Are There Differences Between the Upper and Lower Parts of the Superficial Musculoaponeurotic System

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DOI: 10.1177/1090820X14528947 2014 34: 661 originally published online 17 April 2014Aesthetic Surgery Journal

Xiaoqing Hu, Zhijun Wang, Qi Wang, Chen Zhang, Gang Hu and Hongzhi QinSystem? A Preliminary Biomechanical Study

Are There Differences Between the Upper and Lower Parts of the Superficial Musculoaponeurotic

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Facial Surgery

Although absorbable filler injections and suture suspen-sion are increasingly popular approaches for facial rejuve-nation, traditional rhytidectomy has unique advantages. This procedure is often the only choice for elderly patients who display obvious signs of skin relaxation and for patients who seek a very significant change in their appear-ance. Tightening the preauricular or the lateral area of the superficial musculoaponeurotic system (SMAS) in differ-ent directions is the key component of rhytidectomy proce-dures. Plastic surgeons tend to view the SMAS as 1 mechanical unit, and whether there are any biomechanical differences between the upper and the lower parts of the

528947AESXXX10.1177/1090820X14528947Aesthetic Surgery JournalHu et alresearch-article2014

Dr X. Hu is a Clinical Attending Surgeon and Drs Z. Wang and Zhang are Professors of Plastic Surgery in the Department of Plastic Surgery, Affiliated Xinhua Hospital of Dalian Medical University, LiaoNing, China. Dr Q. Wang is a Clinical Attending Physician in the Department of Pediatric Intensive Care, Childrens Hospital of Zhengzhou, HeNan, China. Drs G. Hu and Qin are Professors of Plastic Surgery in the Department of Plastic Surgery, First Affiliated Hospital of Dalian Medical University, LiaoNing, China.

Corresponding Author:Dr Zhijun Wang, Department of Plastic Surgery, Affiliated Xinhua Hospital of Dalian Medical University, No. 148 Wansui Rd, Shahekou District, Dalian, 116021, LiaoNing, China. E-mail: [email protected]

Are There Differences Between the Upper and Lower Parts of the Superficial Musculoaponeurotic System? A Preliminary Biomechanical Study

Xiaoqing Hu, MMS; Zhijun Wang, MD; Qi Wang, MB; Chen Zhang, MD; Gang Hu, MD; and Hongzhi Qin, MMS

AbstractBackground: The superficial musculoaponeurotic system (SMAS) becomes thinner and gradually disappears from the midface. In rhytidectomy, manipulation of the SMAS occurs in the lateral area, and previous research has focused primarily on the SMAS region as a whole.Objectives: In this preliminary study, the authors compared the viscoelasticity of the upper and lower regions of the SMAS using biomechanical techniques.Methods: Two adjacent projection regions of the SMAS were designated: region 1 and region 2, representing the upper and lower parts, respectively. The SMAS tissues from 8 fresh-frozen cadaver heads were cut into 64 samples before biomechanical testing, and the following variables were recorded for subsequent analysis: stress-strain curve, elastic modulus, ultimate strength, and elongation at break.Results: The stiffness of region 1 was markedly greater than that of region 2. Energy dissipation was greater in region 2. Elastic modulus and ultimate strength were significantly higher for region 1, and elongation at break was longer in region 2. The fit curve of the 2 regions deviated markedly.Conclusions: The biomechanical properties of the upper and lower regions of the lateral SMAS are functionally different. Such knowledge will help refine the planning and design of facial surgery and improve outcomes for patients who undergo rhytidectomy.

Keywordssuperficial musculoaponeurotic system (SMAS), biomechanics, rhytidectomy, rejuvenation, facelift, viscoelasticity

Accepted for publication October 21, 2013.

INTERN

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662 Aesthetic Surgery Journal 34(5)

SMAS has not been reported previously. Our research pro-vides a preliminary examination and analysis.

MethodSPreliminary work was performed on multiple specimens to determine the optimal assessment technique. For the study, 8 fresh-frozen human cadaveric heads, which had been stored at 60C, were selected. The age range of the cadavers was 45 to 65 years (average age, 58.3 2.6 years); 5 were male, and 3 were female. As deep scarring could have affected the evaluation, specimens without facial scars or surgical operations, including facelifts, were selected. Dissections were performed after the cadaveric heads had thawed for 48 hours at 4C.

