Patent Publication Number: US-11660233-B2

Title: Systems and methods for tissue dissection in corneal transplants

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application No. 16/054,066, filed on Aug. 3, 2018, which claims the benefit of, and priority to, U.S. Provisional Patent Application Ser. No. 62/541,233, filed Aug. 4, 2017, the contents of each of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     Field 
     The present disclosure pertains to systems and methods for transplanting a cornea to treat disorders of the eye, and more particularly, to systems and methods for dissecting tissue for corneal transplants. 
     Description of Related Art 
     Various disorders of the eye may result from diseased/damaged corneal tissue. The diseased/damaged corneal tissue can affect vision by scattering and/or distorting light and causing glare and/or blurred vision. In some cases, proper vision can only be restored by a corneal transplant which replaces the diseased/damaged corneal tissue with healthy tissue from an organ donor. 
     SUMMARY 
     Systems and methods of the present disclosure employ a manual dissection system to remove diseased/damaged tissue from a cornea according to dimensions that match a corneal implant. For instance, to minimize the removal of the endothelium in a full-thickness transplant, the manual dissection system may remove a volume of diseased/damaged tissue according to a mushroom shape. 
     According to an example embodiment, a dissection system for corneal transplants includes a housing including a contact side configured to be positioned against a cornea. The housing includes an interior passageway with an opening at the contact side. The dissection system includes a blade assembly disposed in the interior passageway of the housing. The blade assembly includes a first blade and a second blade. The first blade includes a first cutting edge and the second blade includes a second cutting edge. The first blade and the second blade are movable relative to the housing such that the first cutting edge and the second cutting edge extend through the opening of the housing and out of the interior passageway. The first cutting edge is configured to produce a first cut in the cornea disposed at the contact side and the second cutting edge is configured to produce a second cut in the cornea. The first cut and the second cut defines a volume of tissue for removal from the cornea. The dissection system includes one or more manipulators configured to move the first blade and the second blade relative to the housing. The system may further include one or more cutting mechanisms configured to make further cuts transverse to at least one of the first cut or the second cut. The one or more cutting mechanisms may include one or more wires, and the one or more manipulators may be configured to move the wires to make the transverse cuts. 
     According to another example embodiment, a method operates a dissection system for corneal transplants. The dissection system includes a housing including a contact side configured to be positioned against a cornea, the housing including an interior passageway with an opening at the contact side. The dissection system includes a blade assembly disposed in the interior passageway of the housing. The blade assembly includes a first blade and a second blade, the first blade including a first cutting edge, the second blade including a second cutting edge, and the first blade and the second blade being movable relative to the housing. The dissection system includes one or more manipulators. The method includes positioning the contact side of the housing against a cornea. The method includes operating the one or more manipulators to move the first blade and the second blade relative to the housing such that the first cutting edge and the second cutting edge extend past the opening of the housing and out of the interior passageway. The first cutting edge produces a first cut in the cornea disposed at the contact side and the second cutting edge produces a second cut in the cornea, the first cut and the second cut defining a volume of tissue for removal from the cornea. The method may further include making further cuts, with one or more cutting mechanisms, transverse to at least one of the first cut or the second cut. The one or more cutting mechanisms may include one or more wires, and the method may further comprise operating the one or more manipulators to move the wires to make the transverse cuts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates example removal of tissue from a cornea according to a mushroom shape and a correspondingly shaped corneal implant received by a bed in the cornea formed by the removal of the tissue. 
         FIG.  2 A  illustrates an example dissection system for precise manual removal of corneal tissue in a corneal implant, according to aspects of the present disclosure. 
         FIG.  2 B  illustrates a top view of the example dissection system of  FIG.  2 A . 
         FIG.  2 C  illustrates a partial perspective view of the example dissection system of  FIG.  2 A . 
         FIG.  2 D  illustrates an example implementation of the example dissection system of  FIG.  2 A . 
         FIG.  3    illustrates another example dissection system for precise manual removal of corneal tissue in a corneal implant, according to aspects of the present disclosure. 
         FIG.  4    illustrates yet another example dissection system for precise manual removal of corneal tissue in a corneal implant, according to aspects of the present disclosure. 
         FIG.  5 A  illustrates a further example dissection system for precise manual removal of corneal tissue in a corneal implant, including an additional cutting mechanism for making cuts after penetration of outer/inner blades into the cornea, according to aspects of the present disclosure. 
         FIG.  5 B  illustrates a bottom view of the example dissection system of  FIG.  5 A . 
         FIG.  5 C  illustrates a partial perspective view of the example dissection system of  FIG.  5 A . 
         FIG.  5 D  illustrates an example coupling between a manipulator and the outer/inner blades for the example dissection system of  FIG.  5 A . 
         FIG.  5 E  illustrates another example coupling between a manipulator and the outer/inner blades for the example dissection system of  FIG.  5 A . 
         FIG.  6 A  illustrates aspects of an alternative cutting mechanism for making cuts after penetration of outer/inner blades into the cornea with a dissection system, according to aspects of the present disclosure. 
         FIG.  6 B  illustrates further aspects of the alternative cutting mechanism of  FIG.  6 A . 
