Letter to the Editor Regarding “Normative Relationships Between Spinopelvic Alignment and Femoroacetabular Orientation: A Cross-Sectional Study”

Dear Editor,

We read with great interest the study by Ludwig et al, which presents normative data delineating associations between spinopelvic and femoroacetabular alignment in a large cohort of healthy young adults. ¹ This work is a valuable addition to the limited body of evidence characterizing pre-degenerative morphometrics that are relevant for arthroplasty and spinal realignment planning. The use of upright full-body EOS imaging, robust exclusion criteria, and a relatively homogenous cohort provides a strong foundation for normative modeling. However, several key methodological and conceptual issues merit further investigation.

First, the primary models for acetabular anteversion (AA) and inclination (AI) revealed adjusted R² values of 0.381 and 0.094, respectively, suggesting that spinopelvic and anthropometric variables only partially accounted for variability in acetabular alignment. While the authors correctly noted this as residual variability, the absence of pelvic rotation or transverse plane metrics, particularly axial pelvic tilt, limits mechanistic interpretability. Given that AA is a three-dimensional parameter sensitive to both coronal and axial pelvic orientations, ² its projection in the patient’s vertical plane may oversimplify this complexity. Furthermore, excluding anterior pelvic plane (APP) analyses from the primary model, despite their relevance to surgical practice, limits translational utility. ³

Second, the study’s linear modeling approach does not account for potential non-linear interactions, which are plausible given the biomechanical interdependence of lumbar lordosis, pelvic tilt, and acetabular orientation. ⁴ Interaction terms (eg, PI × PT or LL × sex) were neither explored nor tested. This may have led to an underestimation of the true explanatory power of spinopelvic dynamics, especially in females, who exhibited statistically significant differences in AA, NSA, and LL. Moreover, considering that multiple morphotypes may represent distinct mechanistic pathways, the pooled regression may obscure subgroup-specific associations.

Third, while the exclusion of participants with an ODI >10 minimizes symptomatic bias, it may have inadvertently selected individuals with lower physical activity levels or altered postural habits not captured in static EOS imaging. The study did not report the participants’ occupation, physical activity level, or dynamic pelvic range, all of which may influence sagittal balance and acetabular coverage. The static, single-posture imaging paradigm fails to capture functional variability in pelvic tilt and hip positioning, which is particularly relevant for preoperative simulation and prosthetic orientation.

Finally, the study identified a novel inverse relationship between weight and AA, yet did not include body fat distribution or waist-to-hip ratio, which are factors known to influence pelvic position and acetabular load vectors. ⁵ Without adjusting for central adiposity, the observed associations may reflect proxy confounding rather than direct biomechanical relationships. Similarly, self-reported ethnicity was not analyzed despite known racial differences in acetabular morphology.

In summary, while Ludwig et al contributed valuable normative data, future studies should incorporate three-dimensional dynamic assessments, consider nonlinear modeling strategies, and stratify analyses by morphotype, sex, and body composition to better characterize the multivariate determinants of femoroacetabular alignment.