Abstract:
A contact lens for application in practice of orthokeratology on an eye, including a curved shell having a concave surface and a convex surface. The concave surface includes a carrier zone and a back shaping zone, the back shaping zone having a first curvature and the carrier zone having at least one second curvature. The curved shell has a geometric center and the back shaping zone has a shaping zone center and the back shaping zone center is offset peripherally from the geometric center. The curved shell can have an overall diameter that approximates a corneal limbal diameter of the eye to which the contact lens is to be applied.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention generally relates to the field of contact lenses. More particularly, the invention relates to contact lenses that are applied to the eye in the practice of orthokeratology to temporarily reshape the anterior corneal surface. 
       BACKGROUND OF THE INVENTION 
       [0002]    Orthokeratology is the practice of applying rigid generally highly oxygen permeable contact lenses to the eye to reshape the anterior corneal surface, thus altering refractive error temporarily or possibly permanently. Orthokeratology is sometimes identified by other names such as corneal shaping contact lenses, overnight vision correction, overnight contact lenses to correct or control myopia, or corneal refractive therapy. Other identifiers beyond these may be utilized as well. Orthokeratology is used primarily in the treatment of myopia or myopia with astigmatism. However, orthokeratology has also been utilized, less commonly, in the refractive treatment of hyperopia or hyperopia with astigmatism. 
         [0003]    Orthokeratology has existed in some form for many decades. Early, hard contact lenses were noted by many to temporarily alter the anterior corneal shape, thus, creating alterations in refractive error when the contact lenses were not on the eye. Early attempts to practice orthokeratology utilizing hard (PMMA or polymethyl methacrylate) contact lenses were hindered by poor technology and limited understanding of corneal physiology and the criteria necessary to predictably correct for myopia by corneal molding. Significant advances have been made in the practice of orthokeratology since the advent of corneal topography measuring instrumentation in approximately the last twenty years. The advent of computerized corneal topography has permitted the creation of contact lens designs for orthokeratology that provide better repeatability and predictability of results because the corneal topographer provides an increasingly accurate map of the surface curvature of the cornea. 
         [0004]    Further improvements in orthokeratology occurred in recent years because rigid gas permeable contact lenses that provide a much higher oxygen permeability have demonstrated the possibility of wearing the orthokeratology lenses overnight rather than utilizing the contact lenses during the day. Further, the introduction of computer numerically controlled precision lathes and other manufacturing equipment have allowed lens designs for orthokeratology to be manufactured to higher accuracy than previously possible. 
         [0005]    Orthokeratology has also been credited by some researchers with slowing or reducing progression of myopia in children. Results of research on this area have been mixed and the effectiveness of orthokeratology in limiting or slowing the progression of myopia is uncertain. 
         [0006]    Because the cornea provides a large fraction of the eye&#39;s focusing power, and because of the high refractive power and steep curvature of the cornea, very small changes in the curvature of the cornea or corneal thickness can result in substantial changes in refractive error. Accordingly, in orthokeratology, specially shaped contact lenses are used to lightly press on or mold the cornea, causing the cornea to gradually change shape to correct refractive error. The corrective effect may last up to approximately  72  hours once the eye is initially adapted to the lenses. Accordingly, in a usual orthokeratology procedure, according to current practice, contact lenses are worn during sleep and removed during the day. The corneal molding effect after the lenses are removed provides a reduction or elimination of refractive error. Typically, the rigid high oxygen permeability contact lenses are worn for six to eight continuous hours during sleep. Sometimes soft contact lenses are worn during the day at least during the adaptation period for correction of residual refractive errors. Some patients experience a return of some refractive error in the evening for which the orthokeratology lenses or soft contact lenses may be worn to correct refractive error under these circumstances. 
         [0007]    Improvements in the accuracy of corneal topography analysis has also provided additional information about the shape of the anterior cornea and has made it apparent that the optical center of the cornea does not always coincide with the visual axis or optical axis of the eye. 
         [0008]    There is still room for improvement in the area of orthokeratology contact lenses. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention disclosed and claimed in this application addresses a problem apparent in certain orthokeratology circumstances. 
         [0010]    Example embodiments of the invention include a high oxygen permeability rigid gas permeable contact lens for orthokeratology that is sized to approximate the limbal diameter and that has a displaced molding zone as compared to the geometric center of the contact lens. According to embodiments of the invention, the displaced molding zone orthokeratology lens may include a curved shell having a concave surface and a convex surface. The concave surface includes at least a carrier zone and a molding zone. The molding zone is on the concave surface of the shell and has a first curvature and the carrier zone has at least one second curvature, possibly one or several additional curvatures. 
