18 / 242
J. Robin, T. Zakaria, P. Neyret
18
equal gaps. All techniques also employ either
dependent or independent bony resections.
More recent technology has aimed to improve
accuracy of TKAby using computer navigation
and patient specific instrumentation (PSI).
With a classical technique of dependent bony
cuts, the tibia is cut first with the femoral cuts
being linked to the tibial cut. This allows for a
balanced bony resection of the posterior femur
to obtain the flexion gap (and rotation) and the
distal femur to obtain the extension gap. The
extension gap can be adjusted to the flexion
gap or vice versa. With independent cuts, the
distal femur, the posterior femur and the tibia
are all cut independently and the soft-tissues
are balanced to further equalize the flexion and
extension gap.
There are benefits and down-sides to all of
these techniques, however, a poignant fact is
that if the majority of TKA balancing is done
early in the procedure, this leaves less to
chance and reduces difficulties in balancing
the soft-tissues after all bony cuts have been
performed. This gives greater control to the
surgeon and reduces the possibility of having
to perform large releases late in the procedure
or to have to use a larger polyethylene spacer
than initially anticipated. Minimalizing the
uncertainty of TKA balancing during the
procedure is the key.
This concept can be explained utilizing the
different balancing techniques of measured
resection compared to the gap balancing
technique.
When the measured resection technique is
performed either using a classical instrumented
technique, via computer navigation, or PSI,
three independent bone cuts
are made based
on a measured resection amount afforded by
the jigs or computer simulated plan. Soft-tissue
balancing is then performed to allow
equalization of the flexion and extension gaps
prior to trialing and implantation of the
components. The flexion and extension gaps
can be checked at the appropriate stages during
the procedure, however, there is no other way
of controlling the balance of the gaps prior to
all bony resections being made. Minor gap
balancing alterations may be achieved with
soft-tissue releases, however, this method
leaves very limited opportunity to correct for
any major balancing issues should they occur
prior to prosthesis implantation.
Alternatively, when the gap balancing method
is used, irrespective of whether the flexion or
extension gap is produced first,
two bone cuts
are made followed by ligament balancing and a
linked or
dependan
t third bone cut to match
the first gap. This allows for ligament balancing
earlier in the procedure, prior to establishing
both the flexion and extension spaces. Linking
the femoral bone cuts after the first gap is
created allows earlier appreciation of the soft-
tissue restraints and reduces the educated
estimation that would otherwise be required to
balance the gaps afterwards. This gap balancing
technique can be performed using classical
instrumented method or with a computer-
navigated version, which can simulate the
balanced gaps.
Computer navigation may be utilized to
perform gap balancing in a third way. This third
technique uses only
one bone cut
prior to
simulated gap balancing. After standard tibial
bone resection, navigation is used to simulate
the flexion and extension gaps prior to making
any femoral bone cuts. The soft-tissues can be
balanced in flexion to obtain the simulated
flexion gap, and then in extension for the
extension gap. Once the soft-tissues have been
balanced and the flexion and extension gaps
equalized on the computer navigation, the
definitive posterior femoral and distal femoral
bone cuts can be executed according to the
planned simulation. This technique gives the
surgeon greater control on balancing the flexion
and extension gaps. After the simulated gaps
have been planned and carried out with the
prior ligament balancing as necessary there
should be little need to perform further
balancing later in the procedure. This theory is
summarized in Figures 1 and 2.