Axial length changes after scleral buckles

The axial length of the eye may change after placement of a scleral buckle. Radial soft silicone sponges appear to induce little change in the axial length of the eye. Segmental scleral buckles may cause a hyperopic shift while encircling scleral buckles may produce increases or decreases in axial length, depending on the scleral buckle material, the location of the buckle, and the height of the buckle.^1,2 Hard silicone encircling buckles most commonly increase the axial length of the eye,^3–5 although some eyes show no apparent change in axial length after placement of an encircling buckle.^6,7 Occasionally, high encircling silicone buckles may decrease the axial length of the eye.^4,8

The changes in axial length induced by circumferential scleral buckles can be understood by analyzing the geometry of the eye with an encircling buckle in place. If a circumferential buckle is tightened around the equator of the eye, the first effect is to decrease the circumference of the eye in coronal cross-section, causing the eye to assume an elliptical shape in horizontal cross-section. The normally spherical human eye acquires the shape of a prolate spheroid after placement of a broad circumferential buckle (Fig. 99.1). A prolate spheroid is the result of the rotation of an ellipse about its major (longer) semiaxis. The eye is still circular in coronal cross-section, because the encircling buckle constricts the equator of the eye into a circle of smaller circumference. The eye becomes more elliptical in sagittal and horizontal cross-section. A decrease in the circumference of the eye caused by indentation from a scleral buckle in the coronal plane is accompanied by an increase in the anteroposterior dimension of the eye (Fig. 99.1) in the sagittal and horizontal planes as the eye becomes elongated by the equatorial constriction from the encircling scleral buckle. This change from a sphere to a prolate spheroid occurs primarily because of the relative inelasticity of the sclera at physiologic intraocular pressures in the fluid-filled eye. If the circumferential buckle is tightened high enough, the eye will assume the shape of a dumbbell. The anteroposterior axial length of the eye decreases at very high circumferential buckle heights because part of the circumference of the sclera in the horizontal and sagittal cross-sections is used to create the dumbbell-shaped indentation in the sclera (Fig. 99.1).

Fig. 99.1 (A) The normal spherical shape of the eye. (B) The spherical eye acquires the shape of a prolate spheroid after placement of a moderately high circumferential buckle. Horizontal and sagittal cross-sections of an eye with a broad circumferential buckle at moderate buckle heights show an ellipse. Coronal cross-section of an eye with a circumferential buckle shows a circle. The anteroposterior axial length of the eye increases at moderate buckle heights. (C) The eye acquires a dumbbell shape at very high circumferential buckle heights. Coronal cross-section of an eye with a very high circumferential buckle still shows a circle, but the sagittal and horizontal cross-sections show a dumbbell shape. The axial length of the eye decreases at very high buckle heights.

Placement of two mattress sutures per quadrant to invaginate the sclera beneath a circumferential buckle has additional effects on the geometry and axial length of the eye. Indentation of the sclera by tightening mattress sutures around a circumferential hard silicone exoplant causes a decrease in the axial length of the eye if there is no concomitant circumferential shortening of the encircling buckle.⁴ The mattress sutures pull together the anterior and posterior sclera where the suture bites are anchored, decreasing the axial length of the eye. As the mattress sutures are tightened, the sclera is indented by the circumferential buckle beneath the mattress sutures, causing the eye to acquire a dumbbell shape in horizontal and sagittal cross-sections.

These two effects, circumferential shortening and scleral invagination by mattress sutures, must be considered in understanding the geometry of the eye after placement of an encircling scleral buckle. The first effect, circumferential shortening, increases the axial length by changing the shape of the eye from a sphere to a prolate spheroid with mild to moderate circumferential tightening of an encircling buckle. The second effect, invagination of the sclera around a broad encircling element with mattress sutures, contributes to a decrease in the axial length of the eye.

The increases in axial length from circumferential shortening of the eye predominate over decreases in axial length from scleral invagination at low to moderate buckle heights, producing net increases in axial length and a shift toward myopia. The decreases in axial length from scleral invagination tend to predominate over increases in axial length at very high buckle heights, causing a net decrease in axial length. Some eyes with moderate to high circumferential buckles have no net change in axial length when the circumferential shortening and scleral invagination precisely balance. It would be desirable to try to adjust circumferential buckle to minimize axial length changes, but this is difficult to achieve, since other factors, such as support of the retinal breaks and relief of vitreoretinal traction, are more important surgical goals.

