Abstract:
A bone plate and to a fixation system comprising two types of bone screws. The bone plate has two intersecting stepped round holes which are different in size and in the upper regions of which a circumferential rib extends. The upper region of the smaller round hole is designed to slidably guide a bone screw having a partially spherical heads and a screw having a thread or conical head. The lower region of the larger round hole is used to support a conical head.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a Continuation of U.S. patent application Ser. No. 13/805,444, incorporated herein by reference. 
     U.S. patent application Ser. No. 13/805,444 is a 35 U.S.C. 371 filing of International Application Number PCT/EP2011/003111, filed on Jun. 24, 2011, incorporated herein by reference. 
     International Application Number PCT/EP2011/003111 in turn claims priority to U.S. Provisional Application No. 61/358,171 and German Patent Application Number 10-2010-025-001.5, filed on Jun. 24, 2010, incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a bone plate with several holes arranged in the direction of the plate longitudinal axis, for accepting bone screws, as well as a fixation system for bones, having a bone plate. 
     BACKGROUND OF THE INVENTION 
     With a known bone plate of this kind (EP 0760632 B1), the holes are all around spherically formed towards the upper side of the bone plate, in order to be able to support a bone screw with a head having a spherical lower side, in different angular positions. At the plate lower side, adjacent to the bone, the holes have a region of smaller diameter with partial thread in order to be able also to accept a bone screw with cylindrical threaded head which screw shall be inserted, vertically to the plate plane. 
     With a further bone plate of the specified kind (EP 1158915 B1, and EP 1198916 B1), elongated holes with an internal thread are available which thread extends from the upper side to the lower side of the bone plate, at one end of the elongated hole, and has a peripheral or central angle in the range from 190 to 280°. The internal thread extends over the whole depth of the elongated hole, tapers towards the lower side of the bone plate, and has a cone angle in the range from 5 to 20°. 
     With a further known bone plate (EP 1255498 B1), elongated holes are provided in the bone plate which holes may be formed oval, ellipsoidal, or rectangular, or may have a combination of such forms. Only circular holes are explicitly not covered by this definition of an elongated hole. The elongated hole is combined with a circular hole, and the latter is provided with a three-dimensional structuring which is available in the form of an internal thread, or a peripheral plate or lip. A conical internal thread is shown which extends from the upper side to the lower side of the bone plate, and has a peripheral or central angle in the range from 190 to 280°. 
     A fixation system for bones, having a bone plate is known from DE 19858889 A1 which system has elongated holes which have lugs closely to their lower sides adjacent to the bone. The lugs extend in the lower part of the elongated hole, in parallel to the plate plane. There are bearing surfaces for spherical heads of the bone screws, towards the upper side of the bone plate. For cooperating with the lugs of the bone plates, the bone screw has a short piece of thread, below the spherical head which thread is able to deform the lugs in the elongated hole, and to adapt. It is thereby possible to screw the bone screw in different angle positions with respect to the axis of the passage holes. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object to provide a bone plate with which various bone screws—such with conical bearing surfaces, and such with spherical bearing surfaces—may be used, in order to meet the different requirements, when fixing a fractured or damaged bone. In particular, bone fragments may be able to be shifted relatively to each other during fixation. 
     The bone plate in detail comprises a preferably lengthy plate body of tissue-compatible, rigid material, the plate body defining an upper side, a lower side, and a longitudinal axis. Transversely to the plate plane, shaped hole are provided which consist of a first, larger round hole, and a second, smaller round hole, wherein the round holes intersect while forming edges between which a passage for the screw shaft of a bone screw is formed. A circumferential radial rib is provided around both round holes which rib starts at the hole wall, and extends in a plane towards the round hole centre. It is within the scope of the invention that this rib extends in a plane, and is not provided with a lead, like with a thread. In case of embodiments with a circumferentially closed or annularly closed rib, an improved strength, in particularly a higher rigidity may be provided, that is principally not possible with a thread. 
     A bone screw having a thread at the screw head may rest on the rib of the smaller round hole, and makes an engagement for the rib of the larger round hole possible, whereby mutual clamping takes place. 
     Within the meaning of the invention, this clamping may also cause an elastic deformation which, however, does not essentially leave a permanent, deformed part after separating the bone screw from the bone plate. This means that this deformation does not also still comprises a plastic deformation, in excess of the purely elastic part. After separating the bone screw from the plate, no ridges or grooves may therefore be seen at the bone screw, or also at the bone plate. If surface changes are generated by improper handling, these changes typically are not present as grooves, channels, or ridges, but rather as laminar rubs. 
