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INTRODUCTION
The anterior cruciate ligament (ACL) has been
found to be the primary restraint to tibia antero-
posterior displacement, allowing knee stability
and control of joint kinematics. Because of that
diagnosis of tibio-femoral joint laxity was
historically based on Lachman and anterior
drawer tests, measuring static laxity, involving
only one degree of freedom.
Contrasting to unidirectional tests, pivot shift
test evaluates dynamic laxity (defined when
more than two degrees of freedom are
involved), by applying multi-directional loads
through a range of movement of the knee. A
positive pivot shift is described as the anterior
subluxation of the lateral tibial plateau and its
reduction during flexion associated with
internal rotation and valgus stress.
This sign is closely related to a functional ACL
insufficiency and its grade is proportional to
clinical symptoms [1], reduced sport activity
[2], articular cartilage [3] and meniscal
damages [4, 5]. The limit of pivot shift is its
great variability in the performance and
interpretation, making it a highly surgeon-
subjective clinical examination.
In the last years, to solve this limit, various
devices have been developed to measure the
pivot shift; many of them, like open MRI,
complex software, special markers, robot and
electromagnetic software, are too complex and
expensive to be used in daily clinical practice.
Instead, inertial sensors have been spread in the
clinical practice thanks to its low cost,
simplicity and reliability [6].
DISCUSSION
Inertial sensors are specialized non-invasive
devices constituted of an accelerometer to
evaluate linear acceleration and a gyroscope to
quantify angular velocity; they contain an
internal mass attached to a spring. When the
sensor is accelerated by the forces acting on the
knee during a pivot shift test, it begins to move
in the same direction of the force while the
internal mass do not move because of inertia.
This relative movement will produce a
lengthening of the spring which is directly
proportional to the acceleration. Calculating
the integral of acceleration allows to obtain
velocity.
In 2012, Lopomo
et al.
[7] has reported the
results using a specific type of inertial wireless
sensors linked to a tablet PC equipped with
dedicated software (KiRA, Orthokey LLC,
Lewes, DE, USA) while pivot shift maneuver
was performed on 66 consecutive ACL-injured
ACCELEROMETER FOR PIVOT
SHIFT ASSESSMENT
S. ZAFFAGNINI, G. CARBONE, A. GRASSI, F. RAGGI,
T. ROBERTI DI SARSINA, C. SIGNORELLI