound, and central endothelial cell count (SP8000; Konan, Hyogo, Japan).

Stimulus AC/A ratio was assessed using the gradient method, previously described by von Noorden and Campos [5]. Through fully corrected glasses, the baseline angle of ocular alignment for the accommodative target at 6 m was measured with the alternate prism cover test. Afterward, upon placing a -3 diopters (D) lens before the eyes with full correction, the deviated angle was measured. The measurements were repeated three times, and the average of the measurements was considered to be the representative value. The convergence amplitude was measured using a handheld rotary prism. Placing it before the eye, a near (33 cm) target of Snellen E chart, size of 20 / 200, was fixed, and at a speed of 4 prism/sec, the prism was rotated. As the prism rotated, the moment of diplopia appearance was defined as a 'break point,' and the moment after the break point where the diplopia disappears was defined as a 'recovery point.' NPC was assessed as previously described by Maples and Hoenes [6]. A Bernell Accommodative Rule, which is commonly used in research, was utilized [7]. One end of the accommodative rule was placed just above the nose at the brow, between the two eyes. NPC measurements were repeated three times and the average of the measurements was considered to be the representative value.

A total of 37 patients followed the study. The mean age of three men and 34 women was 24.3 ± 4.8 years old, ranging from 19 to 34 years. The preoperative refractive error (RE) ranged from -5.75 D to -11.25 D (mean, -8.92 ± 4.13 D) in spherical equivalent. Mean residual RE was -1.75 ± 1.47 in the right eye and -2.21 ± 2.57 in the left eye at postoperative one week, -0.75 ± 0.15 in the right eye and -0.64 ± 0.51 in the left eye at postoperative one month, and -0.50 ± 0.30 in the right eye and -0.52 ± 1.33 D in the left eye at postoperative three months. The mean residual RE was -0.54 ± 0.37 in the right eye and -0.57 ± 1.36 D in the left eye at six months after surgery. All values showed statistical significance compared to preoperative refractive error (p < 0.05).

The convergence amplitude was measured as 24.31 ± 4.12/17.12 ± 3.99 (break/recovery) prism diopters (PD) before surgery. Upon six months of follow-up, the convergence amplitude showed only slight changes with statistical significance (Table 1).

The stimulus AC/A ratio showed changes after surgery. The preoperative stimulus AC/A ratio was 4.39 ± 0.79 PD/D. After one week and one month, the AC/A ratio (4.91 ± 1.22 PD/D at one week, 5.11 ± 0.71 PD/D at one month) increased with statistical significances (p < 0.05). However, afterward, the AC/A ratio seemed to stabilize, and was comparable at three or six months after surgery to the preoperative values (Fig. 1).

The preoperative NPC was 3.82 ± 1.37 cm. No significant differences in the NPC were observed at any of the follow-up visits, suggesting no effect of the Artisan PIOL implantation on the convergence power (Fig. 2).

In this study, we evaluated the change in the accommodation-convergence parameters in myopic patients who had bilateral Artisan PIOL implantation. With their glasses removed, the patients may experience not only an improvement of visual acuity, but also a change in accommodation-convergence response required for daily activities.

There have been few reports describing alteration of the stimulus AC/A ratio after refractive surgery. After LASIK, no significant change of AC/A ratio between the group wearing glasses before surgery and naked after surgery have been reported [8]. Prakash et al. [4] reported a change in the stimulus AC/A ratio after bilateral LASIK following stabilization a few months after surgery. In the present study, we found an initial increase in the AC/A ratio early in the postoperative period. As previously reported, myopic individuals had a higher AC/A ratio than emmetropic individuals [9]. The residual RE after Artisan PIOL implantation might influence the AC/A ratio during the early postoperative period in this study. The residual RE one week postoperative was greater than the residual RE at postoperative one month, so it might be possible to underestimate the AC/A ratio at one week postoperative.

The stimulus AC/A ratio started to decrease at three months after the surgery, and progressively stabilized. This could be evidence of a stable accommodation-convergence relationship, which was altered after the Artisan PIOL implantation. We speculate the stabilization of the accommodation-convergence relationship would occur within the first three months after surgery.

The relation of AC/A ratio to refractive errors has been previously reported in numerous reports, revealing general increase in myopes. Rosenfield and Gilmartin [10] reported greater accommodative convergence and stimulus AC/A ratios in myopes than in emmetropes. In the younger population, myopic children had been found to have a reduced accommodation, whether measured in the laboratory [11,12] or clinic [13,14], which resulted in a greater AC/A ratio. Without emmetropic control group, these findings were impossible to reveal in this study. The AC/A ratio was increased for the first month, subsequently the AC/A ratio was decreased to the near the preoperative value. These findings might be due to unstable myopic residual refractive error during the initial postoperative period.

There might be possible explanations as to why the AC/A ratio was increased during early postoperative periods. Accommodation and vergence demands vary for ametropes depending on whether their refractive error is corrected with single vision contact lens or spectacle lens [15]. Increased accommodation and greater convergence effort are required when wearing contact lenses in myopic patients [16]. Thus, the AC/A ratio might be increased during the early postoperative period, because myopic patients remove their glasses after Artisan PIOL implantation.

In conclusion, this prospective study revealed a stabilization of the accommodation-convergence relationship after the Artisan PIOL implantation within three months postoperatively. Being newly adapted to emmetropic status, the patient seemed to accommodate more, thus increasing the convergence amplitude and stimulus AC/A ratio until one month postoperatively. However, the AC/A ratio gradually returned to near the preoperative value within six months. Larger studies are recommended to evaluate the difference of the AC/A ratio developed by different surgical techniques to correct myopia. Furthermore, the effect of residual refractive error change on the AC/A ratio would be worthwhile to evaluate.