     According to preferred embodiments, the first, larger round hole has three sections, namely an upper, rounded flute shaped section above the circumferential rib, a central rounded flute shaped section below the circumferential rib, and a lower, lower truncated cone shaped section tapering to the lower side of the plate body, and having a largest diameter which is smaller than the diameter of the upper or central section. The smaller round hole comprises an upper section having a sloping transitional region towards the plate surface, further a central section below the plane of the circumferential rib, having a sloping rounded surface, and a lower, cylindrical or conical section having a diameter which is smaller than the diameter of the upper or central section. 
     With all embodiments, bone screws having a head may be used which head is provided with a screw thread in its upper region, and with a conical bearing surface in its lower region, and indeed in this way that, when inserting vertically into a bone, shifting the head relatively to the shaped hole stakes place, in such a way that bone fragments may be drawn near to each other, when fixing. The new bone plate, however, also allows using bone screws with spherical bearing surfaces at the lower side of the screw head. Such spherical head screws may be screwed in, in an angle to the bone plate, as it is necessary, sometimes. 
     Further details of the invention result from the description of the shown embodiments and the attached claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view onto a shaped hole in a bone plate, 
         FIG. 2  is a sectional view of the shaped hole, 
         FIG. 3  is a perspective drawing of the shaped hole, and 
         FIG. 4  shows a bone screw engaging a shaped hole, 
         FIG. 5  shows a second embodiment of a bone plate in perspective drawing, 
         FIG. 6  is a sectional view of the shaped hole, 
         FIG. 7  is a lateral view of the shaped hole, 
         FIG. 8  shows a bone screw engaging a shaped hole, together with an enlarged sectional view, 
         FIG. 9  shows a further embodiment of a bone plate having a circumferential rib with various heights in radial direction, 
         FIG. 10  shows an enlarged sectional view of a part of a sectional drawing along line AA, and therewith extending in parallel to the longitudinal axis of the further embodiment of the bone plate with inserted bone screw, shown in  FIG. 9 , 
         FIG. 11  is a sectional drawing along the line AA, and therewith extending in parallel to the longitudinal axis of the further embodiment of the bone plate with inserted bone screw, shown in  FIG. 9 , 
         FIG. 12  is an enlarged sectional view of a part of a sectional drawing along the line BB, and therewith extending transversely to the longitudinal axis of the further embodiment of the bone plate with inserted bone screw, shown in  FIG. 9 , 
         FIG. 13  is a sectional drawing along the line BB, and therewith extending transversely to the longitudinal axis of the further embodiment of the bone plate with inserted bone screw, shown in  FIG. 9 , 
         FIG. 14  shows a still further embodiment of the a bone plate having a circumferential rib with different heights in radial direction with which embodiment the symmetry axis of the circumferential rib is arranged offset relatively to the symmetry axis of the conical section of the larger round hole, positioned under the rib, and with which corresponding radii around the respective symmetry axis are dash-dotted, for making their relative displacement easier visible, 
         FIG. 15  shows the still further embodiment of the bone plate with circumferential rib with different height in radial direction, shown in  FIG. 14 , but without the corresponding radii around the respective symmetry axis, for making their real form easier visible, 
         FIG. 16  is a sectional drawing of the still further embodiment shown in  FIGS. 14 and 15  along a line which would correspond to the line AA in  FIG. 9 , and is therefore extending in parallel to the longitudinal axis of the still further embodiment of the bone plate without, however, inserted bone screw, shown in  FIGS. 14 and 15 , 
         FIG. 17  is a sectional drawing of the still further embodiment shown in  FIGS. 14 and 15  along a line which would correspond to the line BB in  FIG. 9 , and therefore is extending transversely to the longitudinal axis of the still further embodiment of the bone plate without, however, inserted bone screw, shown in  FIGS. 14 and 15 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of preferred embodiments, same reference signs essentially describe same parts in or at these embodiments, for the sake of clarity. 
     The figures show the part of a bone plate comprising a preferably lengthy plate body  1  made of tissue compatible, rigid material, and in which body a row of shaped holes is placed of which a shaped hole  2  is shown. The plate body  1  may also be formed, besides lengthy, oval, round, or polygonal, or its form may be fitted to the respective use. 
