Abstract:
The microkeratome comprises a holder ( 1 ) with a suction ring ( 11 ) for attachment to the sclera of a patient&#39;s eye and having means ( 23 ) for connection to a suction source. A slide ( 2 ) is displaceably mounted on the holder ( 1 ) in a linear guide thereof. The slide ( 2 ) has a plane contact surface for contacting the cornea of the patient&#39;s eye and slideable over the cornea in a direction parallel to the contact surface. The slide contains a cutting blade with a cutting edge which is parallel to the contact surface. A first motor moves the slide on the holder. The blade is mounted in the slide for oscillating movement in a direction parallel to the cutting edge. A second motor oscillates the blade. The two motors are mounted on a common motor unit which is detachably mounted to the slide. The holder has a grip portion surrounding the motor unit on two lateral sides and a rear side.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. application Ser. No. 09/247,528, to Frank Ziemer, filed Feb. 10, 1999, now U.S. Pat. No. 6,165,189. 
    
    
     FIELD OF THE INVENTION 
     The present invention concerns a microkeratome for performing LASIK surgery. 
     BACKGROUND OF THE INVENTION 
     LASIK (Laser Assisted In-Situ Keratomileusis) is a procedure commonly used to treat Myopia (nearsightedness), Hyperopia (farsightedness), and Astigmatism through the use of the Excimer Laser. LASIK is an operation which is performed in the Excimer Laser suite. The whole procedure is done under topical anesthesia (numbing drops) and its entire duration is seldom longer than 10 minutes. A suction ring of about 20 mm diameter is placed over the sclera (the white part of the eye) to hold the eye firmly. In performing LASIK, the surgeon first uses a microkeratome, presently with a special oscillating steel blade, to make a partial cut through the front surface of the cornea. This creates a flap of clear tissue on the front central part of the eye. So the automated microkeratome passes across the cornea to create a thin flap This part of the operation usually takes only a few seconds. The suction ring is then retired from the eye, and the flap is lifted back to leave enough room for the usage of the Laser. The Excimer Laser, which has been previously programmed specifically for the correction of the desired amount of the visual effect, is then applied. A rapid, continuous emission of Laser pulses removes very small, precise amounts of corneal tissue. Depending on the type of refractive error, this part of the surgery takes between 30 to 60 seconds. The cornea is then irrigated with saline solution, and the flap is folded back to its original position. Within minutes, the flap adheres itself to rest of the cornea and the LASIK procedure is done. In a couple of days, the cornea will be crystal clear and almost an imperceptible scar will barely be seen. 
     U.S. Pat. No. 5,133,726 describes a microkeratome. It comprises a holder with a suction ring for attachment to the sclera of a patient&#39;s eye. A suction source is connected to the suction ring. A slide is displaceably mounted on the holder in a linear guide. The slide has a plane frontal surface including a transparent plate for contacting the cornea of the patient&#39;s eye and slideable over the cornea in a direction parallel to the frontal surface. Attached to the slide is a flexible shaft which is driven by the motor. The end of the shaft has a threaded area which engages a pinion. The pinion drives via transmission gears a drive gear that engages in a rack on the holder to move the slide. At the extreme end of the shaft an eccentric is formed which engages a slot in a sled displaceably mounted on the slide. On the sled a steel cutting blade is mounted with a cutting edge which is parallel to the frontal surface. In operation, when the motor is started it simultaneously oscillates the blade parallel to the cutting edge and moves the slide on the holder. With this microkeratome the surgeon needs a lot of practice to position and to fix the abutting surface provided to stop the motor at the right time, i.e. at the right place. The abutting surface is to guarantee that the desired width of the remaining hinge linking the cut flap of tissue with the remaining cornea is obtained. The speed and oscillation frequency are fixed and have a fixed ratio determined by the gearing. 
     SUMMARY OF THE INVENTION 
     The problem to be solved with the present invention is to provide an improved microkeratome. This problem is solved by the combination of features disclosed hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention is hereinafter described with reference to the drawings, in which 
     FIGS. 1 and 2 show perspective views of the microkeratome, 
     FIGS. 3 to  5  show longitudinal sections, 
     FIGS. 6 is a perspective view of the holder with grip unit, 
     FIGS. 7 a - 7   b  are a perspective view of the parts of the slide, 
     FIGS. 8 a  to  8   c  show the parts of the sled, 
     FIGS. 9 and 10 show a tonometer in a dismounted and mounted position, 
     FIGS. 11 and 12 schematically illustrate a control unit, 
     FIG. 13 is a cross section along the top surface of part of the holder, and. 
