Patent Publication Number: US-2010121330-A1

Title: Surgical bone milling instrument

Description:
TECHNICAL FIELD 
     The present invention relates to a surgical bone milling instrument. 
     BACKGROUND ART 
     In general, the purpose of the instrument is the removal of areas of bone in a controlled manner in proximity to delicate parts of bone, typically to complete holes in bones in which the a terminal portion of the hole is located close to particularly delicate organs. 
     A typical application is in the surgical technique of mini-lifting of the maxillary sinus cavity, which involves the raising of the floor and filling with biomaterial of the maxillary cavity through a hole prepared in the bone for the insertion of a dental implant. 
     The cavity in the maxillary sinus is the largest of the pneumatic cavities in the cranial bone and it is located in the rear areas above the upper jaw. 
     The formation of the maxillary sinus floor can be influenced by the presence of the roots of premolar and molar teeth. At the apex of these roots there is a thin layer of compact cortical bone (maxillary sinus floor). The maxillary sinus cavity is normally unitary and contains air. The inside of the cavity is coated entirely with a mucus membrane known as the Schneider membrane. 
     The insertion of dental implants in the rear areas of the upper jaw is almost always conditioned by the presence of the maxillary sinus, which limits the availability of bone in height, especially in patients who have been edentulous for a long time. 
     Following the loss of the rear teeth of the jaw, a process of reabsorption of “external” bone begins, combined with a like reabsorption of the floor of the “internal” maxillary sinus, causing expansion of the maxillary sinus cavity and the approach of the sinus floor progressively closer to the “outside” edge of the alveolar crest, reducing the height of bone available for implantation. 
     In the 1990s the above-mentioned surgical techniques for the mini-lifting of the maxillary sinus were developed, substantially involving the boring of holes in the jaw for the insertion of dental implants and the raising of the maxillary sinus floor and its filling with biomaterial of the maxillary cavity through the hole. 
     These techniques penetrate to the Schneider membrane, depending only on tactile sensation and radiographic investigations for its identification, since the membrane is not directly but only indirectly visible. 
     These techniques attracted the interest of dental surgeons less familiar with advanced surgery. The maxillary sinus mini-lifting technique offers functional advantages in view of the incidence of the operation. 
     The main difficulty of these techniques for raising the maxillary sinus is the creation of the final part of the bone cavity with decollement/detachment of the Schneider membrane from the “internal” bone surface in order to permit the filling of the sinus cavity with bone tissue (biomaterial). The limited thickness of the membrane or the slightest error of the operator can result in lacerations that compromise the success of the filling of the sinus cavity. 
     The current techniques for mini-lifting can be divided on the basis of the instruments used, which are either osteotomes or mills. 
     Osteotomes are manual instruments, increasing in size and provided with a concave point with a cutting edge. Osteotomes can be used with depth limit stop rings adjusted manually with fixing screws. The stop is positioned according to the available bone height, established radiographically. An osteotome can be used with manual pressure or, in the case of hard bone, with the help of a surgical mallet. The special conformation of the osteotome means that once it is inserted into the implant hole, prepared to a distance of 1 to 2 mm from the floor of the maxillary sinus, it can remove a small quantity of bone tissue from the walls of the site and concentrate this on the terminal section of the osteotome. The bone cortex of the floor of the maxillary sinus is then fractured, by way of percussion with a mallet, to raise the floor of the sinus until the required lifting is achieved. 
     This operation requires extreme delicacy to avoid the possibility of lacerating the thin Schneider membrane. 
     In order to reduce the risk of laceration of the membrane, instead of using osteotomes alone, the insertion of biomaterial into the bone cavity has been proposed in order to act as padding to be compacted vertically between the osteotome point and the bone. 
     Mini-lifting mills are rotating instruments (500 rpm) for fitting on an electrical turbine and are fitted with a non cutting point and available in various calibrated lengths (one mill every millimetre) for sequential use. The mills act by wearing away the bone cortex that precedes the floor of the maxillary sinus, and because they are not sharp they can be used on the final portion of bone, limiting the risk of damage to the Schneider membrane. 
     After preparation of the site and after decollement and raising of the membrane using rounded manual instruments, the biomaterial is inserted before the positioning of the dental insert. 
     This technique again depends on the tactile sensitivity and practical experience of the operator. 
     One aim of the present invention is to realize a device capable of overcoming the difficulties mentioned above. 
     These and other aims are achieved by the invention as it is characterized in the accompanying claims. 