         FIG.  7 A  illustrates aspects of an alternative cutting mechanism for making cuts after penetration of outer/inner blades into the cornea with a dissection system, according to aspects of the present disclosure. 
         FIG.  7 B  illustrates a cross-sectional view of further aspects of the alternative cutting mechanism of  FIG.  7 A . 
         FIG.  7 C  illustrates yet further aspects of the alternative cutting mechanism of  FIG.  7 A . 
     
    
    
     DETAILED DESCRIPTION 
     Various disorders of the eye may result from diseased/damaged corneal tissue. The diseased/damaged corneal tissue can affect vision by scattering and/or distorting light and causing glare and/or blurred vision. In some cases, proper vision can only be restored by a corneal transplant which replaces the diseased/damaged corneal tissue with healthy tissue from an organ donor. 
     From the outer (anterior) surface of the eye to the inner (posterior) parts, the structure of the cornea includes five layers: (1) epithelium, (2) Bowman&#39;s layer, (3) stroma, (4) Descemet&#39;s membrane, and (5) endothelium. Penetrating keratoplasty (PK) involves a full-thickness transplant where all layers of the cornea from the epithelium to the endothelium are removed and replaced with a corneal implant. In PK, a manual dissection device known as a trephine may be employed to remove the full thickness of existing corneal tissue. The trephine may also be used to cut a donor cornea to provide the corneal implant that dimensionally matches the removed corneal tissue. The corneal implant is then positioned in place of the removed corneal tissue and sutured into place. 
     Anterior lamellar keratoplasty (ALK) is an alternative treatment that selectively replaces diseased/damaged tissue in an anterior part of the cornea. A type of ALK procedure is deep anterior lamellar keratoplasty (DALK) which removes the epithelium, Bowman&#39;s layer, and the stroma but leaves the native Descemet&#39;s membrane and endothelium in place. In ALK, the surgeon dissects the cornea and removes the anterior part of the cornea. A dimensionally matching corneal implant from a donor cornea is then positioned in a bed formed by the removal of corneal tissue and sutured into place. 
     ALK is less invasive than PK and is preferred when the endothelium is healthy. In contrast to the cells of the epithelium and the stroma, the cells of the endothelium cannot regenerate. With ALK, patients retain their own endothelium so the risk of rejection by the immune system may be dramatically reduced. 
     Although PK involves a full-thickness transplant, certain approaches for PK attempt to minimize the removal of the endothelium. For instance, a patient may have a healthy endothelium, but central corneal scars and full-thickness opacities require a full-thickness transplant. As shown in  FIG.  1   , an example approach for PK removes an anterior portion  2   a  and a posterior portion  2   b  of tissue from a cornea  2 . The approach illustrated in  FIG.  1    can provide more effective and faster healing. The anterior portion  2   a  extends from the epithelial surface  2   c  of the cornea  2  to a depth in the stroma  2   d  to define a first thickness t 1  (along the z-axis as shown). The anterior portion  2   a  has a substantially circular profile along the x-y plane with a first diameter d 1 . For instance, the first thickness t 1  may be approximately 175 μm to approximately 200 μm and the first diameter d 1  may be approximately 9 mm. The posterior portion  2   b  extends from the anterior portion  2   a  through the endothelium  2   e  to define a second thickness t 2  (along the z-axis). t 1 +t 2  is the thickness from the epithelial surface  2   c  through the endothelium. The posterior portion  2   b  has a substantially circular profile along the x-y plane with a second diameter d 2 . For instance, the second thickness t 2  may be approximately 350 μm and the second diameter d 2  may be approximately 6.5 mm (or larger). The first diameter d 1  of the anterior portion  2   a  is greater than the second diameter d 2  of the posterior portion  2   b . The difference between the first diameter d 1  and the second diameter d 2  may be approximately 0.5 mm to approximately 1 mm. As such, the portions  2   a, b  together define a volume of tissue having a mushroom shape. The removal of the posterior portion  2   b  results in the removal of a smaller section of the endothelium than would be the case if the posterior portion  2   b  were to have the same diameter d 1  as the anterior portion  2   a  (corresponding to a removal of corneal tissue having a uniform diameter d 1 ). 
     As also shown in  FIG.  1   , the removal of the portions  2   a, b  forms a bed  2   f  in the cornea  10 . The bed  2   f  also has a mushroom shape. A corneal implant  4  is correspondingly shaped to be received in the bed  2   f . Using a microkeratome or other conventional dissection device to manually remove the portions  2   a, b  may not provide the sufficient precision to ensure a dimensional match between the corneal implant  4  and the bed  2   f  Indeed, the mushroom shape of the corneal implant  4  and the bed  2   f  makes it a greater challenge to achieve a match. Although a femtosecond laser may be employed to cut the portions  2   a, b  precisely from the cornea  2 , practitioners might not be conveniently equipped with a femtosecond laser system to cut the cornea  2  according to matching dimensions. 
     Advantageously, aspects of the present disclosure provide approaches for manually removing corneal tissue with the precision and consistency necessary to match the dimensions of a corneal implant. Such approaches employ devices that are more convenient and cost-effective than a femtosecond laser. With such devices, it is feasible for suppliers to shape a corneal implant with a femtosecond laser or similar high-precision cutting system and for practitioners to remove a volume of tissue manually and form a bed that accurately matches the shape of the corneal implant provided by the supplier. 