     FIGS. 14 to  16  show a mounting unit for a diamond blade. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The microkeratome comprises a holder  1  with a suction ring  11  for attachment to the sclera  12  of a patient&#39;s eye  13  (see FIG.  10 ). The suction ring  11  has two spherical surfaces  14 ,  15  (FIG. 5) and an annular recess  16  between them. A circular ring-shaped insert  17  is detachably mounted in the recess  16  (see FIG.  5  and  10 ). The insert  17  has radially extending notches  18  with contact surfaces  19  which are rounded on all sides for contacting the eye and which lay in the same sphere as the surfaces  14 ,  15 . Between adjacent notches  18  the insert  17  has through holes  20  which communicate with the recess  16  behind the insert  17 . This recess  16  is connected to a channel  22  which at the rear end of the holder communicates with a connector  23  for connecting to a suction pump. 
     A cover plate  24  extends along and covers the channel  22  (see FIG.  2  and  5 ). The removable insert  17  has the advantage that the recess  16  and insert  17  can easily be cleaned and sterilized after use. The insert  17  is of metal and slotted on one spot of its circumference so that it can easily be removed by tweezers (see slot  25  in FIG.  2 ). 
     The holder  1  has two rectilinear directing guide rails  27  (FIG. 10) which are symmetrical to the longitudinal middle plane  28  (FIG. 13) extending through the axis  29  of the ring  11 . The rails  27  face away from the middle plane  28 . The longitudinal extension of the rails  27  is perpendicular to the axis  29 . This particular and uncommon arrangement of the rails  27  has the advantage that the moving slide  2  described below which has guide grooves  30  guided in the rails  27 , on the line where the moving slide  2  meets the holder  1 , does not contact and jam or squeeze the tissue of the eye. Thereby, injuries of the eye and pressure changes during cutting can be avoided. A rear, raised part  31  of the holder has an upper surface  32  which is perpendicular to the axis  29 . The part  31  has a rectilinear groove  33  extending perpendicular to and across the middle plane  28 . On one end the groove  33  merges into a longitudinally extending groove  34  which is open towards the forward end (FIG.  6  and  13 ). 
     The guide grooves  30  of the slide  2  are best seen in FIG. 7 a  which shows the parts of the slide  2 , a sled  3  (FIG. 7,  8   a  and  8   b ) with a cutting blade  4  having a cutting edge  5 , part of a motor unit  6  removeably mounted on the slide  2 , a mounting help  7  and a handle  8  for the sled  3 . The slide  2  has two lateral arms  38  on which a transverse plate  39  is mounted with a plane contact surface  40  for contacting the cornea of the eye  13 . The surface  40  is perpendicular to the axis  29  and therefore parallel to the guide rails  27  and grooves  30 . In a park position shown in FIG. 5 the forward end face  41  of the plate  39  is slightly behind the opening  42  of the ring  11 . This allows free access to the eye in the park position, e.g. for purposes described later. The plate  39  may be transparent, e.g. of glass, and may have marks for reading the diameter of the contact area of the plate  39  with the eye  13  for the purposes more specifically described in U.S. patent application Ser. No. 09/129,365 by Frank Ziemer, filed Aug. 5,1998, and incorporated herein by reference. 
     The slide  2  has a transverse, prismatic opening  44  (FIG. 1 and 7 b ) in which the prismatic, thermoplastic sled  3  is guided. The opening  44  extends perpendicular to the axis  29  and the rails  27  and is open to both sides. In a part  45  of the opening  44  with rectangular cross section the sled  3  is guided. An elastic arm  46  integrally formed on the sled  3  has attachment pins  47  formed on it. The blade  4  (FIG. 8 c ) has corresponding holes  48  which match the pins  47 . The blade  4  is attached to the sled  3  by thermoplastic deformation of the free ends of the pins  47 . 
