Quantification of Gravitational Effects on Nasal Tip Geometry

Introduction

The nasal tip is an important structure for the function and aesthetics of the nose. Controlling its shape and position has been described as one of the greatest challenges in rhinoplasty surgery. In published literature discussing the nasal tip, it is a commonly accepted principle that the downwardly directed gravitational force acts upon the soft tissues of the nose. Thus, the positioning of a patient (supine vs upright) is believed to potentially influence nasal tip geometry. However, it is important to note that many measures of nasal tip geometry include surrounding soft tissue landmarks as reference points, in particular the upper lip. These soft tissues can also be affected by changes in patient positioning and may influence the apparent rotation and projection of the nasal tip. In this study, we sought to validate the principle of gravitational effects on tip position and delineate if there is a true change in nasal tip rotation. In addition, we attempted to quantify the degree of change in tip rotation and projection associated with gravity.

Methods

In this prospective case series, nasal measurements were acquired from 20 volunteer subjects (10 male, 10 female) over the age of 18 at a single institution between December 2020 and January 2021. Exclusion criteria included a history of prior nasal surgery or trauma.

Photographs were obtained according to techniques of standardized clinical photography in facial plastic surgery. Prior to photography, any objects obstructing the face (eyeglasses, piercings, etc.) were removed. A standardized photographic environment was used, which included dual studio lighting and a set photographer-to-subject distance. The same digital single-lens reflex (DSLR) camera was used for all photos, with fixed aperture settings, focal length, shutter speed, and ISO to ensure that all photographs were obtained under identical settings and conditions. The subject was positioned in the Frankfurt horizontal plane, and all photos were obtained in standard rhinoplasty views in addition to a lateral supine photo.

An image processing program available via public domain (Image J, National Institutes of Health and the Laboratory for Optical and Computational Instrumentation) was utilized for photographic measurements. ImageJ is an open-source software for processing and analyzing scientific images. Nasal tip geometry was analyzed using a variety of methods (Figure 1). Nasal tip rotation was measured using the nasolabial angle (NLA), the nasofrontal angle (NFA), and the columellar-facial angle (CFA), in both the upright and supine positions. Nasal tip projection was measured by techniques previously described by Powell and Humphreys, ¹ Goode, ² Simon, ³ Crumley and Lanser. ⁴ These methods utilize ratios of the length measured by various nasal and facial landmarks to provide standardized measurements of tip projection. These measurements were obtained in both the upright and supine positions (Figures 2 and 3). Each individual measurement was performed twice, with the average of the two used in data analysis for each patient in each position. Statistical analysis was performed using a paired samples t-test to assess for any significant differences in nasal tip geometry between these two positions. P values < 0.05 were considered statistically significant. The Pearson coefficient was used to analyze associations between the three different methods in both upright and supine positions.

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Figure 1.

Visual summary of nasal tip rotation and projection measurements. (A) Nasolabial angle: measurement of angle a° is ideally 93.4°–98.5° in males and 95.5°–100.1° in females. (B) Nasofrontal angle: measurement of angle a° is ideally 127°–142°. (C) Columellar-Facial Angle: measurement of angle a° is published previously as an average of 108° in males and 104.2° in females (D) Powell’s ratio: ratio of a: b is ideally 2.8:1. (E) Goode’s Ratio: a, b, and c form a 3:4:5 triangle and the ratio of a:c is ideally 0.55–0.6:1. (F) Simon Ratio: the ratio of a:b is ideally 1:1. (G) Crumley’s Method: the ratio of a:b is ideally 3.53:1.

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Figure 2.

Visualization of the effect of nasal tip geometry in the upright position (left) and supine position (right) in a male subject.

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Figure 3.

Visualization of the effect of nasal tip geometry in the upright position (left) and supine position (right) in a female subject.

Results

A total of 20 subjects were included (10 males, 10 females) with age ranging from 25 to 71 years. The overall mean age was 37.8 years; the mean age of the men and women were 35.3 and 40.3, respectively.

We acquired objective measurements of nasal tip rotation and projection in both the supine and upright positions (Table 1). NLA, NFA, and CFA were used to calculate nasal rotation. Mean NLA was 95.8° in the upright position and 99.0° in the supine position; NLA calculated tip position had a statistically significant positional difference of 3.2° (3.2%, p = 0.002). Mean NFA was 134.5° in the upright position and 133.9° in the supine position; NFA calculated tip position had a positional difference of 0.7° (p > 0.05) which was not statistically significant. Mean CFA was 100.8° in the upright position and 101.3° in the supine position; CFA calculated tip position had a positional difference of 0.5° (p > 0.05) which was not statistically significant.

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Table 1.

Nasal Tip Rotation and Projection Measurements in the Upright and Supine Positions.

