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The reasons for a customized knee prosthesis Stepping outside the Square
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with better stability, keeping the ligament
insertion intact and getting closer to the natural
tension may reduce a significant source of
potential residual pain.
What to customize?
Bone coverage
Aside from the customization of the femoral
condyle contour to restore the biomechanics,
the restoration of bony coverage is aimed to
maintain the natural smooth transitions at the
new articular surface – bone interface, including
the bone cuts created to accommodate the
prosthetic box. The miss-match resulting from
the cuts and the implant generates either
overhang of the implant in some areas or
exposed sharp bony cuts in other places. These
miss-matches are responsible for soft tissue
impingement or overstuffing and may generate
stiffness, irritation, or pain and discomfort that
affect the clinical result. At the trochlea area
the miss match is often large, due to proximal
propagation of the cut, and the possible rotation
or flexion of the cutting guide. Customization
of the femoral contour, along with bony
coverage, eliminates the AP/ML dissociation
issue and many sources of impingement. The
femoral box can also be designed in a more
proportional and bone sparing way, especially
in smaller sizes, where the miss match is
increased when using a fixed, standard amount
of resection.
On the tibial side, the accurate coverage of the
bone surface not only protects against possible
ligament impingement but also enhances
implant fixation. The amount of resection,
slope and the frontal obliquity of the cut do
affect this surface, making the planning
essential to approximate the ideal contour.
Kinematics
The kinematics of the knee is essentially guided
by the articular surface contours but orchestrated
by the ligaments. In the normal knee the
femoro-tibial junction is subtly composed of
cartilage and menisci that creates a complex
and harmonious transfer across the range of
motion. But in knee prostheses, the current
necessity to use a stiff material that has a wear
rate, namely polyethylene, prevents exact
restoration of the native surface contour,
whether it is standard or customized, even in
presence of both cruciate ligaments. Thus one
of the main design challenges in customized
implants is to match the prosthetic femoral
anatomical contour to the polyethylene in a
form that can be reproducible and compatible
with material resistance. In other words, there
is still a need to maintain a given degree of
conformity and to use a mechanism (such as a
cam, post, third condyle, etc.) to provide a
sufficient degree of congruency. It is possible
to achieve this challenge through an algorithm
that will match a particular type of stabilization
mechanism with a given medial/lateral femoral
contour from a number of knee sub-groups,
based on a family of similar anatomical
features. Thus the kinematics cannot be totally
customized, but adopted and adapted, from a
proven reliable solution.
Apart from femoro-tibial kinematics, patella
tracking is probably the most important area
which offers a large amount of room for
improvement. The reproduction of the native
patellar-trochlear anatomy is undoutably one
of the more promising areas of progress. In a
Fig. 5: Aspect of customized knee prosthesis with a
residual 3° of native deformity and oblique joint line
restitution through asymmetrical femoral and tibial
design.