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JUEVES / THURSDAY
242
from the medial tibial plateau, which often
leaves the defect on the lateral side of the
tibia that requires a bone graft. Use of a
long-stem tibial component and screws in
the tibial tray securely fi xes the tibial component,
and obviates the use of fi xed structural
bone graft.
Ligament Release Technique and Differential
Balancing
Stability is assessed fi rst in fl exion by holding
the knee at 90° fl exion and maximally
internally rotating the extremity to stress
the medial side of the knee, then maximally
externally rotating the extremity to evaluate
the lateral side of the knee. A medial opening
greater than 4 mm, and a lateral opening
greater than 5 mm, is considered abnormally
lax, whereas an opening less than 2 mm on
either side is considered abnormally tight.
The knee then is extended and stability is assessed
in full extension by applying varus and
valgus stress to the knees. A medial opening
greater than 2 mm and a lateral opening
greater than 3mm is considered abnormally
lax, whereas an opening less than 1 mm on
either side is considered abnormally tight.
Tight laterally in fl exion only
In knees that are too tight laterally in fl exion,
but not in extension, the LCL is released in
continuity with the periosteum and synovial
attachments to the bone. When this lateral
tightness is associated with internal rotational
contracture, the popliteus tendon attachment
to the femur also is released. The iliotibial
band and lateral posterior capsule should not
be released in this situation because they
provide lateral stability only in extension.
Tight laterally in fl exion and extension
The only structures that provide passive stability
in fl exion are the LCL and the popliteus tendon
complex, so knees that are tight laterally in
fl exion and extension have popliteus tendon or
LCL release (or both). Stability is tested after
adjusting tibial thickness to restore ligament
tightness on the lateral side of the knee. Ad-
ditional releases are done only as necessary
to achieve ligament balance. Any remaining
lateral ligament tightness usually occurs in the
extended position only, and is addressed by
releasing the iliotibial band fi rst, then the lateral
posterior capsule if needed. The iliotibial band
is approached subcutaneously and released
extrasynovially, leaving its proximal and distal
ends attached to the synovial membrane.
Tight laterally in extension only
In knees that initially are too tight laterally
in extension, but not in fl exion, the LCL and
popliteus tendon are left intact, and the
iliotibial band is released. If this does not
loosen the knee enough laterally, the lateral
posterior capsule is released. The LCL and
popliteus tendon rarely, if ever, are released
in this type of knee.
Finally, the tibial component thickness is
adjusted to achieve proper balance between
the medial and lateral sides of the knee. Anteroposterior
stability and femoral rollback are
assessed, and posterior cruciate substitution
is done if necessary to achieve acceptable
posterior stability.
MEDIAL PIVOT KNEE
ARTHROPLASTY
J. D. 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 (specifi -
cally the cruciate ligaments) guide the motion
of the knee in such a way that “rollback” occurs.
(Rollback is the progressive posterior