     The arm  46  elastically biases the blade  4  against the first guide surface  53  and the second guide surface  54  (FIG. 7 b ) which are perpendicular to each other and are part of the opening  44 . The surface  53  is coplanar with a lower surface of a slot  55  through which the blade  4  extends. The cutting edge  5  extends to about 0,16 mm below the plane of the contact surfaces  40 . Preloading the blade  4  by the arm  46  has the advantage that the play of the sled  3  in the opening  44  and of the blade  4  in the slot  55  does not have any influence on the accurate guidance of the blade  4  in operation. Therefore, the spacing between the cutting edge  5  and the plane of the surface  40  remains accurate to within a few μm. 
     The prismatic mounting help  7  and the handle  8  each have the same cross section as a forward part  56  of the opening  44 . They each have prismatic or cylindrical extensions  57  which formlockingly engage the sled  3  and/or the arm  46 . The mounting help  7  is tapered at the forward end to facilitate introduction into part  56  of openings  44 . The handle  8  has, spaced from the sled  3 ; a step  58 . The sled  3  has a transverse slot  59  extending perpendicular he opening  44 . A conical counters sink  60  extends from the upper surface of the sled  3  with its axis coinciding with the cent I axis of the slot  59 .: 
     The slide  2  further comprises two prismatic extensions  63  which have centering holes  64  and threaded holes  65  in their end surfaces. Two counteracting ring-shaped locking elements  66 ,  67  surround the extensions  63  and are spread apart by two springs  68 . On a wall opposite a gripping surface  69  each element has a locking bar  70  which in the mounted position of the motor unit  6  engage grooves  72  on a cylindrical extension  71  of the motor unit  6 . The elements  66 ,  67  are held on the slide  2  by a cover unit which is centered with pins  75  in the holes  64  and screwed onto the extensions  63  by screws  76 . The unit  74  has a central cylindrical opening  77  in which the extension  71  is centered. 
     The motor unit  6  (FIG. 1,  3 ,  5 ) comprises a casing  81  on which a first motor  82  and a second motor  83  are mounted. The axis of the motor  82  is perpendicular to the rails  27  and the contact surface  40 . Its output shaft  84  drives a disc  85  which carries an eccentric in  86 . The pin  86  carries a shoe  87  which is square in plan view and engages in the grooves  33 ,  34 . The axis of the motor is inclined rearwardly to gain better access and view to the suction ring  11 . The output shaft  88  of the motor  83  has a further eccentric pin  89  which engages in the slot  59  and thereby drives the sled  3 . The motor unit  6  is covered by a cover  90  which is fixed to the unit  6  by screws  91  and has on both sides two guide rails  92  for suspension in a stand (not shown). 
     The grip unit  9  shown in FIG. 2 and 6 consists of metal. The unit  9  has two opposed lateral side walls  95  and a rear wall  96 , the lower end of the walls  95 ,  96  are welded or braced to the top surface  32  of the raised part  31  of the holder  1 . The grip unit  9  surrounds the motor unit  6  on at least part of its lateral sides and on its rear side. The surgeon holds the microkeratome on this grip unit during the operation. This grip unit  9 , via the holder  1 , is fixed with respect to the suction ring  11 , while the motor unit  6 , together with the slide  2 , moves towards the suction ring  11 . The larger parts of the microkeratome, in particular the holder  1 , the grip unit  9  and the casing  81  are manufactured of titanium to save weight. 
     As shown particularly in FIG. 6 the holder  1  has along the sides of the periphery of the suction ring  11  and along the rear part of the guide rails  27  an upstanding narrow rim  98 . The rim  98  guards the slide  2  from coming into contact with the eye lid. Therefore, the surgeon always has one hand free during the operation. 
     FIG.  4  and the upper part of FIG. 3 further show a cable holder  101  which can be docked alternatively either to the motor unit  6  or to the grip unit  9 . In the holder  101  a plug socket  102  is fixed into which a plug  103  is inserted. A plurality of leads  104  of a cable  105  connecting the holder  101  with a control unit are connected to the plug  103 . A two-armed lever  106  is pivotably supported in the holder  101 . The forward arm of the lever  106  has a hook  107  which in the position shown in FIG. 3 engages a shoulder  108  of the casing  81  of the motor unit  6  and therewith docks the holder to the unit  6 . The rear end of the other arm of the lever  106  has a cylindrical extension  109 . A pusher  110  is mounted on the upper part of the rear wall  96  of the grip unit  9 . The pusher  110  is attached to two cylindrical pins  111  which are slideable in corresponding through-borings  112  of the wall  96 . The pins  111  have at the forward end a wedge surface  115  which merges into a cylindrical recess  116 . 
