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Cosmetic: Original Articles

The Effect of Age on Fat Distribution in the Neck Using Volumetric Computed Tomography

Orra, Susan M.D.; Tadisina, Kashyap M.D.; Charafeddine, Ali M.D.; Derakhshan, Adeeb M.D.; Halliburton, Sandra Ph.D.; Hashem, Ahmed M.D.; Doumit, Gaby M.D.; Zins, James E. M.D.

Author Information
Plastic and Reconstructive Surgery: January 2021 - Volume 147 - Issue 1 - p 49-55
doi: 10.1097/PRS.0000000000007424
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Abstract

Fat distribution in the superficial, intermediate, and deep planes of the neck has been described in a number of recent publications.1–6 Neck fat volume and distribution are influenced largely by age, body habitus, and a number of medical conditions. Precise anatomical knowledge of neck fat volume and location may assist the plastic surgeon in planning and executing neck contouring procedures.

The majority of reports detailing this anatomy have been based on cadaver findings,1–6 and although such studies provide accurate and objective data, the information provided is generally from elderly individuals, is limited in number, and represents one point in time. This study uses volumetric computed tomographic imaging to characterize the volume and distribution of fat between musculofascial layers of the neck in vivo, and examine how they change with respect to age—specifically, between young and elderly women.

PATIENTS AND METHODS

Computed tomographic angiography scans of the head and neck taken between 2009 and 2015 were obtained retrospectively in 40 healthy Caucasian women from the radiologic archives at a single institution. Two-millimeter-thick computed tomographic scan slices were the most precise computed tomographic scans available for volumetric analysis. Measurements of computed tomographic scans were chosen based on the anatomical landmarks described later under Patients and Methods. Body mass index ranged from 25 to 35 kg/m2. Patient scans were equally divided by age into two groups of 20 women each: young (aged 20 to 35 years) and elderly (aged 65 to 89 years). Exclusion criteria included patients who were edentulous, had prior head/neck surgery, or used chronic corticosteroids. Patients were included if they met the age criteria and body mass index at the time of the imaging as defined above. Body mass index was used as a measure of body habitus for this study. Transverse computed tomographic images were reconstructed with a thickness of 2 mm and a distance of 2 mm between slices; the centers of consecutive slices were adjacent and nonoverlapping. Measurements were made from the inferior border of the mandibular body to the inferior border of the thyroid cartilage at the level of the inferior cornu.

Computed tomographic angiography of the neck was the imaging study of choice because of its inclusion of the entire head and neck region within the same scan range and its higher spatial resolution compared to routine head computed tomographic scans. The volume of fat in each compartment was calculated by multiplying the total thickness of each slice (0.2 cm) by the sum of the cross-sectional areas (in centimeters squared) measured for each transverse slice of the neck. The formula used was as follows:

The neck was divided into three anatomical regions:

  1. Upper third (inferior border of mandibular body to superior border of hyoid bone).
  2. Middle third (superior border of hyoid bone body to superior border of thyroid cartilage).
  3. Lower third (superior border of thyroid cartilage to inferior border of thyroid cartilage at the level of the cornu).

Quantitative volumetric measurements of fat distribution in the supraplatysmal and subplatysmal planes were measured and compared between the two age groups. The distribution of fat volume was assessed by dividing each supraplatysmal and subplatysmal compartment into upper, middle, and lower thirds. Average volume in the upper, middle and lower thirds of the supraplatysmal and subplatysmal areas were also compared within each age group and between age groups.

All patients had intravenous contrast highlighting the blood vessels, which were subsequently excluded from measurements. This allowed for differentiation between muscle, fat, skin, and blood vessels. The cross-sectional area of fat was measured for each computed tomographic slice. The supraplatysmal fat compartment on transverse image slices was defined as the area of fat from the anterior (superficial) border of the platysmal muscle to the skin of the neck in the anteroposterior plane. The lateral border of our measurement was defined as the medial border of the sternocleidomastoid muscle, the superior border was the inferior mandible, and the inferior border was the superior border of the hyoid. The subplatysmal fat compartment was defined and measured as the fat deep to the platysma muscle but anterior (superficial) to the underlying neck strap muscles. Measurements of the subplatysmal fat compartments were extended bilaterally to the medial borders of the sternocleidomastoid muscles laterally, the mandible superiorly, and the hyoid inferiorly.

