Patent Publication Number: US-2009234333-A1

Title: Microkeratome and cutting head with non-coplanar applanation plate and stromal plate

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 11/366,043, filed on Mar. 1, 2006, and titled “Microkeratome with a Detachable Head”, the content of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to procedures entailing the cutting of corneal tissue and to microkeratomes and related components utilized to cut corneal tissue. In particular, the present invention relates to cutting heads of microkeratomes that include non-coplanar applanation plates and stromal plates. 
     BACKGROUND 
     There have been developed a number of different surgical techniques to correct hyperopic or myopic conditions of a human eye. U.S. Pat. No. 4,840,175 issued to Peyman discloses a procedure wherein a thin layer of a cornea is cut to expose the stroma layer of the cornea. A laser beam is then directed onto the exposed corneal tissue in a predetermined pattern. The laser beam ablates corneal tissue and changes the curvature of the eye. This procedure is sometimes referred to as Laser in situ Keratomileusis (LASIK). 
     U.S. Pat. No. Re 35,421 issued to Ruiz et al. discloses a device for cutting a cornea in a LASIK procedure. Such a device is commonly referred to as a microkeratome. The Ruiz microkeratome includes a ring that is placed onto a cornea and a blade that is located within an opening of the ring. The device also contains a drive mechanism which moves the blade across the cornea in a first direction while the blade moves in a reciprocating transverse direction to cut the eye. The device can create a lamella flap of the cornea which is flipped back so that the stromal bed of the cornea can be ablated with a laser. 
     U.S. Pat. No. 6,051,009 issued to Hellenkamp et al. discloses a microkeratome that is sold under the trademark HANSATOME. The HANSATOME microkeratome moves the blade in an arcuate path about the cornea. The HANSATOME microkeratome includes a disposable blade assembly that can be loaded and removed from the device. The blade assembly includes a blade holder that is attached to a cutting blade. The blade holder has a recess that receives the end of a drive shaft. Rotation of the output shaft moves the blade in an arcuate path, and moves the blade in a back-and-forth motion to create the lamella flap of the cornea. 
     Microkeratomes have three primary components, a hand piece that contains a motor, a head that holds the blade, and a ring that applies a suction to maintain the position of the microkeratome relative to the cornea. Because the microkeratome is in contact with patient tissue it must be cleaned after each procedure, typically involving an autoclave. The head has a number of small cavities that are more difficult to clean. Additionally, the autoclave process may degrade the head after a number of procedures and cleaning cycles. It would be desirable to provide a microkeratome that does not require the head to be sterilized after each surgical procedure. 
     The blades used to cut tissue are replaced after each procedure. The replacement blades are typically loaded into the head of the microkeratome with a pair of forceps. The blade must be loaded accurately so that a drive pin of the motor assembly is inserted into a corresponding slot of a blade holder. Accurately loading the blade with forceps can be a time consuming process. It would be desirable to provide a blade package that can be used to accurately load a blade into a microkeratome in a time efficient manner. 
     A complication may occur while the microkeratome is cutting the lamella flap. It may be desirable to remove the microkeratome in the middle of a cut. Removing the microkeratome requires releasing the vacuum of the suction ring. Releasing the vacuum allows the cornea to move back to its original shape. Movement of the cornea will also cause the blade to move. Movement of the blade may cause damage to the cornea. It would be desirable to provide a microkeratome that allows a surgeon to remove the head while the suction ring is still fixed to the cornea. It would also be desirable to provide a microkeratome that allows the surgeon to vary the thickness of the lamella flap hinge. 
