Ophthalmic viscosurgical devices (OVDs) are a class of clear gel-like material used in eye surgery to maintain the volume and shape of the anterior chamber of the eye, and protect the intraocular tissues during the procedure. They were originally called viscoelastic substances, or just viscoelastics. Their consistency allows the surgical instruments to move through them, but when there is low shear stress they do not flow, and retain their shape, preventing collapse of the anterior chamber. OVDs are available in several formulations which may be combined or used individually as best suits the procedure, and are introduced into the anterior chamber at the start of the procedure, and removed at the end. Their tendency to remain coherent helps with removal, as the cohesive variants tend to be drawn into the aspiration orifice without breaking up. Uses OVDs are used to protect the corneal endothelium from mechanical trauma and to maintain volume and form of the intraocular space during an open incision. The OVD is introduced into the space by syringe through a cannula. At the end of the procedure they are removed by aspiration and the space filled with a compatible fluid. OVDs are commonly used in cataract, cornea, glaucoma, eye trauma, and vitreoretinal surgery. Contraindications Despite the side effects, the advantages of OVDs have made them indispensable in ophthalmic surgery involving the anterior chamber. Adverse effects OVDs can cause excessive post-operative intraocular pressure, particularly if any is left remaining in the eye after surgery. The pressure rise is dose-related. It develops in the first day and will usually resolve spontaneously within three days. This effect is assumed to be a consequence of the large molecules of the OVD causing reduced outflow in the trabecular meshwork. Various drugs have been used to limit pressure spikes, and while effective, are not entirely predictable in their effects. OVD can be trapped behind the IOL in the capsule during normal surgery, and may cause a forward displacement of the IOL, which in turn shifts the focal plane of the IOL towards near vision. Properties The properties of an ideal OVD would include: Ease of placement, low viscosity during placement, and low tendency to entrain bubbles Retention under positive pressure in the eye Retention during phacoemulsification Does not interfere with use of instruments or placement of the IOL. Protects the endothelium Nontoxic, sterile, and does not affect intraocular pressure Does not obstruct aqueous outflow