Objective Analysis of Age-Related Changes in the Superficial Musculoaponeurotic System in Japanese Females Using Computed Tomography

Abstract

Background: The superficial musculoaponeurotic system (SMAS) is an anatomical structure involved in facial aging.

Objectives: This study focused on SMAS thickness and aimed to establish age-related changes in SMAS thickness.

Methods: A total of 100 adult Japanese female participants (aged 20-79 years) were enrolled in the study. The participants were divided into 3 age groups: Y, M, and E, comprising 20 to 39, 40 to 59, and 60 to 79 years, respectively. Anatomical structures were used as landmarks to standardize the SMAS analysis sites. The SMAS in a fixed analysis area (FAA) was quantified using multi-detector computed tomography (MDCT), and the relationship between SMAS thickness and age, as well as BMI, was analyzed.

Results: In 96 participants (four were excluded due to imaging artifacts), a moderate yet significant negative correlation was found between the average (A)-SMAS thickness within the FAA and age. The A-SMAS thickness in groups M and E was significantly lower than that of group Y, and the mean value of group E was significantly lower than that of group M. SMAS thickness was greater in the young population. The SMAS gradually became thinner with aging. A statistically significant correlation was not found between SMAS thickness and BMI.

Conclusions: Using MDCT technology, age-related changes in SMAS were successfully analyzed. This highly objective analysis method corroborated the aesthetic surgical knowledge of the SMAS features related to facial aging. In clinical applications, our findings may help elucidate the mechanisms involved in facial aging.

Figure 1.

Three-dimensional computed tomography (3D CT) image. 3D CT was used to display the anatomical landmarks of the face, as well as the anatomical relationship of the subcutaneous structures. Created to target muscle density, it is superimposed with a transparent 3D CT image of the facial skin. Structures on the surface of the face and subcutaneous structures are simultaneously visible.

Figure 2.

Creation of axial and coronal computed tomography (CT) images. (A) A leading three-dimensional CT(3D CT) image for accurately generating axial CT images. An infraorbitomeatal (IOM) line drawn between the infraorbital margin and external acoustic meatus is a global standard reference for CT. Axial CT images on lines parallel to the IOM line can be accurately created while simultaneously checking the existing anatomical structure. Coronal CT images were created in a plane orthogonal to axial CT images. (B-G) A guideline for creating accurate axial CT images. Axial CT images were cut from the whole-facial 3D CT image and a combination of several axial images of the left halves of the face. The accuracy of the axial CT images for each participant was confirmed using this reference image. The positional relationship between the structures of the facial surface and structures under the skin can be easily comprehended, and the superficial musculoaponeurotic system (SMAS) is visible as a linear structure in the subcutaneous adipose tissue. Its thickness varies depending on the facial region. (B) At the level of the central part of the eyeball: ethmoidal sinus is depicted. In the subcutaneous adipose tissue of the temporal region, membranous structures are visible. In order from the surface, the SMAS, superficial temporal fascia, and deep temporal fascia covering temporal muscle are distributed in layers. (C) At the level of the lower part of the eyeball: the maxillary and sphenoid sinuses are visible. In the subcutaneous adipose tissue of the temporal region, a membranous structure, the SMAS is recognized. (D) An image on IOM: the lower edge of the orbit, auricle and external acoustic meatus are visible. In the subcutaneous adipose tissue of the upper cheek, the SMAS is recognized. (E) At the level of the lower edge of the nose: the zygomatic major muscle and the anterior margin of the parotid gland are visible, and the SMAS is recognized in the subcutaneous adipose tissue between them. The masseter muscle exists deeply them. (F) At the level of the upper mandibula: the SMAS is recognized between the zygomatic major muscle and parotid gland. (G) At the level of the corner of the mouth: the SMAS is recognized between the major zygomatic muscle attaching to the orbicularis oris muscle and parotid gland.

Figure 3.

Quantitative analysis on axial and coronal computed tomography (CT) images of superficial musculoaponeurotic system (SMAS) analysis. (A) Three-dimensional (3D) CT image showing anatomical landmarks for standardization of the analysis site of the SMAS. The inferior margin of the nose and anterior margin of the earlobe were used as anatomical landmarks (large dots) for an axial CT image, and the center of the fixed analysis area (FAA; small dot) was installed at the midpoint connecting them. The horizontal line B indicates the location of the axial CT image. A coronal CT image was created at the location of the small dot in a plane orthogonal to the axial CT image along the vertical dotted line C. (B) Axial CT images for quantitative analysis of the SMAS: the subcutaneous adipose tissue containing the SMAS was delineated using a 9 × 3 mm rectangle, and this rectangular area was defined as the FAA. The subcutaneous adipose tissue of the cheek was sufficiently thick and determined to be optimal for SMAS analysis. Even within the FAA, SMAS thickness is not uniform. The area occupied by the SMAS within the FAA was quantitatively analyzed using ImageJ software with contrast enhancement and binarization. (C) Coronal CT images for quantitative analysis of the SMAS. Using the same method as for the axial CT image, the area occupied by the SMAS within the FAA was quantitatively analyzed using ImageJ software (National Institutes of Health).

Figure 4.

Correlation between average superficial musculoaponeurotic system (A-SMAS) thickness within fixed analysis area (FAA) and age. A moderate yet significant negative correlation was found between A-SMAS thickness (circle) in FAA on axial computed tomography (CT) images and age (r = −0.60, P < .001). A moderate yet significant negative correlation was found between A-SMAS thickness (rhombus) in FAA on coronal CT images and age (r = −0.68, P < .001). Specifically, the SMAS becomes thinner with aging.

Figure 5.

The average superficial musculoaponeurotic system (A-SMAS) thickness within the fixed analysis area (FAA) among the 3 age groups. (A) The A-SMAS thickness within the FAA on axial computed tomography (CT) images among the 3 age groups. The A-SMAS thickness in both groups M (n = 32; 1.00 ± 0.13 mm) and E (n = 30; 0.87 ± 0.17 mm) was significantly lower than the corresponding value in group Y (n = 34; 1.16 ± 0.17 mm; both P < .001). A-SMAS thickness was significantly lower in group E than the corresponding value in group M (P < .001). (B) The A-SMAS thickness within the FAA on coronal CT images among the 3 age groups. The A-SMAS thickness in both group M (n = 32; 0.90 ± 0.14 mm) and group E (n = 30; 0.82 ± 0.19 mm) was significantly lower than the corresponding value in group Y (n = 34; 1.16 ± 0.18 mm; both P < .001). A-SMAS thickness was significantly lower in group E than the corresponding value in group M (P = .004).