U.S. patent application number 09/248792 was filed with the patent office on 2002-01-31 for hip joint prosthesis having a zirconia head and a ceramic cup.
Invention is credited to ABOUAF, MARC, CALES, BERNARD, KWON, OH-HUN, LILLEY, EDWARD, STEFANI, YVES, URFFER, DANIEL.
Application Number | 20020013625 09/248792 |
Document ID | / |
Family ID | 24442002 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020013625 |
Kind Code |
A1 |
ABOUAF, MARC ; et
al. |
January 31, 2002 |
HIP JOINT PROSTHESIS HAVING A ZIRCONIA HEAD AND A CERAMIC CUP
Abstract
This invention relates to a biojoint prosthesis comprising: a) a
prosthetic comprising a first component having an outer surface
comprising at least 90 mol % zirconia, and b) a second component
having a surface shaped to receive the outer surface of the first
component, wherein the outer surface of the first component is
received on the surface of the second component, and wherein a) at
least a portion of the surface of the second component receiving
the first component comprises a ceramic having a surface roughness
of no more than 100 nm, and b) the outer surface of the first
component has a surface roughness of no more than 100 nm.
Inventors: |
ABOUAF, MARC; (WESTBORO,
MA) ; LILLEY, EDWARD; (SHREWSBURY, MA) ;
URFFER, DANIEL; (MORIERES, FR) ; CALES, BERNARD;
(EVREUX, FR) ; KWON, OH-HUN; (WESTBORO, MA)
; STEFANI, YVES; (VANVES, FR) |
Correspondence
Address: |
THOMAS M DIMAURO
SAINT-GOBAIN CORPORATION
1 NEW BOND STREET
P O BOX 15138
WORCESTER
MA
016150138
|
Family ID: |
24442002 |
Appl. No.: |
09/248792 |
Filed: |
February 12, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09248792 |
Feb 12, 1999 |
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08698635 |
Aug 16, 1996 |
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5871547 |
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09248792 |
Feb 12, 1999 |
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08609711 |
Mar 1, 1996 |
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Current U.S.
Class: |
623/23.11 |
Current CPC
Class: |
A61F 2002/30639
20130101; A61F 2002/3621 20130101; A61F 2220/0033 20130101; A61F
2/3609 20130101; A61F 2002/3233 20130101; A61F 2/389 20130101; A61F
2250/0025 20130101; A61F 2/34 20130101; A61F 2/38 20130101; A61F
2310/00029 20130101; A61F 2/3662 20130101; A61F 2310/00203
20130101; A61F 2002/30084 20130101; A61F 2002/30934 20130101; A61F
2002/30349 20130101; A61F 2002/3611 20130101; A61F 2310/00239
20130101; A61F 2002/3493 20130101; A61F 2002/3225 20130101; A61F
2002/30655 20130101; A61F 2/3859 20130101; A61F 2/30767 20130101;
A61F 2002/30332 20130101; A61F 2002/30321 20130101; A61F 2002/30685
20130101; A61F 2310/00317 20130101; A61F 2/32 20130101; A61F
2002/365 20130101; A61F 2310/00023 20130101; A61F 2/468
20130101 |
Class at
Publication: |
623/23.11 |
International
Class: |
A61F 002/36 |
Claims
We claim:
1. A joint prosthesis comprising: a) a prosthetic comprising a
first component having an outer surface comprising at least 90 mol
% zirconia, and b) a second component having a surface shaped to
receive the outer surface of the first component, wherein the outer
surface of the first component is received on the surface of the
second component, and wherein a) at least a portion of the surface
of the second component receiving the first component comprises a
ceramic having a surface roughness of no more than 100 nm, and b)
the outer surface of the first component has a surface roughness of
no more than 100 nm.
2. The prosthesis of claim 1 wherein the joint is a hip joint, the
first component is a substantially spherical ceramic head, the
second component is an acetabular cup having a socket surface
shaped to rotatably receive the outer surface of the ceramic head,
wherein the outer surface of the ceramic head is received in the
socket surface of the acetabular cup, and wherein a) at least a
portion of the socket surface receiving the head comprises a
ceramic having a surface roughness of no more than 50 nm, and b)
the outer surface of the head has a surface roughness of no more
than 50 nm.
3. The prosthesis of claim 2 wherein the portion of the socket
surface comprising a ceramic comprises at least one oxide.
4. The prosthesis of claim 3 wherein the portion of the socket
surface comprising a ceramic comprises at least about 50%
alumina.
5. The prosthesis of claim 3 wherein the ceramic head consists
essentially of zirconia partially stabilized by between 2 mol % and
5 mol % rare earth oxide.
6. The prosthesis of claim 5 wherein the ceramic comprising at
least one oxide is selected from the group consisting of a ceramic
consisting essentially of alumina having a grain size of less than
one micron, YTZP zirconia having a surface roughness of no more
than 15 nm, and zirconia-toughened alumina.
7. The prosthesis of claim 5 wherein the ceramic comprising at
least one oxide is a ceramic consisting essentially of alumina
having a surface roughness of no more than 20 nm.
8. The prosthesis of claim 7 wherein the head consists essentially
of PSZ zirconia having a surface roughness Ra of no more than 10 nm
and the ceramic consists essentially of alumina having a grain size
of between 0.4 and 0.9 .mu.m.
9. The prosthesis of claim 2 wherein the outer surface of the
ceramic head and the socket surface each have a surface roughness
Ra of no more than 15 nm.
