Abstract:
A keratome for performing corneal resectioning that creates a corneal flap having a temporal hinge in which a blade is initially positioned nasally and is drawn across the cornea to create the flap with a temporal hinge.

Description:
RELATED APPLICATIONS AND PATENTS 
       [0001]    The present application is a continuation of Ser. No. 10/668,882 filed on Sep. 23, 2003, which is a continuation-in-part of Ser. No. 09/521,010 filed on Mar. 7, 2000 now U.S. Pat. No. 6,623,497, which is a continuation-in-part of Ser. No. 09/132,987 filed on Aug. 12, 1998 now U.S. Pat. No. 6,083,236 and Ser. No. 10/668,882 filed on Sep. 23, 2003 is also a continuation-in-part of 10/618,279 filed on Jul. 11, 2003 now U.S. Pat. No. 7,207,998, which is a divisional of 09/586,273 filed on Jun. 2, 2000 now U.S. Pat. No. 6,599,305, which is a continuation-in-part of Ser. No. 09/132,987 filed on Aug. 12, 1998 now U.S. Pat. No. 6,083,236. 
         [0002]    It is also a continuation of Ser. No. 10/884,171 filed on Jul. 1, 2004 which is a continuation-in-part of Ser. No. 10/668,882 filed on Sep. 23, 2003 (see above) and is also a continuation-in-part of Ser. No. 10/618,279 filed on Jul. 11, 2003 now U.S. Pat. No. 7,207,998 (see above). 
         [0003]    The contents of all of the aforementioned serials are incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0004]    The present invention pertains to the general field of opthalmologic surgical devices, and more specifically to the field of devices for performing corneal resectioning and methods therefore. 
       BACKGROUND 
       [0005]    Numerous ophthalmic surgical procedures, such as for correcting myopia or hyperopia, require one or more steps of resectioning the cornea of the eye. A variety of devices called keratomes have been developed over recent decades to perform such corneal resectioning. Referring to  FIGS. 1 ,  2   a  and  2   b , a typical resectioning operation will separate flap  6  of corneal tissue  2  from eyeball  4 . The tougher outer layers of epithelial cells  8  are separated and lifted away to expose the more compliant inner layers  12  of cornea  2 , but the separated outer layers are left attached as flap  6 . Once exposed, interior layers  12  of cornea  2  will to some extent adjust themselves, or their shape may be altered through further surgical steps. Such further steps may include, for example, making radial keratotomy cuts or performing a subsequent resectioning which may include removing a contoured layer of corneal tissue. At the conclusion of the various steps of the surgical procedure, flap  6  is typically replaced over inner corneal tissues  12  to protect the healing tissues. 
         [0006]    The representative keratomes described in U.S. Pat. Nos. 5,496,339 issued to Koepnick, and Re. 35,421 issued to Ruiz et al., which are depicted in  FIGS. 3   a  and  3   b , demonstrate many standard features of prior art keratomes. A retaining ring for positioning and retaining the subject eyeball is typically supplied with a source of vacuum. The vacuum pressure draws the eyeball into the retaining ring so that the cornea protrudes through the retaining ring and presses against the surface of a feature, herein referred to as an applanation shoe, which is provided to restrain the protruding cornea. An applanation shoe has been found important in all known prior art. 
         [0007]    However, an applanator impedes access to the eye under surgery. One approach to this problem is to make the applanator pivotable, or otherwise disengageable from contact with the eye, without a need to disengage the entire surgical apparatus from its positioning on the eye. 
         [0008]    In order to resection the cornea, a cutting blade must be drawn through the corneal tissue, and both the thickness and the expanse of the corneal tissue which is cut must be carefully controlled. The separated portion of the cornea is typically left attached along one edge to form flap  6  which can easily be replaced over the cornea after the surgery. 