     Another problem attending the use of microkeratome cutting heads of known designs is the occurrence of a buttonhole (or dimple) in the cornea, which is a result of the cutting procedure. A buttonhole generally is a depression in the central region of the cornea, which results in an uncut island of tissue and is created as a conventionally designed cutting head passes over the cornea. The buttonhole is highly undesirable as it results in the blade cutting only the peripheral tissue that is at a higher elevation than the tissue in the central cornea where the buttonhole exists. Thus, a flap having a hole at its center is thereby created instead of an intended continuous or unbroken corneal flap. The occurrence of the buttonhole and consequent peripheral cutting event typically require that the surgeon lay the corneal flap (having the hole in its center) back down on the stromal bed of the cornea and wait a few months to attempt the intended procedure again. Therefore, there is also a need for providing a microkeratome that prevents the occurrence of such buttonholes, dimples, or depressions in the corneal tissue. 
     SUMMARY 
     To address the foregoing problems, in whole or in part, and/or other problems that may have been observed by persons skilled in the art, the present disclosure provides methods, processes, systems, apparatus, instruments, and/or devices, as described by way of example in implementations set forth below. 
     According one implementation, a microkeratome cutting head includes a head structure, an applanation plate, and a stromal plate. The head structure may include a front side, a back side, a bottom side between the front side and the back side, a top side, a blade cavity disposed in the head structure and configured for receiving a blade, and a blade opening disposed at the bottom side and through which the blade extends. The blade is configured for creating a corneal flap at a corneal flap thickness. The stromal plate may be disposed at the bottom side between the blade opening and the back side, and lies in a stromal plate plane. The applanation plate may be disposed at the bottom side between the blade opening and the front side. The applanation plate is disposed at an elevation above the stromal plate plane, relative to an axis perpendicular to the stromal plate plane and directed generally from the bottom side toward the top side. The elevation from the stromal plate plane to the applanation plate is approximately equal to the corneal flap thickness. 
     According to another implementation, a microkeratome includes a hand piece, a head coupled to the hand piece, and a blade. The head may include a front side, a back side, a bottom side between the front side and the back side, a top side, a blade cavity disposed in the head structure, a blade opening disposed at the bottom side, a stromal plate, and an applanation plate. The stromal plate is disposed at the bottom side between the blade opening and the back side, and lies in a stromal plate plane. The applanation plate is disposed at the bottom side between the blade opening and the front side. The blade may be disposed in the blade cavity and extend out from the bottom side through the blade opening. The blade is configured for creating a corneal flap at a corneal flap thickness. The applanation plate is disposed at an elevation above the stromal plate plane, relative to an axis perpendicular to the stromal plate plane and directed generally from the bottom side toward the top side. The elevation of the applanation plate relative to the stromal plate is approximately equal to the corneal flap thickness. 
     According to another implementation, a method is provided for creating a corneal flap in a cornea. A cutting head of a microkeratome is moved across the cornea. While moving the cutting head, the cornea is flattened by contacting the cornea with an applanation plate of the cutting head such that the applanation plate passes over the cornea. While the cornea is flattened, a corneal flap is created by contacting the cornea with a cutting edge of a blade extending out from the cutting head. While the cornea is being cut, a buttonhole is prevented from forming in the cornea by contacting the cornea with a stromal plate of the cutting head, the stromal plate being located behind the cutting edge relative to the applanation plate. 
     According to another implementation, a method is provided for fabricating a microkeratome cutting head. A head structure configured for coupling to a microkeratome hand piece is formed. A blade cavity and a blade opening are formed in the head structure. The blade cavity is configured for receiving a blade such that the blade extends through the blade opening and the blade is positioned for creating a corneal flap at a corneal flap thickness. An applanation plate is formed in front of the blade opening and at a bottom side of the head structure from which the blade extends, the applanation plate lying in an applanation plate plane. A stromal plate is formed behind the blade opening and at the bottom side, such that the stromal plate is positioned at an elevation below the applanation plate plane. The elevation difference between the applanation plate and the stromal plate is set to be approximately equal to the corneal flap thickness. 
     Other devices, apparatus, systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood by referring to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is an exploded view of a microkeratome assembly of the present invention; 
         FIG. 2  is a perspective view of the microkeratome; 
         FIG. 3  is a top view showing three different aspiration connectors; 
         FIG. 4  is a sectional view showing a latch assembly of the microkeratome; 
         FIG. 5  is a sectional view showing motors of the microkeratome; 
         FIG. 6  is a top sectional view showing a blade being loaded into a head of the microkeratome from a blade shuttle. 