Macroscopic DissectionBecause the SMAS thins and gradually disappears in the midface, we focused on the SMAS in the lateral face to determine clinical applications for aesthetic surgery. In accordance with previous reports1,2 and the practical aspects of SMAS facelift technology, we defined 2 adjacent projection regions of facial skin: region 1 and region 2 (Figure 1).

Region 1Region 1 includes the upper part of the SMAS, just below the zygomatic arch, extending anterior of the tragus to the lateral edge of the zygomatic major muscle, then posteri-orly to the upper edge of the platysma beneath the lobulus auriculae. This triangular projection was denoted as the aponeurotic part of the pretragal SMAS, superficial to the parotid fascia.

Region 2Region 2, a polygonal projection beneath region 1, repre-sents the lower part of the SMAS, superficial to the mas-seter fascia. It extends downward inferolaterally along the anterior edge of the masseter muscle, intersecting with the rim of the mandible.

dissection ProcessOnce the cadaver heads thawed, the skin over the 2 regions was dissected sharply, undermined, and lifted. This pro-cess allowed for full visualization of the SMAS tissue over-lying the parotid fascia and masseter fascia. The SMAS was separated superficial to the parotid fascia through the loose connection between the 2 soft-tissue layers, thus avoiding damage to the parotid fascia and parotid gland. The SMAS was harvested in the lateral face approximately 0.5 cm from the zygomatic and masseteric ligaments. During dissection, we noted that the SMAS of region 2 con-tained substantial muscle fibers resembling compact strips

running transversely and parallel to the platysma muscle in the cervical region.

Preparation of SamplesThe SMAS tissues of region 1 and region 2 were harvested separately and then cut into multiple samples. Each cadaver provided a total of 8 samples: 4 from each region of the SMAS. The preparation of each cadaver sample set, including SMAS dissection, was conducted at a tempera-ture of 4C and completed within 1 hour. Because of the limited number of cadavers, the direction of the sampleeither parallel or perpendicular to the headwas not con-sidered a condition for grouping. The samples were trimmed to a dumbbell shape to ensure that they could be broken in the center. The size of each sample was designed

Figure 1. The 2 adjacent projection regions of the superficial musculoaponeurotic system (SMAS) on facial skin. Region 1, represented by the red triangle A-B-C, includes the upper part of the SMAS, superficial to the parotid fascia and just below the zygomatic arch. Region 1 extends from point A (anterior of the tragus) to point B (lateral edge of the zygomatic major muscle) and then posteriorly to point C at the upper edge of the platysma beneath the lobulus auriculae. Region 2, which lies beneath region 1 and is represented by the blue polygon B-C-D-E, includes the lower part of the SMAS, superficial to the masseter fascia. This area extends downward from point B, inferolaterally along the anterior edge of the masseter muscle to point E, intersecting with the mandibular rim. Point D is the mandibular angle.


Hu et al 663

to coincide with the serrated grips of the Instron universal test-ing machine (model 5567A; Instron, Norwood, Massachusetts). For uniformity, all samples were measured at their mid-point. The length, width, and thickness of each sample were approximately 10 mm, 10 mm, and 3 mm, respec-tively. All dissections and SMAS sample preparations were performed by the primary author (X.H.) to ensure that the samples were as identical as possible.

Biomechanical TestingPrior to each measurement, the Instron machine was cali-brated and zeroed out. The sensing elementa key com-ponent of measurement rangewas 100 N. Testing was performed with uniaxial tension in parallel with each sam-ples long axis. The control mode of test setup parameters was x-y (ie, the display mode of coordinate axes), accord-ing to various biomechanical test schematics. The x-axis indicated tensile displacement (ie, change in length of deformation) or the Green strain (ie, mechanical index of deformation), with = (2 1)/2 (the stretch ratio = L/L

0, where L and L

0 represented the stretched and initial

lengths, respectively). The y-axis indicated tensile loading force or the Lagrange stress (force per unit area).