     DISCLOSURE OF INVENTION 
     The invention makes it possible to complete axial milling of a bone hole (formed previously using a standard instrument) maintaining control of the position of the device relative to the hole. The action of the probe element of the instrument enables the operator to perceive, also visually, the moment when the milling head of the instrument reaches the end of the previously formed hole, or the end of the excavation created by the milling head. 
     From this point onwards the operations of further axial advance of the milling head can be monitored with the probe element; while the milling element is further advanced, to excavate an additional axial section of groove, the probe device remains axially stationary against the central area of the end surface (the part not subjected to milling). Putting millimetric markings on the probe element makes it a means for continuous control and determination of the axial position of the milling head relative to the position of the end of the bone hole. 
     Furthermore, by way of axial pressure applied to the probe element the detachment of the residual portion of the bone wall can be achieved as soon as the residual portion has reached a breaking resistance level that is lower than the force applied to the probe element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in detail below with the aid of the attached figures of the drawings, which illustrate a non-exclusive embodiment of the invention, by way of example, and in which: 
         FIG. 1  is a side view of a first embodiment of the instrument of the invention. 
         FIG. 2  is a cross-section along the axial plane II-II of  FIG. 1 . 
         FIGS. 2A and 2B  show the milling head  11  of  FIG. 2 , in enlarged scale, respectively in the advanced and withdrawn positions. 
         FIG. 3  is a perspective view of  FIG. 1 . 
         FIGS. 4A and 4B  show an enlarged detail of  FIG. 1  in two different operating positions. 
         FIG. 5  is the cross-section along the transverse plane V-V of  FIG. 4A . Figures from  6 A to  6 F show a sequence of operating stages in which the instrument excavates the final section of a hole in a bone. 
         FIG. 7  is an exploded view of the instrument of  FIG. 1 . 
         FIG. 8  is an axial cross-section of a second embodiment of the instrument of the invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The instrument  1  illustrated in figures from  1  to  5  comprises a milling element  10 , in particular, having an elongate body shape with a longitudinal axis A, the forward portion of which can rotate about the axis A and mill the bone. In particular, the extreme forward portion of the milling device  10  forms a milling head  11  which is equipped with sharp raised milling ridges  12 , capable of milling bone in an axial direction when rotating. 
     The instrument  1  comprises a probe element  20 , in particular, with an elongate shape located internally of and coaxial with the milling element  10  and sliding longitudinally, along axis A, through the milling element  10 , with a forward portion  21  projecting beyond the forward portion of the milling element  10 . In particular, the probe element  20  is inserted into an axial through-hole  15  formed in the milling element  10 ; the raised milling ridges  12  thus have a radial extension limited by the presence of the hole  15  in the head  11 . 
     In particular, the probe element  20 , at least in its forward and mid portions, is shaped as a cylindrical rod, possibly with the forward portion  21  rounded in order not to harm the tissues with which it comes into contact. 
     The probe element  20  is capable of indicating to the operator, also visually, when the milling head  11  reaches the end of the hole into which it is inserted during the use of the instrument and, from that point onwards, the position of the milling head  11  in the subsequent operations of further axial advance of the milling head  11 . 
     Further, by way of an axial pressure applied to the probe element  20 , it is possible to cause the detachment of the residual portion of bone wall, when the bone wall has reached a resistance to breakage which is lower than the thrust force applied to the probe element  20 . 
     The instrument  1  comprises a screw-threaded tubular element  30  through which the milling element  10  is inserted coaxially, with the milling head  11  thereof projecting beyond the end of the tubular element  30 . In particular the tubular element  30  has a cylindrical tubular body, an outside surface of which has a circular cross-section and is provided with a thread  31   a,  which engages by helical coupling in the hole formed in the bone. 
     The milling element  10  is associated to the tubular element  30  and can rotate around the longitudinal axis A relative to the tubular element  30 , and can displace axially with respect thereto. 
     In particular, the milling element  10  has a rear portion which coaxially passes through the tubular element; in particular, this comprises a tubular rod  14  of constant cross-section located behind the milling head  11 , the external cylindrical surface of which snugly couples with and matches the size of the internal cylindrical surface  32  of the threaded tubular element  30 , the length of the rod  14  being greater than that of the tubular element  30 . The head  11  has a greater external diameter than the rod  14  and its rear portion has a radial rear edge  11  b designed to stop in striking contact with the circular edge of the front end  30   a  of the tubular element  30 . The external diameter of the head  11  is approximately equal to the external diameter of the tubular element  30 , in particular, it is smaller than the external diameter of the thread  31   a.    
