Gluteal morphology: what MRI and anthropometry actually document

The buttock is one of the most discussed and least carefully measured regions in popular anatomy writing. Almost everything circulated about it treats the trait as a single quantity, more or less of one substance, and assumes the substance is fat. A 2022 imaging study out of the University of Chile is useful precisely because it tested that assumption directly, and found it wrong.

This article covers what the clinical literature documents about gluteal anatomy, what the 2022 study adds, and how both map onto the seven dimensions recorded in the Butt section of the phenotype atlas. As with every trait page, the atlas records modal tendencies for populations and not predictions about individuals. See reading the phenotype catalog.

The study

Danilla and colleagues at the Hospital Clinico Universidad de Chile ran a case-control comparison between women working as buttocks models, taken as the aesthetic reference group, and women with what the paper calls normal buttocks. Rather than relying on photographs alone, which is the usual method in this literature, they combined three instruments: magnetic resonance imaging, direct anthropometric measurement, and photography. MRI is what makes the study worth reading, because it separates structures that a photograph fuses into one silhouette. Bone, muscle, and subcutaneous fat all contribute to the outline, and only imaging can say how much each one contributed.

The finding that matters: the fat was the same

The headline result is a negative one. The amount of subcutaneous fat was equal in the models and the controls.

What differed was everything underneath it. Compared with controls, the models had a narrower waist, a narrower iliac crest, wider hips, a wider C point, and a larger and thicker gluteus maximus. The study attributes the projection of the buttock to that muscle directly: a bigger gluteus maximus adds more projection at the C point, the landmark of maximum projection used in the gluteal aesthetic literature. The paper places that point of maximum projection in lateral view 2.7 cm above the pubic bone.

So the difference between the two groups was skeletal frame and muscle bulk, not fat volume. The paper's own conclusion is a four-component model: an aesthetically ideal buttock is the conjunction of adequate bony shape, muscle development, a subcutaneous fat layer, and tight skin. Read carefully, fat is necessary to the result without being the variable that separated the groups. That is a more interesting claim than either "it is all fat" or "fat does not matter," and it is the one the data supports.

The practical implication is unflattering to a large industry. Fat grafting adds the one component that was already equivalent between the groups, and cannot change iliac crest width, waist width, or muscle mass. That is consistent with the common criticism that volume added without regard to the underlying frame reads as bulk rather than shape.

Why a photograph cannot tell you this

This is the point where the study bears on how the atlas is built, and it is worth being explicit about the limitation rather than quiet about it.

Every one of the seven dimensions in the Butt taxonomy is photo-assessable by design. Buttock shape on the Mendieta four-class system, relative size, projection in lateral view, gluteal fold definition, lateral hip concavity, the buttock-to-hip silhouette, and left-right asymmetry are all read from the outline. That makes them observable at scale, reproducible between observers, and applicable to ordinary photographs, which is exactly why the taxonomy was drawn that way.

It also means the taxonomy describes the result and is structurally blind to the cause. Two women can be recorded identically as round shape, high projection, hourglass silhouette while arriving there by different routes: one through a wide bony frame and a large gluteus maximus, the other through fat distribution over a narrow frame. The silhouette is the same. The anatomy is not. No photo-based scale can separate them, and the atlas should not be read as claiming otherwise.

This is the same caveat that governs the breast section, where volume, density, and glandular-to-adipose ratio are independent axes that a photograph collapses into one impression. Photo-assessable scales are the right tool for population-scale description and the wrong tool for causal claims about tissue.

The hip dip has a skeletal explanation

One atlas dimension gets direct support from the study. Lateral hip concavity, catalogued in the taxonomy under its clinical name, the trochanteric depression, and known popularly as the hip dip or violin hip, is the visible depression between the iliac crest and the greater trochanter. The atlas records it as a common skeletal-anatomy variant.

