Comparison of skin-to-bone distance in the interscapular region when measured in the prone position and in the seated position in a therapy massage chair

Abstract

[Purpose] To investigate whether the skin-to-bone distance at specific points in the interscapular region differs when measured in the prone and seated positions. [Participants and Methods] Eighteen healthy males (age, 22–63 years; height, 161.5–181.5 cm) were recruited. The skin-to-bone distance at Points A (fifth rib, near acupoint BL43) and B (eighth rib, near acupoint BL46) was measured using ultrasonography in the prone and seated positions. In the prone position, the participants laid on a bed with their shoulders abducted at 90°, and in the seated position, a commercial massage chair was used. [Results] The mean distance at Point A in the prone position was 2.7 ± 0.4 cm (range, 1.9–3.4 cm). The mean distance at Point B was lower than that at Point A in the prone and seated positions, with no significant difference in the distance between the two positions. [Conclusions] The skin-to-bone distance in the interscapular region did not differ significantly when measured in the seated and prone positions. Tissue components in some anatomical areas change with posture, whereas others do not. Further studies on posture-induced changes to tissue components are necessary to improve understanding of these variations.

INTRODUCTION

The interscapular region contains the trapezius and rhomboid muscles, which are crucial for scapular movement and postural maintenance. These muscles are often the sources of shoulder and back pain. Consequently, the interscapular area is frequently targeted for acupuncture and physical therapy. For example, several acupuncture points along the medial border of the scapula are used to relieve shoulder stiffness and upper back discomfort.

In acupuncture, precise depth control is crucial for accurate needle insertion into muscles and soft tissues. Particularly in the interscapular region, overly deep needling can reach the pleura, and although rare, poses the risk of pneumothorax¹⁾. Therefore, to avoid the risk of pneumothorax, a method is sometimes employed in the interscapular region, where the ribs are palpated and used as a backstop for needle insertion. However, this technique requires a thorough anatomical knowledge and precise needling skills. In Chapter 52, “Forbidden Points for Needling”, of the Huangdi Neijing: Suwen, a foundational text of ancient Chinese medicine compiled around the 2nd century BCE, it is written that “if the lung is pierced, death will occur within three days, accompanied by symptoms such as coughing”. The danger of such medical errors has been recognized since ancient times, and this remains a critical issue that requires ongoing vigilance in modern practice.

With advances in imaging technologies such as computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound, information on the distance from the body surface to the bones and organs has been elucidated, gradually revealing anatomical insights related to acupuncture needle depth^{2,3,4,5,6,7,8,9,10,11)}. Individual differences in age and body type have also been reported. However, these measurements are typically conducted in the prone or supine position, whereas in actual clinical practice, treatments are performed not only in recumbent positions but also in various postures. Acupuncture treatments of the back are typically performed in the prone position; however, seated or lateral positions may also be used, depending on the patient’s symptoms and condition^{12, 13)}. Seated treatments are particularly beneficial for pregnant women¹⁴⁾ and patients who have difficulty lying down.

In studies of the abdomen¹⁵⁾, MRI in the supine, prone, and lateral positions revealed changes in the contour of the body’s cross-sectional shape. In the prone position, the abdominal cross-section resembled a flattened bun, whereas in the lateral position, it approximated a sagging triangular rice ball. The shape of the right kidney changed from elliptical in the supine position to edamame-like in the prone position and bun-shaped in the lateral position. The distance from the body surface to the right kidney was the shortest in the prone position, intermediate in the supine position, and greatest in the lateral position.

In situations that more closely resemble actual clinical practice, anatomical measurements have shown variations compared to those obtained in previous studies conducted in the supine position^{4, 8, 16)}. Regarding the interscapular region, there are no reported data on how the distance from the skin surface to the ribs changes between the prone and seated positions. In the prone position, the entire abdomen and chest is in contact with the supporting surface and the back muscles are likely to relax. In contrast, in the seated position, gravity acts vertically, and the patient maintains their own posture, which may result in the displacement of soft tissues owing to gravitational forces.

In acupuncture, it is crucial to clarify whether the information on needle insertion depth obtained in the recumbent position can be directly applied to the seated position or if there are position-specific changes that may introduce risks in needling.

