Abstract:
The invention relates to an instrument for preparing and/or machining a femoral head ( 11 ) embodied on a femoral neck ( 19 ). Said instrument comprises tactile forceps ( 91 ) that are provided with two forked levers ( 13, 15 ) and two tactile jaws ( 23, 25 ), each of which is assigned to one of the forked levers ( 13, 15 ). The forked levers are interconnected at a fulcrum ( 93 ). The inventive instrument features a manipulating end and a working end. The tactile jaws are located at the working end of the instrument. A guiding element ( 28 ) is provided which encompasses a centering pin ( 21 ) and is mounted and/or guided at least at two points ( 105, 107 ) spaced apart from each other in the direction of the centering pin ( 21 ) in such a way that the centering pin ( 21 ) always lies on a central plane ( 12 ) of the tactile forceps ( 91 ) regardless of an opening angle of the tactile forceps ( 91 ), said central plane ( 12 ) containing the fulcrum ( 93 ). The bearing and/or guiding points ( 105, 107 ) are directly and/or indirectly hinged to the tactile forceps ( 91 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to European Patent Application No. 05021794.2 filed on Oct. 6, 2005, the disclosure of which is hereby explicitly incorporated by reference herein. 
     SUMMARY 
     The invention relates to an instrument for the preparation and/or machining of a femoral head. 
     The femoral head is formed at the proximal end of a femur and forms a hip joint together with a hip joint socket of a pelvic bone. Prior to the insertion or mounting of an implant which at least partly replaces the femoral head the femoral head is prepared and/or machined by means of instruments of the initially named kind. The instruments serve in particular for the alignment of tools for the machining of the femoral head at the femoral head. The alignment as a rule takes place with reference to the neck of the femur. A machining of the femoral head matched to the respective patient is of particular importance in the case of a so-called “re-surfacing” in which it is not the whole of the natural femoral head which is replaced but rather only an outer layer of bone material is removed in order to subsequently mount a cap-like femur implant on the remaining femur stump, with the femur implant cooperating with the hip joint socket of the pelvic bone, in particular with an artificial hip joint shell. 
     An instrument of the initially named kind is now proposed here which, in addition to a multitude of other advantageous characteristics, ensures a simple and at the same time precise alignment of a tool which is to be used subsequently for the machining of a femoral head. More specifically it should for example be possible to specify an axis, in particular a drilling axis for a tool for the machining of a femoral head. Furthermore, a method for the preparation and/or machining of a femoral head should be set forth. 
     In addition to other advantageous characteristics, the subjects described in the claims are able to fulfill these requirements. 
     The instrument set forth includes probing forceps having two forcep levers connected to one another at a point of rotation and also two probing jaws each associated with a respective one of the forcep levers. The instrument has a manipulation side and a working side with the probing jaws being arranged on the working side of the instrument. In addition a guide element having a central axis is provided. The guide element is supported and/or journalled and/or guided at least two points mutually spaced apart on the direction of the central axis such that the central axis is always disposed in a central plane of the probing forceps independently of an opening angle of the probing forceps, said central plane including the fulcrum. The support and/or guide points are in this arrangement directly and/or indirectly pivotally connected to the probing forceps. 
     The forceps-like design of the instrument offers the operator an access “from above”, i.e. at least approximately in the direction of the axis of the femur neck in that the probing jaws can be opened sufficiently far that they can pass by the femoral head. Working “from above” is extremely simple and above all only requires a little space. The subsequent probing of the femur neck takes place simply by closing the probing jaws. In this connection the operator can reliably probe the transition between the femoral head and the neck of the femur or the start of the neck in that he, for example, “travels” along the contour of the femur in the head region with the probing jaws as it were. For this purpose no space-consuming movements of the instrument are required but rather the instrument merely only needs to be opened and closed to a relatively small degree. The probing jaws can for example be placed proximally and distally at the neck of the femur. 
     When the probing jaws contact the neck of the femur or the start of the neck it is automatically ensured that the axis of the neck of the femur extends at least approximately in the central plane of the instrument which extends through the fulcrum of the forceps levers and centrally between the probing jaws. In this way a center of the neck of the femur can be found, in particular in the proximal-distal direction and indeed independently of the width of the neck of the femur. Furthermore, as a result of the mounting and/or guidance of the guide elements it is automatically ensured that the central axis also lies in the central plane of the instrument, i.e. of the probing forceps. The central axis can for example determine a drilling axis for a drilling wire for a tool for the machining of the femoral head or a reference axis for such a drilling axis. Since both the axis of the neck of the femur and also the centering axis are located in the central plane of the probing forceps an alignment of the centering axis can be achieved with respect to the axis of the neck of the femur. 
