Abstract:
A method for performing intrastromal ophthalmic laser surgery requires Laser Induced Optical Breakdown (LIOB) of stromal tissue without compromising Bowman&#39;s capsule (membrane). In detail, the method creates cuts in the stroma over all, or portions of, a plurality of concentric cylindrical surfaces (circular or oval). Importantly, these cuts are all centered on the visual axis of the patient&#39;s eye. In accordance with the present invention, cuts can be made either alone or in conjunction with the removal of predetermined volumes of stromal tissue. The actual location of cuts in the surgery will depend on whether the treatment is for presbyopia, myopia, hyperopia or astigmatism.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to methods for performing intrastromal ophthalmic laser surgery. More particularly, the present invention pertains to laser surgery wherein stromal tissue is cut on concentric cylindrical surfaces, with the surfaces being oriented parallel to, and centered on, the visual axis of an eye. The present invention is particularly, but not exclusively, useful as a method for performing intrastromal ophthalmic laser surgery wherein reshaping of the cornea is accomplished by inducing a redistribution of bio-mechanical forces in the cornea. 
     BACKGROUND OF THE INVENTION 
     The cornea of an eye has five (5) different identifiable layers of tissue. Proceeding in a posterior direction from the anterior surface of the cornea, these layers are: the epithelium; Bowman&#39;s capsule (membrane); the stroma; Descemet&#39;s membrane; and the endothelium. Behind the cornea is an aqueous-containing space called the anterior chamber. Importantly, pressure from the aqueous in the anterior chamber acts on the cornea with bio-mechanical consequences. Specifically, the aqueous in the anterior chamber of the eye exerts an intraocular pressure against the cornea. This creates stresses and strains that place the cornea under tension. 
     Structurally, the cornea of the eye has a thickness (T), that extends between the epithelium and the endothelium. Typically, “T” is approximately five hundred microns (T=500 μm). From a bio-mechanical perspective, Bowman&#39;s capsule and the stroma are the most important layers of the cornea. Within the cornea, Bowman&#39;s capsule is a relatively thin layer (e.g. 20 to 30 μm) that is located below the epithelium, within the anterior one hundred microns of the cornea. The stroma then comprises almost all of the remaining four hundred microns in the cornea. Further, the tissue of Bowman&#39;s capsule creates a relatively strong, elastic membrane that effectively resists forces in tension. On the other hand, the stroma comprises relatively weak connective tissue. 
     Bio-mechanically, Bowman&#39;s capsule and the stroma are both significantly influenced by the intraocular pressure that is exerted against the cornea by aqueous in the anterior chamber. In particular, this pressure is transferred from the anterior chamber, and through the stroma, to Bowman&#39;s membrane. It is known that how these forces are transmitted through the stroma will affect the shape of the cornea. Thus, by disrupting forces between interconnective tissue in the stroma, the overall force distribution in the cornea can be altered. Consequently, this altered force distribution will then act against Bowman&#39;s capsule. In response, the shape of Bowman&#39;s capsule is changed, and due to the elasticity and strength of Bowman&#39;s capsule, this change will directly influence the shape of the cornea. With this in mind, and as intended for the present invention, refractive surgery is accomplished by making cuts on predetermined surfaces in the stroma to induce a redistribution of bio-mechanical forces that will reshape the cornea. 
     It is well known that all of the different tissues of the cornea are susceptible to Laser Induced Optical Breakdown (LIOB). Further, it is known that different tissues will respond differently to a laser beam, and that the orientation of tissue being subjected to LIOB may also affect how the tissue reacts to LIOB. With this in mind, the stroma needs to be specifically considered. 
     The stroma essentially comprises many lamellae that extend substantially parallel to the anterior surface of the eye. In the stroma, the lamellae are bonded together by a glue-like tissue that is inherently weaker than the lamellae themselves. Consequently, LIOB over layers parallel to the lamellae can be performed with less energy (e.g. 0.8 μJ) than the energy required for the LIOB over cuts that are oriented perpendicular to the lamellae (e.g. 1.2 μJ). It will be appreciated by the skilled artisan, however, that these energy levels are only exemplary. If tighter focusing optics can be used, the required energy levels will be appropriately lower. In any event, depending on the desired result, it may be desirable to make only cuts in the stroma. On the other hand, for some procedures it may be more desirable to make a combination of cuts and layers. 