         FIG. 7  is a side elevation view of a microkeratome cutting head according to another implementation. 
         FIG. 8  is a detailed view of the region designated “A” in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed is a microkeratome that includes a latch assembly that couples a head to a hand piece. The latch assembly allows the head to be readily detached from the hand piece and sterilized. There is no need to also sterilize the hand piece. The microkeratome also has a ring assembly that is coupled to the head and the hand piece. The ring assembly may include a fastener that can be unfastened to allow the hand piece and head to be detached from the ring, even while the ring assembly is fixed to a cornea. 
     The hand piece includes a motor that moves the blade across the ring. The microkeratome may have an aspiration connector with a collar that limits the travel of the blade and the thickness of a resulting lamella flap. The aspiration connector can be replaced with a collar of a different diameter to produce a flap with a different thickness. This allows a surgeon to select a flap hinge thickness. 
     The blade may be loaded into the microkeratome with a blade shuttle. The blade shuttle may have a plunger that pushes the blade into the microkeratome head. The movement of the plunger may be limited by a stop within the shuttle. The stop assists in accurately locating the blade within the head. 
     Also disclosed is a microkeratome head that includes an applanation plate and a stromal plate disposed in a non-coplanar configuration in which the stromal plate is at a lower elevation than the applanation plate. 
     Referring to the drawings more particularly by reference numbers,  FIGS. 1 and 2  show an embodiment of a microkeratome  10  of the present invention. The microkeratome  10  includes a hand piece  12  that is connected to a head  14  and a ring assembly  16 . The microkeratome  10  is typically used to cut a lamella flap in a cornea (not shown) as part of a LASIK procedure. The ring assembly  16  may be attached to a source of vacuum to create a suction pressure between the ring  16  and the cornea. The suction pressure fixes the microkeratome  10  to the cornea. The hand piece  12  has a wire assembly  18  that is connected to an electrical console (not shown). The console provides electrical power to actuate the microkeratome  10 . 
     The head  14  generally includes a front side  102 , a rear side  104 , a bottom side  106 , and a top side  108 . The head  14  has a blade cavity  20  that can receive a blade, or a blade and a blade holder to which the blade is mounted (not shown). The ring assembly  16  may include a helical gear  22  that is coupled to the hand piece  12 . For example, the helical gear  22  may be coupled to an internal gear (not shown) of the hand piece  12  at a coupling location  110 . The hand piece  12  includes a motor (not shown) that cooperates with the helical gear  22  to move the head  14  and blade in a linear direction across an opening  24  of the ring assembly  16 . The opening  24  is formed through a top surface  112  of the ring assembly  16  that faces the bottom side  106  of the head  14  and its blade. The cornea may protrude through this opening  24 . 
     To accurately guide the head  14  and the blade along the linear direction, the head  14  and ring assembly  16  may have one or more corresponding linear bearing members such as, for example, one or more corresponding tongues  26  and grooves  28 , respectively, that create linear bearings. In the illustrated example, the tongues  26  are formed on the head  14  and the grooves  28  are formed on the ring assembly  16 . Alternatively, the tongues  26  may be formed on the head  14  and the grooves  28  may be formed on the ring assembly  16 . In another alternative, the head  14  may include a tongue  26  that movably engages a corresponding groove  28  of the ring assembly  16 , and the head  14  may also include a groove  28  that is engaged by a corresponding tongue  26  of the ring assembly  16 . 
