Discussion
This study found that anthropometric features were able to predict
rhomboid and pleura depth in asymptomatic subjects. These findings could
help clinicians in the use of dry needling or acupuncture by determining
the most appropriate needle length to avoid an accidental pleura
puncture and a potential pneumothorax.
Although this is not the first study assessing the optimal needle
insertion in this location, it is the first calculating the accuracy of
a prediction model including multiple anthropometric features. Seol et
al. 13 provided needle length recommendations based on
the BMI and providing safety margins (which consist in a range between
the mean + 1 SD of rhomboid muscle depth and the mean - 1 SD of rib
depth). This recommendation could be acceptable if clinicians cannot use
ultrasound imaging, but several features with moderate correlation with
rhomboid-pleura depth (e.g., perimeter, and gender) identified in our
study were not considered. Current results would support that the first
parameter to be considered for selecting the needle length is the BMI,
explaining a great percentage of variance in both rhomboid and pleura
depth. This is an expected finding since higher weight is associated
with higher proportion of fat and, therefore, more skin-to- underlying
tissues distance. Interestingly, in the descriptive analysis, we
observed gender differences for all the anthropometric measures,
supporting that men show higher anthropometric scores (i.e., height,
age, weight, and BMI) than women. These gender differences would explain
the relevance of gender in the prediction model.
The identified variables determining rhomboid and pleura depth were
consistent, where BMI, gender or thorax circumference were the most
relevant predictors in both prediction models. Although respiratory
moment and age were associated with pleura depth, they were not included
in the regression model due to their poor significance. Thus,
respiratory moment was not associated with rhomboid lower limit. Since
the rhomboid major is not a ventilatory muscle, its depth did not show
differences between maximum inspiration and expiration; explaining the
lack of association of respiratory moment. Nevertheless, although the
role of serratus posterior superior in the breathing is
controversial19, the safety margin is greater during
maximum expiration due to the intercostal and serratus posterior
superior activation, which should be considered during application of
invasive procedures.
Our data revealed a more explained model for pleura depth than for
rhomboid muscle depth (explaining 69.7% and 52.7% of variance,
respectively). Even if rhomboid deep limit cannot be accurately
predicted, the combination of both models could be potentially useful to
decrease the risk of pneumothorax during dry needling.
Perhaps the most relevant findings from the current study are the
results from the hierarchical regression analysis used to determine
pleura depth, which explained almost the 70% of variance. Clinicians
can use, in absence of ultrasound equipment and inability to manual
identification of the ribs, the BMI and thorax circumference to decrease
the risk of an accidental puncture of the pleura. Based on the current
results, clinicians should use needles no longer than 25mm to avoid an
accidental pleura puncture since the lower limit of rhomboid major lower
limit ranges from 11.6 mm to 16.2 mm and the pleura could be needled at
19.6-27.7 mm of depth.
Although this study has shown promising results, potential limitations
should be recognized. First, this prediction model was based on a sample
of healthy subjects, with several demographic differences. Larger sample
sizes are needed to determine normative values of rhomboid and pleura
depth. Second, we only assessed one measurement point, which could not
be the exact location of the MTrP; therefore, different locations could
have different depths. Further research assessing depth differences
between segments is needed. Finally, this model can be applied just in
healthy subjects with no neuromuscular conditions.