     The milling device  10  is solidly fixed to a drive element  40 , located coaxially and to the rear of the tubular element  30 , causing the rotation of the milling element and translating it axially in relation to the tubular element  30 . In particular, the drive element  40  is coaxially and solidly joined to the rear portion of the rod  14 ; the drive element  40  has a forward shank  41 , of lesser diameter, and a rear portion  42 , of greater diameter, shaped as a circular handle for enabling manual activation of the milling element  10 , in rotation and in axial translation. The rear portion  42  also serves as a handle for the manipulation of the entire instrument  1 . 
     In the embodiment shown in the figures, the drive element  40  also acts to transmit torque to the tubular element  30 , causing it to rotate. 
     In particular, the drive element  40 , is free to slide axially, over a certain distance, relative to the tubular element  30 , and to rotate, again relative to the tubular element  30 , and is torsionally engaged to the tubular element  30  by reciprocal engaging means that leave the drive element  40  free to be rotated by an angle of less than 360 degrees and to be axially displaced in relation to the tubular element  30 . 
     In particular, the reciprocal engaging means comprise profiled raised portions  35  and  45  extending in an axial direction towards each other (one faces back and the other forward), from the extreme rear end circular edge  30   b  of the tubular element  30  and respectively from the extreme forward end circular edge  42   a  of the shank  42  of the drive element, the raised portions of which are designed to reciprocally strikingly contact following reciprocal rotation in order to transmit torque. 
     The instrument comprises means for limiting the axial displacement of the milling element  10 , together with the drive element  40 , with respect to the tubular element  30 . 
     In the embodiment shown in  FIGS. 1 to 7 , these means are determined by the length of the free axial portion of the rod  14  on which the tubular element  30  can slide, in relation to the length of the tubular element  30 . The milling element  10  is free to move axially relative to the tubular element  30  between a fully advanced position, in which the milling head  11  projects forward the maximum possible distance D 1  from the tubular element  30  (see  FIGS. 1 ,  2 , and  2 A), and a fully withdrawn position, in which the milling head  11  is at the minimum possible distance D 2  from the tubular element  30 , in particular it is in contact there-with (D 2  equal to zero) (see  FIG. 2B ). 
     In particular, the above-mentioned profiled raised portions  35  and  45  are conformed such that they maintain the tubular element  30  at a predefined maximum axial distance (which substantially corresponds to the above-mentioned fully withdrawn position of the milling element  10 ) relative to the drive element  40 , when the elements are positioned in reciprocal torsional contact (the position of  FIG. 4A ), or to enable an approach of the two elements  30  and  40  to a minimal axial distance (which corresponds to the above-mentioned fully advanced position of the milling element  10 ) following angular displacement relative to the reciprocal torsional contact position (the position in  FIG. 4B ). 
     In detail, the profiled raised portion  35 , joined to the tubular element  30 , has a widened base  36  with two surfaces  36   a  inclined and converging, and a central raised straight portion  37  with two parallel sides of axial development  37   a  that project axially relative to the base  36 . The other raised portion  45 , joined to the drive element  40 , has a raised central portion with two parallel sides of axial development  45   a  ( FIGS. 4A ,  4 B). 
     Alternatively, the raised portion joined to the threaded tubular element  30  can have the shape described herein for the raised portion united with the drive element  40  and vice-versa. 
     Rotating the drive element  40  relative to the tubular element  30 , the raised portion  45  first comes into contact with the base  36  of the relief portion  35 , then slides along one of the inclined surfaces  36   a;  following this sliding, as it approaches the central raised portion  37 , the two elements  30  and  40  move away from each other axially. When the axial sides  37   a  and  45   a  then come into reciprocal contact ( FIG. 4A ), the tubular element  30  and the drive element  40 , are situated at a predefined distance (corresponding to the fully withdrawn position of the milling head  11 ), without the possibility of moving closer, due also to the fact that at the base of the central raised portion  37  two sections  36   b  are located on transversal planes (at 90 degrees relative to the axial direction) serving as end stops against the highest extremity of the raised portion  45 , preventing its approach to the tubular element  30 . 