         [0009]    Keratomes must have a mechanism by which the knife blade is guided. Proximate to the cutting location, the prior art keratomes all have blades rubbing on guides, or metal rubbing on metal, such as drive gears. Unfortunately, such rubbing can result in shavings being created and entering the surgical site. Referring to  FIG. 3   a , the keratome of Ruiz et al. has an intricate mechanism with metal-on-metal gears rubbing in the surgical vicinity. For example, pinion  834  rides on track  891  which is part of positioning ring  890 ; and endless pinion  822 , along with its eccentric shaft and associated pinions, operates directly above the blade cutting site (not shown). In  FIG. 3   b , the keratome of Koepnick is seen to have blade  954  which rubs directly on the insert  948  and slides in surfaces defined along line  991 . The sliding surfaces at  991  are located directly above positioning suction ring  990 , and the rubbing surface between blade  954  and insert  948  is directly adjacent regions of intimate contact between the corneal tissue and insert  948 . Thus, these two prior art keratome examples have rubbing between the cutting blade and other surfaces, and rubbing of gears, very close to the surgical site. 
         [0010]    Another drawback of existing keratomes is the inconvenience of maintaining surgical cleanliness. Since parts of the keratome must be in intimate contact with tissues around and including the surgical site, it is necessary to ensure a high degree of cleanliness and sterility. The relatively intricate mechanisms which prior art keratomes position near the surgical site, as described above, have not been well-adapted for ease of cleaning and autoclaving. 
         [0011]    Thus, a need exists for an easily used keratome able to perform precise resectioning operations, while facilitating surgical cleanliness by avoiding creation of shavings which might contaminate the surgical site, and by being easily cleaned, sterilized, and replaced. 
       SUMMARY OF THE INVENTION 
       [0012]    A keratome in accordance with the present invention enables an opthalmologic surgeon to perform corneal resectioning, separating a flap of corneal tissue for later surgical device near the surgical site. 
         [0013]    In accordance with the present invention, the surgical tissue for later replacement, without a need for an applanator, and without any rubbing of parts of the surgical device preferably includes a surgical unit having cutting head elements mounted on a drive assembly, and also includes a control unit and a foot pedal. During surgery, the cutting head elements are in intimate contact with the subject eye, for positioning and cutting. The drive assembly element supports and drives the cutting head elements. The control unit is the preferred source of power and vacuum for the surgical unit, and it supplies power and vacuum according to settings entered by the user. The foot pedal allows the user to give commands to the surgical device without requiring use of hands. The surgical unit is preferably hand-held and easily positioned over the subject eye. 
         [0014]    The preferred surgical unit includes three distinct elements. Two of these are “cutting head” elements which must contact the eye during corneal surgery—a positioning ring assembly and a blade fork assembly. These two cutting head elements extend from the third element, a drive assembly, in such a way that interference and rubbing between the cutting/head elements proximal to the surgical site is minimal or entirely absent. Preferably, the two cutting head elements are easily removed and as easily replaced onto the third element, the drive assembly, without a need for tools, so the surgeon can ensure sterility by simply attaching fresh and sterile replacements for the cutting head elements. 
         [0015]    In a preferred embodiment of the present invention, a blade fork assembly suspends a cutting blade between the positioning ring and the applanation shoe and guides the cutting blade near to the applanation shoe. The thickness of the cut is preferably controlled by a guide, which is disposed a controlled distance away from the cutting blade. The outer layer of corneal tissue is separated by the blade as it passes between the blade and the guide, so that the thickness of the separated layer is controlled by the spacing between the blade and the guide. 
         [0016]    The blade fork assembly is caused to move by the drive assembly, which imparts two distinct movements to the blade fork assembly during cutting action. One movement is a high-speed lateral oscillation, and the other, imparted at the same time, is a slow smooth forward movement. The drive arm impel the blade fork forward as long as it is commanded to do so through the control unit, until the drive arm impinges on an adjustable stop mechanism, thereby causing a clutch to slip and preventing further forward displacement of the drive arm, 
         [0017]    The blade assembly is preferably entirely suspended and does not touch any part of the mechanism which is near to the surgical site except indirectly by way of the blade fork drive arm which supports the blade assembly. 
         [0018]    In another embodiment of the invention the blade is mounted on the mounting assembly so that it faces rearwardly, that is, toward the drive mechanism. With this reverse mounting and by constructing the drive mechanism to operate in reverse, the blade can be moved from a beginning point away from the drive mechanism, toward the drive mechanism. In this way a corneal flap can be made that begins proximate the nasal side of the cornea with its hinge proximate the temporal side of the cornea with the drive mechanism being positioned at the temporal side of the patients face. Also, in this embodiment a guide member is provided in a substantially fixed relationship to the blade in particular above and forward of the blade edge to define a space which will control the depth of cut and the flap thickness. With the reverse mounting a corneal flap may be created with a temporal hinge by placing the device to begin cutting on the nasal side of the cornea toward the temporal side. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a cross-section of an eye. 