10. The prosthesis of claim 2 wherein the outer surface of the head
has a monoclinic zirconia content of less than 10%.
11. The prosthesis of claim 7 wherein the outer surface of the head
and the ceramic socket surface each have a surface roughness Ra of
no more than 15 nm.
12. The prosthesis of claim 2 wherein the head consists essentially
of PSZ zirconia and the socket surface consists essentially of
alumina, and wherein the surface roughness Ra of the outer surface
of the head is at least twice the surface roughness Ra of the
socket surface.
13. The prosthesis of claim 5 wherein the outer surface of the head
and the ceramic socket surface each have a surface roughness Ra of
no more than 10 nm.
14. The prosthesis of claim 1 wherein the articulating surfaces of
the first and second components are selected such that each has a
wear factor of less than 10.sup.-7 mm.sup.3/Nm when subjected in
tandem to ASTM G99 wherein the pin is the material of the outer
surface of the first component and the plate is the material of the
surface of the second component.
15. The prosthesis of claim 2 wherein the outer surface of the
ceramic head and the socket surface of the acetabular cup are
selected such that each has a wear factor of less than 10.sup.-7
mm.sup.3/Nm when subjected in tandem to ASTM G99 wherein the pin is
the material of the outer surface of the ceramic head and the plate
is the material of the socket surface of the acetabular cup.
16. The prosthesis of claim 11 wherein the outer surface of the
ceramic head and the socket surface of the acetabular cup are
selected such that each has a wear factor of less than 10.sup.-7
mm.sup.3/Nm when subjected in tandem to ASTM G99 wherein the pin is
the material of the outer surface of the ceramic head and the plate
is the material of the socket surface of the acetabular cup.
17. An acetabular cup having a socket surface for receiving a
ceramic femoral head having a surface roughness Ra of no more than
50 nm, wherein the improvement comprises at least a portion of the
socket surface comprises a ceramic having a surface roughness of no
more than 50 nm.
18. The cup of claim 17 wherein the ceramic socket surface has a
grain size of less than one micron.
19. The cup of claim 18 wherein the ceramic consists essentially of
alumina.
20. A method of maintaining a low wear condition, comprising the
step of: a) sliding a first ceramic surface having a surface
roughness of no more than 100 nm against a second ceramic surface
having a surface roughness of no more than 100 nm with a force of
between 5N and 25N, thereby providing a wear factor of no more than
10.sup.-8 mm.sup.3/Nm.
21. A hip joint prosthesis comprising: a) a femoral prosthetic
comprising a substantially spherical ceramic head having a
diameter, and b) an acetabular cup having a ceramic socket surface
shaped to rotatably receive the ceramic head, the socket surface
having a diameter, wherein the ceramic head is received in the
socket surface of the acetabular cup, and wherein the diameter of
the socket surface is larger than the diameter of the head, thereby
providing rolling contact between the head and the socket
surface.
22. A hip joint prosthesis comprising: a) a trunnion having a stem
having a taper and b) a substantially spherical ceramic head having
a recess having about the same angle as the taper, wherein the head
is fitted onto the trunnion by a Morse taper lock between the stem
and the recess, wherein the recess completely traverses the
head.
23. A knee joint prosthesis comprising: a) a femoral component
having a base and a plurality of tynes extending therefrom in the
same direction, and b) a tibial plate having a surface shaped for
receiving the tynes, wherein the tynes have an outer surface
comprising zirconia partially stabilized by between 2 mol % and 5
mol % rare earth oxide and a surface roughness of no more than 100
nm, and wherein the surface of the tibial plate receiving the tynes
is a ceramic having a surface roughness of no more than 100 nm.
Description
BACKGROUND OF THE INVENTION
[0001] In surgeries requiring a total hip joint replacement, both
the acetabulum and the upper portion of the femur must be replaced,
and appropriate materials must be selected as the replacement
components. During the 1970's, the femoral prosthetic component was
typically made of a metal such as stainless steel, alloys of
Cr--Co--Mo and titanium, while the mating acetabular cup was
typically made of ultra high molecular weight polyethylene
(UHMWPE). However, it soon became apparent that the metal-UHMWPE
coupling produced significant amounts of polyethylene wear debris
in vivo. This debris has been heavily implicated in the osteolytic
destruction of periarticular tissues and the subsequent loosening
of the hip joint prosthesis. Consequently, the medical community
began to consider replacement materials or the metal heads.
[0002] Because of the wear and debris problem, standardized testing
methods were also developed to help compare the wear rates of
candidate materials for hip joint prosthesis components.
Determining the wear-related suitability of components for use in a
hip joint prosthesis typically involves performing a standard
pin-on-disc wear test, such as ASTM F 732 82, and characterizing
its results by a normalized wear factor, k. The wear factor, k, is
defined as the wear volume V (in mm.sup.3) of a material, divided
by the product of the load P (in N) and the sliding distance X (in
m). According to Saikko, Wear 166 (1993) 169-178, a wear factor of
10.sup.-9 mm.sup.3/Nm is "extremely low" while a wear factor of
10.sup.-7 mm.sup.3/Nm is "considerable".
