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PFOA is disparately distributed across the
patellofemoral joint itself, with the patella side
demonstrating more severe degeneration earlier
in the disease process compared to the
juxtaposed femoral groove [13, 14]. Several
hypotheses have been suggested based on these
observations, including differences in load
duration, chondrocyte metabolism, and
histological, material and compositional
properties [13]. The etiology of PFOA has
proposedly three major pathologic pathways:
posttraumatic, history of instability and primary
(or idiopathic) [18, 19]. Long-term studies
showed no difference in PFA survival between
these diagnostic groups [1], although no
prospective data are available. Post-traumatic
PFOA can result from direct trauma to the
patellar cartilage, non-anatomic reduction of
displaced patellar fractures, excessive callus
formation in comminuted fractures or
suboptimal refixation of the patellar tendon
after partial patellectomy [20]. Patellofemoral
arthroplasty cohort studies included patients
with end-stage post-traumatic osteoarthritis in
12-32% [1, 21]. Although patellar dislocation
can result in traumatic joint cartilage lesions,
the risk factors for instability itself also
predisposes a patient to the development of
osteoarthritis [12]. These risk factors include
deficiencies in one of three anatomical
structures that stabilize the patellofemoral
joint: trochlear groove geometry, the medial
and lateral retinacula (including the medial
patellofemoral ligament (MPFL)) and the
alignment of the extensor apparatus (including
the quadriceps muscles, patellar tendon, and
tibial tuberosity). Primary isolated patello
femoral osteoarthritis is diagnosed when no
history of trauma or findings of instability are
noted. This subgroup is probably over-
represented in clinical studies due to
unrecognized instability.
When asymptomatic, the natural course of
PFOA can be benign, as shown by Guilbert et
al. in a cohort of patients treated nonoperatively
for isolated patellofemoral osteoarthritis; 90%
had not been operated upon after a follow-up of
9 years [23]. They concluded that PFOA is well
tolerated as long as the femorotibial
compartments were not involved in the arthritic
process. If symptomatic PFOA is not well
tolerated and the indication for PFA is set, there
still is the possibility that OA progresses
femorotibial. This occurs less in posttraumatic
PFOA then in patients after a history of PF
instability or with idiopathic (primary) PFOA.
The diagnosis of PFOA is based on a typical
history of anterior knee pain after prolonged
sitting or upon rising from a chair, and pain
when descending and ascending stairs. The
pain usually is less severe when walking on
level ground. Clinical findings are not specific
and include quadriceps wasting, pain, and
crepitus emanating from the anterior compart
ment [15]. Compared to the operative findings
of patellofemoral osteoarthritis, patellofemoral
crepitus as a sign has a sensitivity of 89% and
specificity of 82% [16]. Findings of patello
femoral instability are common. Regardless of
the presenting age or etiology, symptomatic
PFOA is rare [17]. Radiological findings of
PFOA using the lateral and axial (skyline) view
typically include patellofemoral joint space
narrowing without signs of femorotibial
osteoarthritis. The skyline view has been
suggested to be more sensitive than the lateral
view for assessing patellofemoral osteoarthritis,
though over 75% of orthopedic surgeons in the
United Kingdom do not use the skyline view in
the routine investigation of knee osteoarthritis
[24]. Compared to intra-operative findings, no
significant differences in terms of sensitivity
and specificity were found between lateral
(sensitivity 82%, specificity 65%) and skyline
(sensitivity 79%, specificity 80%) radiographs
[16]. A clinical study comparing lateral and
skyline radiographic views with intra-operative
findings demonstrated that a normal skyline
radiograph can be used to reliably exclude
clinically significant (Collins grade 4)
patellofemoral osteoarthritis [25]. Magnetic
resonance imaging (MRI) studies have shown
that joint space narrowing in axial view
radiographs has a high specificity for cartilage
defects detected by MRI in the same joint [26].
A significant association has been found
between the presence of an osteophyte in the
patellofemoral joint and knee pain; other MRI
findings, including focal or diffuse cartilaginous
abnormalities, subchondral cysts, and bone
marrow edema were not associated with knee
symptoms [27]. In the majority of cases,