           [0020]      FIG. 2   a  shows a cornea with a flap of epithelial tissue lifted. 
           [0021]      FIG. 2   b  is a representation of the variation of corneal tissue beginning at the outermost layers. 
           [0022]      FIG. 3   a  shows the prior art keratome of Ruiz et al. 
           [0023]      FIG. 3   b  shows the prior art keratome of Koepnick. 
           [0024]      FIG. 4  shows the control unit with connections to the surgical unit and to a foot pedal. 
           [0025]      FIG. 5  shows the surgical unit, with the cutting head elements attached to the drive assembly. 
           [0026]      FIG. 6  shows the drive assembly front end with the cutting head elements detached therefrom 
           [0027]      FIG. 7  shows an eye in a positioning ring and a blade cutting a corneal flap with thickness controlled by a guide. 
           [0028]      FIG. 9   a  shows a blade fork assembly with a cam lever securing it to the blade fork drive arm. 
           [0029]      FIG. 9   b  shows a blade fork assembly secured to the blade fork drive arm with a thumb screw. 
           [0030]      FIG. 8   a  shows details of section  8   a - 8   a  of  FIG. 9   a , including the blade. 
           [0031]      FIG. 8   b  shows details of section  8   b - 8   b  of  FIG. 9   b , including a stainless steel blade with guide. 
           [0032]      FIG. 8   c  shows an alternative dual blade and guide in a section similar to  8   b - 8   b.    
           [0033]      FIG. 8   d  shows an alternative angled blade and guide in a section similar to  8   b - 8   b.    
           [0034]      FIG. 8   e  shows an alternative blade and bearing guide in a section similar to  8   b - 8   b.    
           [0035]      FIG. 10  shows the positioning ring releasably attached to the drive assembly. 
           [0036]      FIG. 11  shows details of positioning ring restraint at section  11 - 11  of  FIG. 10 . 
           [0037]      FIG. 12  shows a cross-section of a surgical unit using motor driven blade oscillation. 
           [0038]      FIG. 13  shows alternative features for the surgical unit to permit field-driven blade oscillation. 
           [0039]      FIG. 14  shows an isometric view of an alternative embodiment. 
           [0040]      FIG. 15  shows a front view of the alternative embodiment of  FIG. 14 . 
           [0041]      FIG. 16  shows section A-A of  FIG. 15 . 
           [0042]      FIG. 17  shows an enlarged detail of  FIG. 16 . 
           [0043]      FIG. 18  shows the assembly of the alternative embodiment. 
           [0044]      FIG. 19  shows the starting position of the alternative embodiment. 
           [0045]      FIG. 20  shows the ending position of the alternative embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    The present invention is described below by examples which include the best mode known, but such description is not to be taken as limiting the invention, which is defined separately in the claims. 
         [0047]    Referring to  FIGS. 4 &amp; 5 , the present invention is preferably embodied in three separate components: surgical unit  100 , foot pedal  300 , and control unit  400 . Surgical unit  100  has three subsections including drive assembly  110  and two cutting head elements: 
         [0000]    positioning ring assembly  20  and blade fork assembly  60 . Foot pedal  300  communicates user commands to control unit  400  via cable  310 , and surgical unit  100  is connected to control unit  400  by electrical cable  410  and vacuum hose  412 . Each of these items are discussed in more detail below. 
       Control Unit 
       [0048]    The following describes a preferred embodiment of the invention with reference to  FIG. 4 . Control unit  400  is a microprocessor-controlled unit enabling the user to direct operation of the actuators within drive assembly  110  and the level of vacuum supplied to positioning ring assembly  20  of surgical unit  100 . The user controls operation by means of two pedal switches of foot pedal  300 , in conjunction with three rotary input devices  450 ,  452  and  454  and two pushbuttons  456  and  458  on the front panel of control unit  400 . Operating parameters are displayed on the front panel for the user by means of numeric readouts  412 ,  414  and  416  and by multiple character alpha-numeric display  440 , while speaker  434  gives audible information. 
