Background/Purpose:   While patellofemoral
(PF) OA has been found to have substantial impact on pain and function, most
studies continue to focus on tibiofemoral (TF) OA.  These compartments differ in
pathomechanics and the profile of factors contributing to disease course.  Depending on where and how it is
transmitted, increased joint stress, force transmitted through the joint averaged
over articular contact area, may overload and injure cartilage in either compartment.  Knee sagittal
plane dynamic joint stiffness (DJS) is the interaction between the external
knee flexion moment and knee flexion joint excursion during gait.  In theory, greater knee sagittal DJS may
increase joint stress and accelerate disease progression through elevated
external knee flexion moment, reduced sagittal joint excursion and contact area,
or both.  We hypothesized that
greater baseline knee sagittal DJS is associated with cartilage damage
progression in PF and TF compartments 2 years later in persons with knee OA.

Methods:   Participants
all had knee OA (KL grade ³ 2) in at least one knee.  Three-dimensional knee kinematics and
kinetics during walking on a 35 x 4 foot walkway were captured at a rate of 120
Hz, using external passive reflective markers, an 8-camera Digital Real-Time
Eagle motion analysis system, and 6 AMTI force plates.  Baseline sagittal DJS of both knees was
computed as the slope of the linear regression line for external knee flexion
moments vs. flexion angles during the loading response phase of gait.  Participants underwent 3.0T MRI of both knees at baseline and 2 years later using
double oblique coronal and axial FLASH sequences, coronal T1-weighted spin-echo
(SE), and sagittal, coronal and axial fat-suppressed turbo spin echo sequences.
 Cartilage damage progression was
defined as WORMS score worsening for whole, medial, or lateral TF and PF.  We assessed the association
between baseline DJS and 2-year cartilage damage progression (yes vs. no),
using logistic regression with GEE to account for correlations between knees, adjusting
for demographics and disease covariables.  Results are shown as odds ratios (ORs) and
95% confidence intervals (CIs).

Results:  The study sample consisted of 391
knees from 204 persons: mean age 64.2 years (SD 10.0); BMI 28.4 kg/m²
(5.7); 156 (76.5%) women.  Table 1a
shows the percentage of knees with cartilage damage progression. Table 1b shows
the ORs for progression unadjusted, then adjusted for covariables.  Greater baseline DJS was strongly associated
with 2-year structural progression in the PF, specifically the lateral surface,
but not in the TF.

Conclusion:   Elevated baseline knee sagittal DJS was
associated with increased risk of PF disease progression.  Unlike frontal-plane joint mechanics,
which predominantly influence the medial-lateral TF load distribution, sagittal
knee mechanics during ambulation may determine PF load and disease course.