     The lever  106  is urged into its position shown in FIG. 3 by two laterally spaced pins  117  which are pre loaded upwardly by springs  118 . FIG. 4 shows a cross section parallel to the longitudinal section of FIG. 3 but containing the axis of one of the pins  117 . 
     When the holder  1  is inserted onto the slide  2  to the position shown in FIG. 3, the pusher  110  is pushed in. The wedge surface  115  pushes the extension  109  upwardly so that the lever  106  pivots counterclockwise against the force of the springs  118 . In the fully pushed in position the extension  109  rests in the recess  116  of the pins  111  (shown in dashed lines in FIG.  4 ). The lower ends of the two pins  117  in that position engage in corresponding borings  119  of the rear wall  96 . The hook  107  is out of engagement with the shoulder  108  and the holder  101  is now coupled to the grip unit  9  and therewith to the holder  1 . This has the advantage that the cable  105  does not move when the slide  2  with the motor unit  6  moves forward. During cutting the cable does not transmit any forces to the moving unit. 
     Part of the pins  122  of the socket  102  are connected (not shown) to one end  123  of a band-shaped, flexible electrical cable  124 . The end  123  is clamped in a cylindrical drum  125  which is fixed to the connection holder  101 . In the park position shown in FIG. 3, the cable  124  surrounds the drum  125  in a loose loop. Its other end is fixed to the motor unit  6  and its various leads are connected to the motors  82 ,  83  and to an angle sensor  126  which is coupled to the output shaft  84  of the motor  82  as a feedback of the position of the slide  2  on the holder  1 . 
     An electronic circuit plate  129  comprising a hall sensor  130  is mounted on the holder  101  above the lever  106 . The extension  109  contains a permanent magnet  131  cooperating with the hall sensor  130 . The plate  129  is connected by leads  132  to some of the pins  122 . 
     FIG. 9 and 10 show a tonometer  136  for use with the microkeratome of the present invention. The tonometer  136  comprises a housing  137  of titanium with a cylindrical boring  138  which, in the mounted position shown in FIG. 10, is coaxial to the axis  29 . On the upper and lower end of the boring  138 , two low-friction thermoplastic slide rings  139  are inserted in corresponding grooves. Two cylindrical sleeves  140  are slideably guided by the rings  139 . The sleeves  140  are pressed or bonded onto a cylindrical, transparent sapphire body  141  which has a plane lower end surface  142  perpendicular to the axis  29  and a spherical upper surface  143 . The lower surface  142  has engraved or etched cross lines  144  of which one, in the mounted position, is parallel to the guide rails  27  and the other one is perpendicular to it. The lines  144  cross the axis  29 . The lower surface further has a circular mark  145 . A pin  146  screwed into the housing  137  engages a longitudinal groove  147  in one of the sleeves  140  and guarantees an accurate angular position of the cross lines  144 . 
     The upper face of the suction ring  11  has two diametrically opposed upstanding bosses  151  (not shown in FIG. 1 to  6 ) with plane upper surfaces  152  which contain spherical or cylindrical recesses  153 . Each boss  151  further comprises a boring  154  parallel to the rails  27 . The housing  137  has corresponding contract surfaces  155  for contacting the surfaces  152  and pins  156  for insertion into the borings  154 . The housing  137  further has two inserts  157  with spring loaded balls  158  which snap into the recesses  153  when the tonometer  136  is mounted on the suction ring  11 . 
     The microkeratome further comprises a control unit  162  (FIG. 11) with a touch screen  163 , a socket  164  for plugging in a plug at the end of the cable  105 , a connector  165  for connecting the unit  162  with the connector  23  by a vacuum house (not shown), and a socket  166  for plugging in a cable to two foot switches (not shown). The control unit  162  contains a vacuum pump, a vacuum vessel, a vacuum switch, a vacuum sensor and a programmable microprocessor. 
