WO2008098728A2

WO2008098728A2 - Fixation device

Info

Publication number: WO2008098728A2
Application number: PCT/EP2008/001025
Authority: WO
Inventors: Bernd Simon; Carsten Hoffmann; Jakob Kemper
Original assignee: Stryker Trauma Gmbh
Priority date: 2007-02-12
Filing date: 2008-02-11
Publication date: 2008-08-21
Also published as: WO2008098728A3

Abstract

A hip fracture device providing distance limited dynamization, load controlled dynamization and combinations of both dynamization methods by varying components. The components may be varied interoperatively. A bone plate having a plurality of openings for receiving a compression bone screw or a cortical screw. An end cap, threadably insertable in the opening and having a layer of polymeric material interposed between the end cap and the top of the head such that the compression of the polymeric material would allow slight axial movement of the screw. Alternatively, a locking ring adapted to attach to the head of the screw and having ridges that have shape complimentary to the depressions and fit in the depressions when the locking ring is attached to the head.

Description

Fixation device

Field of the Invention

The present invention relates to an fixation device, in particular to an apparatus and method for the treatment of fractures of the proximal femur including the neck of the femur and the intertrochantric region, and more particular to a hip fracture device with barrel and end cap for load control. Further, the invention relates to screw fastenings in the technical field of bone plates, e.g. for the treating of femur fractures or fractures of the neck of the femur.

Background of the Invention

In treatment of the fracture of the femoral neck it is necessary to maintain angular stability of the head fragment to maintain an anatomical reduction postoperatively. It is also desirable to compress fracture site intra-operatively and then to stabilize the bone fragments by not allowing any further axial or angular movement. Since axial movement of the bone fragment resulting in shortening of the neck of the femur will result in reduced physical functioning, particularly in younger patients, it is desirable to stabilize the fracture postoperatively.

Many locking plates are available that allow stabilization of bone fragments. Conventional locking plates (also known as bone plates) have a plate that is attached to the fragments of the fractured bone via screws that are inserted in the bone through screw holes in the plate. The screws of the conventional locking plates have threads on the head portion in addition to the threads on the shaft. The threads on the head portion have a greater core diameter than the threads on the shaft but both threads have same pitch. When the screw is advanced in the bone and the head of the screw is in the screw hole of the bone plate, the threads on the screw head engage matching threads in the screw hole. This locks the screw in place and prevents it from moving in the axial direction post operatively. However, such bone plate system can not be used to compress the fracture site. In another conventional bone plate system used for femoral neck fracture a compression screw is used. The compression screw head does not have the threads and therefore may be rotated further after its head has reached the final axial position thereby compressing the fracture site. A separate end cap is then screwed in the compression screw hole of the bone plate to prevent the screw from moving back in the axial direction.

These bone plate systems require a separate step of installing an end cap to prevent post operative axial movement of the screw. Therefore, there is a need for further improvement in bone plate systems to provide an easy to use plate system that facilitates intra-operative compression and at the same time provides angular and axial stability post operatively.

The development of the mechanism described below is based on the locking mechanism described in US 2005/143742.

Summary of the Invention

It may be seen as an object of the present invention to provide an improved fixation device.

The object is solved by the subject matter of the independent claims, wherein embodiments thereof are incorporated in the dependent claims.

According to en exemplary embodiment of the present invention, a fixation device may form a mechanism which ensures a fixed or stable angle between a bone plate and the fixation screw. The inventive fixation device may enabling a so-called dynamic movement of the screw along the longitudinal axis, even when the screw is implemented into a bone. In order to ensure such a longitudinal movement of the screw a sliding ring may be arranged between the locking ring and the screw. This sliding ring may form a counter bearing when the locking ring is immobilized, fixed or clamped, while the screw may still be able to move along the longitudinal axis. In order to prevent that the screw may move out of the bone plate a distance clip may be attached to the screw, e.g. in a groove of the screw. Thus, a migration of the screw may be limited.

According to an exemplary embodiment of the invention, there is provided a hip fracture device allowing distance limited dynamization, load controlled dynamization and the combining of both dynamization methods. The hip fracture device has a plate and screw assembly. By replacement of modular components in the screw assembly the extent of axial travel and the force resisting travel may be adjusted interoperatively.