     The above explained alignment of the instrument at the femur or the alignment of the centering axis with respect to the axis of the neck of the femur is in most cases regarded as a first alignment step, which can be followed, if required, by at least one further alignment step and/or at least one fine correction of the position of the instrument relative to the femoral head, or of the centering axis relative to the axis of the neck of the femur. Through the design of the instrument an ideal alignment in respect of the axis of the neck of the femur is always ensured, at least with respect to the central plane, in each position with the probing jaws contacting the neck of the femur or the start of the neck, whereby the alignment of the machining instruments which are to be used subsequently is considerably simplified for the operator. 
     In one embodiment a functional unit is provided which includes a drilling jig the drilling axis of which extends parallel to a lever plane spanned by the two forcep levers. After completed alignment of the instrument at the femur a bore coinciding with the drilling axis can in particular be formed in this manner in the femur head, into which a wire-like or bar-like element can then be introduced with the aid of which the machining tools, in particular milling cutters, can be guided in a reliable manner into an ideal orientation with respect to the position of the femoral cap relative to the femoral head and positioned and moved during the machining. 
     In one embodiment the forceps levers are rotatable relative to one another about a fixed axis of rotation and are additionally connected to one another at a distance from the axis of rotation via at least one articulated lever arrangement. 
     In a further embodiment the guide element on which the linear guide is formed carries the functional unit and connects the latter to the forcep levers. The linear guide is for example provided in the form of an elongate hole. 
     In a further embodiment the probing jaws are formed on pivotable end portions of the forcep levers. In this connection the pivotable end portions can be connected to one another via an articulated lever arrangement. This articulated lever arrangement can be provided in addition to an articulated lever arrangement disposed above the pivot region which in particular has a linear guide. 
     This articulated lever arrangement connecting the pivotable end sections to one another is executed in a further embodiment as a parallelogram linkage. In particular it is possible to achieve in this way a situation in which the pivotable end portions of the forcep levers—i.e. those parts of the pivotable end portions which are disposed parallel to a plane spanned by the forcep levers—always have the same orientation relative to one another and in particular extending parallel to one another during the opening and closing of the instrument independently of the opening width. For this purpose the space requirement of the instrument is minimized with optimized handling thereof. 
     The functional unit includes for example at least one aiming outrider which can be coupled with the at least one aiming bar in such a way that the aiming bar extends parallel to the centering axis. In this connection an aiming outrider can be pivotally designed and indeed about an axis which extends offset from and parallel to the centering axis or coincides with this axis. 
     In a further embodiment the instrument includes in particular the functional unit, a probing arm for the probing of the femur neck and/or of the transition between the femur head and the neck of the femur, with this probing arm being pivotable about the centering axis. With a probing arm of this kind “circular tracing” can be carried out whereby the operator can check the alignment of the instrument relative to the femur head while taking account of at least the substantial part of the total periphery. 
     In an exemplary embodiment aiming pins are attached to the probing jaws, with the connection line of the aiming pins standing perpendicular to the central plane and intersecting the central axis. These aiming pins enable a central position to be found in the neck region of the femoral head when the instrument is set in place. 
     In a further exemplary embodiment the instrument is provided with an adjustable auxiliary abutment with which an additional reference point can be provided for the operator during the alignment process. This auxiliary abutment can be adjustable in the central plane, for example in two directions which extend perpendicular to one another. 
     Further embodiments are set forth in the dependent claims, in the description and also in the drawing. 
     The various embodiments of an instrument that are set forth in accordance with the independent patent claim directed to the instrument, or the features realized there, can naturally be combined with one another. 
     In the method which is set forth for the preparation and or machining of a femoral head, which is formed on a neck of a femur which has a femur neck axis, an instrument is selected for the preparation and/or machining of the femoral head which includes probing forceps with two forcep levers and two probing jaws and also a guide element having a centering axis which always lies in a central plane of the probing forceps independently of an angle of opening of the probing forceps. The probing forceps are closed in such a way that the probing jaws symmetrically contact the neck of the femur on opposite sides with respect to the axis of the neck of the femur, in particular proximally and distally, whereby the centering axis is automatically aligned centrally with respect to the two oppositely disposed sides of the neck of the femur. 
     The invention will be explained in the following in more detail with reference to embodiments illustrated in the drawing. In this connection the embodiments in the drawing are only to be understood as instructive and are not to serve to restrict the subjects described in the claims. The representations in the drawings are simplified; details not necessary for an understanding of the invention have been omitted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-4  show various views of an embodiment of an instrument. 
         FIG. 5  shows the instrument of  FIGS. 1-4  at a femoral head to be machined. 
         FIGS. 6 and 7  show various views of a further embodiment of an instrument. 
