Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Patent
1998-05-20
2000-08-15
Smith, Jeffrey A.
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
A61F 232
Patent
active
061029564
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a modular endoprosthesis, particularly a femur prosthesis, which has a conical socket connection, with a first shaft element having a substantially conical bore, a second shaft element having a substantially conical pin adapted to fit the bore in the first shaft element, for connecting the two shaft elements together, wherein the first shaft element containing the bore has a circumferential indentation on its outer wall, close to the mouth end, for the purpose of reducing the mechanical load occurring on the edge of the mouth of the bore under bending stress.
DE 33 40 767 A1 discloses a prosthesis of this kind which consists of at least two shaft portions connected by fitting together. The mechanical connection between two shaft elements is achieved here by pressing a conical pin formed on one shaft element into a conical bore provided in the other shaft element so as to obtain a press fit.
As the endoprosthesis is made up of different shaft elements, endoprostheses of different shaft lengths can advantageously be produced in this way.
Moreover, the endoprosthesis can easily be removed, for example in the case of a repeat operation, by separating the shaft elements from one another and removing them one by one.
A disadvantage of the known endoprosthesis is, however, that the shaft element which contains the conical socket may have pronounced wear caused by microscopic movements in the region of the rim of the mouth, which may lead to cracking in extreme cases.
SUMMARY OF THE INVENTION
The aim of the invention is therefore to prevent these manifestations of wear in a modular endoprosthesis of this kind.
The aim is achieved by means of a modular endoprosthesis, comprising: a first shaft element having a first end, a second end, a longitudinal axis, an outer wall, a circumferential indentation and a substantially conical bore, wherein the bore includes an open mouth and a closed base; and a second shaft element having a first end, a second end, a longitudinal axis, an outer wall and a substantially conical pin adapted to fit the bore in the first shaft element, for joining the two shaft elements together with a conical socket connection. The circumferential indentation of the first shaft element is located on the outer wall close to the open mouth of the conical bore for the purpose of reducing mechanical stresses in the vicinity of the mouth of the bore when the endoprosthesis is subjected to a bending force.
The invention includes the technical teaching of shaping the shaft element of an endoprosthesis of this kind with a conical bore so as to avoid local peaks in mechanical stress in the region of the rim of the mouth of the bore.
The invention starts from the finding that the known shaft element containing the conical socket was too rigid in construction in the rim area, so that, in the event of bending stresses on the endoprosthesis with a corresponding offsetting of the longitudinal axes of the pin and bore, there was a reduction in the effective frictional forces between the pin and the bore, resulting in local microscopic movements.
Thus, the stress-absorbing contact area when the endoprosthesis is subjected to bending stress corresponds substantially to the entire surface area of the conical pin. As the bending stress increases and the longitudinal axes of the pin and bore are offset more and more as a result, the transmissible frictional forces are reduced. This leads to local movements at the rim of the mouth of the bore, which may lead to abrasion and cracking.
Since the bending stress acting on the endoprosthesis is not constant over time but is subject to fluctuations in its degree and direction depending on the natural loading conditions of the endoprosthesis, oscillating microscopic movements occur between the pin and bore. These microscopic movements together with the local stress peaks occurring at the rim of the mouth of the bore lead to the abrasion of material and hence to premature wear, which is also known as fretting.
According t
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patent: 3846846 (1974-11-01), Fischer
patent: 4676797 (1987-06-01), Anapliotis et al.
patent: 4808186 (1989-02-01), Smith
patent: 4878917 (1989-11-01), Kranz et al.
patent: 5074879 (1991-12-01), Pappas et al.
patent: 5645600 (1997-07-01), Bimman
patent: 5653764 (1997-08-01), Murphy
Artos Medizinische Produkte GmbH
Kinberg Robert
Robert Eduardo C.
Smith Jeffrey A.
Spencer George H.
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