Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Utility Patent
1999-05-27
2001-01-02
Isabella, David J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Utility Patent
active
06168632
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a femur component of a hip-joint endoprosthesis with a shaft to be anchored in the femur's marrow cavity having a distal segment and a proximal segment joined by a neck with a stub to receive a hinge head or with a hinge head firmly adjoining the neck and also including a posterior surface, a lateral side, a mesial side and a plane of symmetry, longitudinal ribs extending in the proximal-to-distal direction on the anterior surface and on the posterior surface in the proximal segment of the shaft.
BACKGROUND OF THE INVENTION
Femur components of this general type are known from the state of the art, but they incur various drawbacks.
As regards un-cemented femur prostheses for hip-joint replacement, the primary stabilization of the femur shaft is implemented by frictionally and geometrically locking onto the enclosing bone. The femur shaft is configured in such a way that loading it entails its being wedged into the bony support. In particular, during the first loading phase wherein some seating shifts of the femur shaft are likely, a suitable configuration must assure reliable primary affixation. In the event of a seating shift, new stabilization must be assured by suitable reconfiguration. In the absence of adequate primary stability, loading will entail repeated shifts at the boundary surface between femur shaft and bone, preventing reliable implant bodily incorporation. On the other hand, if the primary anchoring is reliable, the implant can be enclosed by the bone tissue during the healing process and offer good long-term prospects.
Preferably, the primary affixation is in the upper portion of the prosthesis shaft enclosed by the spongy bone. A large support surface can be achieved in the big bone volume present therein. Seen biomechanically and clinically, it has been advantageous to apply the force through this region.
Illustratively, a femur shaft is on the market wherein the proximal shaft portion intended to be anchored in the spongy bone structure continuously tapers conically in the lateral-to-mesial direction in order to secure renewed, automatic clamping in the event the bone yields in the mesial direction. The region of the trochanter major with the anchoring space, however, does not have a cross-sectionally triangular or trapezoidal shape, but rather an oval one. Accordingly, this known femur shaft suffers from the drawback that the laterally much enlarged proximal shaft portion may crack the bone. In addition, this known femur shaft comprises solid ribs which when displacing bone volume raises the pressure and may further contribute to the cracking effect.
A longitudinal section of the proximal femur with an inserted femur shaft shows that the spongy substance is not sharply delimited to the trochanter region but instead partly continues as far as the zone of the diaphyseal bone tube. However, as much as possible of this bone structure should be used to transmit the load. But the ribs located in one position of the known femur shaft do not optimally meet this requirement because of the little differentiated configuration. The point of contact and the elongation of the ribs at the shaft should be designed in such manner that as much as possible of the spongy volume of the proximal femur is used for anchoring.
SUMMARY OF THE INVENTION
An object of the invention is to provide a femur component of a hip-joint endoprosthesis optimally corresponding to the spongy architecture in the proximal femur part and entailing cementless, primary shaft anchoring in the femur in the most stable possible manner to secure thereby good likelihood of bone healing.
The double-wedge or ellipsoidal shape of the proximal shaft segment offers the advantage that the prostheses shaft can wedge itself both laterally and mesially in the event of a seating shift. The oval envelope curve of the ribs matching the cross-section of the proximal femur minimizes the danger of cracking the proximal femur due to direct pressure on the hard cortical bone.
In a preferred further development of the invention, wherein the ribs are cross-sectionally triangular, these ribs easily penetrate the spongy bone volume and, as a result, the pressure is reduced during the insertion procedure. Because, preferably, the triangular ribs extend conically, additional wedging is achieved that is lacking in rectangular ribs such as are used in the state of the art.
The straightness of the shaft together with the increasing height in the proximal direction of the ribs extending in the direction of the shaft axis allows secure positioning of the femur shaft and knocking it into place with guidance by the self-cutting ribs. If, on the other hand, the ribs are partly or all mounted at an angle to the shaft axis, no seat enclosing the ribs can be realized when installing the femur shaft. Because the rib projection varies along the shaft, the stress on the spongy volume is more homogeneous than in known shafts with ribs beginning at a given height which then continuously increases.
Another preferred development consists in that the combs of the longitudinal ribs subtend an angle ½&dgr; of at least 1°, preferably at least 2° with the plane of symmetry. The individual combs of the longitudinal ribs subtend different angles ½&dgr; in the range of 3 to 8° with the plane of symmetry, preferably the longitudinal ribs situated closer to the lateral and the mesial side subtending a larger angle ½&dgr; than those in between. Such a rib geometry functionally stimulates the enclosing bone, whereas such a stimulus is not achieved with the dull rib shape of the state of the art. This functional stimulus causes bone regeneration in the stressed zone with ensuing compaction and hence bone healing. The blood supply to the regenerated bone can optimally form in the troughs of this rib structure.
Appropriately, the anterior and posterior surfaces form a wedge tapering toward the distal segment, the central plane being the plane of symmetry, the angle &egr; of the wedge being in the range of 0.5 to 3.0°, preferably within 1.0 and 2.0°. On account of this geometry, the wedging effect is continued also along the upper shaft zone. In case subsequent intervention is due on a solidly integrated shaft, the shaft is more easily knocked free if its geometry is conical in all directions, that is, also proximally in the intra-rib zone, than if the geometry were other than conical.
Seen in a section orthogonal to the plane of symmetry, the envelope curve of the combs of the longitudinal ribs is approximately in the form of a kite quadrilateral of which the sides may approximately represent straight lines or arcs of circular or elliptical segments.
Relative to the mesial side, the kite quadrilateral should subtend an inside angle &agr; larger than 10°, and preferably larger than 12°. In addition, the inside angle &agr; should be less than 22°, preferably less than 20°.
Toward the lateral side, the kite quadrilateral should subtend an inside angle &bgr; larger than 8°, preferably larger than 9°. Moreover the inside angle &bgr; should be less than 45°, preferably less than 40°.
Appropriately, the longitudinal-rib combs are sharp and, seen in a section orthogonal to the plane of symmetry, are preferably triangular. Illustratively, the longitudinal ribs may assume the shape of three-sided pyramids of which the vertices point distally. The longitudinal-rib combs, however, may also be rounded and, seen in a section orthogonal to the plane of symmetry, preferably are semi-circular. On the other hand, uniformly thick longitudinal ribs of rectangular cross-section are to be avoided.
Preferably in a continuous manner, the width of the longitudinal ribs appropriately decreases from the proximal to the distal sides. This also applies to the height of the longitudinal ribs which preferably continuously decrease in the proximal-to-distal direction.
Surprisingly especially good clinical results were observed when at least one of the longitudinal ribs runs as far as the distal half of the shaft becaus
Cotting Anton
Moser Walter
Isabella David J.
Mathys Medizinaltechnik AG
Pearne & Gordon LLP
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