       FIGS.  2 A-D  illustrate an example dissection system  10  for manually removing corneal tissue. As shown in  FIG.  2 A , the dissection system  10  includes a housing  100 , a blade assembly  200 , and a syringe  300 . The housing  100  has a contact side  100   a  that can be placed against the cornea  2 . The contact side  100   a  may be contoured to accommodate the general anterior shape of the cornea  2 . The housing  100  includes a substantially cylindrical outer wall  102  that extends upwardly from the contact side  100   a  (in the positive-z direction). The housing  100  includes an interior passageway  108  with an opening  110  at the contact side  100   a.    
     The housing  100  includes one or more vacuum chambers  104  that can be coupled to the syringe  300  or other negative pressure source via a tube  302 . (The vacuum chambers  104  are selectively shown in  FIG.  2 A  with dashed lines.) The plunger of the syringe  300  may be drawn back or otherwise operated to provide a negative pressure in the vacuum chambers  104  via the tube  302 . One or more vacuum openings  106  for the vacuum chambers  104  are arranged along the periphery of the opening  110  at the contact side  100   a . The vacuum openings  106  can engage the epithelial surface  2   c  of the cornea  10 . Negative pressure in the vacuum chambers  104  generates suction between the epithelial surface  2   c  and the housing  100  at the vacuum openings  106 , thereby securely fixing the housing  100  to the cornea  2 . To decouple the housing  100  from the cornea  2 , the syringe  300  can be operated in an opposite manner to create positive pressure in the vacuum chambers  104  and release the suction at the vacuum openings  106 . 
     The housing  100  includes positioning elements  110  that extend radially outward from the outer wall  102 . The positioning elements  110  provide sufficient surface area that the practitioner can use to hold and position the housing  100 , e.g., between his/her fingers. 
     The blade assembly  200  is disposed in the interior passageway  108  of the housing  100 . The blade assembly  200  includes a manipulator  202 , which can be operated to cut the cornea  2  with the blade assembly  200 . For instance, a threaded coupling may be provided between the housing  100  and the blade assembly  200 . The manipulator  202  may be rotated about the z-axis to cause rotation of the blade assembly  200  relative to the housing  100 . As it rotates, the blade assembly  200  also rides along the thread of the coupling, which thus causes the blade assembly  200  to move axially (along the z-axis) relative to the housing  100  and the cornea  2 . As shown in the top view of  FIG.  2 B , the manipulator  202  includes a plurality of radially extending rods  204  which the practitioner can use to rotate the manipulator  202 , e.g., with his/her fingers. The practitioner may simultaneously use the positioning elements  110  to hold the housing  100  stably in position while rotating the manipulator  202 . 
     Accordingly, the manipulator  202  can move the blade assembly  200  in the negative-z direction and through the passageway opening  110  until the blade assembly  200  penetrates the cornea  2  positioned against the contact side  100   a  of the housing  100 . The housing  100  is securely coupled to the cornea  2  so that the blade assembly  200  is restricted to predictable and precise movement along the z-axis into the cornea  2 . 
     The blade assembly  200  includes an outer blade  210  and an inner blade  220 . (The outer blade  210  and the inner blade  220  are selectively shown in  FIGS.  2 A , C with dashed lines.) As shown in the partial view of the dissection system  10  in  FIG.  2 C , the outer blade  210  and the inner blade  220  are substantially tubular. The outer blade  210  includes a central passageway  212  with a substantially circular cutting edge  214 . The inner blade  220  is disposed in the central passageway  212  of the outer blade  210  and extends past the cutting edge  214  of the outer blade  210 . The inner blade  220  also includes a central passageway  222  with a substantially circular cutting edge  224 . 
     The outer cutting edge  214  and the inner cutting edge  224  are substantially concentric. Correspondingly, the outer blade  210  and the inner blade  220  create substantially concentric circular cuts into the cornea  2 . When the blade assembly  200  penetrates the cornea  10 , the circular cut made by the outer cutting edge  214  has a larger diameter than the circular cut made by the inner cutting edge  224 . For instance, as shown in  FIG.  2 D , the outer cutting edge  214  may have a diameter that makes a substantially circular outer cut with the first diameter d 1 , which corresponds to the anterior portion  2   a  removed from the cornea  2 . Additionally, the inner cutting edge  224  may have a diameter that makes a substantially circular inner cut with the second diameter d 2 , which corresponds to the posterior portion  2   b  removed from the cornea  2 . 
     The manipulator  202  moves the outer blade  210  and the inner blade  220  simultaneously. As shown in the top view  FIG.  2 B , the manipulator  202  includes an aperture  206  that aligns with the central passageway  222  of the inner blade  220 . As such, the cornea  2  can be seen through the aperture  206  and the central passageway  222 . Cross-hairs  208  or other positioning guides may be disposed in the aperture  206  and/or the central passageway  222  to mark the center of the outer blade  210  and the inner blade  220 . The practitioner may employ the cross-hairs  208  to fix the housing  100  to the cornea  2  and center the blades  210 ,  220  over a desired location, e.g., the center, of the cornea  2 . As such, the location of the cuts made by the outer cutting edge  214  and the inner cutting edge  224  can be controlled. 