The SMAS samples of each cadaver were stored in a moist, room-temperature environment (25C) throughout the experiment. Testing of each cadaver was completed within 1 hour. Testing for viscoelastic characteristics dem-onstrated the hysteresis effect and stress-strain procedure.3 In the typical phenomenon of hysteresis loop, the strain lags behind stress and energy loss in the process of succes-sive loading and unloading cycles. For stress-strain testing, the samples were pulled at a displacement rate of 10 mm/min until failure, at which time the Lagrange stress required to rupture the tissues was measured. Ultimate strength is defined as the maximum level of stress (mea-sured in megapascals) that the tissue can sustain, calcu-lated as force (measured in newtons) divided by the original, unstrained cross-section area of the specimen (measured in square meters). The following measurements were automatically collected by the computer for subse-quent analyses: stress-strain curve, the elastic modulus (Youngs modulus E), ultimate strength, and the percent-age of elongation at break. Youngs modulus E was defined as the slope of the elastic region (near linear part) of the stress-strain curve.

Statistical AnalysisThe t test was used to identify differences in the biome-chanical properties of the SMAS between region 1 and region 2 (df = 31, paired samples t test, 2-tailed). Statistical significance was defined as P < .05 (SPSS, version 13.0; SPSS, Inc, an IBM Company, Chicago, Illinois).

ReSultS

Hysteresis LoopEach sample was stretched at a rate of 10 mm/min to the same displacement point. A series of hysteresis loops were shown with loading and unloading cycles until a stable con-dition was reached; therefore, the sample was precondi-tioned. The loading curves lay outside the unloading curves, and the stretch ratio of the loading curves was lower than that of the unloading curves (Figure 2). Because a sample was loaded in the elasticized region, it returned to its origi-nal shape through the unloading process. While all loops decreased gradually to an equilibrium level, the stiffness between the 2 regions was markedly different. For clarity, the 2 representative curves at the third circle of each test for regions 1 and 2 were isolated in a load-vs-displacement pre-conditioned curve (Figure 3). Measured stiffness and energy dissipation are shown in Table 1. No statistically significant difference was found within the same region. Average stiff-ness was markedly greater in region 1 (0.21 0.02 vs 0.14 0.03 in region 2). The area enclosed by the curve was 0.57 0.09 for region 1 and 2.10 0.76 for region 2.

Stress-Strain CurveThe average curve (Figure 4) showed that the elastic mod-ulus (Youngs modulus E) and ultimate strength of region 1 were significantly higher than those of region 2; however, the percentage of elongation at break was shorter in region 1. The average values of all specimens from both regions are summarized in Table 2. As with hysteresis testing, no significant difference was found among samples within the same region.

Figure 2. The hysteresis loop in successive cycles: the loading curves lay outside the unloading curves. All loops decreased gradually to a level of equilibrium. Region 1 (left) exhibited greater stiffness than did region 2 (right). SMAS, superficial musculoaponeurotic system.



We evaluated the stress-strain curve of the 2 regions with a 5-order polynomial model, which provided a good fit for the experimental data. The various parameters are listed in Table 3. Results showed obvious deviation between the curves of the 2 groups. The A

0 of region 1

(34.86 E-03) was approximately 10 times that of region 2 (3.08 E-03), but the subsequent parameters (A

1-5) were

similar for the 2 regions.

diScuSSionTightening the SMAS has become an essential part of rhyt-idectomy procedures: in most facelift techniques, surgeons tighten the SMAS to counteract the sagging force of the facial soft tissue. Because the SMAS has customarily been regarded as a whole, and previous reporting4 has focused on biomechanical properties of the SMAS as a whole, less is known about the mechanical behavior of the different parts of this system. Better understanding of the viscoelas-tic properties of the SMAS can guide surgeons in selecting optimal rhytidectomy techniques. Our study was con-ducted to elucidate the biomechanical differences between the upper and lower parts of the lateral SMAS (regions 1 and 2, respectively). The study compared stiffness, energy dissipation of hysteresis loops, the elastic modulus

(Youngs modulus E), ultimate strength, and the elonga-tion of the stress-strain curve at break. The SMAS tends to thin out from the lateral face to the central face, and the properties mentioned above may be changed gradually, and thus our data are intended to provide general guidance for the biomechanics of the lateral SMAS. Examination of specific gradients is beyond the scope of this study.

