U.S. patent application number 11/147930 was filed with the patent office on 2006-01-26 for prosthesis device for the ankle articulation.
Invention is credited to Fabio Catani, Sandro Giannini, Alberto Leardini, John J. O'Connor.
Application Number | 20060020345 11/147930 |
Document ID | / |
Family ID | 11343982 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060020345 |
Kind Code |
A1 |
O'Connor; John J. ; et
al. |
January 26, 2006 |
Prosthesis device for the ankle articulation
Abstract
A prosthetic ankle joint device for replacement of articular
bearing surfaces of the natural joint comprises: a first component
(2) for attachment to a first tibial bone segment (2a) having a
first articulating bearing surface (5); a second component (3) for
attachment to a second tarsal bone segment (3a) having a second
articulating bearing surface (6) opposite to the first articulating
bearing surface (5). The first articulating bearing surface (5) and
the second articulating bearing surface (6) are substantially
different in shape to the corresponding shapes of the articular
bearing surfaces of the natural joint. The first surface (5) being
concave, or flat, or convex. Each articulating bearing surface (5,
6) presents a respective shape designed on the basis of the shape
of the opposite articulating bearing surface (5, 6) according to
the common normal to their point of mutual contact at every
position over the range of passive plantar and dorsi-flexion. The
common normal theorem being applicable because of a four-bar
linkage mechanism identified at this joint in the sagittal plane,
formed by the tibio-calcaneal calcaneo-fibular ligaments.
Inventors: |
O'Connor; John J.; (Oxford,
GB) ; Leardini; Alberto; (Bologna, IT) ;
Giannini; Sandro; (Viareggio, IT) ; Catani;
Fabio; (Bologna, IT) |
Correspondence
Address: |
Richard J. Minnich, Esq.;Fay, Sharpe, Fagan, Minnich & McKee, LLP
Seventh Floor
1100 Superior Avenue
Cleveland
OH
44114-2579
US
|
Family ID: |
11343982 |
Appl. No.: |
11/147930 |
Filed: |
June 8, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10019994 |
Nov 9, 2001 |
6926739 |
|
|
PCT/IB00/00638 |
May 12, 2000 |
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11147930 |
Jun 8, 2005 |
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Current U.S.
Class: |
623/21.18 |
Current CPC
Class: |
A61F 2002/30892
20130101; A61F 2002/4658 20130101; A61F 2/4202 20130101; A61F
2/4606 20130101; A61F 2002/4205 20130101; A61F 2002/4207 20130101;
A61F 2230/0069 20130101; A61F 2/4684 20130101; A61F 2002/30616
20130101; A61F 2002/30604 20130101; A61F 2310/00011 20130101; A61F
2/4657 20130101; A61F 2250/0064 20130101; A61F 2/30767 20130101;
A61B 17/1775 20161101; A61F 2002/30224 20130101 |
Class at
Publication: |
623/021.18 |
International
Class: |
A61F 2/42 20060101
A61F002/42 |
Foreign Application Data
Date |
Code |
Application Number |
May 13, 1999 |
IT |
BO99A000253 |
Claims
1. A prosthetic ankle joint device for replacement of articular
bearing surfaces of the natural joint comprising: a first component
for attachment to a first tibial bone segment said first component
having a first articulating bearing surface; a second component for
attachment to a second tarsal bone segment, said second component
having a second articulating bearing surface opposite to the first
articulating bearing surface; said first articulating bearing
surface and said second articulating bearing surface being each
substantially different in shape to the natural corresponding
shapes of said articular bearing surfaces of the natural joint, the
first articulating bearing surface being concave, or planar, or
convex; each articulating bearing surface presents a respective
shape designed on the basis of the shape of the opposite
articulating bearing surface according to the common normal theorem
to their point of mutual contact at every position over the range
of passive plantar and dorsi-flexion.
2. A prosthetic ankle joint device according to claim 1, wherein
the common normal to the point of mutual contact of the
articulating bearing surfaces at every position over the range of
passive plantar and dorsi-flexion passes close to the intersection,
in the sagittal plane, of the most nearly isometric fibres in the
tibio-calcaneal ligament on the medial side of the joint and
calcaneo-fibular ligament on the lateral side of the joint.
3. A prosthetic ankle joint device according to claim 1, wherein
the common normal to the point of mutual contact of the
articulating bearing surfaces at every position over the range of
passive plantar and dorsi-flexion passes through the intersection,
in the sagittal plane, of the most nearly isometric fibres in the
tibio-calcaneal ligament on the medial side of the joint and
calcaneo-fibular ligament on the lateral side of the joint.
4. A prosthetic ankle joint device according to claim 1, wherein
said second bearing surface (5, 6) presents a radius of curvature,
in the sagittal plane, up to 75% longer than that of the natural
talar articulating bearing surface.
5. A prosthetic ankle joint device according to claim 1, wherein
said second articulating bearing surface (6) presents a radius of
curvature equivalent to a value close to the value of the radius of
the corresponding natural surface of the natural ankle joint.
6. A prosthetic ankle joint device according to claim 5, wherein
the radius of curvature of said second articulating bearing (6)
surface is comprised between 22 mm and 34 mm.
7. A prosthetic ankle joint device according to claim 1, wherein
said first component comprises a first portion for attachment to
the first tibial bone and a second portion fastened to said first
portion, said second articulating bearing surface being defined on
said second portion of said first component.
