Calf circumference assessment in osteoporotic vertebral fractures: screening utility versus prognostic validity

To the editor

We read with great interest the article by Lu et al. published in Aging Clinical and Experimental Research, comparing calf circumference (CC) with chest-CT–derived T12 skeletal muscle index (SMI) for the assessment of osteosarcopenia and prediction of one-year outcomes in patients with osteoporotic vertebral fractures (OVFs) [1]. The authors are to be commended for addressing an important clinical problem in spine practice, namely how to operationalize sarcopenia assessment in fracture settings where advanced body-composition techniques may be variably available.

While the authors appropriately describe CC as a screening tool and acknowledge the superior prognostic performance of CT-derived SMI, we believe that the study raises a broader measurement-science issue that warrants explicit clarification. Specifically, the comparison between CC and CT-derived muscle indices is largely framed around diagnostic concordance, whereas the results themselves demonstrate that these measures represent distinct clinical constructs with different intended purposes.

Diagnostic concordance versus outcome validity

A central methodological issue concerns the criterion used to judge performance. Calf circumference, as defined in the Asian Working Group for Sarcopenia (AWGS) 2019 framework, is primarily intended for case-finding, optimizing sensitivity to identify individuals who warrant further assessment, rather than to serve as a direct surrogate for imaging-based muscle quantity [2]. In contrast, CT-derived SMI is a quantitative anatomical measure designed to capture muscle mass with sufficient precision to support prognostic inference [3].

In this context, the authors’ finding that CC-defined osteosarcopenia does not independently predict refracture, while CT-defined osteosarcopenia does, should be interpreted not as a limitation of CC, but as confirmation of its construct validity as a screening instrument rather than a prognostic marker. Evaluating CC primarily by its concordance with CT-derived indices risks applying an inappropriate success criterion to a tool designed for a different measurement purpose. From a measurement-science perspective, the more appropriate evaluation framework would anchor CC performance to clinically meaningful outcomes, rather than to anatomical agreement with imaging-based reference standards.

Adiposity as a modifying variable, not an exclusion criterion

The authors’ identification of abdominal circumference (AC) as the principal driver of discordance between CC- and CT-based definitions is an important observation [1]. This finding is biologically plausible, as CC captures both muscle and subcutaneous fat, whereas CT-derived SMI isolates contractile tissue. However, the proposed solution of excluding patients with markedly elevated AC introduces a secondary methodological concern. Exclusion addresses misclassification at the cost of external validity, particularly in aging spine populations where central adiposity is common and clinically relevant.

From a construct-validity standpoint, adiposity should be treated as a modifying variable that alters the meaning of CC, rather than as a reason to remove patients from analysis. Emerging sarcopenic obesity frameworks increasingly emphasize combined anthropometric indices, such as waist-to-calf or AC/CC ratios, which better reflect muscle–fat imbalance while preserving feasibility [4].

Implications for outcome-oriented spine care

Taken together, these considerations support a two-stage assessment paradigm in OVF care. Calf circumference can serve as an efficient first-line screening measure in acute fracture settings, whereas CT-derived muscle assessment—often already available from routine imaging—should be preferred when the objective is risk stratification, prognostication, and therapeutic decision-making[5].

As an illustrative next step, future analyses could examine whether AC-stratified CC thresholds or simple waist-to-calf indices improve refracture prediction compared with CC alone, using the same outcome data already collected in similar cohorts.

Conclusion

In summary, Lu et al. provide important evidence that CT-derived muscle quantity possesses superior outcome validity for refracture prediction in OVF patients, while CC retains value primarily as a screening tool [1]. The key methodological implication is not that CC performs inadequately, but that it should not be evaluated or applied using the same criteria as imaging-based muscle measures.

We suggest that future studies explicitly distinguish screening validity from prognostic validity, and evaluate CC within outcome-anchored frameworks that treat adiposity as a modifying variable rather than an exclusion criterion. Such an approach would align sarcopenia assessment with measurement-science principles and support more precise, outcome-driven application in spine practice.

Author contributions

Both authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Audai Abudayeh and Iakiv Fishchenko. The first draft of the manuscript was written by Audai Abudayeh and co-author commented on previous versions of the manuscript. Both authors read and approved the final manuscript.

Funding statement

The authors did not receive support from any organization for the submitted work.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.

Ethical approval and informed consent

Not applicable.