| MOBILITY
CHARACTERISTICS OF TOTAL ANKLE REPLACEMENTS |
Steven
M. Raikin, M.D. Orthopaedic Research Laboratories Christine
S. Heim, B.Sc. Lutheran Hospital Nicholas A. Plaxton, M.S.
Cleveland Clinic Health System A. Seth Greenwald, D.Phil.(Oxon)
ABSTRACT
Ankle
joint replacement is again emerging as an alternative to arthrodesis
in the surgical treatment of ankle joint arthritis. Restoration
of normal ankle joint function through arthroplasty can result
in relief of pain and return of normal gait biomechanics.
Despite promising early results in the 1970s, long-term follow-up
studies have been fraught with high failure rates leading
to their abandonment. Prosthetic loosening has been the predominant
cause of failure with rates ranging between 52% and 95% at
10 years. This has been linked to the excessive intrinsic
constraint offered by these systems, often in conjunction
with compromised subtalar joint motion and alignment. To be
successful, prostheses need to be able to withstand the considerable
forces which act across the ankle during normal walking while
allowing for a functional range of motion. Contemporary ankle
replacement designs seek to offer this balance without being
excessively constrained, thereby decreasing the risk of loosening.
This
study characterizes total ankle replacement devices in terms
of the force generated during a prescribed displacement. The
four systems evaluated include the Agility (DePuy Inc.), Buechel-Pappas
(Endotec, Inc.), STAR (Link Orthopaedics, Inc.) and TNK (Kyocera
Corporation). Currently, only the Agility system is available
for clinical use in the United States, with the remaining
representing a growing international presence.
While
analyzing total ankle replacements it is important to appreciate
the actual displacements allowed by the geometric constraint
of each design. Within the envelopes of normal displacement
the systems studied demonstrate relatively low force and torque
values which should contribute to their longevity. The one
exception is the medial displacement realized in the Kyocera
design, however, only clinical reports will gauge the significance
of this finding. It is important to appreciate the dramatic
reduction in device constraint realized by these contemporary
designs in comparison to first generation devices. Clinical
longevity of total ankle arthroplasty is dependent upon a
correct balance between the intrinsic mobility allowed by
design geometry and patients presenting pathology.
These
ongoing laboratory evaluations assist an understanding of
the anticipated performance of contemporary total ankle replacements.
The results are intended to aid the surgeon in device selection
when considering patient factors. Further, they provide the
manufacturer with design criteria and assist regulatory agencies
in determining the safety and efficacy of specific ankle designs.
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