Abstract:
A cartridge for an IOL delivery system that has an injector tip geometry designed to reduce stresses at the incision wound edges generated during insertion of an IOL into an eye is disclosed. The injector tip geometry includes a tubular nozzle having a modified elliptical cross section. The injector tip geometry reduces stresses generated reduce the likelihood of tearing or overstretching the wound during cartridge tip insertion and residence time in the wound while the lens is being delivered into the eye.

Description:
This application claims priority under 35 USC §119 to U.S. provisional application Ser. No. 61/026,805, filed on Feb. 7, 2008. 
     This invention relates to intraocular lenses (IOLs) and more particularly to cartridges for use with devices used to inject IOLs into an eye. 
    
    
     BACKGROUND OF THE INVENTION 
     The human eye in its simplest terms functions to provide vision by transmitting and refracting light through a clear outer portion called the cornea, and further focusing the image by way of the lens onto the retina at the back of the eye. The quality of the focused image depends on many factors including the size, shape and length of the eye, and the shape and transparency of the cornea and lens. 
     When trauma, age or disease cause the lens to become less transparent, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. The treatment for this condition is surgical removal of the lens and implantation of an artificial lens or IOL. 
     While early IOLs were made from hard plastic, such as polymethylmethacrylate (PMMA), soft, foldable IOLs made from silicone, soft acrylics and hydrogels have become increasingly popular because of the ability to fold or roll these soft lenses and insert them through a smaller incision. Several methods of rolling or folding the lenses are used. One popular method is an injector cartridge that folds the lenses and provides a relatively small diameter lumen through which the lens may be pushed into the eye, usually by a soft tip plunger. The most commonly used injector cartridge design is illustrated in U.S. Pat. No. 4,681,102 (Bartell), and includes a split, longitudinally hinged cartridge. Similar designs are illustrated in U.S. Pat. Nos. 5,494,484 and 5,499,987 (Feingold) and U.S. Pat. Nos. 5,616,148 and 5,620,450 (Eagles, et al.). In an attempt to avoid the claims of U.S. Pat. No. 4,681,102, several solid cartridges have been investigated, see for example U.S. Pat. Nos. 5,275,604 (Rheinish, et al.) and U.S. Pat. No. 5,653,715 (Reich, et al.). 
     These prior art devices were intended to inject an IOL into the posterior chamber of an aphakic eye through a relatively large (approximately 3.0 mm or larger) incision. Surgical techniques and IOLs have been developed that allow the entire surgical procedure to be performed through much smaller incisions, 2.4 mm and smaller. Such small incisions require that the IOL be compressed very tightly, and that the nozzle used on the injection cartridge have very thin walls. The combination of a tightly compressed lens traveling through a very thin walled nozzle often results in the nozzle splitting during use. In addition, although the surgeon may make the incision a specific size, insertion and manipulation of the cartridge and the lens frequently stresses the incision walls, increasing the size of the incision as well as causing trauma to the surrounding tissue. 
     Accordingly, a need continues to exist for an intraocular lens injection cartridge capable of injection an IOL through a relatively small incision with reduced trauma to the tissue. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention improves upon the prior art by providing a cartridge for an IOL delivery system that has a injector tip geometry designed to reduce applied stresses on the incision during insertion of the cartridge tip through the wound to reduce the likelihood of tearing or overstretching the wound during cartridge tip insertion and residence time in the wound while the lens is being delivered into the eye. 
     It is accordingly an objective of the present invention to provide a cartridge for a lens delivery system that has an injector tip geometry designed to reduce stresses on the wound incision. 
     It is a further objective of the present invention to provide a cartridge for a lens delivery system that reduces post insertion wound trauma. 
     Other objectives, features and advantages of the present invention will become apparent with reference to the drawings, and the following description of the drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph comparing the arc length for a circle, an ellipse and a straight line; 
