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The Principles of an Ideal Patellofemoral Arthroplasty
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(Reconstructive) Ltd, Memphis, USA), offers
patellar resurfacing as optional. The unique
oxidised zirconium material of the trochlear
component has a significantly lower coefficient
of friction compared with cobalt chrome and
therefore the risk of wear will theoretically be
considerably reduced. The Zimmer® Gender
Solutions™PFASystem offers a guided milling
system for preparation of the trochlea which
allows accurate depth control and contouring,
thus avoiding the pitfalls of freehand technique.
In addition, it is the only PFA that incorporates
anatomical differences between genders into
the design. The trochlear sulcus angle for four
out of five of the available sizes is greater to
accommodate the larger Q angle found in
females who are more predisposed to develop
patellofemoral arthritis, presumably for this
very reason. Whilst these innovations appear
fairly logical and plausible it is important to
bear in mind that there is no clinical evidence
to suggest that they are of any benefit or
advantage. Similarly, customised prostheses
developed as a solution to the issue of size
limitations with standard prostheses have not
been shown to be of clinical or cost effective
benefit. Future robust studies are required to
investigate these theoretical advantages.
The most recent Report from the National Joint
Registry for England and Wales [17] revealed
the Avon as the most popular PFA, with the
lowest revision rate at 5 years reported as 7%
compared with 12% for the other brands (FPV,
Journey, Zimmer Natural-Knee® II PFJ and
Zimmer Gender Solutions PFJ).
Generally these more advanced designs have
better results than the first-generation pros
theses. Fewer failures secondary to patello
femoral dysfunction have been reported, with
progression of tibiofemoral degeneration being
the most significant mode of failure.
Complications such as loosening and anterior
knee pain are also far less common [18, 19].
Instrumentation
Studies have shown the importance of aligning
the trochlear prosthesis in external rotation to
improve patellar tracking and minimise forces
causing lateral translation [20]. A jig that sets
the external rotation using femoral alignment is
ideal, and freehand sculpting of the trochlea
may result in less accurate preparation. Ideal
instrumentation will provide accurate orienta
tion and consistent depth of the entire cut
surface to allow the prosthesis to sit flush on
the anterior surface of the distal femur, and
have a smooth transition from the prosthetic
surface to the native femoral articular surface.
Most current PFA systems are fully instru
mented. They claim not to require freehand
bone preparation and therefore offer consistent,
accurate alignment and precise bone cuts.
However, it is difficult to provide instrumenta
tion which can accommodate the deficient bone
which is often encountered in patients with
advanced patellofemoral degenerative changes
or dysplasia.
The current instrumentation systems are not
without limitations. For instance, the FPV
patellar spring loaded clamp is only designed
to be used with a medial parapatellar approach.
Surgeons intending to perform a lateral
parapatellar approach may attempt to overcome
this predicament by using the contralateral
patella component in reverse. However this
risks erroneous alignment which will have a
negative impact on patellar tracking. Currently,
most of the systems do not offer a short
intramedullary anterior cut guide. In patients
who have existing intramedullary fixation, the
guide is unable to contact the distal femur and
therefore the risk of notching and incorrect
external rotation are high. Another example is
the milling device and cutting jig used in the
Zimmer Gender Solutions PFJ system.
Although a very useful instrument it can be
challenging (particularly in the presence of
hard bone) to maintain orientation of the mill
within the guided tracks (see fig. 3A-B). Loss
of perpendicular mill alignment relative to the
jig could result in loss of the “set position”, the
jig rising off the bone and ultimately an uneven,
shallow bone preparation. This will offset
component positioning and potentially affect
the patellofemoral biomechanics. Furthermore,
once the jig is pinned into place the depth of the