Abstract:
A fixation apparatus comprises a holder and a tension sleeve receiving a bone screw, wherein the bone screw has a shaft with a front thread and a head at the rear end, and the holder has a through borehole with a receiving seat, in which the tension sleeve is mounted and able to swivel, while the shaft has a thickening in a rear region, to the front of the head, whose width increases toward the head, and the width of the tension sleeve is larger than the width of the shaft immediately before the thickening and smaller than the greatest width of the thickening. When screwed in, the thickening widens the tension sleeve so that it is pressed by its envelope against the wall of the receiving seat. In this way, an angle-stable pressing is achieved for any given orientation of the bone screw.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/418,662, filed Dec. 1, 2010. This application also claims the priority of European Patent Application No. 10015172.9, filed Dec. 1, 2010. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a fixation apparatus for fastening by means of a bone screw. The fixation apparatus serves in particular for connection to a support rod for an implant, but it can also be used by itself. 
     BACKGROUND OF THE INVENTION 
     Screws are often used for a removable yet also sufficiently firm attachment of implants. Depending on the application, a rigid-angle positioning between the implant and the bone screw is sufficient; in other cases, however, a variable-angle (polyaxial) arrangement of the bone screws is required. Such a polyaxially mounted bone screw for a fixation apparatus is known in its basic features from EP 0 614 649 B1. The bone screw is led through a support sleeve, which has a screw-on nut for the securing. In order to attach the rod of an implant, a transverse borehole is provided in the support sleeve, through which the rod can be passed in an intermediate region between the nut and the bone screw. By tightening the nut, the rod is braced against the support sleeve. Large tightening forces can twist the support sleeve, which endangers the fastening safety. 
     In a further developed design, as is known from U.S. Pat. No. 7,641,674 B2, a pressure screw screwed into the rear end of the support sleeve acts on a tensioning element by a support rod of the implant that is inserted through a transverse bore, which has at its front end a dome-shaped recess to receive a spherically thickened screw head of the bone screw. The tensioning element can move lengthwise and is pressed by the force of the pressure screw against a snap ring arranged at the front end of the support sleeve. Thus, it forms an abutment for the pressing force applied by the pressure screw. This design limits the pressing forces which can be applied, and thus the clamping forces needed to achieve an angle stability. 
     SUMMARY OF THE INVENTION 
     A problem on which the invention is based is to create an improved fixation apparatus, starting from the above-mentioned prior art, that achieves a better angle fixation. 
     This can be achieved by a fixation apparatus with the features as broadly disclosed herein. Advantageous further developments are described in the detailed embodiments below. 
     In a fixation apparatus comprising a holder and a tension sleeve receiving a bone screw, wherein the bone screw has a shaft with a front thread and a head at the rear end, and the holder has a through borehole with a receiving seat, in which the tension sleeve is mounted and able to swivel, the invention specifies that the shaft has a thickening in its rear region, to the front of the head, whose width increases toward the head, and the width of the tension sleeve is larger than the width of the shaft immediately before the thickening and smaller than the greatest width of the thickening. 
     By “front” is meant the direction toward the tip and by “rear” is meant the direction toward the head of the bone screw. 
     The thickening increasing from front to rear reaches its greatest width on the head, which is larger than the clear width of the tension sleeve. Preferably, the thickening extends over a length that is at least two and a half times the length of the tension sleeve. It is especially preferable when the thickening is conical, namely, with a cone angle of at most 15 degrees, preferably at most 5 degrees. 
     By being able to swivel is meant here that the axis of the bone screw fixated by means of the tension sleeve can take up a freely adjustable angle to the axis of the through borehole in the holder. The adjustment range is preferably at least 15 degrees in each direction about the center axis of the through borehole. 
     The invention is based on the notion of using a combination of two features to substantially enhance the force securing the screw in its angle position. On the one hand, the arrangement of the thickening forward from the head of the bone screw prevents the bone screw from slipping out of the tension sleeve even under very high force loading. Thus, the bone screw can be tightened at the implant site, whereupon it is displaced axially forward. On the other hand, thanks to this axial movement, the thickening is moved into the tension sleeve and presses against its inner wall. In this way, the tension sleeve with its preferably spherical envelope surface is elastically widened and seizes the preferably complementary shaped, i.e., spherically domed, wall of the receiving seat. Depending on the type of thickening, one achieves a substantial force transmission in this way, over tenfold for a cone angle of 5 degrees. Thus, a pressure between tension sleeve and receiving seat can be effectively achieved. The bone screw is thereby fixed in the desired angle position by a press fitting. 