     Before starting an operation, the motor unit  6  with cover  90  and connection holder  101  are suspended on the rails  92  in a stand. These units need not to be sterilized because they do not come into contact with the patient or with the surgeon&#39;s hands. They may, however, be sterilized with gas. The holder  1  with grip unit  9 , the slide  2  and the tonometer  136  are sterilized in an autoclave. The sled  3  with blade  4 , mounting help  7  and handle  8  are contained in a sterile sachet or box. The patient&#39;s eye  13  is marked by a conventional stamp known as LASIK-marker with cross lines crossing the axis of the eye and with a small circle around that axis. The surgeon enters his name to the control unit  162 , whereupon his last used set of parameters appears on the touch screen. He enters the patient&#39;s name and changes the parameters if required. When he touches one of the fields with parameter denomination, a new keyboard appears which allows the corresponding entry. For the numerical parameters a numerical keyboard is displayed and for names an alphanumerical one. The cables and the hose are connected to the control unit  162 . 
     For the assembly of the microkeratome, first the sled  3  is inserted into the slide  2  up to abutment of the step  58  of the handle  8  on the slide  2 . In this position the slot  59  is centered in the axis of the opening  77 . The slide  2  is then pushed onto the extension  71 . The bars  70  automatically snap into the grooves  72 . The pin  89  enters the conical recess  60  and automatically centers the slot  59  to the pin  89 . The mounting help  7  and the handle  8  are removed. Now the holder  1  is inserted with its rails  27  into the grooves  30  of the slide  2  from behind. The shoe  87  enters the groove  34  which is best seen in FIG. 13 which shows a cross section along the upper surface  32  of the raised part  31  of the holder  1 . The axis  169  of the disc  85  and the output shaft  84  of the motor  82  is marked with a circle. This axis  169  is parallel to the middle plain  28  but offset to one side. The rotary position of the shoe  87  with respect to the axis  169  is shown in solid lines in FIG.  13 . The holder  1  is pushed forward until the shoe  87  abuts the rear face  170  of the groove  33 . 
     Now the pusher  110  is pushed in so that the holder  101  is decoupled from the motor unit  6  and coupled to the gripping unit  9 . The signal of the sensor  130  initiates a rotation of the output shaft  84  by about 10° so that the shoe  87  enters the groove  33  and reaches the position shown in dashed lines in FIG.  13 . The slide  2  is now in its parking position shown in FIG. 3 and 4. The microkeratome still hangs with its rails  92  in its stand and is ready to start operation. 
     The tonometer  136  may now be attached to the suction ring  11  and the surgeon grips the microkeratome and places the suction ring  11  on the patient&#39;s eye  13  such that the cross lines  144  are exactly aligned with the cross lines marked on the patient&#39;s eye. The vacuum is now switched on with the right foot switch so that the suction ring adheres to the eye  13  and is immobilized relative to the eye. The eye pressure is checked. The gravitational force of the body  141  and sleeves  140  creates a certain, visible contact area which should not be larger than the area marked with the circle  145 . 
     If this is the case, the tonometer  136  is removed and the slide  2  is moved to its initial position by pressing the right foot switch. The shoe  87  -it then in the position shown in dashed lines in FIG.  13  and the plate  39  has moved forward so that the end face  41  is somewhat ahead of the axis  29 . The cutting edge  5  is a specified distance h away from the axis  29 , e.g. 6 mm. 
     Some surgeons have so much experience that they do not need a tonometer but feel the eye pressure with a finger. In that case, cross lines  170  on the suction ring are used for centering the suction ring on the eye  13 . The plate  39  is transparent and has a set of marks  171  with increasing radius on its top or bottom surface around the axis  29  in the initial position of the slide  2 . The surgeon reads the diameter of the contact surface of the plate  39  with the eye  13  with the help of these marks  171  and enters the value on the touch screen  163 . He checks whether all parameters are correct and confirms them by pressing the right foot switch. By pressing, that switch again, the cut is started and the microkeratome now operates the way described in U.S. patent application Ser. No. 09/129,365 mentioned above. The control unit  163  varies the angular speed of the disc  85  such that the linear speed of the slide  2  is constant and corresponds to the set value. The total angle of rotation is calculated from the distance h, the flap diameter and the hinge width. 
     After the cut is finished, the slide  2  returns to the park position and the vacuum is switched off with the right foot switch. Disassembly of the microkeratome for cleaning and sterilizing is done in the reverse order of the assembly. The sled  3  and blade  4  are disposed. 