     As shown in  FIG.  2 C , the inner blade  220  extends farther downward in the negative-z direction than the outer blade  210 . Thus, when the blade assembly  200  penetrates the cornea  10 , the inner blade  220  penetrates the cornea  10  to a greater depth than the outer blade  210 . The blade assembly  200  includes an outer blade depth controller  216  to control the penetration depth of the outer blade  210  and an inner blade depth controller  226  to control the penetration depth of the inner blade  220 . For instance, the outer blade depth controller  216  and the inner blade depth controller  226  may be separately rotated about the z-axis as manual dials to set the respective penetration depths. 
     As shown in  FIG.  2 D , the outer blade depth controller  216  may be operated so that the outer blade  210  moves past the contact side  100   a  of the housing  100  by a distance that makes an outer cut in the cornea  2  with a depth of t 1 . This provides the first thickness t 1  of the anterior portion  2   a . Additionally, the inner blade depth controller  226  may be operated so that the inner blade  220  moves past the contact side  100   a  of the housing  100  by a distance that makes an inner cut in the cornea  2  with a depth of t 1 +t 2 . This provides the second thickness t 2  of the posterior portion  2   b.    
     According to an example embodiment, the outer blade depth controller  216  can move one or more adjustable stops  112  to a position along the interior passageway  108  of the housing  100 . One or more corresponding stops  218  are coupled to the outer blade  210 . (The stops  112 ,  218  are selectively illustrated by dashed lines in  FIG.  2 A .) Thus, when the manipulator  202  is operated, the outer blade  210  can move downwardly in the negative-z direction and penetrate the cornea  2  until the stops  218  of the outer blade  210  reach the stops  112  at the set position. The outer blade depth controller  216  may provide numerical markers to allow the practitioner to dial a position for the stops  112  corresponding to the desired penetration depth t 1  for the outer blade  210 . 
     Meanwhile, the inner blade depth controller  226  can adjust the distance between the inner cutting edge  224  and the outer cutting edge  214 . For instance, the inner blade  220  may be adjustably coupled to the outer blade  210 , e.g., by a threaded coupling, and the inner blade depth controller  226  may be operated to adjust the coupling and set the distance. The inner blade depth controller  226  may provide numerical markers to allow the practitioner to dial the desired thickness t 2  for the posterior portion  2   b . This thickness is equivalent to the distance between the cutting edges  214 ,  224 . 
     Once the distance between the cutting edges  214 ,  224  is set with the inner blade depth controller  226 , the manipulator  202  may be operated to move the outer blade  210  as described above. Correspondingly, the inner blade  220  moves with the outer blade  210  at the set distance. As illustrated in  FIG.  2 D , when the outer blade  210  is stopped from further movement by the stops  112 , the outer cutting edge  214  stops its cut at the desired penetration depth t 1  and the inner cutting edge  224  stops its cut at the set distance t 2  from the outer cutting edge  214 . Accordingly, the outer blade  210  creates an outer cut with the depth and diameter to remove the anterior portion  2   a , and the inner blade  220  creates an inner cut with the depth and smaller diameter to remove the posterior portion  2   b . (The inner cut of the inner blade  220  also passes through the anterior portion  2   a  but does not affect the outer cut of the outer blade  210 .) 
     After making the desired outer and inner cuts, the housing  100  and the blade assembly  200  can be released from the cornea  2  by operation of the manipulator  202  and the syringe  300 . With the precise outer and inner cuts, a separate dissection device or other manual instrument may be employed to remove the anterior portion  2   a  and the posterior portion  2   b . In particular, to remove the anterior portion  2   a , an annular cut is made at substantially the penetration depth of t 1  for the outer blade, between the outer cut to the inner cut. Additionally, to remove the posterior portion  2   b , a circular cut defined by the circumference of the inner cut is made at substantially the penetration depth of t 1 +t 2  for the inner blade. As described above, the removal of the portions  2   a, b  produces a bed  2   f  for receiving the corneal implant  4 . With the precision of the cuts by the dissection system  10 , the bed  2   f  provides a dimensional match with the corneal implant  4 . 
     Aspects of the present disclosure are not limited to the embodiment described in  FIGS.  2 A-D . For instance,  FIG.  3    illustrates another example dissection system  20  including an alternative blade assembly  400  with an outer blade  410  and an inner blade  420 . In contrast to the outer blade  210  and the inner blade  220  described above, the movement of the inner blade  420  is not coupled to the movement of the outer blade  410 . As such, the blade assembly  400  includes a first manipulator  402   a  to move the outer blade  410  along the z-axis and a second manipulator  402   b  to move the inner blade  420  separately along the z-axis. 
     The blade assembly  400  includes an outer blade depth controller  416  that can move one or more adjustable stops  432  to a position along the interior passageway  108  of the housing  100 . One or more corresponding stops  418  are coupled to the outer blade  410 . Similar to the manipulator  202 , when the manipulator  402   a  is operated, the outer blade  410  can move downward in the negative-z direction and penetrate the cornea  2  until the stops  418  of the outer blade  410  reach the stops  432  at the set position. The outer blade depth controller  416  may provide numerical markers to allow the practitioner to dial a position for the stops  432  corresponding to the desired penetration depth t 1  for the outer blade  410 . 
