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REVIEW 5 major objections 5 minor 47 references

Analysis of in-vivo skin anisotropy using elastic wave measurements and Bayesian modelling

T0 review · 5 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read This paper reports that in-vivo skin anisotropy increases logarithmically with age and that stiffness along Langer lines rises with age, based on elastic-wave measurements from 78 volunteers aged 3 to 93, and proposes ellipse eccentricity…

desk verdict A useful new anisotropy metric and a plausible age trend, but the functional-form claims and unquantified ellipse-fit failures need referee attention before the headline holds up. read the letter →

arxiv 2506.02248 v1 pith:AA4MAKQA submitted 2025-06-02 physics.bio-ph cond-mat.soft

Matt Nagle , Susan Price , Antonia Trotta , Michel Destrade , Michael Fop , Aisling Ní Annaidh This is my paper
classification physics.bio-phcond-mat.soft
keywords skinanisotropyLangerlinesin-vivotensionReviscometerRayleighsurfacewaveBayesianmodellingellipseeccentricityageing
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper uses elastic surface waves to measure how human skin's direction-dependent mechanics change across life, with data from 78 volunteers spanning ages 3 to 93. The central claim is that skin anisotropy, quantified by the eccentricity of an ellipse fitted to arrival-time measurements at 36 angular positions, increases logarithmically with age, with the steepest rise from childhood into adulthood. It also claims that stiffness along the skin's tension lines (Langer lines) increases with age, while average all-direction stiffness shows no significant age trend. The authors introduce ellipse eccentricity as a more outlier-resistant alternative to the classic max/min anisotropic ratio, supporting this with a simulation study. If correct, the results would give surgeons and cosmetic practitioners a quantitative, age-dependent description of skin anisotropy instead of relying on generic maps.

What carries the argument

The load-bearing object is an ellipse fitted by direct least squares to polar-plot RRT data measured at 10-degree increments around the skin site. From the fitted ellipse, the semi-major axis a and semi-minor axis b give the eccentricity e = $\sqrt$(1 - $b^{2}$/$a^{2}$) as the anisotropy measure, the area A = pi*a*b as the average all-direction arrival time, and the semi-minor axis as the arrival time along the fastest direction, i.e. the Langer line. The conversion from arrival time to stiffness uses the Rayleigh-wave relation E = rho*$v^{2}$*(3.284). The statistical machinery is a Bayesian multivariate outcome regression on log eccentricity, area/1000, and semi-minor axis, with a correlated error term and Markov chain Monte Carlo inference, which accounts for the dependence among the ellipse-derived outcomes; a projected-normal circular regression tests whether stretching changes the ellipse angle.

What would settle it

Compute a per-subject ellipse goodness-of-fit measure, such as residual standard deviation or an asymmetry index, regress it on age, and re-run the Bayesian model excluding poor fits; if the 0.95 HPDI for the age coefficient on log eccentricity still excludes zero after exclusion, the age trend survives, but if the excluded cases cluster in children or the elderly, the reported logarithmic increase could be an artifact of fit quality.

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Extended reading notes

Core claim

The discovery is that in-vivo human skin anisotropy, measured as the eccentricity of an ellipse fitted to angular Reviscometer arrival-time data, increases logarithmically with age: the posterior mean age coefficient on log eccentricity is 0.007 with 95% HPDI [0.005, 0.009]. In the same multivariate model, stiffness in the Langer-line direction, represented by the ellipse's semi-minor axis, increases with age (coefficient -0.391, HPDI [-0.696, -0.085]), while the ellipse area, representing average stiffness across directions, does not show a significant age effect. Gender does not significantly change anisotropy, but males show significantly stiffer skin both on average and along Langer lines. Applied stretch in the Langer-line direction significantly increases eccentricity and decreases the semi-minor axis while conserving the ellipse tilt angle, evidence that the level of skin tension strongly affects both measurements. The paper further claims that the eccentricity measure is more robust than the anisotropic ratio, because in simulations the ratio drifts upward and develops extreme outliers under noise while the eccentricity remains near its true value.

