Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Patent
1992-04-27
1993-09-21
Isabella, David
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
A61F 238
Patent
active
052464601
DESCRIPTION:
BRIEF SUMMARY
This invention concerns prosthetic components for replacement of the femoral facet of the patella.
Components of this kind have been in routine use for some years, typically, although not exclusively, for total knee joint replacement in association with other components to replace the patellar and tibial facets of the distal femur and also the femoral facets of the proximal tibia. However there is a current concern with the rates of failure found to arise with such components due to component wear and other causes, and this applies not least of all to patellar components.
The cause of these failure rates can be explained, at least in part, by a new analysis of the knee joint which is expressed in a paper entitled "Kinematics and Mechanics of the Cruciate Ligaments of the Knee" by O'Connor J. J. and Zavatsky A. presented at the First World Congress on Biomechanics, Symposium of Biomechanics of Biarthrodial Joints, San Diego, Calif., 1990. This paper is now published as Chapter 25 in the related proceedings, Vol. 2, pages 197-252, edited by V. C. Mow, A. Ratcliffe and S. L-Y. Woo, Springer-Verlag, New York, 1990. In any event, this analysis is thought to be consistent with the natural function of the knee joint as so far observed and understood, patellar components as previously used to date are not adequately consistent with the analysis, and the failure rates so far as patellar components are concerned can be explained as a result of this inconsistency.
An object of the present invention is accordingly to provide a prosthetic patellar component which is more consistent with the analysis in question and, to this end, such a component comprises a fixation part adapted for securement to the bone of a patella, and an articulation part defining a surface to assume the role of the femoral facet of that patella, said parts being mutually engageable in such a manner as to allow rotation therebetween about a medio-lateral axis relative to said surface.
In order to clarify the nature of the proposed component and the analysis on which it is based, the same are further described below with reference to the accompanying drawings, in which:
FIGS. 1-3 diagrammatically illustrate, in a sagittal plane and respectively different positions of flexion-extension, a model of the knee joint on which the analysis is based, and
FIGS. 4 and 5 similarly illustrate respectively different forms of component according to the invention.
In FIGS. 1-3 the joint model is respectively at full extension, 50.degree. flexion and 100.degree. flexion.
The distal femur is denoted at 10 and the proximal tibia at 20. The patella is denoted at 30 by way of a circular arc representing the femoral facet, this arc being centred at S and engaged with the femur at Y. This engagement is located just beyond the proximal extremity of the trochlea at full extension as seen in FIG. 1, the trochlea being denoted at 11 as a circular arc centred at T, with the engagement progressing distally around the trochlea with flexion movement as seen from FIGS. 2 and 3.
The remaining structural elements of direct interest in FIGS. 1-3 are the quadriceps tendon which is denoted at 40 and connected to the patella at F, and the patellar tendon which is denoted at 50 and connected at its opposite ends respectively to the patella and tibia at E and R.
According to the analysis under consideration, determination of the spatial position of the patella and its engagement with the femur must satisfy the geometrical conditions that the point E remains at a constant distance from R, that point Y lies on the line from centre S which is perpendicular to the femur, and that the quadriceps tendon remains substantially parallel to the femur over which it passes. However, these conditions are not sufficiently determinative and it is additionally appropriate to apply the mechanical condition that the lines of action of the forces on the patella intersect. For this purpose the lines of action of the forces in the two tendons should intersect on the line of action SY of the patello-femoral
REFERENCES:
patent: 3964106 (1976-06-01), Hutter et al.
patent: 4007495 (1977-02-01), Frazier
patent: 4085466 (1978-04-01), Goodfellow et al.
patent: 4634444 (1987-01-01), Noiles
Goodfellow John W.
O'Connor John J.
British Technology Group Ltd.
Isabella David
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