     The hand piece  12  may contain another motor (not shown) that moves the blade in a lateral reciprocating (or oscillating) manner (i.e., orthogonal or transverse to the linear direction along which the head  14  moves) so that the blade cuts corneal tissue and creates a lamella flap. For example, this other motor may drive the rotation of a shaft  114  that includes an eccentric cam or pin  116 . When the head  14  is attached to the hand piece  12  in this example, the shaft  114  extends through a coupling member  118  generally disposed at or near the top side  108  of the head  14  and the eccentric pin  116  engages a slot (not shown) of the blade (or blade holder). The coupling member  118  or some other portion of the structure of the head  14  may have a groove  44  utilized to attach the head  14  to, and detach the head  14  from, the hand piece  12  in a manner described below. The coupling member  118  of the head  14  may be oriented such that the hand piece  12  is oriented at a non-zero angle relative to the top surface  112  of the ring assembly  16 . 
     The ring assembly  16  may include one or more side walls  122  that extend upward from the top surface  112  of the ring assembly  16  on either side of the opening  24  of the ring assembly  16 . The linear bearing member(s) associated with the ring assembly  16  (e.g., tongues  26  and/or grooves  28  as described above) may be formed in the side wall(s)  122  as illustrated in the example of  FIGS. 1 and 2 . A bore  124  may be formed in one of the sidewalls  122  to receive the helical gear  22 . The ring assembly  16  may further include a nut  30  that is attached to the helical gear  22 . The nut  30  can be removed to allow the hand piece  12  and head  14  to be detached from the ring assembly  16 . This allows the hand piece  12 , head  14  and blade to be removed even while the ring  16  is applying suction to a cornea. By way of example, the microkeratome  10  may be actuated to initiate cutting of a cornea by the blade. A complication may occur which causes the surgeon to de-actuate the microkeratome  10  and stop the cutting process. Under suction the ring  16  flattens out the cornea. If the suction is removed the cornea may move back to its natural shape. This movement may cause undesirable movement between the blade and corneal tissue. The nut  30  allows the hand piece  12 , head  14  and the blade to be removed from the cornea without removing the suction of the ring  16 . 
     The ring assembly  16  may include an aspiration connector  32 . The aspiration connector  32  is connected to an aspiration tube (not shown) and is coupled to aspiration openings (not shown) in the ring. The aspiration connector  32  may have a collar  34  that limits the travel of the head  14  and the blade. The aspiration connector  32  may have a threaded shaft (not shown) that screws into a corresponding threaded opening (not shown) of the ring assembly  16 . 
       FIG. 3  shows a plurality of aspiration connectors  32 ,  32 ′ and  32 ″ that each have collars  34 ,  34 ′ and  34 ″. Each collar  34 ,  34 ′ and  34 ″ has a different diameter. The thickness of the lamella flap can be varied by attaching different connectors  32 ,  32 ′ or  32 ″ to the ring assembly  16 . For example, connector  32  may create a relatively thin flap. Connector  32 ′ may create a thicker flap and connector  32 ″ may create an even thicker flap. The different connectors  32 ,  32 ′ and  32 ″ allow the surgeon to vary the thickness of a lamella hinge. 
     Alternatively, the helical gear  22  can provide a stop function. The stop function may be provided by the end of the threads near the fastener  30 . The stop function could also be provided by a nut attached to the threads of the gear  22 . The size of the flap hinge can be varied by changing gears  22 . 
       FIG. 4  shows a latch assembly  40  that connects the head  14  to the hand piece  12 . The latch assembly  40  may include a latch  42  that is pivotally connected to the hand piece  12  and fits within the groove  44  of the head  14 . In the illustrated example, the groove  44  is formed in the coupling member  118  of the head  14 . The latch assembly  40  may include a return spring  46  that biases the latch  42  into the groove  44 . 
     The latch assembly  40  may further have an actuator  48  that can be depressed by a user to move in a downward direction as indicated by an arrow  402  to rotate the latch  42  out of the groove  44 . Alternatively, the actuator  48  may be configured to be actuated by the user in a lateral direction (i.e., transverse to the arrow  402  and the linear movement of the hand piece  12  and the head  14  relative to the ring assembly  16 ), which may assist in preventing accidental detachment of the head  14  from the hand piece  12  during a surgical procedure. The assembly  40  may include a return spring(s)  50  to move the actuator  48  back when released by the user. As shown in  FIGS. 1 ,  2  and  4 , the actuator  48  may be located on a first face  52  of the hand piece  12 . 