         FIG. 2  is a graph illustrating theoretical incision size for various tip sizes; and 
         FIG. 3  is an enlarged perspective view of the lens delivery system cartridge of the present invention. 
         FIG. 4  is a graph illustrating a circle used to form a blend radius for forming a modified elliptical cross-sectional shape of an injection tip, where the circle is tangent to both a curve above and below an inflection point and that has a 90 degree tangent line at a point crossing the x-axis. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is directed to cartridge  10  having a body  11  and tip  12 , tip  12  having a geometry designed to reduce stresses generated during insertion of an IOL into an eye. Although any incision size can be used, the dimensions given in the following discussion are based on a 2.0 mm incision or wound in the eye. 
     The action of inserting a cartridge tip  12  through an incision wound develops stresses at the wound edges that can result in trauma and tearing of the incision. The inventors have discovered that a correlation exists between the degree of wound stresses and cartridge tip geometry. Based on this discovery, the inventors determined that the incision or wound can be modeled as a deformable body having roughly an elliptical outer dimension with a major axis of approximately 2.0 mm and a minor axis of approximately 0.25 mm. In addition, the inventors determined that a cartridge tip  12  nozzle can be modeled as a rigid body with the assumption that no deformation of the tip  12  nozzle occurs during the insertion of an IOL into an eye and that any deformation occurs in the wound. As no actual tissue material properties are available, the material properties of the wound tissue can be modeled using the Arruda-Boyce material model. Assuming that the area of the tip  12  nozzle is larger than the area of the wound, the inventors applied a theoretical load to the inside of the wound to “stretch” the wound large enough to allow the nozzle tip  12  to enter. By lowering the theoretical load until the interior wound margins contact the entire outer peripheral surface of the tip  12 , the residual strain, stress, stress distribution and contact pressure can be determined. 
     One skilled in the art understands that a circle or round cartridge tip has an aspect ratio of 1 because the height and width are equal. However, as the aspect ratio is reduced by shortening the height, the arc length changes which serves to reduce the degree of wound stretching by reducing the applied stresses at the wound edges as shown in  FIG. 1 . One skilled in the art also understands that a straight line connecting height and width results in the shortest distance between those points and represents the shortest “arc” length possible relative to applied stresses on the wound without creating a negative arc. Negative arc is undesirable because lens damage or undesirable folding can occur when a negative arc or non-curved geometry is used. 
     The cross-sectional form of an ellipsoid, representing the injection tip  12  geometry, can be analyzed by using the ellipse shape factor. This shape factor ‘ε’ can remain constant to maintain the same form as the tip size is varied from 3.0 mm and below. By varying other parameter values, the form is maintained while reducing the periphery and resultant theoretical incision size as shown in Table 1 below and  FIG. 2 . 
       FIG. 4  shows a Cartesian coordinate system  40  having an abscissa axis (interchangeably referred to as “x-axis”)  42  and an ordinate axis (interchangeably referred to as “y-axis”)  44 . The Cartesian coordinate system  40  defines a first quadrant (also referred to as quadrant I)  46 , a second quadrant (also referred to as quadrant II)  48 , a third quadrant (also referred to as quadrant III)  50 , and a fourth quadrant (also referred to as quadrant IV)  52 . 
     This shape factor ε, also known as eccentricity, is further discussed below starting with the ellipse equation. A cross section of an ellipsoid in a plane parallel to coordinate axes forms an ellipse. In general, this 2D ellipse can be represented by the following equation: 
                       (     x   -   h     )     2       a   2       +         (     y   -   k     )     2       b   2         =   1     ;         
Where h, k represent the center of the ellipse, ‘a’ is the major axis and ‘b’ the minor axis. The shape of an ellipse can be represented by its eccentricity, ε, defined as follows:
 
