Our study showed that the V4c Visian ICL had comparable objective accommodation compared to LASIK surgery for myopic correction, within similar AMP and accommodative lag outcomes. The HFC increased significantly after 3 months of ICL implantation, which represent increased tension of ciliary muscles after the ICL operation, whereas there were no HFC changes after LASIK surgery. In addition, the vault of the ICL was correlated to postoperative HFC in the ICL group.

Based on previous study [16], the accommodation of a near target in a myopic patient is less than an emmetrope, which could lead to lower activity of the ciliary muscles and longer preservation of accommodation. Thus, after refractive correction, AMP or accommodative function may be improved. However, due to the dysfunction of the ciliary body, zonular fibers, or lens, this improvement may be not observed in patients over 30 years [16]. Moreover, as shown in Prakash’s study [17], although there was an improvement in accommodation in the early period after LAISK, the accommodation could stabilize and approximate the preoperative state at 3 months after the operation. These studies also showed that the AMP could be similar to the preoperative baseline after 3 months of LASIK. In our study, the AMP and the accommodative lag after LASIK may have no identical changes, which was agreement with previous work [4]. In the previous study, Liu et al. [4] also found that there were no changes in the AMP and HFC after LASIK surgery, and they suggested that the LASIK produced no significant effect on accommodation.

Generally, ICL has been considered to maintain the accommodation of eyes because its anterior vault and uncontacted crystalline lens design [18]. Meanwhile, the optic of the ICL needs to be secured in the ciliary sulcus, and there is a possibility that the ICL lens or its footplates may influence the ciliary muscle or tissues around the sulcus. Sheng et al. [19] found that as in the non-accommodative state, more than 53.7% of eyes with footplates rested outside the ciliary sulcus using ultrasound bio-microscopy. They also found that when in an accommodative state, the position of the ciliary-sulcus outside-resting footplate moved closer to the ciliary body or even the zonules. As shown in publications from Kamiya et al. [12], a transient decline of the accommodative amplitude in the early period after ICL implantation was found, and they hypothesized that due to impaction from ICL fixation on the ciliary muscles, the ICL may cause transient dysfunction of the ciliary muscles even if the crystalline lens remained untouched. They did not analyze and evaluate the function of the ciliary muscles directly.

Compared to previous studies on ICL, there was a contradictory outcome related to subjective accommodative function. As shown in Tang’s [20] and Kamiya’s [12] publish, there was a decline of AMP after ICL implantation, while Cheng et al. [11] found that 1 month after ICL surgery, the accommodative function was significantly enhanced, resulting in an increase in AMP, near point convergence, and accommodation facility. Our outcomes showed that the objective amplitude of accommodation had no significant changes after ICL implantation. This discrepancy may be related to different refractive characteristics. As showed in Wan’s study [21], there were different recovery reactions in terms of accommodation when treating myopia with different degrees after ICL surgery. Patients with high myopia may display more obviously accommodative changes after ICL correction. As shown in our study, only eyes with myopia ≤ − 8.5 D were recruited, hence the accommodative function may cause a non-significant change after ICL implantation. Moreover, during the accommodative reaction in the eye with the ICL lens, except when the lens power changes for a specific distance, there were other biometric changes (e.g., in the vault and pupil size) occurring [22, 23]. The power of the eye may be different from expected if the optic eye system cannot remain static [24].

The MFs reflect the influence of the constraints set by the physiological components of the basic mechanism of accommodation; while, it is still unclear what role the MFs play in accommodation. However, one aspect seems clear: the HFC elements of the MFs are not under neurological control and were less dependent on the stimuli conditions (i.e., pupil diameter) [25]. Previous research demonstrated that there was only a certain correlation between HOA and LFC, while, not apparent in the HFC [26]. As shown in our study, there were no significant correlations between HFC and the change of ocular total HOA in either the ICL or the LASIK group.

In a state with ciliary muscle tension, a small accommodative stimulus could cause a large fluctuation and lead to an increased HFC [27]. It should be noted that the HFC may be used to reflect the function of the ciliary muscle. Our study found that there were significant differences in the objective MFs outcome between the ICL and LASIK surgery. This indicated that the HFC of the MFs increased significantly; whereas, there was no change after LASIK surgery. The impaction from ICL fixation may cause transient dysfunction of the ciliary muscles compared to well-balanced LASIK for myopic correction. We further indicated that the ICL lens may produce a reversed force to the ciliary body upon which it was rested and this may increase the tension of the ciliary muscle, resulting in increased HFC. Meanwhile, due to the soft character of the material and the appropriate vault of the ICL lens, the morphology change of the crystalline lens during the maximum and minimum accommodative states would not be impacted, resulting in unchanged AMP. Our study found that there was a slightly positive correlation between HFC and vault; that is, the higher the vault was, the greater the HFC was. This is in accordance with the suggestion above: that ICL with a higher vault may produce a greater reversed force to the ciliary body, which may cause increased HFC compared to ICL with a low vault. In eyes with preoperative MRSE of less than − 8.50 D, there were no significant difference between ICL (− 0.11 D) and LASIK (− 0.09 D) at 3 months after operation in our study, and this refractive outcomes were simialr as previous study [28]. Moreover, in our study, we had found no relationship between MRSE and HFC in both ICL and LASIK groups. Previous literatures show that the degree of myopia is positively correlated with the amount of accommodative HFC [27]. While the effects were influenced by the age and myopic-progression status [25]. In our study, only patietnts with stable myopic progression statues, and their age were older than 18 years were recruited. Then, the HFC may had no significantly changing in LASIK group.

Based on a study within 4 years follow-up, the lense thickness would gradually increase after 12 months of ICL implantation and turn less remarkbale, and they also found that there were a negative raletionship between vault and lens thickness [29]. Richdale et al. foud that accommodation were correlated with lens thickness [30]. So in a long follow-up, the lens rise could be a reason that affacted accommodation after ICL operation.

There were some limitations in our study. First, we observed accommodation and micro-fluctuation only within 3 months follow-up postoperatively. Over this time period, there will be some adaptions. As shown by Kamiya et al. [12], the postoperative accommodative function was impaired in early follow-up after surgery, then recovered gradually. Thus, studies with a longer follow-up period are needed in future work. Second, in this study, we only assessed the MFs outcomes, which reflected the function of the ciliary muscles. A greater value of the HFC of accommodative micro-fluctuations was associated with thinner ciliary bodies using optical coherence tomography [27]. Thus, more measurements containing ciliary muscles or ciliary zonules would be helpful to validate and clarify the structural differences under accommodative reaction in our future work. In addition, we did not objectively evaluate visual discomfort in the patients, and this discomfort when near-working might be negligible in some patients. It would be better if visual discomfort was evaluated with a questionnaire in future work [31].

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