[0003] From these wear tests, alumina was identified as a candidate
replacement material. The reported wear factors k of the
UHMWPE.backslash.alumina couple of about 10.sup.-7 mm.sup.3/Nm to
10.sup.-9 mm.sup.3/Nm were found to be superior to the
metal.backslash.UHMWPE couple. Accordingly, modular femur
components comprising an alumina head taper fit to a metal stem
were developed, and these components were coupled with both UHMWPE
and alumina cups. Although the alumina-UHMWPE and alumina-alumina
couplings produce less wear debris, the low strength of alumina
(only about 600 MPa) has hindered its widespread acceptance.
Because of the unreliability associated with the low strength of
alumina, other ceramic materials have been considered.
[0004] Over the past five years, artificial hip joint prostheses
having zirconia heads have gained acceptance in the medical
community. As the strength of partially stabilized zirconia is
typically between about 900-1300 MPa and its toughness is at least
about 5 MPa ml/.sup.1/2, it is more reliable than alumina. In fact,
the superior mechanical properties of zirconia has even enabled its
use in small 22 mm heads. In all hip joint prostheses using
zirconia heads, UHWPE cups have been used. See, for example, U.S.
Pat. No. 5,181,929, assigned to Ceramiques Techniques Desmarquest;
Willmann (of Cerasiv), Biomedizinische Technik 39 (4) (1994) 73-78;
Saikko, Wear 166 (1993) 169-178; Derbyshire, Med. Eng. Phys. 16
(1994) 229-236; Tateishi Mat. Sci. Eng. C1 (1994) 121-125; Japanese
Patent Publication (Kokoku) 5-75423; Streicher (of Sulzer)
"Bioceramics, Vol. 4", (1991) 9-16; and Schwartz (of Richards),
"36th Ann. Mtg. ORS"1990 483. Since the reported production of
UHWPE debris has been extremely variable in this case (a wear
factor k of 10.sup.-7 mm.sup.3/Nm according to Streicher and
Derbyshire to 10.sup.-9 mm.sup.3/Nm according to Saikko), there
remains a continuing need to identify materials for use in
acetabulum cups which, when coupled with zirconia heads, produce
even smaller amounts of debris than a UHMWPE cup.
[0005] Researchers studying the wear of zirconia-ceramic couplings
have reported extremely high wear. For example, the results
reported by Ludema, in "Advanced Ceramics for Structural and
tribological Applications" (1995) 37-45, yield an alumina
plate-zirconia pin total wear factor of about 10.sup.-5 mm.sup.3Nm,
and a zirconia plate-alumina pin total wear factor of over
10.sup.-5 mm.sup.3/Nm. The results reported by Medevielle et al.,
in J. Eur. Cer. Soc. 15 (12) (1995) 1193-1200, yield an alumina
plate-zirconia pin total wear factor of about 10.sup.-6 mm.sup.3/Nm
(assuming a test period of 30 minutes). Tucci, in Wear, 172, (1994)
111-119, reports yttria tetragonal polycrystal ("YTZP") zirconia
plate-alumina pin wear factors ranging from 10.sup.-5 mm.sup.3/Nm
to 10.sup.-3 mm.sup.3 Nm. Lastly, the results of Sudanese, in
Alumina vs Zirconium Oxide: A Comparative Wear Test, in
"Bioceramics 1989", pp. 237-240, Oonishi H, Aoki M, Sawai L (eds),
yield a wear factor for a zirconia ring-zirconia disc coupling
which was over 5000 times worse than an alumina-alumina
coupling.
[0006] Because of the high wear factors associated with
zirconia-ceramic couplings, the hip joint prosthesis field has
considered and specifically rejected a zirconia head-ceramic cup
coupling. For example, in Clarke, Clin. Orthop. 282 (1992) 19-30,
the author points to the relative inferiority of zirconia in wear
tests and warns that "there may be a cause for concern given the
potential to mix and match alumina ceramic cups with zirconia
balls". In Willmann, supra, Table 3 specifically discourages using
a zirconia head and an alumina cup and bases that conclusion upon
clinical and technical studies. Sudanese, supra, concludes that ".
. . zirconium oxide cannot be used for ceramic-ceramic coupling
prosthesis, due to its low wear resistance. It may be the
bioceramic material of choice to make ceramic heads in hip
prosthesis with [polyethilene] sockets." Although Kyocera JP Patent
Publication 4303443 discloses an alumina head coupled with a cup
having either an alumina or zirconia mating surface, its reference
to a zirconia cup is merely prophetic and it does not disclose a
zirconia head with a cup having an alumina mating surface. Since
the wear mechanisms for the head and cup are different (see e.g.,
Medevielle's discussion and Ludema's results when the couples are
switched), Kyocera's silence with respect to the zirconia
head-alumina cup combination is notable.
[0007] Accordingly, there exists a continuing need for an
acetabulum cup for use with a zirconia head which produces a wear
factor of less than 10.sup.-7 mm.sup.3/Nm.
SUMMARY OF THE INVENTION
[0008] In accordance with the present invention, there is provided
a joint prosthesis comprising:
[0009] a) a prosthetic comprising a first component having an outer
surface comprising at least 90 mol % zirconia, and
[0010] b) a second component having a surface shaped to receive the
outer surface of the first component,
[0011] wherein the outer surface of the first component is received
on the surface of the second component, and
[0012] wherein a) at least a portion of the surface of the second
component receiving the first component comprises a ceramic having
a surface roughness of no more than 100 nm (preferably, no more
than 50 nm), and b) the outer surface of the first component has a
surface roughness of no more than 100 nm (preferably, no more than
50 nm).