     Metals and their alloys are considered to be tissue compatible material, as they are usually used form producing implants. Preferred metals comprise each kind of titanium, preferably also its alloys TiAl6V4, and TiCp. Also steels such as implant steels, for example the alloy 1.4441 are preferably useable. 
     A further material class for this purpose also comprises absorbable materials, like magnesium, or absorbable synthetic material, like PLA. PLA is a bio-compatible and absorbable synthetic material made of lactic acid molecules chemically bond to each other which material may also be used like other absorbable synthetic materials. 
     A bone plate is considered to be rigid, if the plate structurally provides the rigidity necessary for its intended use and operation purpose. Depending on the used tissue compatible material, this may be ensured by the thickness and the width of the plate, and usually is within the scope of activities of a person skilled in the art. 
     Bone plates for small and less stressed body parts such as hand bones and foot bones for example are thinner and often less wider than bone plates of the larger and stronger stresses parts such as parts of the shank and of the thigh. 
     The bone plate has an upper side  11  and a lower side  12  which usually extend in parallel to the plate plane, wherein the lower side  12  is adjacent to the bone to be fixed. The shaped holes  2  are strung along the longitudinal axis of the lengthy plate body  1 , and consist of two stagely formed round holes,  21  and  22 , extending transversely to the plate plane, and whose axes  21   a ,  22   a  intersect the longitudinal axis of the bone plate. The smallest diameter of the first round hole  21  is greater than the smallest diameter of the second round hole  22 , and the distance of the two axes  21   a  and  22   a  from each other is smaller than the smallest diameter of the first round hole. Edges  23 ,  24  result from the intersection of the holes  21 ,  22  which edges delimit the regions of the round holes from each other, and leave open a passage for the relatively thin screw shaft of a bone screw, if this screw extends into the other round hole, in case of inclination of a bone screw. The peripheral angles of the wall soffits of the round holes  21 ,  22  are 250° of the larger hole, and 220° of the smaller hole. Variations of 10° smaller and 20° greater are possible. 
     Due to the stage form of the round holes  21 ,  22 , one may distinguish an upper region  25 , and a lower region  26  of the round hole  21 , and an upper region  27 , as well as a lower region  28  of the round hole  22 . 
     The upper regions  25  and  27  are formed bowl-like with extended radial range, while the lower regions  26  and  28  form generated surfaces with straight surface lines. The upper regions  25  and  27  have larger diameters with respect to the lower regions  26  and  28 . The lower region  26  forms a truncated cone shaped section which tapers towards the lower side of the plate body  1 . The lower region  28  is formed cylindrical, but it may also be formed truncated cone shaped. 
     While the two upper regions  25  and  27  of the two round holes  21 ,  22  are in total formed bowl-like, a radial rib  33  extends, starting at the hole walls, in a plane around the shaped hole  2 . Rib  33  is formed as a circumferential web, and has a wedge-shaped cross section which tapers towards the centre of the respective round hole. The rib  33  seems to be similar to an “eight”, in top view. 
     While the rib  33  may uniformly extend around, a ridge  61  is arranged for the rib part fixed to the larger round hole in parallel to the plane of the lug  33  at the outer edge of the larger round hole by what an edge  24  towards the second round  22  hole is formed. Starting from the longitudinal axis of the plate body  1 , the height of the lug  61  increases towards the edge of the plate body  1  what results best from comparing  FIG. 4  with  FIG. 3 . Between the lug  61  and the rib  33 , a guidance is formed which favours the engagement of the thread  41  of the head screw  40 . 
     Each of the two round holes  21 ,  22  is divided into three sections: Above the lug  33 , the larger round hole  21  has an upper rounded flute shaped section  31  with or also without ridge  61 . Below the rib  33 , a central, round flute shaped section  36 , and a lower, truncated cone shaped or conical section  26  are provided. The smaller round hole  22  comprises an upper section  35  with a leading-in slope  62 , a central section  36  having a sloping rounded surface  63  below the plane of the rib  33 , and a lower section  28  which is preferably formed cylindrical, but may also be conical. 
     The bone plate is formed for cooperating with at least two kinds of bone screws. 
     The one kind has a bone screw head with a partially spherical lower side, and may rest on the sloping rounded surface  63  with the head lower side. Thereby, an inclination of the screw axis with respect to the plate plane also is possible, and indeed in longitudinal direction as well as (in a minor degree) transversely to the bone plate. This is made possible by the distance of the edges  23  from each other which distance is selected, accordingly to the intended purpose. 