     A user can attach the head  14  to the hand piece  12  by moving the head  14  until the latch  42  snaps into the groove  44 . The head  14  can be removed from the hand piece  12  by depressing the actuator  48  to pull the latch  42  out of the groove  44 . The head  14  may then be sterilized and re-attached to the hand piece  12 . Alternatively, the head  14  may be replaced. By way of example, the head  14  may be constructed from a low cost plastic material that is replaced after every procedure. The hand piece  12  may also be constructed from a plastic material. The head  14  may be constructed from the same plastic material as the hand piece  12 . By way of example, the plastic may be a polycarbonate or polysulphone. 
       FIG. 4  also illustrates the shaft  114  and the eccentric pin  116  of the motor that drives the lateral oscillatory motion of the blade (or blade holder). The shaft  114  may extend through the bore of the coupling member  118  of the head  14 . The eccentric pin  116  engages a slot  404  of the blade (or blade holder)  406 . 
       FIG. 5  shows a pair of motor assemblies  60  and  62  within the hand piece  12 . Motor assembly  60  may move the blade in a lateral reciprocating manner. Motor assembly  62  may be coupled to the elongated helical gear  22  of the ring assembly  16  to pull the head  14  and blade across the ring opening  24 . In the present context, the term “assembly” indicates one or more components (e.g., motor, shaft, linkage, gear, etc.) as needed to effect the movement of the blade in the linear and lateral directions via a source of power disposed in or coupled to the hand piece  12 . In the illustrated example, the motor assembly  62  that drives the linear motion of the head  14  and blade may include a shaft  502  and one or more internal gears  504  and  506 . In this example, the internal gear  506  includes threads in mating engagement with the threads of the helical gear  22 . The internal gear  506  may, for example, have an annular structure in which internal threads engage the helical gear  22  and external threads engage another internal gear  504  (or directly to threads provided on the shaft  502 ). In this example, the helical gear  22  does not itself rotate. Consequently, the internal gear  506  functions as a rotating, linearly moving worm gear that is driven by the motor assembly  62  to travel along the length of the helical gear  22 . By way of the mating engagement between the moving internal gear  506  and the stationary helical gear  22 , the hand piece  12 , head  14  and blade are pulled forward in the linear direction along which the helical gear  22  is oriented. 
     As shown in  FIG. 6 , a blade (or a blade mounted to a blade holder)  70  may be packaged within a blade shuttle  600 . The blade shuttle  600  may include a housing  74  having a housing interior  72  that holds the blade  70 . In advantageous implementations, the housing  74  encloses the blade  70  in a sealed, sterile condition prior to use of the blade  70 . The blade shuttle  600  may further include a plunger  76  located in the housing  74 . The plunger  76  may be operated to push the blade  70  into the head  14  along a blade-loading direction  602 . For this purpose, a portion  604  of the plunger  76  (e.g., a tap, button, finger grip, or the like) may be accessible from outside the housing  74  for manipulation by the user. The blade shuttle  600  may further include a drawer  610  that slides out from the housing  74  to the open position illustrated in  FIG. 6 . The drawer  610  has a drawer opening  612  communicating with the housing interior  72 . The drawer opening  612  receives the head  14  in preparation for operating the plunger  76  to push the blade  70  into the blade cavity  20  of the head  14 . 
     The blade shuttle  600  may have an alignment pin (not shown) that is inserted into a corresponding alignment hole (not shown) of the head  14  to align the blade  70  with the head cavity  20 . The housing  74  may include a stop (not shown) that limits the travel of the plunger  76  and the location of the blade  70  within the head  14 . The stop provides a feature that allows for the blade  70  to be accurately located within the head cavity  20  in a repeatable manner. It is desirable to accurately locate the blade  70  within the blade cavity  20  so that the eccentric pin  116  ( FIG. 4 ) of the motor assembly  60  ( FIG. 5 ) is properly coupled to the corresponding slot  404  ( FIG. 4 ) of the blade  70 . 