[0013] Preferably, the joint prosthesis is a hip joint prosthesis
comprising:
[0014] a) a femoral prosthetic comprising a substantially spherical
ceramic head having an outer surface comprising at least 90 mol %
zirconia, and
[0015] b) an acetabular cup having a socket surface shaped to
rotatably receive the ceramic head,
[0016] wherein the outer surface of the ceramic head is received in
the socket surface of the acetabular cup, and
[0017] wherein a) at least a portion of the socket surface
receiving the head comprises a ceramic having a surface roughness
Ra of no more than 100 nm (preferably, no more than 50 nm), and b)
the outer surface of the head has a surface roughness of no more
than 100 nm (preferably, no more than 50 nm).
[0018] Preferably, the portion of the socket surface comprising a
ceramic comprises at least one oxide, more preferably it consists
essentially of either a biomedical grade alumina having a surface
roughness of no more than 20 nm or a zirconia partially stabilized
with between 2 and 5 mol % rare earth oxide and having a surface
roughness of no more than 20 nm.
[0019] Preferably, the ceramic head consists essentially of a
zirconia partially stabilized with between 2 and 5 mol % rare earth
oxide ("PSZ"), and preferably has a surface roughness Ra of no more
than 15 nm, more preferably no more than 10 nm.
DESCRIPTION OF THE FIGURES
[0020] FIG. 1 shows a cross section of an embodiment of the present
invention utilizing conventional hip joint prosthesis geometry.
[0021] FIG. 2 shows a cross section of an embodiment of the present
invention wherein the frustoconical cavity of the zirconia head
completely traverses the head.
[0022] FIG. 3 shows a cross section of a head-cup combination in
which the diameter of the cup is at least about 10% larger than the
diameter of the head, thereby allowing rolling contact.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0023] It has been found that a pin-plate combination of hipped
YTZP zirconia-hipped YTZP zirconia having surface roughnesses of
about 11 nm and 8 nm, respectively, produced low wear rates in
pin-on-discs wear tests designed for hip joint prosthesis
applications. In particular, the wear factor k of the zirconia pin
was found to be only 3-6.times.10.sup.-8 mm.sup.3/Nm. In a related
test, an alumina ball having a surface roughness Ra of about 40 nm
was slid against the same YTZP plate and a wear factor of
8.5.times.10.sup.-9 mm.sup.3/Nm was found for the ball and
negligible wear was found for the YTZP plate. Concurrently, in an
independent test, it has also been found that a YTZP-alumnia couple
produced lower wear rates than either YTZP-YTZP or alumina-alumina
couples. Although the pin-on-disk test is only a screening test
(and the zirconia head-alumina cup and zirconia head-zirconia cup
couplings should still be validated on a joint simulator machine),
the positive results indicate that ceramics in general, and smooth
surfaced oxides such as alumina and zirconia in particular, are
potential candidate materials for use with smooth zirconia surfaces
in bioprosthetic joints such as hips, knees, toes, wrists, elbows,
ankles and fingers in general, and in the acetabular cup of a hip
joint prosthesis having a zirconia head in particular.
[0024] The present invention is surprising in light of the prior
art teachings on ceramic wear. In general, the prior art taught
that intrinsic factors such as toughness, hardness and thermal
conductivity determine the initial wear, and that third body debris
determines the wear rate thereafter. Without wishing to be tied to
a theory, it is believed that the primary factor in producing the
vastly superior wear factors k of the present invention is the
heretofore unappreciated criticality of maintaining the surface
roughness Ra of both of the articulating surfaces below 100 nm
(preferably, below 50 nm) in order to achieve a wear factor of
10.sup.-7 mm.sup.3/Nm. The present invention relies on the belief
that surface damage is less manageable than the art admits and that
maintaining the surface roughness Ra at each surface below 100 nm
can substantially prevent surface damage nucleation when the
articulation is in the low force regime common to hip joint
prosthesis articulating surfaces. Simply, it is believed that, in
order to attain low wear rates in zirconia-ceramic hip joint
prosthesis couples, the surfaces must be made smooth and kept
smooth. In contrast, the prior art routinely controlled only one of
the articulating surfaces' roughness, not both. For example,
Medevielle et al. used as-sintered balls in their ball on ring
tests. Ludema et al. not only reported just the surface roughness
of their plates, they also used a low porosity alumina having a 217
nm. Tucci et al. reported that only the surface under tensile
stress (i.e., the plate) was polished to a mirror finish and did
not report the Ra of their low density alumina pins. Sudanese did
not report the surface roughness for the zirconia ring.
[0025] The head component preferably consists essentially of a
ceramic comprising at least about 90 mol % zirconia, and more
preferably is a partially stabilized zirconia (PSZ). The PSZ is
typically partially stabilized by a rare earth oxide at a
concentration of between about 2 mol % and about 5 mol %. Most
preferably, the PSZ is yttria stabilized tetragonal zirconia
polycrystal (YTZP). Preferably, the YTZP has a mean grain size (SEM
using ASTM E 112/82) of no more than 1 micron (.mu.m), preferably
between 0.3 and 0.8 .mu.m. The bulk of the head should have a four
point flexural strength of at least about 920 MPa, preferably at
least 1300 MPa. Its density should be at least 99.7% of theoretical
density, preferably at least 99.8%. In some embodiments, it has an
elasticity modulus (ASTM C 674) of no more than 220 GPa; an open
porosity of no more than 0.1%; less than 1% impurities; and a
fracture toughness (as per Chantikul) of at least 5 MPa
m.sup.1/2.
