M. Odumenya, S.J. Krikler, A.A. Amis
296
Patellar Component
In broad terms there are two types of patellar
component design in current use: the non-
anatomical dome-shaped (axisymmetrical)
‘button’ and the conforming asymmetrical
component which is either functional or
anatomical.
The Avon and FPV both have conforming
geometry. The most recent Avon patella
component is anatomical, mimicking the seven
facets of the natural patella. The design is based
on the Kinemax patella and has a central crest
offset towards the medial side by 3mm. The
medial and lateral sides each have a superior,
central and inferior facet. The medial side is
shorter than the lateral, and so the medial edge is
thicker than the lateral, due to the offset crest.
This edge is smoothed to create an articular
surface similar to the odd facet. In deep knee
flexion the patella rotates internally and this
surface articulates with the lateral aspect of the
medial femoral condyle. In the original design
the medial edge scored the medial condyle,
hence the change. The designer recommends
leaving a 1-2mm rim of cancellous bone so that
a new rimof fibrous tissue (a ‘patellar meniscus’)
can form and protect the component, and to use
a smaller size thus reducing the degree of wear
of the button by widening the fibrous tissue rim.
The FPV patellar component is sided and
facetted with an off-centre longitudinal ridge
that becomes increasingly more medialised
from proximal to distal. The base is oval shaped
to minimise the uncovered resection surface of
the patella. The two proposed advantages of
such a design are the increased surface area
comparedtothedomeshapedpatellarcomponent
and the greater resistance to lateral subluxation.
However, one could argue that the FPV patellar
component needs to be more resistant against
such a broad sulcus angle compared to theAvon.
The main concern with these components is
achieving the correct alignment. If fixed in the
wrong orientation the component will not
articulate in the desired fashion and the patient
may become symptomatic.
The axisymmetric button does not need to be
fixed in a particular rotation, therefore fixation
is less challenging. However, contact pressures
are known to be higher with an axisymmetric
patellar component compared to a more
anatomical bearing surface. Previous studies
have shown that the contact stresses at the
button-trochlear interface exceed the yield
strength of the
ultra high molecular weight
polyethylene
(UHMWPE) causing plastic
deformation [24, 25].
In vivo
some of the load
may be transmitted to the adjacent fibrous
tissue, so the contact force may actually be
lower than previously measured
in-vitro
.
The Low Contact Stress
(LCS) patellofemoral
prosthesis (Depuy Orthopaedics, Warsaw,
Indiana) had a modular two-part patellar
component, consisting of a metal plate for bone
fixation and a mobile polyethylene bearing
[26]. The concept was that the patella would be
self-aligning within the trochlear groove to
enhance tracking. The early (two-year) results
from an independent centre were discouraging
in that 31%(17 out of 51) had already undergone
revision of which only two were related to
tibiofemoral disease progression [27]. This
study reported a dismal 46% (95% CI 30 to 63)
survival at three years (severe pain/revision
endpoint). The high failure rate was most likely
due to themodular patellar component. Multiple
complications have been reported including
dissociation of the polyethylene from the metal
base and loss of mobility. Significant metallosis
has also been found in a number of revised
knees as a result of metal-on-metal articulation
of the trochlea with the metal base of the
patellar component when it became exposed at
extremes of motion [27].
It is crucial that there is overlap of the patellar
and trochlear articular surfaces in full knee
extension. The amount of overlap should be
greater than 30%; if it is less than 30% there is
a risk of subluxation [28]. If this is not achieved,
patella alta is created and the patella may not
engage in the initial stages of knee flexion.
Once flexion commences the patella becomes
increasingly more engaged within the trochlear
groove. At approximately 90° knee flexion the
patella is most stable, beyond this it begins to
internally rotate and the odd facet articulates
with the lateral aspect of the medial femoral