According to an exemplary embodiment of the invention, the hip fracture device uses a fixed barrel and modular end caps to variably limit the extent of axial travel of the screw within the barrel while restraining the screw to be coaxial with the barrel.

According to an exemplary embodiment of the invention, a spring pin mounted to an end cap progressively engages a bore in the screw to provide load controlled dynamazation.

According to a further exemplary embodiment of te invention, a bone plate having one or a plurality of openings for receiving a compression bone screw or a cortical screw. An end cap, threadably insertable in the opening and having a layer of polymeric material ma be interposed between the end cap and the top of the head such that the compression of the polymeric material would allow slight axial movement of the screw. Alternatively, a locking ring adapted to attach to the head of the screw and having ridges that have shape complimentary to the depressions and fit in the depressions when the locking ring is attached to the head. The locking ring and the bone screw being assembled together and being insertable in the bone simultaneously using a dedicated instrument. Compression may be applied to a bone - A -

fracture by turning the bone screw alone after the locking ring has reached its final axial position.

A screw and plate system typically applies a static compressive force across the fracture. It has been found that allowing the screw to travel along its axis in response to loading by the patient further encourages the growth of strong bone to heal the fracture. Screws of this type, known as dynamic compression screws, must provide axial movement while preventing angular rotation or lateral movement across the fracture. One shortcoming of dynamic compression screws is that, unless the travel is appropriately limited, the neck of the femur may be undesirably shortened.

According to a further exemplary embodiment of the invention, there is provided a bone plate for use with fractures of the femur. Screws attach the bone plate to the femur. The compression screws that are inserted in the neck of the femur may be parallel to the axis of the neck of the femur. Inserting the bone screws in the neck region of the femur provides compression and angular and rotational stability to the head of the femur. Cortical interlocking type screws may be used in a distal portion of the bone plate in the subtrochantric shaft region of the femur. The compression screws may stabilize bone fragments when used with end caps and prevent the shortening of the femoral neck resulting in improved postoperative function of the hip. The end cap may be inserted in a threaded plate hole and contact the top of each screw. A polymer buffer may be placed in the screw hole between the end cap and the head of the compression screw. The polymer buffer may allow small movement of the screw.

In use, the compression bone screw may be inserted in the screw hole and screwed into the neck of the femur until the underside of the bone screw sits on the flat face formed in screw hole. Next the screw is rotated further to apply compression to the fracture site. Once the desired amount of compression is applied, the end cap is inserted in screw hole. The end cap prevents the screw from moving back in the axial direction.

In another embodiment, a compression screw having a different head design is used with a split locking ring. The locking ring may have a smooth circular outer surface that fits in the screw hole. The inner surface of the locking ring may have a saw blade like or similarly functioning geometry. The saw blade geometry on the inner surface is preferably asymmetric. The compression screw head has a saw blade geometry that can mate with the saw blade geometry on the inner surface of the locking ring.

In use, the screw and the split locking ring may be assembled together and inserted into the screw hole. The assembly of the screw and the locking ring is then screwed into the bone using a dedicated insertion instrument that holds and rotates the screw and the locking ring simultaneously. When the head of the screw reaches the terminal axial position in the screw hole, both the screw and the locking ring can be rotated further to apply compression to the fracture site. After the compression is applied, the screw alone is turned. The locking ring is thereby clamped between the head of screw and the bone plate. This results in fixing the screw in place such that the screw can not back out in axial direction.

