ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

(a) (b)
Fig. 4. Maximum von Mises stresses (MPa) in lateral (L) and medial (M) side of the PF
joint (femur cartilage (Fig. 4a) and patellar cartilage (Fig. 4b) for healthy and PFPS
subjects at different knee positions.
Higher contact stresses occurred in the lateral side of the healthy joint and shifted to the
medial side in the PFPS joint (Fig. 3). Similar trends were obtained for the von Mises
stress (Fig.4).
5. DISCUSSION
The purpose of this study is to quantify the contact stresses in the PF joints for healthy
and symptomatic joints. To fulfil this purpose, FEA of the PF joints for healthy and
pathological subjects at two different knee flexion angles were investigated. Besides the
FEA, another geometric approach has been developed to investigate the joint contact
behaviour for both healthy and PFPS subjects. From the current study we demonstrated
that the contact deformation is related to the virtual penetration depth (PD) of the joint’s
femoral and patellar cartilage surfaces measured using the 3D registration and linear
mapping of the patella from 15° (reference position) to 30° and 45° knee flexion. Five
different measurement techniques were used to measure PD in three healthy and three
PFPS female knee joints.
Contact areas between femur and patellar cartilage surface are extended from the medial
to the lateral side of the PF joints [19]. Therefore, the location of the maximum contact
stress could be in the lateral, central, or medical side of the contact surface. In the
present study, the location of the peak contact stress was found in the lateral side of the
patellar cartilage surface for healthy subjects using finite element models which are
consistent with those reported in the literature for healthy subjects [17,18]. Contact
stresses and von Mises stresses increased with knee flexion from 30° to 45°.
Using the alternate geometric method, higher values for PD for the healthy subjects at
30° knee position were obtained in the lateral side compared to the medial side of the
PF joint. Contact stresses reported for PFPS subjects are also higher in the lateral side of
the PF joints [17] which are completely opposite compared with the current study. One
recent study has found increased bone metabolic activity in the posterior side of the
patella [14]. They also found increased bone metabolic activity in both medial and
lateral side of the patella of subjects with chronic knee pain [14].
It was reported that due to the muscle imbalance, the patella shifts and tilts laterally
causing an overloading in the lateral facet [7-10, 20]. Our study, confirmed the lateral
shift of the contact area using the alternate geometric method and the FEA. However,
peak stresses and PD location were observed in the medial side for PFPS subjects; in
contradiction with the current literature. However, Sawatsky et al. (2012) [5] showed