     By rotating the drive element  40  relative to the tubular element  30  in the opposite direction so as to distance the two relief portions  35  and  45  from each other, the rotary movement of the element  40  is independent relative to element  30 , up to an angle M ( FIG. 5 ) less than 360 degrees; further, these elements  30  and  40  can be axially moved towards each other until the upper apex of the relief portion  37  stops in contact with the edge  40   a  of the element  40  (or the upper apex of the relief portion  45  stops in contact with the edge  30   b  of element  30 ), which determines the minimum distance between elements  30  and  40  ( FIG. 4B ). 
     The axial through-hole  15 , extends with a coaxial hole  46  of substantially the same diameter and passing through the drive element  40 ; the probe device  20 , of matching diameter, is located inside the holes  15  and  46 , the probe device  20  being subject to the action of elements acting to push it axially so that the forward portion  21  thereof projects with respect to the forward portion of the milling element. 
     The probe element  20  has a rear portion  22  which is visible to the operator. 
     In particular, the rear portion  22  of the probe element is designed to be visible at the rear of the handle defined by the rear portion  42  of the drive element  40 . 
     The probe element  20  can be moved axially relative to the milling element  10  and relative to the handle  42 . 
     In particular, this displacement is produced by pressure applied to the rear portion  22 , such as to cause the forward portion  21  to project for a predefined distance beyond the forward portion of the milling element  10 . 
     In the embodiment shown in the figures, the rear portion  22  of the probe element  20  has a stop surface  23   a  for axial movement thereof towards the forward end and it is subject to the axial thrust pressure of a precompressed helical spring  16  acting to push the probe element  20  forward. In particular the portion  22  has a greater diameter than the axial hole  46 , thus forming the stop surface  23   a.    
     In particular, the rear portion  22  is housed in a cylindrical concavity  43  formed in the rear portion  42 , which is coaxial with axis A and faces rearwards; the spring  16  is located inside the concavity  43  and is compressed between a radial raised portion  23  formed in the rear portion  22  and a stop element  44  coupled helically to the concavity  43 , with the possibility of adjusting the axial position of the spring  16  and thus its level of precompression, and consequently the axial pressure acting on the probe element  20 . 
       FIG. 8  shows a second embodiment of the invention. 
     The second embodiment differs from the first principally due to the fact that the drive element  40  rotates only the milling element  10  and not also the tubular element  30 ; consequently the described profiled raised portions  35  and  45  are absent and the drive element  40  can rotate freely relative to the tubular element  30 . 
     The tubular element  30  is instead rotated by a dedicated second drive element  50 , independent of the first drive element  40 . 
     In particular the drive element  50  is located forward of the drive element  40  and it is coaxially and solidly fixed to the rear end portion of the tubular element  30 ; the rod  14  of the milling element  10  passes through a matching axial through-hole  57  formed in the drive element  50 . The second drive element  50  comprises a tubular shank  51  fixed coaxially and solidly to the rear end portion of the tubular element  30  and a rear portion  52 , of greater diameter relative to the shank  52 , shaped as a circular handle serving to enable the operator to manually activate the milling element  10 , with their fingers, in rotation and in axial translation. Preferably, the rear portion  52  has a diameter that is greater than the corresponding portion  42 . 
     The two drive elements  40  and  50  are free to rotate reciprocally, even if they are in contact. 
     This embodiment also includes elements acting to limit the axial movement of the milling element  10  relative to the tubular element  30  to a predetermined extent. 
     In particular, the milling element  10 , together with the drive element  40 , is free to move axially relative to the tubular element  30  and the second drive element  50 , between a fully advanced position ( FIG. 2A ), in which the milling head  11  projects forward the maximum possible distance D 1  from the tubular element  30 , and a fully withdrawn position ( FIG. 2B ), in which the milling element  11  is at the minimum possible distance D 2  from the tubular element  30 , and in particular is in contact there-with (D 2  equal to zero). 
     The axial displacement between the elements  10  and  30  is delimited between the contact of a forward edge  30   a  of the tubular element  30  with a rear edge  11   b  of the milling head  11  (which determines the fully advanced position) ( FIG. 2A ) and the contact of a transversal rear surface  52   b  of the rear portion  52  with a forward circular protrusion  42   a  of the shank  42  (which determines the fully withdrawn position) ( FIG. 2B ). 
     There follows a description of use of the instrument. 
     A typical use of the instrument illustrated is to extend the final part of an initially blind hole, realized previously in a jaw bone.  FIG. 6A  shows the preliminary blind section  71  of the hole, which is realised in the jaw bone  75  in a known way (for example, using a motorized milling instrument, possibly with the use of radiographic viewing means), the end surface  72  of which lies at a relatively short, though safe, distance (a few millimetres) from the Schneider membrane  76  located at the bottom of the maxillary sinus  77 . 
