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The Principles of an Ideal Patellofemoral Arthroplasty
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A number of authors have reported significant
clinical improvements and a reduction in
patellofemoral complications following the
revision of Lubinus prostheses to the Avon
Patello-Femoral Joint Replacement System
(Stryker® Howmedica Osteonics, Allendale,
New Jersey), a second-generation PFA [6, 7]. A
retrospective case series found that 17% of
patients with the Lubinus prosthesis suffered
from either subluxation, catching or severe
pain. This rate was significantly reduced to 4%
following conversion to the Avon [6]. Another
study replaced 14 failed Lubinus prostheses
with Avon implants for patellar component
wear and trochlear component malposition.
The authors reported substantial improvement
in functional outcomes and pain, with no cases
of misalignment or instability at five years
follow-up [7]. These improvements are
probably attributable to the trochlear design of
the Avon.
The Avon, unlike the Lubinus or Richards, has
an anterior flange which is straight near
extension, in the sagittal plane, and then a
radius of curvature distally, through an arc
close to 90°; this shape is closer to the normal
geometry of the distal femur (see fig. 2B). This
allows for the trochlear prosthesis to lie flush
within the intercondylar notch, medially,
laterally and on the surface of the anterior
femoral cortex. The broader sulcus angle of the
Avon (approximately 125°) compared with the
Lubinus (approximately 110°) is less
constraining in extension and thus less sensitive
to slight patellar tilt or subluxation (see fig. 1A-
B). This concept is based on previous
biomechanical studies [8, 9] performed during
the development phase of the Kinemax total
knee replacement which suggested that the
patella is unconstrained in full extension and its
movement is dictated by the quadriceps, mainly
vastus lateralis, and hence there is often a
lateral tilt. The patellar component has enough
freedom to find its course smoothly along the
trochlear groove, whereas the narrower Lubinus
prosthesis was much less forgiving.
TheAvon prosthesis was based on the Kinemax
Plus total knee replacement, with a symmetrical
component; one geometric design exists for
both left and right. The justification [10] for
this design was that the prosthesis aligns with
the mechanical axis, not the anatomical one,
and therefore sided prostheses are not necessary.
The functional outcomes for this prosthesis are
excellent, the incidence of patellar maltracking
is less than 1% and survivorship has been
reported as 100% at mean 5 year follow-up
[11] and 94% at 8-14 year follow-up [12].
Despite these encouraging results, other authors
have postulated that the lack of asymmetry
Fig. 1A-B : [A] The skyline radiograph shows the
trochlear sulcus angle of the Lubinus prosthesis is
approximately 110°. [B] The skyline radiograph
shows the broader trochlear sulcus angle of the
Avon prosthesis is approximately 125°.
Permission to use image granted by copyright owners
LippincottWilliams &Wilkins. Lonner JH, Patellofemoral
arthroplasty: pros, cons and design considerations.
Clin Orthop 2004; 428: 158-65.
A
B