MAXIMIZATION OF THE DESIGN OF A

The focusing ability of the human eye declines universally with age. This age related decline in the amplitude of accommodation results in difficulty in reading at a normal working distance in the 40’s and is called presbyopia. Conventionally, bifocals, reading glasses or bifocal contact lenses are used to facilitate reading in presbyopic patients. A new method for correcting presbyopia involves placing an implant, a scleral expansion segment, in the sclera (white of the eye) to alter the position of the underlying ciliary muscle.

Multiple scleral expansion segment designs were analyzed and compared to determine ease of insertion, and maximum scleral lift obtained for a given depth, and location. The results of the analysis were compared to clinical outcomes.

The human eye focuses by the change in shape of the crystalline lens induced by contraction of the ciliary muscle. The ciliary muscle is attached to the crystalline lens equator by zonules (Figure 1). The crystalline lens grows throughout life and its equatorial diameter increases at 20 microns per year. This linear growth of the crystalline lens reduces the effective working distance between the ciliary muscle and the crystalline lens equator and accounts for the age-related decline in the amplitude of accommodation that results in presbyopia.^1-8 The ciliary muscle is attached to the sclera.

By stretching the sclera, using the scleral expansion band segment, in the plane of the crystalline lens equator, the effective working distance of the ciliary muscle is increased which increases the amplitude of accommodation in presbyopes allowing them to read without an optical aid.

The surgical procedure involves cutting belt loops into the sclera 3.00 mm posterior to the limbus (the point where the clear cornea meets the white of the eye). Two parallel 300 micron deep by 1.5 mm long incisions, which are separated by 4 mm, are made with a guarded square diamond blade. The parallel incisions are then connected using a 5 mm long lamella diamond blade. The polymethylmethacrylate scleral expansion band segment is inserted into the scleral belt loop so that the ends protrude from each side of the belt loop (Figure 2). This process is repeated in the four oblique quadrants of the eye.

We analyzed the surgical procedure using nonlinear finite element analysis (ANSYS Release 5.5.3). The scleral belt loop width and thickness, the stability, dimensions, and best location for the scleral expansion band segment were evaluated.

The original successful surgical procedure involved making 900 micron wide by 300 micron deep by 4 mm long belt loops, 2 mm posterior to the limbus. The inserted scleral expansion band segment was 4.5 mm long and had top and bottom surface radii of curvatures of 9 mm, respectively. The central height was greater than the width. This scleral expansion band segment had a 10%-15% incidence of rotation. When the scleral expansion band segment rotated the effect was lost.

In order to improve stability and scleral lift a new scleral expansion band segment was designed. Each of the surgical and design parameters was analyzed. Figure 3 shows the geometric changes made between the original and newer band segment design.

Figure 2. Artist's Rendering of Scleral Expansion Band Segments in Belt Loops - Postoperative Appearance

The exact stiffness of the various parts of the eye is unknown because almost all of the material properties of the eye have been measured post mortem. After reviewing the available literature,^9-14 the elastic constants chosen for the cornea, sclera and limbus were 1, 3, 24 N/mm², respectively.

Hyperelastic elements were used to represent the various components of the eye due to the large strains that are induced when inserting the scleral expansion band segment into the belt loop. A common technique of approximating the Mooney-Rivlin constants when the elastic modulus is known is to set A=E/6 and B=0. Using this guideline, the corresponding M-R constants were input for the materials for a 2-point curve.

The scleral expansion band segment is made of polymethylmethacrylate and is very stiff relative to the tissues of the eye. The scleral expansion band segment was assigned an elastic modulus at least 1000 times stronger than the sclera.

A representative finite element model at the beginning of the analysis is shown in Figure 4. The model consisted of HYPER58 elements for portions of the eye, SHELL181 elements for the implant (with added BEAM4 elements for stiffness) and the general contact elements CONTA174 and TARGE170 are used to model the flexible to flexible nonlinear contact between the implant and eye. This model is a 1/16^th symmetric representation of a human eye.

Symmetry boundary conditions are applied to all of the boundary surfaces. The scleral belt loop was implemented by creating coincident nodes representing a crack in the finite element model. This crack would then allow separation within the model forming a belt loop. The placement of the scleral expansion band segment into the scleral belt loop was simulated as follows:

A small amount of pressure was applied to each side of the crack to open the scleral belt loop.
The scleral expansion band segment was inserted into the scleral belt loop.
The pressure used to open the belt loop was reduced to zero.
Internal pressure was applied to the inside surface of the eye to represent the normal intraocular pressure.

A schematic representation of the steps used in inserting the scleral expansion band segment into the scleral belt loop is shown in Figure 5.

The size and position of the scleral belt loop and the dimensions of the scleral expansion band segment verses scleral lift was evaluated.

The amount of scleral lift obtained by the original scleral expansion band segment was compared with the newly designed segment. The new scleral expansion band segment had the same curvature on the top surface as the old segment, but had a flat bottom with grooves.

The width of the new scleral expansion band segment was greater than its central height. A simple calculation of the center of gravity of the two segments demonstrated that the new segment had 50% less chance of rotation. The new wider scleral expansion band segment produced significantly better results for a given incision width as indicated in Figure 6.

The analysis of the effect of the width of the scleral belt on the amount of scleral lift for the newly designed scleral expansion band segment demonstrated that the narrower the scleral belt loop the greater the scleral lift (Figure 7). The graph also demonstrates that the scleral lift has a steep slope near the belt loop that reaches a maximum and declines gradually as the limbus is approached. Because of the gradual decline in the slope of the scleral lift, the scleral expansion band segment is placed more posterior; so that, any variation in the position of the crystalline lens equator will have less effect on the result.

This analysis work also revealed that there is not a linear relationship between the central height of the scleral expansion band segment and scleral lift (Figure 8). Although a central height of 1060 microns produced the maximum amount of scleral lift, this size scleral expansion band segment can not be readily inserted into the scleral belt loop. A central height of 925 microns was chosen as the best compromise for scleral lift and ease of insertion.

The thickness of the scleral belt loop was evaluated. A thin 150 micron belt loop produces only a minimal effect, while a 450 micron thick belt loop produces a dramatic effect. Figure 9 shows views of the sclera after the implant has been inserted for loops of 150 and 450 microns. Figure 10 is a graph of the lift obtained in this series of tests. As a result of these findings the surgeon must err on making the belt loop thicker; i.e., the deeper the incision the better the results.

The use of this nonlinear finite element analysis of the scleral expansion band segment demonstrated the critical parameters for design and surgical technique. The proper compromise in height, width and placement of the scleral expansion band segment was predicted for maximum stability and effect. The important surgical variables of scleral belt loop thickness and width were quantified.

Surgery using the original scleral expansion band segment resulted in a mean increase in the amplitude of accommodation in presbyopes of approximately 3.00 diopters with a range of 0.7 to 7 diopters. The newly designed scleral expansion band, along with the suggested changes in the surgical technique, has increased the mean amplitude of accommodation to 4.5 diopters. The minimum and maximum of the effect have increased. The present range is 2.0 to 8 diopters. These presbyopes no longer need bifocals or reading glasses.