Load-bearing premise

The entire age trend rests on the assumption that an ellipse adequately fits every subject's angular arrival-time data, so that its eccentricity truly summarizes the anisotropy; the authors note that some subjects' raw data lacked the required symmetry and produced poor ellipse fits, but they do not quantify how many or how those cases were handled.

Editorial extensions

If this is right

  • Surgical incision orientation and wound-closure planning could be tailored by age, since older skin is more direction-dependent and therefore more sensitive to the orientation of cuts.
  • Elastic-wave measurements may become a practical way to infer in-vivo skin tension, because an applied stretch changes eccentricity and semi-minor axis in a consistent direction while conserving the Langer-line angle.
  • Universal cosmetic surgery practices applied to very young or elderly patients are called into question, since the level of anisotropy varies strongly across the lifespan.
  • The eccentricity metric, being less sensitive to outliers than the anisotropic ratio, could replace the ratio in future angular RRT studies.
  • The results imply that directional stiffening and overall stiffness are partly decoupled: with age, stiffness rises only along Langer lines, not on average across directions.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The authors do not test whether the age trend generalizes beyond the volar forearm site; a natural extension would be to repeat the protocol on additional body locations and see whether the logarithmic anisotropy trajectory is site-specific.
  • The ellipse-fit assumption is the main unresolved risk; a systematic check of per-subject fit residuals by age group, not reported in the paper, would show whether the steep childhood-to-adulthood rise is partly an artifact of poorer fits in younger subjects.
  • The calibration relation E = rho*v^2*(3.284) comes from linear-elastic, isotropic, unstressed materials; extending the wave-speed model to include pre-tension could convert the observed stretch effect into an actual numerical estimate of in-vivo skin tension, which the paper stops short of doing.
  • The finding that gender affects stiffness but not anisotropy suggests that overall connective-tissue stiffness and directional organization are governed by at least partly separate biological mechanisms, so interventions targeting one may not alter the other.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

5 major / 5 minor

Summary. This manuscript proposes using the eccentricity of a least-squares ellipse fitted to angular Reviscometer Resonance Running Time (RRT) measurements as a measure of in-vivo skin anisotropy, compares this metric with the classic anisotropy ratio through simulations, and then fits a Bayesian multivariate regression of log eccentricity, ellipse area, and semi-minor axis on age, gender, and stretch configuration for 78 subjects aged 3–93. The authors report that anisotropy increases with age, that stiffness increases along Langer lines with age, that gender does not affect anisotropy but affects stiffness, and that applied stretch changes both anisotropy and stiffness while preserving the direction of Langer lines.

Significance. The paper has two potentially useful contributions. First, the eccentricity metric is simple, and the simulation study (10,000 replicates per condition) provides credible evidence that it is more robust to noise and outliers than the max/min anisotropy ratio; this is a practical contribution for skin-mechanics measurement. Second, the dataset spans a wider age range than most comparable studies, and the code is publicly available, which makes the results reproducible. If the statistical issues listed below are fixed, the study could be a useful reference for age-related skin anisotropy; the Bayesian framing and explicit reporting of posterior intervals are strengths.

