Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MONOPOLAR CONSTRAINED ACETABULAR COMPONENT
3
4 The present invention relates to improved acetabular components, such
as liners, assemblies and complete replacements, for use in the body. The
6 improved acetabular components can be used, for example, in hip
arthroplasties
7 in both a primary and revision setting. The present invention provides a
8 decreased risk of dislocation while facilitating greater range of motion
9 ("ROM").
Description of the Field
11
12 Dislocation is a major source of morbidity at re-operation after total hip
13 arthroplasty. Dislocation rates have been quoted at I - 10% for primary
total
14 hip arthroplasties. Many studies have shown increased rates of dislocation
after
revision total hip arthroplasty when compared with primary total hip
16 arthroplasty. Currently, the chance of "successful surgical management of a
17 recurrent dislocation" is 70% when a cause for the dislocation can be
identified.
1s Dislocation results in significant patient anxiety. In addition, the costs
19 associated with surgical management of dislocation are significant.
A variety of approaches have been used to address dislocation, including
21 changing the angle of the femoral component, changing the angle of the
22 acetabular component, using extended lip liners, using off-set liners,
advancing
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1 the greater trochanter, and using constrained acetabular components. Another
2 approach is to use femoral heads of larger diameter.
3 Some of these approaches have distinct disadvantages, however.
4 Altering the orientation of the components can increase stability in one
direction (for example, posteriorly)' but at the same time decrease stability
in the
6 opposite direction (for example, anteriorly). Use of extended lip liners and
7 currently available constrained acetabular components which utilize
additional
8 polyethylene to surround the femoral head increases joint stability but this
9 additional material also can, depending on the design, constrict the range
of
1o motion of the joint and can lead to subsequent problems related to
11 impingement. When the hip joint is articulated (for example, flexed,
extended,
12 rotated, etc.) to its limit, impingement between the femoral neck and
acetabular
13 liner may occur.
14 Component impingement is undesirable for several reasons. The
femoral neck impinging against the polyethylene liner can produce damage to
16 the polyethylene as well as increasing the stresses at the component / bone
17 interface. Additionally, the point of impingement acts as the fulcrum
through
18 which the force produced by the leg could lever the femoral head out of the
19 liner and hence lead to dislocation.
Many current methods of reducing the risk of dislocation, including
21 recurrent dislocation, employ the use of so called "constrained liners,"
which
22 are complex bipolar or tripolar articulations. In a study of twenty one
23 constrained acetabular components to treat preoperative or intraoperative
24 instability, six patients had recurrent dislocations. It was postulated
that this
increase was caused by the impingement of the femoral stem on the rim of the
26 insert due to decreased range of motion within the design. Orthopaedic
27 Knowledge Update, Chapter 38, page 474 (James H. Beatty, M.D., editor,
28 AAOS).
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1 Constrained systems that are currently available use ultra high molecular
2 weight polyethylene (UHM)APE) which is not crosslinked. A major limitation
3 of the material is it's high wear rate and the risk of periprosthetic
osteolysis.
4 Also, their stated range of motion in flexion is between 72 and 90 . Such
devices are available from Zimmer, Inc., Johnson & Johnson/Depuy Inc., and
6 Howmedica-Osteonics, Inc.
7 The current products have additional limitations. For example, the
8 Osteonics system utilizes only 22, 26 and 28 mm heads. The minimum
9 diameter of the acetabular shell is 50 mm. This design is "bipolar." A
bipolar
1o system is made up of a acetabular shell with a polyethylene liner, and
inside of
11 that liner is a metal head. Inside of that metal head is another
polyethylene
12 liner, which in turn captures a 22, 26, or 28 mm head that is attached to
the
13 femoral stem. Thus, the Osteonics system has multiple polyethylene on metal
14 articulations. The multiple articulations generate polyethylene wear debris
at a
much faster rate than designs with a single metal on polyethylene
articulation.
16 Additionally, the range of motion only is approximately 90 degrees.
17 A constrained system developed by Zimmer, Inc. uses a metal shell with
18 a two-piece polyethylene liner. One piece is inserted into the acetabular
metal
19 shell followed by placement of the other portion around the neck on the
femoral
component before the femoral head has been put on the Morse taper. The head
21 is then reduced into the acetabular component. The polyethylene portion
with a
22 metal band that is around the femoral neck is then brought into contact
with the
23 acetabular polyethylene component and the metal band then secured to
24 "capture" the head within the assembled polyethylene liner. Again, this
design
uses ultra high molecular weight polyethylene and can generate significant
26 debris as well as a limited range of motion. The limited range of motion
results
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1 in impingement in flexion, which generates a rotational torque at the bone
shell
2 interface, potentially inhibiting bony in-growth into the acetabular shell.
3 In the late 1960's to early 1970's, others attempted to implant a
4 cemented conventional UHMWPE liner with greater than 50% head coverage.
Lagrange and Letournel, Int Surg. 60(l):21-4 (1975); Lanzetta, Arch Ortop.
