Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2001-02-28
2002-01-29
Philogene, Pedro (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S023480
Reexamination Certificate
active
06342077
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a cementable shaft prosthesis comprising a centering apparatus of plastic which can be pushed on at its distal shaft end and which forms a cavity to the distal shaft end, with a conical clamping connection existing between distal shaft end and the centering apparatus which permits a deeper sinking in of the distal shaft end into the cavity in the distal direction.
A centering apparatus for the distal end of a cementable prosthesis shaft which can be pushed onto the distal end and forms a cavity for the distal end in order that the distal end can sink in deeper into the cavity to avoid fissures in the hardened cement is shown in patent application GB-A-2 104 391. Plastics such as polyethylene are used as the material for the centering apparatus.
This proposed embodiment has the disadvantage that the distal end, which is formed conically, must be machined to within very narrow tolerances in its outer contour in order to ensure a clamping in the axial direction and a subsequent slipping when the axial force increases. This kind of clamping at the same time causes such a strong sealing action that a ventilation bore at the lowest point of the cavity is proposed. A further disadvantage consists in that thick shaft ends, which would be desirable for obtaining a uniformly thin cement jacket in the marrow chamber cavity, have large surface portions with a relatively large half cone angle of the surface to the shaft axis in the distal region. If a clamping with guide surfaces which lie closely in contact is provided in this region, large axial forces and ring stresses arise at a cement pouch which surrounds the shaft and the centering apparatus after the hardening of the bone cement due to the relatively large half cone angle in the event that the shaft sinks in further into the cavity.
SUMMARY OF THE INVENTION
The object of the invention is to improve the prosthesis and the centering apparatus in this regard. This is achieved in accordance with the independent claim
1
in that the distal shaft end has a conical bore in the proximal direction in which a spigot which is formed on at the centering apparatus in the cavity forms the clamping connection.
The invention has the advantage that the outer contour of the distal prosthesis end has no guiding function any longer and that the cavity of the centering apparatus can be held without cement through a weak line contact between its proximal circumferential edge and the surface of the shaft. A further advantage consists in the reliability of the clamping and positioning of the centering apparatus on the spigot, since the cone angle can be chosen freely and since a conical bore can be economically produced in the distal shaft end to within narrow tolerances.
Advantageous further developments of the invention result from the subordinate claims
2
to
10
.
In the manufacture of the centering apparatus in an injection molding tool particularly narrow pairing tolerances result in the clamping region between the spigot and the conical bore, since the geometry of the spigot is likewise produced as a bore due to the shape inversion in the injection mold. The manufacture and the monitoring of the tolerances are thus simpler at both parts, shaft end and centering apparatus.
The half cone angle of the conical bore can be chosen between 0.5° and 2.5° depending on the material of the shaft and the centering apparatus and in accordance with the geometry of the spigot. In the case of a metallic shaft and a centering apparatus of plastic, for example of PMMA (polymethyl methyl acrylate), the half cone angle can lie between 1.2° and 1.6°.
If the spigot has two stages with different diameters and the graduation in relation to the axial distance is chosen such that it coincides with the half cone angle of the conical bore, then two axially spaced guiding and clamping zones arise during the pushing on of the centering apparatus which oppose a resistance force F in the axial direction corresponding to the friction and to the half cone angle between the conical bore and the spigot which increases over-proportionally with increasing dipping in of the shaft tip. A dipping in is practically possible only because the stages of the spigot deform radially inwardly. If the spacing of the stages is chosen such that at first only one stage is contiguous at the conical bore and if this stage can be more easily deformed radially, because for example it is formed of only a few ribs, then at first only a slight axial resistance force arises in the pushing on of the centering apparatus, which only increases much more strongly when the second stage is also radially inwardly deformed during the further dipping in of the distal shaft tip. This means that during the pushing on the centering apparatus holds itself on the shaft tip and develops a noticeable resistance when the centering apparatus has reached a predetermined position aligned with its spigot. The prosthesis shaft can then be introduced with the centering apparatus into a marrow chamber of a tubular bone which is filled with bone cement and is guided at its distal end by at least three fins of the centering apparatus.
The desired position of the centering apparatus is signalled in an even more marked way during the pushing on if the conical bore has a run-in cone with a somewhat larger half cone angle and the second stage is contiguous at this steeper cone and must first be radially deformed at this steeper cone for the further penetration into the conical bore. The cavity in the centering apparatus and the axial resistance force are dimensioned such that the distal shaft end can later dip in into the centering apparatus by several millimeters after the pushing on and cementing in without the jacket of the centering apparatus, which is enclosed by the bone cement, experiencing impermissible forces in the axial direction.
In a femur shaft prosthesis it is advantageous due to the medially lying femur head when in the introduction of the shaft in the marrow chamber cavity the centering apparatus supports the shaft towards the lateral by a fin. For this reason it is furthermore advantageous when a rotational securing which is independent of the dipping in depth and which permits a pushing on of the centering apparatus only when a fin is oriented laterally is provided between the shaft tip and the centering apparatus in the cavity. When the prefabricated centering apparatus itself consists of PMMA, a homogeneous cement pouch with the bone cement results after the cementing in of the prosthesis which permits a later sinking in of the shaft, without impermissible stresses arising in the cement pouch in the region of the distal shaft tip.
REFERENCES:
patent: 5074879 (1991-12-01), Pappas et al.
patent: 5108439 (1992-04-01), Morscher et al.
patent: 5458651 (1995-10-01), Lawes
patent: 6241772 (2001-06-01), Ling et al.
Güttinger Werner
Mettler Felix
Philogene Pedro
Sulzer Orthopedics Ltd.
Townsend and Townsend / and Crew LLP
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