         FIG. 8  shows various views of another embodiment of an instrument. 
         FIG. 9  shows a guide element of the instrument of  FIG. 8 . 
         FIGS. 10-12  show the instrument of  FIG. 8 , each with an adapter device. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION 
     The embodiment of an instrument shown in  FIGS. 1-5  includes probing forceps  91  with two probing levers or clamps  13 ,  15  which are pivotally connected to one another as in a pair of scissors or pliers at a fulcrum  93  shown in  FIG. 2  which lies on a pivot axis  17  of the probing forceps  91 , so that the two forcep levers  13 ,  15  can be mutually rotated relative to one another about the axis of rotation  17 . The instrument is to this extent of X-like shape, with the pivot region which determines the axis of rotation  17  not lying at the center of the two forcep levers  13 ,  15 , but rather dividing them approximately in the ratio of 1:2 to 1:3. Through the pivot axis  17  the instrument is divided into a manipulating side and a working side, with the manipulating side including actuation portions  33 ,  35  which will be explained subsequently in more detail and with the working side including probing portions  43 ,  45  which will be explained in more detail in the following. 
     One of the two longer upper actuating portions  33 ,  35  is provided with a latching arm  71 , which is provided at its side facing the pivot axis  17  with a toothed latch arrangement, by which the latching arm  71  cooperates with the free end of the other actuating portion  35  in order to retain the respectively reached position during a closing movement by latching. The biasing of the latching arm  71  which is required for this is achieved by means of a spring  73  secured to the actuation portion  33 . 
     The forcep levers  13 ,  15  span a lever plane  14  ( FIG. 2 ) with probing portions  43 ,  45  extending perpendicular to it, and with the probing portions being formed at their mutually confronting inner sides as probing jaws  23 ,  25  which are set in front of the lever plane  14 , with the lever plane being orientated in  FIG. 2  perpendicular to the plane of the drawing. A central plane  12  of the probing forceps  91  and the lever plane  14  spanned by the two forcep levers  13 ,  15  stand perpendicular to one another, with the central plane  12  in  FIG. 1  being orientated perpendicular to the plane of the sheet. 
     A guide element  28  and a rear aiming outrider  47  are located on different sides of the lever plane  14  and are connected to one another by means of a plug connection extending through bores in the forcep levers  13 ,  15  in the pivot region. The plug axis coincides with the pivot axis  17 . The rear aiming outrider  47 , which is stationary relative to the centering axis  21 , has an angled end piece through which an aiming bar  51  indicated in broken lines in  FIGS. 1 and 2  can be pushed which extends parallel to the lever plane  14  and lies in a central plane  12  ( FIG. 1 ) which will subsequently be explained in more detail in the following. 
     The guide element  28  has a T-shaped basic shape, with the perpendicular leg of the “T” being formed by a carrier arm  29  for a centering axis  21  and for a functional unit  27 , which will subsequently be explained in more detail, that extends along the pivot axis  17  and perpendicular to the lever plane  14 . The upper part of the cross beam of the “T” which is disposed on the actuation side of the pivot axis  17  is provided with a linear guide  41  in the form of an elongate slot which extends parallel to the lever plane  14  and lies in the central plane  12  ( FIG. 1 ). The linear guide  41  and the probing jaws  23 ,  25  are located on different sides of the pivot region. The central plane  12  extends through the axis of rotation  17  and centrally between the two forcep levers  13 ,  15 . The ends of two guide levers  38  which are pivotally connected to one another via a fulcrum point  95  are compulsorily guided along the elongate slot guide  41  and are respectively connected at the other ends to the two forcep levers  13 ,  15 . The fulcrum  95  is determined by a pin  97 . 
     Consequently, the guide element  28  is journalled at a bearing point  105  at the fulcrum  93  and at a guide point  107  at the fulcrum  95 . 
     Through this geometry a situation is achieved in which the two probing jaws  23 ,  25  of the forcep levers  13 ,  15  each have the same spacing from the central axis  21  independently of the pivotal position of the two forcep levers  13 ,  15 . This signifies that the plane of symmetry of an article held by the two probing jaws  23 ,  25  always coincides with the central axis  21  of the instrument independently of how far the instrument has been opened. 
     The lower part of the cross beam of the “T” lying on the working side of the pivot axis  17  is essentially formed by a separate extension  30  which is held at the guide element  28  by a clamping screw  77 . An auxiliary abutment  61  is held at the extension  30  by means of a further clamping screw  79  ( FIG. 4 ) and is adjustable in the central plane  12  parallel to the lever plane  14 . The auxiliary abutment  61  has a separate abutment element  83  which is in turn adjustable in the central plane  12 , now however perpendicular to the lever plane  14  and thus parallel to a probing plane  44  ( FIG. 2 ) spanned by the probing sections  43 ,  45  of the forcep levers  13 ,  15 , with the probing plane  44  in  FIG. 2  being orientated perpendicular to the sheet plane. The abutment element  83  is held at the auxiliary abutment  61  by means of a further clamping screw  81 . 