6 83(l):49-53 (1970). These designs had a 35mm inner diameter and was
7 available in only two (47 and 58mm) outer diameters. These approaches,
8 however, did not yield a replacement with acceptable performance. Long-term
9 data is not available for the LaGrange/Letournel design, likely because it
did
not become a widely-accepted product. Its head design (35 mm diameter) in
11 conjunction with conventional UHMWPE available at the time would have had
12 a substantial wear rate.
13 Thus, current and past designs have utilized various approaches,
14 including countersinks, in which the center of rotation of the femoral head
is
below the level of the flat surface of the liner. Yet even the use of a
16 countersink design using large head diameters is less than optimal because,
17 among other things, these liners did not have an orifice that was less than
the
18 diameter of the femoral head, and therefore have no mechanical capture, and
19 took no other steps to avoid or minimize dislocation.
It has been shown that larger heads are effective in reducing the rate of
21 recurrent dislocations, even when used with conventional polyethylene.
22 Amstutz et al., 12th Annual International Symposium for Technology in
23 Arthroplasty, ISTA'99, Chicago, IL (1999). However, hip simulator studies
24 have shown that the wear of conventional polyethylene with the larger heads
is
excessive. This disadvantage was demonstrated in vivo by the results of
surface
26 replacement as well. Amstutz et al., Clin. Orthop. 213: 92-114 (1986). In
27 addition, simply using a large head diameter does not produce mechanical
28 capture of the femoral head.
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1 In achieving some of the advantages of the invention, in accordance with
2 one aspect of the invention, there are provided liners with one or more
recesses.
3 The type of recesses according to the present invention, however, are
different
4 than that of the unconstrained Modell St. Georg / Mark I & II and ENDO-
Modell liner, which was to "prevent irritation of the psoas muscle and
enable(s)
6 removal of cement,". rather than for reducing dislocation or impingement.
See
7 Englelbrechht E, Siegel A, Kluber D: The Modell St. Georg/Mark I/Mark II
8 Prosthesis. p. 66. In: Finerman G, Dorey F, Grigoris P, McKellop H (eds):
Total
9 Hip Arthroplasty Outcomes. Churchhill Livingstone, New York, 1998.
In view of the limitations of the commercialized designs, new
1 t approaches are needed that would increase the inherent stability of the
head-
12 liner articulation while at the same time increasing range of motion, which
13 would result in a decreased rate of dislocation for both primary and
revision
14 total hip arthroplasties. Such approaches would decrease patient anxiety,
eliminate the need for postoperative dislocation precautions, and reduce the
16 number of revisions performed for dislocation and recurrent dislocation
17 resulting in a net cost savings to the healthcare system. Such risks of
is dislocation are markedly reduced while range of motion is increased,
19 particularly when a large-head design and/or recesses are employed..
Cut away monopolar constrained acetabular liners (U.S. Patent No. 7,169,186
21 filed May 15, 2002) for use in total hip replacement to help reduce the
22 potential of dislocation of the hip while still providing sufficient range
of
23 motion for daily activities have been developed. The invention disclosed
herein
24 also provides a separate constraining ring that can be used in conjunction
with
the monopolar constrained acetabular liner and the cut away monopolar
26 constrained acetabular liner to provide additional support for the femoral
head
27 to stay with the acetabular liner and avoid dislocation.
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1
2 Summary of the Invention
3
4 It is an object of the invention to provide improved prostheses for use in
the hip. In accomplishing this and other objects, there is provided in
6 accordance with one aspect of the present invention a monopolar acetabular
7 liner that can enclose a femoral head to form a hip replacement prostheses,
s wherein the liner has a rim that creates an orifice, and wherein the orifice
has a
9 diameter that is smaller than the diameter of the femoral head. The rim
extends
io beyond the center line of the head (such as the center of rotation) when
the head
11 is enclosed by the liner, and thereby the head is constrained from
dislocation.
12 The liner can, in certain embodiments, include one or more recesses or cut-
13 aways to facilitate additional range of motion or be without recess or cut
away
14 for use with liners that likewise lack a recess or cut away. The liner, in
another
embodiment, is designed to accommodate proper fitting of a constraining ring
16 that can be used in conjunction with the monopolar constrained acetabular
liner
17 and the cut away monopolar constrained acetabular liner. In accordance with
is another embodiment, there is provided a constraining ring that can be used
in
19 conjunction with the monopolar constrained acetabular liner and the cut
away
monopolar constrained acetabular liner. The constraining rings provide
21 additional support for the femoral head to stay with the acetabular liner
and
22 avoid dislocation. Preferably, the liner comprises ultra high molecular
weight
23 polyethylene, that more preferably is all or in part cross-linked.
Preferably, the
24 cross-linking is performed via irradiation. The femoral head can have a
large
diameter. Preferably, the femoral head has a diameter of 32 mm or larger.
26 The constraining ring can, in certain embodiments, include one or
27 more recesses or cut-aways to be compatible with the corresponding liner.
28 Alternatively, the constraining ring will be without a recess or cut away
for use
with liners that likewise lack a recess or cut away.
