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M. Saffarini, P.G. Ntagiopoulos, G. Demey, B. Le Negaret, D. Dejour
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soft tissues that stabilize the patella are often
unbalanced, and the trochlea is usually
dysplastic [27], thus the PFA trochlear
component must provide a groove with normal
anatomic depth to guide the patella, and must
realign the extensor mechanism to ensure
normal patellar tracking.
Of the five implants measured, four had a
sulcus angle greater than 144° in the “Merchant
view” and all 5 specimens had a sulcus angle
greater than 143° in the “Brattström view”. In
two of the specimens the sulcus angle exceeded
these radiographic indicators of trochlear
dysplasia by more than 10º. Ahigh sulcus angle
indicates a shallow or dysplastic trochlea,
observed in the majority of patients suffering
from patellofemoral disorders [33]. Trochlear
components with high sulcus angles require a
specific and adapted surgical technique
including ligament balancing and extensor
mechanism realignment according to the TT-
TG value to prevent any further patellar
maltracking in early flexion.
Of the five implants measured, three specimens
had a facet less than 5mm high through the
entire range of early flexion (0° to 30°), and
two specimens had a facet less than 5mm high
beyond early flexion (30° to 45°). The lateral
facet is essential to align the patella within the
trochlea during knee flexion, and to prevent
lateral subluxation and tilt [34, 35]. In a
radiographic study of 200 normal knees,
Brattström reported the range of lateral facet
height to be 4.2 to 6.5mm (at 30º of flexion)
[20]. In a more recent cadaver study of 33
femora, Shih
et al
. reported the mean height of
the lateral facet to be 6.6 ± 1.8mm (at 0º of
flexion) [24]. A low facet would predispose to
lateral patellar dislocation, while an elevated
facet could exacerbate tension in the lateral
patellar retinaculum, and potentially lead to
excessive patellofemoral contact pressures and
impingement [34].
In all specimens, the trochlear groove was
oriented laterally (range 1.6º to 13.5º). There is
general consensus that the trochlear groove is
bilinear, with different orientation in its
proximal and distal portions [37-40], but there
are debates on whether its orientation is lateral
[41], parallel [42, 43], or medial [44, 45] to the
femoral anatomical axis. The position of the
trochlear groove is fundamental as it influences
the final alignment for correction of the TT-TG.
The closer the position of the prosthetic groove
to normal anatomy, the better the correction of
the alignment, and the less the surgeon needs to
deal with implant orientation.
Two of the specimens appeared relatively
narrow, while the other three specimens were
relatively wide.
With a narrow implant, some of the native
medial and lateral facets are preserved, which
is an advantage as there is less bone resection.
However, implant positioning is not adjustable
after cuts, and in case of high-grade trochlear
dysplasia, where the entire trochlea is abnormal,
the cut and implant would not fully correct the
abnormality [16, 15]. With wider implants, the
required bone resection is greater, but the
surgeon has more freedom to move the trochlear
groove medially or laterally as necessary to
correct alignment [7, 8, 10, 12]. Ideally, a
trochlear component should extend far enough
distally to allow proper coverage of the diseased
trochlea and facilitate proper implant
positioningandorientation,withoutencroaching
into the intercondylar notch [15], as this could
lead to impingement against the anterior
cruciate ligament and lead to ligament damage
and lack of extension.
The present study invokes a discussion on the
common classification of PFA implants.
Numerous authors distinguished implants as
first – or second – “generation” based on the
year they were released on the market [16, 46,
8, 12, 11]. First generation implants included
the Lubinus, Blazina, Richards Mod II and III
systems, while second generation implants
included the Avon, Autocentric, and LCS
systems. We find it more appropriate to classify