26.1 Pars Plana Vitrectomy (PPV), Scleral Buckling & Intraocular Tamponades (Gas/Silicone Oil)

Pars Plana Vitrectomy (PPV), Scleral Buckling & Intraocular Tamponades (Gas/Silicone Oil)

Core Clinical Mandate: Vitreoretinal surgery demands an exact command of micro-architectural ocular anatomy, high-speed cutting fluidics, and the physical chemistry of intraocular tamponades. Ophthalmic medical technologists must master the anatomical landmarks of the pars plana, the mechanics of small-gauge vitrectomy, the biomaterials of scleral buckling, and the life-threatening and vision-threatening hazards associated with intraocular gas expansion.

Surgical Anatomy of the Pars Plana & Sclerotomy Placement

The pars plana (also termed the orbiculus ciliaris) represents the posterior, flattened zone of the ciliary body. Anatomically, it extends from the posterior margin of the ciliary processes (corona ciliaris) approximately 2.0 mm posterior to the corneoscleral limbus back to the ora serrata, which marks the anterior termination of the neurosensory retina (located approximately 6.0 mm posterior to the limbus nasally and 7.0 mm temporally).

Because the pars plana is composed of collagenous connective tissue, smooth muscle fibers, and double-layered non-pigmented/pigmented ciliary epithelium without overlying neurosensory retina or major radial blood vessels, it provides the safest anatomical corridor for entering the vitreous cavity without inducing retinal tears, dialysis, or intraocular hemorrhage.

Anterior-to-Posterior Surgical Landmarks at the Limbus:
[Corneoscleral Limbus] ──► 2.0 mm: Corona Ciliaris (Ciliary Processes)
                       ──► 3.0 to 4.0 mm: PARS PLANA (Surgical Safe Zone)
                       ──► 6.0 mm (Nasal) / 7.0 mm (Temporal): ORA SERRATA (Retina Begins)

Strict Sclerotomy Distance Landmarking

The distance from the surgical limbus to the entry sclerotomy is measured using a surgical Castroviejo caliper perpendicular to the limbus:

1. Phakic Eyes: 3.5 mm to 4.0 mm Posterior to the Limbus
   • Anatomical Rationale: In a phakic eye, the crystalline lens has an equatorial diameter of approximately 9.0 to 10.0 mm. Entering closer than 3.5 mm places micro-instruments dangerously close to the lens equator. Even microscopic mechanical touch of the crystalline lens capsule induces immediate irreversible focal cataractogenesis or capsular rupture. Placing ports at 3.5–4.0 mm ensures entry through the mid-to-posterior pars plana safely behind the lens equator while remaining safely anterior to the ora serrata.
2. Pseudophakic or Aphakic Eyes: 3.0 mm to 3.5 mm Posterior to the Limbus
   • Anatomical Rationale: Because the native crystalline lens has been removed, the anterior vitreous space is expanded and the risk of crystalline lens strike is eliminated. Sclerotomies are shifted anteriorly to 3.0–3.5 mm to maximize posterior clearance from the vitreous base and ora serrata, reducing the risk of peripheral retinal dialysis or anterior retinal tears during instrument manipulation.

The Standard Three-Port Vitrectomy Geometry

Traditional and modern pars plana vitrectomy utilizes three distinct transscleral entry ports arranged in an arc:

• Inferotemporal Port (Infusion Cannula): Positioned 3.0 to 4.0 mm posterior to the limbus in the inferotemporal quadrant. This quadrant keeps the flexible infusion tubing clear of the surgeon's hands and instruments, which operate from superior and temporal positions. Mandatory Safety Check: Before initiating active fluid infusion, the surgeon and technologist must directly visualize the cannula tip through the pupil with an indirect ophthalmoscope or surgical microscope to confirm it has fully penetrated the non-pigmented ciliary epithelium into the vitreous cavity. Infusing fluid under the ciliary epithelium or into the suprachoroidal space causes massive bullous choroidal detachment or subretinal infusion disaster.
• Superotemporal Port (Active Instrument Port): Accommodates the dominant hand instruments (vitreous cutter, micro-scissors, subretinal cannulas, extrusion needle, intraocular forceps).
• Superonasal Port (Illumination / Bimanual Port): Accommodates the endo-illuminator (light pipe) or secondary instrument (membrane pick, illumination chandelier).