     In light of the above, it is an object of the present invention to provide methods for performing ophthalmic laser surgery that result in reshaping the cornea to achieve refractive corrections for improvement of a patient&#39;s vision. Another object of the present invention is to provide methods for performing ophthalmic laser surgery that require minimal LIOB of stromal tissue. Still another object of the present invention is to provide methods for performing ophthalmic laser surgery that avoid compromising Bowman&#39;s capsule and, instead, maintain it intact for use in providing structural support for a reshaped cornea. Yet another object of the present invention is to provide methods for performing ophthalmic laser surgery that are relatively easy to implement and comparatively cost effective. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, methods for performing intrastromal ophthalmic laser surgery are provided that cause the cornea to be reshaped under the influence of bio-mechanical forces. Importantly, for these methods, a tissue volume for operation is defined that is located solely within the stroma of the cornea. Specifically, this operational volume extends posteriorly from slightly below Bowman&#39;s capsule (membrane) to a substantial depth into the stroma that is equal to approximately nine tenths of the thickness of the cornea. Thus, with the cornea having a thickness “T” (e.g. approximately 500 μm), the operational volume extends from below Bowman&#39;s capsule (e.g. 100 μm) to a depth in the cornea that is equal to approximately 0.9 T (e.g. approximately 450 μm). Further, the operational volume extends radially from the visual axis of the eye through a distance of about 5.0 mm (i.e. the operational volume has a diameter of around 10.0 mm). 
     In general, each method of the present invention requires the use of a laser unit that is capable of generating a so-called femtosecond laser beam. Stated differently, the duration of each pulse in the beam will approximately be less than one picosecond. When generated, this beam is directed and focused onto a series of focal spots in the stroma. The well-known result of this is a Laser Induced Optical Breakdown (LIOB) of stromal tissue at each focal spot. In particular, and as intended for the present invention, movement of the focal spot in the stroma creates a plurality of cuts, with each cut being made on portions of a respective cylindrical surface. 
     Geometrically, the respective cylindrical surfaces on which cuts are made are concentric, and they are centered on the visual axis of the eye. And, they can be circular cylinders or oval (elliptical) cylinders. Further each cylindrical surface has an anterior end and a posterior end. To maintain the location of the cylindrical surface within the operational volume, the posterior end of the cut is located no deeper in the stroma than approximately 0.9 T from the anterior surface of the eye. On the other hand, the anterior end of the cylindrical cut is located in the stroma more than at least eight microns in a posterior direction from Bowman&#39;s capsule. These “cuts” will each have a thickness of about two microns. 
     In a preferred procedure, each cut is approximately two hundred microns from an adjacent cut, and the innermost cut (i.e. center cut) may be located about 1.0 millimeters from the visual axis. There can, of course be many such cylindrical cuts (preferably five), and they can each define a substantially complete cylindrical shaped wall. Such an arrangement may be particularly well suited for the treatment of presbyopia. In a variant of this procedure that would be more appropriate for the treatment of astigmatism, portions of the cylindrical surfaces subjected to LIOB can define diametrically opposed arc segments. In this case each arc segment preferably extends through an arc that is in a range between five degrees and one hundred and sixty degrees. Insofar as the cuts are concerned, each pulse of the laser beam that is used for making the cut has an energy of approximately 1.2 microJoules or, perhaps, less (e.g. 1.0 microJoules). 
     For additional variations in the methods of the present invention, in addition to or instead of the cuts mentioned above, differently configured layers of LIOB can be created in the stromal tissue of the operational volume. To create these layers, LIOB is performed in all, or portions, of an annular shaped area. Further, each layer will lie in a plane that is substantially perpendicular to the visual axis of the eye. For purposes of the present invention the layers are distanced approximately ten microns from each adjacent layer, and each layer will have an inner diameter “d i ”, and an outer diameter “d o ”. These “layers” will have a thickness of about one micron. As indicated above, the present invention envisions creating a plurality of such layers adjacent to each other, inside the operational volume. 