6
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I In accordance with another aspect of the present invention, there is
2 provided a monopolar acetabular assembly comprising (A) a monopolar
3 acetabular component that encloses a femoral head, wherein the liner has a
rim
4 that creates an orifice, and wherein the orifice has a diameter that is
smaller
than the diameter of the femoral head; and (B) a metal shell. The rim extends
6 beyond the center line of the head (such as the center of rotation) when the
head
7 is enclosed by the liner, and thereby the head is constrained from
dislocation.
8 The liner can, in certain embodiments, include one or more recesses or cut-
9 aways to facilitate additional range of motion or be without recess or cut
away;
to and a constraining ring which can be used in conjunction with the monopolar
1 t constrained acetabular liner and the cut away monopolar constrained
acetabular
12 liner to provide additional support for the femoral head to stay with the
13 acetabular liner and avoid dislocation. The constraining ring, which can be
14 made from a number of high strength materials (for example, cobalt chrome
is alloy, titanium, stainless steel, etc.), is designed to fit around the
perimeter of
16 the liner. The constraining ring, in another embodiment, is designed to fit
17 properly in conjunction with the monopolar constrained acetabular liner and
the
18 cut away monopolar constrained acetabular liner. Preferably, the liner
19 comprises ultra high molecular weight polyethylene, that preferably is all
or in
20 part cross-linked. Preferably, the cross-linking is performed via
irradiation.
21 Preferably, the femoral head has a large diameter. Preferably, the femoral
22 head has a diameter of 32 mm or larger. The constraining ring can, in
23 certain embodiments, include one or more recesses or cut-aways to be
24 compatible with the corresponding liner. Alternatively, the constraining
ring
25 will be without a recess or cut away for use with liners that likewise lack
a
26 recess or cut away. In accordance with the invention, the constraining ring
is
27 preferably made of high strength materials such as cobalt chrome alloy,
titanium, or stainless steel.
7
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1 In accordance with still another aspect of the present invention, there is
2 provided a hip Joint replacement assembly comprising (A) a monopolar
3 acetabular liner that encloses a femoral head, wherein the liner has a rim
that
4 creates an on ice, and wherein the orifice has a diameter that is smaller
than the
diameter of the femoral head; (B) a metal shell; and (C) a femoral head. The
6 rim extends beyond the center line of the head (such as the center of
rotation)
7 when the head is enclosed by the liner, and thereby the head is constrained
from
s dislocation. The liner can, in certain embodiments, include one or more
9 recesses or cut-aways to facilitate additional range of motion or be without
recess or cut away; and a constraining ring which can be used in conjunction
11 with the monopolar constrained acetabular liner and the cut away monopolar
12 constrained acetabular liner to provide additional support for the femoral
head
13 to stay with the acetabular liner and avoid dislocation. The constraining
ring, in
14 another embodiment, is designed to fit properly in conjunction with the
monopolar constrained acetabular liner and the cut away monopolar constrained
16 acetabular liner. Preferably, the liner comprises ultra high molecular
weight
17 polyethylene, that preferably is all or in part cross-linked. Preferably,
the cross-
es linking is performed via irradiation. The metal shell may be made of
titanium,
19 stainless steel, or a cobalt chrome alloy. Preferably, the femoral head has
a
large diameter. Preferably, the femoral head has a diameter of 32 mm or
21 larger. The constraining ring can, in certain embodiments, include one
22 or more recesses or cut-aways to be compatible with the corresponding
liner.
23 Alternatively, the constraining ring will be without a recess or cut away
for use
24 with liners that likewise lack a recess or cut away.
In accordance with the invention, metal shells preferably are made of
26 titanium, cobalt chrome alloys, or stainless steel. The femoral stems
preferably
27 also may be made of a cobalt chrome alloys, stainless steel, or titanium.
Preferably, the femoral heads are made of a cobalt chrome alloys, stainless
steel
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1 or ceramic. Where the femoral head and stem are one piece, preferably they
are
2 made of a cobalt chrome alloy or stainless steel.
3 In accordance with still a further aspect of the invention, there are
4 provided methods of replacing a hips in patients in need thereof, comprising
the
step of surgically implanting in a given patient a monopolar acetabular liner
6 that can enclose a femoral head to form a hip replacement prostheses,
wherein
7 the Iiner has a rim that creates an orifice, and wherein the orifice has a
diameter
8 that is smaller than the diameter of the femoral head, and thereby the head
is
9 constrained from dislocation. The liner can, in certain embodiments, include
one or more recesses or cut-aways to facilitate additional range of motion or
be
11 without recess or cut away; and a constraining ring which can be used in
12 conjunction with the monopolar constrained acetabular liner and the cut
away
13 monopolar constrained acetabular liner to provide additional support for
the
14 femoral head to stay with the acetabular liner and avoid dislocation. The
constraining ring, in another embodiment, is designed to fit properly in
16 conjunction with the monopolar constrained acetabular liner and the cut
away
17 monopolar constrained acetabular liner. Implantation of assemblies and
total
1s replacements, such as shells, femoral heads and femoral stems, also are
19 provided.