[0026] Preferably, the head has an outer surface having a roughness
of no more than 15 nm, more preferably no more than 10 nm. It has
been found that controlling the surface roughness Ra of each
surface to less than 15 nm reduces wear even more significantly. In
one side-by-side comparison of YTZP pins having surface roughness
Ra's of 11.5 nm and 25.6 nm, respectively, the wear rate of the
smoother pins was found to be about seven times lower than the
coarser pin. Generally, the outer surface of the head contains less
than 10% monoclinic zirconia, preferably less than 5%. Most
preferably, the outer surface consists essentially of 100%
tetragonal zirconia.
[0027] Preferably, the outer surface of the head is made of the
same material as the bulk of the head (i.e., the head is a
monolith) However, it is contemplated that the outer layer of the
head can be another material. In such a case, both the bulk and the
outer surface preferably possesses mechanical characteristics
similar to those disclose in U.S. Pat. No. 5,181,929, the
specification of which is incorporated by reference.
[0028] In one preferred method of making the YTZP zirconia, the
rare earth oxide powder and submicron zirconia powder are mixed,
the mixture is cold isostatically pressed at between 50 and 400 MPa
and appropriately green machined to form a green sphere which is
then sintered at between about 1300.degree. C. and 1500.degree. C.
for about 1 to 4 hours to achieve a density of at least 95%; and
the sintered piece is hipped in an inert gas such as argon at
between 1300.degree. C. and 1500.degree. C. for between 0.5 and 4
hours to produce a sintered sphere having a density of at least
99.9%, and a grain size of at most less than one micron. Without
wishing to be tied to a theory, it is believed the hipping allows
the densification to take place at a lower temperature, thus
preventing substantial grain coarsening. It is also believed the
hipping closes porosity and heals cracks which can promote
wear.
[0029] In preferred embodiments, both the socket surface and the
outer surface of the head each have a surface roughness of no more
than 20 nm, preferably less than 10 nm, more preferably no more
than 5 nm. When a relatively hard material such as alumina is used
as the socket surface, the preferred levels of roughness for the
socket surface are smaller than those of the outer surface of the
head because alumina is harder than zirconia and so is more apt to
become a mini-grinding wheel and create a fracture in the zirconia
surface. Therefore, in some embodiments in which the outer surface
of the head is a PSZ and the socket surface consists essentially of
alumina, the PSZ outer surface of the head has a surface roughness
Ra of at least twice that of the alumina socket surface.
[0030] In some embodiments, the ceramic comprising at least a
portion of the socket surface has a grain size of less than two
microns, preferably less than one micron (by linear intercept
method). Preferably, this ceramic has a density of at least 3.9
g/cc, more preferably at least 3.97 g/cc; and a grain size of
between 0.4 and 0.9 .mu.m. In some embodiments, it has a 4 point
flexural strength of at least 400 MPa, more preferably at least 550
MPa. Most preferably, the ceramic comprising at least a portion of
the socket surface consists essentially of alumina.
[0031] A preferred alumina can be produced by sintering Ceralox
APA-0.5 MgO alumina, available from Ceralox Corp. of Tuscon, Ariz.
at about 1400 C. for about 60 minutes and then hipping at 1350 C.
and 200 MPa for 45 minutes. In addition, sol gel processes such as
those disclosed in U.S. Ser. No. 07/884,817, now abandoned, or U.S.
Pat. No. 4,657,754, the specifications of which are incorporated by
reference, can also be used to make fine grained alumina.
[0032] If zirconia toughened alumina is selected as the ceramic
comprising at least a portion of the socket surface, then it is
preferable to use a material comprising at least 20 vol % zirconia
as disclosed in U.S. Pat. No. 4,316,964, the specification of which
is incorporated by reference.
[0033] In one especially preferred embodiment, the head consists
essentially of a YTZP zirconia, stabilized by between about 2 and 5
mol % rare earth oxide, having a flexural strength of at least 900
MPa, and preferably 1200 MPa, and surface roughness of no more than
10 nm, and a density of at least 99.7% of theoretical density; and
the socket surface consists essentially of alumina having a
flexural strength of at least 500 MPa, a surface roughness of no
more than 10 nm, and a density of at least 99.8% of theoretical
density. Without wishing to be tied to a theory, it is believed
that maintaining these surface roughnesses on these materials will
provide a total wear factor of no more than about 10.sup.-7
mm.sup.3/Nm in zirconia pin-alumina disk tests using otherwise
standard ASTM G99 test conditions.
[0034] The surfaces of each of the articulating surfaces should be
polished by carefully staged grinding, lapping and finishing steps
which insure that subsurface damage is minimized in achieving the
smooth surface. One preferred method of polishing includes the
procedure using diamond grit shown below in Table I.
1TABLE I Slice Grit Size Wheel type Speed Pressure Duration Ra
.mu.m rpm psi min .mu.m A #320 blank 0.110 B 40 alumina 100 20 4
0.130 C 30 platen 25 25 3 0.028 D 9 platen 125 25 2 0.020 E 3
texmet 200 30 1.2 0.012 F 1 cloth 300 45 1.2 0.008 G 0.25 cloth 400
60 <1 0.005
[0035] Preferably, the geometry of the head is essentially a
spherical ball having a diameter of between about 22 and 32 mm and
a single frustoconical cavity whose total angle at its apex is
about 6 degrees. Examples of some preferred cavity designs are
found in U.S. Pat. Nos. 4,964,869 and 5,181,929, the specifications
of which are incorporated by reference. The sphericity of the head
should at least be comparable to a sphericity of less than 5 .mu.m
for a 28 mm head, as measured by Mitutoyo apparatus (BHN 305).
