Note: Descriptions are shown in the official language in which they were submitted.
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FEMORAL COMPONENT FOR USE IN REPLACEMENT HIP JOINT
This invention relates to a femoral component for use in a
replacement hip joint.
The "Exeter" type femoral component of the kind shown in
British Patent No. 1 409 054 is well known and comprises a neck which
carries a ball head for cooperation with an acetabular socket. The
neck is connected to a tapered collarless stem. Thus there is no
collar for resting either on the bone or the cement in the area where
the stem joins the neck of the implant. This type of stem has evolved
so that the stem can be given a polished finish to help it slide down
inside the bone cement and the present invention relates to this type
of femoral component.
In certain cases, for example where the medullary canal is
particularly narrow, it is difficult to ensure that a stem of standard
shape is firmly housed. In other cases, it may be preferable to
encourage proximal load transfer to the bone.
The present invention is intended to overcome some of the above
disadvantages and comprises a femoral component of a replacement hip
joint which has a tapered collarless stem for fixing in a medullary
canal by cement, and in which said stem has a proximal portion which
has a straight or curved taper, and an extended elongated distal
portion which is adapted to extend into the shaft of the bone.
The extended distal portion can act to centralise the stem and
the tapered surfaces act to encourage proximal load transfer to the
bone.
Preferably the elongated distal portion of the stem is
substantially circular in cross-section although it can be other cross-
sections if desired.
This elongated distal portion can be at least the same length
as said proximal portion and in a preferred embodiment is substantially
twice as long as said tapered proximal portion.
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The taper angle of the proximal portion can be slightly steeper
than in known constructions on the proximal part of the stem.
At the hip joint bearing surfaces there is a load transfer from
the acetabular component into the femoral component of the implant.
Beyond the distal end of the femoral component stem all this load has
been transferred into the bone. Between the cut end of the femur
proximally and the distal tip of the femoral stem the load gradually
transfers from the implant into the bone. The distribution of this
load transfer along the length of the femoral stem is influenced at
each cross-sectional level by the relative stiffness of the implant,
the bone cement mantle and the surrounding bone. Many femoral hip stem
implants have high cross-sections near their distal tip giving high
sectional stiffness and this causes a high proportion of the load from
the bearing surface to be retained within the implant and transferred
out through the bone cement mantle into the bone near the distal end of
the implant. Conversely, stems which are very flexible distally (by
virtue of the choice of material modulus or sectional geometry) cause a
greater proportion of the load to be transferred into the bone at or
near the proximal end of the femur.
Subsidence of the stem within the cement includes an increase
in stem section to be accommodated by he cement proximally leaving a
residual hoop strain, thus causing proximal load transfer into the
bone.
The arrangement according to the invention therefore provides
relative rigidity at the top end of the implant which is greater than
the distal end. This transfers more load from the hip stem onto the
bone through the bone cement mantle at the top and less passes down the
stem and is transferred out into the bone at the distal end. Transfer
of load onto the top end of the femur is thought to be beneficial in
order to avoid bane resorbtion.
A further advantage is that at the present in some patients a
point about half way up the stem in existing devices is found to come
very close to the internal bone wall and leaves very little space for
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cement. Thus, although the existing type of stem has straight tapering
edges, the inside form of the cortical bone is more trumpet shaped.
Therefore, by providing concave surfaces on the stem, the thickness of
the cement mantle thickness laterally in this area can be increased to
a more acceptable thickness, at least 1 mm and preferably l~ mm - 2 mm.
It is important in the present arrangement that the edges and
side faces of the stem never become parallel over the proximal part of
the stem because this will lose the advantage of taper locking
engagement either before or after any subsidence. The stem must be
always narrowing as it progresses downwards but it is not necessary to
maintain a constant taper angle.
Preferably the surface of the stem is polished and it can be
used with a distal void centraliser.
In the constructions referred to above, the proximal part of
the femoral component and the distal stem are in one piece but, in an
alternative construction, the proximal portion is provided with means
for connection to a separate elongated distal portion.