In yet another embodiment, a compression screw having a different head design is used with a locking ring. The locking ring has a threaded circular outer surface that fits in the screw hole. The top wall of the locking ring projects towards the center of the screw hole and has a hexagonal internal periphery. The bottom surface of the top wall has ridges. The screw has a head that has an outer peripheral surface that slidably fits into the locking ring. The top surface of the head of the screw has depressions that correspond to the ridges. Thus when the screw is assembled in locking ring, the ridges sit in the depressions. The top surface of the screw head also has a hexagonal depression to allow engagement of a suitable screw driver. In use, the compression screw and the locking ring are assembled together and inserted into the screw hole. The assembly of the screw and the locking ring is then screwed into the bone using a dedicated insertion instrument that holds and rotates the screw and the locking ring simultaneously. When the head of the screw reaches the terminal axial position in the screw hole, the screw can be rotated further to apply compression to the fracture site. When the screw is rotated further the ridges loose contact with the depressions. This forms a small gap of approximately 0.1 -0.4 millimeters between the screw and the locking ring. As soon as the body weight is applied post-operatively, the femoral head fracture fragment presses the screw back to the lateral side until the movement is stopped by the locking ring. The polymer buffer may also be used with any of the above described embodiments.

With the present invention, it is possible to adjustably control the extent of axial movement (distance limited dynamization) and to adjustably provide a force that resists travel (load controlled dynamization). It is especially advantageous if the resisting force increases with the extent of travel.

As used herein, when referring to bones or other parts of the body, the term

"proximal" means closer to the heart and the term "distal" means more distant from the heart. The term "inferior" means toward the feet and the term "superior" means towards the head. The term "anterior" means towards the front part of the body or the face and the term "posterior" means towards the back of the body. The term "medial" means toward the midline of the body and the term "lateral" means away from the midline of the body. It should be noted that the above features may also be combined. The combination of the above features may also lead to synergetic effects, even if not explicitly described in detail.

These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiments described hereinafter.

Brief description of the Drawings

Exemplary embodiments of the present invention will be described in the following with reference to the following drawings.

Fig. 1 illustrates a femural neck

Fig. 2 illustrates a cross sectional view of a locking mechanism according to an exemplary embodiment of the invention.

Fig. 3 illustrates a cross sectional view of a detail of a locking mechanism according to an exemplary embodiment of the invention.

Fig. 4 illustrates a plan top view on the fixation device, in particular the locking mechanism according to an exemplary embodiment of the invention.

Fig. 5 illustrates a frontal elevation view of a hip fracture device implanted in a proximal femur according to an exemplary embodiment of the invention.

Fig. 6 illustrates a close up view of a portion of Figure 5. Figure 7 illustrates a sectional lateral view as shown in Figure 5 with the end cap removed.

Figures 8-10 illustrate views as in Figure 6 showing various end cap and spring pin configurations.

Figures 11-14 illustrate views as in Figure 6 showing the operation of the device during force limited axial travel.

Fig. 15 illustrates an anterior elevation view of a bone plate mounted on a femur.

Fig. 16 illustrates an isometric sectional view of a screw hole in the bone plate of Fig. 15 with a bone screw and an end cap inserted therein.

Fig. 17 illustrates an isometric view of a first locking ring embodiment.

Fig. 18 illustrates a sectional view of the bone plate of Fig. 15 with a locking ring and a screw installed therein.

Fig. 19 illustrates a lateral view of a portion of a bone plate assembly showing the bone plate, a screw and the locking ring of Fig. 17.

Fig. 20 illustrates a sectional view of the bone plate of Fig. 15 with a second embodiment of a locking ring and a screw installed therein.

Fig. 21 illustrates another sectional view of the embodiment of Fig. 20. Detailed Description of Exemplary Embodiments

In fractures of the femural neck, as illustrated in Fig. 1, in particular Gaarden III/IV, it is desired to develop a plate that provides angular stability ASy of the head fragment to maintain an anatomical reduction postoperatively. Intraoperativly this mechanism should be polyaxial to facilitate implantation in a broad variety of anatomies. However, axial dynamisation (in direction AxS) meaning sliding of the screw through the plate might be necessary to allow for postoperative compression of the fracture site. However, it might be useful to limit the distance of dynamisation to avoid too much collapse of the neck and excessive deformation of the proximal femur. Also initial compression should be feasible. Thus, a polyaxial mechanism may be provided for connecting a screw with a plate in which the angle can be locked by the user (the surgeon) while axial movement of the screw is still possible. In addition to that a mechanism was to be developed to limit the axial movement of the screw to a defined amount. Moreover, the choice of the amount of dynamisation in discrete steps should be done by the user. Polyaxial mechanisms that allows for locking intra operatively to define the angle have been previously developed and patented. The development of the mechanism described below is based of the locking mechanism described in US 2005/143742.