major comments (5)
  1. [Section 3.2, Eq. (5)] The statement that "the eccentricity increases logarithmically with age" is the opposite of what Eq. (5) implies. With log(Eccentricity) as the outcome and Age as a linear predictor, the model is log(e) = a + 0.007·Age + ..., so e increases exponentially with age (e ∝ exp(0.007·Age)), not logarithmically. The interpretation in the abstract, Section 3.2, and the Discussion should be corrected; if the authors intend to claim a logarithmic dependence, they should instead fit eccentricity against log(Age). The statement that the steepest increase occurs "from childhood into adulthood" also does not follow from the fitted model on the untransformed scale.
  2. [Section 3.2, Table 3] The claim that "the skin stiffness increases linearly with age" in the Langer-line direction is not supported by the fitted model. The outcome is the semi-minor axis length in RRT units (an arrival time), not stiffness. Combining Eq. (1) with the linear model gives E ∝ (semi-minor axis)⁻² under the authors' calibration, which is not a linear function of age. The manuscript should either model stiffness directly after converting RRT to wave speed, or explicitly state that the linear relation is for arrival time / semi-minor axis, not for stiffness.
  3. [Section 4, Section 2.2] The Discussion concedes that "the raw measurements from some subjects did not exhibit this symmetry and thus the ellipse fit was poor," yet the paper reports no count of such subjects, no measure of per-subject fit quality, and no analysis of whether poor fits are related to age. Because eccentricity is the outcome variable in Eq. (5) and is treated as error-free, poor ellipse fits introduce unmodeled measurement error that can bias the age coefficient. A sensitivity analysis that quantifies per-subject fit residuals, tests their correlation with age, and re-estimates the model after excluding poor fits is needed before the headline age trend can be accepted.
  4. [Section 2.4, Eq. (5)] Each subject contributes two rows (natural and stretched configurations), but the model has no subject-level random effect and therefore treats these repeated measurements as independent. This ignores within-subject correlation and can make the reported 0.95 HPDIs too narrow. The model should include a subject-level random intercept or otherwise account for the repeated-measures structure.
  5. [Section 2.1, Table 2, Section 1] The sample-size reporting is internally inconsistent: the Introduction states 72 subjects, Section 2.1 states 78 subjects (37 female, 41 male), and Table 2, as printed, does not sum to 78 and contains rows where the number of females exceeds the total (e.g., "11-20 3 10"). The age range also varies between the abstract (3–93) and Section 2.1 (3–92). These discrepancies must be corrected and the demographic table verified, as the "sizeable dataset" claim depends on this information.
minor comments (5)
  1. [Section 2.4] Specify the actual prior distributions used in rstanarm rather than only "default non-informative priors," and report MCMC diagnostics such as R-hat and effective sample size.
  2. [Table 1] Table 1 has formatting and unit issues: ρ is presumably in kg/m³, not kg/m², and the "RRT Conversion" values should be defined more clearly.
  3. [Figure 7] Figure 7 reports R² = 0.5105 from a simple log-linear model, but the Bayesian regression in Eq. (5) includes additional covariates; the relationship between this figure and the main model should be clarified.
  4. [Section 3.2] The gender coefficient is described with female as the baseline, but the coding of the gender covariate is not stated explicitly; it should be defined in the model description.
  5. [Section 2.2] The phrase "axisymmetric measurements" in Section 4 may be misleading; the data are expected to be symmetric under 180° rotation, not axisymmetric in the strict sense. Consider using "two-fold rotational symmetry" or similar.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the age-anisotropy finding is inferred from subject data with an external calibration constant; the acknowledged ellipse assumption is a stated modeling premise, not a concealed input.

full rationale

The derivation chain is self-contained with respect to its empirical claims. The central result (posterior mean age coefficient 0.007 on log-eccentricity, and -0.391 on semi-minor axis RRT; Section 3.2, Table 3) comes from fitting the multivariate regression in Eq. (5) directly to per-subject eccentricity, area, and semi-minor-axis measurements derived from Reviscometer data; no fitted parameter from that regression is recycled as a prediction. The stiffness calibration in Eq. (1) uses Young moduli measured by tensile tests on three elastomers (Table 1), an external input independent of the in-vivo age regressions. The simulation study (Section 2.3) is an in-sample performance comparison: data are generated from ellipses and the proposed eccentricity metric is benchmarked against that same parametric family, so it is a self-consistency check rather than independent validation of the ellipse assumption; however, the paper states this assumption explicitly ('Assuming an ellipse is an appropriate model for our data', Section 2.2) and concedes the limitation that 'the raw measurements from some subjects did not exhibit this symmetry and thus the ellipse fit was poor' (Section 4). This is a transparent modeling premise and robustness concern, not circularity. The only overlapping-author citation used in the reasoning (Deroy et al. 2017 for associating the fast-wave direction with Langer lines) is not load-bearing for the magnitude/anisotropy-age claim, since eccentricity is direction-independent. Separate correctness risks exist but are not circular: the phrase 'eccentricity increases logarithmically with age' inverts the functional form implied by Eq. (5), where log(e) is linear in age so e grows exponentially in age; and the number/systematic age-dependence of poor ellipse fits is unquantified. These are interpretation and robustness issues, not self-referential derivations.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

