Chapter 7: Surgery for Cataract Flashcards
Who invented the corneal knife used for a cleaner incision?
Albrecht von Graefe- 1828-1870
Who invented intracapsular extraction?
Samuel Sharp - 1753
Who invented phaco and when?
Charles Kerman -1967
Describe retrobulbar anesthesia.
Lidocaine injection into muscle cone via 25G, 1.5 inch (38 mm) blunt retrobulbar needle.
What are the complication of retrobulbar anesthesia?
Retrobulbar hemorrhage, globe penetration, optic nerve trauma, EOM toxicity, inadvertent intravenous injection causing cardiac arrhythmia; inadvertent intradural injection associated with seizures/respiratory arrest/brain stem anesthesia
What is peribulbar anesthesia?
Shorter - 1 inch- 25G or 27G needle used to inject anesthetic external to the muscle cone, underneath Tenon’s capsule
How does peribulbar anesthesia compare with retrobulbar anesthesia?
Slightly less effective; eliminates risk of complications of ON injury/intradural injection
What can be used for supplemented anesthesia during surgery?
Sub-Tenon’s — small posterior incision made through anesthetized conjunctiva and Tenon’s —> small cannula used to administer anesthetic.
What is used for topical anesthesia for cataract surgery?
- propracaine or tetracaine drops
- cellulose pledgets soaked in anesthetic
- lidocaine jelly
- +/- intracameral preservative free 1%/2% lidocaine; IV sedation
What are the disadvantages of topical anesthesia for cataract surgery?
Blepharospasm, lack of amines is, potential patient discomfort
What type of anesthesia can be used in patients with essential or reactive blepharospasm?
Facial nerve block
List and describe the types of facial nerve blocks.
- O’Brien block = directed proximally and peripherally at the nerve trunk
- Van Lint block = directed proximally and peripherally at the terminal branches
- Atkinson block = directed between these two regions
What are the indications for GA for cataract surgery?
Paediatrics patients, dementia, head tremour, deafness, language barrier, MSK disorder (inability to lie flat), restless leg syndrome, claustrophobia
What infectious RFs should be identified and treated before CEIOL?
Coexisting lid disorders, conjunctivitis, blepharitis, hordeolum, chalazion, systemic infections
MOA preoperative antibiotic drops?
Association of pre-op gtts and reduction in ocular surface bacterial counts and lower incidence of positive aqueous cultures after surgery
What do you do pre-op for patients with history of herpetic eye disease?
Pre-op prophylactic antivirals
What is the most important structure to sterilize before surgery?
Fornix
Describe the process of prepping the eye before CEIOL.
- 5% Povidone-iodine solution (not scrub/soap) placed in conjunctival fornix
- prep of skin with 10% povidone-iodine
- draping eyelashes out of operative field
What are some principles during surgery that aid to lower risk of endophthalmitis?
- limit number of times instruments introduced into the eye
- check for signs of lint/cilia/debris on tips of instruments
- minimize intraoperative manipulation
- check for wound closure
What % of cataract surgeries result in bacterial inoculation of AC? How does this fit with endophthalmitis rates?
7-35%
- ability of AC to clear itself
List some intraoperative complications that increase risk of endophthalmitis?
Posterior lens capsule tear
Vitreous loss
Prolonged surgery
Which study describe use of adding antibiotics to irrigating solution or injecting them into AC after CEIOL? Which antibiotic was used?
Endophthalmitis Study Group
- intracameral cefuroxime
Which substances are found in OVD?
Sodium hyaluronate
Chondroitin sulfate
Hydroxypropyl methylcellulose
What is sodium hyaluronate? Where is it found and isolated from? What is its half life in aqueous and vitreous?
- bio polymer occurs in many connective tissues in the body such as synovial fluid and vitreous
- isolated from human umbilical cord and rooster combs
- 1/2 life is 1 day in aqueous and 3 days in vitreous
What is chondroitin sulfate and where is it found?
Sulfated glycosaminoglycan; found in cartilage
What is hydroxypropyl methylcellulose - HPMC? How is it metabolized? How is it eliminated?
- doesn’t occur naturally
- cellulose widely distributed in plant fibres such as cotton and wood —> addition of hydroxypropyl and methyl groups increase its hydrophilic property
- methylcellulose non physiological compound —> not metabolized intraocularly; eventually eliminated in aqueous but can be easily irrigated from the eye
What are the 4 properties of OVDs?