Measurements of the cross-sectional area of each computed tomographic slice were made by only one individual to minimize measurement bias, interrater discordance, and experimenter bias. Measurements were performed on a standalone imaging workstation (Leonardo; Siemens Medical Systems, Erlangen, Germany). Fat regions of interest were outlined using a region-of-interest free-hand tool. The cross-sectional area within each region of interest was automatically calculated by the computer software. An example of the method of free-hand region-of-interest measurements completed on each slice for supraplatysmal and subplatysmal fat volume measurements is shown in Figure 1.

Fig. 1.
Fig. 1.:
Axial computed tomographic image of a sample patient demonstrating the technique used to quantify the volume of fat in the supraplatysmal and subplatysmal compartments. Fat regions of interest were outlined using a region-of-interest free-hand tool. Cross-sectioned area within each region of interest was automatically calculated.

Excel (Microsoft Corp., Redmond, Wash.) statistical software was used to conduct the statistical analysis with a significance level set at 0.05. A normality assumption was performed within each age group and the data were found to follow a normal distribution; thus, we used parametric tests, t test, and analysis of variance for our analysis. A two-tailed t test was used to compare total fat volume in the supraplatysmal and subplatysmal fat compartments between age groups. A two-tailed t test was also performed to compare fat volumes within each of the three regions of the neck between age groups. An analysis of variance was used to compare average volumes among the three regions of the neck within each age group. Data were presented as means ± SD unless otherwise noted. Average age was reported and compared between the two age groups. Average body mass index was also compared between the two age groups and was used as the best measure for body habitus; we aimed to have patients with similar body habitus between the two groups so that their neck fat was comparable.

RESULTS

Patient Demographics

Computed tomographic angiograms of the neck were obtained for 40 female Caucasian patients and divided into two equal groups of 20 women. Group 1 (young patients) had a median age of 26.5 years (range, 20 to 33 years), and group 2 (old patients) had a median age of 74 years (range, 65 to 89 years). The average age of the young patients was 26.1 ± 3.99 years, and average age of the old patients was 74.7 ± 6.61 years (p < 0.0001). The statistically significant difference between the two age groups allowed for statistical comparison of the neck fat compartments without confounding factors associated with similar age.

Average body mass index was 30.05 ± 2.83 kg/m2 in the young patients and 28.91 ± 3.17 kg/m2 in the old patients (p > 0.05). There was a significant difference between the two groups with regard to age, but no significant difference between the two age groups with regard to body mass index. Equivalency in sex, ethnicity, and body mass index between groups allowed for statistical comparison between the two age groups.

Neck Fat Compartment Volumes

In group 1 (young patients), the average total supraplatysmal fat volume was 71.89 ± 21.78 cm3, which was significantly more than the average total subplatysmal fat volume of 23.53 ± 8.72 cm3 (p < 0.0001). In group 2 (old patients) the average total supraplatysmal fat volume was 56.71 ± 17.82 cm3, which was significantly more than the average total subplatysmal fat volume of 25.49 ± 9.81 cm3 (p < 0.0001). Therefore, there was more total supraplatysmal fat volume than subplatysmal fat volume in both age categories. Paired, two-tailed t test analysis between supraplatysmal and subplatysmal fat compartments in the young and old groups are depicted in Table 1.

Table 1. - Paired t Test Comparing Total Supraplatysmal and Subplatysmal Fat in the Young and Old Age Groups*
Young Group Old Group p (Old vs. Young)
Total supraplatysmal fat volume, cm3 71.89 ± 21.78 56.71 ± 17.82 <0.05
Total subplatysmal fat volume, cm3 23.53 ± 8.72 25.49 ± 9.81 >0.05
p < 0.05 (supraplatysmal vs. subplatysmal) <0.0001 <0.0001
*There is significantly more supraplatysmal fat in the young group compared to the old group. Both the young and the older age groups have more supraplatysmal than subplatysmal fat.

When dividing the neck anatomically into upper, middle, and lower thirds based on the mandible, hyoid, and thyroid cartilage as defined above, analysis of variance was used to compare the quantity of fat between neck regions in the supraplatysmal and subplatysmal planes in both age groups separately. In group 1 (young patients), there was no significant difference in fat volume when each third of the neck was compared in the supraplatysmal plane: upper third, 20.35 ± 10.96 cm3; middle third, 26.29 ± 11.58 cm3; and lower third, 25.37±13.29 cm3 (p > 0.05). That is, the fat volume was relatively evenly distributed within each supraplatysmal region in the young patients. In comparing the fat volume of the subplatysmal space in group 1 (young patients), we found that the middle third had significantly more fat volume (10.40 ± 4.78 cm3) than the upper (6.87 ± 5.08 cm3) and lower thirds (6.39 ± 3.53 cm3) (p < 0.01) (Table 2).