     A further kind of usable bone screw  4  is shown in  FIG. 4 . This bone screw  4  has a screw head  40  with interior engagement and with external thread  41  at the upper end, as well as a conical support plane  42  at the lower end. The cone inclination of the support plane corresponds to the cone inclination of the lower region  26  of the larger round hole  21 . 
     The cone inclination of the lower region  26  of the larger round hole  21 , as well as the cone inclination of the conical supporting plane at the lower end of the screw head  40  of the bone screw  4  has an angle in the range of 3 to 30°, relatively to the longitudinal of the symmetry axis, or of the round hole, each. This cone angle preferably is in a range of 5 to 20°, and most preferably in a range from 8 to 12°. A mostly preferred design has very proven of value, with a cone angle of about 10°, with high values of endurance strength, and a good detachability of the connection between the bone screw and the bone plate. 
     In an angle range from about 8 to 12°, very good stability values are provided against tilting of the bone screw relatively to the bone plate, while simultaneously by providing only moderate self-locking. 
     The external thread  41  may be cylindrical, but a conical thread is preferred. The screw head  40  is followed by a screw shaft  42  which is intended for being fastened in a bone element to be fixed. The circumferential rib  33  extends in a plane, preferably in parallel to the plate plane, while the thread  41  extends along screw surfaces which extend transversely opposite to the plane of the radial rib  33 , and indeed also, if the bone screw  4  is brought into engagement with in parallel, or unidirectionally with the axis  21   a  of the round hole  21 . When doing so, the clamping takes place between the threads  41  and the rib  33 . In this regard, the ridge  61  is useful, because it offers a counter bearing for the thread of the screw head, and thereby provides a defined clamping with a defined self-locking effect. 
     In connection with the shown bone plate, the bone screw  4  may be used for approaching mutually, and for pressing together bone fragments. To this end, the bone screw  4  is placed with its axis in parallel to the axis  22   a  of the round hole  22 . As soon as the lower edge of the conical supporting surface  42  reaches the leading-in slope  62  of the smaller round hole  22 , a lateral force onto the screw head  40  is effected, while screwing-in the screw what causes a displacement of the bone fragment to be fixed, relatively to the bone plate. If a bone fragment is thus already being fixedly connected with the bone plate, this bone fragment is shifted against the bone fragment to be fixed, as it is wanted. 
     It is pointed out that due to the large peripheral angle of the cone surface of the lower region  26  which angle is in the range from 250 to 290°, a sufficiently strong connection between the plate body  1  and the bone screw  4  is effected, in case of a seized screw head  40 , because the circumferential rib  33  generates enough elastic tension force after clamping the screw threads  43  for keeping the conical surfaces being pressed onto each other at  25  and  42 . 
     By means of  FIGS. 5-8 , a further embodiment of the bone plate is shown, wherein the same reference signs are used for corresponding parts. 
     The main difference is the design of the circumferential, radial rib  33 . This rib is partially cut off in the region of the smaller round hole  22 , in order to provide a sloped, rounded sliding surface  35   a , and a transitional surface  35   b  which is useful for guiding the head of a bone screw with partial spherical lower side of the head. A residual rib  37  is formed which increases to the full dimension of the rib  33 , in the region of the larger round hole  21 . The remaining border edge  34  extends by less than 180°, and so allows the lateral insertion of bone screws from the smaller round hole  22  into the larger round hole  21 . 
     In the following, reference is made to  FIG. 9  which shows a further embodiment of a bone plate  1  having a circumferential rib  33 , wherein this circumferential rib  33  has a different height in radial direction, therefore towards its symmetry axis which, provided with the reference sign  71 , is for example approximately defined by the intersection of plane AA and plane BB. 
     In  FIG. 9 , a bone screw  4  is shown inserted into this bone plate  1 . 
     The circumferential rib  33  of the bone plate  1 , in this further embodiment, has a wedged cross section with which the height of the wedged cross section in radial direction along one circular, however, is not constant around the circumference of the hole. This means that it does have the same value for each section. 
     In this further embodiment, the wedged cross section of the circumferential rib  33  is flattened step by step, as it may well be seen in  FIG. 10  which shows an enlarged sectional view of a part of a sectional drawing along line AA, and therewith extending in parallel to the longitudinal axis of the further embodiment of the bone plate with inserted bone screw, shown in  FIG. 9 . 