The imaging results back that framing. If iliac crest width and gluteus maximus bulk are the measurements that differentiate the reference group, then the depression between the crest and the trochanter is governed by the distance between those two bony landmarks and by how much muscle sits across the gap. It is a consequence of skeletal proportions, and it is not a fat deposit that can be reduced or a deficiency that can be trained away. Readers arriving at the atlas looking for the hip dip are usually asking whether it can be changed. The honest answer, and the one the anatomy supports, is that its presence is largely structural.

What the study does not establish

Two limits matter, and the second one matters more on this site than anywhere else.

The methodological limit is ordinary for the genre. The paper carries Level of Evidence III, the rating this journal assigns to case-control designs. The published abstract does not state the sample size and the full text sits behind a paywall, so the precision of the reported differences cannot be assessed from the public record. Studies in gluteal aesthetics typically run small, single-center samples, and this one should be read with that expectation until the full text is available.

The deeper limit is in the design itself. The case group was defined as buttocks models, which means the aesthetic judgment was made before any measurement was taken. The study therefore measures the anatomy of a professional aesthetic convention, at one center, in one country, at one time. It is a description of what a particular beauty standard is made of, not evidence that the standard is universal, biologically privileged, or a target anyone should adopt. The title asks what makes a beautiful buttock beautiful and the design can only answer what the women already called beautiful have in common.

That distinction is load-bearing for a phenotype catalog. The atlas documents variation across populations. It does not rank it, and a clinical ideal derived from one reference group is not a scale that other populations can be scored against. Anyone treating the study's numbers as a global standard is using it for something its design cannot support.

How the atlas applies gluteal data

The Butt section records seven qualitative dimensions per group, drawn from the gluteal aesthetic surgery literature, principally Cuenca-Guerra, Mendieta, and Roberts. They are categorical and ordinal rather than metric, which is a deliberate choice: the published population-level data for this region is far thinner than for stature or body composition, and assigning numeric modal values where no random-sample measurement exists would manufacture precision the sources do not have.

Coverage is also uneven for a mundane reason worth stating plainly. The catalog's portrait corpus is overwhelmingly head-and-shoulders photography, which is what documentary and biographical sources produce. Gluteal dimensions are among the least populated in the atlas as a result, and pages carry the trait's taxonomy and general description rather than a group-specific modal value in most cases. That is a limit of the available imagery, not a judgment about the groups, and it is recorded rather than papered over. The same constraint applies across the below-the-neck categories generally.

For readers working on anatomically plausible rendering, the study is more useful than any silhouette scale. It names the structures that produce the shape: the waist-to-iliac-crest relationship, hip width, the height of maximum projection relative to the pubic bone, and gluteus maximus bulk. Generators that treat the region as a single volume control reliably produce a shape without a frame, which is what makes the output read as wrong even when no individual element is obviously off.

References

• Danilla S, Troncoso E, Jara R, Dominguez C, Albornoz C, Erazo C, Sepulveda S, Nielsen J, Serra S, Yamada N. What Makes a Beautiful Buttock Beautiful? A Case-Control Study Comparing Buttocks Models versus Normal Women by Magnetic Resonance Imaging, Photography and Anthropometry. Aesthetic Plastic Surgery 47(5):1896-1904, 2023. doi:10.1007/s00266-022-03222-w. PMID 36542093.
• Mendieta CG. Classification system for gluteal evaluation. Clinics in Plastic Surgery 33(3):333-346, 2006. PMID 16818092.
• Cuenca-Guerra R, Quezada J. What makes buttocks beautiful? A review and classification of the determinants of gluteal beauty and the surgical techniques to achieve them. Aesthetic Plastic Surgery 28(5):340-347, 2004.
• Roberts TL 3rd, Weinfeld AB, Bruner TW, Nguyen K. "Universal" and ethnic ideals of beautiful buttocks are best obtained by autologous micro fat grafting and liposuction. Clinics in Plastic Surgery 33(3):371-394, 2006. PMID 16818095.

Where to start

• The Butt section of the phenotype atlas has the full seven-dimension taxonomy with valid values and sources.
• For the neighboring regions that share a skeletal frame, see body shape and weight.
• For how the catalog handles scales in general, and which ones are clinical versus descriptive, see peer-reviewed classification scales.