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In accordance with a further aspect of the
invention, there is provided a use of a monopolar acetabular
liner or monopolar component for replacing a hip in a
patient in need thereof, wherein the monopolar acetabular
liner or monopolar component is capable of enclosing a
femoral head to form a hip replacement prostheses, and the
liner has a rim that creates an orifice having a diameter
that is smaller than the diameter of the femoral head,
wherein the femoral head has a diameter of 32 mm or larger.
In contrast to current bipolar and tripolar
constrained components the monopolar constrained design has:
(i) increased range of motion, (ii) decreased wear of the
articulation, (iii) simpler intraoperative assembly,
(iv) decreased risk of impingement of the femoral neck upon
the liner rim, (v) decreased stress transfer to the
acetabular component-cement interface, (vi) decreased stress
transfer to the cement-bone interface, (vii) decreased
stress transfer to the metal shell-bone interface,
(viii) eliminates the thin polyethylene articulating
surfaces associated with multi-polar designs,
(ix) eliminates multipolar locking rings, which have had
reported malfunctions and failure, (x) allows for the use of
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1 large head diameters, (xi) has smaller possible acetabular shell outer
diameters,
2 (xii) utilizes thicker polyethylene bearing surfaces, (xiii) and further
facilitate
3 range of motion in preferred directions.
4 In contrast to the Legrange /Letournel design, the monopolar constrained
design according to the invention has: (i) a different and superior bearing
6 surface, (ii) larger head diameters, (iii) decreased wear, (iv) increased
ROM, (v)
7 the capacity to be use in cemented or bony in-growth acetabular
replacements,
8 (vi) thicker polyethylene bearing surfaces, (vii) modularity for use with
9 uncemented acetabular shells, (viii) can be easily exchanged at the time of
1o primary or revision surgery, and (ix) and can be designed to facilitate
11 movement in preferred directions.
12 Although large head diameters with standard UHMWPE bearing
13 surfaces have been used for the treatment of dislocation and recurrent
14 dislocation by Amstutz et al. In contrast to the designs employed by
Amstutz,
the monopolar constrained has: (i) has a mechanical capture of the femoral
16 head, (ii) reduced wear, (iii) decreased periprosthetic osteolysis, (iv)
modularity
17 for easy conversion between different amounts of constraint, and (v) can be
1s designed to facilitate movement in preferred directions.
19 In embodiments employing the recesses, the liner can have one or more
recess. A recess, often referred to herein as a cut away, can be positioned to
21 further facilitate movement in a desired direction.
22 Embodiments employing a constraining ring can be used in conjunction
23 with the monopolar constrained acetabular liner and the cut away monopolar
24 constrained acetabular liner to provide addition restraint against
dislocation of
the hip joint while at same time not impeding the range of motion of the hip
26 joint. The constraining ring, in another embodiment, is designed to fit
properly
27 in conjunction with the monopolar constrained acetabular liner and the cut
2s away monopolar constrained acetabular liner.
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1 The constraining ring can be secured to the acetabular liner by means of
2 a number of locking mechanisms that are similar to those that are currently
used
3 to secure acetabular liners into their mating metal acetabular shell.
4 Embodiments of this invention have used a locking mechanism which relies on
the spatial interference between the polyethylene acetabular liner and the
metal
6 acetabular shell. This invention is not limited by the use of the locking
7 mechanism used in such embodiments and the use of other locking mechanisms
8 are possible.
9 In another embodiment of this invention have used a locking mechanism
which relies on the spatial interference between the polyethylene acetabular
11 liner and the metal constraining ring. As the constraining ring is being
seated
12 onto the acetabular liner the polyethylene must deform as it moves past a
ridge
13 on the constraining ring. Once the ring is fully seated then the deformed
14 polyethylene relaxes behind the ridge hence securing the constraining ring
to
the liner. This invention is not limited by the use of the locking mechanism
16 used in such embodiments and the use of other locking mechanisms are
17 possible. Yet in another the constraining ring can also be fastened to the
18 acetabular liner by direct screw fixation where screws are passed directly
19 through the constraining ring into the polyethylene liner.
In another embodiment of this invention the constraining ring can be
21 secured to the liner by means of a retaining ring which is designed in an
22 unstressed state to sit in grooves in both the constraining ring and
acetabular
23 liner. Upon setting the constraining ring onto the liner the retaining will
deform
24 until the constraining ring is fully seated onto the liner at which point
the
retaining ring relaxes into the designed grove in the liner.
26 The constraining ring can, in certain embodiments, include one or more
27 recesses or cut aways to be compatible with the corresponding liner.
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Alternatively, the constraining ring will be without a
recess or cut away for use with liners that likewise lack a
recess or cut away.
According to another aspect of the present
invention, there is provided a monopolar acetabular liner
that can enclose a femoral head to form a hip replacement
prostheses, wherein the liner has a rim that creates an
orifice and the liner has at least two recesses, and wherein
the orifice has a diameter that is smaller than the diameter
of the femoral head wherein the femoral head has a diameter
of 32 mm or larger.
