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.