A geometric model to explain the effects of circumferential scleral buckles on the axial length and to predict these axial length changes was developed to improve the predictability of these axial length changes. The geometric model of the effects of scleral buckles on axial length is based on the following assumptions: First, the overall contour of the eye with a scleral buckle is assumed to be an ellipse. Second, the circumference of the eye is constant, because the buckle does not stretch or shrink the sclera substantially. Figure 99.2 gives a gross estimate of the circumference of the indented eye wall (which equals π times axial length) after a scleral buckle, based on the preoperative perimeter of the eye and the amount of indentation of the eye wall by the scleral buckle at the equator. The axial length of the eye can then be calculated following placement of the scleral buckle, using the formula to solve for the axial length to predict the effect of the scleral buckle on refraction.

Fig. 99.2 (A) The axial length changes induced by a circumferential scleral buckle may be estimated grossly by this formula by solving the equation for ‘a’ if the original circumference of the eye, the amount of buckle indentation, and equatorial shortening of the eye are known. (B) Geometric model relating circumferential buckle shortening, scleral indentation, and axial length.

Refractive errors caused by scleral buckles

Three major types of refractive error can be induced by scleral buckles used for retinal reattachment. The first type is an astigmatic error caused by changes in the corneal curvature; the second type is a change in the spherical equivalent induced by changes in axial length, anterior chamber depth, or position of the crystalline lens. The third is higher-order aberrations which were found to be greater when segmental scleral buckles are used rather than circumferential buckles.⁹ The higher-order aberrations persist at least 3 months but may improve over time.

Spherical equivalent errors

Changes in refraction from scleral buckles caused by changes in axial length and lens position are more common than are astigmatic errors. Most eyes with encircling buckles have a small shift toward myopia; this is associated with an increase in axial length.^5,8,16 Shallowing of the anterior chamber associated with displacement of the lens anteriorly in phakic eyes with an encircling buckle may also contribute to a shift toward myopia.⁸ Anterior displacement of the lens becomes less pronounced several months after retinal reattachment in most eyes.¹⁶ The buckle height decreases substantially with radial buckles over a period of months, while buckle height does not decrease as much in eyes with circumferential buckles.¹⁷ Some eyes with high circumferential scleral exoplants have a shift in refraction toward hyperopia.⁷ The changes in axial length and refractive error with different scleral buckles are reported in the literature and are summarized in Table 99.1. When scleral buckles are placed in children, the scleral buckle may retard growth-related increases in the axial length of the eye, causing the eye to develop less myopia than the fellow eye.²¹ This has been considered as a possible method to slow progressive myopia in children if it could be made more predictable.

Table 99.1 Changes in axial length and refractive error with different scleral buckles

Scleral chord versus scleral arc length

The curvature of the spherical globe must be considered when distances on the surface of the globe are measured. Calipers are commonly used to measure distances for placement of scleral sutures to hold the scleral buckle in place. Calipers measure the shortest line between two points on the spherical globe, which is called the scleral chord length (Fig. 99.3). The distance measured along the curved surface of the globe between two points is the scleral arc length. The scleral chord length measured by the calipers is always shorter than the scleral arc length.²² The scleral arc length can be calculated from the scleral chord length, and vice versa, using the formula in Figure 99.4. Scleral arc length and scleral chord length are similar when the chord length is a small percentage of the radius of the globe. A caliper setting (chord length) of 8 mm corresponds to a scleral arc length of 8.16 mm, a 2% error; a caliper setting of 13 mm corresponds to a scleral arc length of 13.74 mm, a 5.7% error. The discrepancy between scleral chord length and scleral arc length increases nonlinearly with larger caliper measurements and should be considered whenever calipers are used to measure large distances on the globe.

Fig. 99.3 Calipers are used to measure distances for placement of sutures around scleral buckles and to measure the shortest distance between two points on the sclera (scleral chord length). The distance between two points on the surface of the sclera (scleral arc length) is always longer than the scleral chord length.

