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Within the knee prosthesis family posterior
stabilized (PS) knees are characterized by a
coupling mechanism between the femoral
component and the tibial articular surface.
This mechanism was primarily meant to com-
pensate for a lost or resected posterior cruciate
ligament. The concept was introduced about
40 years ago [10]. The PS-knee is amongst the
designs that have produced favorable long-
term results [5]. At present there is a high and
increasing interest in PS-knees because they
allow for a more “natural” kinematic of the
knee after total knee replacement [7]. But the
coupling mechanism might lead to additional
problems in PS-knees, not seen in the cruciate
retaining prosthesis type [17]. These problems
are specific for different designs of PS-knees.
Today we see an enormous variance between
different designs within the PS-knee family.
DEFINITIONS
In general PS-knee prosthesis are characterized
by a mechanism that prevents a posterior dislo-
cation (posterior drawer) of the tibia.
Throughout this article all movements will be
described for a mobile tibia versus a fixed
femur except for the so called “femoral roll-
back”, because this is a very commonly used
and well defined term. The protuberance on the
tibial articular surface will be called “post”
because the term “spine” is an anatomical des-
cription for a part of the genuine tibial plateau.
The femoral counterpart that accommodates the
post will be referred to as “box” being aware
that this can be a closed or an open box. The
posterior and anterior borders of the box that are
meant to engage with the post will be addressed
as “cam”. Whereas the posterior cam plays the
much more important role, the anterior cam is
only mentioned for the so far experimental bi-
cruciate stabilizing knee prosthesis [18].
INTERACTION BETWEEN
POST AND BOX
The interaction between the post and the box
can be divided in a post-cam mechanism and
an additional effect occurring between the
medial and lateral walls of the femoral box and
the post. Function of the post-cam mechanism
is influenced by design factors (Table 1,
fig. 1). Whereas the post-cam mechanism is
probably the most important effect in a PS-
knee, box and post will restrict AP translation,
ML translation, rotation around the long tibial
axis as well as varus/valgus rotation [15].
In
vivo
measurements with patients after PS total
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DIFFERENT TYPES OF POSTERIOR-
STABILIZATION IN PS-KNEES
D. KOHN