Because it is intended that the elongated distal portion should
not carry any of the load, it can be provided by what is in effect an
elongated centraliser, and this can be made from metal or, for example,
from a synthetic plastics material.
The elongated distal portion can be solid or hollow and, if
desired, it can be provided with centraliser means in the form of
flanges or other projections.
The invention can be performed in various ways and two
embodiments will now be described by way of example and with reference
to the accompanying drawings in which
Figure 1 is a side elevation of a femoral component according
to the invention located in a femur;
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Figure 2 is a front elevation of the component shown in
Figure 1;
Figure 3 is a cross-sectional end view on the line III-III of
Figure 2; and
Figures 4 and 5 are similar views to Figures 1 and 2 but of an
alternative construction.
As shown in Figures 1 to 3 of the drawings the femoral
component of a replacement hip joint has a collarless stem 1 of
substantially rectangular cross-section. The stem 1 is intended for
fixing in position in a medullary canal 2 of a femur indicated by
broken lines 3 by cement in well known manner. The proximal portion of
the stem has a continuous taper from a point indicated by broken line 4
to a portion 5 where it merges into a neck 6. The neck 6 communicates
with a boss 7 to receive a ball head indicated by broken lines 8 which
will co-operate with an acetabular socket.
The anterior side face 9 of the stem including portion 5 is
substantially flat until it merges into the neck 6 which is of circular
cross-section. The posterior side face 10 is of similar
configuration. These faces are radiussed with a longitudinally
extending curve so that they are concave. The shaping extends from the
line 4 and up through the portion 5 on these faces to the neck 6. At
the upper end there is a high concave radius as indicated by reference
numeral 11, the radius decreasing thereafter and finally running out at
the point 4.
The centre line of the tapering straight stem portion is
indicated at 14.
The lateral face 15 and the medial face 16 of the stem are also
tapered below the portion 5. The curving inner medial face 17 of the
portion 5 is of a greater angle to the axis 14 than normal and merges
into a straight cylindrical portion of the stem at the point 4 and the
concave surface 19 on the lateral face of the portion 5 is also at a
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greater angle and merges into a straight portion of the stem at the
same point.
Below the point 4 the stem is in the form of an elongated
cylindrical distal portion 20.
Below the portion 5 the medial faces 15 and 16 are also shaped
concave down to the point 4.
As shown in the drawing, the faces 17 and 19 are curved but
they could be straight.
As shown in the drawings the elongated distal portion 20 is at
least the same length as the tapered proximal portion above the line 4
and which is indicated by reference numeral 21. Preferably, and as
shown in the drawings, the elongated distal portion 20 is substantially
twice as long as the tapered proximal portion 21.
The cross-section of the extended portion 20 can be any
convenient shape but, as shown in the drawings, it is circular.
The cross-section of the stem above the line 4 is as shown in
Figure 3 and is substantially rectangular with radiussed corners. The
surfaces can be flat but in the arrangement shown they are slightly
bowed outwardly.
In the arrangement shown the femoral component is intended for
use with a removable ball head 8 but if desired the ball head could be
integral with the stem.
The surface of the stem incorporating the portion 5 is highly
polished and if desired a distal void centraliser (not shown) for
example of the kind set forth in British Patent No. 2 104 391 can be
used with it.
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In the construction described above, the proximal portion and
elongated distal portion 20 are integral and made as a single element
but, in the arrangement shown in Figures 4 and 5, the two portions are
made separately, but the same reference numerals are used to indicate
similar parts.
As shown in Figures 4 and 5, the tapered proximal portion 21 is
made from metal and has a short tail 30, the distal end of which is
tapered as shown at 31. An elongated distal portion 32 has a hollow
proximal end 33 which is a tight push fit over the tapered portion 31.
The distal portion 32 can be made from metal or a plastics material and
it can be provided with centraliser flanges or abutments 34. If
desired, the distal portion 32 can be hollow over the whole of its
length or substantially solid as shown in the drawings. Its outer
surface is smooth apart from the centraliser flanges 34 and the whole
femoral component sink into the cement in known fashion.
The distal portion 32 can be relatively flexible because the
main loading on the component is carried beneath the proximal shoulder.