Table 2. - Analysis of Variance Comparing Neck Fat Regions in the Young Age Group*
Upper Third Total Volume Middle Third Total Volume Lower Third Total Volume§ p
Total supraplatysmal fat volume, cm3 20.35 ± 10.96 26.29 ± 11.58 25.37 ± 13.29 >0.05
Total subplatysmal fat volume, cm3 6.87 ± 5.08 10.4 ± 4.78 6.39 ± 3.53 <0.01
*Upper third, middle third, and lower third anatomical landmarks are defined in the table. Values are represented by means ± SD. Values of p < 0.05 were considered significant.
†Inferior mandibular margin to superior margin of hyoid bone.
‡Superior margin of hyoid bone to superior margin of thyroid cartilage.
§Superior margin of thyroid cartilage to inferior margin of thyroid cartilage.

In comparing the fat volumes of the supraplatysmal space in group 2 (old patients), we found that the middle third had significantly more fat volume (28.58 ± 20.01 cm3) than the upper (18.93 ± 10.35 cm3) and lower thirds (15.46 ±11.57 cm3) (p < 0.01). As for the subplatysmal space, we found that the middle third had significantly more fat volume (13.91 ± 9.65 cm3) than the upper (8.95 ± 5.06 cm3) and lower thirds (5.23 ± 3.74 cm3) (p < 0.01). Group 2 (old patients) had the highest volume of fat in the middle third of the supraplatysmal space (28.58 ± 20.01 cm3), followed by the upper third of the supraplatysmal space. In addition, group 2 had the lowest amount of fat in the upper and lower thirds of the subplatysmal spaces. Results of analysis of variance in the old patients are listed in Table 3.

Table 3. - Analysis of Variance Comparing Neck Fat Regions in the Old Age Group*
Upper Third Total Volume Middle Third Total Volume Lower Third Total Volume§ p
Total supraplatysmal fat volume, cm3 18.93 ± 10.35 28.58 ± 20.01 15.46 ± 11.57 <0.01
Total subplatysmal fat volume, cm3 8.95 ± 5.06 13.91 ± 9.65 5.23 ± 3.74 <0.01
*Upper third, middle third, and lower third anatomical landmarks are defined in the table. Values are represented by means ± SD. Values of p < 0.05 were considered significant.
†Inferior mandibular margin to superior margin of hyoid bone.
‡Superior margin of hyoid bone to superior margin of thyroid cartilage.
§Superior margin of thyroid cartilage to inferior margin of thyroid cartilage.

A paired t test was used to further compare the three regions of the neck in each platysmal plane between both groups. In other words, we used the paired t test to further compare our analysis of variance results that are displayed in Tables 2 and 3. We first analyzed the supraplatysmal space and found that there was no difference in the amount of fat in the upper and middle thirds in young versus old patients. However, the young patients had significantly more fat in the lower third of the neck than the old patients (p < 0.01), as depicted in Table 4. In analyzing the subplatysmal space, there was no significant difference between the three regions of the neck in the young versus the old (Table 5).

Table 4. - Paired t Test Comparing Supraplatysmal Neck Fat Volume of the Upper, Middle, and Lower Thirds of the Neck between Young and Old Age Groups*
Third Young Old p
Upper, cm3 20.35 ± 10.95 18.95 ± 10.35 >0.05
Middle, cm3 26.3 ± 11.55 28.58 ± 20.05 >0.05
Lower, cm3 25.35 ± 13.3 15.47 ± 11.55 0.01
*Values are represented by means ± SD. Values of p < 0.05 were considered significant.

Table 5. - Paired t Test Comparing Subplatysmal Neck Fat Volume of the Upper, Middle, and Lower Thirds of the Neck between Young and Old Age Groups*
Third Young Old p
Upper, cm3 6.85 ± 5.1 8.95 ± 5.05 >0.05
Middle, cm3 6.93 ± 4.8 13.9 ± 9.9 >0.05
Lower, cm3 6.4 ± 2.37 5.25 ± 3.7 >0.05
*Values are represented by means ± SD. Values of p < 0.05 were considered significant.