     Usually the thickening will have a smooth envelope surface. This allows for a friction-free tightening of the bone screw and a simplified axial movement relative to the tension sleeve. But it can also be provided to configure a secondary thread on the thickening. This preferably has the same lead as the thread on the shaft. It is especially preferable for the secondary thread to be a multiple thread. This simplifies the screwing in process, especially when the mating thread is but short. The mating thread can be formed in the tension sleeve, preferably on a shoulder running around the preferably cylinder-shaped inner wall of the tension sleeve. The shoulder prevents the bone screw from wandering out, and the inner thread arranged there facilitates the spreading of the tension sleeve and thus further increases the fastening safety. 
     In one preferred embodiment, the envelope of the thickening is provided with its own threading (secondary thread). The secondary thread is preferably configured with a lead that is smaller than the lead of the thread on the shaft, advisedly in the range of 0.4 to 0.6. Thus, the secondary thread is more fine than the thread on the shaft and therefore preferably a single-flight thread. But a multiple-flight design is not ruled out. In front of the envelope on which the secondary thread is formed there is preferably provided a conical segment by which the width increases from the width of the shaft to the width of the envelope. The envelope itself is advisedly cylindrical, but this is not compulsory. Preferably, the head is conical in design, so that it directly adjoins the thickening and broadens toward the rear. 
     It is not absolutely necessary for the thickening to rise up uniformly from the shaft. It can also be provided that the thickening forms a flange at its front end. It is especially expedient in this case to combine the thickening structurally with the head of the bone screw, and then the outer envelope of the thickening will have a conicity that can furthermore advantageously extend across to the head. 
     Moreover, it can be provided that the bone screw has a conical thread with a core diameter increasing toward the head. With such a thread, a good cutting action in the bone can be achieved with good fastening safety of simple implantation techniques. 
     The tension sleeve is preferably slotted. This can be accomplished in various ways. Thus, a slot can be provided running the entire length of the tension sleeve. This produces a good widening capacity, yet at the expense of a rotationally symmetrical transfer of force to the wall of the receiving seat. Alternatively (or also in addition), it can be provided to have several slots open at one end. Advisedly, they are open toward the front end. 
     The tension sleeve can be provided with a complex through opening. By this is meant that the through opening has a conical segment to accomplish the spreading function and a threaded segment that is preferably short (at most two turns of the screw) and serves for tensioning. 
     In an alternative embodiment providing special protection, the fixation apparatus comprises a screw and a holder, wherein the screw has a threaded shaft and a ball head, and the holder comprises a holding sleeve, a tensioning element and a pressing element, and the holding sleeve is a continuous inner borehole provided with a front snug fit for the tensioning element and a rear inner thread for the pressing element, and wherein the tensioning element is a tension cage, and in the installed state the ball head is enclosed beyond its region of greatest width and interacts with the snug fit via a conical boundary surface so that the tension cage is squeezed together in the forward direction under an axial displacement and is pressed against the ball head. 
     This is based on the notion of providing for a substantial boosting of the force securing the screw in its angle position by a combination of two features. This is achieved, on the one hand, in that the tension cage encircles the ball head of the screw around its equator, and thus the ball head is prevented from slipping or being forced out even under very high force loading. This is combined with the invention&#39;s specified conicity between the tension cage and its snug fit in the holding sleeve, whereby the pressing force applied by the pressing element causes a narrowing of the tension cage with the ball head of the screw accommodated inside. The envelope surface of the tension cage enclosing the ball head is pressed into the ball head with substantially enhanced force, thanks to the wedge effect, so that a pressing fit results. Thanks to this pressing fit, combined with the protection against slipping out even under very high force application, a very stable angle fixation is achieved. 
     In order to increase the pressing forces exerted by the tension cage, the tension cage is preferably provided with slots on its envelope surface that are open toward the front end. In this way, the tension cage even when substantially compressed can transmit the pressing forces generated by the conicity without hindrance to the ball head, and without any unwanted internal constraint being produced in the tension cage. 
     Advantageously, the pressing element acts on an outer edge of the end face of the tension cage. When the pressing force acts on the outer edge and not the center of the tension cage, there is a direct force transmission to the conical boundary surface between outer envelope surface of the tension cage and the snug fit. Moreover, this prevent the tension cage from being indented at the center under the load of the pressing force, which would create a force on the ball head pushing it out of the tension cage. This enhances the fastening safety. 