Fig. 99.4 The scleral arc length (S) can be calculated from the scleral chord length (C) and vice versa. The inverse sin in the formula must be calculated in degrees to obtain the scleral arc length. The difference between the scleral arc length and the scleral chord length increases nonlinearly with larger distances measured by the calipers. R, radius of eye.

Effects on the internal geometry of the eye

It is the buckling effect created by an episcleral or intrascleral buckle that is desirable in closing retinal breaks and relieving vitreoretinal traction. The geometry of the scleral buckle and the technique of scleral buckle and suture placement determine the shape and height of the indentation. It may be desirable to create a high buckling effect in the treatment of certain retinal detachments, and suture placement is important in these eyes. Deep suture bites of at least 4–5 mm long are needed to allow maximum tension to be placed on the suture and transmitted to the scleral buckle. Superficial or short scleral suture bites tend to tear through the sclera and decrease the effective buckle height. The tearing strength of 4–0 silk sutures through the sclera is significantly greater for deep suture bites (50–90% depth) than for superficial suture bites (20–30% depth) (Thompson, unpublished data).

The major variables that determine the internal geometry of indentation induced by the scleral buckle exoplant include: (1) shape of the buckle; (2) composition of the buckle (silicone sponge versus hard silicone); (3) suture placement with respect to the dimensions of the buckle; (4) suture tension; (5) distribution of tension from the suture to the buckle; and (6) intraocular pressure. An analysis of scleral indentation from a 5-mm radial silicone sponge showed that the following factors decreased indentation: (1) placement of the suture bites too close or too far apart; (2) high intraocular pressure; (3) short suture bites in the sclera; (4) loose sutures; and (5) use of a half-thickness sponge compared with a full-thickness sponge. Factors that increased scleral indentation included: (1) low intraocular pressure and (2) tight sutures.^23,24

The orientation of the scleral buckle also helps to determine the topography of the indentation in the sclera. Radial buckles appear to offer advantages in the support of a solitary horseshoe-shaped retinal tear.^25–27 Moderate to high circumferential encircling scleral buckles cause radial folding of the retina. This radial folding occurs because the encircling buckle forces a reduction in the normal circumference of the eye in the equatorial meridian. The sclera and retina are unable to shrink to the new, smaller circumference, so the “excess” retina, choroid, and sclera are thrown into radial folds to conform to the smaller circumference of the eye induced by the encircling buckle (Fig. 99.5). The circumferential shortening of the eye beneath an encircling buckle is the basis of the fishmouth phenomenon (Fig. 99.6).^28,29 Wedge-shaped buckles and radial scleral buckles minimize the risk of the fishmouth phenomenon because they cause less circumferential shortening over the retinal tear than do encircling scleral buckles.

Fig. 99.5 Radial retinal folds on a broad encircling buckle induced by shortening the circumference of the eye. The sclera, choroid, and retina are unable to shrink to the smaller globe circumference beneath the scleral buckle indentation, so the “excess” retina on the buckle is thrown into folds.

Fig. 99.6 The fishmouth phenomenon results when a radial fold bisects a retinal tear. The radial fold tends to keep the tear from settling on the buckle and may lead to persisting retinal detachment. The fishmouth phenomenon is most commonly produced by circumferential shortening of the globe from an encircling buckle, causing a radial fold to pass through a horseshoe-shaped retinal tear. Minimizing the circumferential shortening of the eye with a radial or wedge-shaped buckle decreases the likelihood of the fishmouth phenomenon.

Circumferential encircling buckles are necessary in some eyes because of multiple retinal breaks, circumferential vitreoretinal traction, or circumferential shortening of the retina caused by epiretinal membranes. Anterior circumferential vitreoretinal traction caused by cell-mediated contraction of the vitreous base or peripheral epiretinal membranes associated with funnel-shaped retinal detachments in eyes with proliferative vitreoretinopathy decrease the effective circumference of the peripheral retina. A broad circumferential encircling buckle is preferable in this situation because it reduces the circumference of the eye wall, allowing reapposition of the retina against the retinal pigment epithelium (RPE). The risks and benefits of radial or encircling circumferential buckles must be evaluated in each circumstance so as to choose the optimal geometry of the scleral buckle for the particular retinal tear and retinal detachment configuration.