Small-Gauge Vitrectomy Instrumentation & High-Speed Fluidics

The evolution of vitreoretinal surgery has transitioned from legacy large-bore 20-gauge systems to minimally invasive, transconjunctival sutureless vitrectomy systems (MIVS).

Gauge Caliber and Technical Characteristics

Trocar-Cannula Insertion Dynamics

Modern 23G, 25G, and 27G vitrectomy utilizes micro-trocar blade and cannula assemblies:

• Displacement of Conjunctiva: The conjunctiva is manually displaced over the underlying sclera with pressure calipers or toothed forceps prior to insertion. This creates a non-aligning "step-incision" between the conjunctival wound and the scleral entry once the cannula is removed, preventing direct fistulous tract formation and post-operative endophthalmitis.
• Bi-Planar / Angled Insertion: The trocar blade enters at an acute angle of approximately 30° parallel to the limbus, tunneling lamellarly through scleral collagen for 1.0 mm before turning perpendicular (90°) toward the center of the vitreous cavity. This produces a self-sealing, valved scleral flap that seals autonomously under internal intraocular pressure upon cannula removal.
• Valved Cannulas: Cannula hubs incorporate integrated silicone elastomeric slit valves. These prevent passive fluid egress and chamber collapse during instrument exchanges, maintaining constant intraoperative IOP and preventing retinal incarceration.

Cutting Mechanics, Traction Physics, and Dual-Blade Cutters

In vitreous surgery, traction on the mobile neurosensory retina is governed by fluidic flow resistance and the volume of vitreous gel drawn into the cutter port per cut cycle:

$\text{Bite Volume} = \frac{\text{Aspiration Flow Rate (mL/min)}}{\text{Cut Rate (cuts/min)}}$

• Traction Dynamics: Vitreous gel is an elastic, non-Newtonian viscoelastic polymer composed of type II collagen fibrils interwoven with hyaluronic acid coils. When a cutter operates at low cut rates (e.g., 1,500 cpm), a large "bite" of gel enters the port before being sheared. As this large bolus is sucked into the port, tensile forces are transmitted along connected collagen fibers directly to the peripheral vitreous base, causing secondary retinal tears.
• High Cut Rates (10,000 to 20,000 cpm): By dramatically accelerating cut speed, the duration of port opening per cycle decreases. Consequently, the vitreous bite volume is microscopic, shearing collagen fibrils into tiny fragments without dragging downstream gel. This reduces pulse-induced vitreoretinal traction to near zero.
• Dual-Blade / Twin Duty Cycle (TDC) Cutters: Conventional cutters utilize a single guillotine blade that cuts only during the forward stroke. Dual-blade cutters feature two cutting edges operating within an oscillating sleeve, cutting on both the forward and backward movements. This achieves an effective cut rate double the pneumatic drive frequency (up to 20,000 cpm) while maintaining a 100% open-port duty cycle, preventing fluidic surges.

Intraoperative Dyes & Vitreous Visualization

• Triamcinolone Acetonide (Triesence / Kenalog): Preservative-free synthetic corticosteroid micro-crystals. When injected into the vitreous cavity, hydrophobic triamcinolone crystals electrostatically bind to transparent cortical vitreous fibrils, turning the transparent hyaloid into an opaque white scaffold. This enables clear visualization of the posterior hyaloid face and ensures complete separation during posterior vitreous detachment (PVD) induction.
• Indocyanine Green (ICG): Binds selectively to the collagen type IV of the internal limiting membrane (ILM), staining it dark green to facilitate macular hole and macular pucker peeling. Clinical Caveat: ICG exhibits potential retinal phototoxicity and RPE toxicity if left in contact under intense illumination.
• Brilliant Blue G (BBG) & MembraneBlue-Dual: High-affinity triphenylmethane vital dye that selectively stains the ILM bright blue with significantly lower retinal toxicity than ICG. Often combined with trypan blue (which stains epiretinal membranes and scarred collagen violet-blue) to differentiate ERM from native ILM.