     Two aiming pins  87 ,  88  are each arranged on a respective probing jaw  23 ,  25 . Their connection line stands perpendicular to the central plane  12  and crosses the central axis  21 . The probing jaws  23 ,  25  are so displaced when closing the instrument that they lie, when viewed from the side, approximately at the center of a neck  19  of the femur shown in  FIG. 5 . The abutment element  83  is displaceably adjustable perpendicular to the connection line of the aiming pins  87 ,  88 . It has a scale  89  which respectively indicates the double distance (in numbers) from the auxiliary abutment  61  to the connection line of the aiming pins  87 ,  88 . When the neck diameter is present in numbers in the central plane  12 —for example through a prior measurement with a sliding caliper—then this diameter can be set in numbers with the scale  89  and the auxiliary abutment  61  ensures that the later drilling axis  21  coincides with the neck axis  101  at least in the region of the probing jaws  23 ,  25 . 
     A functional unit  27  carried by the guide element  28  at the free end of the carrier arm  29  which includes a front aiming outrider  46  and also a drilling jig  31  is mounted in front of the lever plane  14  and is connected to the forcep levers  13 ,  15  via the carrier arm  29  lying in the central plane  12 . The functional unit  27  and the probing jaws  23 ,  25  are located on the same side of the lever plane  14 . 
     The free end of the carrier arm  29  is formed as a circular ring-like guide region  103  for the drilling jig  31 . The front aiming outrider  46  is pivotally and releasably pushed onto this end with a likewise circular ring-like portion, with the projecting aiming arm of the aiming outrider being provided with a plurality of bores which extend parallel to the lever plane  14  and thus perpendicular to the probing plane  44 . The bores serve to receive a further aiming bar  49 . 
     The drilling jig is a centering sleeve  31  which is guided through the circular ring portions of the front aiming outrider  46  and of the guide region  103  of the guide element  29 . The lower end side of the centering sleeve  31  is provided with fixing means, a toothed or serrated rim, which forms a fixing portion  75 . Instead of a serrated rim, pointed projecting pins can also be provided. Furthermore, the centering sleeve  31  has a central through-bore which determines the drilling axis  21 , which lies in the central plane  12  and extends parallel to the lever plane  14  and thus perpendicular to the probing plane  44 . The aiming bars  49 ,  51  and a drilling instrument which is not shown but which is pushed through the centering sleeve  31 , for example a Kirchner wire, thus extend parallel to one another. 
     In the state ( FIG. 5 ) mounted on the femur, the probing sections  43 ,  45  contact the femur neck  19 . The femur head  11  is located in a receiving space  85  of the instrument which is bounded at one side by the probing plane  44  ( FIG. 2 ), at the opposite side by the functional unit  27  and perpendicular thereto by the lever plane  14  ( FIG. 2 ). Thus the instrument requires very little space in the region of the femur head  11 . 
     Not shown in  FIG. 5  is the auxiliary abutment  61  ( FIG. 4 ) the abutment element  83  of which lies in the probing plane  44  ( FIG. 2 ). The free end of the abutment element  83  is thus likewise located at the level of the neck  19  of the femur and thus makes an additional alignment aid available. 
     The instrument serves for the determination of the position and direction of a bore which is to be formed in the femoral head  11  for a guide element, which is in particular wire-like or pin-like, along which instruments can subsequently be guided for the machining of the femoral head  11 . Using the instrument, the operator can probe the transition between the femoral head  11  and the neck  19  of the femur via the probing jaws  23 ,  25  formed on the probing portions  43 ,  45  and in this connection operates advantageously and in space-saving manner from above. 
     The alignment of the instrument at the femur can be checked with the aid of the aiming bars  49 ,  51  of which the front aiming bar  49  disposed on the same side of the lever plane  14  as the functional unit  27  can be pivoted about the drilling axis  21 . It is of particular advantage that this checking can take place with the two aiming bars  49 ,  51  in two planes standing perpendicular to one another. On reaching the desired position, the instrument can be fixed via the fixing portion  75  on the femur head by slight hammer blow on the centering sleeve  31 . 
     The operation of the instrument is extremely simple for the operator and can take place with one hand. The latching of the achieved closing position which takes place during a closing movement can be released by slight lifting of the latching arm  71  in order to be able to reopen the instrument and set it in place anew if required. The auxiliary abutment  61  secures the instrument against tilting in the central plane  12 . A rough pre-adjustment of the auxiliary abutment  61  can take place as a result of the operation planning. 
