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
What are accommodative IOLs?
- Special monofocal IOLs which focus light on a SINGLE point
- Exhibits anterior movement to improve near vision using the eye’s accommodative effort
- (+) Hinged haptics: enable movement when the lens capsule dimensions change during accommodation
Are images magnified when using artificial lenses?
Yes.
Contact Lens: 7 - 12% magnified
IOL: 3 - 4% magnified
Lenses CLOSER to the anatomic site of the crystalline lens causes LESS magnification.
What is the effect of IOL location to the resultant refractive power?
PCIOL in bag: IOL inside the capsular bag in the posterior chamber
- anatomic location of crystalline lens
- NO adjustment to power
CAP FAM: Closer Add Plus, Farther Add Minus
Moved ANTERIORLY:
- pulls convergence point forward anterior to the retina = myopia
- ADD Minus (less PLUS) to diverge rays back into the retina
Ciliary Sulcus (PCIOL in sulcus) - decrease power by 0.5 D Anterior Chamber (ACIOL) - decrease power by 3.0 D
Moved POSTERIORLY:
- pushes convergence point backward posterior to the retina = hyperopia
- ADD Plus (less MINUS) to converge rays closer into the retina
Scleral-fixated PCIOL (behind capsule) - increase power by 1.0 D
What is the target correction for IOL implants?
Emmetropia is NOT always the goal.
IOL power should depend on the patients needs.
Target: Myopia
- increase IOL power
- distance vision will suffer (rays will converge anterior to retina)
- near vision is excellent (far point; divergent light rays with MINUS vergence)
1. Pt is previously myopic and is used to near vision
2. Frequent near-vision use (detailed work, focused work, heavy reader)
3. Elderly whose near function is more important for ADL
4. If pt is able to wear spectacles as needed for distance vision
Target: Emmetropia
- near vision will suffer (rays will converge posterior to the retina)
- distance vision is excellent (far point; parallel light rays with ZERO vergence)
1. Frequent distance-vision use (driver)
2. If pt is able to wear reading adds as needed for near vision
