Notas / Notes - Active Congress.......

VIERNES / FRIDAY
188
ligament should be released first. This leaves the posterior
oblique portion intact to provide stability both in flexion and
extension.
TIGHT IN EXTENSION, BALANCED IN FLEXION
In some cases the posterior medial structures are tight and
the anterior MCL is normal after insertion of the trial components.
These knees are tight in extension, but balanced normally in
flexion. Knees that are tight only in extension after total knee
arthroplasty first should have release of the posterior oblique
fibers of the MCL, and release of the posterior capsule if medial
contracture persists in extension. This procedure leaves
the anterior portion of the MCL intact to stabilize the knee.
TIGHT MEDIALLY IN FLEXION AND EXTENSION
In many cases with a long-standing varus deformity and
medial ligament contracture, the knee is tight medially both
in flexion and extension. This indicates that the entire MCL
is contracted. The posterior capsule and PCL also may be
contracted, but the primary contracture is the MCL in these
cases. The PCL and posterior capsule cannot be evaluated
until the MCL contracture has been corrected. Knees that
are tight in flexion and extension have release of the anterior
and posterior portions of the medial collateral ligament. This
is done by first stripping the anterior portion of the MCL in
line with the tibial long axis, then directing the osteotome
posteriorly to release the posterior portion of the ligament.
Those knees that remain tight in full extension after release
of the posterior oblique MCL have release of the posterior
medial capsule from the femur and tibia. If inappropriate posterior
femoral rollback occurs, or if medial ligament tightness
remains in flexion after release of the anterior portion of the
MCL, the PCL is released from its tibial attachment.
TIGHT POPLITEUS TENDON
Occasionally the popliteus tendon and its surrounding structures
are tight in the varus knee after the medial side has been
corrected. This often is difficult to detect, but rotational stability
testing of the tibia demonstrates that the tibia is held anteriorly
on the lateral side and pivots around the lateral edge of the
tibial component. The popliteus tendon is released from its
bone attachment when the knee is flexed. It is found just distal
and posterior to the lateral collateral ligament (LCL) attachment,
and care must be taken to avoid release of the LCL
during this procedure.
COMPENSATORY LATERAL RELEASEEXTENSION ONLY
Occasionally, after full MCL release, the knee is excessively
loose on the medial side in extension, and tight laterally.
Compensatory lateral release corrects the imbalance, and
a thicker tibial component brings the knee to correct stability.
COMPENSATORY LATERAL RELEASEFLEXION AND
EXTENSION
In some cases after full release of the MCL, the secondary
stabilizers are inadequate to provide medial stability in flexion
and extension, and the knee is too loose medially after the
tibial component has been sized to bring the lateral ligaments
to their normal tension. In those cases the LCL and popliteus
tendon are released to create more laxity both in flexion and
extension, and a thicker tibial component is used to tension
the medial structures.
MEDIAL PIVOT KNEE ARTHROPLASTY
J. David Blaha, M.D.
University of Michigan. Medical School,
(USA)
Design of total knee prostheses is predicated on knowledge
of the kinematics of the normal knee. Designs that more closely
mimic the normal might reasonably be expected to perform
more normally for the patient. For many years the knee joint
has been viewed as a “four-bar link” in which the ligaments
(specifically the cruciate ligaments) guide the motion of the knee
in such a way that “rollback” occurs. (Rollback is the progressive
posterior movement of the contact point between the femur
and the tibia with increasing flexion.) Proponents of the fourbar
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 flexion-extension.
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 “four-bar 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 fluoroscopic 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-in-socket configuration of the implant appears to provide
a clinical result characterized by enhanced anterior-posterior
stability both to clinical examination and in functional use.