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patients. Actually the system, an evolution of
the originally developed, consists of a sensor
inlaid into a three - axial accelerometer and
three orthogonal gyroscope Bluetooth
connected to a tablet PC. The sensor is fixed,
completely non-invasive, by a strap on the
tibia, between the lateral aspect of the anterior
tuberosity and Gerdy’s tubercle. It’s the optimal
position to reach a good stability of the sensor
and to minimize skin artefacts during pivot
shift execution. Furthermore this position is
located in the lateral compartment of the knee,
which is the most influenced by the presence of
the pivot shift phenomenon. To validate this
system and evaluate its reliability, it was
compared to an invasive navigation system, by
measuring knee joint kinematics during pivot
shift concomitantly by an accelerometer fixed
to the skin and a navigation system [8]. The
authors found good intra-rater reliability in the
acceleration range and in the mean acceleration
waveform, justifying the use of inertial sensor
in the daily clinical practice. The limitations of
the methodology is the intrinsic variability of
pivot shift depending on variance among
examiners and muscular resistant offered by
the patient, as demonstrated by a recent
multicenter cohort study reporting significant
differences in the grading of the pivot shift in
awake and anesthetized patients [9].
A different inertial sensor, The MEMSense™
sensor, has been tested by Labbé
et al.
[10] to
quantify pivot shift in 13 ACL-injured patient.
This device uses an embedded micro
electromechanical system sensor integrating
a triaxial accelerometer, gyroscope and
magnetometer to evaluate acceleration and
velocity of the tibiofemoral joint during pivot
shift execution. Authors demonstrated that
both acceleration and velocity of femur and
tibia correlate well with the clinical grade of
the pivot shift.
Petrigliano
et al.
[11] validated the use of
another device, The ITG-3200 (ITG-3200,
Invensense, CA), to quantify the pivot shift
phenomenon in cadaveric specimens. It’s a
non-invasive microelectromechanical gyro
scope, but, unlike MEMSense™, it’s a single
axis device that can be site on the lower
extremity to measure tibial external rotation in
the transverse plane during pivot shift. Authors
found that the angle of rotation was higher in
the ACL deficient knees compared to the intact
specimens, but tibial rotation and rotational
velocity are not closely related to the clinical
grade of pivot shift. So these findings,
confirmed by Borgstrom
et al.
[12] who used
the same device to correlate analytic data with
clinical grade, suggest that tibial rotation and
rotational velocity alone can not define clinical
grade of pivot shift and gyroscope data could
be associated to acceloremeter data to give a
more definite assessment of pivot shift.
CONCLUSION
Among several tests proposed to evaluate
laxity of the knee joint, pivot shift test is the
most specific test for ACL-injury, being closely
correlates to clinical symptoms. Because its
complexity and its inter-individual variability,
its quantification represent a challenge among
orthopaedics involved in ACL surgery.
Development of several systems to assess pivot
shift could help surgeon to quantify pivot shift,
improving diagnostic capabilities. Although
navigation systems increased our understanding
about knee kinematics, they are invasive,
complex ad expensive; for these reasons their
use is proposed for intra-operative analysis.
Unlike navigation systems, inertial sensors are
non-invasive, intuitive, simple to use, in
expensive and reliable way to quantify pivot
shift phenomenon. Their use allows to compare
the injured knee to the healthy one, using it as
reference. Measuring acceleration, velocity
and rotation of the tibia relative to the femur
during pivot shift, it’s possible correlates
analysis data with clinical grade of symptoms.
Use of inertial sensors, thanks to its reliability,
could be encouraged in the daily clinical
practice both in diagnostic phase and in
postoperative evaluation; moreover it could be
use as a teaching tool in instructing young
surgeon to perform the pivot shift in a more
standardized way.