The central claims rest on five main assumptions: the validity of the wave-speed-to-stiffness conversion for skin, the ellipse model for angular data, the identification of Langer lines with the fastest wave direction, the independence assumption in the regression (which ignores repeated measures), and the chosen linear/log-linear functional forms. One calibration parameter (0.284 microseconds) is fitted to elastomer data and affects absolute stiffness values.

free parameters (1)
  • RRT-to-time conversion factor = 0.284 microseconds per RRT unit
    Average of three elastomer calibrations (0.249, 0.282, 0.322 microseconds); used to convert Reviscometer RRT readings to wave speed and stiffness via E = rho * v^2 * 3.284. Affects absolute stiffness values, not relative comparisons.
assumptions (5)
  • domain assumption RRT readings are proportional to Rayleigh wave travel time, and E = rho * v^2 * 3.284 applies to in vivo skin
    The calibration relation assumes an unstressed, isotropic, linear elastic material. The paper acknowledges this is not strictly valid for pre-tensioned skin (Section 4).
  • domain assumption Angular RRT data are adequately modeled by an ellipse
    All anisotropy estimates derive from the ellipse fit. The paper notes some subjects' data lacked the assumed symmetry and produced poor fits (Section 4).
  • domain assumption The fastest wave direction corresponds to the Langer line
    Used to define the stretch axis and to interpret the semi-minor axis as stiffness along Langer lines (Section 2.2).
  • domain assumption Natural and stretched measurements are independent conditional on covariates
    Equation (5) contains no subject-level random effects, so within-subject correlation is ignored.
  • ad hoc to paper Age effects are linear on the log-eccentricity scale and on the semi-minor axis
    These functional forms are chosen for tractability and are not compared to alternatives; they directly produce the logarithmic and linear conclusions.

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Cite this review

Pith. "Pith review of Analysis of in-vivo skin anisotropy using elastic wave measurements and Bayesian modelling." pith.science (2026). https://pith.science/paper/AA4MAKQA

@misc{pith2026250602248,
  author       = {Pith},
  title        = {Pith review of: Analysis of in-vivo skin anisotropy using elastic wave measurements and Bayesian modelling},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/AA4MAKQA}},
  note         = {Machine review of arXiv:2506.02248}
}
read the original abstract

In vivo skin exhibits viscoelastic, hyper-elastic and non-linear characteristics. It is under a constant non-equibiaxial tension in its natural configuration and is reinforced with oriented collagen fibers, giving rise to anisotropic behaviour. Understanding the complex mechanical behaviour of skin has relevance across many sectors including pharmaceuticals, cosmetics and surgery. However, there is a dearth of quality data characterizing human skin anisotropy in vivo. The available data is usually confined to limited population groups and/or limited angular resolution. Here, we use elastic waves travelling through the skin to obtain measurements from 78 volunteers from 3 to 93 years old. Using a Bayesian framework, we analyse the effect that age, gender and level of skin tension have on the skin anisotropy and stiffness. First, we propose a new measurement of anisotropy based on the eccentricity of angular data and conclude that it is a more robust measurement compared to the classic ``anisotropic ratio". We then find that in vivo skin anisotropy increases logarithmically with age, while the skin stiffness increases linearly along the direction of Langer Lines. We also conclude that gender does not significantly affect the skin anisotropy level, but does affect the overall stiffness, with males having stiffer skin on average. Finally, we find that skin tension significantly affects both the anisotropy and stiffness measurements, indicating that elastic wave measurements have promising applications in determining in vivo skin tension. In contrast to earlier studies, these results represent a comprehensive assessment of the variation of skin anisotropy with age and gender using a sizeable dataset and robust modern statistical analysis. This data has implications for the planning of surgical procedures and the adoption of universal cosmetic surgery practices for young or elderly patients.

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