Primary Knee Replacement is, as the term suggests, the name
given to a family of knee implants designed to be used as a first
knee replacement. This type of knee replacement allows the remaining
soft-tissue to provide some of the constraint to the knee joint
and is usually implanted in patients who have not had a previous
Total Knee. The term constraint, does not imply limiting
movement, rather it simply allows some of the normal stabilizers
in the knee (namely the collateral ligaments, and sometimes the
PCL) to still perform their function.
Revision Knee Replacement
Revision Knee Replacement is designed to replace a worn-out
and/or loosened Primary Knee Replacement. This type of knee replacement
has more intrinsic constraint and can "play the role"
of some of the natural stabilizers which may be absent or no longer
functioning. In some cases, a revision knee implant may be used
as a patient's first knee implant if the disease is advanced enough.
PS Knee Implants
PS, Posteriorly Stabilized, or PCL Substituting Knee Implants
are designed to be implanted with the Posterior Cruciate Ligament
or PCL removed. In this type of knee replacement, a protrusion
on the tibial insert is designed to mate with a specially shaped
bar (cam) on the femoral component. This "cam and post"
interaction substitutes for the normal function of the PCL by:
Limiting anterior or "frontward" movement of the
femur (thigh) relative to the tibia (shin).
Promoting posterior or backward movement of the femur relative
to the tibia as you bend or flex your knee. This movement
is necessary to allow for deep flexion of the knee.
Studies have shown that PS knees tend to more predictable and
provide for slightly higher degrees of flexion compared to CR
knees (see CR section). This is because the function of the
knee is dependent on a fixed metal and plastic mechanism rather
than the native PCL whose function and balance can be variable
in a diseased knee.
For some photos and description of a PS knee you can
visit:
CR or Cruciate Retaining Knee Implants are designed to be implanted
with an intact Posterior Cruciate Ligament or PCL. Often this
ligament is still functioning to some degree in patients with
advanced osteoarthritis. In this type of knee replacement, the
PCL is allowed to act as the primary stabilizer limiting anterior
or "frontward" movement of the femur (thigh) relative
to the tibia (shin). This is the primary function of the PCL in
the normal knee. Another function of the PCL is to promote posterior
or backward movement of the femur relative to the tibia as you
bend or flex your knee. This movement is necessary to allow for
deep flexion of the knee.
Studies have shown that CR knees tend to have less predictable
and slightly lower degrees of flexion compared to PS knees (see
PS section). This is because the function of the knee is dependent
on the native PCL whose integrity and "health," if you
will, can be somewhat variable. That being said, there are studies
that have shown that the performance of CR knees and PS knees
are comparable. If you are so inclined, you can view abstracts
of publications at:
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.