     In use, the blade shuttle  600  may be initially provided to the user as a package that contains the blade (or blade holder)  70  in a sterile condition within the housing interior  72 . To install the blade  70  into the head  14 , the user may slide out the drawer  610  to reveal the drawer opening  612 , position the head  14  in the drawer opening  612  such that the blade cavity  20  is in proper alignment with the blade  70 , and operate the plunger  76  (such as by manipulating the exposed portion  604 ) to transfer the blade  70  from the housing  74  into the blade cavity  20 . The blade shuttle  600  may be discarded thereafter. 
       FIG. 7  is a side elevation view of a microkeratome cutting head  700  according to another implementation. The cutting head  700  may share many of the same or similar features as those illustrated in  FIGS. 1 ,  2 ,  4  and  5 , and accordingly such features are designated by similar reference numerals in  FIG. 7 . In  FIG. 7 , a blade assembly  750  including a blade  752  mounted to a blade holder  754  is shown installed in the blade cavity  720  of the cutting head  700  for use during a surgical procedure. A blade opening  756  formed at the bottom side  706  of the cutting head  700  communicates with the blade cavity  720 . At the installed position of the blade assembly  750 , at least a portion of the blade  752  that includes its cutting edge extends out from the bottom side  706  of the cutting head  700 . The cutting head  700  includes an applanation plate  762  and a stromal plate  764 , which are generally located at the bottom side  706  of the cutting head  700 . The applanation plate  762  is located in front of the blade opening  756  and the stromal plate  764  is located behind the blade opening  756 . The applanation plate  762  and the stromal plate  764  may be formed as an integral part of the main structure of the cutting head  700 , or alternatively may be physically separate components attached to the structure of the cutting head  700  by any suitable means. 
     In use during a procedure for creating a corneal flap, the cutting head  700  is driven in the linear direction to the left from the perspective of  FIG. 7 , and is guided by the ring assembly as described above. As the cutting head  700  moves forward, the cornea first encounters the applanation plate  762 , and then the blade  752 , and then the stromal plate  764 . As the applanation plate  762  comes into contact with the cornea, the applanation plate  762  flattens the cornea and creates intraocular pressure. The applanation plate  762  maintains these flattening and pressurized conditions as it passes over the cornea. While the cornea is flattened, the blade  752  begins to cut a corneal flap, which action is enhanced by driving the blade  752  into oscillatory motion along a lateral axis (i.e., into and out from the drawing sheet of  FIG. 7 ) as described above. With continued forward movement of the cutting head  700 , the stromal plate  764  passes over the region of the stroma (or stromal bed) that has been exposed as a result of the cutting. 
     As noted above, a problem attending the use of microkeratome cutting heads of known designs is the occurrence of a buttonhole (or dimple) in the cornea, which is a result of the cutting procedure. A buttonhole generally is a depression in the central region of the cornea, which results in an uncut island of tissue and is created as a conventionally designed cutting head passes over the cornea. The buttonhole is highly undesirable as it results in the blade cutting only the peripheral tissue that is at a higher elevation than the tissue in the central cornea where the buttonhole exists. Thus, a flap having a hole at its center is thereby created instead of an intended continuous or unbroken corneal flap. The cutting head  700  illustrated in  FIG. 7  prevents the occurrence of a buttonhole and attendant complications, as will now be described. 
     We have now discovered that a major cause for the occurrence of buttonholes relates to the respective elevations of the applanation plate and the stromal plate of a cutting head. In cutting heads of known designs, the applanation plate is typically disposed at the same elevation as the stromal plate or at a lower elevation than the stromal plate. While not wishing to be bound by any particular theory, we have determined that the higher the stromal plate is in relation to the applanation plate, the more likely a buttonhole may occur. We have found that the stromal plate, if properly located, can serve to maintain an intraocular pressure of sufficient magnitude and constancy that prevents the cornea from losing its firmness. These findings, the effect of the relative elevations or heights of the applanation plate and the stromal plate, and the role of the stromal plate have not been appreciated by persons skilled in the art prior to the present teachings. 
