Vitrectomy and intraocular tamponades

Evolution and Mechanics of Pars Plana Vitrectomy (PPV)

Pars plana vitrectomy was conceptualized and pioneered by Robert Machemer in 1970 using a 17-gauge full-function instrument. Ophthalmic vitreoretinal surgery has since evolved from conventional 20-gauge vitrectomy (which required conjunctival peritomies, large scleral incisions, and sutured wound closure) to Micro-Incision Vitrectomy Surgery (MIVS):

• MIVS Calibres: 23-gauge ($0.64\,\text{mm}$), 25-gauge ($0.50\,\text{mm}$), and ultra-high-speed 27-gauge ($0.40\,\text{mm}$) systems.
• Transconjunctival Sutureless Cannulas: Trocar-cannula entry systems are inserted through the conjunctiva and sclera via angled, beveled incisions that create self-sealing, valved tunnels. This avoids conjunctival dissection, minimizes postoperative corneal astigmatism, accelerates wound healing, and reduces postoperative ocular inflammation and patient discomfort.

Anatomical Entry Sites: Sclerotomy Placement Landmarks

The pars plana of the ciliary body is an anatomically privileged, avascular zone measuring approximately $4\,\text{mm}$ in width (extending from the posterior edge of the ciliary processes at the pars plicata, situated $2\,\text{mm}$ posterior to the limbus, to the ora serrata, situated $6-7\,\text{mm}$ posterior to the limbus temporally and $5.5-6\,\text{mm}$ nasally):

• Adult phakic eyes: Entry is commonly about 4 mm behind the limbus, adjusted for anatomy and technique; avoid lens and retinal injury.
• In Pseudophakic or Aphakic Eyes: Sclerotomies are placed $3.5\,\text{mm}$ posterior to the corneal limbus. Because the native crystalline lens is absent, entering at $3.5\,\text{mm}$ maximizes surgical maneuverability while avoiding the anterior vitreous base and ora serrata.
• Children: Entry planning depends on age and measured anatomy, with examination/ultrasound as needed. Adult distances and a generic paediatric distance cannot be applied to every eye.

Modern Vitrectomy Fluidics and Instrumentation

• High-Speed Cutters: Contemporary vitrectomy probes feature dual-pneumatic drive mechanisms achieving cut rates of $5,000$ to $10,000$ cuts per minute (cpm) (up to $20,000\,\text{cpm}$ with dual-blade technology). Higher cut rates yield smaller vitreous bites, minimize tractional drag on the peripheral retina, and suppress intraocular pressure turbulence.
• Wide-Angle Non-Contact Viewing Systems: Non-contact systems (e.g., BIOM, Resight) utilize wide-angle inverter optics to provide up to $130^{\circ}$ panoramic visualization of the peripheral retina, independent of corneal magnification or pupil constriction.
• Chandelier Endoillumination: Dedicated transscleral fiber-optic light pipes anchored through independent trocars liberate both hands of the surgeon, enabling bimanual techniques for complex membrane peeling and proliferative vitreoretinopathy (PVR) dissection.

Intraocular Endotamponades: Gases vs. Silicone Oil

Intraocular tamponades provide surface tension and buoyancy across retinal breaks, sealing them against fluid ingress while chorioretinal retinopexy adhesion matures (which requires 10 to 14 days).

1. Intraocular Expansile Gases

Intraocular gases are inert, water-insoluble fluorochemicals. When injected as pure (100%) gas into the eye, nitrogen and oxygen from surrounding tissues rapidly diffuse down their partial pressure gradients into the bubble faster than the high-molecular-weight fluorochemical can diffuse out, causing predictable volumetric expansion:

2. Silicone oil is non-resorbable and provides prolonged tamponade in selected complex detachments. Indication, later removal, posture and pressure/retinal surveillance depend on the individual eye. It does not provide universally immediate functional vision or require permanent retention.

• 1,000 cSt vs. 5,000 cSt Viscosity:
  • 1,000 Centistokes (cSt): Lower viscosity; significantly easier and faster to inject and aspirate through small-calibre MIVS cannulas. However, it undergoes earlier emulsification.
  • 5,000 Centistokes (cSt): Higher viscosity; exhibits superior resistance to shear stress and significantly delays emulsification, making it preferable for long-term tamponade ($>6$ months).
• Complications of Silicone Oil:
  • Emulsification: Over time, mechanical shear stresses break the oil bubble into micro-droplets ("tiny bubbles" in the anterior chamber / "inverse hypopyon" floating superiorly), clogging the trabecular meshwork and inciting intractable secondary open-angle glaucoma.
  • Pupillary Block Glaucoma in Aphakic Eyes: Silicone oil has a specific gravity of $0.97\,\text{g/cm}^3$ (lighter than water) and floats superiorly. In aphakic eyes, it can herniate anteriorly to block the pupil. To prevent pupillary block, a surgical inferior peripheral iridectomy (Ando iridectomy at 6 o'clock) is mandatory, allowing aqueous humor to circulate from the inferior posterior chamber into the anterior chamber.
  • Refraction: Silicone oil changes the posterior lens-interface power, commonly producing a hyperopic shift in phakic/pseudophakic eyes. Magnitude depends on lens shape and IOL design; aphakic oil-filled eyes can shift myopically. It does not reverse the crystalline lens into a divergent lens.
  • Other Complications: Band keratopathy, corneal endothelial decompensation, and accelerated nuclear sclerotic cataract formation in many phakic eyes within $12-24$ months.
• Heavy Silicone Oil (e.g., Densiron, Oxane HD): Formulated by blending silicone oil with high-density fluorinated alkanes (specific gravity $>1.0\,\text{g/cm}^3$). Because it sinks to the bottom of the vitreous cavity, heavy oil provides targeted inferior tamponade for complex inferior PVR without requiring prolonged prone posturing.