         [0049]    A microprocessor on printed circuit board  460  executes operating firmware which is held in reprogrammable non-volatile memory and can be reprogrammed in the field. The firmware allows the microprocessor system to read switch closures and the rotation of the rotary controls. These electronics translate operator actions into tool control voltages which are applied to the drive unit actuators and can be stored as presets to be recalled as required by the operator. The microprocessor system also interprets the sensors and controls the actuators to maintain vacuum at a level set by the user. 
         [0050]    Control unit  400  provides electric control signals to surgical unit  100  via cable  410 . 
         [0051]    Vacuum pressure for positioning ring assembly  20  is supplied from control unit  400  via vacuum hose  412 . Control unit  400  contains vacuum reservoir  422  in which vacuum pressure is established by vacuum pump  420  and released by vacuum release solenoid  426 , and the vacuum pressure is sensed by vacuum transducer  424  to give feedback to the control electronics. Electric control for the actuators (not shown) within drive assembly  110  is provided by electronic switches  436 - 438 . Persons skilled in the art will appreciate that there is no limit to the variations by which control unit components may control the surgical unit actuators and vacuum. 
       Surgical Unit 
       [0052]    Referring to  FIG. 5 , surgical unit  100  includes drive assembly  110  for supporting and driving the cutting head elements which contact the eye during surgery, including positioning ring assembly  20  and blade fork assembly  60 . Surgical unit  100  is supplied electrically via cable  410 , and vacuum is supplied to positioning ring  30  via vacuum hose  412  which attaches to vacuum connection tube  22 . Blade  66  will cut the corneal tissue in a flap of a thickness controlled by the spacing from blade  66  to guide  76 . 
         [0053]      FIG. 6  more clearly delineates the cutting head elements, positioning ring assembly  20  and blade fork assembly  60 , as they are separated from front end  112  of drive assembly  110  without a need for tools. Since the cutting head elements ordinarily come into direct contact with a subject eye, it is preferable that they be removable and replaceable on drive assembly  110  without a need for tools, in order to facilitate the use of clean and sterile elements. For the same reason, it is also preferable that these cutting head elements be either sterilizable or sterile disposable. 
         [0054]    Positioning ring support  32  preferably has tapered edges to mate with receiving feature  106  in drive assembly  110 , with retention feature  34  also mating to a feature (not shown) of drive assembly  110 . Positioning ring  30  may be restrained by thumbscrew  114 . Blade fork  70  mates to drive arm  140 , preferably using spring loaded ball detent assemblies  64  having a spring-loaded ball  62  to mate to drive arm notch  141 . The three elements  20 ,  60  and  110  of surgical unit  100  are each described in more detail below. 
       Surgical Cutting Action 
       [0055]      FIG. 7  shows the cutting head elements resectioning cornea  2 . Vacuum pressure delivered to vacuum chamber  36  of positioning ring  30  will draw sclera  3  and cornea  2  of eye  4  upward to a stable position. Blade fork drive arm  140  ( FIG. 5 ) supports blade fork  70  and imparts a compound movement to it. Blade fork  70  is oscillated rapidly in a direction parallel to the cutting edge of blade  66  (in and out of the page of  FIG. 7 ), and simultaneously moved slowly forward (from right to left in  FIG. 7 ), while maintaining blade  66  at a controlled distance from positioning ring  30 . Blade  66 , suspended from blade fork tines  68  along with guide  76 , thereby separates a layer of corneal tissue  2  to form flap  6 . The thickness of flap  6  is determined primarily by the spacing between blade  66  and guide  76 , and to some extent by the guide and blade orientation and position. The forward travel of blade fork  70  continues until the formation of flap  6  is completed. 
       Blade Fork Assembly 
       [0056]      FIG. 6  shows some details of blade fork assembly  60 . A typical blade  66  and a representative guide  76  are shown suspended from blade fork tines  68 . Optional spring detent insert  64  and the detent ball  62  of another spring detent insert are also shown. The detent ball of insert  64  will nest in notch  141  to releasably position blade fork  70  with respect to fork drive arm  140 . 
