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The Principles of an Ideal Patellofemoral Arthroplasty
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Despite these relatively minor issues, these
instrumented systems are far superior compared
with those of the past. Systems which required
significant freehand sculpting of the femur
were daunting to surgeons trained to perform
knee surgery with sophisticated guides and
cutting blocks, and there was a feeling that
these systems required a long learning curve.
It is plausible to believe that the high
reproducibility that more modern instruments
provide together with theoretically better
prosthetic designs would have a positive impact
on clinical outcomes. However, to date, no
other prosthesis has equalled or exceeded the
excellent functional results [11, 12] or revision
rate [11, 12, 17] of the Avon. Time will tell
whether these newer prostheses are able to
improve on these highly commendable results.
Biomechanicaland
Material Science
Considerations
The ideal PFA should attempt to reproduce
patellofemoral biomechanics and correct co-
existing pathology causing instability.
The patellofemoral joint has a complex
configuration. Knowledge of the joint reaction
forces, kinematics and stability are required to
create an effective PFA. Patients suitable for
PFAoften have instability symptoms secondary
to the maltracking or malalignment pathology
that may have caused the arthritis to occur.
Therefore, it is of paramount importance that
the articular geometry of the femoral and
patellar components provides stability in the
medial-lateral direction (transverse plane).
The patella is pulled laterally in the final 20° of
knee extension and disengages from the
proximal trochlear groove. The line of motion
is estimated as the direction of the force vector
of the vastus lateralis, the largest muscle of the
quadriceps. This direction of motion is reversed
during early knee flexion, where the patella
moves ‘relatively medially’ and the lateral
facets of both patella and trochlear groove
engage. In patients with patellar maltracking
the patella tends to displace laterally. The
severity of displacement depends on the
integrity of the surrounding soft tissue stabi
lisers and bony alignment.Afemoral component
with a prominent proximal lateral facet could
prevent this lateral displacement by causing the
maltracking patella to engage, capture and
maintain engagement as it passes into the distal
portion of the trochlear groove in deep flexion.
The mechanics of medial-lateral stability is of
crucial importance for reasons already
discussed. However, the other degrees of
freedom that make up the complex movement
pattern of the patella must be considered. This
includes rotation in the sagittal plane (that is:
flexion-extension) and rotation and translation
in the coronal plane [21]. During the initial 30°
of tibiofemoral flexion there is a lag discrepancy
between flexion occurring at the tibiofemoral
joint and that of the patella due to the distal
translation of the patella that occurs (10° of
patellar flexion at 20° of tibiofemoral flexion).
Beyond this, the patella rotates around the arc
of the femoral articular geometry in the sagittal
plane, with 55° of patellar flexion at 90°
tibiofemoral flexion [14]. A femoral component
with a low-profile anterior flange would enable
this motion to occur smoothly and thus avoid
the ‘catching’ or ‘snapping’ symptoms caused
by a bulky anterior flange that forces the patella
into extension during initial engagement.
In deep flexion femoral roll-back occurs and
the distal end of the patella is raised out of the
trochlear groove, bridging the intercondylar
notch. Therefore the transition between the
trochlear component and normal articular
surface should be smooth, i.e. no step, to avoid
‘clunking’ during knee motion.
In patients with severe misalignment symptoms
such as lateral patellar tilt, chronic lateral
subluxation or lateral maltracking, the increased
prominence of the lateral facet of the trochlear
component may not be sufficient to correct the
abnormality. Soft tissue reconstruction such as
medial patellofemoral ligament reconstruction
or even tibial tuberosity anteromedialisation
may be necessary to correct the pathology.