     The operation of the manipulator  402   a , however, does not move the inner blade  420 . Thus, the blade assembly  400  includes an inner blade depth controller  426  that that can move one or more adjustable stops  442  to a position along a central passageway  412  of the outer blade  410 . One or more corresponding stops  428  are coupled to the inner blade  420 . When the manipulator  402   b  is operated, the inner blade  420  can move in the negative-z direction and penetrate the cornea  2  until the stops  428  of the inner blade  420  reach the stops  442  at the set position. The inner blade depth controller  426  may provide numerical markers to allow the practitioner to dial a position for the stops  442  corresponding to the desired penetration depth t 1 +t 2  for the inner blade  420 . Accordingly, the practitioner operates each of the manipulators  402   a, b  separately to make the respective outer and inner cuts. 
       FIG.  4    illustrates another example dissection system  30  including an alternative blade assembly  500  as well as the housing  100  and the syringe  300 . In contrast to the blade assemblies  200 ,  400  described above, the blade assembly  500  includes an outer blade  510  and an inner blade  520  with constant respective penetration depths. In other words, blade assembly  500  does not employ depth controllers that allow the respective penetration depths to be adjusted. For instance, one or more stops  532  are fixedly positioned along the interior passageway  108  of the housing  100 . One or more corresponding stops  518  are coupled to the outer blade  510 . The blade assembly  500  includes a manipulator  502  that can be operated to move the outer blade  510  in the negative-z direction and penetrate the cornea  2  until the stops  518  of the outer blade  510  reach the stops  532  at the set position. The position for the stops  532  corresponds to the desired penetration depth t 1  for the outer blade  510 . 
     Additionally, the position of the inner blade  520  relative to the outer blade  510  cannot be adjusted. The inner blade  520  has an inner cutting edge  524  that is fixedly positioned at a distance t 2  from an outer cutting edge  514  of the outer blade  510 . As such, when the outer blade  510  reaches the desired penetration depth t 1 , the inner blade  520  reaches a desired penetration depth t 1 +t 2 . Accordingly, the practitioner operates the manipulator  502  to make the same outer and inner cuts. 
     As described above, a separate dissection device or other manual instrument may be employed to remove the anterior portion  2   a  and the posterior portion  2   b  after a blade assembly  200 ,  400 ,  500  has been manipulated to make cuts in the cornea with the outer blade and the inner blade. In alternative embodiments, however, the blade assembly may be configured to make further cuts to remove the anterior portion  2   a  and the posterior portion  2   b . Such a blade assembly eliminates the need for a separate dissection device or other manual instrument. In particular, to remove the anterior portion  2   a , the blade assembly can make a cut (e.g., an annular cut) at the penetration depth of t 1 , between the outer cut to the inner cut. Additionally, to remove the posterior portion  2   b , the blade assembly can make a cut (e.g., a circular cut) defined by the inner cut at the penetration depth of t 1 +t 2 . The annular and circular cuts are generally transverse to the outer and inner cuts, respectively. 
       FIGS.  5 A-C  illustrate an example dissection system  40  employing a blade assembly  600 . Like the blade assembly  500  described above, the blade assembly  600  includes an outer blade  610  and an inner blade  620  with constant respective penetration depths. The inner blade  620  has an inner cutting edge  624  that is fixedly positioned at a distance t 2  from an outer cutting edge  614  of the outer blade  610 . As such, when the outer blade  610  reaches the desired penetration depth t 1 , the inner blade  620  reaches a desired penetration depth t 1 +t 2 . 
     The dissection system  40  includes the housing  100  and the syringe  300 . As described above, the housing  100  can be positioned securely against the cornea  2  with the use of a negative pressure provided by the syringe  300 . The blade assembly  600  is disposed in the interior passageway  108  of the housing  100 . The housing  100  thus positions the blade assembly  600  relative to the cornea  2 . 
     The blade assembly  600  includes a manipulator  602  that can be rotated about the z-axis to cause the outer blade  610  to move relative to the housing  100  and the cornea  2 . Such movement of the outer blade  610  results in corresponding movement of the inner blade  620 , which is fixed relative to the outer blade  610 . The manipulator  602  can be rotated to cause penetration of the outer blade  610  to a desired depth t 1  and penetration of the inner blade  620  to a desired depth t 1 +t 2 . The manipulator  602  includes a plurality of radially extending rods  604  which the practitioner can use to rotate the manipulator  602 , e.g., with his/her fingers. The practitioner may simultaneously use the positioning elements  110  to hold the housing  100  stably in position while rotating the manipulator  602 . 
     The blade assembly  600  can make an annular cut at the penetration depth t 1  between the cuts made by the outer blade  610  and the inner blade  620 . Additionally, at the penetration depth t 1 +t 2 , the blade assembly  600  can make a circular cut with a circumference defined by the inner blade  620 . Together, the annular cut and the circular cut allow the anterior portion  2   a  and the posterior portion  2   b  to be removed. 