         [0057]      FIG. 9   a  shows blade fork assembly  60  suspending blade  66  and guide  76  from blade fork tines  68 . Blade  66  and guide  76  are shown in cross section  8   a - 8   a  in  FIG. 8   a , and variations of the blade and guide arrangement are shown in  FIGS. 8   b ,  8   c ,  8   d , and  8   e . In  FIG. 9   a , blade fork  70  is attached to drive arm  140  using a trapezoidal mating construction, and the trapezoidal attachment between blade fork  70  and drive arm  140  is secured using a locking lever  144  which actuates a locking cam (not shown) by rotating about pivot  146 . 
         [0058]      FIG. 9   b  shows blade fork assembly  60  alternatively secured to blade fork drive arm  140  by thumbscrew  142 . Spring loaded ball detent assembly  64  helps establish and hold the positioning of blade fork  70  with respect to drive arm  140 . As above, fork tines  68  suspend blade  66  and guide  76 , which can be seen in cross section  8   b - 8   b  in  FIG. 8   b .  FIGS. 8   a ,  8   c ,  8   d  and  8   e  show alternative examples of blade and guide arrangements which may be used. 
         [0059]    Blade fork  70  is preferably composed of titanium but many other materials are suitable, including stainless steel. For a steam sterilizable blade fork, dimensionally stable plastics such as polycarbonate or polysulfone are suitable, and gas or gamma ray sterilization is compatible with additional plastics, such as polypropylene. 
         [0060]    Blade  66  is preferably sapphire or similar crystalline materials, which is hard and strong and desirably transparent for the best visibility as the cutting operation progresses. Alternatively, and particularly for disposable versions, the blade may be surgical stainless steel or other suitable material. 
         [0061]    The overall position of blade  66  and guide  76  with respect to positioning ring  30  is established by the combined positioning of blade  66  and guide  76  in blade fork assembly  60 , by the relative positioning of drive arm  140  to positioning ring mounting features  106  ( FIG. 6 ), and by the positioning ring  30  dimensions. However, this is a less critical relationship than in many keratomes, because the relationship between blade  66  and guide  76  primarily determines the corneal flap thickness. 
         [0062]      FIG. 8   a  shows details of section  8   a - 8   a  of  FIG. 9   a , including guide  76  disposed parallel to blade  66 . The spacing between guide  76  and blade  66  controls the thickness of corneal tissue cut, enabling the cut thickness to be controlled very precisely and also to be set under controlled conditions at the factory. Guide  76  has a cross-section defined in a plane perpendicular to the longitudinal axis of blade  66 . 
         [0063]    The perimeter of the cross-section of guide  76  is advantageously small, preferably less than 2 mm or less than 6 mm. A small cross-sectional perimeter conveys several advantages: it reduces the frictional interaction between the guide and the cornea, it localizes a deformation  5  ( FIG. 7 ) of the cornea to avoid pressure on the eye generally, and it reduces the likelihood of trapped bubbles distorting the cornea to cause inaccurate cuts. 
         [0064]      FIG. 8   b  shows section  8   b - 8   b , an arrangement of blade and guide for the blade fork assembly  60  shown in  FIG. 9   a . The leading edge of guide  76  is positioned very slightly forward (in the direction that the cutting head elements extend from the drive assembly) of the cutting edge of blade  66 . Dimension x 1  is the distance in the direction of blade travel between the leading edge of blade  66  and the leading edge of guide  76 . The optimum length of dimension x 1  depends on the orientations of the plane of blade  66  and, if applicable, of guide  76 . Dimension x 1  is preferably greater than zero, for example 0.20+/−0.05 mm or 0.30+/−0.05 mm. Dimension y 1 , the distance between guide  76  and blade  66  in a direction perpendicular to the travel plane of blade  66 , will vary depending upon the surgeon&#39;s needs, but will typically be made nominally 0.150 mm, 0.160 mm, 0.170 mm, or 0.180 mm, each nominal dimension being controlled to within a tolerance of preferably 0.030 mm or even more preferably 0.015 mm. 
         [0065]      FIG. 8   c  shows, in a cross section similar to that of  8   a - 8   a  ( FIG. 9   a ), an arrangement of blades  66  and  67  which may be suspended from blade fork tines  68 . Here, lower blade  66  utilizes upper blade  67  as a guide for one flap of corneal tissue, while upper blade  67  utilizes guide  76  to control the thickness of a second flap of corneal tissue. Using this arrangement, a slice of corneal tissue of precise dimensions may be separated and then removed to accommodate an implant, leaving another flap  6  of the harder outer corneal tissue to cover the surgical site. 
