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The reasons for a customized knee prosthesis Stepping outside the Square
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tibial trays have also been emphasized, along
with gentle 1mm liner thickness increment.
The tibiofemoral joint is asymmetric in shape
and dimension, and correct positioning of the
tibial component must accommodate both
optimal bone coverage and satisfactory
patellofemoral tracking. As such a compromise
must be found during the operation to meet
these two requirements, as best bone coverage
often internally rotates the tibial tray [59].
Asymmetrical trays reflect the tibial torsion
more accurately, and may offer the best
compromise for optimal bone coverage and
patellofemoral tracking [58]. However symme
trical trays have also been reported to provide
the best compromise for coverage and more
kinematic rotation and tracking [60, 61].
Bony resection
Irrespective of individual patient’s bone size
and characteristics, bone resection requires a
minimum bony cut that is not proportional to
the patient’s anatomy, in order to accommodate
the prosthesis and bearing. This has greater
consequence in smaller bones as the resection
is at a level of poorer bone quality and in closer
proximity to the level of the collateral
ligaments.
On the femoral side a fixed resection level can
encroach upon the collateral ligament insertions
in small femurs, with the potential risk of
prosthetic impingement upon the soft tissue
envelope. On the tibial side a relatively large
distal resection level results in a smaller
component size for that knee, and overall a
relatively posterior and peripheral displacement
of the implant, and strain increases significantly
in the proximal tibia during loading [62]. The
deep MCL is a distinct medial stabilizer and
plays an important role in rotational stability.
With a standard 9-mm tibial resection up to
54% of the deep MCL insertion area may be
resected, and it is resected in at least 1/3 of
cases of conventional TKA. However it may
have implications in future designs of both
unicondylar and total knee arthroplasty [63].
Patello-femoral joint
Patello-femoral function and stair climbing has
been shown to improve with more anatomic
trochlea designs of the femoral component
[64]. Trochlear designs have also been gradually
modified to better accommodate the patellar
articular facets, with broad extended
asymmetrical trochlear grooves. The literature
has conflicting evidence how effective this is in
improving patella tracking [65, 66].
Patella resurfacing
Anterior knee pain remains a factor in patient
dissatisfaction, and furthermore the role of
patella resurfacing during primary total knee
arthroplasty remains controversial. Whilst
resurfacing may reduce actual revision rates
[67]. The literature has shown no benefit from
resurfacing of the patella in terms of outcomes
[67, 68, 69]. This may be by differences in
design between TKAbrands. However a review
of five popular primary knee designs
demonstrated that patella resurfacing has no
improvement in overall knee function or
anterior knee-specific function irrespective of
TKA brand or for cruciate retaining versus
sacrificing designs [70].
Kinematics
Aside from sizing variations there are many
variations available to try and improve the
kinematics and function of the knee replace
ment by different means. Orthopaedic device
companieshavedevelopedtechnicaldifferences
including; single radius of curvature femoral
components, graduated radius of curvature
components, medial pivot designs, third
condyle and high flexion femoral component
designs to attempt to achieve kinematics
matching the native knee. Third condyle
designs
have
demonstrated
similar
anteroposterior and medial-lateral ligamentous
stability compared to the native knee [71].
There is some evidence single radius designs