     A test program may be run prior to use by touching the screen  163  on the field “test”. An insert is then shown on the touch screen instead of the fields “database”, “help” and “service” and the program, which checks the various functions and the vacuum by pinching at a certain stage the vacuum tube is run through with the foot switches. This test only takes a few seconds. 
     The overwhelming majority of surgeons use steel blades  4  in microkeratomes. These blades are rather expensive since they are used only a single time. Another possibility is the use of diamond blades which are cheaper in the long run and have a number of advantages, in particular a much sharper cutting edge which results in a cleaner cut. The rpm of the motor  83  can be set lower or it may not even be necessary to vibrate the blade  4 , thus reducing vibrations. A diamond blade can be used indefinitely if properly handled. 
     However, the cutting edge of a diamond blade is extremely vulnerable. The blade must be disassembled from the side after use for claiming and sterilizing. 
     FIG. 14 to  16  show a holder and mounting unit  176  for such a diamond blade. The blade  177  and sled  178  are actually designed for the microkeratome according to U.S. patent application Ser. No. 09/129,365 (without the elastic arm  46  of the sled  31  and therefore with a different shape of the opening  44 ) but it is intended to later use a sled comparable to the one show in FIG. 7 and 8. The elastic arm  46  would in this case have to include a cylindrical body to accommodate the female thread, and the extension  57  may need a snap action fit to the arm  46  (FIG.  7  and  8 ). 
     The blade  177  consists of a narrow diamond blade  179  vacuum-hard-braced onto a steel blade  180  of the same thickness. The blade  180  is screwed onto the prismatic sled  178  by screws  181 . The slot  182  and the wedge-shaped countersink  183  correspond to the slot  59  and countersink  60  of FIG.  8 . On one of its face surfaces the sled  178  has two borings  184  and in the center a female thread  185 . 
     The unit  176  has a sleeve-shaped holder  188  with a cup-shaped end  189  and a thoroughgoing boring  190 . A shaft  191  is slideable in the boring  190  and fixed against rotation by a pin  192  engaged in a longitudinal groove  193 . A spring loaded ball  194  engages in one of two axially spaced circumferential grooves  195  of the shaft  191  to fix its two longitudinal positions. The shaft  191  has a thoroughgoing stepped longitudinal boring  196 , and on its forward end two pins  197  engaging the borings  184 . 
     A pin  199  with a gripping end  200  is slideable and rotatable in the boring  196 . The axial position of the pin  199  is held by a spring urged ball  201  engaging one of two grooves  202 . The forward end of the pin  199  has a male thread  203  matching the thread  185 . Two pins  204 ,  205  of different diameters project from the plane end face  206  parallel to the axis of the pin  199 . These pins match with corresponding borings of the slide. Before use a cover with corresponding borings  208 ,  209  is slid onto these pins  204 ,  205 . 
     After use of the microkeratome the unit  176  is docked to the slide of the microkeratome by the two pins  204 ,  205 . The pin  199  is pulled back and the shaft  191  moved to its forward position. The pin  199  is then moved forward and rotated so that the screw  203  screws into the thread  185 . The motor unit is now removed from the slide and the shaft  191  pulled back. The sled  178  with the blade  177  is cleaned e.g. in an ultrasound bath and sterilized, and the end  189  is covered by the cover  207 . For mounting the sled  178  the unit  176  is again docked to the slide of the microkeratome and the shaft  191  and pin  199  moved to their forward position. In this position the slot  182  is centered in the middle of the slide so that the motor unit can be mounted as described above. 
     The dashed lines in FIG. 16 indicate the slide  213  corresponding to the slide  30  of U.S. patent application Ser. No. 09/129,365 but adapted for docking of the unit  176 . To that end one side of the slide  213  has two borings  214 ,  215  for receiving the pins  204 ,  205  and aligning mounting unit  176  on slide  213 . The sled  178  has on its end surface a female thread  185  and borings  184  for coupling sled  178  to mounting unit  176 , 
     With this mounting unit  176  it is ensured that the blade  177 , and in particular the diamond blade  179 , is handled extremely carefully when mounting and dismounting the sled  178  and when cleaning it. There is no risk of touching the cutting edge  5  with an object or with a finger.