Scleral Buckling Mechanics, Materials & Surgical Placement

Scleral buckling remains a foundational surgical modality for rhegmatogenous retinal detachments (RRD), particularly in young phakic patients, pediatric detachments, dialyses, and inferior retinal breaks where vitreous tamponade gases are less effective.

Biomechanical Principles of the Scleral Buckle

A scleral buckle is an exoplant secured to the external scleral surface that mechanically indents the eyewall inward toward the detached retina. This inward indentation achieves three critical therapeutic effects:

1. Relief of Vitreomacular and Vitreobasal Traction: By indenting the sclera and choroid toward the vitreous base, dynamic traction exerted by contracting vitreous strands on the edges of retinal tears is neutralized.
2. Apposition of RPE and Neurosensory Retina: Mechanical indentation physically approximates the underlying retinal pigment epithelium to the sensory retinal flap, facilitating cellular adhesion.
3. Alteration of Intraocular Fluid Dynamics: Indentation changes ocular fluid currents during saccadic eye movements, preventing fluid currents from driving under the tear margins into the subretinal space.

Scleral Buckling Materials & Profiles

Scleral Buckling Exoplant Configurations:
[Band #240 (2.5 mm flat solid)] ──► 360° Encircling Cerclage
[Tire #276 / #287 (Asymmetric grooved)] ──► Broad Quadrantic / Equatorial Indentation
[Sponge #506 (3.0 mm) / #507 (5.0 mm)] ──► Segmental / Radial Break Support
[Watzke Sleeve] ──► Friction Locking Collar for Encircling Bands

• Solid Silicone Rubber Elements:
  • Bands (e.g., Style #40, #240): Smooth, solid silicone strips (2.0 to 2.5 mm wide) used for 360° encircling cerclage. Bands provide uniform, mild equatorial indentation to support multiple peripheral tears or counteract vitreous base contraction.
  • Tires (e.g., Style #276, #287): Wide (7.0 to 10.0 mm), contoured solid silicone exoplants. The internal surface features a concave channel that seats an encircling band (#240), while the outer surface is convex or asymmetric to match scleral curvature. Tires generate broad, smooth, high-profile buckles that support large horseshoe tears, dialyses, or anterior PVR.
• Silicone Sponges (e.g., Style #506 [3 mm], #507 [5 mm]):
  • Flexible, closed-cell porous silicone cylinders or half-cylinders. The closed-cell structure prevents fluid absorption into the sponge matrix. Sponges can be placed segmentally (oriented circumferentially or radially beneath an isolated break) without requiring a full 360° encircling band.
  • Radial Orientation: Preferred for large horseshoe tears because radial sponges support the entire length of the tear without "fish-mouthing" (the phenomenon where circumferential buckles cause posterior radial folding of tear edges, allowing persistent fluid leak).
• Watzke Silicone Sleeve: A small, hollow silicone collar through which the two overlapping ends of an encircling band are threaded. The sleeve friction-locks the band ends together, allowing the surgeon to adjust circumferential tension and calibrate the buckle height.

Suture Fixation and Depth Calibration

Exoplants are secured to the sclera using non-absorbable synthetic sutures—typically 5-0 polyester (Mersilene or Dacron) or 5-0 nylon mounted on spatulated micro-needles:

• Mattress Suture Configuration: Sutures are placed as horizontal mattress loops spanning the buckle element. The suture bites must be placed approximately 1.5 to 2.0 times the width of the exoplant to ensure effective inward displacement upon knot tying.
• Scleral Depth (50% Thickness): Scleral thickness varies widely (from 1.0 mm at the posterior pole to 0.3 mm just behind the rectus muscle insertions). Suture passes must penetrate precisely 50% of the scleral depth. Entering too shallowly causes suture cheese-wiring and buckle extrusion; entering too deeply perforates into the vascular suprachoroidal space or retina, causing catastrophic choroidal hemorrhage, retinal puncture, or vitreous loss.