     In yet another variation of the present invention, “radial cuts” can be made in the stroma. Specifically, the radial cuts will be located at a predetermined azimuthal angle θ and will be substantially coplanar with the visual axis of the eye. Each radial cut will be in the operational volume described above and will extend outwardly from the visual axis from an inside radius “r i ” to an outside radius “r o ”. Further, there may be as many “radial cuts” as desired, with each “radial cut” having its own specific azimuthal angle θ. 
     As intended for the present invention, all “cuts” and “layers” (i.e. the cylindrical cuts, the annular layers, and the radial cuts) will weaken stromal tissue, and thereby cause a redistribution of bio-mechanical forces in the stroma. Specifically, weaknesses in the stroma that result from the LIOB of “cuts” and “layers” will respectively cause the stroma to “bulge” or “flatten” in response to the intraocular pressure from the anterior chamber. As noted above, however, these changes will be somewhat restrained by Bowman&#39;s capsule. The benefit of this restraint is that the integrity of the cornea is maintained. Note: in areas where layers are created, there can be a rebound of the cornea that eventually results in a slight bulge being formed. Regardless, with proper prior planning, the entire cornea can be bio-mechanically reshaped, as desired. 
     With the above in mind, it is clear the physical consequences of making “cuts” or “layers” in the stroma are somewhat different. Although they will both weaken the stroma, to thereby allow intraocular pressure from aqueous in the anterior chamber to reshape the cornea, “cuts” (i.e. LIOB parallel and radial to the visual axis) will cause the cornea to bulge. On the other hand, “layers” (i.e. LIOB perpendicular to the visual axis) will tend to flatten the cornea. In any event. “cuts,” alone, or a combination of “cuts” with “layers” can be used to reshape the cornea with only an insignificant amount of tissue removal. 
     In accordance with the present invention, various procedures can be customized to treat identifiable refractive imperfections. Specifically, in addition to cuts alone, the present invention contemplates using various combinations of cuts and layers. In each instance, the selection of cuts, or cuts and layers, will depend on how the cornea needs to be reshaped. Also, in each case it is of utmost importance that the cuts and layers be centered on the visual axis (i.e. there must be centration). Some examples are: 
     Presbyopia: Cylindrical cuts only need be used for this procedure. 
     Myopia: A combination of cylindrical cuts (circular or oval) and annular layers can be used. In this case a plurality of cuts is distanced from the visual axis beginning at a radial distance “r c ”, and a plurality of layers is located inside the cuts. Specifically, “d i ” of the plurality of layers can be zero (or exceedingly small), and “d o ” of the plurality of layers can be less than 2r c  (d 0 &lt;2r c ). In an alternative procedure, radial cuts can be employed alone, or in combination with cylindrical cuts and annular layers. If used, the radial cuts are each made with their respective azimuthal angle θ, inside radius “r i ” and outside radius “r o ”, all predetermined. 
     Hyperopia: A combination of cylindrical cuts and annular layers can be used. In this case, the plurality of cuts is distanced from the visual axis in a range between and inner radius “r ci ” and an outer radius “r co ”, wherein r co &gt;r ci , and further wherein “d i ” of the plurality of layers is greater than 2r co  (d o &gt;d i &gt;2r co ). 
     Astigmatism: Cylindrical cuts can be used alone, or in combination with annular layers. Specifically arc segments of cylindrical cuts are oriented on a predetermined line that is perpendicular to the visual axis. Layers can then be created between the arc segments. 