8. A prosthetic ankle joint device according to claim 7, wherein
said first portion of said first component is made of a metallic
material.
9. A prosthetic ankle joint device according to claim 7, wherein
said second portion of said first component is made of a plastic
material, preferably ultra-high molecular weight polyethylene.
10. A prosthetic ankle joint device according to claim 1, wherein
one of the articulating bearing surfaces is substantially planar,
in the sagittal plane while the other articulating bearing surface
is convex.
11. A prosthetic ankle joint device according to claim 1, wherein
one of the articulating bearing surfaces is concave, in the
sagittal plane, while the other articulating bearing surface is
convex.
12. A prosthetic ankle joint device according to claim 11, wherein
both articulating bearing surfaces are complementary to each other,
in the frontal plane.
13. A prosthetic ankle joint device according to claim 12, wherein
both articulating bearing surfaces present an undulated profile, in
the frontal plane.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 10/019,994, filed Nov. 9, 2001, which
application is the U.S. national phase of International Application
No. PCT/IB00/00638 filed May 12, 2000, which claims priority from
and the benefit of Italian application BO99A 000253 filed May 13,
1999. The disclosure of application Ser. No. 10/019,994 filed Nov.
9, 2001 is also hereby expressly incorporated by reference into
this application.
TECHNICAL FIELD
[0002] The present invention relates to a prosthetic joint for
replacement of the human ankle and to methods for the design of
shapes and sizes of the relevant components, particularly for a
two-component prosthesis with components fixed to the talus and to
the tibia respectively, and designed to allow motion compatible
with the functions of the retained ligaments.
BACKGROUND ART
[0003] In total ankle replacement, the degenerated articular
surfaces of the natural bones are removed and replaced with an
artificial joint called a prosthesis. The goals for the replaced
joint are a) to relieve the pain, b) to restore the original
mobility, and c) to restore the original stability.
[0004] Therapy resistant ankle pain and the disadvantages of ankle
arthrodesis (i.e. fusion of the talus to the tibia with consequent
loss of mobility) led to the development of numerous ankle joint
prostheses. At least 18 designs were introduced throughout the
Seventies of the previous century but, after early encouraging
results, ankle arthroplasty soon acquired a bad reputation based on
many long-term follow-up clinical-radiographic studies. The
frequent failures of the previous implants have been related by the
present inventors mainly to the inadequate restoration of the
original mobility and stability of the ankle complex, caused by
poor knowledge of the guiding/stabilizing role played by the
ligaments involved. The relative contributions of the ligamentous
structures and articular surfaces of the joint to its passive and
active stability had not yet been fully understood.
[0005] The designs devised by the pioneers (1970-1979) mostly
featured two-component prostheses with one component attached to
the talus and the other to the tibia. These designs have been
classified as constrained, semi-constrained and non-constrained.
The two-component designs have been also equivalently categorized
as congruent (spherical, spheroidal, conical, cylindrical,
sliding-cylindrical) or incongruent (trochlear, bispherical,
concave-convex, convex-convex), according to extent of the
conformity of the shapes of the two prosthetic articular surfaces.
The non-constrained incongruous type can be enable restoration of
the normal multi-axial three-planar motion exhibited by the healthy
joint compatible with its ligaments but leads to poor resistance to
wear and deformation of the prosthetic surfaces due to high local
stresses resulting from small contact areas; this type also
exhibits poor inherent stability of its non-conforming articular
surfaces. The constrained congruent designs can be expected to
provide better stability and larger contact areas between the
components and better performance in terms of resistance to wear
and deformation of the prosthetic surfaces due to a better pressure
distribution; but they also provide inadequate restoration of the
characteristic multi-axial three-planar motion of the healthy
joint, which also involves rolling as well as sliding at the
articulating surfaces and may be incompatible with the retained
ligaments. Cylindrical or conical designs can also provide high
stability since the surfaces are forced into total conformity under
load, restricting motion to a single plane. The highest failure
rates were shown by the constrained designs. Non-constrained
designs with incongruent articular surfaces showed only slightly
better results. Failure of non-constrained incongruous designs may
also be attributed to the poor quality of the ultra-high molecular
weight polyethylene used for the tibial component.
[0006] Ankle joint replacement design therefore has to address the
following traditional dilemma also encountered in knee prosthesis
design. An unconstrained or semi-constrained type of two-component
prosthesis that allows for the necessary axial and transverse as
well as flexion/extension mobility requires incongruent contact,
which leads to inadequate load-bearing capacity at the interface
between its articular surfaces. Conversely, a congruent type of
prosthesis produces undesirable constraining forces that overload
the anchoring system needed to secure fixation of the prosthetic
components to the bones. Meniscal bearing prostheses can resolve
the dilemma by providing complete congruence over the entire range
of positions of the joint with minimally constrained fixed
components to enable the soft tissues still to restore the
physiological multi-axial motion of the joint. However, potential
problems pertaining to the risk of subluxation and/or dislocation
can be envisioned. These risks depend respectively on the degree of
distracting stability of the joint and the degree of entrapment of
the bearing element in between the bone-anchored components.
[0007] Although most of the more recent ankle prosthesis designs
feature three components, a few of these still have two components.
