Elastic rod having different degrees of stiffness for the surgical treatment of the spine

Abstract

For a differentiated treatment of individual motion segments of the spine, a plastic rod (41, 42, 46) for the dynamic stabilization of the spine has different degrees of stiffness in its longitudinal direction. The different rod segments are interconnected along an oblique plane (13).

Claims

1. A plastic connecting rod assembly for a surgical treatment of a spine, comprising a clamp having two opposed clamping ridges, and a plastic rod having at least two rod segments having different properties, wherein the at least two rod segments are interconnected at respective planar end faces thereof to form a connection, wherein said planar end faces lie along an oblique plane which is oblique with respect to a longitudinal axis of the plastic rod, wherein said two opposed clamping ridges are arranged in a clamping plane to engage opposite sides of the plastic rod; wherein an obliquity angle of said oblique plane is large enough so that at least one end of said oblique plane lies outside the clamping plane, the clamping plane being defined perpendicular to said longitudinal axis, wherein the oblique plane forms an angle in the range 45 to 85 with the longitudinal axis of the plastic rod.

2. The plastic connecting rod assembly according to claim 1, wherein the connection is welded.

3. The plastic connecting rod assembly according to claim 2, wherein the connection is vibration-welded.

4. The plastic connecting rod assembly according to claim 1, wherein the connection is bonded.

5. The plastic connecting rod assembly according to claim 1, wherein the connection is welded and bonded.

6. The plastic connecting rod assembly according to claim 1, wherein the at least two rod segments of the plastic rod are limited to two.

7. The plastic connecting rod assembly according to claim 1, wherein the plastic rod is made of a polycarbonate urethane.

8. The plastic connecting rod assembly according to claim 1, wherein the at least two rod segments are rehydrated to saturation after their connection.

9. The plastic connecting rod assembly according to claim 1, wherein the at least two rod segments have different stiffness properties, respectively.

10. The plastic connecting rod assembly according to claim 1, wherein the plastic rod has two ends and an outer surface defined between the two ends, and the at least two rod segments are interconnected at a line defined in the outer surface, and said line defines said oblique plane.

11. A method for producing a plastic connecting rod assembly for a surgical treatment of a spine, including a clamp having two opposed clamping ridges, and a plastic rod having at least two rod segments having different properties, the method comprising: interconnecting the at least two rod segments at respective planar end faces thereof to form a connection, wherein said planar end faces lie along an oblique plane which is oblique with respect to a longitudinal axis of the plastic rod, arranging said two opposed clamping ridges in a clamping plane to engage opposite sides of the plastic rod; providing an obliquity angle of said oblique plane that is large enough so that at least one end of said oblique plane lies outside the clamping plane, the clamping plane being defined perpendicular to said longitudinal axis, wherein the oblique plane forms an angle in the range 45 to 85 with the longitudinal axis of the plastic rod.

12. The method of claim 11, wherein the at least two rod segments are dried prior to being connected along the oblique plane and the plastic rod is then rehydrated.

13. The method of claim 11, wherein the at least two rod segments are rehydrated to saturation after their connection.

14. The method of claim 11, wherein the connection is welded.

15. The method of claim 14, wherein the connection is vibration-welded.

16. The method of claim 11, wherein the connection is bonded.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is explained in more detail hereinafter by way of preferred examples with reference to drawings which merely illustrate exemplary embodiments and in particular do not show the maximum number of different consecutive degrees of stiffness.

(2) The Figures schematically show:

(3) FIG. 1a: a point-symmetrical rod having sections of different degrees of stiffness connected via an oblique plane;

(4) FIG. 1b: a rod having an arbitrary cross-section (e.g. with two plane-parallel sides) and sections of different degrees of stiffness connected via an oblique plane;

(5) FIGS. 2a and 2b: the reaction forces for a rod connected by a butt-joint;

(6) FIGS. 3a and 3b: the reaction forces for an oblique connection according to the invention and the angle of the connecting plane;

(7) FIG. 4: clamping ridges on a butt joint; and

(8) FIG. 5: clamping ridges on an oblique connection.

DESCRIPTION OF EMBODIMENTS

(9) FIG. 1a shows a left 1 and a right 2 segment of a rod 41 having a point-symmetrical cross-section 4 and the oblique weld seam 3 connecting them.

(10) FIG. 1b shows a left 1 and a right 2 segment of a rod 42 having an arbitrary cross-section 5, e.g. with two plane-parallel sides, and the oblique weld seam 3 connecting them.

(11) FIGS. 2a and 2b show an upper 6 and a lower 7 segment of a rod 44 connected by a butt joint 8 according to the prior art. In FIG. 2b, arrow 9 represents the external force and arrow 10 the reaction force in a sectional view. Here the reaction force exclusively consists of a normal force according to arrow 10.

(12) In contrast, FIGS. 3a and 3b show an upper 11 and a lower 12 segment of rod 46 interconnected by an oblique joint 13. In FIG. 3b, arrow 14 represents the external force and arrow 15 the reaction forces in a sectional view. Here the reaction forces consist of a normal force 15a and of a shear force 15b. The angle denotes the angle between the plane of the joint 13 and the longitudinal axis 47 of rod 46, or, as illustrated, the side face of rod 46.

(13) FIG. 4 shows an upper 6 and a lower 7 segment of a rod 46 connected by a butt joint 8. If clamping ridges 18 apply on the connecting plane, they will apply a load to the weld seam of joint 8 from both sides.

(14) FIG. 5 shows an upper 11 and a lower 12 segment of an obliquely connected rod. Clamping ridges 18 apply on one side of the weld seam 13 at the most.

(15) Consequently, the disclosed invention offers multiple advantages, as explained below.

(16) An oblique connection 13 with an angle of inclination between 5 and 85, preferably 45, always represents a larger surface than a connection that is perpendicular to the rod axis 47. In this manner, tensions in the connection are reduced. Furthermore, shear stresses result, which are generally more suitable for connections than normal stresses i.e. stress perpendicular to the connecting plane.

(17) If a clamping device comprises ridges 18 located in one and the same clamping plane and if these ridges apply in a connecting plane 8 that is orthogonal to the rod axis, they will penetrate into the connecting plane from both clamping sides. In the case of a bonded joint or a weld seam, a critical splitting force may result.

(18) If a clamping device comprises ridges 18 located in one and the same clamping plane and if these ridges apply to an oblique connecting plane 13, only one ridge 18 may apply a load to the connection on one clamping side, and no force is exerted on other sections of the connection plane 13. Moreover, the clamping does not act in the oblique connecting plane and thus has no direct splitting or cleaving effect either.

(19) This connection method allows connecting rod segments with large differences in longitudinal or transversal stiffness. To this end, the rod segments have to be dry. In order to allow the subsequent clinical application of rod segments of high stiffness, these have to be rehydrated after their connection and delivered in a preferably saturated condition.

(20) The description of the examples enables the one skilled in the art to perceive modifications and alternations without leaving the scope of protection defined by the claims.