The plotted trend of neck fat volume compared to age depicted in Figure 2 illustrates a negative slope for the supraplatysmal fat with increasing age, indicating a decrease in fat volume with increasing age at a rate of 0.84 cm3/year. A similar linear regression model was plotted for the subplatysmal space. This graph illustrates a very small positive slope with an increase in fat volume at a rate of 0.005 cm3/year in aging women, indicating that although there may be an increase in fat volume in the subplatysmal space, the rate of change over time is very small.

Fig. 2.
Fig. 2.:
The plotted trend of neck fat volume compared to age illustrates a negative slope for the supraplatysmal space of the aging neck, indicating a decrease in fat volume with increasing age at a rate of 0.84 cm3/year. A similar linear regression model was plotted for the subplatysmal space. This graph illustrates a very small positive slope with an increase in fat volume at a rate of 0.005 cm3/year; indicating that although there may be an increase in fat volume in the subplatysmal space, the rate of change is very small over time.

DISCUSSION

Previous studies have defined the fat compartments of the neck. Anatomically, this fat distribution has been divided into the superficial (supraplatysmal), intermediate (subplatysmal), and deep (deep to the deep fascia) planes, with further subcompartments in each plane.1,4 In the supraplatysmal space, two portions—the suprahyoid and infrahyoid compartments—have been defined. In the subplatysmal space, six total compartments have been defined: the central suprahyoid, the central infrahyoid, the lateral suprahyoid bilaterally, and the lateral infrahyoid bilaterally. The deep plane held only one compartment and is clinically insignificant. Dye injection suggests that the supraplatysmal and subplatysmal planes were continuous in nature, with the lateral infrahyoid subplatysmal and very deep compartments appearing to be separate5. In terms of fat distribution, the majority of fat has been found to be in the supraplatysmal plane, with approximately one-third present in the subplatysmal compartment, although large variation was noted. The submandibular gland is a significant contributor to soft-tissue weight from the subplatysmal compartment. Less than 1 percent of weight is contributed by the very deep compartment of fat in the neck. It should be noted that these descriptions of musculofascial planes within the neck, and the distribution of fat between these planes, were derived almost exclusively from cadaver studies.5

In this study, we used volumetric computed tomographic angiography of the neck to characterize the neck fat volume and distribution in a patient series. Computed tomographic imaging, along with magnetic resonance imaging, has long been known to be the gold standard modality for capturing soft-tissue anatomy and its relationship with the facial skeleton. Computed tomographic imaging modalities are used daily to track progress across many medical and surgical conditions. Sakamoto et al. used magnetic resonance imaging to evaluate changes in free flap volume in head and neck reconstruction and found it to be a reliable technique, especially for fatty tissue characterization.7 Rohrich and Pessa successfully used cadaver dissections and computed tomography to assess aging in the face of cadavers and were able to effectively characterize the fat compartments of the face using such techniques.8,9 Rohrich and Pessa and others have also shown subplatysmal fat hypertrophy in disease states such as obesity and human immunodeficiency virus.10,11

Raveendran et al. outlined changes in the aging neck, including fat deposition, bone resorption, increases in laxity of muscle retaining ligaments, and loss of elasticity of the skin. These changes may lead to an obtuse angle of the neck with facial aging.12 Characterizing the variation in anatomy of subplatysmal fat should be of interest for the plastic surgeon, as resection and contouring of this area is one of the more important aspects of an aesthetically pleasing neck lift.8,9,13,14 Although the distribution of fat in the neck has been previously studied in detail, there have been no studies to date evaluating how the volume and distribution of fat in each compartment of the neck changes with age. Our current study attempts to extend the work of our prior study12 in addition to Rohrich, Pessa, Raveendran, and others, by using volumetric computed tomographic imaging to analyze neck fat volumes and its distribution in young and old age groups.

Our present study documented a significant difference in the volume of fat in the neck between young (aged 20 to 33 years) and old (aged 65 to 89 years) patients. We found both young and old patients have significantly more fat in the supraplatysmal plane than in the subplatysmal plane. Furthermore, when comparing the two age groups, we specifically found that young patients have a significantly greater amount of total fat in the supraplatysmal plane compared with elderly patients (p < 0.05). Although there is little literature regarding fat quantification in the necks of young patients, previously published cadaveric studies7 do corroborate our findings in our elderly individuals. In these cadaver studies, the majority of the fat was found in the supraplatysmal plane. Nearly half (44.7 percent) of the total fat in the neck was found in this superficial layer in these reports.10

Although this and other studies have documented that significantly more total fat is found in the superficial rather than the intermediate plane, it should be noted that approximately one-third to one-half of total fat was found in the intermediate compartment. This suggests that opening the neck and exploring the subplatysmal plane is indicated for complete neck contouring. In addition, because the most subplatysmal fat was found in the middle third of the subplatysmal plane, particular attention might be paid to this area.

