208 / 324
PROBLEMS OF THE PS
DESIGN
The PS design can cause dislocation of the
TKA that does not reduce spontaneously
(fig. 4 right). A dislocation safety factor has
been proposed as a predictor for stability of the
respective prosthetic design [12] (fig. 5). On
the femoral side additional bone resection is
necessary to accommodate the box [6, 2]. But
the foremost problem is the high stress on the
tibial post [16]. Contact stress at the post-cam
mechanism can be as high as 120 MPa in the
worst case scenario, when the femoral compo-
nent is rotated and tilted [15, 1]. Lower but
still critical stresses have been shown by other
authors being as high as 33 MPa [16]. It has
been suggested that a more conforming post-
cam geometry would reduce contact stress by
increasing contact area. In addition it has been
suggested that the contact location at the post
should migrate, not to overload one singular
location at the post. Wear of the post unfortu-
nately cannot be assessed radiologically [4],
but can only be critically evaluated in retrie-
vals [8]. Wear or even breakage of the post are
a predominant problem of PS TKA. Post wear
has been found in 100 % of retrievals of
implants of all designs [5]. Wear can occur not
only on the posterior side where the post-cam
engagement takes place, but as well on the
anterior side if the post-cam mechanism
blocks hyperextension or compensates for the
ACL. The latter has been considered difficult
DIFFERENT TYPES OF POSTERIOR-STABILIZATION IN PS-KNEES
207
Fig. 3 : ML-Fit (arrows) between tibial post
and femoral box will determine whether the
post-box mechanism can compensate for
varus-valgus instability or not.
Fig. 4 : PS prosthesis (I
NTERAX
®, Stryker Howmedica). The tibia dislocated posteriorly when
the patient bent his knee in bed (right). It had to be reduced under anaesthesia.