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R. SEIL, C. MOUTON, D. THEISEN
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lower values for rotational laxity than the first
prototype of the device due to improvements in
the standardization of the patient installation
and joint fixation. The Minimum Detectable
Change (MDC) has been determined to reach
4.2° for internal rotation and 5.9° for external
rotation (Mouton). Individualized normative
references have been established taking into
account gender and body mass.
PHYSIOLOGICAL LAXITY
Systematic evaluations of patient and control
cohorts have shown that contralateral knees of
ACL-injured patients display greater anterior
and rotational knee laxity than knees of healthy
individuals (Branch, Mouton). As such,
increased physiological laxity has been
determined as a potential risk factor for ACL
injuries. In addition, it has been shown that
exercise and fatigue increases anterior and
rotational knee laxity in such patients. Gender
has a big influence on rotational knee laxity,
with women having up to 40% higher knee
rotation in comparison to men. It may represent
one of the factors explaining the higher risk for
ACL injuries in females. Body mass also
influences rotational laxity with increased body
mass being related to lower knee rotation.
Neither age nor the menstrual cycle seem to
influence rotational knee laxity measurements
in adults.
Although laxity measurements overestimate
knee laxity, normative references must be
established to define normal laxity for each
device. Mouton
& al.
proposed a methodo
logical approach to calculate standardized
laxity scores for anterior and rotational knee
laxity taking into account influencing
individual characteristics. Sex and body mass
were found to significantly influence rotational
laxity and to explain a non-negligible amount
of the variability in internal and external
rotation (46 to 60%). As a consequence, the
latter parameters were taken into account to
calculate an individualized score which has the
advantage to allow for the direct comparison of
individuals, regardless of differences in sex or
body mass.
KNEE LAXITY IN THE
INJURED KNEE
Diagnosis of ACL injuries
The diagnosis ofACLinjuries with arthrometers
is based on the side-to-side difference (SSD)
observed in anterior laxity measurements
between the injured and the healthy knee.
According to the IKDC objective score, a SSD
greater than 3 mm relates to an ACL injury
regardless of the device used to measure
anterior knee laxity. At this threshold, the KT-
1000
®
performed at a maximal manual force
seem to display the highest sensitivity and
specificity for the diagnosis of complete ACL
injuries compared to other devices. It is
important to highlight that most studies
reported the sensitivity and the specificity of
arthrometers to diagnose ACL injuries by only
considering complete ACL tears, which are the
easiest to detect. With newer devices like the
GNRB
®
, for all types of ACL tears (including
total tears, partial tears, and ligament remnants)
and regardless of associated meniscocapsular
injuries, the sensitivity and specificity of the
GNRB
®
reached, respectively, 75 and 95% for
the ATD at 200 N and an optimal threshold of
1.2 mm (Mouton).
To improve the diagnosis of ACL injuries,
additional analysis of rotational knee laxity has
been proposed. Cadaveric studies revealed that
the section of the ACL led to an increase of 2.4
to 4° in internal rotation in knee flexion angles
below 30° (Lane, Nielsen). This accounts for
approximately 10-15° of the internal rotational
range. Subtypes ofACL tears like posterolateral
bundle injuries induced an increase of 3° at
5 Nm in internal rotation (Lorbach). Similarly,
recent findings showed an increase in internal
rotation after sectioning of the ACL + antero
lateral ligament of 3° at 20° of knee flexion
(Sonnery-Cottet).
Although they may be clinically relevant, these
differences induced by sequential sectioning of
different intra- and extraarticular structures are
relatively minor. This is getting problematic
when the measurements need to be performed
in vivo
. So far, the only device measuring static