According to another aspect of the present
invention, there is provided a hip joint replacement
assembly comprising: (A) a monopolar acetabular liner that
can enclose a femoral head, wherein the liner has a rim that
creates an orifice and the liner has at least two recesses,
and wherein the orifice has a diameter that is smaller than
the diameter of the femoral head, wherein the femoral head
has a diameter of 32 mm or larger; (B) a metal shell; and
(C) a femoral head.
According to another aspect of the present
invention, there is provided a use of a monopolar acetabular
liner or monopolar component of replacing a hip in a patient
in need thereof, wherein the monopolar acetabular liner or
monopolar component is capable of enclosing a femoral head
to form a hip replacement prostheses, wherein the liner has
a rim that creates an orifice having a diameter that is
smaller than the diameter of the femoral head and the liner
has at least two recesses, wherein the femoral head has a
diameter of 32 mm or larger.
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According to still another aspect of the present
invention, there is provided a monopolar constrained
acetabular liner system with constraining ring assembly
comprising: (A) a monopolar acetabular liner that is capable
of enclosing a femoral head, wherein the liner has a rim
that creates an orifice, wherein the orifice has a diameter
that is smaller than the diameter of the femoral head,
wherein the femoral head has a diameter of 32 mm or larger,
wherein a constraining ring fits around the perimeter of the
monopolar constrained acetabular liner, wherein the
constraining ring is capable of being set onto the
acetabular liner during an operative procedure after the
femoral head has been reduced into the acetabular liner; and
(B) a metal shell.
According to yet another aspect of the present
invention, there is provided a hip joint replacement
assembly comprising: (A) a monopolar acetabular liner that
is capable of enclosing a femoral head, wherein the liner
has a rim that creates an orifice, wherein the orifice has a
diameter that is smaller than the diameter of the femoral
head, wherein the femoral head has a diameter of 32 mm or
larger, wherein a constraining ring fits around the
perimeter of the monopolar constrained acetabular liner,
wherein the constraining ring is capable of being set onto
the acetabular liner during an operative procedure after the
femoral head has been reduced into the acetabular liner;
(B) a metal shell; and (C) a femoral head.
According to another aspect of the present
invention, there is provided a use of a monopolar acetabular
liner or monopolar component for replacing a hip in a
patient in need thereof, wherein the monopolar acetabular
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liner or monopolar component is capable of enclosing a
femoral head to form a hip replacement prostheses, and the
liner has a rim that creates an orifice having a diameter
that is smaller than the diameter of the femoral head,
wherein the femoral head has a diameter of 32 mm or larger
and wherein a constraining ring, in use with the monopolar
constrained acetabular liner or a cut away monopolar
constrained acetabular liner, is capable of fitting around
the perimeter of the constrained acetabular liner, and is
further capable of being set onto the acetabular liner
during an operative procedure after the femoral head has
been reduced into the acetabular liner.
According to a further aspect of the present
invention, there is provided a cut away monopolar
constrained acetabular liner system with constraining ring
assembly comprising: a monopolar acetabular liner that is
capable of enclosing a femoral head, wherein the liner has a
rim that creates an orifice, wherein the orifice has a
diameter that is smaller than the diameter of the femoral
head, wherein the femoral head has a diameter of 32 mm or
larger, wherein a constraining ring is used with the
monopolar constrained acetabular liner or the cut away
monopolar constrained acetabular liner, wherein the
constraining ring fits around the perimeter of the
constrained acetabular liner, and wherein the constraining
ring is capable of being set onto the acetabular liner
during an operative procedure after the femoral head has
been reduced into the acetabular liner.
According to yet a further aspect of the present
invention, there is provided a hip joint replacement
assembly comprising: (A) a monopolar acetabular liner that
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is capable of enclosing a femoral head, wherein the liner
has a rim that creates an orifice, wherein the orifice has a
diameter that is smaller than the diameter of the femoral
head, wherein the femoral head has a diameter of 32 mm or
larger, wherein a constraining ring fits around the
perimeter of the monopolar constrained acetabular liner,
wherein the constraining ring is capable of being set onto
the acetabular liner during an operative procedure after the
femoral head has been reduced into the acetabular liner; and
(B) a femoral head.
According to another aspect of the present
invention, there is provided a use of a monopolar acetabular
liner or monopolar component for replacing a hip in a
patient in need thereof, wherein the monopolar acetabular
liner or monopolar component is capable of enclosing a
femoral head to form a hip replacement prostheses, and the
liner has a rim that creates an orifice having a diameter
that is smaller than the diameter of the femoral head,
wherein the femoral head has a diameter of 32 mm or larger,
and a constraining ring, in use with a monopolar constrained
acetabular liner or a cut away monopolar constrained
acetabular liner, is capable of fitting around the perimeter
of the constrained liner and setting onto the acetabular
liner during an operative procedure after the femoral head
has been reduced into the acetabular liner.
These and other aspects of the present invention
will become apparent to the skilled person in view of the
description set forth below.
Brief Description of the Figures
Figure 1 is a schematic view of a monopolar liner
in functional relation with a femoral head and neck.