     The compulsory coupling of the actuation side portions of the forcep levers  13 ,  15  via the guide lever  38  with the linear guide extending in the central plane  12  automatically ensures a symmetrical opening and closing of the instrument with respect to the centering axis  21 . In this way it is ensured that the drilling axis  21  lies in this central plane  12  in which the axis  101  of the neck of the femur lies, at least approximately, when the probing jaws  23 , contact the neck  19  of the femur or the start of the neck. 
     If the correct alignment of the instrument at the femoral head  11  has been found, then the centering on the femoral head  11  of the centering sleeve  31  can take place by blows on the centering sleeve which is provided at the bottom with serrations or with pointed pins, whereby the instrument is adequately reliably fixed to the femoral head  11  in order to introduce the desired bore in the femoral head through the central bore of the centering sleeve  31  along the aligned drilling axis  21 . 
     A further embodiment of an instrument in accordance with  FIGS. 6 and 7  is distinguished from the previously explained embodiment firstly in that the probing portions  43 ,  45  are provided on the pivotable end portions  53 ,  55  of the forcep levers  13 ,  15 , i.e. the forcep levers  13 ,  15  are not made rigid over their entire length but are provided with a hinge. The pivot axes of the end portions  53 ,  55  extend perpendicular to the lever plane  14  shown in  FIG. 2 . 
     The pivotal movement of the end portions  53 ,  55  is compulsorily guided by a further articulated arrangement  39  disposed below the pivot axis  17  which is formed as a parallelogram linkage. Two guide levers  40 , which are respectively pivotally connected at their one end to a part section  53   a ,  55   a  of the pivotable end portion  53 ,  55  extending parallel to the lever plane  14 , are jointly pivotally connected at their two other ends to the guide element  28 . 
     A straight connecting line between the common hinge point for the two guide levers  40  and the pivot axis  17 , i.e. the fulcrum  93  of the probing forceps  91  which lies in the central plane  12  explained in connection with  FIG. 1 , extends parallel to the part sections  53   a ,  55   a , whereas the guide levers  40  respectively extend parallel to that part of the forcep lever  13 ,  15  which extends between the pivot axis  17  and the hinge point of the pivotable end portion  53 ,  55 . In this way a four bar linkage in the form of a parallelogram is formed on each side of the central plane  12 . During opening and closing of the instrument the two part sections  53   a ,  55   a  of the pivotable end portions  53 ,  55  thus extend during opening and closing of the instrument always parallel to one another independently of the pivotal position of the two forcep levers  13 ,  15 . 
     Furthermore, in this embodiment, the front aiming outrider  46  for the front end bar  49  is not pivotable but it rather forms a rigid arm of the guide element  28 . The aiming outrider  46  is of U-shape or hook-like shape, lies in a plane extending parallel to the probing plane  44  explained in conjunction with  FIG. 2  and is guided around a pivot region for a probing arm  57  which will be described in more detail in the following 
     The U-shaped probing arm  57 , the open side of which faces the centering axis  21 , is a component of the functional unit  27  and is pivotally mounted by means of a pivot part  58  above the pivot axis  17  around the drilling axis  21 . The probing arm  57  extends downwardly in trunk-like manner until shortly above the probing plane  44  spanned by the probing sections  43 ,  45  and in doing so around the receiving region  85  ( FIG. 4 ) of the instrument for the femoral head  11 . 
     At the free end of the probing arm  57  a spherical probing head  59  is formed approximately at the level of the probing jaws  23 ,  25 . By pivoting of the probing arm  57  the so-called “circular tracing” can be carried out in which the probing head  59  is guided around the drilling axis  21  along the start of the neck of the femoral head  11  in order to check the position of the instrument in a direction extending parallel to the pivot axis  17  and relative to the femoral head  11 . In this connection the instrument with the probing sections  43 ,  45  is fixed below the start of the neck at the femur so that the probing head  59  can move at the level of the start of the neck during the circular tracing. 
     Furthermore, in distinction to the above-explained embodiment, the fixing portion  75  is not provided here in the form of a “crown-like” end face of a sleeve. On the contrary the fixing portion  75  is formed here by three parallel pins which converge to a point of the bottom and which are arranged equally spaced from one another in the peripheral direction at the same radial spacing from the drilling axis  21 . 
     A circular tracing unit in accordance with the embodiment of  FIGS. 6 and 7  can also be provided in the embodiment of  FIGS. 1 to 5 . Furthermore, in both embodiments, the other respective centering sleeve  31  and/or the other respective fixing portion  75  can be provided. A parallelogram linkage with pivotable end portions corresponding to the embodiment of  FIGS. 6 and 7  can also be provided in the embodiment of  FIGS. 1 to 5 . 
