5. Fluidics, Fast Cutting, and Wound Construction
Steve T. Charles (Memphis)
Wound Leak Issues
Factors in Wound Leaks
• Thin Sclera (myopia, Marfan’s, collagen disorders)
Wound Leak Prevention Tactics
• Angulated Wound Construction (I have used on all cases since 9/06 because of Kupperman burst pressure
data)
• Fluid-Air Exchange (Charles, usually unnecessary with angulated wound)
• Leave Vitreous in Wound (DeJuan)
– Increases Peripheral Vitreoretinal Traction & Post-Operative Retinal Breaks
– – Increased Risk of Endophthalmitis from Vitreous Wick Angulated Wound Technique
Insert Trocar-Cannula Limbus Parallel, Bevel Up, Near Tangential to Sclera Until Cannula in Contact with Conjunctiva; Then Continue
Insertion Perpendicular to Sclera
Remove Cannulas in Near Tangential Orientation While Applying Pressure on Wound with Back of Smooth Forceps (like withdrawing needle
from vein)
IOP at >20 mmHg During Removal (leave infusion on)
Eliminates Need for FAX & Sutures
Rationale of Fluid-Air Exchange
Air Reduces or Eliminates Wound Leaks Leading to Hypotony & Vitreous Wicks (endophthalmitis)
Air Allows Visualization of Leaks Via Air Under Conjunctiva
Air Prevents Sub-Conjunctival Antibiotics from Entering Eye Causing Retinal Toxicity
Use >2/3 Fill, Not 1/3 Fill as I Previously Reported, So Inferotemporal Sclerotomy Will Be Covered When Patient Seated or Standing
Air Bubble Obstructs Vision for 1-3 Days (problem for patients with poor vision in other eye)
Suture Sclerotomies Thru Conjunctiva With 8-0 Biosorb If Concern About Leak & Poor Vision in Other Eye Prohibiting Air Bubble
Sub-Conjunctival Antibiotics
Sub-Conjunctival ABs Should Not be Omitted (topical ABs do not produce MIC drug levels in vitreous cavity of phakic or IOL eye, cataract
intervention is in A/C, PPV intervention is in vitreous cavity)
Sub-Conjunctival AB Injection at the End of the Case Pressurizes the Sub-Conjunctival Space & Can Cause Antibiotics to Enter the Eye &
Cause Retinal Toxicity
• ABs Should Be Injected Slowly at the Maximum Possible Distance From the Sclerotomies (inferior cul de sac)
Piston & Pressure Difference Effects
Tool Withdrawal from Cannulas Acts Like Pulling Back on Syringe & Produces Acute Vitreoretinal Traction
Withdraw Tools from Cannulas Very Slowly to Reduce Acute Vitreoretinal Traction
Support Eye Wall by Applying Pressure to Wound While Removing Cannulas to Avoid IOP Increase and Wound Eversion Which Can Cause
Vitreous Wick
Fluidics & Fast Cutting
“Fast Cutting”
Is Fast Cutting Better for All Cases & All Tasks? Yes
Does It Cut Vitreous Better? No
• Why? Because the Frequency (cuts/minute), Not the Velocity (mm/sec) is Higher
Is “Fast Cutting” More Efficient (higher vitreous/infusion fluid ratio) No
• Removal Rate is Technique & Flow Rate Driven
Why “Fast Cutting” Is Better Even Though It Does Not Cut Vitreous Better & Is Not More Efficient is Explained on Upcoming Slides
Type of Cutting
Sharpness is High Pressure/Area Achieved By Thin Section (diamond knife), Vitreous Cutter Is Not Sharp, Has Square Edges
Inertial Cutting – Based on High Velocity of Blade and Tissue Inertia, Inertial Cutting is Phaco Mode of Action, Inertial Cutting Much Less
Significant Factor in Vitrectomy Cutters Than Shear
Shear is Accomplished by Having Two Rigid, Squared-Off Edges Moving Past Each Other, Shear is Vitreous Cutter Primary Mode of Cutting
Cutter “Speed”
Cutter Frequency is Measured in cuts/minute
