| BUECHEL-PAPPAS™
TOTAL ANKLE REPLACEMENT SYSTEM |
Total
Ankle Design Rationale
Biocompatability
Co-Cr porous coating is characterized by a lack of full bone
ingrowth. As shown in the figure above a fibrous layer is always
present between the bone and the metal. Further, the ingrowth
is limited in its coverage of the porous coating. It is characterized
by a "spot weld" type ingrowth pattern.
Titanium
porous coating, on the other hand, is characterized by direct
bone apposition onto the titanium. Our observation of retrieved
titanium implants has shown a much greater ingrowth coverage
of the porous coating than we have observed onto Co-Cr implants.
Thus
titanium, due to its better biocompatability, is superior
to Co-Cr as a biological fixation surface. TiN ceramic is
also biologically neutral and thus one sees the same ingrowth
characteristics onto the ceramic as one sees on titanium.
Prosthetic
Fixation
The dual fin fixation of the talar component is for the purpose
of obtaining fixation on both sides of the talus so as to
eliminate the resorption, and associated talar component tilt,
encountered in the earlier single fin design as well as to
avoid disruption of the talar blood supply produced by the
single, central fin. The primary blood supply of the talus
is inferior and central to the talus. The single, central
fin appears to substantially disrupt this source of blood
supply causing talar necrosis, whereas the short, off-center
fins do not.
The short
fixation peg of the tibial component is designed to help resist
tilting forces on the talus resulting from off-center loads.
Early designs with a dual fin fixation were originally used
and worked well in the relatively few cases of trunion ankles
implanted. During 1977, our analysis of meniscal bearing and
rotating platform knee component indicated that the short
stem provided superior fixation to fin fixation. Therefore,
this concept was adapted to the ankle tibial fixation in 1978,
abandoning the dual fin design. Our long term, 25 year, experience
with tibial knee components verifies the analysis of 1977.
The fins of the bicruciate retaining tibial component normally
show radiographic lucencies around the fins. These lucencies
are rare with the posterior cruciate retaining tibial platform
where off center loading of the tibial component is greater
than in the bicruciate platform. Although loosening is not
a significant problem with either knee design, radiological
loosening is more apparent in the finned design.
The same
situation therefore is likely to exist with the tibial ankle
component. Fractures of early designed tibial plate corners
indicate substantial off center loads in the ankle. Although
the dual fins design should provide acceptable results, the
short stem seems to be superior. In a study comparing a comparable
single fin talar, dual fin tibial mobile bearing ankle with
the B-P dual finned talar, stemed tibia, loosening was more
apparent in the single fin talar, dual finned tibia design.
The fixation
surfaces are three-layer BioCoat® commercially pure titanium
sintered bead porous coating on a titanium alloy substrate.
The mean pore size is about 325 microns. The porous coating
is itself covered with a coating of TiN ceramic.
More
than 15 years of clinical experience with the Mark II design
has demonstrated that the talar dome collapse problem of the
Mark I has been solved. The tibial peg fixation has been successful
for more than twenty years in both the Mark I and II designs.
Thus the fixation configuration has been found to be effective.
Prosthetic
Ankle Motion
The
B-P ankle provides near normal plantar and dorsiflexion well
beyond the motion required for walking.
It also
provides unlimited axial rotation.
The B-P
ankle also provides limited inversion-eversion of the tibiotalar
joint without loss of congruity of the contact surfaces.
Prosthetic
Ankle Stability
The
A-P stability of the B-P ankle is primarily extrinsic, as
is the natural ankle. A seven degree posterior inclination
of the tibial platform provides posterior shear resistance
of about 0.12 times the joint reaction force or a maximum
of about 0.6 times body weight. This is most of the estimated
posterior shearing force.
Since
the B-P Ankle resurfaces the tibiotalar joint with near natural
articulating surfaces it is similar in these stability modes
to the normal ankle.
Prosthetic
Force Resistance
Equations, sufficient for use in knee and ankle prostheses,
for the computation of the contact stress of two bodies in
contact were developed in the 1930's.These have been applied
to the evaluation of knee prostheses.
Since
the B-P mobile ankle bearing allows Congruency and Mobility
just as does the NJ knee bearing, the calculation of the contact
stress in the B-P ankle articulation predicts low contact
stress.
The mobile
bearing Mark II B-P Ankle is the culmination of almost twenty
five years of development. It fully exploits the mobile bearing
concept by maintaining complete congruity for all phases of
motion. Further it provides normal ankle motion and stability
along with this congruity. The low wear ceramic coating of
superior biocompatability; along with its porous coated ingrowth
fixation geometry provide a realistic expectation for a long
life and perhaps a lifetime joint replacement.
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