     Advantageously, the tension cage has a radially projecting edge at its forward end. This enables a preassembly of the tension cage in the holding sleeve so that the tension cage can be shoved from the front into the holding sleeve and—after passing the narrow point at the front edge of the holding sleeve—snap into its position, while the radially projecting edge is still situated outside of the narrow point and the tension sleeve is prevented from wandering backward toward the thread. The ball head of the bone screw can be pressed into this unit. With such a preassembly, the usefulness to the surgeon is substantially facilitated, and furthermore the high risk to the patient of a possible loss of parts is averted. 
     The conical boundary surface can be configured in any given way. It is preferable for the tension cage to have a conical outer edge. This can cooperate with a narrow point provided at the forward end of the inner borehole. Such a narrow point is easy to fabricate, since the remainder of the inner borehole can be essentially cylindrical in shape. However, it is not ruled out that the inner borehole tapers conically in the region of the snug fit as an alternative or additional feature. 
     In a preferred embodiment, the snug fit has two zones, wherein one segment is arranged on the sleeve and the other on the tension cage. The segment on the sleeve is the front segment, and immediately behind this the segment adjoins the tension cage. The rear segment on the tension cage has its greatest width at the transition to the front segment. The rear conical segment preferably adjoins smoothly, i.e., without a change in width. The tension cage according to this preferred embodiment is arranged in the rear segment and interacts with the segment formed as a hollow spherical shell such that the ball head, as described above, is enclosed beyond the equator. In the variant described here, the tension cage itself does not go beyond the equator. This two-part design is easy to assemble and ensures a good fastening of the ball head with little sensitivity to tolerances. 
     The tension cage has, at its rear end face, preferably a groovelike concavity formed to receive a support rod. More details on the support rod are discussed further below. The concavity ensures a form-fit with the support rod and, thus, an exact supporting of its position. 
     The tension cage is advisedly slotted laterally in its rear zone, and this preferably as a double slot. This creates spreadable tongues. They are advisedly dimensioned such that they engage with window like recesses at the inside of the sleeve. This affords an additional fixation of the tension cage along with the bone screw supported by it, preventing an unintentional loss of its positioning. 
     Preferably, the width of the inner bore in the rear thread region is greater than the width in the region of the snug fit. This enables an easy preassembly of the tension cage in the holding sleeve, namely, by introducing the tension cage from behind through the threaded segment into its position in the region of the snug fit. This allows the width of the front opening of the inner bore to be smaller than the width of the tension cage. In this way, the tension cage is protected against an unintentional wandering to the front. 
     Advisedly, the holding sleeve has a seat for a support rod of an implant. This allows one to achieve a secure fixation of the implant in the region of the bone in which the bone screw is inserted. Even complex implants can be fastened securely and stable in angle by this method. Advantageously, the seat is configured as a transverse borehole to the inner borehole of the holding sleeve, and this in the region of the transition between the rear inner thread and the front snug fit. In the installed condition, the rod is thus situated between the pressing element and the end face of the tension cage, on which the pressing element can act directly without the rod. When the rod is installed, the pressure is transmitted from the pressing element to the tension cage, i.e., indirectly, namely across the cross section of the rod. This accomplishes at the same time a fixation of the rod relative to the fixation apparatus with no further expense. 
     The transverse borehole preferably has a nonround cross section. In this way, a form-fitting connection can be achieved between rod and holding sleeve. This creates additional safety against unwanted twisting. The transverse bore does not necessarily have to have the same cross sectional shape as the rod, but rather advisedly the transverse bore is extended lengthwise in the direction of the longitudinal axis of the inner bore. This offers further adjustment room and ensures a secure fastening even when the tension cage is positioned quite far forward (to counterbalance a relatively thin ball head of the bone screw). 
     Preferably, the pressing element has a buttress thread, wherein the rearward pointing load flank is steeper than the other forward pointing flank. A load flank angle of 0° or a negative load flank angle has proved to work especially well. 