Intraocular Tamponades: Gases vs. Silicone Oil

Intraocular tamponades provide surface tension and buoyancy to seal retinal breaks from the inside of the eye, preventing fluid from passing into the subretinal space while chorioretinal adhesions (cryotherapy or laser photocoagulation scars) mature (which requires 10 to 14 days).

Physical Properties and Expansion Kinetics of Intraocular Gases

Intraocular gases function via two distinct physical principles: surface tension (interfacial surface tension between water and gas is ~73 mN/m, preventing the bubble from escaping through microscopic retinal breaks) and buoyancy (specific gravity of gas is ~0.001 g/cm³, exerting an upward buoyant force that flattens superior and posterior breaks against the RPE).

Following intraocular injection of pure gas, nitrogen and oxygen from surrounding capillary blood diffuse down their concentration gradients into the gas bubble far faster than the large, insoluble fluorocarbon molecules can diffuse out. Consequently, the bubble expands dramatically until an osmotic equilibrium is established with circulating blood gases.

Formulation Rule: When non-expansile gas is required (e.g., following complete fluid-air exchange in PPV), the assistant or technologist prepares a 20% SF₆ mixture (e.g., 10 mL pure SF₆ mixed with 40 mL sterile air in a 50 mL syringe) or a 14% C₃F₈ mixture (7 mL pure C₃F₈ mixed with 43 mL sterile air). Injecting pure (100%) gas without fluid egress leads to explosive intraocular hypertension.

Silicone Oil Tamponade (Polydimethylsiloxane)

When long-term intraocular tamponade is required—such as in proliferative vitreoretinopathy (PVR), viral necrotizing retinitis (CMV retinitis, ARN), traumatic retinal breaks, or in patients unable to position post-operatively or who must fly—silicone oil is the agent of choice.

• Physical Properties: Chemically inert polymer of polydimethylsiloxane. Specific gravity is 0.97 g/cm³ (slightly lighter than water/aqueous; floats to the top of the vitreous cavity to tamponade superior breaks). Interfacial surface tension against water is approximately 40 mN/m (lower than gas [73 mN/m], meaning silicone oil can squeeze through larger retinal breaks more easily than gas).
• Viscosity Formulations: 1,000 cSt vs. 5,000 cSt:
  • 1,000 Centistokes (cSt): Lower viscosity. Easier and faster to inject and aspirate through small-gauge (23G/25G) cannulas. However, lower molecular cohesion leads to a higher rate of droplet emulsification over time (breakdown into tiny sub-droplets that migrate into the anterior chamber, causing "inverse hypopyon" or secondary open-angle glaucoma from trabecular meshwork clogging).
  • 5,000 Centistokes (cSt): Higher viscosity, larger polymer chains. Requires high-pressure automated viscous fluid injection systems to inject and aspirate. However, it exhibits a significantly lower rate of emulsification, making it ideal for chronic, long-term tamponade (months to years).
• Surgical Removal: Unlike intraocular gases, silicone oil is non-absorbable. It must be surgically removed via secondary pars plana vitrectomy, typically 3 to 6 months post-operatively once the retina is permanently attached, to prevent corneal endothelial decompensation, oil-induced glaucoma, and band keratopathy.
• Heavy Silicone Oil (e.g., Densiron, Oxane HD): Formulated by blending silicone oil with fluorinated alkanes, increasing specific gravity to >1.02 g/cm³ (heavier than water). This enables the oil to sink to the bottom of the vitreous cavity, providing direct tamponade for recalcitrant inferior retinal breaks and inferior PVR without requiring prolonged inverted prone positioning.

