Typically, the top surface of the tibia is resected creating
a planar surface that is perpendicular to the shaft of the tibia.
In a "PCL retaining" procedure, a "bony island"
is preserved around the insertion site of the Posterior Cruciate
Ligament in the tibia. In a "PCL sacrificing" procedure,
the PCL and the insertion area are removed.
About 8mm of bone is removed as measured off of the "good"
side of the joint surface of the tibia known as the tibial plateau.
In the arthritic knee, there is usually uneven wear of the joint
surface. The joint basically collapses on the diseased side. The
resection level is typically a couple of millimeters below the
surface of the worn side of the joint, or 8mm off of the "good"
side. Creating a flat tibial surface therefore requires a wedge
shaped piece of bone to be removed.
Once the resection is made, trial components are placed in the
joint. With the femoral and tibial prep complete (and often the
patella prep as well), and a trial femur and tibia implant in
place, the surgeon basically moves and tenses the joint through
a range of motion. The surgeon can use tibial insert trials (also
called tibial bearings or tibial spacers) of various thicknesses
until the correct stability is achieved. The tibial insert trial
corresponds to the tibial insert implant that will eventually
sit in between the metal femoral and tibial
implant components.
Often, the surgeon must make surgical corrections of certain
soft-tissue structures to help align and stabilize the knee.
Once trialing is complete, the appropriate "keel"
preparation is made. The keel is a fin or cross shaped protrusion
on the implant that helps it stay fixed and in
the bone. This can be punched, chiseled, broached or cut into
the tibia and a set of instruments is used so that the resulting
shape matches the implant. Here, the surgeon must take care in
aligning the keel preparation so that the correct rotation of
the implant about the axis of the tibia is achieved. Correct rotational
alignment of the implant promotes better distribution of load
across
the implant and better motion of the knee joint.
Next, the "tibial baseplate" portion of the implant
is simply pressed or cemented in place.
A plastic tibial insert is then snapped into the tibial baseplate
(see the Implant section for more details). The insert comes in
the various thicknesses that correspond to the trial used previously.
Medical Disclaimer:
The contents of this website and the links are for information only
and are not a substitute for medical advice. This information should
not replace necessary
medical consultations with a qualified physician or health care
provider.