Measurement	Published/ideal	Upright	Supine	Δ	P
Nasolabial angle (NLA)	Male: 93.4°–98.5° Female: 95.5°–100.1°	95.8°	99.0°	3.2°	0.002*
Nasofrontal angle (NFA)	127°–142°	134.5°	133.9°	0.7°	Not significant
Columellar-facial angle (CFA)	Male: 108° Female: 104.2°	100.8°	101.3°	0.5°	Not significant
Powell’s ratio	2.8	3.22	3.19	0.03	Not significant
Goode’s ratio	0.55 – 0.60	0.55	0.56	0.01	Not significant
Simon’s ratio	1.00	1.36	1.47	0.11	0.0016*
Crumley’s ratio	3.53	4.11	3.95	0.16	0.0011*

*

Statistically significant result.

Nasal tip projection was calculated by the Simon, Crumley, Powell, and Goode ratios, with the Simon and Crumley methods showing a statistically significant positional difference in tip projection. The Simon ratio was 1.36 for upright position and 1.47 for supine, with a positional difference of 0.11 (p = 0.0016), indicating a relative tip deprojection in the upright position. Crumley ratio was 4.11 for upright and 3.95 for supine, with a positional difference of 0.16 (p = 0.0011), again indicating a relative tip deprojection in the upright position. By contrast, the Powell and Goode methods, which do not incorporate the upper lip length in nasal tip projection calculation, did not show any statistically significant positional difference in their nasal tip projection. The Powell method had an upright ratio of 3.22 and a supine ratio of 3.19 (mean difference = 0.03, p > 0.05). The Goode method had an upright ratio of 0.55 and a supine ratio of 0.56 (mean difference = 0.01, p > 0.05).

Discussion

Anticipating and controlling the long-term shape and position of the contoured nasal tip is one of the most challenging aspects of rhinoplasty surgery. Didactic approaches to the understanding of nasal tip structure and support began with JR Anderson et al ⁵ description of the tripod theory in the 1960s. Fundamentally, this model defines a nasal tripod resting on the anterior nasal spine formed by the combined medial crura and the paired lateral crura; modifications to this pyramid by shortening or lengthening arms lead to predictable alterations of tip position. In the 1970s, Janeke and Wright ⁶ introduced the concept of fibrous and ligamentous connections that supported the nasal tip. Tardy and Zide ⁷ refined these findings in 1990 by classifying major and minor support mechanisms of the nasal tip and detailing consequences of their alteration on tripod stability.

Qualitatively, tip support can be evaluated by depressing the nasal lobule and gauging the forces that resists this deformation. ⁸ Many attempts have been made to quantify the biomechanical properties of the nasal tip. Tools have been used to measure reaction forces of the nasal tip to displacement before and after rhinoplasty. ⁹ Gassner et al ¹⁰ developed a rhinomanometric device to calculate nasal tip resilience in cadavers. This device, and others like it, typically only measure force application perpendicular to the facial plane and would not be able to capture the downward effects of gravity. Other groups have used computer modeling and finite element analysis (FEA) to model nasal tip mechanics and to estimate relative contributions of various nasal tip support mechanisms. ¹¹ Common surgical techniques to manipulate the nasal tip structure involve the tongue-in-groove suture technique, caudal septal extension grafts (CSEG), or columellar strut grafts.^{12 -14} In each of these, the technique strives not only to set the tip at a preferred position, but also to counteract post-operative ptotic-forces acting on the tip.

To ensure long-lasting success, the surgeon must create an aesthetically pleasing and functional result while anticipating resultant changes over time from various forces, including gravity. In their 2008 modification to the tripod theory, termed “the cantilevered spring model,” Westreich and Lawson ¹⁵ state that the post-operative nasal tip must produce an upward force vector that is equal and opposite in nature to the downward direction of gravity. Presumably, if this additional force is not properly accounted for, it could affect the final postoperative tip position. Currently, there is a need for more clearly defined data points quantifying the changes in nasal tip geometry secondary to patient positioning and the effects of gravity. In the current study, we sought to quantify the magnitude of these gravitational forces on tip rotation and projection.

For assessing positional changes in tip rotation in our study, the NLA, NFA, and CFA were utilized. In this study, the difference between the upright and supine NFAs was not statistically significant. This is unsurprising, given that this angle is primarily determined by the bony anatomy of the glabella, nasion, and nasal dorsum. The NFA is not a commonly utilized measure of tip rotation and is more often applied in the evaluation of dorsal aesthetics, and thus may not be as important in the assessment of nasal tip geometry. Therefore, we would not expect gravity to exert a statistically significant change on its measurement. Only one previous study performed by Kim et al ¹⁶ in 2017 assessed the topic of nasal tip geometry in supine and upright positions; they found only a statistically significant difference in NFAs, with it being greater in the upright position. This may be in part due to the difference in methods used to measure upright and supine NFAs, with upright measurements occurring using facial CT scans and supine measurements using lateral cephalograms, both of which were analyzed retrospectively and thus may not have been consistent in imaging methodology as patients underwent their respective studies.