     In all embodiments the bone screw can be provided with a continuous borehole running along its central axis, i.e., it is cannulated. Thus, it can be inserted along a guide wire that is pushed through, accomplishing a very good positioning accuracy even under difficult access conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained in reference to the enclosed drawing, which shows an advantageous sample embodiment. There are shown: 
         FIG. 1 , an exploded view of the fixation apparatus according to a first embodiment of the invention; 
         FIG. 2 , an assembly view with rod of an implant installed; 
         FIG. 3 , a cross sectional view of the holding sleeve of the fixation apparatus; 
         FIG. 4 , a side view of a fixation apparatus according to a second embodiment of the invention; 
         FIG. 5 , a detail enlargement from  FIG. 4 ; 
         FIG. 6 , perspective views of tension sleeves; 
         FIG. 7 , sectional representations of tension sleeves; 
         FIG. 8 , various sample embodiments for thickenings of the bone screw; 
         FIG. 9 , a multiple holder for two bone screws. 
         FIG. 10 , an exploded view of the fixation arrangement according to a variant of the first embodiment; 
         FIG. 11 , a cross sectional view of the holding sleeve of the variant per  FIG. 10 ; 
         FIG. 12 , a representation of a bone screw for a variant of the second embodiment; and 
         FIG. 13 , a sectional view of a tension sleeve of the variant of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The fixation apparatus comprises a holder  2  for a bone screw  1 . The bone screw  1  is itself of conventional configuration with a shaft  10 , on which a bone thread  11  is provided in its forward region or throughout. At the rear end of the shaft  10  is provided a screw head  12 , which is spherically configured with an equator  13  at its largest diameter. At its rear end a star-shaped depression  14  can be provided as a seat for a correspondingly configured screwdriver. 
     The holder  2  comprises a sleeve  3 , a tension cage  4  and a pressing element  5 . The sleeve  3  has a hollow cylinder configuration with an inner opening  30  running from a rear to the front end. The inner opening  30  has an inner thread  31  in its rear area and a snug fit  32  provided with a smooth wall in its forward area. The core diameter of the inner thread  31  is chosen such that it is a bit smaller (around 1 mm) than the diameter in the region of the snug fit. The configuration of the wall in the region of the snug fit  32  is essentially cylindrical with a narrow point  33  at the forward end. Thus, a front mouth is formed with a decreased diameter. 
     The sleeve  2  furthermore has two diametrically opposite longitudinal slots  35 . They extend from the rear end of the sleeve  3  across the entire region of the inner thread  31  as far as the transitional region  34  between inner thread  31  and snug fit  32 . There, the slot  35  has a broadening in the tangential direction. 
     The tension cage  4  has an envelope surface  40  provided with a conical outer surface. this is divided by six slots  41  extending from the front end over around ⅘ of the length of the tension cage into a corresponding number of segments  42 . These segments  42  are joined together by a holding ring  45 , formed in the region of the rear end  43 , and bordering a central installation opening  44 . At the front ends of each of the segments  42  is formed an inwardly facing projection  46 . The inner diameter of the tension cage  4  is chosen so that the ball head  12  of the screw  4  is accommodated therein with a slight play. The projection  46  protrudes so far inward that the segments  42  can only be moved under elastic spreading across the equator  13  of the ball head  12 . This protects the ball head  12  of the screw  1  from an unintentional dropping out from the tension cage  4 . 
     The pressing element  5  is configured as a headless screw with a buttress thread  51  on its outer envelope. At the rear end face, a star-shaped depression  54  is formed at the center to receive a screwdriver. The buttress thread  51  is configured such that the rearward pointing, load-bearing thread surface  52  is perpendicular to the envelope surface, while the other forward pointing thread surface  53  is inclined at an angle of around 70°. With this asymmetrical sawtooth configuration of the thread  51 , a spreading of the holding sleeve  3  is prevented, even under heavy loading. 
     For the assembly process, the cage  4  is guided from the front through the narrow point  33  under elastic compression of the segments  42 , until the tension cage  4  comes to a stop in the region of the snug fit  32 . In the installed state, the front ends of the segments  42  with the projection  46  stick out from the narrow point toward the front. In the next step, the screw  1  with its ball head  12  is inserted into the cage  4  under elastic spreading of the segments  42 . Thus, these parts are captively preassembled. After this, the pressing element  5  can be installed by several turns in the inner thread  31 . The forward part of the slot  35  with the widening  36  remains free. The fixation apparatus is now ready for use. 
