• In a young eye, the crystalline lens compensates for the positive spherical aberration (SA) induced by the cornea

  • ​This SA remains unchanged throughout life 

 

• With age:

  • ​Lens becomes more positive

  • Total SA of eye increases 

    • ​As a result, the optical quality of eye worsens, reducing contrast sensitivity and resolution

      • ​Therefore, affects functional vision 

 

 

• SPHERICAL IOLs also provide a poor image quality due to an increase in SA induced by the IOL

 

• Method to minimise/avoid this is to progressively decrease the lens power towards the periphery, in the form of an ASPHERIC IOL

 

 

Different aspheric designs have varying amounts of asphericity to resolve SA:

 

 

TECNIS IOL (AMO)​

 

    - One of the first aspheric IOLs to compensate for all the positive SA

   - ​This corrected both the CORNEAL and IOL SA 

   - Corneal topography values show that the average human cornea SA is +0.27µm

    - ​The Tecnis IOL was designed with an altered prolate frontal surface, introducing -0.27µm of SA

   - ​Therefore 'CORRECTING' the aberration

 

 

 

 

 

 

 

 

 

 

 

ACRYSOF IQ 

 

- Leaves the patient with a small amount of SA (-0.2µm)

           - Reduction of SA carried out by the posterior aspheric surface which deals with the effects of light rays            that are over-refracted at the periphery

 - Over-refraction causes patients to have decreased image quality due to a region of defocused light

 

 

 

 

 

 

 

 

 

 

  SOFPORT AO (Bausch & Lomb)

 

  - Described as 'SA neutral’ or ‘SA free’

  - No alteration is made to the corneal SA

   - Achieved with a prolate aspheric curve on either the anterior or posterior surface 

   - Advantageous as it takes into consideration the variations in corneal asphericity between patients 

 

 

 

 

 

 

 

 

 

 

 

• In a comparison between Tecnis and an equivalent spherical design, the Tecnis design showed:

 

​- A better contrast sensitivity at all spatial frequencies

- A reduction in SA of entire eye 

- A better modulation transfer function  

 

• However, showed high image quality degradation in comparison to any other aspherical or spherical designs 

• As a result of mean spherical aberration removal, the Tecnis IOL has provided improvements such as:

  • Enhanced functional vision under mesopic and photopic conditions. 

 

• However, when comparing three aspheric IOLs with their standard spherical counterparts:

  • Although the Tecnis IOL has an increased contrast sensitivity and reduces spherical aberrations, they do not improve functional vision significantly.

 

To correct the actual amount of SA:

- The eyes refractive power and position of the IOLs principle planes should be considered

 - As power varies with eccentricity, IOLs should be well centred and resistant to tilt within the eye.

 

 

• Aberration- free IOLs (e.g. Sofport) have shown less sensitivity to a wide range of displacement and tilt compared to aberration-correcting IOLs (e.g. Tecnis)

 

• Sofport also provides a better image quality 

  • This therefore provides an IOL with a satisfactory compromise between good image quality and the ability to resist decentration and tilt

 

• IOLs such as the Acrysof IQ, which leave the patient with a residual amount of spherical aberration have improved the depth of focus

  • However, generally SA correction causes a decrease in depth of focus