The supraplatysmal and subplatysmal fat in young patients was quite evenly distributed among the three regions of the neck; however, the elderly patients had more fat in the middle third of the neck in both supraplatysmal and subplatysmal planes. This suggests that perhaps fat redistribution occurs with age and is perhaps in part responsible for the obtuse cervical mental angle seen with aging. Raveendran et al.12 conducted a cadaveric study examining fat in the superficial and deep planes of the neck, along with the superficial portion of the submandibular gland. They demonstrated that in the elderly, neck there is skin, fat, and muscle laxity contributing to “turkey gobbler” deformity and vertical bands. They also introduced the concept of pseudoptosis of the submandibular gland, a consequence of increased weakness of the floor of the mouth, both of which lead to increased exposure of submental structures.

It seems to be that total fat volume in the neck tends to decrease with age (Fig. 2). We found that it decreases at a rate of 1.68 cm3/year in the supraplatysmal region, whereas the rate of change of total fat volume with age is negligible in the subplatysmal plane.

In summary, our study found that total supraplatysmal fat volume decreases with age, whereas total subplatysmal fat volume remains relatively stable. In addition, the elderly have significantly more fat in the upper and middle thirds of the neck, whereas the young have more evenly distributed fat volume between the three regions.

Limitations of our study were numerous. Our initial hope was to perform a longitudinal study of the same individuals over a prolonged period, which would have provided the ideal control group of a patient’s initial younger computed tomographic scan versus her own future, elderly scan. This proved impossible because of computer software changes at our institution and the limitations in obtaining archived radiographic images of the same patients over an extended period. Therefore, a retrospective review of patients assigned randomly into young and elderly groups was the best alternative.

With regard to our age groups selected for this study, our aim was to attempt to demonstrate whether or not neck volume and distribution changed with age. Therefore, we chose two disparate age groups: the young (aged 20 to 35 years) and the elderly (aged 65 to 89 years), thus providing the greatest likelihood of significant change. Had we included a third group (aged 36 to 64 years), this would have provided additional data points; this omission could be considered a weakness of the study.

A larger sample size would have been ideal. However, we had limited computed tomographic scans available that met inclusion criteria and that provided precise millimeter thickness images. It should be noted that previous published studies regarding facial and neck fat volumetric analyses had sample sizes similar to ours. To minimize differences in body habitus among individuals, patients were selected with a narrow average body mass index window of 28 to 30 kg/m2. Although not an ideal measure of body morphology, this was the best tool available.

Finally, the ideal position for measurement of neck fat compartments would have been in the standing position, and not the standard supine position used to obtain computed tomographic images. It is known that elderly patients demonstrate more platysmal laxity than younger cohorts. Therefore, measuring fat distribution in the supine rather than the standing position may have had an effect on our upper, middle, and lower third measurements. However, it seems unlikely that this would have had any effect on fat compartment volume and location.

Other possibilities to be investigated include comparison between sex and ethnicity; this study was limited to Caucasian women. In addition, our method of analysis used computed tomographic scanning; however, the method of performing computed tomographic scanning requires that patients are in the supine position, which may affect the neck fat compartments, distributions, and measurements. Furthermore, we analyzed the computed tomographic scans of only 40 Caucasian women; a larger study could result in more significance and allow for formulating more inferences about the aging neck. Finally, the method used to measure the cross-sectional area of neck fat volume in each computed tomographic scan slice was not automated, a limitation of our radiology software. As a result, measurements are prone to human measurement error. However, all measurements were obtained by only one experimenter, which allowed us to minimize the bias of interrater discordance and experimenter bias.

CONCLUSIONS

According to our findings, total supraplatysmal fat volume decreases with age, whereas total subplatysmal fat volume remains stable. In addition, the elderly had significantly more fat volume in the upper and middle thirds of the neck, whereas the young have more evenly distributed fat volume between the three regions. This suggests but does not prove that fat deposition and redistribution in the neck may occur with age and may be a contributing factor to the obtuse cervicomandibular angle of the elderly. The clinical implication of this study is that the loss of fat above the platysma may be a contributing factor to neck laxity in the older population. Persistent subplatysmal fat volume in the elderly supports the concept of subplatysmal fat exploration and possible removal at the time of cervicoplasty and face lifting.

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