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Once the alignment has been set, the
instrumentation allows reproducible
bone resection of the articular
surfaces. The distal femoral and
proximal tibial cuts are achieved by
linked resection guides. The cuts
are therefore parallel and result in
a preset space that is calculated to
match the thickness of the implants
and reproduce the selected alignment.
These linked, precise cuts reduce
the potential need for recutting and
may help to preserve tibial bone
stock. Because the tibial resection
level corresponds to the selected
polyethylene thickness, the likelihood
of needing a tibial articular surface
that is between available component
thicknesses is reduced. Also, the
EM instruments eliminate the need
for intrusion into the medullary
canal. By not drilling the canal, the
associated blood loss and possibility
of fat embolism are reduced.
Patient Preparation
With the patient in the supine
position, test the range of hip and
knee flexion. If unable to achieve
120° of knee flexion, a larger incision
may be necessary to create sufficient
exposure. Wrap the ankle area with
an elastic wrap. Do not place bulky
drapes on the distal tibia, ankle, or
foot. A bulky drape in this area will
make it difficult to locate the center
of the ankle, and will displace the
Tibial Resector, which may cause
inaccurate cuts.
Be sure that the proximal femur is
accessible for assessing the femoral
head location. Use anatomic landmarks
to identify the location of the femoral
head. Alternatively, the surgeon may
prefer to reference the anterior-superior
iliac spine.
Technique tip: Place a marker, such
as an EKG electrode, over the center
of the femoral head. Then confirm
the location with an A/P radiograph
or fluoroscopy.
Preoperative Planning
This technique is written with the distal
femoral resection performed first.
However, if preferred, the tibia can be
resected first. With either option, the
tibial apparatus must be assembled
and applied to the tibia first. Steps
three and four can then be performed
in the order preferred. Take standing
weight-bearing A/P and lateral
radiographs of the affected knee, and
a skyline radiograph of the patella.
Then take a supine A/P radiograph
showing the center of the femoral
head, the knee, and as much of the
tibia as possible (preferably including
the ankle). This radiograph is used to
determine limb malalignment.
The goal of the procedure is to
establish mechanical alignment that
is slightly undercorrected relative to a
neutral mechanical axis (see Fig. 1).
Do
not overcorrect the alignment. It is
better to be slightly undercorrected
than to risk overcorrection.
An additional radiograph while
stressing the limits of the tissues
may be helpful in assessing the
appropriate correction.
When evaluating the patient and
planning for the procedure, consider
TKA if:
• Degenerative changes are present
in the contralateral compartment
and/or the patellofemoral joint.
• The ACL is deficient.
• A significant flexion contracture exists
• Slight undercorrection is not
attainable
• There is significant overcorrection
with a valgus stress.
• There is an existing valgus or varus
deformity ≥15°