Volume changes in the eye after scleral buckles

Indentation of the eye wall by a scleral buckle displaces fluid from the vitreous cavity, causing a reduction in the volume of the vitreous cavity. This occurs because a sphere contains the largest volume of fluid with the least surface area. The amount of fluid displaced depends on the buckle type and configuration. The amount of volume displacement is small for most buckles but can be substantial for broader encircling buckles. Estimation of the intraocular volume of an eye with a scleral buckle is important in several circumstances: (1) estimation of how much fluid must be withdrawn from the vitreous cavity or drained from the subretinal space to permit placement of a specific scleral buckle; (2) injection of pharmacologic agents such as antibiotics or antimetabolites into the vitreous, when therapeutic and toxic concentrations must be considered and (3) injection of expansile gases into the vitreous. The volume displacement of a scleral buckle can be predicted as a function of the following variables: (1) the axial length of the eye; (2) the buckle width measured anterior and/or posterior to the equator; (3) the buckle circumference; and (4) the buckle height. The formula for determining volume displacement by a scleral buckle is given in Figure 99.7. A 5-mm radial silicone sponge displaces only about 0.2 mL, or 5%, of the vitreous cavity volume.³⁰ This is why placement of a radial sponge in a nondrainage procedure only occasionally elevates the intraocular pressure substantially. A 2.5-mm wide silicone encircling band (#240 style) displaces about 0.5 mL, or 12%, of the vitreous cavity volume.³⁰ A 7-mm wide hard silicone encircling buckle (#287 style) displaces from 1.3 mL (33%) to 1.7 mL (43%) of the vitreous cavity volume of a phakic eye, depending on the buckle geometry and height.³⁰ The decrease in vitreous cavity volume increases with increasing buckle width and height for circumferential buckles, as shown in Table 99.2. Magnetic resonance imaging has been used to confirm the decreased volume of the eye induced by scleral buckles. An encircling band in this study reduced the vitreous cavity volume by an average of 1.7 mL.³¹

Fig. 99.7 Indentation of the sclera by a scleral buckle displaces fluid from the vitreous cavity. The ocular volume displacement from a scleral buckle is a function of the circumference of the buckle, the radius of the eye, the height of the buckle, and the width of the buckle (1 mL = 1000 mm³).

Table 99.2 Estimated vitreous cavity volume displacement of scleral buckles

(Data from Thompson JT, Michels RG. Volume displacement of scleral buckle. Arch Ophthalmol 1985;103:1822–4.³⁰)

Scleral buckles and ocular rigidity

Placement of a scleral buckle changes the normal ocular rigidity. Ocular rigidity is the change in intraocular pressure for a given change in intraocular volume and is a measure of the elasticity of the eye. Intraocular pressure normally increases rapidly as microliter volumes are injected into the eye. The increase in intraocular pressure (ocular rigidity) is decreased in eyes with an encircling scleral buckle because the volume of the vitreous cavity was decreased with placement of a scleral buckle.^32,33 This decrease in volume is related to changes in the shape of the eye caused by the scleral buckle.³⁰ As the intraocular pressure is increased by injection of saline solution or gas into the eye, the eye becomes less elliptical and more spherical as the sutures holding the buckle are stressed. The net effect is to decrease the buckling effect and to increase the intraocular volume slightly so that the intraocular pressure does not rise as rapidly as in the normal eye. This effect can be better understood by considering an eye with an encircling buckle and no invagination by scleral sutures. If water is injected into the eye, the encircling buckle will stretch as the eye assumes a more spherical shape. Once the buckle has stretched so that the eye returns to its original spherical shape (before placement of the buckle), the intraocular pressure will increase rapidly. The changes in ocular rigidity have several important clinical ramifications. First, methods of measuring intraocular pressure that depend on a standard ocular rigidity, such as the Schiøtz tonometer and Tonopen® are less accurate in eyes with scleral buckles. Second, an injection of fluid or gas into an eye with a scleral buckle will cause less elevation of intraocular pressure than injection of the same volume into a normal eye, if all other factors are equal. Placement of an intraocular gas bubble into the vitreous also reduces ocular rigidity because the gas in the vitreous cavity is more compressible than the vitreous fluid it replaces. Eyes with reduced ocular rigidity from an intraocular gas bubble and an encircling scleral buckle require even larger volumes of fluid aspiration to reduce the intraocular pressure than normal eyes with elevated intraocular pressure.³⁴