     Whenever a combination of cuts and layers are required, the energy for each pulse that is used to create the cylindrical cuts will be approximately 1.2 microJoules. On the other hand, as noted above, the energy for each pulse used to create an annular layer will be approximately 0.8 microJoules. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
         FIG. 1  is a cross-sectional view of the cornea of an eye shown in relationship to a schematically depicted laser unit; 
         FIG. 2  is a cross-sectional view of the cornea showing a defined operational volume in accordance with the present invention; 
         FIG. 3  is a perspective view of a plurality of cylindrical surfaces where laser cuts can be made by LIOB; 
         FIG. 4  is a cross-sectional view of cuts on the plurality of cylindrical surfaces, as seen along the line  4 - 4  in  FIG. 3 , with the cuts shown for a typical treatment of presbyopia; 
         FIG. 5A  is a cross-sectional view of the plurality of cylindrical surfaces as seen along the line  5 - 5  in  FIG. 3  when complete cuts have been made on the cylindrical surfaces; 
         FIG. 5B  is a cross-sectional view of the plurality of cylindrical surfaces as seen along the line  5 - 5  in  FIG. 3  when partial cuts have been made along arc segments on the cylindrical surfaces for the treatment of astigmatism; 
         FIG. 5C  is a cross-sectional view of an alternate embodiment for cuts made similar to those shown in  FIG. 5B  and for the same purpose; 
         FIG. 6  is a cross-sectional view of a cornea showing the bio-mechanical consequence of making cuts in the cornea in accordance with the present invention; 
         FIG. 7  is a perspective view of a plurality of layers produced by LIOB in accordance with the present invention; 
         FIG. 8  is a cross-sectional view of the layers as seen along the line  8 - 8  in  FIG. 7 ; 
         FIG. 9A  is a cross-sectional view of a combination of cuts and layers as seen in a plane containing the visual axis of the eye, with the combination arranged for a treatment of hyperopia; 
         FIG. 9B  is a cross-sectional view of a combination of cuts and layers as seen in a plane containing the visual axis of the eye, with the combination arranged for a treatment of myopia; 
         FIG. 9C  is a cross-sectional view of a combination of cuts and layers as seen in a plane containing the visual axis of the eye, with the combination arranged for a treatment of astigmatism; and 
         FIG. 9D  is a top plan view of radial cuts that are coplanar with the visual axis. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , it will be seen that the present invention includes a laser unit  10  for generating a laser beam  12 . More specifically, the laser beam  12  is preferably a pulsed laser beam, and the laser unit  10  generates pulses for the beam  12  that are less than one picosecond in duration (i.e. they are femtosecond pulses). In  FIG. 1 , the laser beam  12  is shown being directed along the visual axis  14  and onto the cornea  16  of the eye. Also shown in  FIG. 1  is the anterior chamber  18  of the eye that is located immediately posterior to the cornea  16 . There is also a lens  20  that is located posterior to both the anterior chamber  18  and the sclera  22 . 
     In  FIG. 2 , five (5) different anatomical tissues of the cornea  16  are shown. The first of these, the epithelium  24  defines the anterior surface of the cornea  16 . Behind the epithelium  24 , and ordered in a posterior direction along the visual axis  14 , are Bowman&#39;s capsule (membrane)  26 , the stroma  28 , Descemet&#39;s membrane  30  and the endothelium  32 . Of these tissues, Bowman&#39;s capsule  26  and the stroma  28  are the most important for the present invention. Specifically, Bowman&#39;s capsule  26  is important because it is very elastic and has superior tensile strength. It therefore, contributes significantly to maintaining the general integrity of the cornea  16 . 
     For the methods of the present invention, Bowman&#39;s capsule  26  must not be compromised (i.e. weakened). On the other hand, the stroma  28  is intentionally weakened. In this case, the stroma  28  is important because it transfers intraocular pressure from the aqueous in the anterior chamber  18  to Bowman&#39;s membrane  26 . Any selective weakening of the stroma  28  will therefore alter the force distribution in the stroma  28 . Thus, as envisioned by the present invention, LIOB in the stroma  28  can be effectively used to alter the force distribution that is transferred through the stroma  28 , with a consequent reshaping of the cornea  16 . Bowman&#39;s capsule  26  will then provide structure for maintaining a reshaped cornea  16  that will effectively correct refractive imperfections. 
     While referring now to  FIG. 2 , it is to be appreciated that an important aspect of the present invention is an operational volume  34  which is defined in the stroma  28 . Although the operational volume  34  is shown in cross-section in  FIG. 2 , this operational volume  34  is actually three-dimensional, and extends from an anterior surface  36  that is located at a distance  38  below Bowman&#39;s capsule  26 , to a posterior surface  40  that is located at a depth 0.9 T in the cornea  16 . Both the anterior surface  36  and the posterior surface  40  essentially conform to the curvature of the stroma  28 . Further, the operational volume  34  extends between the surfaces  36  and  40  through a radial distance  42 . For a more exact location of the anterior surface  36  of the operational volume, the distance  38  will be greater than about eight microns. Thus, the operational volume  34  will extend from a depth of about one hundred microns in the cornea  16  (i.e. a distance  38  below Bowman&#39;s capsule  26 ) to a depth of about four hundred and fifty microns (i.e. 0.9 T). Further, the radial distance  42  will be approximately 5.0 millimeters. 