     As shown in  FIGS.  5 B-C , the blade assembly  600  includes wires  651  (or similar cutting structures) that extend between the outer blade  610  and the inner blade  620  within the central passageway  612  of the outer blade  610 . The wires  651  are aligned with the outer cutting edge  614  of the outer blade  610  (i.e., generally, at the same position on the z-axis as the outer cutting edge  614 ). Additionally, the blade assembly  600  includes a wire  652  (or similar cutting structure) that extends across the central passageway  222  of the inner blade  620 . The wire  652  is aligned with the inner cutting edge  612  of the inner blade  610  (i.e., generally, at the same position on the z-axis as the inner cutting edge  612 ). 
     When the outer cutting edge  614  of the outer blade  610  penetrates the cornea  2  to the desired depth t 1 , the wires  651  also penetrate the cornea  2  to the desired depth t 1 . Meanwhile, when the inner cutting edge  624  of the inner blade  620  correspondingly penetrates the cornea  2  to the desired depth t 1 +t 2 , the wire  652  also penetrates the cornea to the desired depth t 1 +t 2 . The wires  651 ,  652  have sufficient tension and sharpness to cut through the cornea  2  and do not generate significant resistance against the movement of the outer blade  610  and the inner blade  610 . Although  FIGS.  5 B-C  illustrate two wires  651  and one wire  652  as an example, embodiments may employ different numbers of wires  651  and/or wires  652 . The wires  651 ,  652  can penetrate the cornea  2 , because the outer blade  610  and the inner blade  620  do not rotate relative to the housing  100  and the cornea  2  when penetrating the cornea  2 . 
       FIG.  5 D  illustrates an example configuration for coupling the manipulator  602  to the outer blade  610  and the inner blade  620 . The example configuration allows the manipulator  602  to be operated so that the outer blade  610  and the inner blade  620  move axially along the z-axis to penetrate the cornea  2  without rotating about the z-axis. 
     As shown in  FIG.  5 D , the manipulator  602  is coupled to the housing  100 . The manipulator  602  can rotate about the z-axis relative to the housing  100 , but cannot move according to other degrees of freedom relative to the housing  100 . For instance, the manipulator  602  may include engagement structures  605  that can snap into an annular track  105  running along a surface (e.g., top surface) of the housing  100 ; the engagement structures  605  can move within the annular track  105  to allow rotation of the manipulator  602 . 
     The manipulator  602  includes a central passageway  603 . The outer blade  610  is disposed within the central passageway  603 . The outer blade  610  includes an outer surface  617  that faces an inner surface  607  of the manipulator  602  within the central passageway  603 . The manipulator  602  includes a thread  662  that spirals along the inner surface  607 . The outer blade  610  includes tabs  664  that are biased to extend radially outward from the outer surface  617  and engage the thread  662 . When the manipulator  602  is rotated in a first direction about the z-axis, the thread  662  applies a force against the tabs  664  in the negative-z direction. This force causes the outer blade  610 , as well as the inner blade  620  fixed to the outer blade  610 , to move in the negative-z direction and penetrate the cornea  2 . The movement of the outer blade  610  and the inner blade  620  does not involve rotation about the z-axis relative to the housing  100  and the cornea  2 . In some cases, the housing  100  may include one or more guide structures to engage the outer blade  610  and prevent such rotation while allowing movement along the z-axis. Rotation of the manipulator  602  in the first direction stops when the outer blade  610  and the inner blade  620  reach their respective desired penetration depths t 1  and t 1 +t 2 , respectively. 
     Once the outer blade  610  and the inner blade  620  reach the desired penetration depths, the manipulator  602  can be further operated to make additional cuts (e.g., transverse cuts) to allow the anterior portion  2   a  and the posterior portion  2   b  to be removed. In particular, the manipulator  602  can be rotated in a second direction about the z-axis to cause the wires  651 ,  652  to rotate about the z-axis. This second direction is opposite from the first direction in which the manipulator  602  is rotated to move the outer blade  610  and the inner blade  620  in the negative-z direction. Rotation of the wires  651  makes an annular cut at the penetration depth t 1 , between the outer cut to the inner cut. Meanwhile, rotation of the wire  652  makes a circular cut at the penetration depth of t 1 +t 2 . 
     As shown in  FIG.  5 D , when the manipulator  602  is rotated in the second direction about the z-axis, the outer blade  610  and the inner blade  620  do not move in the positive-z direction. Although the thread  662  may apply a force against the tabs  664  in the positive-z direction, the tabs  664  are shaped (e.g., with an angled surface) so that such force also pushes the tabs  664  radially inward. The force overcomes the radially outward bias of the tabs  664 , causing the tabs to move radially inward. This inward movement of the tabs  664  prevents the force in the positive-z direction from pushing the outer blade  610  and the inner blade  620  in the positive-z direction. 
     The manipulator  602  includes tabs  663  that engage the tabs  664  of the outer blade  610  as the manipulator is rotated in the second direction. The engagement between the tabs  663 ,  664  causes the outer blade  610  as well as the inner blade  620  to rotate in the second direction with the manipulator  602 . The wires  651 ,  652  rotate correspondingly with the outer blade  610  and inner blade  620 . Because the thread  662  does not move the outer blade  610  and inner blade  620  along the z-axis, the wires  651 ,  652  rotate on the x-y planes at the depths t 1  and t 1 +t 2 , respectively, to produce the desired cuts. 
