Defining sagittal imbalance in axial spondyloarthritis: considerations beyond C7 sagittal vertical axis

To the Editor,

Sagittal alignment is a fundamental determinant of spinal biomechanics and patient-reported outcomes in inflammatory and deformity-related spinal disorders. In this context, the study by de Oliveira et al. provides valuable EOS-based insight into sagittal balance and compensatory mechanisms in radiographic axial spondyloarthritis (axSpA) [1]. The authors report that sagittal imbalance—defined as a C7 sagittal vertical axis (SVA) ≥ 50 mm—was independently associated with spinal mobility (BASMI), but not with functional status (BASFI) or quality of life (ASQoL). While methodologically rigorous, this conclusion deserves careful interpretation in light of how sagittal imbalance was defined and measured. A central methodological consideration is the reliance on C7 SVA as a sole, dichotomous marker of sagittal imbalance in EOS imaging acquired with the upper limbs supported at iliac crest height. Arm position during standing radiography has been shown to induce measurable changes in thoracic alignment and whole-spine posture, including global sagittal parameters such as SVA. Recent EOS-based data confirm that arm elevation alone can modify spinal alignment independent of intrinsic deformity [2]. When a posture-sensitive variable is dichotomized at a fixed threshold, misclassification near the cutoff becomes likely, with a well-recognized tendency to attenuate associations with clinical outcomes. This issue is particularly relevant in axSpA, where sagittal alignment reflects a complex interaction between spinal ankylosis, pelvic retroversion, hip extension capacity, and whole-body compensation. The 50-mm SVA threshold originates largely from adult spinal deformity frameworks, in which SVA is not interpreted in isolation but alongside pelvic parameters such as pelvic tilt and PI–LL mismatch. Global angular measures such as the T1 pelvic angle (TPA) were specifically developed to overcome the posture- and compensation-dependence of linear measures like SVA and have demonstrated robust associations with health-related quality of life [3]. Importantly, several high-quality ankylosing spondylitis cohorts with thoracolumbar kyphosis report that global sagittal alignment parameters—including SVA-related and angular measures—correlate with functional impairment and quality of life, both pre- and post-operatively [4, 5]. These data support the broader concept that sagittal malalignment is clinically meaningful when comprehensively assessed. Accordingly, the absence of an independent association between SVA-defined imbalance and BASFI or ASQoL in the present study should be interpreted as conditional on the chosen exposure definition and acquisition posture, rather than as evidence that sagittal alignment lacks relevance to patient experience in axSpA. Incorporating additional sagittal modifiers and sensitivity analyses may further refine this important line of investigation.

Author contributions

AA. and IF wrote the main manuscript text and revised the full text. Both authors read and approved the final manuscript.

Funding

The authors received no financial support for the research, authorship, and publication of this letter.

Data availability

No datasets were generated or analysed during the current study.

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Competing interests

The authors declare no competing interests.