They comprise a curved metal component fixed to the talus and a
metal component fixed to the tibia, with articulating surfaces
similar in shape respectively to that of the natural talar and
tibial articular surfaces when seen in the sagittal plane. The
latter component has a plastic bearing for articulating with the
former, and a metal on the back for fixation to the bone. These
designs claim original natural shapes to allow restoration of the
original mobility, and potentially stability at the replaced joint
(see documents U.S. Pat. No. 5,326,365, U.S. Pat. No. 3,889,300,
U.S. Pat. No. 3,896,503, U.S. Pat. No. 3,975,778, U.S. Pat. No.
3,839,742, U.S. Pat. No. 3,886,599, U.S. Pat. No. 3,896,502, U.S.
Pat. No. 3,872,519, U.S. Pat. No. 4,021,864, U.S. Pat. No.
4,069,518, U.S. Pat. No. 4,156,944). These designs also attempt to
allow the multi-axial nature of the rotation at the ankle and
minimize the loads transmitted to the bones by the means of
fixation elements. They allow plantar- and dorsi-flexion (motion in
the sagittal anatomical plane) and very partially the
characteristic internal/external axial rotation (in the transverse
plane) at the ankle joint. With these designs, some abduction and
adduction movement (in the frontal plane) of the talus relative to
the tibia is also allowed, though resulting in partial loss of
contact between the two components. However, no attempt at all was
made to relate the shapes of the surfaces of the implanted
components to the geometry of the retained ligaments or to
understand which joint structures should enable and control the
mobility of the prosthetic joint as well as of the natural joint.
The present inventors have demonstrated mathematically that the use
of non-physiological shapes can still be compatible with the
retained ligaments and result in physiological motion at the ankle
joint as well (Leardini A, Catani F, Giannini S, O'Connor J J.
Computer-assisted design of the sagittal shapes of a
ligament-compatible total ankle replacement. Med Biol Eng Comput.
2001; 39(2):168-175). Recent improvements in the manufacture of
ultra-high molecular weight polyethylene may tempt designers to
return to two-component configurations which are compatible with
the ligaments in the belief that the problems of wear and
deformation exhibited by early two component designs will be
avoided because of improvements in material properties.
[0008] Despite the increasing number of total ankle prosthesis used
in current clinical practice, to this day there are no designs with
clinical results comparable to those achieved with ankle
arthrodesis or to clinical results obtained with total hip and
total knee replacements. Aseptic loosening of the tibial and/or of
the talar components is the most frequent cause of failure but
complications also include impingement and wear together with
subluxation of the mobile meniscal bearing, deep infection,
dehiscence of the surgical wound, lateral talofibular joint
impingement, subsidence of the talar component. The relationship
between cause of failure and the etiology of the degenerative
disease has been studied by many authors with a large variation of
the results reported.
[0009] When seen in the sagittal plane, the natural articular
surface of the tibia at the ankle joint appears to be concave, with
a radius of about 2.4-2.6 cm. The natural articular surface of the
talus when seen in the sagittal plane appears to be convex, with a
radius of about 2.0-2.2 cm so that the natural ankle joint appears
to be a slightly incongruous articulation. The present invention
uses shapes for articular surfaces of the fixed components which
each differ significantly from the shapes of the natural articular
surfaces while nonetheless reproducing natural strain patterns
within the retained ligaments and minimizing subsequent wear of the
components and loosening of the implant.
[0010] The design of an ankle joint prosthesis can aim to replicate
exactly the complete original anatomical geometry of both natural
articular surfaces, a path followed by many of the designers of
constrained two-part prostheses in the 1970's. Studies by the
inventors showed that when the shape of the articular surface of
one of the fixed components is chosen to be non-physiological, the
shape of the articular surface of the other fixed component should
also be non-physiological but can nonetheless be designed so as to
restore natural strain patterns in the ligaments. These patterns
can be restored only when the shapes of the prosthetic articular
surfaces and the geometry of the retained ligamentous structures
are compatible or approximately compatible, i.e. during passive
motion of the ankle, the articular surfaces are designed to move in
mutual contact while maintaining some ligament fibers at or close
to a constant length over the range of dorsi-plantar flexion.
[0011] In summary, one of the main innovative elements of the
present invention is that it uses non-physiological shapes for the
surfaces of both the components attached to the bones but
nonetheless allows restoration of the functions of the ligaments in
controlling and limiting the movement of the ankle joint complex
while minimizing the risk of wear, dislocation or loosening of the
components.
DISCLOSURE OF INVENTION
[0012] The presently proposed use of surface shapes for the two
components fixed to the bones both differing significantly from the
natural was not anticipated in U.S. Pat. No. 4,087,466 or in any
later patent specifications.
[0013] In order to restore the natural articular load bearing
pattern, an ideal human joint prosthesis should reproduce the type
of motion and the original pattern of ligament
slackening/tightening exhibited by the healthy natural joint.
Previous studies by the present authors on intact cadaver ankle
joints (Leardini, A. and O'Connor, J. J. and Catani, F. and
Giannini, S. Kinematics of the human ankle complex in passive
flexion--a single degree of freedom system, J Biomechanics 1999,
32-2:111-118. Leardini, A. and O'Connor, J. J. and Catani, F. and
Giannini, S. A geometric model for the human ankle joint, J
Biomechanics 1999, 32-6:585-591) seem to be qualitatively
consistent with what has been already observed in the knee joint.