     The instrument  1  is suitable for extending the final part of the hole  71  up until it breaks or destroys the thin upper wall  74  of bone cortex which separates the end surface  72  of the hole  71  from the membrane  76 , without causing harmful lesions to the membrane. 
     For this purpose, first ( FIG. 6A ), the tubular element  30  is rotated inside the hole  71 , such that the helical thread  31 a thereof engages with the cylindrical surface of the hole  71 , into which the thread penetrates thanks to the sharp edges thereof. At this stage the milling head  11  is in the above-mentioned withdrawn position relative to the tubular element  30  (in particular, they are in contact). 
     In a first phase (advancement phase), by rotating the element  30  the penetration of the forward end of the instrument  1  (the milling head  11  and the forward portion of the tubular element  30 ) is induced inside the hole  71 . The instrument is made to advance axially in the hole until the forward portion of the milling head  11  comes into contact with the end surface  72  ( FIG. 6B ). At this point the forward portion  21  of the probe  20  is stationary against the surface  72 , aligned with the forward portion of the milling head  11 ; the rear portion  22  of the probe  20 , is now in a position relative to the rear portion of the drive element  40  which can be used as an initial reference position. 
     The point at which the milling head  11  comes into contact with the end surface  72  is actually perceived by the operator, especially because the probe element  20  is pushed back into the hole  15  such that the rear portion  22  thereof moves back into the reference position and this movement is seen by the operator, and the advance of the instrument is halted. Furthermore, the operator notices an increased resistance to rotation of the tubular element  30  together with the fact that the milling head  11  is axially locked in the above-mentioned withdrawn position and cannot perform forward axial movement. 
     In the first embodiment of the instrument, the rotation of the tubular element  30  is produced by manually rotating the drive element  40 , which, through the reciprocally engaging elements  35  and  45 , transmits torque to the tubular element  30 . 
     In the second embodiment of the instrument, the rotation of the tubular element  30  is produced by manually rotating the drive element  50 , which drives only the element  30 , and the milling head  11  is not set in rotation. In the second phase (the milling phase), only the milling element  10  is rotated, and it is also pushed forward relative to the tubular element  30 , in order to create on the end of the hole  71 , by way of the action of the raised milling ridges  12 , a groove  73  in the form of a circular channel surrounding a circular area  72   a  of the end surface  72 , facing the hole  15 , which remains unaffected by the action of the milling ridges  12  (see  FIG. 6C ). 
     In the first embodiment, operation is manual using the drive element  40 , the raised portion  45  is distanced from the raised portion  35 , and the milling element  10  is rotated relative to the tubular element  30 , with alternating rotation by an angle included within the above-mentioned angular range M which does not interfere with the raised portion  35 . At the same time the milling head  11  is pushed forward manually so that the combined actions (axial pressure and rotation) cause removal of bone tissue at the point of contact of the raised milling ridges  12 . 
     In the second embodiment, operation is manual using the drive element  50  on the milling head  11 , rotating it and at the same time pushing it manually forward such that the combined actions (axial pressure and rotation) cause the removal of bone tissue at the point of contact of the raised milling ridges  12 . 
     The milling action continues at most until the milling head  11  reaches the maximum advanced position, this being to a depth equal to the difference D between the maximum distance D 1  and the minimum distance D 2 . 
     Consequently, a peripheral circular groove  73  is created on the end surface  72  with an axial depth of maximum depth D, which develops around the circumference of the end surface  72 , axially approaching the membrane  76 . 
     In the subsequent phase (third phase, advancement), the tubular element  30  is further rotated in order to cause a further axial advance of the instrument proportional to the imposed rotation. 
     In particular, in the first embodiment, the tubular element  30  is again rotated manually, using the drive element  40  in order to act on the tubular element  30  through the reciprocally engaging elements  35  and  45 , causing a further axial advance of the element  30  proportional to the imposed rotation. In this phase, following the approach into contact of the raised portions  35  and  45  the milling element  10  is first returned into the fully withdrawn position in relation to the tubular element  30 , and its raised milling ridges  12  are withdrawn from the groove  73  (see  FIG. 6D ) so that, while the tubular element  30  rotates and screws into the hole  71 , the milling ridges  12  do not mill the bone. 
     Alternatively the rotation of the tubular element  30  is imposed by the drive element  50  (in the second embodiment) which acts directly on the tubular element  30 ; in this case the milling head  11  does not rotate and the raised milling ridges  12  do not mill the bone. 
