D. Brian Kim, MD
Advanced intraocular lens (IOL) fixation is predicated on several important steps, one of which is anterior vitrectomy. Anterior segment surgeons have traditionally performed this from a limbal approach, either coaxial or bimanual. Similar to the phacoemulsification handpiece, the coaxial vitrector contains the components of infusion, aspiration, and cutting, all in one. Although this is familiar and comfortable for the anterior segment surgeon, this approach is problematic because coaxial vitrectomy needs to be performed through a keratome incision. By default, this is inefficient because fluid from the irrigation port pushes vitreous away from the cutter. Moreover, such a large incision is prone to fluid egress, which potentially invites more vitreous to the wound. In contrast, bimanual vitrectomy is performed through 2 separate limbal incisions with infusion in one line and the vitrector in the other. Splitting infusion from the cutter helps to propel vitreous away from the infusion and toward the cutter for enhanced removal, and the smaller and tighter incisions reduce the likelihood for vitreous prolapse to the wound.
Vitreoretinal surgeons have long performed bimanual vitrectomy, but from a posterior approach. In principle, this makes a lot more sense because the vitreous will be drawn away from the anterior segment and toward the posterior segment where it belongs. The history of vitrectomy is beyond the scope of this text, but it is important to know that the 20-gauge 3-port system1,2 has been used for many years preceding the advent of the transconjunctival, sutureless vitrectomy trocar system. In 1990, 25-gauge3 vitrectomy was introduced, which is compatible with a small-gauge incision through a self-sealing sclerotomy. This improves surgical efficiency, patient comfort, and visual recovery; however, 25-gauge instrumentation had some limitations resulting in slower infusion flow and lower vitreous cut rates per minute (cpm). In 2005, the 23-gauge system4,5 was introduced, which maintains the small-incision benefits of 25-gauge vitrectomy but with improved fluidics and a higher cpm for more efficient vitreous cutting and removal. For these reasons, I believe anterior segment surgeons should consider using the 23-gauge system when performing pars plana vitrectomy.
Safe trocar placement requires a general understanding of pars plana anatomy. The avascular pars plana terminates anywhere between 3 to 4 mm posterior to the limbus and is the target tissue for trocar insertion. Careful placement is important because anterior to the pars plana is the vascular pars plicata and posterior to the pars plana is the retina. Pars plana vitrectomy in the phakic eye requires an incision entry 4 mm posterior to the limbus to avoid damaging the crystalline lens. However, for IOL explantation or fixation in aphakic or pseudophakic eyes, trocar placement should be made 3.0 to 3.5 mm posterior to the limbus. One exception is in the nanophthalmic eye, where the pars plana incision should be aimed more anteriorly about 2.0 to 2.5 mm posterior to the limbus. Care must be taken to identify and avoid areas of scleral thinning, which can result from scleral inflammation, previous complex surgery, or a ruptured globe.