         [0066]    In  FIG. 8   d , blade  66  is shown having a small angle to the direction of travel, the angle preferably being about 25 degrees. Blade  66  is captured by screw  72  and washer  74 , or suitable fastener. Flap thickness is controlled by the spacing from blade  66  to guide  76 . 
         [0067]      FIG. 8   e  differs from  FIG. 8   d  in that guide  76  comprises central core  75  and outer cylindrical bearing  77 , which is preferably made of a tough, low friction material such as a plastic containing TEFLON™ material. If bearing  77  is shorter than guide core  76  by an amount equal to the maximum lateral oscillation amplitude of the blade assembly, then with this arrangement bearing  77  may slide very little, or not at all, on the corneal tissue. Rather, sliding may occur at the interface between core  76  and bearing  77 , and bearing  77  may only roll on the corneal tissue. 
       Positioning Ring Assembly 
       [0068]      FIG. 6  shows positioning ring assembly  20 , including positioning ring  30 , vacuum connection nipple  24 , vacuum tube stop  26 , and vacuum connection tube  22 . These items supply vacuum to assembly  20  to draw a subject eye into position and restrain it. 
         [0069]      FIGS. 10 and 11  depicts details of positioning ring assembly  20 . Positioning ring  30  is provided with vacuum to vacuum chamber  36  so that an eyeball placed against it may be drawn in and restrained. The vacuum is furnished through vacuum connection tube  22 , with the vacuum hose (not shown) placed over vacuum connection nipple  24  and stopped by vacuum tube stop  26 . Alternatively, vacuum could be ducted through ring support  32  and drive assembly  110  to obviate vacuum connection tube  22 , with the vacuum hose in that case connected only to drive assembly  110  at any convenient location, such as adjacent to or even within control hose  410  ( FIG. 5 ). 
         [0070]    Referring to  FIG. 10 , which is a bottom view, and cross-section  FIG. 11 , positioning ring support  32  preferably includes retention feature  34  having detent  35 . Retention feature  34  slides into matching recess  120  in drive assembly  110 . Captured ball  117  settles into detent  35  under the pressure of captured spring  115  to properly locate positioning ring assembly  20 . Then, thumbscrew  118  secures retention feature  34 , seating it firmly against the sides of recess  120  formed in head  112  of drive assembly  110 . Alternatively, thumbscrew  114  (e.g.  FIG. 5 ) may be used from the opposite side of drive unit head  112  to secure positioning ring assembly  20 . 
         [0071]    As discussed with regard to blade fork assembly  60 , a variety of materials may be used for positioning ring  20 . The choice depends on whether sterility is to be ensured by reuse of the element in conjunction with a sterilization method, or by using sterile disposable elements. Suitable materials include metals, such as stainless steel, and plastics, such as polycarbonate, polysulfone, polypropylene or others. 
       Drive Assembly 
       [0072]      FIGS. 12 &amp; 13  show details of a preferred embodiment for surgical unit  100 , and in particular shows details of a preferred embodiment for drive assembly  110 , which is largely enclosed by drive assembly cover  160 . 
         [0073]    Referring to  FIG. 12 , the primary actuators within drive assembly  110  are travel motor  180  and oscillation motor  170 . Travel motor  180  drives shaft  184  through gear train  182 . Clutch  190  couples a limited torque to screw  192 . The rotational motion of screw  192  is converted to linear motion by threaded traveller  194 . Pivot assembly  196  couples the motion from the forward end of traveller  194  to blade fork drive arm  140 , while permitting drive arm  140  to oscillate rotationally about the pivot of pivot assembly  196 . Blade travel stop adjust knob  150  preferably rotates a threaded member which adjustably stops blade fork drive arm  140  travel. 