     Yet another embodiment of the instrument in accordance with  FIG. 8  is distinguished from the previously explained embodiments first of all in that no rear aiming outrider and no rear aiming bar are provided in the pivot region of the probing forceps  91 . Instead, a lower aiming bar  109  which can be coupled to the guide element  28  is provided in the axial extension of the guide axis  99  and extends in the state mounted to the guide element  28  beyond the ends of the forcep levers  13 ,  15 . Moreover, a set of abutment elements  83  for the neck of the femur is provided instead of an abutment element adjustable perpendicular to the lever plane  14  and the abutment elements are respectively capable of being coupled to the guide element  28 , with the abutment element  83  respectively mounted on the guide element  28  extending transversely to the centering axis  21  and having a free abutment end remote from the centering axis. The abutment elements are distinguished at least in that in the state mounted on the guide element  28  their free abutment ends are spaced by different amounts from the centering axis  21 . For the attachment to the guide element  28  the respective abutment element  83  is plugged onto the guide element  28  via a latch device and/or snap device, in particular via a ball latch. Furthermore, the embodiment of  FIG. 8  is distinguished from the previously explained embodiments by the guide region  103 . The guide region  103  of the embodiment of  FIG. 8  admittedly includes, as do the previously explained guide regions, a mount  117  for the centering sleeve  31 , with the mount  117  being bounded by a wall section  119  of the guide region  103 , as can be seen in  FIG. 9 . However, the guide region  103  of the embodiment of  FIG. 8  is slotted, i.e. a slit  111  is provided in the wall section  119  in such a way that the wall section  119  only partly surrounds the mount  117 . 
     Otherwise the other embodiment of the instrument in accordance with  FIG. 8  is formed essentially analogously to the previously explained embodiments, with some of the common features being repeated again below and/or being illuminated from a different perspective. Thus, in the instrument in accordance with  FIG. 8  the centering axis  21 , the mounting point  105  hinged to the fulcrum  93  and the guide point  107  arranged at the hinge point  95  always lie in the central plane  12  of the probing forceps  91  independently of the opening angle of the probing forceps  91 . Moreover, the guide axis  99  determined by the mounting point  105  and the guiding point  107  is likewise orientated parallel to the centering axis  21 . Furthermore, a pair of guide levers  28  of the same length is likewise provided which are arranged symmetrically to the central plane  12  which are each rotatably connected to one of the forcep levers  13 ,  15  and also to one another at a hinge point  95 , with the guide point  107  being arranged at the hinge point  95  which connects the guide levers  38  to one another. Furthermore, the guide element  28  likewise has an elongate hole  41  with the articulated connection between the guide levers  38  being formed by a pin  97  which is guided in the elongate hole  41  of the guide element  28 . Furthermore, the guide levers  38  are likewise pivotally connected at the manipulation side of the probing forceps  91  to the forcep levers  13 ,  15 . Furthermore, the guide element  28  likewise includes a guide region  103  for a centering element  31 , in particular a centering sleeve, which specifies the centering axis  21 . Furthermore, the centering element  31  is likewise displaceable in the direction of the centering axis  21 . Furthermore, the centering element  31  likewise includes fixing means  75  at one end for the fixing of the instrument to the femoral head  11 , with the fixing means in particular being arranged towards the working side. Furthermore, the guide region  103  is likewise formed at a carrier arm  29  of the guide element  29  extending transverse to the centering axis  21 . Furthermore, the guide region  103  is likewise capable of being coupled to an aiming outrider  46  and to an aiming bar  49  associated with the aiming outrider  46  and/or to an adapter device  113  shown in the  FIGS. 10 to 12  to fix a drilling axis  115  inclined relative to the centering axis  21 . Furthermore, latching means  71  are likewise provided by which the forcep levers  13 ,  15  can be adjustably fixed to one another in their relative position. Furthermore, the latching means likewise include a latching arm  71  which is pivotally connected to the one forcep lever  13  and can be releasably latched to the other forcep lever  15  via an adjustable toothed latch. 
     The  FIGS. 10 to 12  show the instrument from  FIG. 8  with an adapter device  113  through which a drilling axis  115  inclined relative to the centering axis  21  can be fixed. The adapter device  113  includes first of all a probing device  121  with a probing element  123  for the probing of the femoral head  11 . The probing device  121  is, in this arrangement, formed in such a way that the probing element  123  is adjustable in a direction parallel to the centering axis  121 . In this way it is made possible for the probing element  123  to be placed at different points on the femoral head  11 . The probing element  123  is arranged in such a way that a probing of the femoral head  11  takes place from anterior. 