Fig. 2 illustrates a cross sectional view of a locking mechanism according to an exemplary embodiment of the invention. The locking mechanism provides a connection of a plate 27 and a screw 24. The locking mechanism is located at an opening 26 of the plate 27, wherein the plate may have plurality of though holes 29 for fixing the plate to the bone.

Fig. 3 illustrates a cross sectional view of a detail of a locking mechanism according to an exemplary embodiment of the invention. In the locking mechanism an asymmetrical slotted locking ring 50 is the main element. It may be inserted in an asymmetrical opening 26 in a plate 27. Before locking, it can still be rotated by an angle of ±15°. After the screw 70 is inserted the locking ring 50 is turned so that it is deformed and thus jammed in between the plate 27 and the screw head. Now the screw is blocked in the locking ring 50, it can neither change the relative angle nor move axially through the plate. The new mechanism works in a similar way. The inner contour of the opening in the plate 27 and the locking ring 50 stay principally the same. Dimensions of these components might be changed to account for different mechanical loads but the principle of these two interacting parts remains. However, the counterpart for the locking mechanism is no longer the screw (screw head) itself. An additional element, the so called "sliding ring" 76 is inserted in between the locking ring 50 and the screw 70. The sliding ring 76 may be an unslotted solid element that is the counterpart for the locking ring in the locking mechanism. It provides the angular stability against the plate 27 (through locking mechanism) and against the screw (running fit) but allows for the screw 70 to move axially. The "distance clip" 91 is an addional feature. After sling the sliding ring 50 on the screw 70 the slotted distance clip 91 is attached into one of several circular grooves in the screw. This assembly is then screwed into the bone until the sliding ring reaches the locking ring. Initial compression can be applied before locking. The distance clip limits the maximal distance of dynamisation if desired.

Fig. 4 illustrates a plan top view on the fixation device, in particular the locking mechanism according to an exemplary embodiment of the invention. The head of the screw 70 is surrounded by the sliding ring 76 and the sliding ring is surrounded by the locking ring 50.

Referring to FIG.5, the femur 101, otherwise known as the thigh bone, generally comprises an elongate shaft extending from the hip to the knee. The proximal end of the shaft 103 includes a head 105, a neck 107, a greater trochanter 108 and a lesser trochanter 109. Internal fixation of femoral fractures in general is one of the most common orthopedic surgical procedures. Fractures of the proximal portion of the femur (hip fractures) generally include femoral neck fractures and intertrochanteric fractures. Fractures of the femur which extend into the neck of the bone are often treated with screws that thread into the femoral head and extend generally parallel to the femoral neck axis A-A to a plate on the lateral side of the shaft 103.

Referring to Figure 5, a hip fracture device 121 includes a locking plate 1 11 and one of more (preferably three) screw assemblies 131.

The locking plate 111 generally conforms to the lateral portion of the proximal femur 101 and is attached to the femur by at least one cortical interlocking screw 115 passing through holes 113 in the subtrochanteric shaft region 103 of the femur. The interlocking screws 115 serve to attach the plate to the femur 101. The plate 111 also has one or more stepped bores 117 for each screw assembly 131. The major diameter of the stepped bore 117 incorporates a screw thread for fastening the screw assembly 131. The minor diameter of the stepped bore 117 creates a shoulder 119. Each stepped bore 117 is aligned with the axis of each of the screw assemblies 131.

The screw assemblies 131 incorporate a hip screw 133, a barrel 141, an end cap 151 and an optional spring pin 161. At least one screw assembly 131, in conjunction with the plate 1 1 1, provides angular stability in the indicated direction to counteract the moment created on the femoral neck 107 by the normal force F resulting from loads on the femoral head 105. Rotational stability about the head axis A-A is achieved if more than one screw assembly 131 is connected to the plate. Typically the hip screw assembles 131 are oriented parallel to the femoral neck axis A-A as shown.

