(51) Int.Cl.6:

Description

The invention relates to a fixation system for bones, as disclosed in the preamble of claim 1.

Such fixation systems are used in osteosynthesis. In this case the bone screws are connected to the fragments; and the bone plates span the fracture. Therefore, it is desirable to insert the bone screw at different angles into the bone plate, while simultaneously adapting to the nature of the bone fragment to be connected. To this end, the bone screws in a fixation system of the prior art have heads with a somewhat hemispherically shaped seat surface, of which one seat surface in passage holes is allocated to the bone plate. If, for example, in the case of a simple fracture of the tibia the two bone fragments have to be connected to one another, the metallic bone plate is placed on the aligned bone fragments. Thereafter the screws are screwed into the bones in such a way that the bearing surfaces of the screw heads and of the plate holes come to rest on one another, and the plate is pressed against the bone. The result is that a secure connection is formed between the bone fragments, the bone plate and the bone screws.

It has been demonstrated that the connection between the bone screws and the bone plates can become loose. One of the reasons lies in the insufficient stability of the angular connection between the bone screw and the bone plate, because this connection is secured only by the friction forces between the screw head and the plate hole. Conversely, an angularly stable connection between the bone screw and the bone plate leads to a gain in the stability of the whole assembly. There exist a plethora of solutions to achieve this type of stable connection.

According to the EP 0 201 024 A1, this stable connection may be achieved, for example, by allocating a pressure plate to the bone plate. This pressure plate can be tensioned with the screw heads and securely holds said screw heads in a selected angular position. Owing to their relatively large volume, such complicated fixation systems with a pressure plate are restricted in their usability.

The WO 89/041 50 A1 proposes another solution, according to which the screw head is widened with a spreading screw in a slotted region and is, thereby, pressed into the plate hole. In this case the screw head or the insert, surrounding said screw head, as well as the plate hole have spherical seat surfaces, which permit an alignment at various angles. Therefore, this fixation system is also complicated in its manufacture and application.

Finally, it is also known to provide the screw head with an outer thread and the plate hole with an inner thread. If then the screw is screwed into the bone, the result is, owing to the threaded joint, an angularly stable alignment of the plate and the screw. However, this solution has the serious drawback that the screw may not be screwed into the plate hole at any angle, but rather only in the alignment predetermined by the thread axes.

The invention is based on the problem of providing a fixation system of the type, described in the introductory part, with selectable and fixable angles between the bone plate and the bone screw, so that said fixation system requires less space and is less complicated.

This problem is solved by means of a fixations system, as disclosed in claim 1. Advantageous embodiments of the system are disclosed in the dependent claims.

In a fixation system, according to the invention, the seat surfaces of the bone plate and the bone screw are configured in such a way that they permit a mutual alignment at various angular positions. To this end the bone screw may have a spherical seat surface at one screw head; and the bone plate may have a cylindrical, conical or spherical seat surface in a passage hole. Furthermore, the seat surfaces are provided with means for fixing the bone screw and the bone plate at a specific angular orientation. Said bone screw and bone plate exhibit a preformed thread on at least one of the seat surfaces. When the bone screw is screwed into the bone at a specific angle, the at least one thread forms a threaded joint of the seat surfaces. This threaded joint secures the screw at its screwing angle on the plate. The threaded joint may be the result of a deformation of the material and may be secured by a force fit (friction fit) and/or a material fit (friction weld) between the seat surfaces. At the same time the material deformation may be due to the adaptation of the preformed thread of a seat surface to its contact surfaces of the other seat surface, said contact surfaces being defined by the screw angle, and vice versa. The friction fit and/or material fit connection may be the result of a material deformation. Instead of a plate, other surgical/orthopedic connection carriers of various shapes may also be taken into consideration.

The pitch of the preformed thread may be slightly smaller than the pitch of the bone thread of the screw, in order to achieve a pressing force of the plate against the bone surface.

In a preferred embodiment, the preformed thread has threaded segments, which are spaced apart from each other; and the spacing areas of the thread segments are conducive to the penetration of a mating thread at various screwing angles. The threaded segments have an inlet area having a thread profile that converges in the direction of the inlet end. Analogous to a sled runner, these segments, which are designed in such a manner, facilitate the accommodation and/or threading of a mating thread. The number of threaded segments per turn may be selected so as to be different and may vary in a thread. Preferably each turn of the preformed thread has 2 to 4 threaded segments. The threaded segments in specific peripheral areas of the preformed thread may be arranged in groups, especially for production reasons.

Both the seat surface of the bone plate and the seat surface of the bone screw may have a preformed thread composed of threaded segments that are spaced apart from each other. The penetration of the bone screw into the bone plate may result in the segments of the various threads locking together so that a secondary loosening and unscrewing would be difficult.

Instead of the above configuration, however, a seat surface may also be provided with threaded segments; and the other seat surface may be provided with a preformed continuous thread, a feature that is especially advantageous for production purposes. In this case the threaded segments may be configured on the screw or the plate; and the continuous thread may be configured on the plate or the screw.