       FIG. 3  illustrates a plurality of cuts  44  envisioned for the present invention. As shown, the cuts  44   a ,  44   b  and  44   c  are only exemplary, as there may be more or fewer cuts  44 , depending on the needs of the particular procedure. With this in mind, and for purposes of this disclosure, the plurality will sometimes be collectively referred to as cuts  44 . 
     As shown in  FIG. 3 , the cuts  44  are made on respective cylindrical surfaces. Although the cuts  44  are shown as circular cylindrical surfaces, these surfaces may be oval. When the cuts  44  are made in the stroma  28 , it is absolutely essential they be confined within the operational volume  34 . With this in mind, it is envisioned that cuts  44  will be made by a laser process using the laser unit  10 . And, that this process will result in Laser Induced Optical Breakdown (LIOB). Further, it is important these cylindrical surfaces be concentric, and that they are centered on an axis (e.g. the visual axis  14 ). Further, each cut  44  has an anterior end  46  and a posterior end  48 . As will be best appreciated by cross-referencing  FIG. 3  with  FIG. 4 , the cuts  44  (i.e. the circular or oval cylindrical surfaces) have a spacing  50  between adjacent cuts  44 . Preferably, this spacing  50  is equal to approximately two hundred microns.  FIG. 4  also shows that the anterior ends  46  of respective individual cuts  44  can be displaced axially from each other by a distance  52 . Typically, this distance  52  will be around ten microns. Further, the innermost cut  44  (e.g. cut  44   a  shown in  FIG. 4 ) will be at a radial distance “r c ” that will be about 1 millimeter from the visual axis  14 . From another perspective,  FIG. 5A  shows the cuts  44  centered on the visual axis  14  to form a plurality of rings. In this other perspective, the cuts  44  collectively establish an inner radius “r ci ” and an outer radius “r co ”. Preferably, each cut  44  will have a thickness of about two microns, and the energy required to make the cut  44  will be approximately 1.2 microJoules. 
     As an alternative to the cuts  44  disclosed above,  FIG. 3  indicates that only arc segments  54  may be used, if desired. Specifically, in all essential respects, the arc segments  54  are identical with the cuts  44 . The exception, however, is that they are confined within diametrically opposed arcs identified in  FIGS. 3 and 5B  by the angle “α”. More specifically, the result is two sets of diametrically opposed arc segments  54 . Preferably, “α” is in a range between five degrees and one hundred and sixty degrees. 
     An alternate embodiment for the arc segments  54  are the arc segments  54 ′ shown in  FIG. 5C . There it will be seen that the arc segments  54 ′ like the arc segments  54  are in diametrically opposed sets. The arc segments  54 ′, however, are centered on respective axes (not shown) that are parallel to each other, and equidistant from the visual axis  14 . 
       FIG. 6  provides an overview of the bio-mechanical reaction of the cornea  16  when cuts  44  have been made in the operational volume  34  of the stroma  28 . As stated above, the cuts  44  are intended to weaken the stroma  28 . Consequently, once the cuts  44  have been made, the intraocular pressure (represented by arrow  56 ) causes a change in the force distribution within the stroma  28 . This causes bulges  58   a  and  58   b  that result in a change in shape from the original cornea  16  into a new configuration for cornea  16 ′, represented by the dashed lines. As intended for the present invention, this results in refractive corrections for the cornea  16  that improves vision. 
     In addition to the cuts  44  disclosed above, the present invention also envisions the creation of a plurality of layers  60  that, in conjunction with the cuts  44 , will provide proper vision corrections. More specifically, insofar as the layers  60  are concerned,  FIG. 7  shows they are created on substantially flat annular shaped surfaces that collectively have a same inner diameter “d i ” and a same outer diameter “d o ”. It will be appreciated, however, that variations from the configurations shown in  FIG. 7  are possible. For example, the inner diameter “d i ” may be zero. In that case the layers are disk-shaped. On the other hand, the outer diameter “d o ” may be as much as 8.0 millimeters. Further, the outer diameter “d o ” may be varied from layer  60   a , to layer  60   b , to layer  60   c  etc. 
