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prostheses had a deep constraining trochlear
design which required exact alignment of the
patellar and trochlear components; failure to do
so resulted in maltracking and catching of the
patellar component on the rim of the trochlea.
Due to the unforgiving nature of these prosthe
ses, a high number of additional operations
have been reported. De Winter
et al.
[2] found,
at a mean of eleven years follow-up, that 27%
of patients (7 out of 26) had required further
surgery to treat misalignment or maltracking;
two had patellar realignment, three had patel
lectomy and two had total knee arthroplasty.
Kooijman
et al.
[5] reported, at a mean of
seventeen years follow-up, 27 reoperations in
25 out of 45 PFA, of which 26% involved
corrective surgery for either patellofemoral
symptoms such as catching or prosthesis mal
positioning.
The Lubinus (Waldemar Link, Hamburg,
Germany) PFA was also associated with a high
number of patellofemoral complications. This
prosthesis had a narrow medial-lateral width
and a deep constraining groove in the axial plane
(see fig. 1A). The anterior flange did not extend
proximally, leaving the patella to articulate with
the anterior femoral cortex in full knee extension
before engaging with the trochlear component
in the initial 30° of flexion (see fig. 2A).
Unfortunately, this transition from the femoral
articular cartilage to the prosthesis was not
smooth due to its shape in the sagittal plane not
matching that of the distal femur. Therefore, to
avoid impingement of the intercondylar aspect
of the trochlear component on the tibia or
anterior cruciate ligament when the knee reached
full extension, the component had to be inserted
with an offset from the anterior femoral cortex.
It was this offset position that caused the patella
to catch and sublux on the proud proximal
anterior flange at 30° of tibiofemoral flexion [3,
4, 6], resulting in poor clinical outcomes. Of the
seventy-six Lubinus arthroplasties reviewed by
Tauro et al. [4] twenty four had patellar
misalignment and a further 21 knees required
revision surgery of which 15 were for patellar
maltracking. Therefore 51% of knees had
patellofemoral dysfunction, matching the high
rate of unsatisfactory clinical outcome, of 55%
of knees.
The developers of newer prostheses have taken
into consideration these design flaws of the
past as signified by the improvements in clinical
performance and lower rates of patellofemoral
dysfunction.
Second and Third
Generation
Patellofemoral
Prostheses
The most distinctive design difference between
first-generation and newer PFAs is the design
of the trochlear component. The first-generation
prostheses had a characteristic inlay design, in
which the implant was inset into the trochlea.
These prostheses were significantly smaller,
thinner in depth and narrower in width with a
deeper, more constraining groove than more
recent designs. The philosophy behind these
designs was to preserve bone and avoid
overstuffing the patellofemoral joint, allowing
for simple revision if required. In contrast, the
second- and third-generation prostheses are
onlay designs which have thicker and wider
trochlear components. The aim of the increased
thickness is to restore normal trochlear offset,
that is, the offset that existed before arthritis if
the trochlea was not dysplastic, in order to
maintain soft tissue tensions and the mechanical
advantage of the extensor mechanism. Unlike
the inlay design, the shape of the trochlea does
not completely govern the positioning of the
onlay device. Instead, the trochlea can be
placed in the correct position in the presence of
abnormal trochlear morphology without the
risk of the component sitting prominent relative
to the adjacent articular surface. This still
requires the surgeon to avoid step-like
discontinuities of the surface at the distal end
of the prosthetic articulation, which can cause
patellar catching and clunking. The anterior
flange of onlay designs extends far more
proximally and covers up to and beyond the
articular portion of the anterior femoral cortex,
allowing the patellar component to remain in
contact with the trochlear component in full
knee extension.