Letter to editor: Proposal of a new indicator of hip compensation for spinopelvic–hip mismatch: a retrospective study in Japan

Aanchal Sharma; Reema Rasotra

doi:10.31616/asj.2025.0502.r1

To the Editor,

We read with interest the study by Fujita et al. [1] proposing pelvic femoral angle (PFA)–(pelvic incidence [PI]–lumbar lordosis [LL]) as a novel, radiograph-based indicator of hip compensation in spinopelvic mismatch. This is a practical contribution, especially in settings without EOS imaging. However, some points, guided by STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) recommendations [2], deserve consideration.

Study population and generalizability

The cohort (mean age, approximately 62 years; 84% female; osteoporosis outpatient) may not reflect sagittal mechanics in younger or more active individuals. Age and posture changes significantly influence lumbar alignment and mobility [3].

Model validation

Predicted sagittal vertical axis=663–3.43×[PFA–(PI–LL)] was the formula that was developed and tested using the same dataset. In the absence of external validation or internal split-sample, predictive performance could be exaggerated.

Measurement scope and confounding

Key variables such as thoracic kyphosis, pelvic tilt, body mass index, gait, and muscle strength were omitted despite their influence on sagittal alignment [4].

Tables and statistical reporting

Table 1: 95% confidence intervals (CIs) are absent from intraclass correlation coefficients. Table 2: Several uncorrected comparisons without effect sizes and with different group sizes. Correlations provided without confounder adjustment or CIs are shown in Tables 3 and 5. Table 4: R2 provided without diagnostics or regression coefficients.

The imaging and thresholds

Although intriguing, the cut-offs (166° and 182°) need prospective validity. Knee and ankle compensations were not included; only femoral inclination was measured [5]. Without repeatability checks, standard lateral radiographs can introduce positional inaccuracy [6].

In conclusion, PFA–(PI–LL) shows promise as an economical instrument. Before routine implementation, multicenter prospective trials with verified criteria and enhanced statistical clarity are required.