However, a preformed thread may be present only in one of the seat surfaces, which is made of a harder material than the other seat surface. Then the screwing in of the screw allows a thread to be formed in the plate seat surface as a result of deformation processes and leads to a threaded seat, which is secured by a friction fit and/or material fit. The preformed thread is formed preferably on the screw and may be a continuous thread. Therefore, the screwing process is facilitated through the use of suitable materials for the plate and the screw. In the case of stainless steel, the thread-bearing seat surface of a screw may be hardened. An additional solution consists of the seat surface of the screw being made (in the case of titanium) of a hard titanium alloy and the plate being made of a softer pure titanium. At the same time the thread on the screw has to be deformed in such a way that the screw cutting / deformation process is simplified.

The use of a multiple threaded thread makes it possible, especially in the case of preformed threads with threaded segments, to achieve greater variability of the angular orientation.

The seat surface of the bone screw is formed preferably on the jacket of a screw head. In order to prevent the screw head from continuing to pass through the bone plate, said screw head may have a stop in order to rest against a counter-stop of the bone plate. In contrast, the bone plate has formed its seat surface preferably in a passage hole.

Against this background, the invention has, above all, the following advantages over the prior art solutions of an angularly stable connection to a fixation system for bones.

- The system comprises only two components - that is, the bone plate and the bone screw.

- Other components, which, for example, apply or make the assembly complicated, are avoided.

- The securing of the screw at a specific angle on the plate exhibits high reliability. The risk of detaching from the spherical seat surface is avoided.

- The surgical technology is altered by the solution of the invention, because the angle between the screw and the plate is determined simultaneously with the screwing into the bone. The tightening of additional parts of angularly stable connections of the art is dispensed with. The use of torque wrenches must be taken into consideration, in order to avoid a destruction of the thread of the connection as a result of stripping. Torque wrenches are already used in the implant systems of the art.

Other details and advantages of the invention are disclosed in the following description of the attached drawings, depicting preferred embodiments.

Figure 1 is a partial sectional view of a bone screw prior to being connected to the various bone plates a-c.

Figure 2 is a partial sectional view of a bone screw with a stop.

Figure 3 is a partial sectional view of a bone screw with a continuous multiple threaded thread.

Figure 4 is a partial sectional view of a bone screw with a two threaded, interrupted thread.

Figure 5 is a partial sectional view of a bone screw with a hardened self-forming thread.

Figure 6 is a top view of a section of a bone plate with a continuous inner thread.

Figure 7 is a top view of a section of a bone plate with an inner thread that is interrupted four times.

Figure 8 is a top view of a section of a bone plate with an inner thread that is interrupted three times.

In a fixation system, according to Figure 1, the shank 2 of a bone screw 1 bears a screw head 3, which has an essentially spherical seat surface 4 on the underside and a flattening 5 on the top side. An outer thread 6 is preformed in the seat surface 4. The outer thread 6 has a pitch that is slightly less than the pitch of a bone thread on the broken off portion of the shank 2. The flattening 5 exhibits a receptacle 7 for a shape engaging counter-part of a screwing tool.

This bone screw 1 may be assigned to a variety of bone plates 8, which are shown in the sections a-c of Figure 1. The bone plates 8 share the common feature that they have a passage hole 9 for the passage of the shank 2 and for receiving the head 3 of the screw 1. All of the passage holes are provided with an inner thread 10. The inner threads 10 are formed on the seat surfaces 11, which are, according to the sectional view a, cylindrical; according to the sectional view b, conical; and, according to the sectional view c, spherical.

It is possible to screw the bone screw 1 into one of the bone plates 8 at various angular orientations. In situations, in which the screw axis is inclined in the direction of the hole axis, there is a deformation of the material in the area of the threads 6, 10, as a result of which the screw 1 and the plate 8 are connected by a friction fit and/or friction weld. At the same time the plate 8 rests against the bones to be treated.

The only difference between the bone screw 1, according to Figure 2, and the above described screw is that the screw head 3, adjacent to the top-sided flattening 5, exhibits a laterally protruding stop 12. The stop 12 is supposed to prevent the screw 1 from passing through the passage hole of a plate 8 in that said screw rests with the stop at its top side.

The major distinction between the bone screw 1, according to Figure 3, and the bone screw in Figure 1 lies in a multiple threaded thread 6 on the spherical seat surface 4. As in the case of the above described bone screws 1, the thread turns 6 are designed so as to be continuous.

In contrast, in the variant of a bone screw 1, according to Figure 4, the screw head 3 exhibits a two-threaded thread 6 on a spherical seat surface 4, where each turn of the thread consists of threaded segments 13, which are spaced apart from each other. Each threaded segment 13 is configured in the shape of a lens - that is, has a profile, which converges in the direction of its inlet end 14 and its outlet end 15. In the case of this screw 1 the threaded segments 13 have a constant spacing, but are arranged at different peripheral positions in the various turns of the thread.