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Figure 2 is a perspective view of a monopolar
liner containing 2 recesses to facilitate greater range of
motion.
Figure 3 depicts the components of the cut away
monopolar constrained acetabular system with a constraining
ring: metal acetabular shell (52) which is secured to the
acetabulum, polyethylene cut away monopolar constrained
acetabular liner (54), and constraining ring (56). Note the
metal acetabular shell is not essential for this invention
as the acetabular liner can be directly secured into the
acetabulum with bone cement.
Figure 4 depicts constraining rings (56) in
different perspectives.
Detailed Description of Aspects of the Invention
The present invention decreases the risk of
dislocation of total hip arthroplasty in both the primary
and revision setting. See U.S. Patent No. 7,169,186, filed
May 15, 2002. In conjunction with large diameter femoral
heads, such as disclosed in PCT/US99/16070 (the entirety of
which is hereby incorporated by reference), the present
invention can achieve a range of motion that is greater than
is
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1 currently available so called "constrained" systems using 22, 26, 28 and 32
mm
2 heads. A decrease in the rate of dislocation will have a positive effect on
3 patient satisfaction as well as hospital re-admission for treatment of
dislocation.
4 The increased range of motion in this invention will afford patients a more
normal life style than the currently available constrained systems. The
present
6 invention can be surgically implanted in a patient in the same or similar
manner
7 as currently employed implants. Thus, the present invention results in an
s improved quality of life as well as improved patient satisfaction.
9 The present invention advantageously employs UHMWPE liners,
preferably using UHMWPE that is cross-linked, including highly cross-linked
11 UHMWPE. UHMWPE can be cross-linked by a variety of approaches,
12 including those employing cross-linking chemicals and/or irradiation.
Preferred
13 approaches for cross-linking employ irradiation, and are taught in
14 PCT/US97/02220.
According to the invention, there is preferably a two part acetabular
16 assembly, namely a metal shell for bony in-growth with UHMWPE liner,
17 preferably where the bearing surface of the liner that comes into contact
with
1s the femoral head is comprised of crosslinked UHMWPE.
19 The liner of the present invention preferably is "monopolar" system.
The monopolar constrained acetabular preferably has a one piece design and
21 has a single metal to polyethylene articular surface, and thus preferably
is not
22 bipolar or tripolar in design. The monopolar design allows for the use of
23 thicker polyethylene in the acetabular liner. Compared to the bipolar and
24 tripolar constrained/captured designs, the monopolar design decreases the
surface area of contact between metal and polyethylene because there is only
26 one metal-polyethylene articulation, rather than two or more found in other
27 designs. Moreover, the use of crosslinked UHMWPE will decrease the amount
28 of debris particles generated to articulation. Moreover, in the modular
form of
13
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1 this acetabular design, since the polyethylene component is not cemented
into
2 place, some revision surgeries are more feasible. Moreover, with the modular
3 design different types of heads and liners can be used with the shell, and
can be
4 selected by the surgeon during surgery.
According to one aspect of the invention, more than 50% of the volume
6 of the femoral head is enclosed within the polyethylene liner. The diameter
of
7 the opening/orifice of the polyethylene liner is less than the diameter of
the
8 femoral head that is inserted, as shown in Figure 1.
9 Figure 1 depicts at (10) a liner (12) in functional relation with a femoral
head (14) and neck (20). The rim surface (16) of the liner (12) extends beyond
11 the center line (18), representing the center of rotation of both the
femoral head
12 and the acetabular liner. Preferably the liner rim surface is flat in this
13 embodiment. The inner portions of the rim circumscribe the orifice. If
desired,
14 a metal shell, not pictured, would be positioned between the liner and the
bone
at the mount site, and preferably is configured so as to promote in-growth of
the
16 bone.
17 Figure 2 depicts at (30) a type of recessed or cut away liner (32). In the
18 pictured embodiment, the rim surface (34) is interrupted by two cut aways
(36)
19 which permit greater range of motion while preserving a constraint to hold
a
head in place by a snap fit. Region (38) shows that the rim surface (34) of
the
21 liner (32) extends beyond the center line (not specifically depicted) in
such a
22 manner that the opening/orifice of the liner (32) is less than the diameter
of the
23 femoral head that is inserted therein. Thus, the inner portions of the rim
24 circumscribe the orifice in such a manner that the head is held within the
liner.
If desired, a metal shell, not pictured, would be positioned between the liner
26 and the bone at the mount site, and preferably is configured so as to
promote in-
27 growth of the bone.
14
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1 The embodiments that employ one or more recesses can have the
2 recesses positioned to further facilitate movement in a desired direction.
For
3 example, a liner might have a single recess to provide increased range of
4 motion in flexion. Another arrangement can be a liner with two recesses with
the second recess to provide increased range of motion in extension and/or
6 extension plus external rotation. Moreover, the increased range of motion
7 provided by the cut-away design also allows for greater tolerance for the
8 variations in the orientation of the acetabular component that can occur
during
9 insertion.
