Note: Descriptions are shown in the official language in which they were submitted.
W091/17723 ` PCT~GB91~00708
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DUMMY DEVICE FOR MOU~DING BONE ~AVITY FORM
This invention relates to a dummv device for mouldin~
a bone cavity form. and to a method of manufacture of a
prosthetic device to be implanted i n any ~iven bone cavity.
The invention has been develope~ primarily in
connection with the implantation of replacement hip
prostheses. though it should be understood that the
principles of the invention may be applied ~enerallv to
prosthetic joint implantation.
It is known from EP0129531 to provide a method of
manufacture of a prosthetic joint. in which a bone cavity
is formed to receive the prosthetic joint. and then a
flexible ba~ is introduced into the cavity and is filled
with a mouldable composition which expands the ba8 to come
into intimate contact with the wall of the cavity. The
moulded form is then removed from the cavity. and the ba8
is stripped-off ~since the mouldin~ composition is chosen
to be non-adherent to the material of the bag). The
moulded form can then be installed in a profile detector
apparatus which follows the profile of the moulded form and
converts this into si~nals which control the operation of a
formin~ machine in which a suitable standard size of
prosthesis is mounted. The formin8 machine machines down
the outer profile of the standard prosthesis to correspond
with that of the moulded form. with suitable correction
bein8 built-in to take account of the removal of the outer
skin (the bag) of the moulded form.
The prosthesis can then be fitted snugly into the bone
cavity, with all parts of the prosthesis en8a8in8 firmly
~ith correspondin~ parts of the wall surface of the bone
cavity. This minimises the risk of the prosthesis rocking
in the cavity in service. so that tissue ingrowth can take
place over a period of time. and also to reduce the
~eneration of localised eNcessive pressure loadings which
otherwise mi8ht create discomfort or actual pain to the
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WO 91/17723 PCI/GB91/00'708
patient.
This known technique is a significant improvement over
existing "cementless" implantation techniques. but there
are at least two areas in which the technique could be
modified to provide still further improvement over existin~
techniques.
In particular. the necessity to use a mouldin~
composition which is non-adherent to the ba~ involves an
additional process step in the subsequent removal or
unpeelin~ of the ba~. and the need to compensate for this
reduction in size of the profile in the setting-up of the
operatin~ parameters of the profile detector system and the
machinin~ of these standard prostheses.
Further. the location of the distal end of the
prosthesis in the bone cavity is not always a sufficiently
tiBht fit. which can ~ive rise to an unacceptable amount of
rocking and I or flexing of the prosthesis in service.
This arises because the distal end of the moulding baB is
closed by a rubber plug secured to one end of a mouffting
rod. and this may be undersized relative to the diameter of
the bone cavity formed at the distal end, and therefore the
prosthosis manufactured from the moulded form ~includin~
the rubber plug) will have a similarly undersized distal
end.
With an in~ecure location of the distal end of the
prosthesis. the latter is liable to rock in service.
thereby applyin~ load to the proximal end of the bone
cavity. which is liable to wear and to be a cause of
discomfort or pain to the patient.
Accordingly. the invention provides an improved method
of manufacture of a prosthetic joint to be implanted in a
bone cavity of any particular form. and also an improved
method of formin~ a moulded form from which can be derived
the required profile of the prosthetic joint to be
implanted in the cavity.
Accordin~ to one aspect of the invention there is
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provided a method of forming a moulded form correspondin~
at least partly with the shape of a bone cavity which is to
receive a prosthetic joint and which cavity has an enlar~ed
proximal end and a narrower distal end. and said moulded
form enablin~ the required profile of the prosthetic joint
to be derived therefrom in order to be implanted in the
bone cavity:
in which the method involves the use of a model havin~
a pre-formed proximal dummy portion to fit in the proximal
end of the bone cavity. a distal end portion to be received
by the distal end of the cavity. and a bas to enclose at
least the proximal dummy portion of the model. and the
method comprisin~:
introducing the proximal dummy portion and the distal
end portion into the cavity until the model is seated in
position:
filling the space between the outer surface of the
dummy portion and the inner surface of the ba8 with a
moulding composition so as to expand the ba8 into intimate
contact with the wall surface of the cavity in at least the
proximal end portion of the cavity:
allowing the composition to set into a moulded form
corresponding at least with the enlar~ed proximal end
portion of the cavity: and.
removing the moulded form from the cavity to form a
moulded model from which the required profile of the
prosthetic joint can be derived.