         [0011]    According to an embodiment of the invention, the curved shell has a geometrical center centered on the perimeter of the circular lens and the molding zone has a molding zone center that is displaced from the geometrical center. As discussed above, the curved shell may have an overall diameter that approximates the corneal limbal diameter of the eye to which the contact lens is to be applied. 
         [0012]    According to embodiments of the invention, the curved shell is structured to be stabilized in rotation relative to the eye to when the lens is worn on the eye during sleep. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a cross-sectional view of the anterior segment of a human eye; 
           [0014]      FIG. 2  is a cross-sectional view of the anterior segment of the human eye with a schematically depicted orthokeratology lens in place; 
           [0015]      FIG. 3  is a cross-sectional view of the anterior segment of the human eye schematically depicting the effect of an orthokeratology lens on the shape and structure of the human cornea with corneal changes exaggerated for clarity; 
           [0016]      FIG. 4  is an anterior to posterior view of an orthokeratology lens according to an embodiment of the invention including a displaced molding or shaping zone; 
           [0017]      FIG. 5  is an anterior to posterior view of an orthokeratology lens according to an embodiment of the invention including orientation features of the lens; 
           [0018]      FIG. 6  is anterior to posterior view of an orthokeratology lens according to an embodiment of the invention including a displaced molding or shaping zone; and 
           [0019]      FIG. 7  is a cross sectional view of an orthokeratology lens according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Referring to  FIGS. 1-3 , orthokeratology lens  20  is schematically depicted along with eye  22 . Eye  22  includes an anterior segment  24  which further includes cornea  26 . Cornea  26  has multiple layers and a structure familiar to those skilled in the art. The eye&#39;s structure includes corneal epithelium  28 . According to recent theories of the mechanism of action of orthokeratology, orthokeratology lens  20  provides a positive pressure on the tear film  30 , centrally and a neutral or negative pressure peripherally. In the context of orthokeratology correction for myopia, this causes compression of central corneal epithelium  28  and reorganization of corneal epithelial cells such that the corneal epithelial cells are displaced more peripherally and less centrally, thus subtly reshaping the anterior corneal epithelium  28  surface resulting in a change in refractive error. In the context of hyperopia orthokeratology correction, pressure is applied to the corneal epithelium more peripherally and less centrally thus causing migration or remodeling of corneal epithelial cells to create a steepening rather than a flattening of the anterior cornea. Known orthokeratology lens designs include a molding or shaping zone that is located centered in the rigid oxygen permeable contact lens. 
         [0021]    Referring now to  FIGS. 4 and 7 , displaced molding zone contact lens  32  according to an example embodiment of the invention is depicted. Displaced molding zone contact lens  32  is a generally unitary structure formed of rigid curved shell  34  forming lens body  36 . Rigid curved shell  34  is formed, for example, from a high oxygen permeability gas permeable rigid contact lens material. According to an example embodiment of the invention, rigid curved shell  34  is intended for overnight wear and accordingly, is formed of an oxygen permeable material having an ISO/Fatt Dk of 85 or more. Rigid curved shell  34  is generally formed of a material approved for overnight wear by the U.S. Food and Drug Association. For example, rigid shell  34  may be formed of a siloxane-fluorocarbon polymer. Lens materials may also include other fluorocarbon polymers and any rigid oxygen permeable contact lens material now available and materials to become available in the future. 
         [0022]    Lens body  36  is defined by peripheral edge  38 , anterior convex surface  40  and posterior concave surface  42 . Peripheral edge  38  defines overall diameter  44  of displaced molding zone contact lens  32 . According to an example embodiment of the invention, overall diameter  44  approximates the limbal diameter  46  of cornea  26 . Accordingly, overall diameter  44 , according to an example embodiment, is approximately 11.5 mm but may range from approximately 10.5 mm to approximately 12.5 mm. These diameter values are examples and should not be considered limiting unless recited in the claims. Human cornea  26  is generally understood to have a horizontal diameter in a range of 10.76 mm to 12.60 mm and a vertical diameter in a range of about 10.6 mm to about 11.7 mm. Corneal diameter is commonly measured based on the so-called horizontal visible iris diameter (HVID) which is the measured width of the transparent cornea horizontally across the cornea from the white edge of the sclera temporally to the white edge of the sclera nasally. According to example embodiments of the invention, overall diameter  44  may range between approximately 10.5 mm and 13 mm. 
         [0023]    Displaced molding zone of contact lens  32 , according to an example embodiment of the invention, may be manufactured in a reverse geometry design. Reverse geometry designs include lenses of three zone and four zone designs. These terms are understood by those skilled in the orthokeratology arts. 
