The accommodation amplitude of the human eye usually decreases to less than 1 D after 50 years old, resulting in the inability of eyes to focus near object accurately. This dysfunction is called presbyopia, which makes it impossible for human eyes to obtain clear vision over a continuous object range. Achieving a continuous clear vision is of great importance for pseudophakic presbyopia to improve their living quality. The implantation of intraocular lens (IOL) to replace the crystalline lens is a mature method in clinical practice.
Many attempts of special designed IOLs have been made in this research area to obtain clear vision over a range of object distance. The bifocal IOLs are attempted to provide clear vision in both far and near distance, leaving the intermediate vision uncorrected. The multifocal IOLs are attempted to correct far, near, and intermediate vision simultaneously, yet it fails to provide a continuous clear vision. The accommodative IOLs aims to overcome the defects of bifocal and multifocal IOLs by the axial movements of the special designed haptics or the use of special materials. Unfortunately, it can only provide very limited accommodation.
To regain a continuous range of clear vision for pseudophakic presbyopia, the research group of Yongji Liu from Nankai University proposed a double-zone aspheric diffractive IOL with an extended depth of focus (DOF) of 1.5 D. The modulation transfer function at 50 cycle/mm remained above 0.29 within ± 5° field of view for object distance ranging from 6 m to 0.66 m in photopic vision. In addition, the imaging qualities are robust for pupil changes, different corneal asphericities and polychromatic light, which makes it valuable for real clinical application. The designed IOL was manufactured and the optical performance was evaluated in a continuous range of object distance. The manufactured IOL exhibits the ability to extend DOF. This work has been published in Chinese Optics Letters, Volume 16, No. 9, 2018 (Yayan Bian et al., Design of double-zone aspheric diffractive intraocular lens with extended depth of focus).
"The goal for presbyopia correction is to obtain a continuous clear vision as younger eyes do." says the corresponding author Yongji Liu, "The proposed IOL gives a new possible way to achieve this ultimate goal."
Further work will focus on achieving an IOL with larger range of DOF, good image quality over this distance and easy to be manufactured, because such characters are of great value for clinical practice.
南开大学刘永基课题组提出了一种具有1.5 D焦深的双区域非球面衍射型人工晶状体的设计，可以使得植入人工晶体的老视人眼重新获得连续清晰的视力。理论分析结果表明此款人工晶体在明视觉（3 mm瞳孔）环境下，当物距在0.66～6 m范围内变化时，空间频率为50 cycle/mm处的调制传递函数（MTF）值在±5°视场范围内均高于0.29。在实际临床应用中，大焦深人工晶体的成像性能应尽可能不受人眼瞳孔大小变化、角膜非球面度的影响，同时在白光下成像性能良好。而此款人工晶体均满足以上要求，具有临床应用的价值。该研究团队还对所设计的人工晶体进行了加工检测，测试了此款人工晶体在连续焦深范围内的成像质量。实验结果表明此款人工晶体具有良好的扩展焦深性能。相关研究结果发表于Chinese Optics Letters 2018年第16卷第9期（Yayan Bian et al., Design of double-zone aspheric diffractive intraocular lens with extended depth of focus)。