Post-Operative Positioning & Critical Systemic Contraindications

Strict Post-Operative Head Positioning

Because intraocular gas and standard silicone oil float upward due to buoyancy, the patient's head must be positioned so that the buoyant bubble directly presses against the specific anatomical quadrant of the retinal break:

• Prone / Face-Down Positioning: Mandatory for macular hole repair and posterior pole retinal detachments. The patient must maintain a strict nose-down face-down posture for 8 to 16 hours per day for 3 to 14 days post-op. This places the buoyant gas bubble squarely over the fovea, mechanically sealing the macular hole edges against the RPE and isolating them from vitreous fluid.
• Lateral Decubitus / Tilt Positioning: Used when retinal breaks reside at 3 o'clock or 9 o'clock meridians (patient lies on the ipsilateral side of the tear).
• Supine Positioning STRICTLY CONTRAINDICATED in Phakic Gas-Filled Eyes:
  • If a patient with an intraocular gas bubble lies flat on their back (supine), the buoyant bubble floats forward against the posterior capsule of the crystalline lens.
  • Consequence 1 (Lens Feathering Cataract): Gas contact with the posterior lens capsule disrupts metabolic nutrient exchange from aqueous humor, inducing rapid, irreversible posterior subcapsular "feathering" lens opacification within hours.
  • Consequence 2 (Pupillary Block Glaucoma): In aphakic or pseudophakic eyes, a forward-floating bubble wedges against the iris, causing total pupillary block, extreme shallowing of the anterior chamber, and acute IOP escalation (prevented by creating a surgical inferior peripheral iridectomy at 6 o'clock—the Ando iridectomy—which allows aqueous to bypass the floating bubble).

The Aviation & High-Altitude Disaster: Boyle's Law

$\text{Boyle's Law: } P_1 V_1 = P_2 V_2$

• Mechanism of Disaster: Commercial passenger aircraft cabins are pressurized to altitudes between 6,000 and 8,000 feet above sea level (equivalent to an ambient atmospheric pressure drop from 760 mmHg to ~565 mmHg). If a patient with an intraocular gas bubble boards an aircraft or drives up a high mountain pass, the drop in atmospheric pressure causes the intraocular gas volume ($V_2$) to expand by 25% to 35%.
• Clinical Sequelae: Because the rigid ocular coats (cornea and sclera) cannot expand to accommodate this volume surge, the intraocular pressure spikes instantaneously to 60 to 90 mmHg.
• Catastrophic Outcome: Immediate collapse of ocular perfusion, Central Retinal Artery Occlusion (CRAO), ischemic anterior optic neuropathy, and permanent, irreversible blindness within 60 to 90 minutes of flight.
• Clinical Protocol: Patients with an intraocular gas bubble must receive a prominent neon-colored hospital wristband warning: DO NOT FLY / NO HIGH ALTITUDE ASCENT. Air travel is strictly prohibited until the bubble is 100% resorbed or verified on slit lamp by the ophthalmologist (<0.5 mL residual bubble). In contrast, silicone oil-filled eyes can fly safely, because silicone oil is an incompressible liquid that does not expand under atmospheric pressure changes.

Nitrous Oxide (N₂O) Anesthesia Contraindication

• Pharmacological Mechanism: Nitrous oxide ("laughing gas") is an inhalation anesthetic agent. Nitrous oxide is 34 times more soluble in blood than nitrogen.
• The Disaster Scenario: If a patient with a preexisting intraocular gas bubble receives N₂O general anesthesia (or if N₂O is administered during vitrectomy while gas is introduced), N₂O diffuses from retinal capillaries into the intraocular gas bubble vastly faster than the insoluble fluorocarbon gas or nitrogen can diffuse out into the blood.
• Explosive Expansion: Within 15 to 20 minutes of N₂O inhalation, the intraocular bubble expands by 200% to 300%, driving IOP above systolic ophthalmic arterial pressure, causing acute globe rupture (if recent wound) or immediate CRAO.
• Safety Protocol: Nitrous oxide must be terminated at least 15 to 20 minutes before intraocular gas injection during vitrectomy. Furthermore, patients carrying residual gas bubbles must never receive N₂O anesthesia for any subsequent medical or dental procedure.