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J. Robin, T. Zakaria, P. Neyret
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PSI is the most recent advance in TKA
technology. It utilizes various imaging
modalities to produce cutting blocks matched
specifically to the patient’s knee anatomy. In
theory, this is a very accurate technique that
should be reliable and reproducible. The
advantages of PSI include the production of an
available plan for the surgeon to check prior to
surgery, the avoidance of instrumenting the
intramedullary canal which can be particularly
useful with large extra-articular deformities
and reducing the necessary operating inventory
and
theoretically
reducing
instrument
processing costs.
Many different designs of PSI are currently
available for use, and each have their unique
method of producing their blocks. Despite the
theoretical accuracy of these blocks, the
majority of these companies rely on a measured
resection technique with three independent cuts
followed by ligament balancing. PSI therefore
theoretically produces accurate
orientation
of
bony cuts, however does not determine the
level of
the cuts needed for gap balancing.
Few companies, however, have been able to
combine the accuracy of PSI with a gap
balancing technique. This may ultimately
provide the surgeon with the greatest accuracy
and control over TKA balancing.
In one such technique, the PSI tibial cutting
block is used to cut the tibia first. Following
this, a patient-specific distal femoral cutting
block is used. This block is unique as it is
created with an in-built spacer. This block and
spacer device occupies the predetermined
depth of tibial bone cut. This technique allows
the surgeon to verify ligament balancing in
extension prior to making any femoral bone
cuts. Ligament balancing can be checked
directly by coronal plane movement of the knee
in extension with the cutting block in situ. This
also provides the surgeon with an on-table,
real-time assessment of coronal and sagittal
plane alignment in extension.
If there is unexpected extension gap imbalance,
the surgeon can deal with this prior to making
any further bone cuts. There may be need for
limited ligament release if the gap is
asymmetricallytight.Ifthekneeissymmetrically
loose, spacers can be used in order to distalize
the femoral cut. If the knee is symmetrically
tight, the distal femur may need to be re-cut
later. The distal femoral cut created by the block
is parallel to the distal femur and the tibial cut,
providing a rectangular extension gap.
The flexion gap is then balanced by using the
classical instrumentation for cutting the
posterior femur. This allows for a reliable,
balanced flexion gap to be created whilst
simultaneously setting the correct amount of
rotation.
In summary, there are three ways of obtaining
equal gap balancing during TKA. A technique
using three independent bone cuts may give the
surgeon least control over balancing a TKA.
The technique of having two bone cuts and
soft-tissue balancing would appear to give
greater control to the surgeon. The greatest
control over gap balancing, however, may be
from a third method where the tibial bone cut is
made first and gap balancing is simulated with
navigation. The use of PSI in conjunction with
ligament balancing may provide great control,
accuracy and reliability in balancing a TKA.