Hip screw 133 is typically cannulated with a bore 137. Non-cannulated versions may have a blind bore 137 at the distal end. The screw 133 has a central shaft 133 defining a minor external diameter and an external flange 138 defining a major external diameter at the distal end of the screw. Formed internal to flange 138 are rotational features such as a hex socket 139. Threads 135, suitable for anchoring to bone, are formed at the proximal end of the screw 133 and engage the cancelleous bone of the femoral head 105.

Referring to FIG. 6, Barrel 141 is generally cylindrical in shape with an external diameter 143 corresponding to the minor diameter of the stepped bore 1 17 in plate 111 to allow a sliding fit with shoulder 119. Located at the distal end of barrel 141 is an external flange 149 that is a sliding fit with the major diameter of stepped bore 117 and engages shoulder 119 to prevent movement of the barrel 141 in the proximal direction along the screw assembly axis. The barrel 141 has a stepped bore 145 with major diameter 146 and minor diameter 147. The minor diameter 147 creates a shoulder 148. The minor diameter 147 is a sliding fit with central shaft 134 of the screw 133 and shoulder 148 engages external flange 138 to limit movement of the screw 133 in the proximal direction along the screw assembly axis.

End cap 151 has a head 152 in a distal portion with external machine threads on a major diameter 153 for fastening with the mating threads of the bore 117 of the plate 111. Formed internal to head 152 are rotational features such as a hex socket 159. The proximal region of the end cap 151 is a shaft 155 with a minor diameter 156 providing a slip fit with major diameter 146 of the barrel 141. The shaft 155 has a proximal end 158 which may abut the end of the flange 138 to limit movement of the screw 133 in the distal direction along the screw assembly axis. The end 158 has a blind bore 157.

A spring pin 161 is provided for load controlled dynamization. The spring pin 161 is typically a roll pin with a slot 167 (FIG. 7) that, when present, is press fit in bore 157 and is also a sliding interference fit with the bore 137 of the screw 133. The bore 157 is sized to firmly retain the spring pin. The bore 137 is sized to provide a controlled factional resistance to resist movement of the screw 133 in the distal direction along the screw assembly axis as will be further described in conjunction with FIGS. 11-14.

All the various diameters and bores of the screw assembly 131 are concentric about the axis of the assembly as depicted in FIG. 7, which does not show the end caps 151 or the hex socket 139. The various concentric sliding fits allow the screw 133 to move only along its axis, that is, parallel to the axis A-A.

Assembly of the device 121 proceeds as follows. First, the plate 111 is fixed at the proximal femur 101 at the lateral region of the shaft 1013, and the bone is prepared by drilling for the screw 133 and the barrel 141. The barrel 141 is then inserted into the bore 117 of the plate 111 until its final position where the flange 149 is seated against the shoulder 149 formed between the major and minor diameter of bore 1 17. The screw 133 is then inserted into the barrel 141 and turned into the bone until the screw flange 138 is seated against the barrel shoulder 148. By several additional turns of the compression screw 133 the fracture fragment that includes the femoral head 105 is pulled against the distal fracture surface of the femur 101 and the fracture is initially compressed.

By selecting from a kit of various configurations of end caps 151 and spring pins 161, the extent and force required for dynamization can be adjusted by the surgeon at this point in the operation. Should the surgeon desire static locking of the fragment in order to strictly limit travel and prevent shortening of the femoral neck, an end cap 151a with a longer shaft 156a is used to prevent distal motion of the screw 133 as shown in FIG. 8. FIG. 9 shows how caps 151 with various lengths of shaft 156 may be used to allow distance limited sliding of the screw 133. This sliding allows enhanced fragment opposition and postoperative dynamic fracture site compression by weight bearing while limiting excessive femoral neck shortening.

As, shown in FIG. 10, when spring pin 161 is added, the screw assembly 131 provides load controlled sliding of the screw 133. This sliding allows fragment opposition and postoperative dynamic fracture site compression by weight bearing while limiting the load on the fracture site, limiting the travel based on the load, and preventing stress induced resorption of the bone. The initial friction created by the spring pin 161 and the bore 137 can be varied by selecting from a kit of pins with varying spring rates and diameters according to the patients weight, bone structure and the type of fracture. Thus a heavier patient with larger bones may be fitted with a pin that creates more friction.