[0036] In some embodiments, substantially all of the acetabular cup
comprises the ceramic comprising at least one oxide. Accordingly,
cup designs such as those described in U.S. Pat. Nos. 3,924,275 and
4,636,218, the specifications of which are incorporated by
reference, may be used. However, in other embodiments, the socket
surface of the acetabular cup comprising the ceramic comprising at
least one oxide is metal backed. In these situations, designs such
as those described in EPO Patent Publication Al 0,278,205, may be
used.
[0037] During surgery, the head is fitted to the metal stem portion
of the femoral prosthetic by friction fitting the head's cavity
upon the cone of the stem. Referring now to FIG. 1, there is
provided a femoral prosthesis according to the present invention.
The first end 3 of metal femoral rod 2 is implanted into femur 1.
The second end of the rod 2 is shaped to a truncated cone 4. The
cavity of the zirconia head 5 having about the same tape angle as
cone 4 is press fitted onto cone 4. Side wall 6 of the head 5
defined by the frustoconical cavity is in contact over its
substantial length with the lateral wall 7 of the male cone 4. A
space 8 between the top of the cone 4 and the top of the cavity is
also shown, thereby forming corners 12. Concurrently, the
acetabular cup 13 having a socket surface 14 for receiving the head
5 is fitted into the pelvic bone 15. Lastly, the head 5 is
positioned in the socket surface 14 of the acetabular cup 13 to
form the hip joint.
[0038] In some embodiments of the present invention, the head's
frustoconical cavity extends fully through the head. See FIG. 2. It
has been found that designing the cavity to completely traverse the
head provides the head with a very high rupture strength. It is
believed that the deep corners 12 of the cavities in conventional
heads are regions of tensile stress concentration and that
eliminating the corners eliminates this stress, thereby providing a
more uniform hoop stress distribution along the sidewall 6.
[0039] In other embodiments, the cavity geometry includes a chamfer
11 which narrows inwardly from the surface of the head which
intersects the metal cone. See FIG. 2. It has been found that
providing this chamfer reduces the stress at the edge of the
head.
[0040] Preferably, the metal femoral rod is a titanium alloy,
Cr--Co--Mo, or any other metals or alloys used for making
orthopedic implants.
[0041] When articulation occurs between the zirconia head and the
ceramic socket surface, the sphericity mismatch results in point
contact therebetween with stresses as high as 500 MPa. Because the
ceramic socket is routinely designed to be hemispherical and just
slightly larger than the head, the only relative movement is
sliding movement and at least one of the contacting surfaces
remains in contact with the opposing surface, thereby sustaining
the high stress on that point. However, designing a cup to allow
more freedom of movement for the head will create the rolling
contact needed to allow different points to contact and relieve
those particularly stressed initial points of contact. Therefore,
in accordance with the present invention, there is also provided a
hip joint prosthesis comprising:
[0042] a) a femoral prosthetic comprising a substantially spherical
ceramic head having a diameter, and
[0043] b) an acetabular cup having a socket surface shaped to
rotatably receive the ceramic head, the socket surface having a
diameter,
[0044] wherein the ceramic head is received in the socket surface
of the acetabular cup, and
[0045] wherein the improvement comprises the diameter of the socket
surface is larger, and preferably at least about 10% larger, than
the diameter of the head, thereby providing rolling contact between
the head and the socket surface. It is believed this design will
also assist in expelling debris from the articulation
interface.
[0046] Also in accordance with the present invention, there is
provided a method of maintaining a low wear condition, comprising
the step of:
[0047] a) sliding a first ceramic surface having a surface
roughness of no more than 100 nm (preferably, no more than 50 nm)
against a second ceramic surface having a surface roughness of no
more than 100 nm (preferably, no more than 50 nm) with a force of
between 5 N and 25 N, thereby providing a wear factor of no more
than 10.sup.-8 mm.sup.3/Nm.
[0048] Moreover, it is believed that the criticality of controlling
surface roughness Ra of both ceramic surfaces is also applicable to
knee joint prostheses having zirconia femoral components.
Therefore, in accordance with the present invention, there is also
provided a knee joint prosthesis comprising:
[0049] a) a femoral component having a base and a plurality of
tynes extending therefrom in the same direction, and
[0050] b) a tibial plate having a surface shaped for receiving the
tynes,
[0051] wherein the tynes have an outer surface comprising zirconia
partially stabilized by between 2 mol % and 5 mol % rare earth
oxide and a surface roughness of no more than 100 nm (preferably,
no more than 50 nm), and
[0052] wherein the surface of the tibial plate receiving the tynes
is a ceramic having a surface roughness of no more than 100 nm
(preferably, no more than 50 nm).
[0053] The requirements set out above for the hip joint zirconia
head and ceramic socket should also be considered to be applicable
for the knee femoral zirconia femoral component and ceramic tibial
plate surface.