On the other hand, NLA has historically been used as the main measure of nasal tip rotation. In our study, the mean upright NLA was 95.8°, as would be expected based on the published ideal ranges for male (93.4°–98.5°) and females (95.5°–100.1°). ¹⁷ The mean supine NLA was 99.0° indicating a significant rotation of the nasal tip when compared to the upright position. Our study confirmed that the difference between the upright and supine NLAs was statistically significant. This roughly 3.2° difference suggests that gravity’s downward force on the nose does indeed create a quantifiable change in tip rotation when the lip is included in the measurement. Kim et al ¹⁶ found no statistically significant difference in positional NLAs in their 2017 retrospective study, which may again be due to potential inconsistencies across imaging methodologies, thus affecting the angle measurements.

One plausible explanation for the significant difference in NLAs we found is due to the position change of the upper lip in upright and supine positions, which naturally would also affect the NLA. These positional changes would not affect measurements in which the lip wasn’t included, such as the NFAs, which our data supports as there was no significant difference in positional NFAs. This argument is further strengthened by the results when using CFAs to compare positional nasal tip geometry. CFAs were introduced by Kim and Egan ¹⁸ in 2006 as a method of nasal tip geometry measurement that didn’t rely on neighboring anatomic landmarks to approximate nasal tip rotation and thus excluded the upper lip. When we analyzed this angle, there was also no statistically significant difference in supine versus upright positional measurements. This further strengthens the argument that nasal tip positioning does not truly change with positioning but may only appear to do so relatively when the lip is included in measurements and can give the illusion of nasal tip positional changes. Given this potential illusion, it is important for surgeons to be aware of this effect prior to any surgical decision-making, and to ensure an adequate assessment of true nasal tip rotation, while keeping in mind the relative effects upper lip position may have on measurements.

The Powell and Humphreys, ¹ Goode, ² Simon, ³ Crumley and Lanser ⁴ methods were used to calculate nasal tip projection. The Simon and Crumley method showed statistically significant changes in ratio between the upright and supine positions, while the Powell and Goode methods did not. This discrepancy can be accounted for based on the methodology of these measurements. One possible explanation for the differential findings between methods is that the Simon and Crumley methods incorporate upper lip landmarks and length into their ratio measurement of nasal tip projection. On the other hand, the Powell and Goode methods utilize only nasal landmarks in their calculations. The upper lip length was noted to be increased in the upright position due to positional lengthening of the upper lip secondary to gravity. In the supine position, shortening of the upper lip length would result in an illusion of increased tip projection. This theory is bolstered by comparing the Crumley method to the Powell method, which did not show significant positional ratio differences. These two methods are the same formula, except that Crumley adds in upper lip length in the numerator of his equation. When excluding upper lip length, nasal tip projection does not appear to significantly differ between the upright and supine position.

In summary, our study quantifiably supports the principle that gravitational forces do not cause a true change in nasal tip position, but rather exert an effect on nearby structures such as the upper lip, leading to significant changes in certain tip projection and rotation formulas. When using the NLA, tip rotation was significantly decreased by 3.2° in the upright position, but true tip projection was unchanged. Contrarily, when angles were used independent of the upper lip such as the NFA and CFA, there was no significant difference in nasal tip rotation positionally. This is wholly in support of the argument that upper lip changes in the upright and supine position can affect apparent nasal tip rotation but do not actually affect true nasal tip rotation. This potential illusion is important to keep in mind when using NLAs and CFAs to measure tip rotation and in intraoperative decision-making (when the patient is in a supine position) as inaccurate assessment of true versus relative tip rotation may result in undesirable outcomes.

Surgeons should be aware that gravity relatively enacts measurable changes on the nose, especially between the upright pre-/post-operative and supine intraoperative positioning of patients. Our study reinforces the importance of popular tip manipulation methods (tongue-in-groove suture, columellar strut, and the CSEG) in providing the rhinoplasty surgeon more control over difficult and unpredictable surgeries. With these maneuvers, surgeons can fix the nasal tip at the preferred position and counteract unseen forces, such as gravity, in the operating room. If a surgeon disturbs tip support mechanisms and does not use one of these tip manipulation methods during a case, the surgeon must account for gravitational forces causing relative tip derotation when upright.

The study’s limitations encompassed the absence of measurements or assessments of skin thickness and ethnicity. These variables should be more thoroughly examined in future studies, particularly ones involving a larger sample size, to provide a more comprehensive understanding. Additionally, septal support was not assessed as well as the potential impact of age on nasal tip geometry. For future directions, we hope to continue studying the effects of gravity on nasal geometry while stratifying for age, gender, and skin thickness, among other characteristics. We also plan to study the pre-and post-operative effects of gravity on nasal geometry stratified for different rhinoplasty techniques.

Conclusion

In this study, we found that the true nasal tip geometry is not affected by gravity but can be relatively affected when using neighboring landmarks such as the upper lip to measure tip rotation. When using NLA, gravity causes a relative 3.2° derotation on the nasal tip in the upright position but does not appear to have an effect on nasal tip projection. This derotation is not evident when using CFA or NFA, which may be a more accurate measurement of true tip rotation. While the derotation can be counteracted by nasal tip-setting maneuvers, not all surgeons utilize these techniques in their practice and therefore must be aware of the gravitational effects on the nasal tip post-operatively.