     From a different perspective,  FIG. 8  shows that the layers  60  can be stacked with a separation distance  62  between adjacent layers  60  equal to about ten microns. Like the cuts  44  disclosed above, each layer  60  is approximately one micron thick. As mentioned above, the energy for LIOB of the layers  60  will typically be less than the laser energy required to create the cuts  44 . In the case of the layers  60  the laser energy for LIOB of the cuts  44  will be approximately 0.8 microJoules. 
     For purposes of the present invention, various combinations of cuts  44  and layers  60 , or cuts  44  only, are envisioned. Specifically, examples can be given for the use of cuts  44  and layers  60  to treat specific situations such as presbyopia, myopia, hyperopia and astigmatism. In detail, for presbyopia, a plurality of only cuts  44  needs to be used for this procedure. Preferably, the cuts  44  are generally arranged as shown in  FIGS. 4 and 5A . Further, for presbyopia it is typical for there to be five individual cuts  44  that extend from an inner radius of about 1 mm to an outer radius of about 1.8 mm, with a 200 micron separation between adjacent cuts  44 . When hyperopia/presbyopia need to be corrected together, the cuts  44  will then preferably extend further to an outer radius of about 2.3 mm. For hyperopia, a combination of cylindrical cuts  44  and annular layers  60  can be used as shown in  FIG. 9A . In this case, the plurality of cuts  44  is distanced from the visual axis  14  in a range between and inner radius “r ci ” (e.g. r ci =1 mm) and an outer radius “r co ” (e.g. r co =3 mm), wherein r co &gt;r ci , and further wherein “d i ” of the plurality of layers  60  is greater than 2r co  (d o &gt;d i &gt;2r co ). For myopia, a combination of cylindrical cuts  44  and annular layers  60  can be used as generally shown in  FIG. 9B . In this case a plurality of cuts  44  is distanced from the visual axis  14  beginning at a radial distance “r c ”, and a plurality of layers  60 , with decreasing outer diameter “d o ” in a posterior direction, is located inside the cuts  44 . More specifically, for this case “d i ” of the plurality of layers  60  can be zero (or exceedingly small), and “d o ” of each layer  60  in the plurality of layers  60  can be less than 2r c  (d o &lt;2r c ). And finally, for astigmatism, the portions of cylindrical cuts  44  that form arc segments  54  can be used alone (see  FIGS. 5B and 5C ), or in combination with annular layers  60  (see  FIG. 9C ). Specifically arc segments  54  of cylindrical cuts  44  are oriented on a predetermined line  64  that is perpendicular to the visual axis  14 . Layers  60  can then be created between the arc segments  54 , if desired (see  FIG. 9C ). 
     In a variation of the methodologies noted above, the present invention also envisions the creation of radial cuts  66 . The radial cuts  66   a  and  66   b  shown in  FIG. 9D  are only exemplary, and are herein sometimes referred to individually or collectively as radial cut(s)  66 . Importantly, the radial cuts  66  are coplanar with the visual axis  14 , and they are always located within the operational volume  34 . 
     As shown in  FIG. 9D , each radial cut  66  is effectively defined by the following parameters: a deepest distance into the stroma  28 , Z (distal) , a distance below Bowman&#39;s capsule  26 , Z (proximal) , an inner radius, “r i ”, an outer radius “r o ”, and an azimuthal angle “θ” that is measured from a base line  68 . By setting values for these parameters, each radial cut  66  can be accurately defined. For example, as shown in  FIG. 9D , the radial cut  66   a  is established by the azimuthal angle θ 1 , while the radial cut  66   b  has an azimuthal angle θ 2 . Both of the radial cuts  66   a  and  66   b  have the same inner radius “r i ” and the same outer radius “r o ”. The Z (distal)  and Z (proximal)  will be established for the radial cuts  66   a  and  66   b  in a similar manner as described above for the cylindrical cuts  44 . 
     While the particular Method for Intrastromal Refractive Surgery as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.