Example I
[0054] TZ-3Y, a hydrolyzed zirconia powder containing 3 mol %
yttria was formed into tiles by uniaxial pressing at 20,000 lbs
followed by cold isostatic pressing at 30,000 Kip. TZ-3Y was also
formed into cylinders for making balls by pressing at 30,000 Kip.
Both the tiles and cylinders were sintered in air at 1325 degrees C
for 2 hours and then hipped in argon at 1350 degrees C. for 1 hour.
The final density of each was at least 99% of theoretical
density.
[0055] The tiles were finished to about 8 nm using 1-15 .mu.m
diamond paste on a Ectomet 4 Semi-Automatic polisher/grinder,
available from Buehler of Tuscon, Ariz. The cylinders were machined
into balls and then polished to a surface roughness Ra of about
11-12 nm by Chand Kare Technical Ceramics of Worcester, Mass.
[0056] Some of the balls were selected for heat treatment at 1450
degrees C. for 40 hours. The heat treatment caused pores in the
balls of about 1 micron. Post-heat treatment cooling was performed
rapidly to avoid LTD. Intrinsic properties of the heat treated
balls (Balls 2 and 4) and the untreated balls (Balls 1 and 3)are
shown in Table II.
2 TABLE II Balls 1 and 3 Balls 2 and 4 Hardness 12.8 GPa 9.35 GPa
Toughness 5.21 MPa m.sup.1/2 7.24 MPa m.sup.1/2 Grain Size 0.3
.mu.m 0.87 .mu.m
[0057] In addition, some of the heat treated balls and some of the
untreated balls were repolished with 1 .mu.m diamond paste with a
buffing wheel. The resulting surface roughnesses Ra are shown in
Table III below.
[0058] Wear tests were performed on a Falex (TM) Multi-Specimen
Wear Tester using ball on flat geometry. The flat was held
stationary while the ball was rotated to create a circular scar on
the flat. The tests were performed unlubricated in air at room
temperature. A load of 9 N was used. Velocity was 5 mm/sec. The
total sliding distance was 40 meters. Wear volume was calculated
via ASTM G99. The calculated wear factors are provided in Table III
below.
3TABLE III Ball Heat Treatment Repolish Ra (nm) Wear Factor 1 NO
YES 11.1 6 .times. 10.sup.-8 mm.sup.3/Nm 2 YES YES 11.5 6 .times.
10.sup.-8 mm.sup.3/Nm 3 No NO 12.6 1.5 .times. 10.sup.-7
mm.sup.3/Nm 4 Yes NO 25.6 4 .times. 10.sup.-7 mm.sup.3/Nm
[0059] As seen above, balls 1 and 2 have different hardnesses and
toughnesses (intrinsic properties) but had the same surface
roughness and same wear factor. Balls 1 and 3 have the same
intrinsic properties, but the 10% smoother surface gave a three
times lower wear factor. Balls 2 and 4 have the same intrinsic
properties, but a 60% smoother surface gave a seven times lower
wear rate. Accordingly, surface roughness Ra plays as large a role
in the wear rates of zirconia-ceramic couples in this wear regime
as hardness or toughness. The experiments of Example I were
performed without lubrication. Since lubrication often reduces the
wear factor by a factor of about 5, and the actual hip joint
prosthesis articulation may be lubricated by a fluid, controlling
the surface roughness to a level below 25 nm for each YTZP surface
may provide superior protection.
Example II
[0060] A roller-plate was devised to study different combinations
of ceramics in wear having relatively high surface roughnesses The
selected ceramics included alumina, YTZP zirconia, zirconia
toughened alumina (ZTA), and a sintered silicon nitride (SN). The
YTZP was a PROZYR-type zirconia ceramic, a YTZP ceramic available
from Ceramiques Techniques Desmarquest of Evreux, France. Each
roller had an outer diameter of about 35 mm, a length of about 20
mm, and was believed to have a thickness or about 4 mm, and rotated
at about 45 revolutions per minute against the plate. Each plate
had a 20 mm.times.30 mm face and a 8 mm thickness, and moved back
and forward against the roller in a 12 mm span at a speed of 1
mm/second. Total sliding distance was calculated by following a
point on the face of the plate and was found to be about 2967 m.
Roller and plate combinations were subject to wear testing at room
temperature and 200 N in Ringers solution for 24 hours with a
normal force of 200 N. The results are found in Table V below:
4TABLE V Roller Plate Roller Plate Ra Weight Weight Wear Friction
Roller Plate Ra (um) (um) Loss (%) Loss (%) factor k Coeff't
Alumina Alumina 1.2 1.2 0.07 0.08 3.2 .times. 10.sup.-5 0.22 YTZP
ZTA 0.9 0.16 0.30 0.58 1.9 .times. 10.sup.-4 0.72 YTZP SN 0.9 0.17
0.02 0.05 1.1 .times. 10.sup.-4 0.69 YTZP YTZP 0.9 0.17 2.4 2.1 1
.times. 10.sup.-3 0.5 YTZP Alumina 0.9 1.2 0.018 neg. 3.9 .times.
10.sup.-6 0.25
[0061] The results indicate higher wear factors than those
calculated in the other Examples. The reason for this difference is
believed to be the difference in surface roughness Ra. In addition,
the results showed the zirconia roller-alumina place combination to
be superior to the alumina roller-alumina plate combination, and
far superior to the zirconia roller-zirconia plate combination.
