Chapter 8: Lens
CATARACT SURGERY
Cataract surgery has undergone dramatic change during the past 30 years with the introduction of the operating microscope and microsurgical instruments, improvements in suture materials, the development of intraocular lenses, and alterations in techniques for local anesthesia. Further refinements continue to occur, with automated instrumentation and modifications of intraocular lenses allowing surgery through small incisions.
The generally preferred method of cataract surgery in adults and older children preserves the posterior portion of the lens capsule and thus is known as extracapsular cataract extraction. Intraocular lens implantation is part of this procedure. An incision is made at the limbus or in the peripheral cornea, usually superiorly but sometimes temporally. An opening is formed in the anterior capsule, and the nucleus and cortex of the lens are removed. The intraocular lens is then placed in the empty "capsular bag," supported by the intact posterior capsule. In the standard form of extracapsular cataract extraction, the nucleus is removed intact, but this requires a relatively large incision. The cortex is removed by manual or automated aspiration. The technique of phacoemulsification utilizes a handheld ultrasonic vibrator to disintegrate the hard nucleus such that the nuclear material and cortex can be aspirated through an incision of approximately 3 mm. This same incision size is then adequate for insertion of the recently developed folding lenses. If a rigid intraocular lens is used, the wound needs to be extended to approximately 5 mm. The advantages of small-incision surgery are more controlled operating conditions, avoidance of suturing, rapid wound healing with lesser degrees of corneal distortion, and reduced postoperative intraocular inflammation-all contributing to more rapid visual rehabilitation. The phacoemulsification technique does, however, run the risk of posterior displacement of nuclear material through a posterior capsular tear, which generally necessitates complex vitreoretinal surgery. After all forms of extracapsular cataract surgery there may be secondary opacification of the posterior capsule that requires discission using the neodymium:YAG laser (see After-Cataract, above). Lens extraction through the pars plana during posterior vitrectomy is called phacofragmentation. This type of cataract removal is only performed in conjunction with the removal of an opaque or scarred vitreous.
Intracapsular cataract extraction, consisting of removal of the entire lens together with its capsule, is less frequently performed today. The incidence of postoperative retinal detachment and cystoid macular edema is significantly higher than after extracapsular surgery, but intracapsular surgery is still a useful procedure, particularly when facilities for extracapsular surgery are not available.
Intraocular Lens
There are many styles of intraocular lenses, but most prostheses consist of a central biconvex optic and two legs or haptics to maintain the optic in position. The optimal intraocular lens position is within the capsular bag following an extracapsular procedure. This is associated with the lowest incidence of postoperative complications, such as pseudophakic bullous keratopathy, glaucoma, iris damage, hyphema, and lens decentration. The newest posterior chamber lenses are made of flexible materials such as silicone and acrylic polymers. This flexibility allows the lens implant to be folded, thus decreasing the required incision size. Lens designs that incorporate multifocal optics have also been produced. The goal of this design is to provide the patient with good vision for both near and distance without glasses, which current monofocal designs are unable to do.
After intracapsular surgery-or if there is inadvertent damage to the posterior capsule during extracapsular surgery-intraocular lenses can be placed in the anterior chamber or sometimes fixated in the ciliary sulcus.
Methods of calculating the correct dioptric power of an intraocular lens are discussed in Chapter 20. If an intraocular lens cannot be safely placed or is contraindicated, postoperative refractive correction generally requires a contact lens or aphakic spectacles.
Postoperative Care
If a small-incision technique is used, the postoperative recovery period is usually shortened. The patient may be ambulatory on the day of surgery but is advised to move cautiously and avoid straining or heavy lifting for about a month. The eye can be bandaged for a few days, but if the eye is comfortable, the bandage can be removed on the first postoperative day and the eye protected by spectacles or by a shield during the day. Protection at night by a metal shield is required for several weeks. Temporary glasses can be used a few days after surgery, but in most cases the patient sees well enough through the intraocular lens to wait for permanent glasses (usually provided 6-8 weeks after surgery).
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10.1036/1535-8860.ch8