     Once the cuts with the wires  651 ,  652  are completed, the anterior portion  2   a  and the posterior portion  2   b  can be removed from the cornea  2 . In some cases, withdrawal of the dissection system  40  from the cornea  2  also removes the dissected tissue. 
     The outer blade  610  and the inner blade  620  can be reset relative to the manipulator  602  and the housing  100  for a subsequent dissection procedure. As shown in  FIG.  5 D , each tab  664  is disposed on one end of a biasing structure  666  positioned within the outer blade  610 . The biasing structure  666  pushes the tabs  664  radially outward through the outer surface  617  of the outer blade  610 . A button  668  is disposed near the other end of the biasing structure  666  and also extends radially outward through the outer surface  617 . When the button  668  is pushed radially inward, resulting movement of the biasing structure  666  causes the tab  664  to also move radially inward and to disengage the track  662  of the manipulator  602 . Accordingly, the buttons  668  can be squeezed together with fingers to allow the outer blade  610 , as well as the inner blade  620 , to be moved in the positive-z direction, back to a starting position for the subsequent dissection procedure. 
       FIG.  5 E  illustrates an alternative configuration for coupling the manipulator  602  to the outer blade  610  and the inner blade  620 . Similar to the configuration of  FIG.  5 D , the manipulator  602  is coupled to the housing  100  (not shown). The manipulator  602  can rotate about the z-axis relative to the housing  100 , but cannot move according to other degrees of freedom relative to the housing  100 . In addition, the manipulator  602  includes the central passageway  603 . The outer blade  610  is disposed within the central passageway  603 . The outer surface  617  of the outer blade  610  faces the inner surface  607  of the manipulator  602 . 
     As shown in  FIG.  5 E , the manipulator  602  includes the thread  662  which spirals along the inner surface  607 . The outer blade  610  includes tabs  674  that are biased to extend radially outward from the outer surface  617  and engage the threads  672 . When the manipulator  602  is rotated in a first direction about the z-axis, the thread  672  applies a force against the tabs  674  in the negative-z direction. This force causes the outer blade  610 , as well as the inner blade  620  fixed to the outer blade  610 , to move in the negative-z direction and penetrate the cornea  2 . The movement of the outer blade  610  and the inner blade  620  does not involve rotation about the z-axis relative to the housing  100  and the cornea  2 . In some cases, the housing  100  may include one or more guide structures to engage the outer blade  610  and prevent such rotation while allowing movement along the z-axis. 
     Unlike the configuration of  FIG.  5 D , the tabs  674  continue to move along the thread  672  until they enter a groove  676  at the end of the thread  672 . At this point, the outer blade  610  and the inner blade  620  have reached their desired penetration depths t 1  and t 1 +t 2 , respectively. With the tabs  674  positioned in the groove  676 , the thread  672  can no longer apply a force to the tabs  674  and the manipulator  602  can be further rotated in the same first direction about the z-axis to make additional cuts to allow the anterior portion  2   a  and the posterior portion  2   b  to be removed. 
     The manipulator  602  includes tabs  673  that engage the tabs  664  of the outer blade  610  as the manipulator continues to rotate in the first direction. The engagement between the tabs  663 ,  664  causes the outer blade  610  as well as the inner blade  620  to rotate in the first direction with the manipulator  602 . The wires  651 ,  652  rotate correspondingly with the outer blade  610  and inner blade  620 . Because the thread  672  does not move the outer blade  610  and inner blade  620  along the z-axis, the wires  651 ,  652  rotate on the x-y planes at the depths t 1  and t 1 +t 2 , respectively, to produce the desired cuts. As described above, rotation of the wires  651  makes an annular cut at the penetration depth t 1 , between the outer cut to the inner cut. Meanwhile, rotation of the wire  652  makes a circular cut at the penetration depth of t 1 +t 2 . 
     The outer blade  610  and the inner blade  620  can be reset relative to the manipulator  602  and the housing  100  for a subsequent dissection procedure. As shown in  FIG.  5 E , each tab  674  is disposed on one end of the biasing structure  666  positioned within the outer blade  610 . The biasing structure  666  pushes the tabs  674  radially outward through the outer surface  617  of the outer blade  610 . A button  668  is disposed near the other end of the biasing structure  666  and also extends radially outward through the outer surface  617 . When the button  668  is pushed radially inward, resulting movement of the biasing structure  666  causes the tab  674  to also move radially inward and allows the tab  674  to disengage the track  672  of the manipulator  602 . Accordingly, the buttons  668  can be squeezed together with fingers to allow the outer blade  610 , as well as the inner blade  620 , to be moved in the positive-z direction, back to a starting position for the subsequent dissection procedure. 