• Flow Rate & Cutting Frequency Determines Average Vitreous Fiber Travel Between Cuts (average effluent fiber length), Therefore Amount
of Vitreoretinal Traction
• Valve Dynamics, Drive Electronics, and Cutter Friction & Moving Mass
Determine Frequency
Cutter Velocity is Measured in mm/sec
– Velocity at Closure is the Only Measurement Relative to Inertial Cutting, But Inertial Cutting Probably Has Little Relevance to Vitreous Cutters,
– Actuation Pressure, Cutter Friction, Moving Mass, and Spring Rate (except InnoVit) Determine Velocity
Role of Technique in Rate of Vitreous Removal
Old Way: Keep Probe Away from Vitreous and Use High Flow Rate (low cutting rates) to Pull Vitreous to Port
Was Necessary with Older, Low Cut Rate Cutters & Very Slow Fluidics
Perceived as Safer by Some Surgeons But Greatly Increases Vitreoretinal Traction
Better Way: Always Use Highest Cutting Rate, Move Port to Vitreous, Always Advance Port While Cutting (never pull back), and Increase
Proportional Vacuum Until Vitreous Removal Rate is Sufficient, Decrease Cutting Rate Only If Insufficient Removal Rate with 600mmHg
Vacuum
Pulse Flow 25 vs. 20 Gauge
Fluid Resistance
Pressure Difference (P) = Flow (F) x Resistance (R)
Directly Analogous to Electrical E = I x R (Ohm’s Law)
“R” is Resistance to Flow
• Proportional to Lumen Length (tubing is 84 inches long)
• Proportional to 4th Power of Radius
• Resistance to Deformation & Viscosity of Fluid/Tissue Varies Over Very Large Range
• – Air<PFO<BSS<Vitreous<Blood<ERM<Nucleus
“P” is Pressure (voltage, cause)
“F” is Flow (current, effect)
Resistance Issues
Fluidic Resistance From High Cutting Rates & Smaller Cutter Lumen is an Advantage
Infusion System Resistance is a Disadvantage
• Causes Up to 25mmHg Difference Between Infusion Pressure & IOP When Flow (dynamic state)
• VGFI Pressure = IOP Only When No Flow (static state)
• Infusion Pressure Must Be Increased During Flow Using Accurus Foot Pedal VGFI Control (highest flow is with fragmenter & straight extrusion
cannula)
Parameters That Affect
Pulse Flow/Flow Rate
• Cutting Rate
• Port Size
• Duty Cycle
• Lumen Size
• Vacuum
Flow Limiting vs. Flow Control
Port-Based Flow Limiting is Instantaneous
Console-Based Flow Control is Much Slower
• Delay Components, If Surgeon Foot Pedal Control
• – Predictive Cognitive Processing & Neuromuscular Delay
• – Processing and Valve Actuation/Cassette Volume Delay
• – One Way Transmission of Fluidic Response Down 84 inches of Tubing
• Delay Components, If Console Control
• – One Way Transmission of Fluidic Signal Up 84 inches of Elastic Tubing
• – Processing and Valve Actuation/Cassette Volume Delay
• – One Way Transmission of Fluidic Response Down 84 inches of Elastic Tubing
Duty Cycle Issues
Alcon Pneumatic Actuation Produces a Variable Duty Cycle (port open/port closed time dependent on cutting rate) While B&L Electric Cutter
Has Fixed 50 % Duty Cycle.
• Variable Duty Cycle Better Than Fixed Because of the Huge Difference in Viscosity or Resistance to Mechanical Deformation for Air,
PFO, BSS, Vitreous, Hemorrhage, Silicone Oil, Dense ERM/Scar, Sclerotic Lens Nucleus (lowest to highest resistance)
Port Closer to the Tip
Steps for All Cases & All Tasks
• Start with Highest Cutting Rate
• Move Port To Vitreous, Never Suck Vitreous To Port or Withdraw Cutter While Cutting
• Increase Proportional Vacuum Until Vitreous/Tissue Removal Begins
• Decrease Cutting Rate Only If Insufficient Vitreous/Tissue Removal at Max Vacuum