Example III
[0062] In this experiment, an alumina ball having a surface
roughness of about 40 nm replaced the YTZP ball of Example I and
the load was about 20 N. The wear factor of the alumina ball was
found to be about 8.5.times.10.sup.-9 mm.sup.3/NM, while the wear
factor for the YTZP plate was negligible. Although it may have been
more appropriate (for the purposes of evaluating a hip joint
prosthesis having a zirconia head and a ceramic cup) to test an
alumina plate and a YTZP ball, these results suggest the alumina in
a zirconia-alumina couple need not be as smooth as the zirconia in
a zirconia-zirconia couple in order to attain good results.
Example IV
[0063] Alumina and zirconia ceramic specimens were used in wear
testing experiments to screen these materials for possible use as
prosthetic implant articulating surfaces.
[0064] The alumina was made by sintering Ceralox APA-0.5 MgO
alumina at about 1400 C. for about 60 minutes and then hipping at
1350 C. and 200 MPa for 45 minutes. The resulting ceramic had a
density of at least 3.97 g/cc; and a grain size of between 0.4 and
0.9 .mu.m (by linear intercept method), and a 4 point flexural
strength of at least 550 MPa.
[0065] The YTZP zirconia was made by mixing a rare earth oxide
powder and submicron zirconia powder, cold isostatically pressing
the mixture at between 50 and 400 Mpa, appropriately green
machining the body to form a green sphere, sintering at between
about 1300.degree. C. and 1500.degree. C. for about 1 to 4 hours to
achieve a density of at least 95%; and hipping the sintered piece
in an inert gas at between 1300.degree. C. and 1500 C. for between
0.5 and 4 hours. The resulting ceramic had a density of at least
99.9%, a grain size of between 0.3 and 0.8 .mu.m, and a four point
flexural strength of at least about 920 Mpa.
[0066] These materials were tested by the pin-on-disk method,
wherein a pin with a hemispherical tip was slid on the flat surface
of a rotating disk , thus describing a circular unidirectional
path. Four combination were tested: zirconia pin-zirconia zirconia
disk, zirconia pin-alumina disk, alumina pin-alumina disk, and
alumina pin-zirconia disk. Each disk had a surface roughness Ra of
about 12.7 nm (0.5 uin). The zirconia pins had a roughness of about
25.4 nm (1 uin), while the alumina pins had a surface roughness of
about 51-102 nm (2-4 uin).
[0067] The tests were performed on an ISC-200 PC tribometer
(available from Implant Sciences Corp., Wakefield, Mass. 01880)
using pins with a 12.7 mm (half inch) diameter hemispherical tip
and disks 5.08 cm (2.0 inch) in diameter by 0.635 cm (0.25 inch)
thick. The tests were run at room temperature , lubricated with
bovine calf serum containing a bactericide and a precipitation
inhibitor and at a sliding speed of 5 cm/sec. The travel distance
of the pin on the disk was typically 350 meters. The applied load
on the pin was 4.903 N (500 gf), corresponding to an initial
average contact Hertzian stress of 460 MPa for the
zirconia-zirconia pair, 510 MPa for the mixed pairs, and 580 MPa
for the alumina pair. The initial contact diameter was calculated
to be approximately 0.1 mm for all the pairs. Each pair was run at
least three times using the same pin and disk in different
locations.
[0068] After 350 m, no wear could be detected on the disks by
profilometry. The wear tracks on the disks were not visible to the
naked eye except for the zirconia-zirconia pair, and then only
faintly. In contrast, the wear on the pins was readily observable,
as they exhibited the typical round to elliptical wear scars. There
was no evidence of microcracking or brittle fracture in the wear
areas, as examined by SEM.
[0069] The values reported in Table IV below for each pair
represent the average of the wear factors values obtained in each
run. Good reproducibility was obtained for each run.
5TABLE IV Wear Friction Run Pin Disk Factor k Coeff't 1 Zirconia
Alumina 1.6 .times. 10.sup.-8 0.079 2 Alumina Zirconia 1.6 .times.
10.sup.-8 0.119 3 Alumina Alumina 1.8 .times. 10.sup.-8 0.134 4
Zirconia Zirconia 2.2 .times. 10.sup.-7 0.186
[0070] This example shows that the mixed pairs have superior wear
factors over the even the alumina-alumina pair, and that the
zirconia pin-alumina disk combination has the lowest coefficient of
friction.
Example V
[0071] These two experiments were performed in a manner
substantially similarly to Example I, except that each substrate
was subjected to a heat treatment of only two hours. In experiment
A, the ball had a surface roughness Ra of about 10 nm and the plate
had a surface roughness of about 10 nm. In Experiment B, the ball
had a surface roughness Ra of about 10 nm and the plate had a
surface roughness of about 100 nm. Simply, the plate of Experiment
B was only machined and therefore was rougher than its counterpart
in Experiment A. The combinations were test under a load of 10
N.
[0072] The results of the test are shown in Table VI below:
6TABLE VI Experiment Ball Ra (nm) Plate Ra (nm) Wear Factor k A 10
10 5 .times. 10.sup.-8 B 10 100 2 .times. 10.sup.-6
[0073] These results are further evidence that each counterface
must have a fine surface finish in order to produce wear factors
considered desirable for articulating surfaces in a hip joint
prosthesis.
[0074] For the purposes of the present invention, surface roughness
Ra is determined via contact profilometry with a 2 .mu.m radius
diamond stylus and a cutoff length of 0.08 mm.
* * * * *