The type of motion is complex and multi-axial, including rolling as
well as sliding between the articular surfaces. In addition to a
relative sliding, the talus rolls forward on the tibial mortise
during dorsi-flexion and backwards during plantar-flexion. These
studies revealed that the articular surfaces of the natural joint
are slightly non-congruous to allow the rolling component of their
relative motion. It has been shown that, under passive
minimally-loaded conditions, the articular surfaces and the
ligaments of the natural joint prescribe a unique envelope for the
position of the axis of rotation. The changing positions of the
axis of rotation suggests that the hinge-like fixed-axis concept
for the ankle joint commonly described in the literature is an
oversimplification and does not reflect the actual kinematic
pattern of motion. The aforementioned studies by the inventors and
several other studies have described a nearly-isometric pattern of
rotation of the calcaneo-fibular ligament on the lateral side of
the joint and tibio-calcaneal ligament on the medial side in
comparison with all the other ankle ligaments (Colville, M. R. and
Marder, R. A. and Boyle, J. J. and Zarins, B., Strain measurement
in lateral ankle ligaments, Am J Sports Med 1990, 18(2), 196-200;
Bruns, J. and Rehder, U., Ligament kinematics of the ankle joint,
Zeitschrift fur Orthopadie und lhre Grenzgebiete 1993, 131(4),
363-369). A few recent studies also claim an anterior shift of the
contact area at the tibial mortise during dorsiflexion (Kitaoka, H.
B. and Kura, H. and Luo, Z. P. and An, K. N., Contact features of
the ankle joint, Proceedings of 42nd Annual Meeting of Orthopaedic
Research Society, Atlanta (Ga.), 19-22 February 1996, 396) as
described in the aforesaid publications by the inventors. These
observations have been confirmed very recently by the inventors in
specifically designed experiments (Stagni R, Leardini A, Ensini A.
Ligament fibre recruitment at the human ankle joint complex in
passive flexion. J Biomech. 2004 December; 37-12:1823-9; F.
Corazza, R. Stagni, V. Parenti Castelli, A. Leardini Articular
contact at the tibiotalar joint in passive flexion, Journal of
Biomechanics, 2005 June; 38(6):1205-12).
[0014] In conclusion, it has been shown that a) the most anterior
fiber of the calcaneo-fibular and of the tibio-calcaneal ligaments
of the healthy human ankle joint remain exactly or nearly isometric
during passive motion and that these fibers control and guide
passive ankle motion in its predefined and preferential passive
path, whereas the other ligament fibers slacken and lengthen so as
to limit but not to guide motion; b) the axis of rotation in the
sagittal plane passes through or close to the intersection as seen
in the sagittal plane of the most isometric fibers of the
calcaneo-fibular and tibio-calcaneal ligaments and therefore moves
anteriorly and proximally during dorsiflexion when these fibers
change direction as they rotate about their origins and insertions
on the bones; c) the contact area between the natural articular
surfaces moves anteriorly on the tibial mortise during passive
dorsi-flexion and posteriorly during passive plantar-flexion. As
with the knee, the slackening and the tightening of the ankle
ligaments may be explained in terms of their instantaneous
positions with respect to the moving axis of rotation.
[0015] These observations imply that the natural articular surfaces
of the bone segments must fulfill the requirement that they can be
moved passively in mutual contact while fibers within the
calcaneo-fibular and tibio-calcaneal ligaments remain at or close
to constant length. This condition is achieved in the natural joint
because the common normal to the articular surfaces at their point
of contact passes through or close to the axis of rotation at the
intersection as seen in the sagittal plane of those fibers in each
position of the joint (this theorem is called the common normal
theorem). For ankle joint replacement, it is suggested that the
shapes of the articular surfaces of the prosthesis components must
also satisfy the common normal theorem in order to be compatible
with the geometry of the retained ligamentous structures.
[0016] When the geometry of the two isometric ligament fibers is
known in terms of the locations of their origins and insertions on
the bones and when the shape in the sagittal plane of one articular
surface is chosen by the designer, the shape in the sagittal plane
of the complementary surface of the other articular segment for a
two component prosthesis can be deduced from the common normal
theorem for it to be compatible with ligament isometry: to avoid
interpenetration or separation of the two bones during passive
motion of the joint, the normal at the contact point on the
complementary second surface should ideally pass through the
intersection as seen in the sagittal plane of the isometric fibers
of the calcaneo-fibular and tibio-calcaneal ligaments.
[0017] When the articular surface of the talar component as seen in
the sagittal plane is chosen to be a concave circle with a radius
close to that of the natural surface, the talar surface determined
from the common normal is found to be a convex surface with
curvature very similar to that of the natural talus. However, when
the designer chooses a non-physiological surface shape in the
sagittal plane for one of the components of a two component
prosthesis and applies the common normal theorem, the result is a
design with non-congruent surfaces in the sagittal plane and a
non-physiological surface shape for the second component. When the
surface of the tibial component in the sagittal plane is chosen to
be convex, flat or concave, the calculated shape of the compatible
talar component is found in each case to be convex but with a
radius of curvature up to 70% longer than that of the natural
talus.