     The flattening  72  reduces the radial height in a defined manner so that, with this design, decreased clamping forces are caused by the thread pitch of the thread of the bone screw  4 . In the region of the flattening  72 , the thread of the bone screw may freely be arranged in front of the circumferential rib  33  what may well be seen also in  FIG. 11 , with respect to the arrangement of the bone screw  4  relatively to the bone plate  1 . Hereby, only an essentially two-sided engagement of the circumferential rib  33  with the thread of the bone screw  4  is caused, as it may be seen in  FIGS. 12 and 13 , for example. 
       FIG. 12  shows an enlarged sectional view of a part of the sectional drawing along the line BB, and therewith extending transversely to the longitudinal axis of the further embodiment of the bone plate with inserted bone screw, shown in  FIG. 9 . This figure makes it easy to see that the left as well as the right side of the thread of the bone screw engages the rib  33 . By this design, the slope of the thread of the bone screw may be lowered concerning their friction inhibiting effect, and lower friction forces and self-locking forces may be selectively provided than with a rib  33  having the same radial height. 
     In this connection, it also is within the scope of the invention that the height in radial direction of the wedged cross section of the circumferential rib  33  essentially is zero, area by area. 
     In  FIG. 14 , a still further embodiment of the a bone plate  1  having a circumferential rib  33  with different heights in radial direction is shown with which embodiment the symmetry axis  71  of the circumferential rib is arranged offset relatively to the symmetry axis of the conical section  26  of the larger round hole, positioned under the rib, and with which corresponding radii  72  and  73  around the respective symmetry axis are dash-dotted, for making their relative displacement easier visible. 
     The circumferential rib  33  may also have the same radial height along the whole circumference, may more or less be brought into engagement with the thread of the head of the bone screw  4 , by the displacement. 
     Here, the circle formed by the radius  72  extends symmetrically to the symmetry axis  71  of the circumferential, radial rib  33 , and the circle formed by the radius  73  extends symmetrically to the symmetry axis  74  of the conical section  26  of the larger round hole. 
     The displacement (referred to by “x” in  FIG. 14 ) between the symmetry axes  73  and  74  defines the change of the radial height of the rib  33 . 
     This displacement is not limited to direction shown in  FIG. 14 , but may also be arranged in each arbitrary other direction. 
     If this displacement is equal to, or greater than the radial height of rib  33  in one direction, sections  75  are hereby caused in which the radial height of the wedged cross section of the circumferential rib  33  essentially is zero. 
     These sections may particularly well be seen in  FIGS. 15 to 17 . 
     In this context,  FIG. 15  shows the still further embodiment of the bone plate  1  shown in  FIG. 14 , but without the corresponding radii  72 ,  73  around the respective symmetry axes  71 ,  74 , for making their real form easier visible. 
       FIG. 16  shows a sectional drawing of the still further embodiment shown in  FIGS. 14 and 15 , extending in parallel to the longitudinal axis of the still further embodiment of the bone plate  1  without, however, inserted bone screw  4 , shown in  FIGS. 14 and 15 , for making the course of the rib  33  easier visible. 
       FIG. 17  shows a sectional drawing of the still further embodiment shown in  FIGS. 14 and 15 , extending transversely to the longitudinal axis of the still further embodiment of the bone plate  1  however, without, inserted bone screw  4 , shown in  FIGS. 14 and 15 , for making the course of the rib  33  easier visible, also in this case. 
     As with the embodiment shown in  FIGS. 9 to 13 , a defined preselectable, decreasing engagement of the thread of the bone screw  4  into the rib  33  is provided as a function of the displacement x. Also in this case, the friction of thread of the bone screw  4  with the rib  33  may be lowered in a defined manner, in comparison with a rib having a constant radial height. 
       FIG. 17  shows an arrangement with which only a two-sided engagement of the rib  33  into the thread of the bone screw  4  still takes places, essentially similar as shown in  FIG. 13 . 
     The circumferential rib  33  may have the effect of an adaptation which counteracts grooving and rupture tendencies at higher stresses, like bending loadings on the bone plate. 
     In this connection, embodiments with a circumferential rib with which embodiments the radial height of the rib  33  keeps constant are very advantageous with respect to the rigidity and the bending capacity of the bone plate  1 . 
     By means of the invention, a fixation system for bones, having a bone plate and bone screws is created with which embodiment bone screws with a round head may be placed in different oblique angles. Furthermore, the fixation system makes possible the relative shift between the bone fragments to be fixed and the bone plate, when using bone screws. This enables the surgeon to shift bone fragments against each other while fixing the fragments.