     Moreover, the adapter device  113  includes elongate guide means  125  coupled to the probing device  121  for the guidance of a drilling wire  127  for the generation of a bore in the femoral head  11  along the drilling axis  115 . The longitudinal axis of the guide means  125  is in this arrangement orientated such that the drilling axis  115 , the centering axis  21  and a probing plane extending perpendicular to the centering axis  21 , in which a free end of the probing element  123  lies, intersect at a point. The inclination of the drilling axis  115  relative to the centering axis  21  is steplessly adjustable in this arrangement. 
     Through the inclined drilling axis  115  it is made possible to mount an implant tilted relative to the axis  101  of the femur neck, in particular a tilted cap-like implant, on the femoral head  11 . 
     As can be recognized in particular with reference to the  FIGS. 11 and 12 , not only one probing device  121  is provided but rather a set of probing devices  121  with the probing devices  121  being at least distinguished in that the probing of the femoral head  11  takes place in  FIG. 10  by a probing element  123  disposed anterior, in  FIG. 11  by a probing element  123  arranged distal and in  FIG. 12  by a probing element  123  arranged proximal. 
     In the following an exemplary procedure for the fixing of the centering axis  21  and also for any required fixing of the drilling axis  115  which can be optionally inclined relative to the centering axis is described with reference to the drawings. 
     First of all, the diameter of the neck  19  of the femur is determined, for example by means of a caliper. Thereafter an abutment element  83  matching the diameter of the neck  19  of the femur is selected from a set of different abutment elements  83  and is mounted on the instrument. Thereafter the instrument is set in place and aligned at the femoral head or at the neck of the femur such that the neck  19  of the femur comes into abutment at the abutment element  83  of the instrument matched to the diameter of the neck  19  of the femur in order to align the centering axis  21  in the center with respect to the mounting side of the instrument and at its opposite side, in particular in the anterior-posterior direction. The probing forceps  91  of the instrument are subsequently closed such that the probing jaws  23 ,  25  symmetrically contact the neck  19  of the femur at opposite sides with respect to the axis  101  of the femur neck, in particular proximally and distally in order to automatically centrally align the centering axis  21  with respect to the two oppositely disposed sides of the femur neck  19 . On the whole this is achieved in that after the closing of the probing forceps  91  the centering axis  21  coincides with the axis  101  of the femur neck. 
     Thereafter the inclination of the centering axis  21  is checked and if necessary corrected by means of the front aiming bar  49  and also the CCD angle is checked and if necessary corrected by means of the rear aiming bar  51  or the lower aiming bar  109 . Thereafter the forcep levers  13 ,  15  are fixed in their relative position to one another and the instrument is fixed in the aligned position at the femoral head  11  by means of the centering sleeve  31  which determines the centering axis  21 , for example by a light hammer blow on the centering element  31 . 
     If the bore to be produced in the femoral head  11  is to coincide with the axis  101  of the neck of the femur a drilling wire is first inserted through the centering sleeve  31  along the centering axis  21  into the femoral head  11 . The drilling wire can previously have been used in one embodiment of the method as the front aiming bar  49 . Thereafter the front aiming outrider  46  coupled to the instrument through which the drilling wire passes and also the centering sleeve  31  which serves for the guiding of the drilling wire are unthreaded along the drilling wire from the drilling wire and removed. Finally the instrument is removed transverse to the longitudinal extent of the drilling wire, with the drilling wire being guided through the slot  111  formed at the instrument. The mounted drilling wire can now be used for the orientation of a tool for the machining of the femoral head  11 . 
     If the bore to be produced in the femoral head  11  does not however coincide with the axis  101  of the femoral neck but is rather inclined relative to the centering axis  21  the instrument is coupled to the adapter device  113 . Thereafter the femoral head  11  is probed with the probing device  121  of the adapter device  113  in order to determine an equatorial plane of the femoral head  11  perpendicular to the centering axis  21 . Thereafter the inclination of the drilling axis  115  relative to the centering axis  21  is fixed, with the drilling axis  115  being aligned in such a way that it extends through the intersection point of the equatorial plane with the centering axis  21 . In this way it is made possible that the drilling axis  115  is orientated in the direction of the ball center of the femoral head  11 . Next the drilling axis  115  inclined relative to the centering axis  21  is fixed relative to the instrument. Thereafter the drilling wire  127  is inserted along the drilling axis  115  into the femoral head  11 . Finally, the adapter device  113  and the instrument are removed from the femoral head  11 . The placed drilling wire  127  can now be used for the orientation of a tool for the machining of a femoral head  11 . 
     The described instrument enables in particular specific methods which will be described in the following for the preparation and/or machining of a femoral head to be carried out. 