[0018] When the surface of the tibial component in the sagittal
plane is chosen to be either flat or a convex or a concave circle,
exact application of the common normal theorem yields a shape for
the compatible surface of the talar component in the sagittal plane
that is convex but not precisely circular. Motion approximately
compatible with the ligaments can be achieved by replacing the
calculated non-circular shape of the compatible talar surface with
a circular surface which provides a closely fitting approximation
to the ideal shape. The common normal to the articular surfaces at
their point of contact then passes close to but not exactly through
the intersection of the most isometric fibers of the
calcaneo-fibular and tibio-calcaneal ligaments, as seen in the
sagittal plane. However, because all these solutions yield surfaces
for a two-component prosthesis which are non-congruous, a higher
wear rate is possible because of the higher contact stress due to
small contact areas developed between non-conforming surfaces but
recent developments in the manufacture of ultra high molecular
weight polyethylene may make the use of such a two-component
non-congruous prosthesis feasible. Moreover, because of the need to
achieve overall surface/ligament compatibility in the replaced
joint, the accuracy required for implanting two components can be
critical for the overall success of the implant and the risk of
erroneous positioning can be high.
[0019] Compatibility between ligaments and articular surfaces can
be retained even when introducing a third mobile component to the
prosthesis. Again choosing a flat, convex or concave circular shape
for the tibial component as seen in the sagittal plane, the
ligament-compatible shape of the talar component in the sagittal
plane can be approximated by a circular surface. However, the use
of a flat or a concave tibial component with a convex talar
component having a radius in the sagittal plane longer than that of
the natural joint and that of prior art three component prostheses
increases the danger of dislocation of the meniscal bearing because
of reduced entrapment as a consequence of a smaller difference
between the thickest and thinnest regions of the bearing. The
present invention is based mainly on issues related to ankle joint
function in the sagittal plane but it has been developed in three
dimensions to provide a two component prosthetic ankle joint
device, comprising a first component for attachment to the distal
tibia having an articular bearing surface that is generally curved
in a concave manner, and a second component for attachment to the
proximal talus having an articular bearing surface that is
generally curved in a convex manner in the sagittal plane and
curved partly in a concave manner in the frontal plane (a so-called
anticlastic surface having principal curvatures of opposite signs).
The two components are to be secured respectively to the tibia and
to the talus with the articular surfaces of these components in
mutually opposite disposition. The shapes of the articular surfaces
of these components can not be chosen to achieve full congruency at
interface over the range of movement. Rather, the shapes of the
articular surfaces at the interface can be chosen to ensure that
the axis of flexion of the ankle during passive movements passes
through or close to the intersection of the most isometric fibers
of the calcaneo-fibular and tibio-calcaneal ligaments, as seen in
the sagittal plane, thereby providing an articulation that is
substantially ligament-compatible. The thickness of the plastic
bearing fixed to the tibial component can be selected at surgery to
be the most appropriate to restore the original tensioning pattern
of the ligaments: thicker or thinner bearings would involve joint
rigidity or laxity respectively. A thicker bearing would of course
last longer as for the process of the possible wear. As in the
intact joint, the articular surfaces do not constrain the relative
motion of the bone segments but merely allow the replaced joint to
perform the motions directed by the ligamentous mechanism. A
modular thickness of the plastic bearing has also the advantage
that it can accommodate partially for surgical technique errors
entailing the erroneous level of the bone cuts. Lastly, it has been
observed that when total ankle replacement is performed, the
subtalar joint complex is frequently affected too. Total ankle
replacement certainly does have to cope with an affected ankle
(talocrural) joint but also, very often, with an affected subtalar
(talocalcaneal) joint complex, and therefore it should be aimed at
restoring the function of both joints. A full conforming component
articulation in the frontal plane allows abduction/adduction as
well as axial rotation and flexion/extension movements (in the
frontal, transverse and sagittal planes respectively) to occur at
the tibio-talar interface, complementing the abduction/adduction
movements which, in the healthy joint, occur mainly at the
sub-talar joint complex. However smaller contact areas would be
exhibited at the articulating surfaces in the former two cases. In
this two-component solution there is a reduced risk of subluxation
and dislocation for the bearing.
DESCRIPTION OF THE DRAWINGS
[0020] The technical features of the invention, according to the
aforesaid aims, can be clearly noted from the content of the claims
set out below and its advantages shall become more readily apparent
in the detailed description that follows, made with reference to
the accompanying drawings, which show an embodiment provided purely
by way of non limiting indication, in which:
[0021] FIG. 1 is an overall view in the sagittal plane of a
prosthesis device according to the invention;
[0022] FIGS. 2a, 2b and 2c schematically show the mechanism of
relative motion expected between the prosthesis components as
guided by the most isometric fibers of the calcaneo-fibular and
tibio-calcaneal ligaments in an replaced joint in three
characteristic positions: in maximal plantar-flexion, neutral and
maximal dorsi-flexion respectively;
[0023] FIG. 3 is an overall view in the frontal plane of the
prosthesis device shown in the preceding figures. With reference to
FIGS. 1-3, the reference number 1 globally indicates an ankle
prosthesis device comprising two components: a first tibial
component 2, and a second talar component 3 opposite to the first
tibial component 2. The first component 2 is able to be attached to
a tibial bone segment 2a and has a first bearing surface 5
presenting a shape different to the shape of the corresponding
bearing surface of the natural joint. The second component 3 is
able to be attached to the talar bone 3a and has a second bearing
surface 6 presenting a shape different to the shape of the
corresponding bearing surface of the natural joint. The first and
the second bearing surface 5, 6 are arranged to face each other.
The first and the second bearing surface 5, 6 are in mutual contact
to ensure that the original joint movement be restored.