     As shown in  FIGS.  5 A-C , the wires  651 ,  652  in the example dissection system  40  have sufficient tension and sharpness to cut through the cornea  2  as the outer blade  610  and the inner blade  620  penetrate the cornea  2 . Operation of the manipulator  602  to rotate the wires  651 ,  652  can also increase the tension in the wires  651 ,  652 . Alternative embodiments, however, may provide additional support for the movement of the wires  651 ,  652  in the negative-z direction. For instance,  FIGS.  6 A-B  illustrates the outer blade  610  and the inner blade  620 , as well as the wires  651 ,  652  described above.  FIG.  6 A  shows a support structure  653  extending between the outer blade  610  and the inner blade  620  and to the cutting edge  614  of the outer blade  610 . One of the wires  651  is disposed at the end of the support structure  653  and aligned with the cutting edge  614 . The end of the support structure  653  may be recessed or otherwise shaped to engage the wire  651  further. An additional support structure  653  (not shown) may be implemented with the other wire  651 . Meanwhile,  FIG.  6 B  shows a support structure  655  extending to the cutting edge  624  within the central passageway  622  of the inner blade  620 . The wire  652  is disposed at the end of the support structure  655  and aligned with the cutting edge  624 . The end of the support structure  655  may be recessed or otherwise shaped to engage the wire  652  further. The support structures  653 ,  655  move with the outer blade  610  and the inner blade  620  as they penetrate the cornea  2 . Advantageously, the support structures  653 ,  655  help the wires  651 ,  652  to move through the cornea  2 . When the outer blade  610  and the inner blade  620  reach their respective desired penetration depths, the manipulator  602  may be operated as described above to make the additional cuts with the wires  651 ,  652 . In this case, the wires  651 ,  652  disengage from the respective support structures  653 ,  655  to rotate with the manipulator  602 . 
     As shown in  FIGS.  6 A-B , the support structures  653 ,  655  may have a wedge-like or blade-like shapes extending substantially along the length of the outer blade  610  and the inner blade  620 , respectively. In other embodiments, however, the support structures  653 ,  655  may have alternative shapes. For instance, the support structures  653 ,  655  may be shorter cross-bars that extend across and above the wires  653 ,  655  to provide support. 
       FIGS.  7 A-C  illustrate an alternative approach for supporting for the movement of the wires in the negative-z direction. For instance,  FIG.  7 A  illustrates a support structure  683  for a wire  651 . (In contrast to the examples above, a single wire  651  is employed here.) The end  683   a  of the support structure  683  provides a leading edge as the outer blade  610  and the inner blade  620  penetrate the cornea  2 . In  FIG.  7 A , the support structure  683  includes a recess  683   b  that receives the wire  651  above the end  683   a  of the support structure  683 . In contrast, the wire  651  in  FIGS.  6 A-C  is positioned below the support structure  653  and provides the leading edge. Advantageously, the end  683   a  may be sharper than the wire  651  and can cut through the cornea  2  more easily while the wire  251  remains in the recess  683   b.    
     When the outer blade  610  and the inner blade  620  reach the respective desired penetration depths, the manipulator  602  may be operated to disengage the wire  651  from the recess  683   b  in the support structure  683  and to rotate the wire  651  about the z-axis to produce the cuts to help remove the anterior portion  2   a . Although the wire  651  is received in the recess disposed above the end  683   a  of the support structure  683 , the support structure  683  delivers the wire  651  to a depth where the wire  251  can provide an effective cut near the penetration depth t 1  (e.g., within approximately 5 μm). 
     To make the circular cut at or near the penetration depth t 1 +t 2 , a wire  652 ′ as shown in the views of  FIGS.  7 B-C  may be employed. The support structure  685  for the wire  652 ′ may be configured to receive the wire  652 ′ in a recess  685   b  in a manner similar to the support structure  683 . In contrast to the support structure  655  and the wire  652  which extends across the entire diameter of the inner blade  620 , the wire  652 ′ extends across the radius of the inner blade  620 . The wire  652 ′ extends from a center support  657  to an inner wall of the inner blade  620 . The wire  652 ′ can rotate about the center support  657  to make the desired circular cut. 
     In  FIGS.  7 A-C , the rotation of the wires  651 ,  652 ′ starts from one side of the support structures  683 ,  685  (i.e., out of the recesses  683   b ,  685   b ) and ends on the other side of the support structures  683 ,  685 , respectively. As such, the wires  651 ,  652 ′ are blocked by the support structures  683 ,  685  from making complete annular and circular cuts, respectively. The cuts by the wires  651 ,  652 ′, however, are sufficient to allow removal of the anterior portion  2   a  and posterior portion  2   b , respectively. 
     Although the inner and outer blades of the example embodiments above may have substantially circular profiles, it is understood that the other embodiments may employ other profiles to make cuts of different shapes, e.g., elliptical cuts. Additionally, it is understood that the blade assemblies in other embodiments may be configured to make non-concentric inner and outer cuts. Furthermore, it is understood that the blade assemblies in other embodiments may include more than two blades. 
     Although the inner cuts made by the inner blade in the example implementations above may have penetration depths that extend through the endothelium, it is understood that other implementations may employ penetration depths that do not extend completely to the endothelium. Furthermore, although the blade assemblies of the example embodiments above may remove a volume of corneal tissue having a mushroom shape, it is contemplated that blade assemblies in other embodiments may be configured to make cuts that allow corneal tissue to be removed according to other shapes. 
     While the present disclosure has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present disclosure. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the invention. It is also contemplated that additional embodiments according to aspects of the present disclosure may combine any number of features from any of the embodiments described herein.