The control mechanism provides increasing resistance with increasing sliding distance. This is caused by the progressively greater length of the spring pin 161 engaged by the bore 137 during sliding as depicted in FIGS. 11-14. Screw sliding stops when either the resistance becomes equal to the body weight induced force or when the distance limit is reached.

When multiple screw assemblies 131 are used, the installation steps are repeated and the resistance may be varied by using the spring pins in some or all of the assemblies. Typically the distance limits are the same for all the assemblies.

Fig. 15 shows a bone plate 220 mounted on a femur 222. Compression screws 224 attach the bone plate 220 to the head 233 and neck 231 of femur 222. Screws 224 and 225 may be used to attach bone plate 220 to the femur via screw holes 226 in plate 220. Cortical screws 225 may be used to attach a distal portion 227 of bone plate 220 to the subtrochantric shaft of the femur 222. In the preferred embodiment these are locking screws. The compression screw 224 may provide angular and axial stability to the fractured bone pieces. The compression screws 224 may be cannulated or non-cannulated. The compression screws 224 may also provide rotational stability. Rotational stability may be achieved by inserting at least two compression screws 224 through the screw holes 226 and into the neck 231 of the femur 222. The compression screws 224 that are inserted in the neck 231 of the femur 222 may be parallel to the axis of the neck 231 of the femur 222. Cortical interlocking type screws 225 may be used in plate holes 229 in the subtrochantric shaft region of the femur 222. The cortical interlocking screws 225 may have threads (not seen in the figures) on the periphery of the head portion for engaging threads in hole 229. The cortical interlocking type screws 225 may be used to prevent the backout of the screws 225 and the bone plate 220. The compression screws 224 stabilize the neck fracture head fragment and thereby prevent the shortening of the femoral neck 31 resulting in improved postoperative function of the hip.

Figure 16 shows the screw hole 226 in the bone plate 220 with bone compression screw 224 and an end cap 228 inserted in the screw hole 226. The bone compression screw 224 may be a cannulated screw. However, non-cannulated screws may also be used. In a preferred embodiment, the screw hole 226 has a first threaded section 230 having a larger diameter and a second section 232 having a smaller diameter. A flat face 234 is formed at the junction of the first threaded section 230 and the second section 232. Threads (not seen in the figures) may be formed on all or portion of the inner periphery of the first threaded section 230. Inserting one bone compression screw 224 in the neck region of the femur 222 provides angular stability to the head 233 of the femur 222. Three or more bone compression screws 224 may be inserted in the neck region of the femur 222. Inserting more than one bone compression screw 224 provides rotational stability to the head 233 of the femur 222. An end cap 240 may be inserted in screw hole 226 on top of each compression screw 224. A polymer buffer 244 may be placed in the screw hole 226 between the end cap 240 and the head of the compression screw 224. The polymer buffer 44 may allow small movement of the compression screw 224.

In use, the bone compression screw 224 is inserted in the screw hole 226 and screwed into the neck 231 of the femur 222 until the underside of the bone compression screw 224 sits on the flat face 234 formed in screw hole 226. Next the compression screw 224 is rotated further to apply compression to the fracture site. Once desired amount of compression is applied, the end cap 240 is inserted in screw hole 226. End cap 240 has threads (not seen in the figures) on its periphery that mate with threads. End cap 240 is screwed into the screw hole 226 till its bottom is on top of the top surface of the head of the compression screw 224 that was previously installed in that screw hole 226. Thus the end cap 240 prevents the compression screw 224 from moving back in the axial direction. Optionally, the polymer buffer 244 may be placed over the compression screw 224 prior to installing the end cap 240. Cortical bone screw 225 are also installed in screw holes 229 and screwed into the subtrochantric shaft region of the femur 222. The screws 224 and 225 stabilize the bone fracture. The end cap 40 and the bone plate 220 also provide angular stability.