The stiffness of SMAS region 1 was greater than that of region 2, which coincided with comparative findings of the elastic modulus revealed by stress-strain curves (Figure 4). The constant variables helped illustrate the relationship between a given load and the resulting displacement within the elasticized range. It is suggested that region 1 may have maintained stronger elasticity, thus resisting deformation in the direction of the applied load. This find-ing may help to explain why the lower part of the lateral face tends to sag before the upper part. The relationship between stress and strain is actually an interatomic force with atomic spacing. Elastic deformation is a reversible change of atomic spacing under external force. The elastic modulus has a close relationship with interatomic force and atomic spacing, due mainly to the interatomic bonding nature and bonding force.

For nonideal elastic material (ie, stress and strain do not satisfy the linear relationship, as per Hookes law, or the material is a nonlinear elastic body), stress and strain are asynchronous. The unloading curve does not overlap the loading curve; thus, the 2 curves form a closed loop known as the hysteresis loop. The hysteresis loop phenomenon indicates that the deformation work (energy dissipated in the deformation process) consumed by material during loading is larger than that released during unloading. The energy absorbed in the deformation process is known as internal dissipation, which causes damage accumulation. The area encircled by the hysteresis loop, and representing the dissipation, is applied as a damage variable of fatigue accumulation. In this study, energy dissipation was greater in region 2. We speculate that SMAS region 2 is more prone to fatigue damage, and this information may serve as a reference for obtaining better outcomes from primary and secondary rhytidectomy.5

Contrary to another study on facial retaining ligaments,6 the stress-strain curves in our study lacked obvious yield phenomenon. (Beyond the elastic limit, we observed obvi-ous deformation of samples even without increasing stress.) Compared with the ligament, the SMAS showed a

Table 1. Stiffness and Dissipation of Regions 1 and 2

Measurement Region 1 Region 2 t P

Stiffness, N/mm 0.21 0.02 0.14 0.03 4.922 .001

Dissipation, Nmm 102 0.57 0.09 2.10 0.76 4.491 .002

Figure 3. Two representative preconditioned curves at the third cycle in region 1 (left) and region 2 (right). The area representing the dissipation enclosed by the hysteresis loop (shown in yellow) was smaller in region 1. SMAS, superficial musculoaponeurotic system.


Hu et al 665

more inhomogeneous and continuous yield. Ultimate strength, which refers to the maximum stress the tissue can sustain, reflects the resistance to maximum uniform deformation. Our analysis showed that ultimate strength was significantly higher in region 1, which may indicate that overlying skin tension decreased as a result of SMAS support. This may lead to improved long-term results in region 1 relative to region 2. The range of ultimate strength observed in our investigation (1.19 0.20 to 1.59 0.18 Mpa; 172.55 29.00 to 230.55 26.10 psi) is similar to that obtained by Trussler et al7 (224.97 57.9 psi) but falls between the data of the young and the old groups reported by Saulis et al.8 (The average cadaver age was 58.3 2.6 years in our study vs 69 years [range, 53-84 years] in Saulis et al.)

The amount of elongation at break depends on the flex-ibility of the molecular structure (ie, conformational change ability). Among other factors, molecular weight depends on the state of aggregation and multiphase struc-ture, and high molecular weight produces greater flexibil-ity and elongation. When cross-linking density increases, the slippage within the molecular chain will be reduced, tensile strength will increase, and elongation at break will

decrease. Tensile deformation represents an energy- consuming process of conformational changes. Higher elon-gation at break means that the tissues demonstrate greater impact-resistant ability. In this study, SMAS region 2 pro-vided higher elongationand therefore better ductilityand was more conducive to stress and safety reserves.

To take advantage of the high strength exhibited in region 1, SMAS plication would likely be more beneficial than SMAS-ectomy. Plication may counteract fatigue and premature relaxation, provided that the lateral face resec-tion overlaps region 2. Because the SMAS thins in midface, its strength might be reduced gradually from the lateral to the central face. We suggest that the excised or translo-cated part of the lateral SMAS might be overlapped medi-ally to enhance the effect of midface rhytidectomy. Because adhesions or scars form between the SMAS and its adja-cent layers during healing after rhytidectomy, both the SMAS and composite tissues will endure stress. The bio-mechanical performance of the SMAS also will be affected by adjacent tissues, which involves a dynamic, interactive process.