     The cutting head  700  illustrated in  FIG. 7  addresses this problem by setting the elevation of the stromal plate  764  to be lower than the elevation of the applanation plate  762 . Conceptually, the outward-facing surface of the applanation plate  762  lies in an applanation plate plane  772  and the outward-facing surface of the stromal plate  764  lies in a stromal plate plane  774 . According to this implementation, the applanation plate plane  772  and the stromal plate plane  774  are not coplanar. Stated in another way, the applanation plate  762  and the stromal plate  764  lie in different planes  772  and  774 , and the stromal plate plane  774  is lower than the applanation plate plane  772  by an elevation difference designated as  776  in  FIG. 7 . 
     From the perspective of  FIG. 7 , the elevations may be measured or defined along a vertical direction or axis  778 , although it will be understood that no limitation is placed on the orientation of the cutting head  700  relative to any particular direction or plane of reference. In  FIG. 7 , the vertical direction  778  may be considered as being orthogonal to the applanation plate plane  772  or the stromal plate plane  774  and runs generally from the bottom side  706  to the top side  708  of the cutting head  700 . It will be noted here that the applanation plate plane  772  and the stromal plate plane  774  may be generally or substantially parallel to each other. Here, the terms “generally” and “substantially” take into account some degree of imperfection or impreciseness in the process for fabricating the applanation plate  762  and the stromal plate  764 . The elevation difference  776  between the applanation plate  762  and the stromal plate  764  may be measured along the vertical direction  778 . When considering the individual heights or elevations of the applanation plate  762  and the stromal plate  764 , any reference point, line or plane may be utilized. For instance, the elevation of the stromal plate  764  may be taken to be zero and the higher elevation of the applanation plate  762  then considered relative to such zero datum. As other examples, the respective elevations of the applanation plate  762  and the stromal plate  764  may be considered relative to some other surface of, or point on, the cutting head  700 , a surface of or point on the ring assembly  16  ( FIG. 1 ), a point on the cornea, etc. 
     As a result of the configuration illustrated in  FIG. 7 , in operation as the applanation plate  762  passes over the pressurized cornea and the blade  752  begins to cut the corneal tissue, the lower-elevation stromal plate  764  in effect replaces the corneal tissue that has just been removed by the blade  752 . This enables the intraocular pressure to be maintained and prevents the occurrence of a buttonhole. It will be noted that the cutting head  700  is configured to create a corneal flap of a specified thickness. In some implementations, the thickness ranges from 10 μm to 150 μm. As one specific example, the thickness may be 100 μm. In another specific example, the thickness may be 130 μm. We have discovered the setting of the elevation difference  776  between the applanation plate  762  and the stromal plate  764  in proportion to the flap thickness to be an effective implementation. In particular, the elevation difference  776  may be set to be approximately or substantially equal to the flap thickness. Here, terms such as “approximately” or “substantially” encompass a deviation of ±5 μm as between the elevation difference  776  and the flap thickness. Thus, for example, in a case where the cutting head  700  is configured for creating a corneal flap of 100 μm thickness, the stromal plate  764  is set to be 100 μm (or 100±5 μm) lower than the applanation plate  762 . 
       FIG. 8  is a detailed view of the region designated “A” in  FIG. 7 , from which the respective heights of the applanation plate  762  and the stromal plate  764  may be better visualized. 
     The cutting head  700  may be attachable to/detachable from the hand piece  12  and the ring assembly  16 , and may be driven by one or more motor assemblies  60  and  62 , in accordance with the implementations described above and illustrated in  FIGS. 1-5 . Moreover, the blade shuttle  600  described above and illustrated in  FIG. 6  may be utilized to load the blade  752  or blade assembly  750  into the blade cavity  720  of the cutting head  700  illustrated in  FIGS. 7 and 8 . 
     It will be understood that various aspects or details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation—the invention being defined by the claims.