Day 5 (4): Optics of Intraocular Lenses Flashcards
What happens in an emmetropic phakic eye?
Emmetropia = plano refraction/no refractive error
Phakic lens focuses light exactly on the retina
What happens in an emmetropic eye if phakic lens is removed?
Removal of lens removes converging power of eyes –> hyperopia: light focuses posterior to retina
Need to replace the lost PLUS power to focus light back in front to the retina
Options (pre-1949)
- Aphakic: left with no lens (remains hyperopic)
- Spectacles: high power is needed thus constructed to be very thick –> magnified and peripherally distorted image
- Contact Lens: scleral/corneal
Earliest IOLs were made of what material?
Polymethylmethacrylate (PMMA)
- Harold Ridley
- well-tolerated with minimal inflammation and foreign body reaction
Earliest form of IOL design.
Rigid IOLs
- made of PMMA
- rigid
- biconvex
- form of PCIOL: implanted in the bag
- needed ECCE for implantation
- types:
1. Iris-Fixated
2. ACIOL
2. PCIOL - complications very common: glaucoma, uveitis, lens dislocation
What is ECCE?
Extracapsular Cataract Extraction
- Indication: highly-advanced cataracts which are too dense for phaco or when phaco is contraindicated or not possible
- Cataract is removed in one piece instead of being fragmented within the eye
- Disadvantage: larger incision with slower recovery of wound and visual function
Newest and commonly used IOL design at present times.
Foldable IOLs
- Material: Silicone or Acrylic
- Implantation through small incision
- Paved the way for shift from ECCE to phacoemulsification
- Always implanted in the posterior chamber (PCIOL)
- Types:
1. Monofocal
2. Monofocal Toric
3. Multifocal
4. Multifocal Toric
5. Accommodative
What are Monofocal IOLs?
- Focuses light only at a SINGLE point
- Lens power is limited:
+ Clear at distance: only when entering light rays are parallel with ZERO vergence
+ Poor at near or intermediate: divergent light rays with MINUS vergence will focus BEHIND the retina - Need Reading Adds:
+ Spectacles with thick lenses containing additional PLUS power to counteract diverging light rays at near or intermediate
What are Monofocal Toric IOLs?
- For astigmatic eyes with 2 different meridians and focal lines
- Focuses the Circle of Least Confusion to a SINGLE point in retina
- Prescription is spherocylindrical with an axis (denoted in the IOL with a marker)
- Still, lens power is limited:
+ Clear at distance: only when entering light rays are parallel with ZERO vergence
+ Poor at near or intermediate: divergent light rays with MINUS vergence will focus BEHIND the retina - Need Reading Adds:
+ Spectacles with thick lenses containing additional PLUS power to counteract diverging light rays at near or intermediate
What are Multifocal IOLs?
- Focuses light at MULTIPLE focal points
- Aspheric: lenses whose surface profiles are not portions of a sphere or a cylinder; reduces or eliminates spherical aberrations
- Types:
1. Refractive
2. Diffractive
3. Combination
4. With Toric: if with astigmatism
What are spherical aberrations?
- Type of optical aberration found in optical systems that have elements with spherical surfaces
- Light rays that strike a spherical surface off-center are refracted or reflected more or less than those that strike close to the center.
What are Refractive Multifocal IOLs?
- IOLs with concentric annular zones
- Different focal points for distance, intermediate and near
- Pupil-dependent:
+ Zone of IOL utilized dependent on the light conditions and pupil dilatation
+ Central Zone: bright light conditions, small or constricted pupils
+ Peripheral Zones: dark/low light settings, large or dilated pupils - Types:
1. Bull’s Eye lens - concentric rings of different powers
- center: thicker, higher-powered, for near
- periphery: thinner, lower-powered, for distance
2. Annulus Design - decreasing thickness/power from center to margin
- Zone 1 (center): highest-powered/thickest; DISTANCE vision when pupils are fully-constricted (daytime, bright lights)
- Zone 2 (paracentral): NEAR vision for low to moderate light
- Zone 3 (middle): DISTANCE vision for low to moderate light
- Zone 4 (periphery): NEAR vision in all light conditions
- Zone 5 (margin): lowest-powered/thinnest; DISTANCE vision when pupils are fully-dilated (night, dark)
- Zone transitions: INTERMEDIATE vision
- Glare and haloes can still be an issue
- For whom: need intermediate and distance vision in bright light conditions
1. Day drivers
2. Sport players
3. Computer use: intermediate vision with pupil in mid-dilation
4. Light to moderate readers: near vision zone (zone 2) is only available at mid-dilation and NOT at full constriction
What are Diffractive Multifocal IOLs?
- With GRADUAL diffractive steps of decreasing power that create a smooth transition between focal points
- Bends light to multiple focal points depending on the different lighting situations
- Pupil-independent: vision is optimized across a range of distances, lighting conditions and pupil sizes
- Areas:
1. Central: (+) gradient; best for near vision when pupils constricted
2. Intermediate: (+) gradient; best for near vision when pupils mid-dilated
3. Peripheral: (-) gradient to be able to deliver clear distance vision in low-light with pupils fully-dilated - For whom:
1. Frequent users of near vision (heavy reader, detailed craft work)
2. Regular scotopic activities (movies, night-driving, night workers)
What are Combined Multifocal IOLs?
- With properties of both refractive and diffractive IOLs
+ Distance vision: refractive
+ Intermediate and near vision: diffractive - Elongates focal point with seamless transitions
Describe light distribution of the different IOL types.
Monofocal
- SINGLE focal point only
- 100%: DISTANCE vision
Refractive Multifocal
- MULTIPLE focal points due to annular design with CLEAR-CUT zones
Diffractive Multifocal - MULTIPLE focal points with GRADATED gradual zone transitions except at the PERIPHERY - 41%: NEAR vision - 18%: scattered/in-between - 41%: DISTANCE vision
Summary of comparison between diffractive and refractive multifocal IOLs.
Refractive
- 5 clear-cut zones
- DISTANCE and INTERMEDIATE vision
- pupil-DEPENDENT: utilized zone depends on pupil size
- frequent distance vision use in BRIGHT conditions:
+ day drivers
+ sports players
+ light readers or computer (intermediate) work
Diffractive
- gradual zone transitions
- NEAR vision; DISTANCE vision in LOW-light
- pupil-INdependent: optimized vision across different pupil sizes
- frequent near vision users and distance work in low-light settings
+ heavy readers
+ work requiring attention to details
+ night workers/drivers