In another embodiment a compression screw 250 of a different head design is used with a split locking ring 252. Figure 17 shows the locking ring 252. Figure 18 shows a cross sectional view of the bone plate 220 with the locking ring 252 and the compression screw 250 installed therein. Figure 19 is a top view of a portion of a bone plate assembly showing the bone plate 220, the compression screw 250 and the locking ring 252. The locking ring 252 has a smooth circular outer surface 254 that fits in the screw hole 226. The inner surface 256 of the locking ring 252 has a saw blade like geometry. The saw blade geometry on the inner surface 256 is asymmetric. The compression screw 250 has a head 258 that has an outer peripheral surface 260 with a saw blade geometry that can mate with the saw blade geometry on the inner surface 256 of the locking ring 252. The top surface 262 of the screw head 258 has a hexagonal depression to allow engagement of a suitable screw driver. Other known shapes for the depression and corresponding screwdriver may also be used. In use, the compression screw 250 and the split locking ring 252 are assembled together and inserted into the screw hole 226. The assembly of the compression screw 250 and the locking ring 252 is then screwed into the bone using a dedicated insertion instrument that holds and rotates the compression screw 250 and the locking ring 252 simultaneously. When the head of the compression screw 250 reaches the terminal axial position in the screw hole 226, both the compression screw 250 and the locking ring 252 can be rotated further to apply compression to the fracture site. After the compression is applied, the compression screw 250 alone is turned. This makes the compression screw 250 rotate in relation to locking ring 252 which results in disengagement of saw blade geometry on the inner surface 256 of the locking ring 252 from the saw blade geometry on the outer peripheral surface 260. Since the saw blade geometries on both these surfaces are asymmetrical, the disengagement results in spreading of the locking ring 252. The locking ring 252 is thereby clamped between the head of compression screw 250 and the bone plate 220. This results in fixing the compression screw 250 in place such that the compression screw 250 can not back out in axial direction.

To remove the compression screw 250, compression screw 250 is rotated in the opposite direction. This results in the engagement of the saw blade geometries on the on the inner surface 256 of the locking ring 252 and the outer peripheral surface 260. Next the compression screw 250 and the locking ring 252 may be removed simultaneously using the dedicated instrument.

In yet another embodiment a compression screw 270 of a different design is used with a locking ring 272. Figures 20 and 21 show the bone plate 220, compression screw 270 and the locking ring 272 assembled together. The locking ring 272 has a threaded circular outer surface 274 that fits in the screw hole 226. The top wall 276 of the locking ring 272 projects towards the center of the screw hole 226 and has a hexagonal internal periphery. The bottom surface 278 of the top wall 276 has ridges 280. The compression screw 270 has a head 282 that has an outer peripheral surface 284 that slidably fits into the licking ring 272. The top surface 286 of the head of the compression screw 270 has depressions 287 that correspond to the ridges 280. Thus when the compression screw 270 is assembled in locking ring 272, the ridges 280 sit in the depressions 287. The top surface 286 of the screw head 282 also has a hexagonal depression to allow engagement of a suitable screw driver. Other known shapes for the depression and corresponding screwdriver may also be used. The external surface of the locking ring 272 may have threads (not seen in the figures) that engage threads in the screw hole 226.

In use, the compression screw 270 and the locking ring 272 are assembled together and inserted into the screw hole 226. The assembly of the compression screw 270 and the locking ring 272 is then screwed into the bone using a dedicated insertion instrument that holds and rotates the compression screw 270 and the locking ring 272 simultaneously. When the head of the compression screw 270 reaches the terminal axial position in the screw hole 226, the compression screw 270 can be rotated further to apply compression to the fracture site. When the compression screw 270 is rotated further the ridges 280 loose contact with the depressions 287. This forms, for example, a small gap of approximately 0.1 -0.4 millimeter between the compression screw 270 and the locking ring 272. As soon as the body weight is applied postoperatively, the femural head fracture fragment presses the compression screw 270 back to the lateral side until the movement is stopped by the locking ring 272.

To remove the compression screw 270, compression screw 270 is rotated in the opposite direction. This results in the engagement of the ridges 280 in the depressions 287. Next the compression screw 270 and the locking ring 272 may be removed simultaneously using the dedicated instrument.

