The invention relates to an endoprosthesis for replacing a joint, such as a shoulder joint. Such an endoprosthesis includes a base part in which an articulation cavity, tapering towards the bottom, is provided with an axially symmetrical lateral surface. The rotation axis of this lateral surface is positioned in the direction of the joint neck. In the articulation cavity an articulation body is arranged, which interacts at some contact points with the lateral surface by an articulation surface axially symmetrical to a pivoting axis. The pivoting axis of this articulation cuts the rotation axis. Accordingly the articulation body is hinged to the base part rotatable around the rotation axis of the lateral surface and around the pivoting axis. Therefore, by the articulation body supporting a first articulation part, the first articulation part is alignable with respect to its position of inclination and rotation relative to the base part and, in the chosen alignment, fixable within the articulation cavity by a locking means. The artificial first articulation part is provided for the articulation with a natural or with an artificial second articulation part.
An endoprosthesis for a shoulder joint is known from DE-299 18 589 U1, in which an articulation cavity is formed in a base part. The articulation cavity is formed as a cone. A rotating piece is arranged in this articulation cavity. The rotating piece is rotatable around the axis of the cone and connectable to the base part in any rotation position by means of a cone deadlock. A directional piece is hinged to the rotating piece. The directional piece supports a cap and is pivotable with respect to the rotating piece around a cylinder axis cutting the collar axis perpendicularly. This endoprosthesis enables the directional piece to be articulated at the base part not on spherical surfaces but on the surfaces of axial rotation bodies. By that it is possible to adjust the rotation position and the inclination position of the cap independently from each other. The endoprosthesis also includes a locking means for the fixing of the articulation body in the articulation cavity.
The international patent application WO02/39933 (PCT/CH01/00676) describes an endoprosthesis for a shoulder joint in which an axially symmetrical articulation space is formed within the head section of the base part. In this articulation space an axial articulation body is articulated. The axis of the articulation body stands perpendicular to the rotation axis of the articulation space. In an example the articulation space has a conical base. This base serves as a clamping surface, co-operating with an axially symmetrical articulation surface of the articulation body. In order to obtain a good interlock of the articulation surface to the clamping surface it is proposed to form these surfaces by a number of edges. In this endoprosthesis it is achieved that, with easy-to-build axially symmetrical surfaces, both the inclination and the rotation position of a cap connected to the articulation body can be obtained. As compared to the subject of DE-299 18 589 U1, the subject of WO02/39933 does not need a rotation piece. The embodiments of WO02/39933 show a spectrum of possible variations, which at least partly are applicable also to this invention. The content of WO02/39933 is hereby incorporated by reference in its entirety.
In the embodiments described in WO02/39933 the articulation body shows a linear arrangement of the contact points between the articulation body and the base part. For this reason when the locking means is loose the articulation body, under the influence of a respective force, tends to roll with the articulation surface on the clamping surface.
An endoprosthesis is disclosed for the replacement of a joint, in particular a shoulder joint, in which an axial articulation body is fixable within an axially symmetrical articulation cavity in any position desired in terms of inclination and rotation and in which an unintentional rolling movement between the articulation body and the articulation cavity is prevented. In an advantageous exemplary embodiment, a clamping action is provided between the articulation body and the articulation cavity.
The figures illustrate exemplary embodiments of shoulder prostheses, wherein:
In an exemplary embodiment, an articulation surface interacts with the lateral surface at at least three contact points separated one from the other. The articulation surface includes at least one axially symmetrical circular edge around the pivoting axis, which interacts with the lateral surface in two contact points distant one from the other. Both contact points of the circular edge are arranged, with respect to a plane lodging the pivoting axis and the rotation axis, in such a way that a contact point lies on one side and the other contact point lies on the other side of this plane. A geometrically linear or punctual contact is impossible in practice. Physically spoken contact points will always be present as surface areas. Therefore, linear edges are also considered as articulation surfaces.
