Plaque Simulator's 3D plaque models include the toroidally shaped structures of the suture eyelets. The coordinates on the outer surface of the sclera which underly the doughnut hole in the eyelet can be expressed as either a meridian clock hour (in the manner of the retinal diagram), or as an angle (in the manner of toric IOL implants), and a chord distance along that meridian measured from the limbus. Plaque Simulator automatically calculates the scleral coordinates underlying each eyelet of a plaque and the chord distance between eyelets. Manual calculation and/or verification of these coordinates and distances are also supported.
To use the coordinates, the surgeon marks the meridians on the eye near the limbus, either manually or using an axis marker as illustrated below. The chord distances are measured using a Castroviejo caliper as illustrated on the right. The chord distance between the eyelets helps to confirm that the meridians have been correctly projected posteriorly from the limbus.
The Duckworth & Kent Axis Marker model 9-841 is an example of an instrument that features a dial marked in degrees on its anterior surface for setting the desired meridian of the implant while the posterior surface provides two large radial blades which correspond to the axis on the dial. These blades, marked with a sterile marking pen, are aligned with limbal markings made pre-operatively for either the 90° or the 180° axis. Once aligned, the instrument is pressed onto the surface of the eye, leaving two opposing radial marks at the limbus.
Plaque Simulator's retinal diagram is a 2D polar map of the inner surface of the eye (modeled as being 1 mm inset from the outer surface of the sclera) with the posterior pole at the map's center and extending radially to the limbus. The tic marks on the retinal diagrams are spaced at 1 mm of arc increments. Note that the circumferential distance between these marks increases with radial distance from the pole at the center of the diagram. The outermost circle on the diagram is the limbus. The diameter of the limbus circle appears to be much greater than the diameter of the equatorial circle. In 3D space, however, the diameter of the limbus is actually much smaller than the diameter of the eye at its equator.
In reference to the earth, meridians are lines of longitude which are expressed as degrees offset from the prime meridian that passes through Greenwich in the United Kingdom. In reference to the eye and retinal diagram, meridians are also lines of longitude, except that they are usually expressed as either clock hours, or, in the case of toric IOL implants, as angles CCW from the 180 deg. axial bisector (equivalent to the 3 to 9 o'clock meridian).
An outline of the plaque, which generally presses against, but might possibly be offset from the outer surface of the sclera, is projected onto the diagram surface. In the example on the left, we see that the holes in the eyelets are centered along the 1:00 o'clock and 2:30 o'clock meridians which is equivalent to 60 and 15 degrees as seen on the right.
The chord distance between the limbus and the outer sclera underneath an eyelet can be measured by passing a meridian plane through that eyelet in the 2D planar Planar Dosimetry window and enabling the Ruler tool. The illustration to the right is the 2:30 o'clock meridian. In the Planar Dosimetry window, the meridian plane can be advanced from eyelet to eyelet by simply clicking the Eyelet button or by control-clicking the button to display a contextual menu from which a specific eyelet can be selected.
Chord distances to the limbus are calculated automatically for each eyelet and are displayed on the retinal diagram as a coordinate pair with the meridian hour (e.g. 2:30,8.4mm). The Ruler tool in the Planar Dosimetry window enables you to confirm that the automatic calculation is working correctly. Simply drag the ruler into position as illustrated. The suture points are also displayed three dimensionally in the Patient Setup window.
A list of suture eyelet coordinates (either in clock hours or degrees) for each plaque is printed at the lower left corner of the Retinal Diagram document. A list of distance(s) between eyelet pairs, measured on the exterior sclera, appears near the upper right corner. In this example, the distance between eyelets A and B measured on the exterior sclera is 9.3 mm. The coordinates also appear adjacent to each eyelet in the diagram itself.