Preferably. the moulding composition is of such a
nature that it is cold formable, and can form the required
moulded form fairly rapidly to a sufficient de8ree of
rigidity to enable the moulded form to be withdrawn from
the cavity without any appreciable loss of shape.
The composition may also be selected to be non-
adherent with the material of the bag. to enable the ba~ to
be stripped subsequently from the moulded form prior to
monitoring of the outer profile of the moulded form.
WO91/17~ P~T/GB91/00708
The invention is particularly applicable to the
implantation of hip prostheses. and the dummy portion of
the model may be selected from a suitable ran8e of standard
sizes. which will be supplied to suit the size of femoral
rasp employed in the formation of the cavity. The rasp may
be of the type disclosed in more detail in International
Patent Application No PCT/~B89/01188.
The distal end portion of the model also may be
selected from a suitable ran~e of standard sizes to fit
snu~ly in the formed distal end portion of the cavitv.
When a prosthetic joint is manufactured from the derived
profile.of the moulded form. the distal end portion of the
prosthetic joint also will fit snugly into the distal end
portion of the cavity. thereby giving a ri8id implantation
of the prosthetic joint.
However. it should be understood that the moulded form
of the model does not need to conform exactly by its
external profile with all aspects of the internal profile
of the bone cavity into which the model is introduced
(prior to èxpansion of the bag), provided only that at
least the proximal end portion of the moulded form conforms
substantially in external profile with the internal profile
of the enlar~ed proximal end portion of the bone cavity.
~ esirably. the distal end portion of the model is
selected to have a close slidin~ fit in the distal end
portion of the cavity, whereby the prosthetic device
derived from the model can fit in the bone cavity in such a
way that the distal end portion of the device provides
lateral restraint which resists any tendency of the
prosthetic device to rock about its proximal end portion.
but also allows relative axial slidin~ movement in use.
occasioned by loading of the femur of the patient.
An intermediate portion of the moulded form which
interconnects the proximal portion and the distal portion
(and of the prosthetic device derived therefrom) does not
need to be a close fit within its portion of the bone
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WO91/17723 ` PCT/GB91/00708
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- cavity. because its sole purpose is to form a ri8id
intermediate connection between the two portions.
Indeed. it is preferred that the intermediate portion
(of the model) should have lateral clearance with the
. adjacent internal wall of the bone cavity, so that the
correspondi tl~ intermediate portion of the prosthetic device
has similar lateral clearance. and whereby the femur of the
patient is allowed to flex without coming into contact with
the intermediate portion Or the prosthetic device. This
will ensure that axial loading is borne by the proximal
end.
,; The proximal dummy portion and the distal end portion
i;~ are preferably supplied as separate components which can be
: releasably coupled together. to enable at least the distal
. end portion to be re~used.
As mentioned above. the baB encloses at least the
.~ dummy portion. but it may be designed to enclose at least
part of a stem portion forming the intermediate portion by
which the distal end portion is connected to the dummy
portion.
: The proximal portion. which will normally be discarded
after use, is preferably made of suitable plastics
material. Alternatively. it may be retained as a permanent
replica.
To facilitate the introduction of the moulding
composition into the bag. it is preferred that the proximal
dummy portion is formed with a passage. an inlet to the
passa~e. and at least one surface outlet. to enable the
moulding composition to be introduced under pressure into
the bas via the interior of the proximal portion.
AccordinB to a further aspect of the invention there
is provided a method of manufacture of a prosthetic joint
to be implanted in a bone cavity having an enlarged
proximal end and a narrower distal end. and involving the
use of a model having a preformed proximal dummy portion to
f~t in the proximal end of the bone cavity. a distal end
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portion to be received by the distal end of the ca~ity. and
a ba~ to enclose at least the proximal portion of the
model. and the method comprising:
introducing the proximal portion and the distal end
.. portion into the cavity until the model is seated in
position:
filling the space between the outer surface of the
proximal portion and the inner surface of the ba8 with a
,; moulding composition so as to expand the ba8 into intimate
contact with the wall surface of the cavity in at least the
proximal end portion of the cavity:
allowin~ the composition to set into a moulded form
corresponding at least with the enlarged proximal end
portion of the cavity:
removing the moulded form from the cavity to form a
moulded model from ~hich the required profile of the
prosthetic joint can be derived: and.
; following the external shape of the moulded model form
. with a profile detector and causing a machine to shape the
outer profile of a prosthetic joint with a corresponding
shape so that the prosthetic joint can be implanted in the
bone cavity. The invention also includes the model per se
for use in either of the methods of the invention defined
above.