As stated above, recesses can be positioned to further facilitate
11 movement in a desired direction, such as in the common directions of
12 impingement. Recesses can be placed in several different positions, but
13 preferentially would be placed in the position to produce the range of
maximum
14 motion in a desired direction. For example, in a left hip, when looking at
the
acetabular component in position and considering it to be the face of a clock,
16 the recess would be preferentially placed at about 1-2 o'clock. Under
similar
17 circumstances one would place a recess for a right hip at about 10-11
o'clock.
18 Recesses in these positions assist with flexion alone and flexion plus
internal
19 rotation. For embodiments with two or more recesses, at least one recess
also
would be placed in the position to maximize extension alone and extension plus
21 external rotation, and thus these recesses would be placed at about 4-5
o'clock
22 in the left hip and about 7-8 o'clock in the right hip.
23 The recesses preferably are of sufficient size to accommodate the
24 femoral head and stem (including the neck), while still constraining the
head
within the liner. The size of the recess in terms of width and depth depend
26 upon the size of the stem neck and the range of motion of motion sought,
which
27 are readily determinable by the skilled person based upon a patient's size,
age
28 and needs. If the liner is comprised of a cross-linked polymer, like
irradiated
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1 ultra high molecular weight polyethylene, then the recesses preferably are
2 machined into the liner. The liner, in another embodiment, is modified to
3 accommodate a constraining ring that can be used in conjunction with the
4 monopolar constrained acetabular liner and the cut away monopolar
constrained
acetabular liner to provide additional support for the femoral head to stay
with
6 the acetabular liner and avoid dislocation.
7 Figure 3 depicts at (50) a type of constraining ring (56). In the pictured
s embodiment, showing components of the cut away monopolar constrained
9 acetabular system with a constraining ring: optional metal acetabular shell
1o which is secured to the acetabulum (52), polyethylene cut away monopolar
11 constrained acetabular liner (54), and the constraining ring (56). It is
notable
12 that the metal acetabular shell is not essential for this invention as the
13 acetabular liner can be directed secured into the acetabulum with bone
cement.
14 If desired, the metal shell (52), would be positioned between the liner and
the
bone at the mount site, and preferably is configured so as to promote in-
growth
16 of the bone.
17 Figure 4 depicts at (70) a constraining ring (56) shown in different
1s perspectives.
19 The opening in the monopolar constrained acetabular liner and the cut
away monopolar constrained acetabular liner through which the femoral head
21 passes upon reducing the femoral head into the liner is smaller than the
femoral
22 head, which allows a snap-fit. Therefore, the constrained liner, which is
23 typically made of polyethylene, must deform slightly to allow the femoral
head
24 to be embraced by the liner. The constraining ring, which can be made from
a
number of high strength materials (for example, cobalt chrome alloy, titanium,
26 stainless steel, etc.), is designed to fit around the perimeter of the
monopolar
27 constraining acetabular liner and is intended to serve as a structural
support of
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1 the liner to further ensure that the femoral head will stay within the
acetabular
2 liner.
3 The constraining ring is circular in shape and has "cutouts" similar to
4 that of the cut away monopolar constrained acetabular liner (Figure 3 & 4).
This prevents the constraining ring from interfering with the range of motion
of
6 the hip.
7 The constraining ring is preferably designed to be a single piece which
8 can be set onto the acetabular liner during the operative procedure after
the
9 femoral head has been snap-fitted into the acetabular liner. This allows for
the
1o femoral head to stay snapped within the constrained liner.
11 The constraining ring can, in certain embodiments, include one or more
12 recesses or cut-aways to be compatible with the corresponding liner.
13 Alternatively, the constraining ring will be without a recess or cut away
for use
14 with liners that likewise lack a recess or cut away.
Gross sliding or micro motion between separate metallic surfaces of the
16 total hip components can generate wear debris which upon release into the
joint
17 space can elicit a biological response that can lead to the development of
18 osteolysis and hence increasing the of potential of component loosening.
The
19 design of the constraining ring and the acetabular liner are such that to
prevent
contact between metallic surfaces. The constraining ring is designed to fit
21 properly in conjunction with the monopolar constrained acetabular liner and
the
22 cut away monopolar constrained acetabular liner.
23 The components are designed such that the femoral neck will impinge
24 against the polyethylene liner and not the metallic constraining ring in
all
directions of motion. Also, the constraining ring fastens directly to the
26 polyethylene acetabular liner and has no direct contact with the metal
shell into
27 which the liner is set. Another advantage to this feature is that the use
of the
28 constraining ring is not dependent on the use of a metal shell, hence
allowing
17
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1 this system to be used with an acetabular liner which is cemented directly
into
2 the acetabulum.
3 The constraining ring (see Figures 3 & 4) can be secured to the
4 acetabular liner by means of a number of locking mechanisms which are
currently used to secure acetabular liners into their mating metal acetabular
6 shell. Embodiments of this invention have used a locking mechanism which
7 relies on the spatial interference between the polyethylene acetabular liner
and
8 the metal acetabular shell. This invention is not limited by the use of the
9 locking mechanism used in such embodiments and the use of other locking
1o mechanisms are possible.