The radius of the circular edge is larger than the radius of a vertex circle of a cutting curve between a conical sheath surface and a plane perpendicular to the pivoting axis cutting the contact points. The cutting curve is an hyperbole. The conical sheath surface is created through the rotation of a straight line around the rotation axis. This straight line cuts the rotation axis and touches a contact point. This straight line belongs to a tangential plane defined at the contact point by a first and a second tangent. The first tangent lies in the plane perpendicular to the pivoting axis and runs tangent to the circular edge at the contact point. The second tangent lies in the plane perpendicular to the rotation axis and runs tangent to the lateral surface at the contact point.
In an advantageous exemplary embodiment, the articulation surface includes, or alternately consists of, at least two axially symmetrical circular edges round the pivoting axis, at least one of which shares two contact points, distant one from the other, with the lateral surface. One of these contact points is lying on one side of a plane lodging the pivoting axis and the rotation axis, whereas the other contact point is lying at the other side of this plane.
The lateral surface can be preferably formed conically. Therefore, it can have a conical shape with straight lateral surface lines, a trumpet shape with lateral surface lines convex towards the rotation axis or an egg holder shape with lateral surface lines concave towards the rotation axis. In case of a conical shape, the circular edge, at the contact points between the circular edge and the lateral surface, can have a radius, with respect to the pivoting axis, that is larger than the vertex radius of an cutting curve between the lateral surface and a plane perpendicular to the pivoting axis cutting the contact points. In both cases with bent lateral surface lines the radius of the circular edge can be formed larger than the vertex radius of a cutting curve between the plane perpendicular to the pivoting axis cutting the contact points and a conical sheath surface touching the lateral surface. This conical sheath surface is created by the rotation round the rotation axis of a tangent to the conical sheath line at the contact point. By that it is achieved that the articulation body does not co-operate only linearly with a lateral surface but has at least a three-point support. This arrangement, through the appropriate choice of the lateral surface and of the articulation surface, allows a clamping action to be achieved between the articulation cavity and the articulation body.
If two or more circular edges meet the above mentioned criteria, two or more contact points lie on one side and two or more contact points lie on the other side of a plane lodging the rotation axis and the pivoting axis. By this a quadrangular arrangement of the contact points is obtained. This non-linear arrangement of the contact points can prevent, together with the three-point support, the rolling of the articulation body on the lateral surface. The radiuses of the circular edges can be different. In circular edges with the same radius the pivoting axis usually cuts the rotation axis perpendicularly.
In an exemplary embodiment, both circular edges distanced one from the other do not lie on a plane parallel to the rotation axis. The articulation body can, to a limited degree, also spin transversally to the pivoting axis. This results in a certain lateral displacement (offset) of the articulation body with respect to the rotation axis. This offset can be exploited in the orientation of the articulation body, for example to optimize the position of the axis of the joint neck and of the articulation part, respectively, placed on the articulation body in a shoulder joint.
The lateral surface of the articulation cavity can be structured in the shape of a trumpet, of an egg holder or of a cone. The lateral surface can be formed by a surface or by one or more ring edges. The ring edge can be annular or threaded. If the lateral surface has at least one ring edge, the circular edge at the contact points between the circular edge and the ring edge preferably has a radius, with respect to the pivoting axis, which is larger than the vertex radius of a cutting curve between the plane perpendicular to the pivoting axis cutting the contact points of a circular edge and a conical sheath surface. For the construction of this conical sheath surface a plane including the tangent to the ring edge and the tangent to the circular edge at the contact point can be cut through with a rotation axis. A straight line through the cut point obtained on the rotation axis and the contact point can be made to rotate around the rotation axis. The resulting conical sheath surface is now cut at the contact points by a plane perpendicular to the pivoting axis. If the vertex radius of this cutting curve is smaller than the radius of the circular edge, the circular edge adheres to the ring edge in two points.
In the case of a ring edge no offset occurs when the articulation body is pivoted transversally to the pivoting axis. This can be exploited by combining a conical lateral surface with a ring edge to exclude the possibility of an offset. With a lateral surface combined in this way, the articulation body can be configured to lie on all possible contact points only when the pivoting axis stands in a preset angle, usually perpendicularly to the rotation axis. By tightening the locking means the articulation body is brought into this position.