     When the milling head  11  comes into contact with the end surface of the peripheral groove (see  FIG. 6E ), this is perceived by the operator, both due to the increased resistance of the tubular element  30  and because the drive element  40  can no longer slide axially. The operator also perceives that the rear portion of the probe has moved by a measurable distance in relation to the initial reference position. At this point the penetration of the tubular element  30  is stopped. 
     In the subsequent phase (fourth phase, milling) only the milling element  10  is rotated in order to increase by a further axial distance the depth of the circular groove  72  created during the previous phase ( FIG. 6F ). In practice, the milling element  10  is operated in the same way as described above for the second phase. 
     It is possible to proceed with further cycles of advance of the tubular element  30  and subsequent milling as described above, progressively increasing the depth of the groove  73  until a residual portion  74 ′ of the wall  74  is defined, in the form of a small disk delimitated by the groove, remaining attached to the jaw bone  75  by a residual layer  78  of relatively very thin bone (see  FIG. 6F ). 
     In order to finally detach this residual portion  74 ′, which closes the hole  71 , from the bone  75 , a predetermined axial pressure can be applied to the probe element  20 . This pressure can be produced by the spring  16  which automatically breaks the layer  78 , detaches the portion, and opens the hole  71  towards the membrane  76 ; the latter is not damaged because the probe element  20  is designed with a rounded tip. 
     Alternatively, it can be arranged that the axial pressure on the probe element  20  be applied manually by the operator, who, for example, pushes with a finger on the rear portion of the probe element  20  which extends outside of the drive element  40 . 
     Alternatively, when the residual portion of bone wall  74  is sufficiently weakened, the tubular element  30  can be rotated so that the milling head  11  is made to advance axially against the end surface until the residual portion  74  is detached. 
     In a further alternative embodiment (not shown in the figures), the drive element  40  is activated mechanically in rotation, for example by drive transmission elements equipped to the rear part of the same. 
     A similar mechanical drive solution can also be applied to the second drive element  50  of the second embodiment described above. 
     The invention makes it possible to perform axial milling of a bone hole while maintaining control of the position of the device relative to the hole. 
     The instrument  1  enables the operator to perceive, by way of the resistance encountered during axial advance, by screwing, along the hole  71 , possibly also through the action of the probe  20 , the point at which the milling head  11  reaches the end  72  of the previously formed hole  71 , or the end of the cavity produced by the milling head  11 . Furthermore, each phase in which the milling element  10  is rotated while the tubular element  30  is stationary, produces a groove  73 , the depth of which has a constant predefined value; in particular, equal to the value (D 1 -D 2 ) of the axial translation completed by the head  11  passing from the maximum or full withdrawn position to the maximum or full forward position. 
     As the operator can perceive the point which the advancement phase has reached (the milling head  11  reaches the end of the previously formed hole  71 , or the end of the cavity produced by it) and knows the depth of the milling phase (the cavity realized by the milling head  11 ), the depth of the cavity being created can be constantly monitored. 
     Further, the operations of further axial advancement of the milling head  11  can be checked with the probe element  20 ; in particular, while the milling element  10  is made to advance further, to excavate a further axial section of cavity, the probe element  20  remains axially stationary against the central area of the end surface  72  (which is not subjected to milling). 
     Consequently, since the probe element  20  is designed to have millimetre markings, it becomes a means for constant control and for determining the measured position of the milling head  11  relative to the position of the end  72  of the bone hole. 
     Further, by way of an axial pressure applied to the probe element  20 , detachment of the residual portion  74 ′ of the bone wall can be achieved as soon as the residual portion  74 ′ has reached a degree of breaking resistance which is less than the force applied to the probe element  20 . 
     In particular, it can be arranged so that this detachment occurs before the raised cutting ridges  12  have passed beyond, even at a single point, the thickness of the wall  74 ; the detachment thus occurs without the cutting ridges being able to come into contact with the delicate membrane  76 . 
     Furthermore, since the penetration of the tubular element  30  along the axis of the hole  71  is directly dependent on the angle of rotation of the same, by way of the pitch of the thread  31   a,  through the control of the rotation applied to the tubular element  30 , it is possible to maintain control of the extent of axial penetration of the tubular element  30 , and thus of the forward end of the instrument, in the hole  71 . 
     Obviously numerous modifications of a practical-technical nature might be brought to the invention without its forsaking the ambit of the inventive idea as claimed below in the claims.