[0024] Each of the articulating bearing surfaces 5, 6 presents a
respective shape designed on the basis of the shape of the opposite
articulating bearing surface 5, 6 according to the common normal to
their point of mutual contact at every position over the range of
passive plantar and dorsi-flexion. In particular, the shape of each
articulating bearing surface 5, 6 is determined by the common
normal theorem so that the first and second articulating bearing
surfaces 5, 6 be compatible with isometric ligament rotation. For
instance, once the shape of the first articulating bearing surface
5 or the tibial arc has been imposed, the shape of second
articulating bearing surface 6 or tarsal arc is deduced by using
the common normal theorem. Thus, to be compatible with the
isometric rotation of the anatomical calcaneo-fibular and
tibio-calcaneal ligament fibers 9, 10 (FIG. 1, 2a, 2b e 2c) the
tibial and talar surfaces 5, 6 can be of any shape, but when the
shape of one articular surface 5 of the first component 2 is
chosen, the shape of the complementary surface 6 of the other
component 3 is deduced accordingly from the common normal theorem
so that the articulation be ligament-compatible.
[0025] Advantageously, the common normal 26 to the point of mutual
contact of the articulating bearing surfaces 5, 6 at every position
over the range of passive plantar and dorsi-flexion passes close
to, and preferably through, the intersection, in the sagittal
plane, of the most nearly isometric fibres in the tibio-calcaneal
ligament on the medial side of the joint and calcaneo-fibular
ligament on the lateral side of the joint.
[0026] According to the common normal theorem one of the two
articulating bearing surfaces 5, 6 can be substantially planar, in
the sagittal plane, while the other articulating bearing surface 5,
6 can be convex. Alternatively, one of the two articulating bearing
surfaces 5, 6 can be concave and the other convex, like those shown
within the above listed figures, or both articulating bearing
surfaces 5, 6 can be convex, in the sagittal plane.
[0027] To minimise the resection of the talus bone 3a, the talar
component 3 should be convex in the sagittal plane (FIGS. 1, 2a,
2b, and 2c). The inventors have found that curved, convex,
multicentric and multiradial shapes of the articular surface 6 of
the talar component 3 can be ligament-compatible with planar,
concave, or convex shapes of the articular surface 5 of the tibial
component 2. According to a preferred embodiment of the present
invention, both the first articulating bearing surface 5 and the
second articulating bearing surface 6 present respectively radius
of curvature 5a, 6a included within a predetermined range of radius
preferably different, and advantageously close to, the radius of
the corresponding surfaces of the natural joint. In particular, the
first articulating bearing surface 5 presents, in the sagittal
plane, a radius of curvature 5a which can be imposed between a
value of 24 mm (for a concave configuration) and an infinity value
(planar configuration) or between an infinity value (planar
configuration) and a value of 20 mm (for a convex configuration)
while the second articulating bearing surface 6 presents, in the
sagittal plane, a radius of curvature 6a comprised between 22 mm
and 34 mm and in any case to form with the articulating bearing
surface 5 a couple of shapes compatible with the isometric rotation
of the ligaments 9,10.
[0028] Advantageously, the radius of curvature 6a of the second
articulating bearing surface 6 can be up to 75% longer than the
natural talar articulating bearing surface of the natural joint.
Naturally, also in such a case the radius of curvature 5a of the
first articulating bearing surface 5 is determined by the common
normal theorem on the basis of the shape and the radius of
curvature 6a of the second articulating bearing surface 6.
[0029] Always referring to the above figures, the first component 2
is preferably made of two portions 2b, 2c. The first portion 2b,
preferably made of metal, like the second component 3, ensures that
the first component 2 be tightly attached to the tibial bone
segment 2a, whereas the second portion 2c, preferably made of
plastic, advantageously UHMWPE, is fastened to the first portion 2b
by properly connecting means (not shown) and defines the above
mentioned first articulating bearing surface 5. The connecting
means allows that the second portion 2c can be detached from the
first portion 2b to be substituted by another second portion 2c
having different sizes better suited for the installation on the
patient's body and performed bone cut at operation. The connecting
means also ensures that the engagement among the first and the
second portion 2b, 2c be fixed and immovable after implantation of
the prosthesis ankle device.
[0030] The selection of a concave cylindrical or spherical shape
with radius 5a for the articulating bearing surface 5 of the first
tibial component 2 and a convex cylindrical or spherical shape with
radius 6a of the second component 3 can be made for the better
degree of antero/posterior and medial-lateral entrapment of ankle
device with the associated smaller risk of subluxation and
dislocation.
[0031] So, given a convex circular arc in the sagittal plane of the
articular surface 5 of the tibial component 2, together with the
geometry of the two ligament fibers 9, 10 in at a variety of
positions of dorsi-/plantar-flexion, a series of contact points for
the optimal dome of the articular surface 6 of a talar component 3
in the sagittal plane are deduced from the common normal theorem. A
circular arc that best approximates these points is then adopted as
to guarantee large contact areas in all these joint positions. The
radius 6a of the resulting dome-shaped articular surface 6 of the
talar component 3 in the sagittal plane can be even 1.5 times
longer than those of prior three and two-component designs.
[0032] The inventors have found that only with a much more ample
arc radius 6a for the talar component 3 it is possible to reproduce
the characteristic pattern of sliding and rolling during ankle
flexion (FIGS. 2a, 2b and 2c).