The invention will now be described in detail. by way
of exampl0 only. with reference to the accompanying
drawings, in which:
Figure I is a schematic illustration of a model for
~ use in a method of forming a moulded form according to one
: aspect of the invention;
Figure 2 is a detail view of part of the model shown
, in Figure 1:
Fi~ure 3 is an exploded view of the component parts of
the model shown in Figure 2:
; Figure 4 is a schematic illustration of a profile
detector device for use in monitoring the outer profile of
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a moulded form made usin~ the model shown in Figures I to
3: and,
- Figure 5 is a schematic illustration of a machine for
manufacturing a prosthetic joint having a cutting head
controlled by si~nals derived from the profile detector
apparatus of Fi~ure 4.
:~ Referrin~ now to the drawin~s. there will be described
in detail an embodiment of method of formin~ a moulded form
corresponding with the form of a bone cavity which is to
receive a prosthetic joint and having an enlarged proximal
end and a narrower distal end. and which is to receive an
implanted prosthetic joint. and there will also be
disclosed an embodiment of method of manufacture of a
prosthetic joint to be implanted in a bone cavity of any
particular form.
There is shown in Figures 1 to 3 a model which is
designated generally by reference 10 and which has a
preformed proximal dummy portion 11 to fit into the
proximal end of a bone cavity. shown by dashed outline 12
in Figure 1. a distal end portion 13 to fit in the distal
end 14 of the bone cavity. and a baB 15 (shown by dotted
outline in Figure 1. and in solid lines in Figures 2 and 3)
which encloses at least the dummy portion 11 of the model.
In the illustrated arrangement. the internal profile
of the bone cavity is designated by reference 16 (see dash
dot outline in Figure 1) and is formed in a femur in order
to receive an implanted prosthetic hip joint. The distal
end portion 14 of the bone cavity 16 may be formed by any
suitable reamer which will form a cylindrical passage at
the end. and the proximal and enlarged end portion 12 of
the bone cavity can be formed by any suitable means. such
; as particularly the femoral rasp disclosed in more detail
in International Publication No ~09Ot03764.
The invention provides a method of formin~ a moulded
form which corresponds with the form of the bone cavity 16
which is to receive a prosthetic joint. and then after
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WO 91/17723 p~r/GB9l~oo7o8
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extraction of the moulded form. its outer profile can be
followed by a profile detector device. as shown
schematically in Figure 4. and the si~nals derived from the
monitoring of the profile are then used to control the
operation of a machine having a cuttin~ head which shapes
the outer ~rofile of a prosthetic joint to correspond with
that of the moulded form. in order to fit in the bone
cavity of the particular form. as shown schematically in
Figure 5.
However. as can be seen from Figure 1. and as will be
described in more detail later. it is not essential that
the moulded form conforms exactly to the internal profile
of cavity 16. but it is important that there will be
substantial conformity with the internal profile of
enlar~ed proximal end cavity 12. and preferably also
substantial conformity with distal portion 14 of the bone
cavity. '
To form the moulded form which corresponds generally
with the form of the bone cavity. the proximal portion 11
and the distal portion 14 of the model are introduced into
the cavity 16 until the model is seated in position. The
space between the outer surface of the portion 11 and the
inner surface of the ba8 15 is filled with a moulding
composition so as to expand the baB 15 into intimate
contact with the wall surface of the bone cavity in at
least the proximal end portion 12 of the cavity. The
composition is then allowed to set into a moulded form
which corresponds at least with the enlarged proximal end
portion of the cavity. and then the moulded form obtained
thereby is removed from the cavity to present a moulded
model from which the required profile of the prosthetic
joint can be derived.
It is preferred that the moulding composition is of
such a nature that it is cold formable. and can form the
required moulded form fairly rapidly to a suff icient de8ree
of rigidity to enable the moulded form to be withdrawn from
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WO91/~7723 ~ PCT/GB91/00708
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the cavity without any appreciable loss of shape.
The moulding composition may be selected to be non-
adherent with the material of the ba~, to enable the bag to
be stripped subsequently from the moulded form prior to
monitorin~ of the outer profile of the moulded form.
However. this is not essential to the invention. and the
ba8 may remain adherin~ to the moulding composition and
thereby form the outer skin of the moulded form whose
profile can be monitored.