         [0024]    According to an example embodiment of the invention, referring to  FIGS. 4 and 7 , rigid curved shell  34  of displaced molding zone contact lens  32  presents concave surface  42  back shaping zone  48 , second curve  50 , optional third curve  52  and fourth or peripheral curve  54 . Rigid curved shell  34  presents geometric center  56 . As can be seen in the depicted embodiment, shaping zone center  58  is displaced from geometric center  56  as is second curve  50 . 
         [0025]    Base curve  60  of back shaping zone  48  is generally formed to be 0.30 to 1.40 mm flatter than the flattest corneal curvature, also known as the flattest K. Back optic zone diameter  62  of back shaping zone  48  can be of any diameter but generally will fall within the range of 6.0 to 8.0 mm. 
         [0026]    Second curve  50  also sometimes referred to as a reverse curve, generally has a width of 0.5 to 1.0 mm and is curved to be 0.5 to 1.0 mm steeper than base curve  60 . This is the equivalent of approximately 3 to 5 diopters steeper than base curve  60 . Optional third curve  52 , if present, is also sometimes referred to as an alignment zone. Third curve  52 , if present, according to an example embodiment of the invention is 1.0 to 1.5 mm wide. 
         [0027]    Fourth or peripheral curve  54  is generally flatter than third curve  52  if present and flatter than second curve  50  in a three zone design. Fourth or peripheral curve  54  is generally structured to provide adequate edge lift and to aid in lens comfort, movement, tear and debris exchange. 
         [0028]    In the embodiment depicted in  FIG. 4 , back shaping zone  48  and second curve  50  are both displaced as compared to geometric center  56 . According to another embodiment of the invention, back shaping zone  48  is displaced relative to geometric center  56  while second curve  50  shares its center with geometric center  56 . 
         [0029]    Referring now particularly to  FIG. 5 , another embodiment of displaced molding zone contact lens  32  is depicted. Structures similar to those described above with relation to  FIG. 4  are marked with similar reference numerals. In the depicted embodiment, stabilizing structures  64  include superior thin zone  66  and inferior thin zone  68 . Superior thin zone  66  and inferior thin zone  68  are thinner than center zone  70 . According to the depicted embodiment of the lens, orientation marker  72  is also present. Orientation marker  72  may be located inferiorly, nasally or temporally or superiorly so long as it marks a known orientation of displaced molding zone contact lens  32 . 
         [0030]    Referring now to  FIG. 6 , another embodiment of displaced molding zone contact lens  32  is depicted. In the depicted embodiment, rigid curved shell  34  of displaced molding zone contact lens  32  presents concave surface  42  back shaping zone  48 , second curve  50 , optional third curve  52  and fourth or peripheral curve  54 . Rigid curved shell  34  presents geometric center  56 . As can be seen in the depicted embodiment, shaping zone center  58  is displaced from geometric center  56  while second curve  50  is centered on geometric center  56 . 
         [0031]    Referring now to  FIG. 7 , a cross sectional view of displaced molding zone contact lens  32  is depicted. In this view, displaced molding zone contact lens  32  presents convex surface  40 , concave surface  42 , overall diameter  44 , geometric center  56  and shaping zone center  58 . For clarity, various other zones and curves are not depicted in  FIG. 7 . 
         [0032]    In operation, Displaced molding zone contact lens  32  is placed on cornea  26  of eye  22 . Displaced molding zone contact lens  32  is particularly useful for circumstances where the pupil or visual axis of the eye is demonstrated to be displaced from the geometric center of the cornea  26 . Displaced molding zone contact lens  32  orients so that back shaping zone  48  is substantially centered on the pupil or visual axis of the eye when the pupil or visual axis is displaced. Accordingly, shaping zone center  58  generally coincides with the visual axis or pupil center of eye  22 . 
         [0033]    Displaced molding zone contact lens  32  is expected to orient relative the cornea to approximately center back shaping zone  48  on the pupil or visual axis of the eye when the pupil or visual axis is displaced based on alignment between the anterior corneal epithelium  28  and concave surface  42 . 
         [0034]    Stabilizing structure  64 , if present, including superior thin zone  66  and inferior thin zone  68  are met with pressure from eyelids and thus are expected to tend to orient center zone  70  horizontally thus maintaining orientation and location of shaping zone center  58  substantially with the pupillary center or visual axis of eye  22 . 
         [0035]    If present, orientation marker  72  allows the contact lens wearer or practitioner to determine proper orientation of displaced molding zone contact lens  32  so that shaping zone center  58  substantially coincides with the desired location. 
         [0036]    The present invention may be embodied in other specific forms without departing from the spirit of the essential attributes thereof; therefore, the illustrated embodiments should be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.