In an advantageous embodiment the angle between the actual lateral surface or the constructed conical sheath surface and the rotation axis at the contact points lies in an area from 0 to 30°, preferably between 2 and 20°, and especially preferred between 5 and 15°. By this a clamping action is obtained between the articulation surface and the lateral surface. With an angle between 0 and 5 degrees the clamping action is at its highest, while within this range of the angle the plunging depth has greater tolerances. With angles greater than 15 degrees the clamping action is reduced while the plunging depth can be determined more precisely in advance. Larger angles of less than 90 degrees are also possible if the clamping action is given up. The clamping action enables the articulation body to be provisionally fixed in the articulation cavity by a light pressure. By this the position of the collar axis can be controlled before this position is fixed by tightening the locking means and therefore, before the articulation surface or the lateral surface is deformed.
Thanks to the relatively high pressure forces generated at the contact points when tightening the locking means, at least one articulation surface and lateral surface are deformed. Depending on the material pairing it can be obtained that the articulation surface digs into the lateral surface at the contact points or that the articulation surface, i.e. the circular edges of the articulation body is deformed. In the last case the lateral surface flattens the circular edges at the contact points. Thanks to these deformations at the contact points, the pivoting and the rotation of the articulation body around the pivoting axis and around the rotation axis, respectively, are safely prevented. The hardness of the material used for the articulation body, the locking means and the articulation cavity can therefore be chosen according to the intended deformation. Also when these parts can be produced with the same material or with a material of the same hardness, a different pairing can be advantageous. It is particularly advantageous to have the articulation body made of a material that is softer than the articulation cavity and the locking means.
Another possibility to increase the clamping stability is the structuring of the circular edges and/or lateral surface with protruding and sharp edges or points, with notches, grooves or grid relieves. In this case it is advantageous to match the structuring of both surfaces with one another so that they bite during tight clamping.
The articulation body can have a central borehole with an axially symmetrical base. The rotation center or the rotation axis of the base can coincide with the pivoting axis. At the base of the borehole in the articulation body an opening is provided through which a locking means runs. The opening is especially an oblong hole with oblong extension perpendicular to the pivoting axis. The locking means can, for example, be a screw connecting the articulation body to the base part. Other locking means are also possible such as expansion bolts or rivets. The base can be cylindrical, nipple shaped or spindle shaped. A spherical base is preferred due to manufacturing, technical and mechanical reasons. This axially symmetrical structure of the base allows the tightening of a screw head with a circular edge against the base. In this way the circular edge digs into the base. If the base is cylindrical with a cylinder axis coinciding with the pivoting axis of the articulation body the screw head digs on two points. These points lie on a plane transversal to the pivoting axis and at a distance from the pivoting axis. In this way a pivoting movement around the pivoting axis or the articulation body can be counteracted.
For additional safety of the clamping, adequate tools can be used such as pressing lids, counter screws or nuts, plastic or elastic deformable inserts between screw head and base or in the screw threads of the union nut.
Within certain limits the described geometry also allows the swiveling of the pivoting axis around a third axis normal to the pivoting axis and the rotation axis. A limit to this swiveling is created by the angle of the lateral surface at the contact points. As soon as the cutting curve between the plane normal to the pivoting axis through the contact points and the above defined conical sheath surface creates a parabola, a safe clamping between the articulation body and the articulation cavity can no longer be obtained. However, as long as the cutting curve is an hyperbole the articulation body can be safely fixed in the articulation cavity. In these circumstances, as described above, an offset is obtained. For this offset to be exploited, the opening must have a diameter greater than the diameter of the locking screw.
In place of or in addition to a securing through a borehole in the articulation body also other locking media can be used such as screw lids, pressing plates, guiding nuts and the like, as described in the examples of WO02/39933. The securing at the base of a borehole in the articulation body has the advantage that the articulation body can have a very compact structure. This allows a small cross-section area of the articulation body as well as a small diameter of the articulation cavity. This results in a small external diameter of the head section in which the articulation cavity is arranged. To further reduce the external diameter the opening margin of the articulation cavity features a reinforcing rib surrounding the opening margin. In this way the wall thickness of the head section can be minimal without running the risk of the wall collapsing or tearing under stress.
