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Introduction
Results of patellofemoral replacement (PFR)
are poor and compare badly with TKR and
UKR. The inferior results do not relate to
prosthesis failure or loosening and only
partially to progressive degeneration in the
tibio femoral articulation. The poor results
relate to the inability to place a PFR in the exact
position to restore constitutional or natural
tracking in a specific patellofemoral joint.
The patellofemoral joint differs from the rest of
the knee in that it is individualized to each
person, similar to a fingerprint. The reason for
this is that the morphology of both the patella
and its underlying trochlea is the result of the
effect of “form follows function” [1]. This is
the expression of the subtle individual
differences in sagittal, coronal and rotational
alignment of the lower limb as well as the effect
of the angle and forces of the extensor muscles
over the knee joint. As a result of these
individual differences, it is almost impossible
to design instruments and prostheses where
one design fits all.
There should not only be a smooth transition
between the trochlear prosthesis and the
surrounding cartilage, but also the restored
trochlear groove should have proper axial and
rotational alignment. In order to achieve this,
there should be an exact fit distally, medially
and laterally. The fit on the surrounding articular
cartilage will have an effect on the trochlear
groove angle and the rotational alignment in
both the axial and sagittal planes. In practice,
there is often a conflict between good prosthesis
articular transition and the correct trochlear
groove and rotational alignment. In these
situations correct axial and rotational alignment
can often only be achieved at the expense of a
bad prosthesis articular cartilage junction or
vice versa. It is in this respect that:
1)
3D based
preoperative planning and predicted patella
tracking,
2)
patient-specific instrumentation,
3)
surgical robots and
4)
possibly patient-specific
prostheses can be warranted. Each of these
topics will be highlighted and briefly discussed
in the following sections of the report.
3D preoperative
planning
By obtaining full lower limb CT scans,
preferably with 1 mm slices and an MRI of the
knee, it is possible to create a virtual image of
the bony and articular cartilage of the lower
extremity of the knee.
Onthe3Dreconstructionsthroughsegmentation
techniques, it is possible to obtain an exact
measurement of prosthesis-articular cartilage
Robotic surgery and
intelligent intruments
Patellofemoral Arthroplasties
P. Erasmus, K.J. Cho, J.H. Müller