LIVRE JDG 2010

knee arthroplasty have shown 5 to 10 degrees

of internal tibial rotation during flexion [3, 15].

The post-cam mechanism engages at different

degrees of flexion (fig. 2). This is design speci-

fic but is as well different from individual to

individual even when the same type of prosthe-

sis is used, as was shown by

in-vivo

measure-

ments [18]. For most PS-knees the cam

engages with the post at mid-flexion and disen-

gages in deep flexion. In all PS-knees the post-

cam mechanism will prevent dorsal subluxa-

tion of the tibia throughout range of motion

[10, 12]. In addition it will guide femoral roll-

back on the tibia (fig. 2). The rollback allows

for increased flexion. Flexion has been repor-

ted to be better in most PS-designs compared

with the cruciate retaining variant [3]. Rollback

reduces quadriceps force by improving the

labour arm for the extensor mechanism [7].

Independent of the post cam mechanism

mediolateral translation is controlled by box

and post dependant on the fit of the post within

the box (fig. 3). With a tight fit between post

and box rotation is only possible if the cross

section shows a round-on-round geometry.

Varus-valgus rotation is controlled if there is a

tight fit in the frontal plane.

In vivo

measure-

ments have shown a varus-valgus play of up to

4 degrees in a standard PS design [15]. In high

flexion PS prosthesis the post-cam mechanism

enhances femoral rollback only at the mid

range of flexion. In high flexion, geometry of

the femoral condyles and the tibial plateau

play a more crucial role [13]. If impingement

of the anterior side of the post and the anterior

cam are allowed the post/cam mechanism can

substitute for the ACL in hyperextension,

extension and in low flection angles [14].

JOURNÉES LYONNAISES DE CHIRURGIE DU GENOU

206

Fig. 1 : Height and width of the post together with post position and post shape are the deter-

minants of the tibial part of the post-cam mechanism

LPS® (left) and N

LCCK®

(right) articular surfaces; Zimmer, with permission).

- Position

- Height

- Mediolateral width

- AP width

- Shape (quadrangular, round, fit with box)

- Contact point

Table 1 : Factors that influence function and

wear of the post-cam mechanism

Fig. 2 : Interaction of the post-cammechanism.

From 75° on the post-cam contact guides

femorotibial motion and provides femoral roll-

back

EGACY

PS®, Zimmer, with permission).