[0033] Therefore, the criteria pertaining to the restoration of the
original kinematics pattern and to the minimization of the risk of
dislocation for the ankle joint are both met. FIGS. 2a, 2b and 2c
schematically show, with different dimensional scales, in the
sagittal plane, the kinematics of the ankle once it has been
replaced with a prosthesis embodied by the device 1 according to
the invention.
[0034] During ankle flexion (FIGS. 2a, 2b and 2c), the second talar
component 3 slides and rotates in the sagittal plane on the first
tibial component 2. The line of centers 26 joining the center of
curvature 28 of the tibial component 2 to the center of curvature
29 of the talar component 3 passes through or close to the
intersection 27 of the most isometric fibers of the
calcaneo-fibular 9 and tibio-calcaneal 10 ligaments, thus
satisfying the common normal theorem. With ankle joint motion
constrained by this type of mechanism, the estimated elongation of
the two ligament fibers 9, 10 is less than 0.2% of resting length.
Hence, the design allows both for the restoration of the original
pattern of ligament slackening/tightening.
[0035] The tibial component 2 has a highly polished articular
surface 5 and 5, two cylindrical bars 12 (FIGS. 1, 2a, 2b, 2c and
3) covered with a porous coating, and positioned on the upper
surface 2d of the first portion 2b, which is porous coated as well,
and are provided for anchoring the tibial component 2 to the distal
tibia 2a.
[0036] Both articulating bearing surfaces 5, 6, are partly
anticlastic, the central portion of each surface 5, 6 having two
mutually transverse curvatures, in opposite directions, as the
surface of a saddle. The articular surface 5, the lower surface of
the tibial component 2, is a concave arc in the sagittal plane
(FIGS. 1, 2a, 2b and 2c), with a center of curvature 28, and a
generally convex arc in the frontal plane (FIG. 3). The articular
surface 6, the upper surface of the talar component 3, is a convex
arc in the sagittal plane (FIGS. 1, 2a, 2b and 2c), with a center
of curvature 29 in the talar body and a radius that can be about
1.5 times the length of the radius of the natural talar articular
surface, and a generally concave arc in the frontal plane (FIG.
3).
[0037] The upper surface 6 is a surface of revolution, generated by
rotating a generatrix curve about a medial-lateral fixed axis,
orthogonal to the sagittal plane of FIGS. 1 and 2a-2c, i.e.
belonging to the frontal plane of FIG. 3. The generatrix curve is
concave in the frontal plane (FIG. 3), presenting a central concave
circular arc, the sulcus 14, between two lateral convex circular
arcs 15. The articulating surface immediately proximal--coinciding
with the articular surface 5 of the tibial component 2--shows a
circular concave arc in the sagittal plane (FIGS. 1, 2a, 2b and
2c), and has a convex central arc and two concave lateral arcs when
seen in the frontal plane (FIG. 3). These are to be fully
conforming to the equivalent arcs 14, 15 of the talar component 3,
i.e. they have the same radii of curvature when seen in the frontal
plane.
[0038] When the prosthesis is in neutral position (FIG. 2b), the
arc 6 in the sagittal plane is slightly longer posterior than
anterior to the mid longitudinal axis 13 of the tibia 2a (FIG. 2b)
because of the larger range of ankle motion in plantarflexion (FIG.
2a) than in dorsiflexion (FIG. 2c), both in the natural and in the
replaced joint. In this position, the line of centers 26 is
located, in the sagittal plane, slightly forward relative to the
mid longitudinal axis 13 of the tibia 2a (FIG. 2b).
[0039] The consequences of the above arrangements are that:
[0040] 1) The interface between the tibial 2 and the talar 3
components, is capable of independent relative movement by virtue
of the complementary nature of the corresponding coupled surfaces
5, 6. More specifically, the tibial 2 and the talar 3 components
are capable of mutual rotation about an axis passing at the
intersection of the isometric ligaments 9, 10 as seen in the
sagittal plane (FIGS. 2a, 2b and 2c). The deriving ability to
perform relative motions between the tibial 2 and talar 3
components is accordingly extensive and can include rolling,
gliding, twisting, and combinations thereof, such as they take
place in the natural ankle joint complex. The backwards and
forwards motion of the line of centers 26 has significant effect on
the mechanics of the articulation since, in the absence of
friction, it is also the line of action of the compressive force
transmitted across the joint. Its backwards position in
plantar-flexion maximizes the lever-arm available to the
dorsi-flexor muscles running in front of the joint. Its forwards
position in dorsi-flexion maximizes the lever-arm available to the
plantar-flexor muscles running in back of the joint.
[0041] 2) The shapes of the bearing surfaces 5, 6 of the tibial 2
and talar 3 components can reproduce the natural pattern of
relative motion of the corresponding bony segments even though
these surfaces both differ significantly from the shapes of the
natural tibial and talar articular surfaces. Therefore, the
mechanical interactions between the shapes of the surfaces 5, 6 and
the forces in the surrounding muscles and ligaments and
particularly in the related ligament fibers 9, 10 which control the
stability of the joint, will be physiological as well.
[0042] 3) All relative positions of the components once implanted,
under passive conditions, are obtained as positions of minimum
stored energy shared among the ligamentous structures of the joint.
The shape of the bearing surfaces 5, 6 of the two components 2, 3
were designed to allow relative motion without resistance through
mutual sliding without separation or inter-penetration, while the
fibers 9, 10 of the isometric ligament rotate about their origins
and insertion points without stretching or slackening. Other
ligament fibres remain slack during passive motion, except at the
limits of that motion. No input of energy is expected to be
necessary to displace the replaced joint along this neutral passive
path because no tissue deformation is necessary.