In this study, we aimed to examine differences in the distance from the skin surface to the ribs in the interscapular region between the recumbent and seated positions. By elucidating the anatomical changes associated with different postures, these findings can serve as guidelines for improving the safety of acupuncture treatment in a seated position.

PARTICIPANTS AND METHODS

The participants were 18 healthy males with a mean age of 39.2 ± 12.8 years (range: 22–63), height of 171.6 ± 6.1 cm (range: 161.5–181.5 cm), weight of 68.8 ± 11.4 kg (range: 46.0–91.8 kg), and body mass index (BMI) of 23.3 ± 3.7 kg/m² (range: 15.9–30.8 kg/m²). The thickness from the skin surface to the ribs in the interscapular region was measured in both prone and seated positions using ultrasound imaging.

The measurement points were selected based on previous reports^{3, 17, 18)} in the recumbent position, indicating differences in thickness. Two points were chosen: Point A, located on the 5th rib, and Point B, located on the 8th rib, both at 3 cm lateral to the posterior midline, assuming that the distance from the posterior midline to the posterior superior iliac spine was 1.5 cun. Point A was near the acupuncture point BL43 (Gaohuang), which is commonly targeted in treatments for shoulder stiffness. Point B corresponds to acupuncture point BL46 (Geguan), which is the thinnest region of the acupuncture points on the back.

The participants were in the prone and seated postures. In the prone position, the participants laid on a bed with a face hole, their shoulders abducted at 90°, and their upper limbs hanging naturally (Fig. 1a). In the seated position, participants reclined on a treatment chair (JUST Super Lightweight Quick Massage Chair, World JB Corporation, Tokyo, Japan), with the armrests adjusted to 0°, the chest pad tilted backward at approximately 55°, the seat tilted backward at approximately 25°, and the knee pad tilted backward at approximately 55° (Fig. 1b). Because there is a rare risk of orthostatic hypotension during acupuncture treatment in a seated position, a massage chair was used in this study. Sitting in this chair ensured a stable posture, allowing participants to fully relax.

Fig. 1.

Prone and seated positions in a therapy massage chair.

The thickness from the skin to the ribs was measured using an ultrasound device (Hitachi Medical Corporation, Tokyo, Japan). The probe was positioned perpendicularly to the skin and parallel to the sagittal plane. The distance was measured using a scale attached to the ultrasound device. All measurements were performed by an acupuncturist with 3 years of experience in operating ultrasound equipment.

Analyses were performed using the Wilcoxon rank-sum test. Comparisons were made based on the location differences between (1) POINT A (prone position) and (2) POINT B (prone position) and between (3) POINT A (seated position) and (4) POINT B (seated position). Additionally, differences in body position between (1) POINT A (prone) and (3) POINT A (seated) and (2) POINT B (prone) and (4) POINT B (seated) were compared.

As a subanalysis, Spearman’s correlation coefficients were calculated to examine the relationships between the thickness from the skin to the ribs and age, height, and BMI.

The analysis was performed using IBM SPSS Statistics for Windows, version 27.0 (IBM Corp., Armonk, NY, USA), with p-values <0.05 considered statistically significant.

The Tokyo Regional Committee of the Humanities and Social Sciences Research Ethics Committee of the University of Tsukuba approved this study (Approval No. 2021-128). We adhered to the guidelines of the Declaration of Helsinki, and both the authors and participants complied with scientific and ethical standards. Prior to the experiment, we explained the study to the participants and obtained written informed consent based on their voluntary agreement.

RESULTS

The thickness measurements for each of the 18 participants are shown in Table 1.

Table 1. Distance from the skin surface to ribs (n=18).

	Prone position	Seated position
Point A (cm)	2.7 ± 0.4* (1.9–3.4)	2.6 ± 0.6† (1.6–3.7)
Point B (cm)	2.0 ± 0.5* (0.9–3.0)	2.1 ± 0.6† (1.3–3.9)

^*p<0.05, ^†<0.05.

