c.Accuracy in AL measurement is relatively more important in short eyes than in long eyes.

d.During ultrasonic measurement of AL (A-scan), sound travels faster through the aqueous and vitreous than through the crystalline lens and cornea. Therefore, there is a need to make adjustment to the AL “gmeasurement” by correcting for the incorrect velocity of sound.

e.The velocity of sound in an aphakic eye varies significantly between short and long eyes.

Show Answer

Appendix 5.1

History of Intraocular Lens Design

Knowledge of the history of intraocular lens (IOL) design is important for understanding the reasons for the designs currently in use. Since IOLs were first developed, their designs and the location of IOL fixation have changed considerably. The early success of prepupillary lens designs in the 1970s was sufficient to allow IOL implantation to progress. An early IOL design for intracapsular cataract extraction (ICCE), known as the prepupillary Binkhorst iris clip lens, floated freely but maintained centrality by pupil fixation of its anterior and posterior loops (Fig 5-14A). The Binkhorst prepupillary iridocapsular 2-loop lens had posterior loops fixated in the capsular bag after extracapsular cataract extraction (ECCE) (Fig 5-14B). Later designs (eg, the Epstein lens, Fig 5-15; the Medallion and Platina lenses, Fig 5-16A) were sutured or clipped to the iris for fixation. The Fyodorov Sputnik was an extremely popular lens (Fig 5-16B). Prepupillary IOLs are no longer used because of their tendency to dislocate; however, one early loopless design, the Worst “lobster claw” lens (Fig 5-17; renamed the Artisan lens in 1997), which imbricates the iris stroma, has been approved by the US Food and Drug Administration for insertion in phakic eyes to correct high degrees of ametropia. The 2 basic lens designs currently in use are differentiated by the plane in which the lens is placed (posterior chamber or anterior chamber) and by the tissue supporting the lens (capsule/ciliary sulcus or chamber angle) (see Fig 5-1).

Figure 5-14 Schematic illustrations of prepupillary IOL styles. A, Binkhorst iris clip lens and its position in the eye. B, Iridocapsular 2-loop IOL by Binkhorst.

Figure 5-15 Prepupillary Epstein lens by Cope​land. (Courtesy of Rob ert C. Drews, MD.)

Figure 5-17 “Lobster claw” aphakic and phakic intraocular lenses by Worst. (Courtesy of Kenneth J. Hoffer, MD.)

Posterior chamber lenses

The Ridley lens (Fig 5-18) and other early IOL styles were associated with serious complications, prompting ophthalmologists in the 1950s to turn their attention to anterior chamber IOLs, as well as prepupillary lenses. In the late 1970s, posterior chamber IOLs were reintroduced with a planar 2- loop design and continued to evolve, resulting in numerous successful designs. The first 2 design changes were (1) angulation of the loop haptics to prevent pupillary capture, which remains a feature of current designs, and (2) addition of a peripheral posterior annular ridge to prevent posterior capsular opacification. Today, posterior chamber IOLs are by far the most widely used IOLs and are generally employed following phacoemulsification (Fig 5-19).

Figure 5-18 Original Ridley lens. (Courtesy of Rob ert C. Drews, MD.)

Figure 5-19 Posterior chamber IOLs. A, J-loop design. B, Kratz-Sinskey modified J-loop lens. C, Simcoe modified C-loop lens. D, Knolle lens. E, Arnott lens.

C. H. Wooley.)

With a posterior chamber IOL, the optic and supporting haptics are intended to be placed entirely within the capsular bag; in patients with a torn or an absent posterior capsule, the IOL is placed in the ciliary sulcus. The posterior chamber IOL may also be sutured in place (with a nonabsorbable suture) in cases with poor or no remaining capsular support. Alternatively, some surgeons prefer to

use a well-placed, properly sized, high-quality modern anterior chamber lens.

Anterior chamber lenses

Anterior chamber IOLs (eg, Strampelli and Mark VIII lenses; Fig 5-20) sit entirely within the anterior chamber, but the optical portion of the lens is supported by solid “feet” or loops resting in opposite sides of the chamber angle. Anterior chamber IOLs are a popular style for secondary lens insertion in ICCE aphakic eyes. A particular problem with the use of rigid anterior chamber IOLs is inaccurate estimation of the size of the lens required to span the anterior chamber. The haptics must rest lightly in the chamber angle without tucking the iris (which would indicate that the lens is too large) or “propellering” in the anterior chamber from unstable fixation (too small). The “one-size-fits-all” (eg, Azar 91Z and Copeland lenses; Fig 5-21) and closed-loop designs of the 1970s and 1980s were associated with many complications (persistent uveitis, hyphema, cystoid macular edema, iris atrophy, corneal decompensation, and glaucoma), and poor manufacturing led to the UGH (uveitis- glaucoma-hyphema) syndrome.

Figure 5-20 Anterior chamber IOLs. A, Angle-supported lens by Strampelli. B, Mark VIII lens by Choyce. (Courtesy of Rob ert

C. Drews, MD.)

Figure 5-21 One-size-fits-all anterior chamber IOLs. A, Azar 91Z lens. B, Copeland lens. (Courtesy of Rob ert C. Drews, MD.)

These severe problems led to a bias against anterior chamber IOLs that persists to this day. One change manufacturers made that helped improve the status of these IOLs was to provide a supply of these lenses in several diameter sizes. Charles Kelman, MD, resolved other, more crucial problems by designing lathe-cut, single-piece polymethylmethacrylate anterior chamber IOLs with haptics that absorbed minor compression in the plane of the optic; in previous designs, the optic moved anteriorly, toward the cornea, to absorb compression. The original Kelman Tripod (Fig 5-22A) was replaced by the present-day quadripodal Multiflex II (Fig 5-22B) and other, similar designs (Fig 5- 23).

Figure 5-22 Anterior chamber lens designs by Kelman. A, Original Kelman tripod, also known as the “Pregnant 7.” B,

Multiflex II. (Courtesy of Kenneth J. Hoffer, MD. Redrawn b y C. H. Wooley.)

Figure 5-23 Kelman open-looped lens. (Courtesy of Rob ert C. Drews, MD.)

In addition, Kelman strongly urged surgeons to measure horizontal corneal diameter carefully and to check the status and position of the haptics using gonioscopy in the operating room immediately after lens placement. When properly followed, these procedures make modern anterior chamber IOL implantation an excellent alternative when the use of a posterior chamber IOL is not advisable. One drawback is that an eye implanted with an anterior chamber IOL will be tender if rubbed vigorously. Thus, rubbing the eye should be discouraged.