• Monofocal IOLs correct a patient only at distance, where the near vision can only be corrected using positive powered spectacles

 

• Multifocal intraocular lenses (MIOLs)

  • Focus both distance and near images simultaneously

  • They attempt to solve the problem of accommodation in pseudophakic patients, without the use of spectacles

  • These lenses provide at least two different foci on the same axis within the eye, as the IOLs have at least two dioptres of power

 

• Intention is to provide:

  • A full distance visual acuity

  • Increased depth of focus

  • Sufficient near vision acuity

 

• However, there has been controversy over the quality of vision provided by MIOLs 

 

 

 

• Monovision is an alternate approach where the focus of one                                                       monofocal IOL in one eye is set for distance and the other for near.

  •  This design offers some patients with satisfactory vision at both distances

  •  Other patients have found monovision is unacceptable as it rovides only partial binocularity

    • This has led to stereopsis loss in some cases

 

 

DIFFRACTIVE IOLS

 

• Multifocal IOLs adopt the ‘diffraction principle’ where light slows down and changes direction at an edge

• With these lenses, light is diffracted on the posterior surface, where light particles are directed to distance and near focal points 

  • Therefore, providing vision at both distances

 

•  Diffractive multifocal IOLs have a design based on the                                                                         Huygens- Fresnel principle to reduce lens thickness                                                                                     

  • Here, the lens possesses diffractive zones                                                                               (concentric rings) across its posterior surface                                                                                        

  • Light is diffracted at each step and is focused                                                                               onto two different focal points; one for distance                                                                             and the other for near                                                                                                                                     

  • As the pupil aperture increases, further light is                                                                           focused to the distance                                                                                                                         

  • The maximum amount of light that is diffracted is 50% 

    • This value can lessen as the height or number of                                                                       rings is reduced 

 

 

 

 

REFRACTIVE IOLS

 

•  Designed to direct light at varying focal points with the use of concentric zones of different powers              These zones exist in the IOLs optic                                                                                                                   

  • The central region is used for distance and the peripheral for near                                                                                           

  • As the size of the pupil changes, different regions are used

    •  Therefore, the amount of light focused on the distance and                                                      near focal points also change 

    •  Image quality depends on differing pupil sizes  

    • Hence, is pupil dependent 

                                                                            

•  However, these focal points occur on the same axis, thus creating                                                      two superimposed retinal images

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How effective are MIOLs?

 

• Vision improves after implantation of multifocal IOLs for most patients

  • A slight reduction in visual acuity and contrast sensitivity can be expected

 

• With MIOLs, good centration and the absence of tilt are vital

  • This is to prevent astigmatism caused by the implantation

 

• Diffractive IOLs:

  • Effective in ensuring distance and near visual performance

  • However, the dispersion of light caused unwanted symptoms:

    • Glare

    • Haloes

    • Contrast sensitivity reduction

 

• MIOLs cause an increase in spherical aberration

  • This is as a result of the disruption of balance between the cornea and lens

    • Therefore the retinal image quality and vision decreases

 

• The distribution of light through different focal points causes images to be unfocused, causing image confusion, especially at low contrast

 

 

 

ACRYSOF IQ RESTOR MULTIFOCAL IOL (ALCON)

• Designed to provide a wide range of vision

      (distance, intermediate and near), without                                                              

      compromising visual performance.

• This is due to apodisation:

  • Where the height of the diffractive steps

          decrease with increasing eccentricity in the IOL

• ​Hence, providing a smooth transition

 

• With the ReSTOR lens, the diffractive region exists within the optic, but the peripheral region has refractive properties which focuses light towards the focal point for distance

  • This allows more light, as the pupil dilates, directed to the distant focal point, especially under low light levels 

 

 

TECNIS MULTIFOCAL IOL (AMO)

•  Diffractive design only

•  Provides an advanced image quality at all distances

•  The asphericity of the lens optic, similar to that of a                                                                               young crystalline lens, helps to improve the vision 

  •  This allows a sharp focus at both distance                                                                                        and near focal points

  •  Corrects corneal spherical aberration.

  •  However, depth of focus is reduced

 

 

REZOOM MULTIFOCAL IOL 

• Zonal-progressive refractive IOL

• Possesses a range of continual focal points

  •  This enables a controlled transition from                                                                              distance to intermediate to near

• The previous forerunner, Array MIOL caused minor                                                                            visual irregularities such as haloes and glare

• In 2005: ReZoom was introduced

  • Provided an improved visual experience, due to the                                                                   decreased incidence of haloes induced