Surgical reversal of presbyopia: Author reply
Authors: R SCHACHAR
Journal: Ophthalmology (2004)

Nonlinear finite element analysis was used to design and determine proper placement of the scleral expansion band (SEB) and define the dimensions of the scleral tunnel to maximize scleral expansion.1Schachar R.A. Presbyopic surgery.Int Ophthalmol Clin. 2002; 42: 107-118Crossref PubMed Scopus (7) Google Scholar The SEB is placed behind the equatorial plane of the crystalline lens. The SEB expands the sclera by lifting, as well as by applying focal pressure to the eye to produce an outward hydraulic expansion of the surrounding adjacent sclera. Because the ciliary muscle is adherent to this outwardly expanding sclera, the distance between the ciliary muscle and the lens equator is thereby increased. It is important to understand how this inward pressure will produce expansion of the adjacent sclera. The scleral tunnel is shorter than the length of the SEB, so that each foot of the SEB protrudes from the scleral tunnel and applies an inward force on full-thickness sclera. This placement results in scleral expansion by 2 distinct mechanisms. First, the SEB lifts the roof of the tunnel and directs the expansion to the surrounding adjacent sclera. Second, there is hydraulic lift of the surrounding adjacent sclera by transmission of the inward pressure induced by the protruding feet of the SEB. In the normotensive eye, intraocular pressure (IOP) is unchanged after SEB placement. The SEB remains stably positioned, and the effect thereof remains constant over time. This effect is comparable to the hydraulic development and persistence of the secondary increase in axial length, with associated myopia, after scleral buckling procedures. Other types of inward force on the sclera may also produce some reciprocal scleral expansion in the area of the ciliary muscle with physiological reversal of presbyopia, as has been observed after scleral buckling (abstract presented at: The Association for Research in Vision and Ophthalmology meeting, 2002:1487). Incisions in the sclera over the ciliary body will produce scleral expansion in a manner similar to the way radial keratotomy produces flattening of the cornea.2Schachar R.A. Black T.D. Huang T. Understanding Radial Keratotomy. LAL Publishing, Denison, TX1981Google Scholar Incisions made in either the cornea or sclera will weaken the tissue, which will then expand in response to IOP. Dr Kliger suggests that the inward force applied by the SEB reduces zonular tension and, in accordance with the Helmholtz theory of accommodation, this relaxation of the zonules should increase the optical power of the crystalline lens and make the eye myopic. However, this proposal has been refuted by multiple studies that have consistently demonstrated that postoperative SEB eyes have no change in their distance refraction.3Schachar RA, Roy FH, eds. Presbyopia: Cause and Treatment. The Hague, The Netherlands: Kugler Publications; 2001Google Scholar, 4Qazi M.A. Pepose J.S. Shuster J.J. Implantation of scleral expansion band segments for the treatment of presbyopia.Am J Ophthalmol. 2002; 134: 808-815Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar In addition, after maximal zonular relaxation, produced by surgical disinsertion of the anterior ciliary muscle in primates, the primate's eye becomes hyperopic and presbyopic.5Kaufman P.L. Rohen J.W. Barany E.H. Hyperopia and the loss of accommodation following ciliary muscle disinsertion in the cynomolgus monkey physiologic and scanning microscopic studies.Invest Ophthalmol Vis Sci. 1979; 18: 665-673PubMed Google Scholar These changes in refraction are opposite those predicted by Dr Kliger's assertion. In summary, the current design of the SEB, when placed properly, reduces the potential for complications while maximizing, through scleral expansion, the recovery of accommodative function.