     A method for the preparation and/or machining of a femoral head which is formed on a neck of a femur and which has an axis of a femur neck includes selecting an instrument for the preparation and/or machining of the femur head which includes probing forceps with two forcep levers and two probing jaws and also a guide element with a centering axis which always lies in a central plane of the probing forceps independent of an opening angle of the probing forceps and closing the probing forceps in such a way that the probing jaws symmetrically contact the neck of the femur at opposite sides with respect to the axis of the neck of the femur, in particular proximally and distally, whereby the centering axis is automatically aligned in the middle with respect to the two oppositely disposed sides of the femur neck. 
     A further method further includes the selection of an instrument which is an instrument in accordance with at least one of the claims. 
     A further method further includes aligning the instrument prior to closing of the probing forceps such that the neck of the femur comes into abutment, in particular posterior, at an abutment element of the instrument matched to the diameter of the neck of the femur, so that after the closing of the probing forceps the centering axis coincides with the axis of the neck of the femur. 
     A further method further includes selecting an abutment element matched to the diameter of the femur neck from a set of different abutment elements prior to the alignment of the instrument. 
     A further method further includes determining the diameter of the neck of the femur prior to the selection of the abutment element matched to the diameter of the neck of the femur. 
     A further method further includes checking and/or correcting the inclination of the centering axis by means of an aiming bar and checking and/or correcting the CCD angle by means of a further aiming bar. 
     A further method further includes fixing the forcep levers in their relative position to one another. 
     A further method further includes fixing the instrument in an aligned position at the femur head by means of the centering element fixing the centering axis. 
     A further method further includes inserting a drilling wire into the femur head along the centering axis through a centering element formed as a centering sleeve. 
     A further method further includes unthreading and removing from the drilling wire an aiming outrider through which the drilling wire passes and previously coupled to the instrument for checking the inclination of the centering axis and/or to unthread and remove the centering sleeve serving for the guidance of the drilling wire along the drilling wire from the drilling wire. 
     A further method further includes removing the instrument transverse to the longitudinal extent of the drilling wire, with the drilling wire being passed through a slot formed at the instrument. 
     A further method further includes coupling the instrument to an adapter device which includes means for fixing the drilling axis inclined relative to the centering axis. 
     A further method further includes fixing the inclination of the drilling axis relative to the centering axis. 
     A further method further includes fixing relative to the instrument the drilling axis inclined relative to the centering axis. 
     A further method further includes inserting a drilling wire into the femur head along the drilling axis. 
     A further method further includes probing the femoral head with a probing device of the adapter device in order to determine an equatorial plane of the femoral head extending perpendicular to the centering axis. 
     A further method further includes aligning the drilling axis in such a way that it extends through the intersection point of the equatorial plane with the centering axis. 
     The features set forth for the method of the operation can also be combined with one another. 
     In the light of the explanations given here, further embodiments will be made available to the person skilled in the art of the invention characterized in the claims which cannot be exhaustively portrayed here. 
     REFERENCE NUMERAL LIST 
     
         
           11  femoral head 
           12  central plane 
           13  forcep lever, clamp 
           14  lever plane 
           15  forcep lever, clamp 
           17  pivot axis 
           19  neck of femur 
           21  centering axis, drilling axis 
           23  probing jaw 
           25  probing jaw 
           27  functional unit 
           28  guide element 
           29  carrier arm 
           30  extension 
           31  drilling jig, centering sleeve 
           33  actuating portion 
           35  actuating portion 
           37  articulated lever arrangement 
           38  guide lever 
           39  articulated lever arrangement, parallelogram linkage 
           40  guide lever 
           41  linear guide, elongate hole 
           43  probing portion 
           44  probing plane 
           45  probing portion 
           46  front aiming outrider 
           47  rear aiming outrider 
           49  front aiming bar 
           51  rear aiming bar 
           53  pivotable end portion 
           53   a  part section 
           55  pivotable end portion 
           55   a  part section 
           57  probing arm 
           58  pivot part 
           59  probing head 
           61  auxiliary abutment 
           71  latching arm 
           73  spring 
           75  fixing portion 
           77  clamping screw 
           79  clamping screw 
           81  clamping screw 
           83  abutment element 
           85  receiving space 
           87  aiming pin 
           88  aiming pin 
           89  scale 
           91  probing forceps 
           93  fulcrum 
           95  hinge point 
           97  pin 
           99  guide axis 
           101  axis of the neck of the femur 
           103  guide region 
           105  mounting point 
           107  guide point 
           109  lower aiming bar 
           111  slot 
           113  adapter device 
           115  drilling axis 
           117  mount 
           119  wall section 
           121  probing device 
           123  probing element 
           125  guide means 
           127  drilling wire