The articulation body 15 fits into the articulation cavity 21. The articulation body 15 has articulation surfaces 43 which are structured axially symmetrical to the pivoting axis 45. In this example the articulation surfaces are two circular edges 43. This example is not a limitative one. The articulation surfaces 43 can be continuous or interrupted. Interruptions in the articulation surface are obtained by means of cuts or slots in the surface or edge. Therefore, the articulation surface can also be made of a line of tips. Departing from the pure cone shape the articulation cavity 21 can be designed according to a trumpet shape or egg holder shape. In this case the lateral surface lines are not straight like in a cone but are convexly or concavely bent. The lateral surface can also feature circular grooves, threaded grooves or lateral surface line grooves. Lateral surface line grooves are preferably structured in such a way that the wall of the grooves adheres to the front side and the lateral side of the articulation body 15. This favors the tightening of the articulation body in the articulation cavity 21. The material of the lateral surface can be softer than the material of the articulation body.
The lateral surface can be molded with a three-dimensional grid relief. The lateral surface can be built with three or more cylindrical and concentric holes with gradually reducing diameters. In this way, when pressing the articulation body 15 into the holes, the orifice edges are cut in the articulation surfaces 43 of the articulation body 15. The articulation cavity 21 can have two or more conical lateral surfaces 25 in a gradually terraced manner. These can act with the same or with always different articulation surfaces 43 of the articulation body 15.
The articulation body 15 has an external conical surface 47 concentric to a collar axis 48 (
The compression force needed for a long lasting clamping and interlock of the articulation body and articulation cavity is obtained by means of the screw connection. The articulation body 15 is clamped between the tooth tip 41 and the conical lateral surface 25. In order to prevent the force generated at the four contact points 55 from causing a deformation of the head section or leading to a tear in the wall of the articulation cavity 21, the orifice margin 27 of the articulation cavity 21 is reinforced with a ring shaped circular reinforcing rib 57.
The second exemplary embodiment depicted in FIGS. 10 to 13 shows a terraced articulation cavity 22 and an articulation body 15 with two sets of two circular edges 43 and 44. Because of the gradation of the articulation cavity 22 there is a ring tip 67 in the articulation cavity at the margin of the inner and smaller hole 69. The circular edges 43 only interact with the lateral surface 25 of the larger hole 71. With four contact points 55 (which highlight smaller ellipses) between the circular edge 43 and the lateral surface 25 a clamping action is obtained by pushing the articulation body 15 into the articulation cavity 22, as described in the first embodiment example.
The circular edges 44 however have a radius that is larger than the one of the circular edges 43 and are arranged at a shorter distance from each other than the circular edges 43. The circular edges 44 interact both with the lateral surface 25 and the ring edge 67. At the contact points 56 between the circular edge 44 and the lateral surface 25 the angle between the lateral surface and the rotation axis 30 is small.
At the contact points 54 between the ring edge 67 and the circular edge 44, an cutting curve between the rotation surface, formed by the rotation of the conical sheath line 95 around the rotation axis 30 according to
In the schematic representation of the first and second exemplary embodiments, the circular edges 43,44 are represented as edges of cylindrical bodies. These circular edges can also be formed on bodies of other shapes.
As in the first example (
This geometry can be implemented by one single circular edge 43 only.
It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
Number | Date | Country | Kind |
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825/02 | May 2002 | CH | national |
This application claims priority under 35 U.S.C. §119 to Swiss Application CH 2002 825/02 filed in Switzerland on May 15, 2002, and as a continuation application under 35 U.S.C. §120 to PCT/CH03/00295 filed as an International Application on May 7, 2003, designating the U.S., the entire contents of which are hereby incorporated by reference.
Number | Date | Country | |
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Parent | PCT/CH03/00295 | May 2003 | US |
Child | 10984831 | Nov 2004 | US |