The comparison between POINT A and POINT B showed a statistically significant difference in both body positions. However, the comparison between the prone and seated positions at the same location did not show a statistically significant difference (p>0.05).

The correlation between the thickness from the skin to the ribs and individual factors showed a strong positive correlation with BMI (ρ=0.81), but no significant correlation with age (ρ=0.010; p=0.969) or height (ρ=0.073; p=0.772).

DISCUSSION

Previous studies have reported that the thickness from the skin surface to the bone at BL43 in the supine position generally ranges from 1.9 to 2.5 cm^{4, 19)}, and the distance from the skin surface to the pleura ranges from 2.8 to 3.0 cm^{5, 8)}. It has also been established that the distance from the skin surface to both the pleura and bone at BL46 is shorter than that at BL43^{5, 6, 8)}. The measurements obtained in this study are consistent with previous findings.

In recent years, advancements in noninvasive real-time measurements using ultrasound diagnostic devices have made it possible to conduct assessments under various conditions.

In the study by Chu et al.⁷⁾, the distance from the skin surface to the pleura at GB21 (an acupuncture point located on the shoulder) was 4.23 ± 0.57 cm in the seated position, 4.45 ± 0.61 cm in the supine position with both arms raised above the head and elbows bent at a right angle, and 4.59 ± 0.58 cm in the supine position with both arms positioned along the torso. These results confirmed that the thickness increased in the supine position compared to that in the seated position, with the deepest measurement observed in the supine position when the arms were aligned with the torso. Although a difference in depth was observed between maximum inhalation and exhalation, the difference was not clinically significant. Additionally, in the study by Mitchell et al.⁸⁾, placing a bolster under the shoulders in the prone position to retract the scapula resulted in a distance from the skin surface to the bone of 2.2 cm without a bolster and 2.9 cm with a bolster, while the distance from the skin surface to the pleura was 3.0 cm without a bolster and 3.7 cm with a bolster. The use of a bolster showed significant differences across all BMI groups, with greater variability in distance as the BMI increased. These findings suggest that certain distances change depending on the position and condition, whereas others remain consistent.

The results of this study showed that the thickness of the interscapular region did not differ significantly between the seated and prone positions. This suggests that the needle insertion depth data obtained in the prone position can be referenced for treatment of the interscapular region in a seated position. However, these results focus solely on the fact that the thickness remains unchanged. Given the differences in weight distribution and muscle tension between the prone and seated positions, the tactile sensation during palpation and needle insertion may vary. Therefore, careful application of this method is necessary. Furthermore, although these results provide a useful reference, they do not guarantee safety in all cases, and palpation remains a crucial tool. Imaging technologies can provide objective data that complement palpation; however, their use should always be combined with the practitioner’s skills and experience in palpation.

In this study, the seated position was achieved using a massage chair. While the direction of gravity in this seated position is similar to that in a self-supported seated posture, the effects may differ because of the lack of activation of the postural muscles. Additionally, it is necessary to investigate the type of seated posture that is the safest and most effective. Clarifying the optimal seated conditions based on the patient’s body composition, muscle strength, or specific symptoms could further enhance treatment safety.

In physical therapies such as massage, there is a risk of errors related to fractures; however, the relationship between bones and soft tissues, such as muscles, has not been fully elucidated. Gaining insights into whether anatomical distances and positional relationships change with variations in body posture is considered to contribute to the safe application of physical therapies in physiotherapy practice.

Limitation of this study was that the participants were healthy adult males. As this research represents a novel attempt, male participants were selected due to considerations of potential sex-related differences and the requirement for exposing the back. However, different results may be obtained with female participants or under other conditions. Different trends may be observed in older patients, those with reduced muscle strength, and those with specific conditions. The examination of postural changes is limited, and additional studies are needed to investigate other postures and patient populations.

This study demonstrated that body position had no significant effect on interscapular region tissue thickness between the prone and seated positions, supporting the use of seated treatments as a viable option in clinical practice. Future research should focus on validating the applicability of these findings to a more diverse range of patients and treatment environments.

Funding

This study was partially supported by JSPS KAKENHI (grant number JP19K02948).

Conflicts of interest

The authors declare no conflict of interest.