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The Principles of an Ideal Patellofemoral Arthroplasty
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condyle. In deeper flexion, the patella
articulates with the femoral condyles with the
risk of cartilage wear but in the presence of a
‘patellar meniscus’, it bears against the fibrous
tissue rim of the patella and thus prevents
polyethylene wear.Material Science
The ideal trochlear component should be highly
resistant to abrasive wear and have a low
coefficient of friction. The Avon and FPV are
both made from cobalt chrome (CoCr). The
Journey PFJ consists of oxidised Zirconium
(OXINIUM®; Smith & Nephew, Memphis,
USA). This bearing material has been reported
as nearly 5000 times more resistant to abrasion
than CoCr [29]. OXINIUM® also has a
significantly lower coefficient of friction
against UHMWPE due to its more hydrophilic
surface compared with cobalt chrome [30].
This means the synovial fluid has a greater
propensity for the surface of the prosthesis and
disperses evenly, and therefore acts as a more
effective sliding lubricant. For these reasons,
the manufacturers have suggested that
resurfacing of the patella is optional. The use of
UHMWPE for patellar components is standard
practice. Issues of wear are related more to the
tracking motion of the patellar component
rather than the yield strength. Despite advances
in materials for implants, there are no clinical
results of PFA which better a polyethylene
patellar component articulating against a CoCr
trochlea, with both components fixed using
PMMA bone cement.
Patient Selection
Whilst considering the ideal PFA, there must
be a degree of deliberation over the ideal
patient. Patellofemoral arthroplasty should
not be carried out in patients with inflamma
tory joint arthropathy or chondrocalcinosis. A
popular belief growing amongst some
surgeons is to reserve this procedure for
patients with trochlear dysplasia with or
without a degree of patellar tilt or subluxation.
A study supporting this principle showed that
PFA was most effective in patients with
trochlear dysplasia [31]. These findings were
further corroborated by a later study [28]
which found that none of the patients, at a
mean of 7.1 years follow-up, who had
patellofemoral arthritis secondary to trochlear
dysplasia, required revision surgery for
progression of tibiofemoral arthritis compared
to 17% (5 out of 30 knees) of those without
this underlying diagnosis. Those with severe
misalignment due to an abnormal Q angle or
maltracking will require additional proce
dure(s) at the time of PFA to ensure correct
patellofemoral biomechanics are restored. The
complex nature of such combined surgery
requires a surgeon experienced in both
arthroplasty and patellar instability.
Conclusion
The ideal PFAconsists of a trochlear component
that is able to engage the patella within the
trochlear groove during the full range of
flexion-extension without over-constraining
the patella in knee extension, and also possess
a sagittal shape congruent with the distal femur.
The patellar component design is dependent to
a degree on the geometry of the trochlear
component. There are advantages to both
axisymmetric and conforming designs, but the
ease of use of an axisymmetrical button may be
outweighedbythesuperiorbearingperformance
of a more conforming design in a younger
patient. Modular components should be
avoided, given the significant complication
rate. To ensure good fixation both components
should have an adequate number of pegs
appropriately positioned to optimise fixation
and this should be consistent between sizes to
so the best fit can be achieved.
The ultimate goal is for an improved clinical
outcome including greater patient satisfaction.
Ideally, this should be comparable to total knee
arthroplasty while bearing in mind that for
some patients, PFA may be “bridging surgery”
to avoid total knee arthroplasty early in life.
For others it may be a permanent solution.
Tibiofemoral degeneration should be the most
common reason for revision surgery.