         [0074]    Drive arm  140  preferably includes portions of its top and bottom surface which are made closely parallel to each other and a controlled distance apart (the top and bottom surfaces are those most distal from the center of drive arm  140  in the direction parallel to the pivot axis of pivot assembly  196 , with the top surface being the farther from positioning ring  30 ). Drive arm  140  top and bottom surfaces are preferably flat to within 0.005 mm over their travel range of 1.5 cm, and are slidably captured by bearing surfaces  136  and  138  of drive assembly head  112 . The bearing surfaces limit top-to-bottom play of drive arm  140  to preferably 0.01 mm or even more preferably to 0.05 mm. 
         [0075]    Drive assembly head  112  holds positioning ring assembly  20  and blade fork drive arm  140  such that blade fork assembly  60  is maintained a known distance away from positioning ring  30  as the blade fork travels. The distance between blade  66  and applanation shoe  50  is preferably controlled to within +/−0.5 mm, or more preferably within +/−0.25 mm. 
         [0076]    Oscillation is imparted to drive arm  140  by slider  176  which oscillates in a direction perpendicular to the page. Slider  176  interferes with the edges of a groove in drive arm  140 , while the groove allows drive arm  140  to travel in and out of drive assembly  110 . Slider  176  receives oscillation drive from oscillation motor  170  via shaft  172  and eccentric pin  174 . Eccentric pin  174  rides in a slot in slider  176  which absorbs the vertical component of eccentric pin  174 , but transmits the lateral motion. 
       ADDITIONAL EMBODIMENTS OF THE INVENTION 
       [0077]    Corneal flaps may be hinged in more than one place. In particular the hinge may be nasal, temporal, superior or inferior. The apparatus and methods described above are useful for nasal, superior and inferior hinge locations, but can not be easily, if at all, used to create a flap that is temporally hinged. This is because the bulk of the drive mechanism will likely be misaligned due to the patient&#39;s nose preventing proper positioning of the apparatus. The modified method and apparatus now described will allow the basic concepts of the invention be applied to enable a corneal flap with temporal hinging to be created. To accomplish a temporally hinged corneal flap, the cut must commence proximate the nasal side of the cornea and proceed toward the temporal side of the corner. This is accomplished by mounting the blade to face in the reverse direction and the guide similarly to be repositioned above and in front of the blade and to equip the drive mechanism to operate in the reverse direction, that is, the blade assembly will be moved during the cutting action from a point away from the drive mechanism, toward the drive mechanism. In this way while the drive mechanism is positioned temporally, the blade will be placed on the nasal side of the cornea and will cut the flap as it moves toward the drive mechanism toward the temporal side of the cornea, placing the hinge on the temporal side of the cornea. This is shown in  FIGS. 14 through 20 . 
         [0078]      FIGS. 14 and 15  show the blade assembly  200  that includes a blade mounting member  202  in the form of a blade fork of the type previously described. For purposes of defining the positions of the blade assembly, the mounting member  202  has forward portion  204  and a rearward portion  206 . The blade  208  is mounted on the forward portion  204  with its cutting edge  210  facing toward the rearward portion  206 . The guide  212  is mounted on the forward portion  204  in a substantially fixed relationship to the blade, in particular, in front of the blade  208 , forward of the cutting direction (as shown by the arrow A) of the blade  208  and above the blade  208 . This is shown in  FIG. 16  which is a sectional view along line A-A of  FIG. 15 . Also  FIG. 17  shows an enlarged detail and exemplary dimensions for the mounted relationship between the blade  208  and the guide  212 . The blade  208  is shown mounted on the forks  214  and  216  by rivets  218 . The guide is preferably formed as an integral part of the mounting member as can be seen best in  FIGS. 16 and 17 , although with careful control of dimensions and tolerances, it could be a separate piece fastened on the mounting member. By forming the guide integrally with the mounting member tolerance build-up is reduced, a feature that is important in the present invention in order to achieve high accuracy in controlling the depth of the cut and the thickness of the flap. 
         [0079]    As shown in  FIGS. 18 ,  19  and  20  to complete the device the drive mechanism  218 , basically as described above is modified so that it can operate in a “pulling” mode in which the blade assembly  200  starts in a position away from the drive mechanism and is drawn toward the drive mechanism. The positioning ring  222  (with opening  224 ) is attached to the drive mechanism as previously described. All of the features previously described for construction and operation of the device are applicable, and the device may be constructed so that the drive mechanism can select either mode for cutting, that is, moving away from the drive mechanism or pulling toward the drive mechanism, along with the lateral movement as also described above. 
