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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