11 In another embodiment of this invention have used a locking mechanism
12 which relies on the spatial interference between the polyethylene
acetabular
13 liner and the metal constraining ring. As the constraining ring is being
seated
14 onto the acetabular liner the polyethylene must deform as it moves past a
ridge
on the constraining ring. Once the ring is fully seated then the deformed
16 polyethylene relaxes behind the ridge hence securing the constraining ring
to
17 the liner. This invention is not limited by the use of the locking
mechanism
18 used in such embodiments and the use of other locking mechanisms are
19 possible and contemplated by the invention.
Yet in another the constraining ring can also be fastened to the
21 acetabular liner by direct screw fixation where screws are passed directly
22 through the constraining ring into the polyethylene liner.
23 In another embodiment of this invention the constraining ring can be
24 secured to the liner by means of a retaining ring which is designed in an
unstressed state to sit in grooves in both the constraining ring and
acetabular
26 liner. Upon setting the constraining ring onto the liner the retaining will
deform
27 until the constraining ring is fully seated onto the liner at which point
the
2s retaining ring relaxes into the designed grove in the liner.
18
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1 The constraining ring can, in certain embodiments, include one or more
2 recesses or cut-aways to be compatible with the corresponding liner.
3 Alternatively, the constraining ring will be without a recess or cut away
for use
4 with liners that likewise lack a recess or cut away.
Description of the Assembly of Cut Away Monopolar Constrained
6 Acetabular Liner with Constraining Ring (see Figure 3): Components of the
cut
7 away monopolar constrained acetabular system consisting of. i) metal
8 acetabular shell which is secured to the acetabulum, ii) polyethylene cut
away
9 monopolar constrained acetabular liner, and iii) constraining ring. The
metal
to acetabular shell is not essential for this invention as the acetabular
liner can be
11 directed secured into the acetabulum with bone cement. The cut away
12 monopolar constrained acetabular liner is set into the metal acetabular.
The
13 constraining ring is placed over the femoral component and the femoral head
is
14 snap-fitted into the acetabular liner. The constraining ring is then set
onto the
cut away monopolar constraining acetabular liner with the cutouts of the
16 constraining ring aligned with the cutouts of the acetabular liner.
17 Demonstration of the Position of the Acetabular Component within the
18 Hip and Range of Motion: The actual position of the acetabular component
19 within the hip will be set during the operative procedure and will depend
on the
patient's anatomy and gait. The cut away portion of the acetabular component
21 allows for sufficient range of motion in flexion. The cut away portion of
the
22 acetabular component allows for sufficient internal rotation at 90 of
flexion.
23 The cut away portion of the acetabular component allows for sufficient
external
24 rotation at neutral flexion.
In any of the embodiments of the invention, the liners of the invention
26 can be used with hemispherical and non-hemispherical acetabular shells.
27 Also, the invention can be used with any head diameter, including large
28 head diameters (for example, 35 mm or greater, such as 38 mm, 40 mm, 42
19
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1 mm, 44 mm, and any diameter in between or larger) in order to increase the
2 range of motion in comparison with currently available constrained systems.
3 The invention is further demonstrated by the following example, which
4 do not limit the invention in any manner.
6 Example 1
7
8 The load required for insertion and dislocation of femoral heads in a
9 constrained liner according to the invention was quantified by inserting 32
and
38 mm heads into simulated liners of electron beam crosslinked UHMWPE.
11 Two and three dimensional modeling was performed to assess ROM and
12 stability in vitro. The liner had no chamfer.
13 The ROM, and the loads required for insertion and dislocation using
14 different opening diameters for 32 mm and 38 mm heads in simulated
monopolar liners were measured. A 32 mm head with a 31 mm liner opening
16 required 60 lbs. for insertion and 55 lbs. for dislocation. When the
opening was
17 increased to 31.5 mm the forces for insertion and dislocation decreased to
29
18 lbs., and 24 lbs., respectively. Testing of 35.5 mm, 36.5 mm, and 37.5 mm
19 openings with a 38 mm head was performed. The insertional loads were 157,
130, and 28 lbs. The force necessary to produce dislocation of the femoral
head
21 were 135, 126, and 28 lbs., respectively.
22 The ROM for the 38 mm head and a 3 5.5mm orifice liner is 110 . This
23 increases to 118 and 131 as the orifice diameter is increased to 36.5 and
37.5
24 mm. The 32 mm head and 31 mm orifice liner has 116 ROM. The ROM
increases to 124 when a 31.5 mm orifice is used with the 32 mm head.
26 These data demonstrate that the constrained design according to the
27 invention can minimize the occurrence of dislocation, even with heads and
28 liners that are constructed to permit enhanced range of movement.
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1 It is to be understood that the description, specific examples and data,
2 while indicating exemplary embodiments, are given by way of illustration and
3 are not intended to limit the present invention. Various changes and
4 modifications within the present invention will become apparent to the
skilled
artisan from the discussion, disclosure and data contained herein, and thus
are
6 considered part of the invention.
7
8
21