It should be noted that the term 'comprising' does not exclude other elements or steps and the 'a' or 'an' does not exclude a plurality. Also elements described in association with the different embodiments may be combined.

It should be noted that the reference signs in the claims shall not be construed as limiting the scope of the claims.

Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the scope of the present invention.

Claims

Claims:

1. A fixation device, comprising a locking ring for locking a bone screw relative to a bone plate, wherein the locking ring has an outer circumference and an inner circumference; and a sliding ring having an outer circumference and an inner circumference, wherein the sliding ring is arranged in such a way that its outer circumference abuts the inner circumference of the locking ring, wherein the sliding ring is further adapted to receive the bone screw in its inner circumference; and wherein the locking ring and the sliding ring are adapted to immobilize the locking ring relative to a bone plate while enabling the bone screw to be slid along a longitudinal axis therefrom.

2. The fixation device according claim 1, wherein the outer circumference and/or the inner circumference of the locking ring is non-circular.

3. The fixation device according to claim 1 or 2, wherein the outer circumference and/or the inner circumference of the sliding ring is non-circular.

4. The fixation device according to any one of the claims 1 to 3, wherein the locking ring comprises slits, which are preferably adapted to allow a cross-section of the locking ring.

5. The fixation device according to any one of the claims 1 to 4, wherein the sliding ring is substantially incompressible.

6. The fixation device according to any one of the claims 1 to 5, wherein the locking of the bone screw is substantially in respect to a tilting movement.

7. A fixation system, comprising: a fixation device according any one of the claims 1 to 6; a bone screw; and a bone plate, wherein the bone plate comprises a hole adapted to engage with the outer circumference of the locking ring; wherein the screw is adapted to engage with the inner circumference of the sliding ring.

8. The fixation system according claim 7, wherein the hole of the bone plate has a non-circular shape.

9. The fixation system according claim 7 or 8, further comprising a distance clip, wherein the distance clip is adapted to form a stop for a longitudinal movement of the screw.

10. The fixation system according claim 9, wherein the distance clip is attached to a circular groove of the screw.

11. A fixation method, comprising inserting a device according any one of the claims 1 to 6 into a hole of a bone plate; immobilize the locking ring by move the locking ring in such a way that the sliding ring forms a counter bearing.

12. The fixation method according claim 11 , wherein the moving of the locking ring is a turning or rotating of the locking ring.

13. A bone plating system comprising; a bone plate having one or a plurality of openings for receiving a bone screw through the opening and into a bone, the bone screw having a head adapted for fitting in the opening when the bone screw is fully inserted in the bone, the head of the bone screw and the opening in the bone plate having complementary shape such that the bone screw when seated in the opening has angular stability and an end cap being threadably insertable in the opening and having a layer of polymeric material interposed between the end cap and the top of the head such that the compression of the polymeric material would allow slight axial movement of the screw.

14. A bone screw locking system comprising: a bone plate having one or a plurality of openings for receiving a bone screw through the opening and into a bone, the bone screw having a head, the periphery of the head having an asymmetric saw blade geometry that matches the geometry of an inner periphery of a locking ring, the locking ring and the bone screw being assembled together and being insertable in the bone simultaneously using a dedicated instrument, and wherein compression can be applied to a bone fracture by turning the bone screw and the locking ring after they have reached a terminal position and wherein the locking ring is spread by turning the screw clockwise relative to the locking ring thereby clamping the locking ring between the screw head and the plate.

15. A bone screw locking system comprising: a bone plate having one or a plurality of openings for receiving a bone screw through the opening and into a bone, the bone screw having a head, the head having a top surface with depressions formed thereon, a locking ring adapted to attach to the head and having ridges that have shape complimentary to the depressions and fit in the depressions when the locking ring is attached to the head, the locking ring and the bone screw being assembled together and being insertable in the bone simultaneously using a dedicated instrument, and wherein compression can be applied to a bone fracture by turning the bone screw alone after the locking ring has reached its final axial position thereby creating a space between the locking ring and the head, wherein the screw head is pressed through the gap upon application of body weight and the bone screw head is stopped upon contacting the end cap.