- Viscosity
- Elasticity
- Pseudoplasticity
- Surface tension
What is viscosity?
- Resistance to flow; thinness/thickness of a fluid
- determined by MW and concentration
What does a higher viscosity mean?
Better tissue displacement and staying in place
What is elasticity?
Ability of material to return to original shape after being stressed
What does it mean to be a higher elasticity OVD?
Excellent at space maintaining
What is pseudoplasticity?
Ease with which material can change from being highly viscous at rest to watery at increasing rates of shear stress
What is an example of an everyday pseudoplasticity material?
Toothpaste
What is the use of pseudoplasticity in OVD during CEIOL?
At zero shear force, OVD is lubricant and coats tissues well; when forced thru small gauge cannula it functions as a liquid
What is surface tension?
How surface of fluid tends to stick to another surface
What is coatability?
Inversely proportional to surface tension
What does it mean for an OVD to have low surface tension?
Better at coating tissue, but harder to remove from the eye
What are the 2 categories of OVD?
Cohesive and dispersive
Compare and contrast the properties of cohesive vs dispersive OVDs.
- cohesive: long chain, high MW, high viscosity.
- dispersive: short chain, low MW, low viscosity, low surface tension.
Describe how cohesive OVDs behave in the eye.
- maintain space well at no/low shear rates; at high shear rates they are easily displaced.
- easier to remove from the eye because they stick together and are aspirated in long pieces = like spaghetti
- minimal coatability therefore provide less tissue protection
Describe how dispersive OVDs behave in the eye.
- excellent coating and protection at high shear rates
- more difficult to remove from the eye bc don’t stick together and aspirated in short fragments = like macaroni
Which OVD type is more likely to be retained in the eye, causing angle obstruction/reduced outflow and high IOPs post-op?
Dispersive
List the cohesive OVDs.
- Healon, Healon GV by Abbott
- Amvisc, Amvisc Plus, Provisc by Alcon
List the dispersive OVDs.
- OcuCoat by Bausch and Lomb
- Viscoat by Alcon
- Healon Endocoat by Abbott
What are some combination OVDs? Describe them.
- Discovisc by Alcon —> combines dispersive and cohesive properties
- Healon5 by Abbott —> long, fragile chain with high MW that changes behaviour at different flow rates ==> lower flow rate more viscous/cohesive and higher flow rate, acts like pseudodispersive agent
Which OVD can cause extremely high IOP post-op?
Healon5
List the 3 main functions of OVDs.
- space maintenance ability
- coatability
- optical clarity
Describe the functions of space maintenance ability of OVDs.
- keeps AC formed
- expands AC, can do safe manipulations away from k endothelium and posterior lens capsule
- viscomydriasis
- keeps plane of anterior lens capsule flat for better control of CCC
- makes lens implantation less traumatic to zonular fibres and posterior capsule
Describe the functions of coatability of OVDs.
- protects K endophthalmitis
- vitreous tamponade if posterior capsule tear to prevent vitreous prolapse anteriorly.
Describe the functions of optical clarity of OVDs.
- can place on K surface to maintain clarity, prevents drying of K epithelium
- provides slightly magnified view of anterior segment structures
List the main components of phacoemulsification.
Hand piece
Foot pedal
Irrigation system
Vacuum pump
What is the function of the phaco handpiece?
- likened to jackhammer, vacuum, garden hose.
- simultaneous emulsification and aspiration of the lens while keeping tip cool and maintaining AC depth.
What is the mechanical energy of phaco produced by?
To and fro oscillation generated by pizoelectric crystals in the handpiece
What is the amplitude of the movement of the phaco handpiece?
Stroke length
How is stroke length related to phaco power?
Increased stroke length increases power
Describe how the phaco tip results in nucleofractis.
- as tip moves forward, compression of gas atoms in solution occurs
- as tip moves backward, expansion of gas atoms occurs forming gas bubbles (=cavitation)
- bubbles subject to same compression and expansion
- when bubbles implode —> release heat and shock waves —> disassembles lens nucleus
- nonaxial vibrations (torsional or elliptical) can further augment mechanical breakdown of nucleus
What are the 3 positions of the foot pedal?
- position 1: irrigation
- position 2: aspiration at a constant or variable rate
- position 3: phaco power at fixed or variable level
What happens with fixed versus variable foot pedal position 3?
- fixed: power level may be set from 0 to 100%, and chosen power delivered immediately when foot pedal depressed to position 3
- linear ultrasound: surgeon controls amount of phaco power delivered by varying depth of depression of foot while it is in position 3
Define cavitation.
