replace a lens with high refractive error when other methods are not effective. There are risks in addition to cataract procedural risks. A related procedure is the implantation of phakic intraocular lenses in series with the natural lens to correct vision in cases of high refractive errors. Ophthalmic viscosurgical devices Ophthalmic viscosurgical devices (OVDs) are a class of clear gel-like material used in cataract 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. 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, when they are replaced by a buffered saline solution. The tendency of cohesive OVDs to adhere to themselves helps with removal. Intraocular lenses After the removal of the cataract, an intraocular lens (IOL) is usually implanted into the eye, either through a small incision (1.8 mm to 2.8 mm) when using a foldable IOL, or through a larger incision, when using a rigid poly(methyl methacrylate) (PMMA) lens. The foldable IOL, made of silicone, hydrophobic, or hydrophilic acrylic material of appropriate refractive power is folded either using a holder/folder, or a proprietary insertion device provided along with the IOL. The lens implant is inserted through the incision, usually into the capsular bag within the posterior chamber (in-the-bag implantation). Sometimes, a sulcus implantation (in front or on top of the capsular bag but behind the iris) may be required because of posterior capsular tears or because of zonulodialysis. This will require a modification of refractive power because of the more anterior placement on the optical axis. Other designs of multifocal intraocular lens are also available, which focus light from distant as well as near objects, working with similar effect to bifocal or trifocal eyeglasses. Preoperative patient selection and good counselling is necessary to avoid unrealistic expectations and post-operative patient dissatisfaction, and possibly a requirement to replace the lens. Acceptability of these lenses has improved and studies have shown good results in patients selected for expected compatibility. A model of accommodating lens has two hinged struts on