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The movements of the patellofemoral joint are
complex [5, 9]. One part or another of the
patellar cartilage remains loaded throughout
the entire flexion-extension cycle, excepted in
the first degrees of flexion (the patella here
rests on the synovium of the supracondylar fat
pad). The distal portion of the patella is loaded
as the knee flexes, and the contact area migrates
proximally with progressive flexion until 90°,
after which the contact area moves back toward
the central aspect and lateral facet of the patella,
while the medial facet lies in contact with the
synovium overlying the anterior cruciate
ligament.
Most activities involving knee flexion take
place in a closed-kinetic-chain mode, including
bending down, rising from a chair and
ascending stairs, where the forces across the
patellofemoral joint increase as the knee flexes
from 0 to 90°, as do the contact pressures [9].
A improper fit between the two mating areas
leads to an abnormal stress distribution and
eventually blunt trauma.
So, while primary osteoarthritis is signaled by
an imbalance between the synthesis and de
gradation of thematrix components, mechanical
destruction of cells and matrix due to blunt
trauma may be the cause of secondary
osteoarthritis.
Furthermore, the articular cartilage of the
patella consists of a solid phase, made up
mostly of collagen and glycosaminoglycans,
and a fluid phase. The solid phase is slightly
permeable and when a load is applied to the
articular surface, the fluid slowly redistributes
itself within the solid matrix. Disruption of the
articular surface by cracks, fissures or crevices
leads to a loss of pressure within the fluid
phase. High stresses are then borne by the
collagen fibers, which become more prone to
breakdown [6, 9].
Moreover, the ability of collagen to withstand
high stresses has a variable genetic component,
wich accounts for the wide range of clinical
responses to a given joint load among different
patients [9].
Etiologic factors, independently or associated,
are then logical causes of isolatedpatellofemoral
arthritis [5, 8, 9, 12].
Extensor mechanism malalignment
“Malalignment” is a general term encompassing
conditions that lead to a poor positioning and
poor tracking of the
patella.Animproper fit of the
mating surfaces leads to abnormal distribution of
pressure, which in turn can lead to arthritis.
Patellofemoral dysplasia
Patellofemoral dysplasia is highly correlated
with patellofemoral arthritis, and there even is
a specific correlation between arthritis and the
various forms of dysplasia, especially for
trochlear dysplasia [8].
Patella alta, as an isolated factor, was not found
to predispose to arthritis [8].
Instability (symptomatic medio-
lateral displacement, including
dislocation)
Blunt traumas occuring during dislocations
associated with patellofemoral dysplasia in
most cases can explain the occurence of
patellofemoral arthritis.
Trauma
Direct blunt trauma can lead to degenerative
changes in articular cartilage.
Articular fractures of the patella and the
trochlea can be expected to pose the same risk
of arthritis as other intra-articular fractures.
Obesity
Pressures across the patellofemoral joint during
closed-kinetic-chain activities increase with
the person’s weight.