In the bone screw, according to Figure 5, the spherical seat surface 4 of the screw head 3 bears the threaded segments 13 of an outer thread 6. In this case the threaded segments are arranged in groups 15 one above the other in the various turns of the thread. Therefore, meridional, thread-free spacing areas 16 are formed between the groups 15. This screw 1 is a thread former, the head 3 of which is hardened and which can be screwed into a thread-free passage hole of a bone plate 8. Thus, the spacing areas 16 may serve as flutes.

Figure 6 shows a bone plate 8, the passage hole 9 of which exhibits a continuous inner thread 10. This plate 8 matches the plate, depicted in the sectional view a in Figure 1.

The passage hole 9 of the bone plate 8 in Figure 7 has an inner thread 10 composed of threaded segments 17, which are spaced apart from each other. Each turn of the thread exhibits four threaded segments 17, the profiles of which converge in the direction of the two ends 18, 19.

The sole distinction between the bone plate 8 in Figure 8 and the one described above lies in the provision of only three threaded segments 17 per turn of the thread.

The bone plate 8, according to Figure 6, is combined preferably with a bone screw 1, according to Figure 4. The continuous inner thread 10 of the bone plate 8 may pass into the spacing areas of the threaded segments 13 of the bone screw 1 at various screwing angles.

The bone plates 8 of Figures 7 and 8 may be used, for example with the bone screws 1, according to Figure 3 or 4. The spacing areas between the threaded segments 17 allow the passage of both a continuous and segmented screw thread 6.

Patent Claims

1. Fixation system, which is intended for bones and comprises a bone plate (8) having at least one passage hole (9); at least one bone screw (1), which is inserted into a passage hole; seat surfaces (4, 11), which permit a mutual alignment of the bone plate (8) and the bone screw (1) at various angles; and means for fixing the bone screw at a specific angle to the bone plate, characterized in that the fixation means exhibit a threaded joint, which is formed between the seat surfaces of the bone plate (8) and the bone screw (1) by a preformed thread (6, 10) on at least one seat surface (4, 11) and by screwing the bone screw (1) at a specific angle.

2. System, as claimed in claim 1, characterized in that the pitch of the preformed thread (6, 10) is slightly smaller than the pitch of the bone thread of the bone screw (1).

3. System, as claimed in claim 1 or 2, characterized in that the preformed thread (6, 10) has threaded segments (13, 17), which are spaced apart from each other.

4. System, as claimed in claim 3, characterized in that the threaded segments (13, 17) have an inlet area exhibiting a thread profile 1 that converges in the direction of the inlet end (14, 19).

5. System, as claimed in claim 3 or 4, characterized in that each turn of the preformed thread (6, 10) has two to four threaded segments (13, 17).

6. System, as claimed in any one of the claims 3 to 5, characterized in that the threaded segments (13, 17) in specific peripheral areas of the preformed thread (6, 10) are arranged in groups (15).

7. System, as claimed in any one of the claims 3 to 6, characterized in that both the seat surface (11) of the bone plate (8) and the seat surface (4) of the bone screw (1) have a preformed thread (6, 10) composed of threaded segments (13, 17) that are spaced apart from each other.

8. System, as claimed in any one of the claims 3 to 7, characterized in that the seat surface (4) of the bone screw (1) has a preformed thread (6) composed of threaded segments (13), which are spaced apart from each other; and the seat surface (11) of the bone plate (8) has a preformed continuous thread (10).

9. System, as claimed in any one of the claims 3 to 7, characterized in that the seat surface (11) of the bone plate (8) has a preformed thread (10) composed of threaded segments (17), which are spaced apart from each other; and the seat surface (4) of the bone screw (1) has a continuous preformed thread (6).

10. System, as claimed in any one of the claims 1 to 6, characterized in that only one of the seat surfaces (4) has a preformed thread (6) and is made of a harder material than the other seat surface.

11. System, as claimed in claim 10, characterized in that the preformed thread of the one seat surface (4) is a continuous thread (6).

12. System, as claimed in claim 10 or 11, characterized in that the bone screw (1) has the one thread (6).

13. System, as claimed in any one of the claims 1 to 12, characterized in that the preformed thread is a multiple threaded thread (6).

14. System, as claimed in any one of the claims 1 to 13, characterized in that the seat surface (4) of the bone screw (1) is spherical.

15. System, as claimed in any one of the claims 1 to 14, characterized in that the seat surface (4) of the bone screw (1) is designed on the underside of a screw head (3).

16. System, as claimed in any one of the claims 1 to 15, characterized in that the screw head (3) has a stop (12) for resting against a counter-stop of the bone plate (8).

17. System, as claimed in any one of the claims 1 to 16, characterized in that the seat surface (11) of the bone plate (8) is cylindrical, conical or spherical.

18. System, as claimed in any one of the claims 1 to 17, characterized in that the seat surface (11) of the bone plate (8) is formed in a passage hole (9).

-------------------------------

4 sheet(s) of drawings

-------------------------------

Drawings Sheet 1 Number: DE 43 43 117 C2

Int. Cl.⁶: A 61 B 17/58

Publication date: November 4, 1999