     For the fastening of the implant, at first the bone screw  1  is tightened by a screwdriver led through the sleeve  3  to the star-shaped seat  14  in the ball head  12 . In the next step, the rod  7  can be introduced into the transverse bore  35  for assembly. Finally, the pressing element  5  is screwed in and tightened by a screwdriver inserted into the depression  54 . The pressure forces generated in this way are conveyed by the cross sectional rod  7  onto the end face  43  of the tension cage  4 , so that the latter is moved forward in the axial direction. The resulting pressure forces are transformed by virtue of the conicity in the region of the boundary surface between outer envelope  40  of the tension cage  4  and the snug fit  32  in combination with the narrow point  33  into inwardly acting pressing forces that are higher by a multiple in magnitude and firmly press the ball-shaped screw head  12  together with the inside of the segments  42  of the tension cage  4 . This achieves a stable-angle fixation. 
     The dotted line in  FIG. 2  shows one possible region for the different angle that the polyaxial bearing of the screw  1  can take up in relation to the holder  3 . 
     One variant of the first embodiment is shown in  FIGS. 10 and 11 . Similar parts have the same reference numbers. The major difference lies in the configuration of the sleeve  8  and the tension cage  9 . The sleeve  8  is a hollow cylinder with an inner bore  80  running through it from a rear end to the front end, being provided in its rear region with an internal thread  81  to accommodate the pressing element  5  and in its front region with a segment  82 ′ of a snug fit  82  that is shaped as a hollow spherical segment. In the middle region there is provided an essentially smooth inner wall, against which the tension cage  9  bears with its envelope surface  90 . The hollow spherical segment  82 ′ of the snug fit  82  is shaped complementary to the ball head  12  of the bone screw  12 . The opening at the front end of the sleeve  8  is narrowed and lets the shaft  10  pass through, but prevents the ball head  12  from going through. 
     The sleeve  8  furthermore has two diametrically opposite lengthwise slots  85 . These extend from the rear end of the sleeve  8  across the entire region of the internal thread  81  and up to the snug fit  82 . Two windowlike recesses  88  are arranged staggered from the lengthwise slots  85 . 
     The tension cage  9  has an envelope  90  provided with a preferably conical outer surface, as well as an inner space  94 . Its width is adapted to the width of the ball head  12  so as to produce a fit extending over one region. This region forms a second part  82 ″ of the snug fit  82 . It works together with the hollow spherical part  82 ′ in such a way that a continuous snug fit  82  is formed to accommodate the ball head  12 . The joining place between the two parts  82 ′ and  82 ″ lies in the region of greater width and thus where the equator  13  of the bone screw is situated. 
     In the rear zone of the tension cage are arranged pairs of slots  91  opposite each other, thus forming two diametrically opposite tongues  98 . In the installed state of the tension cage  9 , they spread out and fit into the windowlike recesses  88 , thereby preventing the tension cage  9  from falling out from the sleeve  8 . In this way, it is possible to form a preassembled unit from bone screw inserted in the sleeve  8  and tension cage  9 . At its rear end face, the tension cage  9  is provided with a groove  97 . This is preferably in the shape of a semicylinder and serves to receive a support rod of an implant  7  in form-fitting manner. 
     We shall now refer to the second embodiment. As can be seen from  FIG. 4 , the fixation apparatus comprises a holder  2 ′ for a bone screw  1 ′, having a shaft  10 ′ with a bone thread  11 ′ disposed in the forward region. Toward the rear end, the shaft  10 ′ has an increasing thickness  15 ′. It extends as far as a head  12 ′, which has a depression  14 ′ to accommodate a screwdriver (not shown). 
     The holder  2 ′ comprises a base plate  3 ′ and a tension sleeve  4 ′. The base plate  3 ′ has a bearing surface  37 ′ at its front end to bear against a bone or other tissue parts. It can be provided with optional support spikes  38 ′. 
     The base plate  3 ′ has a support opening  30 ′ passing through it, being widened to a snug fit  32 ′ in the shape of a spherical dome in its middle region. This receives the tension sleeve  4 ′, which in turn has a spherical envelope surface  40 ′ with suitable dimensions. The tension sleeve  4 ′ is mounted and able to swivel thanks to the complementary surfaces of the spherical envelope surface  40 ′ on the one hand and the spherical dome-shaped snug fit  32 ′ on the other. In this state, the fixation apparatus is preassembled and ready for implantation. 