Conveniently. the proximal portion 11 of the model may
be supplied in a ran8e of standard sizes to suit the size
of femoral rasp employed in the formation of the cavity.
Furthermore. the distal end portion 14 may be selected from
a suitable ran8e of standard sizes to fit snugly in the
formed distal end portion of the cavity.
It is important to provide a reasonably ri8id location
of the distal end of the prosthetic joint so as to prevent
any rockin~ of the prosthesis and / or undue flexins in
service.
The proximal portion 11 and the distal end portion 14
are preferably supplied as separate components which can be
releasably coupled together. to enable at least the distal
end portion 13 to be re-used.
As illustrated. the ba8 15 encloses at least the
proximal portion 11. but it may be designed to enclose
further parts of the model down to. but not including the
distal end portion 13. Thus. the ba8 may enclose in
addition at least a part of a stem portion 17 which is
detachably coupled with the lower end of the proximal
portion 11. and which also has the distal end portion 13
detachably coupled therewith. As can be seen from Figure
1. there is a substantial lateral clearance 17a between the
inner wall of adjacent portion of cavity 16 and the outer
surface of stem portion 17. This clearance allows the
femur of the patient to flex. without coming into contact
with the prosthetic device derived from the moulded form of
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/091/17723 PCT/GB91/00708
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the mo~dèl-.
The purpose of intermediate portion or stem 17 is
merely to serve as a ri8id intermediate connection between
proximal portion ll and distal portion 13. with proximal
portion 11. with the inflated ba~ 15 thereon. bein~ a close t
fit within the enlarged proximal end portion 12 of the bone
cavity. The corresponding proximal end portion of the
prosthetic device derived from this moulded form of the
model therefore also will fit snugly within the enlarged
proximal end portion 12.
The distal end portion of the prosthetic device
derived from the shape of the selected distal end portion
13 of the model will have a suitable close sliding fit
within the distal end portion 14 of the bone cavity, and
this serves to anchor the proximal end portion of the
prosthetic device a8ainst rockin8 movement on its seating.
but the slidin8 contact permits flexing of the femur.
The proximal portion 11 is made of suitable plastics
material. and to facilitate the introduction of the
moulding composition in to the ba8 IS, it is preferred that
the proximal portion 11 has a central passage formed
therein. an inlet to the passage. and at least one surface
outlet. to enable the moulding composition to be introduced
under pressure into the baB 15 via the interior of the
proximal portion 11. In Figure 1. the outer surface of the
proximal portion 11 is shown having a number of surface
outlets 18. and the moulding composition will normally be
pumped under pressure via an inlet 19 provided at the top
of the proximal 11. The construction of the model is shown
in more detail in Figures 2 and 3.
The invention also includes a method of manufacture of
a prosthetic joint to fit in the bone cavity of any
particular form. and involving the use of the moulded form
obtained from the model described above with reference to
Figures 1 to 3. Figure 4 shows a profile detector
apparatus schematically by reference 20 havin~ a profile
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WO 91/17723 PCl/GB91/00708
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detector head 21 which is movable alon~ the X and Y axes in
order to monitor the outer profile of a moulded form 22
(shown schematically only). and this movement of the head
21 is converted into suitable electrical signals which
control the operation of a machinin~ apparatus having a
cutting head 23. as sho~n in Figure 5. which machines the
outer profile of the prosthetic joint 24 to the required
profile to fit into the bone cavity of the particular form.
Referrin~ now to Figures 2 and 3, the proximal portion
11 has a fenestrated core 27 which communicates with inlet
19. so that the moulding composition e.g. a suitable
silicone. can be introduced under pressure into the baB 15.
The distal end portion 13 is available in a ran8e of
diameters and lengths to suit the reamed isthmus. and has
suitable provision 28 to seal the distal end of ba8 15
The ba~ 15 also is provided with upper and lower
beaded openings 15a and 15b to allow sealable attachment of
the ba~.
The dummy portion 11 is supplied in a ran8e of
proximal inserts which are matched to the rasps used to
prepare the endosteal cavity. and fastener 29 (see Fi~ure
1 3) secures the ba8 15 and prevents proximal leakage of
silicone. Means 30 allows silicone to pass from the
fenestrated core 27 into the ba8 15
A neck portion 31 may be adjustably fixed to the head
15 to allow for anti-retroversion adjustment. Nut 32
clamps the assembly in combination with the distal end
portion 13.
A ran8e of neck adjusters 33 is provided. each
releasably connected to the neck 31 to give variable off-
set.
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