[0043] 4) In particular, to obtain this series of minimum energy
positions between the tibial 2 and talar 3 components, the talar
component 3 is guided by the ligaments to slide forward while
rolling backward with respect to the tibial component 2 during
plantarflexion and to slide backward while rolling forward during
dorsiflexion.
[0044] 5) The interface between tibial component 2 and the talar
component 3 allows ligament-controlled motion in the sagittal
plane. Rotations in the transverse and frontal planes are possible,
but imply further reduction of the contact area and separation of
the components a ball-and-socket joint, providing at least one
degree of rotational movement to the bones. A further degree of
freedom is allowed at the interface between the tibial component 2
and the talar component 3, when the former component can slide
congruently on the latter along the sulcus 14, 15 which extends
along the dome of the components 2, 3 mostly antero-posteriorly.
Dorsi/plantarflexion in the sagittal plane is allowed at the tibial
component 2--talar component 3 interface. Pure translations in the
proximo-distal, antero-posterior and medio-lateral directions are
resisted respectively by tension of the ligaments. In the later
case there is also an inherent resistance of the articulating
surfaces.
[0045] 6) The implantation of the two bone-anchored components 2, 3
has to be carried out most carefully. The center 28 of the tibial
lower spherical arc 5 and the center 29 of the talar upper arc 6
must lie on the same line 26 in the sagittal and frontal plane
(FIGS. 1-3), and this line 26 must pass through or close to the
intersection 27 of the most isometric fibers of the
calcaneo-fibular 9 and tibio-calcaneal 10 ligaments, thus
satisfying the common normal theorem.
[0046] 7) It is here thought that the tibial 2, talar 3 components
will all have to be made in a number of dimensions to accommodate
patients of different size. The number and dimension of different
sizes does not constitute a limitation of the present invention.
Because of its strategic importance in restoring original joint
function, the first portion--of the tibial component 2 should also
be made in various thicknesses. The interval between different
thicknesses can even be very small but in any case it should be
large enough to allow surgeons to detect differences in articular
mobility and stability during the operation.
[0047] Development of the invention since its initial conception
has shown that, while a variety of potentially advantageous forms
are possible within the more general scope of the invention the
above consequences can result from a partly anticlastic-tibial 5
and partly anticlastic-talar 6 prosthetic surfaces. However, they
could also result from a relatively simple form of the invention in
which the talar surface 6 is part-spherically or cylindrically
shaped, and the engaged bearing surfaces 5 of the tibial component
2 is planar. In this way the two-component prosthesis is able to
assist the joint in reproducing the original pattern of ligament
slackening/tightening, still with articulating surfaces designed
according to the present invention.
[0048] The relevance of this general application of the invention
is based on a particular view of the form and function of the
passive structures of the joint, these elements being the articular
surfaces 5, 6 and the adjacent ligaments 9, 10. This view holds
that, during the passive motion of the joint, the articular
surfaces 5, 6 serve to maintain the fibers 9, 10 of the ligaments
at a constant length and that the ligaments themselves act in such
a way as to maintain these articular surfaces in contact. The
articular surfaces 5, 6 serve predominantly to transmit compressive
forces, and the ligaments and the muscle tendons to control and
limit the surface movements while themselves serving to resist and
transmit tensile forces. Thus, there is interdependence between all
the elements of a joint, and this interdependence is vital to the
overall performance of a natural joint having incongruent surfaces
which can provide little inherent stability.
[0049] The advantages and novelty of the illustrated device 1, with
respect to prior designs can be listed as follows: [0050] the
multiaxial pattern of movement of the natural ankle joint can be
closely simulated without significant distortion of the natural
controlling and stabilising mechanism. The necessary compatibility
of the articular surfaces 5, 6 with the isometric rotations of the
ligaments 9, 10, is obtained with arcs of curvature of the tibial
and talar components 2, 3 that significantly different from those
of the natural anatomical shapes, and thus from that of all designs
of the prior art; [0051] the device 1 features conforming surfaces
in both the tibial 2--talar 3 component articulations in the
frontal plane in all the positions of the joint; [0052]
medial-lateral entrapment of the ankle joint device 1 is guaranteed
by the sulcus 14 running on the dome of the tibial and talar
components 2, 3, avoiding the sharpened limiting interfaces as used
in the prior art to prevent dislocation and separation which
certainly entail a high risk of wearing at the sharpened
interfaces; [0053] unlike all previous two-component cylindrical
and ball-and-socket designs, the axis of joint rotation represented
by the point 27 in the sagittal plane is not fixed as imposed by
the congruity of the articular surfaces, but rather is able to move
relative to both tibial 2a and talar 3a bones to assist the joint
in performing the original pattern guided by the isometric rotation
of certain ligament fibers 9, 10 (FIGS. 2a, 2b and 2c).
[0054] In order to restore the original compatibility between
articular surfaces and ligaments at the human ankle joint, not only
should the prosthesis components 2, 3 be designed according to the
criteria set out above, but should be implanted in their definitive
relevant position with great care and precision.
[0055] The invention thus conceived is clearly suitable for
industrial application; moreover, it can be subject to numerous
modifications and variations, without thereby departing from the
scope of the inventive concept. Furthermore, all components can be
replaced by technically equivalent elements.
* * * * *