ε=(1/ a )(√{square root over ( a   2   −b   2 )});
 
where 0&lt;ε&lt;1. The larger the value of ε is, the larger the ratio of a to b and the more elongated the ellipse becomes. Furthermore, for a given eccentricity value, if we know parameter ‘a’ or ‘b’ then the other parameter can be easily calculated using this equation.For completion, note that parameters ‘a’ and ‘b’ are constrained by the following equation:
 
 c   2   =a   2   −b   2 ;
 
where ‘(±c,0)’ represent the foci of the ellipse. Note that the cross section of the ellipse is modified in the sense that the center of the ellipse is not necessarily at the origin and is allowed to float. For example,  FIG. 4  shows ellipse  54  having center  56 .  FIG.4  shows the center  56  located along the negative portion of the y-axis  44 . However, allowing for de-centration of the ellipse leaves a point of inflection  102  (shown in  FIG. 4 ), a sharp feature, when the part of the ellipse lying in quadrant I  46  is revolved around the x-axis  42 . To smooth out this point, a blend in radius is used so that the tangent to the point intersecting the x-axis is at 90 degrees. These two features, the decentered ellipse as well as a blend radius  104  constitute the cross section of the modified ellipsoid configuration.
 
     An alternate form to the above ellipse equation can be represented as,
 
 Ax   2   +Bxy+Cy   2   +Dx+Ey+F= 0;
 
where B 2 &lt;4AC and all coefficients are real. This equation can be converted to the first equation by completing squares and obtaining a form that displays the center of the ellipse as well as the lengths of major and minor axes.
 
     Using the above guidelines, these parameters can be calculated. The elliptic curve in quadrant 1 was fitted to a general ellipse and it was found that (h, k)=(0.0,−0.13), and (a, b)=(0.95, 0.72). The eccentricity ε was then calculated to be 0.65. Given this eccentricity, if either of the two axial dimensions of the ellipse is to be changed, then the other can be calculated with the above equation. Generally, the blend radius at the point of inflection can be chosen to be the smallest possible circle that is tangent to both the curve above and below and has a 90 deg. tangent line at the point crossing the x-axis, such as circle  100  of  FIG. 4 . 
     An example of the application of the determined minimum arc length, aspect ratio and blend radius described above can be seen in Table 1 below where typical values for each of the variables are shown. The table defines typical modified ellipsoid values as a function of incision size. Incision sizes of 1.0, 2.0 and 3.0 mm are used to demonstrate the relationship when the ArcLength width and Ellipse eccentricity are held constant. Applying these values to cartridge tip designs result in the maximum internal volume relative to the minimum arc length which in combination results in significantly reduced strain at the incision wound edges while minimizing the degree of lens compression and resultant lens injection forces. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Modified Ellipsoid typical dimensions as a function of Incision size. 
               
             
          
           
               
                   
                   
                   
                 Ellipse 
                   
                   
                 Blend 
               
               
                 Incision 
                 Ellipse Major 
                 Ellipse Minor 
                 Eccentricity, 
                 ArcLength 
                 ArcLength/ 
                 Radius 
               
               
                 Size (mm) 
                 Axis, a (mm) 
                 Axis, b (mm) 
                 ε 
                 (mm) 
                 Width 
                 (mm) 
               
               
                   
               
               
                 1.0 
                 0.392 
                 0.30 
                 0.65 
                 1.087 
                 1.386 
                 0.2 
               
               
                 2.0 
                 0.784 
                 0.60 
                 0.65 
                 2.174 
                 1.386 
                 0.4 
               
               
                 3.0 
                 1.176 
                 0.90 
                 0.65 
                 3.261 
                 1.386 
                 0.6 
               
               
                   
               
             
          
         
       
     
     The steps outlined above result in a design for cartridge tip  12  that provides the maximum internal volume relative to the minimum arc length. This combination results in significantly reduced strain at the incision wound edges while minimizing the degree of lens compression and resultant lens injection forces. The invention described within provides injector cartridge  10  or nozzle tip  12  shape that reduces the force required to insert cartridge tip  12  through the wound due to the reduced aspect ratio and arc length. In addition, this curved form facilitates reduced wound trauma and potential for lens damage through elimination of sharp external and internal features or transition points. 
     While certain embodiments of the present invention have been described above, these descriptions are given for purposes of illustration and explanation. Variations, changes, modifications and departures from the systems and methods disclosed above may be adopted without departure from the scope or spirit of the present invention.