Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1998-07-17
2001-02-13
Isabella, David J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S023480
Reexamination Certificate
active
06187049
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ceramic femur head for use in an artificial joint such as an artificial hip joint (coxa), artificial shoulder joint or the like for human being. The present invention further relates a femur system using such a ceramic femur head.
2. Description of the Related Art
An example of a prior art artificial hip joint is shown in FIG.
5
. In
FIG. 5
, an artificial hip joint is generally designated by
100
and shown as including a femur head
101
constituting part of a sliding joint, a metallic stem
102
embedded in a femur or thighbone to attach the femur head
101
to a hipbone side end F of the thighbone and a socket
103
made of a high density polyethylene (hereinafter referred to as PE) and fixed to a highbone M side. The femur head
101
is slidably received in the socket
103
.
The femur head
101
has a sufficient strength for practical use and thus has been used to effect a good result but has a problem of being relatively smaller in strength as compared with a metallic femur head.
As a countermeasure for solving this problem, a study has been made for applying a method of increasing the retaining or holding strength in a tapered fit system (i.e., a kind of press fitting in which joining surfaces are tapered correspondingly) which is frequently seen in machine tools, to an artificial joint. That is, in machine tools, it is known that the holding or retaining strength become larger as the finish of the fitting or joining surface becomes higher, so it is considered desirable in the artificial joint to make the degree of finish as high as possible i.e., make the surface roughness as small as possible (so long as the cost permits).
For this reason, the tapered portion
102
a
of the metallic stem
102
and the tapered hole
101
a
of the ceramic femur head
101
are subjected to surface grinding or the like so as to have a higher degree of surface finish, i.e., a smaller surface roughness. In this instance, due to the complicated shape, the tapered portion
102
a
of the metallic stem
102
is generally lower in the degree of surface finish than the tapered hole
101
a
of the ceramic femur head
101
.
However, the femur head
101
having fitted therein the metallic stem
102
reduces markedly in the fracture strength as it reduces in size, i.e., in diameter, notwithstanding the above described countermeasure, so it has been difficult to make such a ceramic femur head
101
that has a sufficiently large strength for its size.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a novel and improved femur system which comprises a ceramic femur head having a taper hole, and a stem having a tapered stem portion fitted in said tapered hole of said ceramic femur head. An inner peripheral surface of the tapered hole of the ceramic femur head has substantially the same surface roughness as an outer peripheral surface of the tapered stem portion of the stem.
The above described stem is a stem-like member which is attached to a thighbone and to which is fitted the femur head.
As the material of the stem, a metal which is softer than the ceramic femur head is desirable since a stem made of such a metal exhibits a good conformability when fitted in the femur head. For example, Ti, Ti alloy, stainless steel, Co—Cr alloy and the like are particularly desirable since they have a good biological affinity and durability and furthermore can reduce the cost.
As the material of the above described ceramic femur head, a ceramic containing, for example, Al
2
O
3
, ZrO
2
, Si
3
N
4
SiC or the like can be used.
By the first aspect of the invention, the ceramic femur head and the stem can be fitted at their tapered portions smoothly and furthermore can be joined in a way as to increase their contact area since the inner peripheral surface of the tapered hole of the ceramic femur head and the outer peripheral surface of the tapered portion of the stem are finished so as to have substantially the same surface roughness. By this, the femur head and the stem are connected or united together firmly, thus resultantly enabling the femur head to have a sufficient strength even in case it is reduced in size.
Heretofore, it has been practiced to finish the stem and the femur head in such a manner that the femur head has a smaller surface roughness than the stem. By the experiments conducted by the applicants, it was revealed and discovered that a higher compression rupture strength can be attained when the stem and the femur head are finished so as to have the same surface roughness, and based on this discovery the present invention was made.
As the reason why such a result was attained, it is considered that a higher degree of surface finish (i.e., a smaller surface roughness) of the tapered hole of the ceramic femur head rather results in an actually reduced contact area with the stem and reversely a lower degree of surface finish (i.e., a larger surface roughness) on the femur head side than the conventional can increase the contact area. For example, as shown in
FIG. 1A
, in case the surface roughness on the femur head side is smaller than that on the stem side, the surface irregularities of the stem are conformed to the surface of the femur head which has a larger hardness, resulting in a small contact leading to an increased stress per unit area and therefore a lowered strength. In contrast to this, in case the surface roughness of the femur head is not so small and nearly equal to that on the stem side, the surface irregularities of the stem are fitted or engaged in those of the femur head as shown in
FIG. 1
b
when the stem is pressed into the femur head, resulting in a large contact area leading to a reduced stress per unit area and therefore a higher strength.
On the other hand, in case the surface roughness on the stem side is too large as compared with that on the femur head side, i.e., the surface of the stem is too rough, smooth fitting of the stem and the femur head at their tapered portions cannot be attained but the stress resulting at the time of their press fitting may be concentrated at a unexpected place to possibly cause breakage of the femur head, so such a stem is not desirable.
In the meantime, in case the degree of surface finish on the stem side is considerably high as compared with that on the femur head side, it is considered that fitting of the stem and femur head at their tapered portions is so smooth as to reversely reduce the contact area, so such a stem is not desirable.
Further, the present invention does not require excessive surface grinding of the tapered hole of the ceramic femur head as the conventional but makes it possible to rather lower its degree of surface finish than the conventional. Thus, the present invention has an advantage of contributing to reduction of the cost.
According to a second aspect of the present invention, the difference in the surface roughness between the outer peripheral surface of the tapered stem portion of the stem and the inner peripheral portion of the tapered hole of the ceramic femur head is equal to or larger than 1 &mgr;m Ra.
Ra which represents a degree of surface finish is a center line average roughness Ra which is defined according to JIS 0601.
By the second aspect, the difference in the degree of surface finish between the tapered stem portion of the stem and the tapered hole of the femur head is small, i.e., 1 &mgr;m Ra or less. When the difference is in such a range, a sufficiently large contact area between the femur head and the stem can be attained at the time of fitting at their tapered portions, thus making it possible to attain a large compression rupture strength and smooth fitting of the tapered portions.
According to a third aspect of the present invention, the difference in the surface roughness between the outer peripheral surface of the tapered stem portion of the stem and the inner peripheral portion of the tapered hole of the ceramic femur head is equal to or larger than 10 &mgr
Fujikawa Kentaro
Miyata Eiji
Foley & Lardner
Isabella David J.
NGK Spark Plug Co. Ltd.
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