         [0080]    The guide  212  as shown by the exemplary dimensions is spaced 0.16 mm (±0.025) above and 0.10 mm (±0.01) forward of the blade cutting edge  210 . The guide  212  begins with a straight portion  226  and commences into a curved portion  220  upwardly and forwardly of the blade cutting edge  210 . This is configured to flatten the cornea locally, just in front of the blade cutting edge so that the cutting operation is controlled by the gap between the blade cutting edge  210  and the guide  212 . The blade and guide are shown as extending straight across the opening defined by the forks  214  and  216 . It is only necessary that they be present and straight in the portion of that opening that will come into contact with the cornea. It is also possible to configure the blade and the guide to not be straight. 
         [0081]    In operation as shown in  FIGS. 18 ,  19 , and  20 , the positioning ring will be put in place with the mounting member in a ready to use position away from the drive mechanism on the far side of the opening  224 . As shown in the figures, the blade  208  and the guide  212  are on the nasal side of the cornea, in the start position. Then the device is activated and the blade assembly  200  is drawn toward the drive mechanism  220  across the positioning ring opening  224 , the blade  208  cutting the corneal flap and stopping to allow a temporal hinge. As described above the guide  212  will press against the cornea leading the blade so that the cut is precisely controlled by the distance between the forward edge of the blade  208  and the guide, the exemplary dimensions in  FIG. 17  being suitable for an accurate corneal flap. It is also notable that, as with embodiments described above, the guide  212  precludes the need for an applanator. 
       ALTERNATIVE EMBODIMENTS OF THE INVENTION 
       [0082]    It will be appreciated by those skilled in the art that many alternative embodiments are envisioned within the scope of the present invention. Some possible variations of the blade fork assembly are discussed in the blade fork assembly section above. Variations of other parts are discussed below, but do not represent an exhaustive survey of possibilities; rather, they serve as examples to show that a wide variety of mechanisms are encompassed within the scope of the invention. 
         [0083]      FIG. 13  shows an alternative embodiment of means to impart oscillating motion to drive arm  140 . In this embodiment drive arm  140  incorporates ferromagnetic material  144  which is acted on by magnetic fields generated by coils  175  positioned along the sides of drive arm  140 . 
         [0084]    Myriad physical configurations of the connection interface surfaces which removably attach the blade fork assembly to the blade fork drive arm can provide the predictable positioning needed to practice the invention. The mating parts of the interface are described herein as trapezoidal or “dove-tail” but may take any form having locating features, including sawtooth, rectangular, eccentric oval, keyhole, or other shapes too numerous to enumerate. 
         [0085]    Similarly, the means for securing the connection interface is shown herein as either a thumbscrew or a cam locking lever, but could be accomplished many other ways. To mention just a few examples, the mating parts could use magnetic attraction, spring-loaded detents, or tapered engaging pieces fitted into a recess formed partly from each of the mating parts. The mating pieces could even interfere snugly under normal conditions, and have a means to temporarily change the shape of one of the pieces to release the interference and thereby permit connecting or separating the interface. Any method known in the art to disengageably secure two pieces in a closely predictable relationship could be used. 
         [0086]    Any blade fork can be used which is able to suspend the blade, and the guide if used, in a properly controlled position with respect to the mounting surface of the connection interface. The blade and the guide may take a multitude of shapes and comprise a multitude of materials; only a few such alternatives are discussed herein. 
         [0087]    A preferred embodiment of this invention includes sterile disposable or sterilizable disposable cutting head elements. A non-limiting variety of material choices suitable for such an embodiment is discussed above with respect to each cutting head element. There is no need for the various cutting head elements to be all disposable or all permanent, but a mixture of disposable and sterilizable types is also suitable. 
         [0088]    Surgical unit actuators may be driven by any known method, including pneumatic drive methods. 
         [0089]    User commands may be recognized in any known way, including voice command reception, and sensing user activation of sensors or switches located on the surgical unit or in other convenient places. The commands thus recognized may exert control through any combination of control elements, which may include mechanical means, direct electrical control, or intelligent electrical control with intelligence provided by any means known to the art. The command recognition and control elements could be physically located at any accessible place, and as an example could be placed largely or entirely within the surgical unit.