Mobile bearing patella prosthesis

#The present invention relates generally to implantable orthopedic prostheses, and more particularly to implantable patellar prostheses for use at the knee joint.

#Generally, the human knee joint comprises three bones, the femur, the tibia and the patella, which each have a smooth surface for articulation on an adjacent surface of at least one other bone. At its distal extremity, the femur has an articulation surface with medial and lateral convex condyles separated posteriorly by an intercondylar groove running generally in the anterior-posterior direction. The condyles join at the distal-anterior face of the femur to form a patellar surface having a shallow vertical groove as an extension of the intercondylar groove.

#The patella includes on its posterior face an articulation surface having a vertical ridge separating medial and lateral convex facets. These facets articulate against the patellar surface of the femur and against the medial and lateral condyles during flexing of the knee joint. The vertical ridge rides within the intercondyler groove to prevent lateral displacement of the patella during flexing of the knee joint.

#The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:

#FIG. 1 is an isometric view of an implantable patellar prosthesis, according to an exemplary embodiment of the present invention;

#FIG. 2 is an isometric view taken from an upper or top position of the bearing member used with the prosthesis of FIG. 1;

#Referring generally to FIG. 1, an implantable patellar prosthesis 30 is illustrated according to one exemplary embodiment of the present invention. Prosthesis 30 comprises an articulation component 32 and a base component 34. Articulation component 32 is permitted to move relative to base component 34. This movement can be better understood by defining the six conventional axes used to describe component motion. As illustrated in FIG. 1, the three translational axes are labeled X, Y and Z and are mutually orthogonal to one another. The rotational axes A, B and C define rotational movement about each of the translational axes. Specifically, rotational axis A defines rotational movement about translational axis X; rotational axis B defines rotational motion about translational axis Y; and rotational axis C defines rotational motion about translational axis Z.

#When prosthesis 30 is implanted, translational reference axes X, Y and Z correspond generally to well known and accepted anatomical directional terms. The X axis extends generally in the medial-lateral direction; the Y axis extends generally in the inferior-superior direction; and the Z axis extends generally in the posterior-anterior direction. For example, if we assume the prosthesis 30 is implanted on the left patella of a human patient, the ends of each of the X, Y and Z axes marked with an arrowhead point generally in the lateral, superior and posterior directions, respectively. During implantation, the patella bone is resected in a plane generally perpendicular to the anterior-posterior direction to remove a posterior portion of the patellar bone, leaving a resected planar bony surface 36. When the implantation is complete, the Z axis lies generally perpendicular to the resected bony surface 36 of a patella 38.

#Articulation component 32 is constructed of an appropriate biocompatible material having desirable wear and bearing friction properties. Combinations of materials may be used when articulation component 32 is formed from multiple components. In the embodiment illustrated in FIG. 1, for example, articulation component 32 comprises a patella bearing portion 40 and a patella insert 42. In this embodiment, patella insert 42 is formed from a biocompatible material, such as titanium, titanium alloys, zirconia ceramics, aluminum oxide ceramics, cobalt chromium alloys and carbon-based materials, e.g. diamond or diamond compound. Patella bearing portion 40 also is made of suitable biocompatible materials, such as polyethylene, e.g. ultra-high molecular weight polyethylene, or ceramic materials.