- formation of gas bubbles arising from aqueous in response to pressure changes at phaco tip.
- bubbles expand and contract, and implosion of bubbles result in emulsification of lens material
What is more efficient wrt cavitation: continuous ultrasound versus intermittent?
Intermittent
Define chatter.
- when ultrasonic stroke overcomes vacuum (“holding power”) —> causes nuclear fragments to repel until vacuum reaches levels to neutralize tip’s repulsive energy —> once again attracts material
What does chatter inhibit?
Followability
What can be done to diminish chatter? How?
Reduction in phaco power —> decreases stroke length of tip excursion, thereby reduces forces that push the fragment away from the tip
Define energy.
Energy = Power x time
How do you decrease energy?
Decreasing either phaco power or length of time that phaco power is on
Define frequency.
Speed at which phaco needle moves back and forth
What does ultrasonic frequency refer to?
Above range of human audibility; >20,000 Hz
What is the frequency of the phaco handpiece?
27,000-60,000 Hz
What is a pizoelectric crystal?
Type of transducer used in handpiece that transforms electrical energy into mechanical energy
How is linear motion generated at the phaco tip?
When a tuned, highly refined crystal is deformed by the electrical energy supplied by the console
Define power.
The ability of the phaco tip to vibrate and cavitation the adjacent lens material
How is phaco power noted?
Percentage of maximum stroke length of the phaco needle
How is phaco power generated?
Foot position 3
Define stroke.
Linear distance that the tip traverses to produce impact on lens material
What is the stroke length in phaco?
0.05 to 0.10 mm
What happens with chatter?
Phaco tip pushes nuclear fragments away even as the aspiration attracts them
What can heat buildup from the delivery of phaco power cause?
Wound burns and damage to the corneal endothelium
What factors can be altered to increase cutting efficiency?
Size and angle of phaco tip; modes of intermittent rather than continuous phaco such as pulse and burst
What can be used to minimize heat generation with phaco?
Torsional and elliptical movement of the phaco tip
What are the available bevels of phaco tips?
0, 15, 30, 45, 60 degrees
What are the available end configurations of phaco tips?
Round, ellipsis, bent, flared
What are the advantages and disadvantages of steeper bevels?
- steep and oval port: larger surface area with greater holding force and greater cutting efficiency
- disadvantages: more difficult to occlude to achieve full vacuum
What is pulsed phaco? What is “pulse”?
Setting the number of pulses per second while in position 3
- pulse = interval of phaco power turned on alternating with interval in which phaco power is turned off.
- amount of phaco delivered depends on foot pedal excursion in position 3
What is the advantage of a pulsed phaco, where there is delivery of phaco power for only a portion of the cycle?
Reduces repulsion of material by the vibrating tip and improves followability
What is a duty cycle?
In pulsed phaco, the ratio of on:off pulses
What is the duty cycle if phaco power on equals the time of phaco power off?
50%
What is burst mode phaco?
Delivery of preset power (0-100%) in single bursts that are separated by decreasing intervals as foot pedal is depressed through position 3 —> at end of position 3 excursion, phaco power is no longer delivered in bursts, but it is continuous
What is the advantage of burst mode phaco?
Allows phaco needle to be buried into lens, an essential step in chopping techniques
What is torsional phaco and its advantages? Name an example.
Pizoelectric crystals of phaco handpiece produces an oscillatory (torsional) movement which is amplified by use of a bent Kelman phaco tip —> greater side to side movement at tip allows for greater shearing forces to assist nucleus disassembly
- Ozil Torsional by Alcon
What system utilizes a combination of transverse and longitudinal phaco modalities?
Elliptical cutting - enhances nucleus emulsification
- Ellips by Abbott
Where is the irrigation of phaco?
Thru sleeve around U/S tip with some egress of fluid thru incisions
What is the function of phaco irrigation?
- Coaxial irrigation with balanced salt solution cools phaco tip, preventing heat buildup and consequent damage to adjacent tissue
- maintenance of AC
How does the surgeon adjust IOP and and AC depth?
Changing height of irrigation bottle, with gravity increasing the force generated to increase IOP
- newer machine have collapsible saline bag which is compressed by pressure plates - sensors on plates provide continuous feedback allowing for active control fluidics and resulting in a more stable AC
What can be added to irrigation solutions during surgery?
- pupil dilators
- antibiotics