     The tension sleeve  4 ′ has a central opening  44 ′, through which the bone screw  1 ′ extends by its shaft  10 ′ upon implantation. The opening  44 ′ has an essentially cylindrical shape, with an inwardly pointing shoulder  46 ′ at the front end (see  FIG. 7 a   ). Other sample embodiments for the opening  44 ′,  44 ″ are shown in  FIG. 7 b  and  c   , namely, one with a complex shaped shoulder  46 ″, on which one or two turns of an inner thread  47 ′ are arranged, or a shoulderless design with opening narrowing conically to the front. Alternatively, however, the opening can also be cylindrical, if the shaft of the bone screw is conical in design. The tension sleeve  4 ′ is preferably slotted, having several slots  41 ′ open at one end (see  FIG. 6 a   ). But one continuous slot  42 ′ can also be provided (see  FIG. 6 b   ). 
     The thickening  15 ′ extends across a region that is around two to three times longer than the length of the tension sleeve  4 ′ as dictated by the length of the opening  44 ′. When the screw  1 ′ is twisted in, it moves axially forward by its shaft  10 ′ through the base plate  3 ′ (symbolized by the single arrow in  FIG. 5 ). The conical thickening  15 ′ comes into contact with the inner wall of the opening  44 ′ and spreads open the tension sleeve upon further axial movement (see double arrow in  FIG. 5 ). In this way, the tension sleeve  4 ′ is forced by its spherical envelope surface  40 ′ against the dome-shaped snug fit  32 . This creates considerable forces on account of the small cone angle of less than 10 degrees, due to the wedge effect, so that this effectively produces a pressing together between tension sleeve  4 ′ and snug fit  32 ′ of the base plate  3 ′. The screw  1 ′ is thus fixed in its particular swiveled angle position relative to the base plate  3 ′. 
     The thickening  15 ′ can be conical, as described above. This is shown in  FIG. 8 a   . Alternative embodiments are shown in  FIG. 8 b  and  c   . Thus, the thickening  15 ′ can be provided with a secondary thread  16 ′. This is a multiple thread, for easier insertion in the inner thread  47 ′ on the tension sleeve  4 ′ and it preferably has the same lead as the thread  11 ′ on the shaft  10 ′. In this way, the axial force and thus the spreading and pressing action can be increased. Furthermore, the protection against unintentional loosening by backing off of the screw  1 ′ in the bone is improved. In another alternative embodiment, the thickening  15 ″ is moved further to the rear and has a forward pointing flange surface  17 ′. The conical region of the thickening can thus be shorter in design, so that a structural integration with the head  12 ′ is made possible. This provides a bone screw  1 ′ that is especially easy to fabricate. Preferably, the latter alternative will be used together with a type of tension sleeve  4 ′ as depicted in  FIG. 7 c   . This enables a unit of especially compact construction, thanks to its likewise conical opening  44 ″, which is preferably complementary to the conicity of the thickening  15 ″. 
     One variant of the second embodiment is shown in  FIGS. 12 and 13 . Similar parts carry the same reference numbers. The bone screw  1 ″ has a shaft  10 ″ with a conical bone thread  11 ″ arranged in the front region. At the rear end, the shaft  10 ″ has a thickening  15 ′″, adjacent to the head  12 ″. The head  12 ″ is designed to broaden conically to the rear, starting from the thickening  15 ′″. The thickening  15 ′″ comprises a conical front segment in the region of the transition to the shaft  10 ″ and an envelope in the rear segment, on which a secondary thread  16 ″ is formed. It has a lead that is somewhat less than half the lead of the bone thread  11 ″ (for example, 7/15). 
     One variant of the tension sleeve  9 ′ is shown in  FIG. 13 . It differs from the tension sleeves  4 ′ shown in  FIG. 7  basically by a complex configuration of the continuous opening  94 ′. This comprises a shorter front cylindrical part  94 ″ and a longer rear conical part  94 ″, and at their transition is arranged an inwardly projecting radial strip  96 ′ with a short internal thread  97 ′. By “short” is meant here up to two turns of the screw. 
     In this variant, the bone screw  1 ″ widens by its thickening  15 ′″ the cylindrical part  94 ′″ of the complex continuous opening, and this under an advancing force that is produced by the secondary thread  16 ″ engaging in the short internal thread  97 ′ during the screwing in process. The conical shape of the head  12 ″, together with the conical part  94 ″, ensures a further spreading and clamping of the bone screw  1 ″, so that it is protected against unintentional loosening. 
     The holder can also be configured so that several bone screws are supported. One example of this is shown in  FIG. 9 . This shows a multiple holder  3 ″, having two receiving seats, in order to additionally accommodate a second tension sleeve  4 ″.