Through previous studies,9-14 the role of the SMAS in facial rejuvenation procedures has gradually become clearer. The

Figure 4. Stress-strain curves without obvious yield points. Samples were pulled at a displacement rate of 10 mm/min until failure. The elastic modulus (or Youngs modulus E), ultimate strength, and percentage of elongation at break were compared. Youngs modulus E and ultimate strength (right) were higher in region 1 (red) than in region 2 (blue); the elongation at break (left) was shorter in region 1. The force level (black arrow) placed on the superficial musculoaponeurotic system (SMAS) in our clinical setting was only a minor portion of the samples ultimate strength.

Table 2. Biomechanic Indices of Regions 1 and 2

Index Region 1 (n) Region 2 (n) t P

Youngs modulus E, Mpa 3.54 1.00 (32) 2.38 0.17 (32) 2.798 .019

Ultimate strength, Mpa 1.59 0.18 (32) 1.19 0.20 (32) 3.648 .004

Elongation at break, % 127.37 20.48 (32) 163.07 17.52 (32) 3.243 .009



SMAS is a continuous, organized fibrous network that con-nects the facial muscles as central aponeurosis,15 a complex tissue comprising collagen fibers, elastic fibers, and fat cells.16 As soft tissue, the SMAS possesses viscoelasticity and thus exhibits nonlinear mechanical behavior as a stress-distribut-ing and load-absorbing structure. The biomechanical differ-ences between region 1 and region 2 may relate to the proportion of muscular and aponeurotic components of the upper and lower parts of the SMAS. Mechanical behavior of the SMAS is primarily determined by the intrinsic properties of its components, which in turn may influence collagen fibers, elastic fibers, and fat cells, resulting in an adaptation of synthesis or arrangement of these components. In our previ-ous pathological studies, we observed a difference in the pro-portion of fibrous components between the 2 regions. However, this finding alone is insufficient for drawing final conclusions; detailed biopsies of both regions of the SMAS are needed.

There are several limitations to this study. Because find-ings from cadaveric samples do not necessarily translate to clinical practice, the biomechanical properties of the 2 SMAS regions may be different for patients undergoing live surgery. This study simulated the SMAS rhytidectomy without verify-ing the parotid fascia. Whether or not the parotid fascia was included, it would be unrealistic to investigate each patient via biopsy during live surgery. Another limitation of our study is the small number of specimens, which does not permit data stratification by age or sex. Therefore, we do not yet know whether age and sex affect strength or the other factors evaluated in our study. This preliminary study warrants fur-ther research and analysis to determine the optimal technique for repositioning facial soft tissue, increasing longevity of the effect, and strengthening the SMAS.

concluSionSRectifying facial relaxation is essential to achieving a youth-ful appearance after rhytidectomy. Various techniques have

been used to accomplish this, including tightening the pre-auricular or lateral part of the SMAS. Rather than simply treating the lateral SMAS as a single unit, it is important to understand the distinct biomechanical differences that exist between the upper and lower regions of this complex sys-tem, as described in this preliminary study. Our findings may be helpful for estimating the distribution of stress and strain in these 2 regions of the SMAS and for refining the design of a suitable vector of elevation or facial shaping in rhytidectomy surgery. Subsequent experiments have been designed to further our knowledge of the distinct biome-chanical properties of these 2 regions and their potential impact on clinical practice.

disclosuresThe authors declared no potential conflicts of interest with respect to the research, authorship, and publication of this article.

FundingThe authors received no financial support for the research, authorship, and publication of this article.

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Table 3. Stress-Strain Curve Fitting Parameters

Region I Region II

Parameter Value SE Value SE

A0

34.86 E-03 3.56 E-03 3.08 E-03 3.04 E-03

A1

16.84 E-03 4.73E-04 7.74 E-03 3.66E-04

A2

1.85 E-03 1.94E-05 1.14 E-03 1.36E-05

A3

2.89E-05 3.25E-07 1.80E-05 2.07E-07

A4

1.71E-07 2.38E-09 1.06E-07 1.37E-09

A5

3.68E-10 6.30E-12 2.25E-10 3.28E-12

Adjusted R2 0.99873 0.99794


Hu et al 667

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Are There Differences Between the Upper and Lower Parts of the Superficial Musculoaponeurotic System