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JUEVES / THURSDAY
244
movement of the contact point between the
femur and the tibia with increasing fl exion.)
Proponents of the four-bar link model point to
studies that have shown a decreasing radius
of curvature of the femoral condyles from
distal to posterior.
Several recent studies of knee joint kinematics
have suggested that the knee can be
modeled as having a single axis of fl exionextension.
Similarly, the internal and external
rotation of the tibia around the femur (i.e.,
the pivot) of the knee can be modeled by an
axis roughly at the central part of the medial
condyle. (van Dijk et al. 1983, Blankevoort et
al. 1988, Hollister et al. 1993, Mancinelli et al.
1994, Blaha et al. 2003) These studies show
that the normal knee does not roll-back, but
rather remains remarkably constant in position
on the medial side (like a ball in a socket)
while varying in contact position on the lateral
side to accommodate internal and external
rotation of the tibia about the femur.
Kinematic studies of total knee prostheses
designed respecting the concept of the “fourbar
link” and providing for roll back have demonstrated
paradoxical kinematics. Instead of
rolling back, these knees demonstrate sliding
forward of the femur on the tibia during in
vivo fl uoroscopic studies. A similar kinematic
study done with a knee joint designed for medial
pivot and medial ball-in-socket kinematics
does not demonstrate paradoxical motion.
(Banks et al. 1997, Dennis et al 1997, Blaha
et al. 1998)
The Advance“ Medial-Pivot (Wright Medical
Technology, Arlington TN USA) total knee
prosthesis has been in clinical use for 5
years (as of January, 2003). Based on the
preliminary results available at the time of
the writing of this abstract the medial ball-insocket
confi guration of the implant appears
to provide a clinical result characterized by
enhanced anterior-posterior stability both to
clinical examination and in functional use.
ROTATION IS THE FINAL
SOLUTION: MOBILE
BEARING TOTAL KNEE
R. “Dickey” Jones, M.D.
U.T. Southwestern Medical Center,
Dallas, TX, USA
Surface wear in TKA remains a problem
producing sub-micron wear particles which
accumulate in periprosthetic tissue leading to
cytokinin release and osteolysis. A physiological
knee joint simulator was used to compare
fi xed bearing to a rotating platform design of
the same knee system. Despite similar contact
areas the rotating platform bearings had
4 times less wear under high kinematics than
the fi xed bearings. Polyethylene orientates
in the principle direction of sliding and strain
hardening with strength increase occurs parallel
to sliding, but is reduced transverse to
sliding. In fi xed bearings the increased multidirectional
motion of the femoral component
relative to the bearing produces the multi-axial
wear path, transverse friction forces and gives
higher wear. With the Rotating Platform,
rotation occurs at the tibial tray, diminishing
multi-directional motion at the femoral
interface, converting to uni-directional wear
paths, decreasing transverse friction forces
and producing signifi cantly lower wear. With
higher kinematic inputs fi xed bearing TKAs
show a fi ve fold increase in surface wear over
lower inputs. The Rotating Platform produced
94% less wear than the fi xed bearing over 6
million cycles.
Dynamic kinematic fluoroscopic studies
confi rm the Rotating Platform moves with
the femoral component and axial rotation
occurs on the inferior aspect. The Rotating
Platform accommodates to the patient’s gait
kinematics, enhancing positive bone and soft
tissue remodeling.