Validity of flounce sign to rule out medial meniscus tear in knee arthroscopy

The meniscal flounce is a fold in the free, non-anchored inner edge of the medial
meniscus 2]–4]. It is like few small ripples in the free edge that disappear at one end. The appearance
of the flounce has been reported at arthroscopy 2]–4], arthrography 5] and MRI scanning 6]–8].

The meniscal flounce is produced when stress maneuvers are applied to the tibiofemoral
joint, resulting in distraction and some rotation of the compartment being assessed.
During these stress maneuvers, meniscal motion and configuration are influenced by
the capsular and ligamentous attachments, relative tibiofemoral movement and loading
of the knee joint. When the meniscus is stressed, the peripheral attachments cause
differential stresses within the body of the intact meniscus. These stresses manifest
as buckling of the free inner margin of the meniscus known as the meniscal flounce.
Pathology affecting the integrity of the meniscus or its attachments can alter the
appearance of this flounce 2].

The meniscal flounce in knee arthroscopy was first described in an illustration in
the Atlas of Arthroscopy by Watanabe et al. 4], the first such collection on the subject of arthroscopic anatomy. Though described
very early in the literature, its significance was not studied till 2006, when Williams
AM et al. 2] and in 2007, Wright RW et al. 3] showed that the presence of this very simple manifestation of stress maneuver and
axial loading can be of great help owing to its high sensitivity and specificity in
diagnosing normal meniscus. Williams AM et al. 2] not only established high diagnostic value but also tried to characterize the size
location and characteristics of normal flouncing on medial meniscus and lateral meniscus.

Similar results as ours was seen in study by Williams AM et al. 2] in which they showed: for an intact medial meniscus the sensitivity, specificity,
and positive predictive value (PPV) were 68.50 %, 92.9 %, and 92.10 % respectively.
Conversely the presence of meniscal pathology correlated closely with either an absent
or abnormal flounce (P 0.0001). In study by Wright RW et al. 3], the presence of a meniscal flounce sign had a positive predictive value of a normal
medial meniscus of 0.97 (63/65 knees). Absence of the meniscal flounce sign had a
positive predictive value of an abnormal meniscus of 0.98 (101/103 knees). Specificity
was 98 % (101/103 knees), and sensitivity was 97 % (63/65 knees).

Our study had large number of false negative cases (17.72 %), meaning neither there
were flounce on first view nor was tear on probing, giving low sensitivity and NPV.
But our study had high specificity and PPV due to very low false positive cases. Since
sensitivity denotes an ability of the test to rule out negative result, number of
false negative cases decreases the diagnostic values. The results are comparable to
the study by Williams AM et al. 2].

For the sign to have high sensitivity and NPV, false negative cases must be less.
Though the surgeons doing arthroscopy in this study are trained Sports Fellow and
experienced, interobserver fallacies can’t be ruled out. Difficulty in visualization
especially of posterior third of medial meniscus is well established 9]. At the same time large number of tears are also found in the same zone. So it is
very likely for tear in this zone to be missed despite thorough visualization. One
way to minimize this inherent difficulty is to use accessory portals in cases of high
suspicion (either clinically or if other diagnostic images like MRI suggests). We
didn’t use any accessory portal for exploring any possible subtle medial meniscus
tear when there was seemingly normal meniscus in absence of flouncing. Also in this
study valgus and external rotation was given manually by an assistant so, there can
be slight variation in the pattern of differential stress created in medial meniscus
and so flouncing was not appreciated.

The influence of other associated intra-articular derangement like ACL insufficiency
or associated lateral meniscus tear were tested by applying logistic regression analysis
and was found to be insignificant (p value =0.278 for ACL and p value?=?0.590 for Lateral meniscus status). Wright RW et al. 3] have also stated that there was no association between chondral injuries or other
ligament injuries, while Williams AM et al. 2] have shown significant association between medial collateral ligament insufficiency
and medial flouncing.

Low false positive cases (3.2 %) in our study increased the specificity and PPV, giving
high diagnostic value to the test in diagnosing normal meniscus. One false positive
case actually had a large wavy pattern without normally decreasing height of wave
(Fig. 7). Probing of that case revealed tear (Fig. 8). Since we have not characterized the appearance or size in our study we included
this case as false positive.

Fig. 7. Medial meniscus free edge as seen in false positive case

Fig. 8. Tear seen on probing of false positive case

Since this is only one of the few studies of its kind, to best of our knowledge, usefulness
of its high specificity and PPV can’t be ignored. Also, despite few false negative
cases, accuracy of the sign is nearly 80 % which is fair enough for clinical purpose
but needs validation in multiple studies, in multiple settings and with larger number
of cases.

Potential observer bias regarding detecting meniscal injury in the medial compartment
based on preoperative clinical and MRI findings, small sample size, study duration
and variation in the ligament laxity in population are some of the limiting factors.

This study has established high specificity and PPV of the flounce sign with fair
sensitivity, NPV and accuracy of the sign. This can be especially useful in two clinical
settings. First, in cases which are clinically diagnosed with medial meniscus tear
and are going for arthroscopic management, non visualization of tear with apparently
normal looking medial meniscus and in the absence of other intra-articular derangement
and absence of flouncing on inner edge, the surgeon can become more vigilant in searching
for tear especially in areas like posterior third. If highly suspected clinically
and diagnostic images like MRI support the diagnosis, the surgeon may use accessory
portal to look for the tears. Secondly, this sign can be used to rule out medial meniscal
tear while doing other knee procedures like ACL and PCL reconstruction and during
surgery for patellofemoral instability. During such procedures, the presence of flouncing
on inner edge of medial meniscus can give idea that the meniscus is normal and there
by aggressive instrumentation can be avoided; limiting iatrogenic chondral injuries
and reducing the surgical time significantly. At no time the sign can replace the
basic recommended methodological procedure.