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

Article information

Asian Spine J. 2025;19(5):867-868

Publication date (electronic) : 2025 October 28

doi : https://doi.org/10.31616/asj.2025.0502.r2

Ryo Fujita, Kohei Takahashi, Toshimi Aizawa

Department of Orthopaedic Surgery, Tohoku University Graduate School of Medicine, Sendai, Japan

Corresponding author: Ryo Fujita Department of Orthopaedic Surgery, Tohoku University Graduate School of Medicine, 1-1 Seiryo-machi, Aoba-ku, Sendai 980-8574, Japan Tel: +81-22-717-7245, Fax: +81-22-717-7248, E-mail: ryo.fujita.e7@tohoku.ac.jp

Received 2025 August 28; Accepted 2025 September 10.

Dear Editor,

We sincerely appreciate the interest and constructive comments regarding our article [1]. As noted in the limitations section, we recognize that the generalizability of our study population is limited and that several potential confounding factors, such as muscle strength and joint mobility, were not included. We also fully understand the importance of conducting a power analysis and external validation. In this respect, we strongly agree with the suggestion that multicenter prospective studies involving broader populations, with clearly defined criteria and rigorous statistical validity, are warranted.

It is evident that the knee and ankle interact with spinal alignment; however, adding parameters inevitably increases the complexity of predictive models. For example, Hasegawa et al. [2] reported a highly accurate formula for predicting ideal lumbar lordosis (R=0.9701, p<0.0001) that incorporated cervical, knee, and ankle parameters. Yet, because of its complexity, the authors themselves concluded that the formula was not practical for application in patients with spinal deformity. We also agree that posterior pelvic shift caused by knee flexion is an important compensatory mechanism; however, as shown by Diebo et al. [3], such mechanisms are secondary and appear only after thoracic and hip compensation have reached their limits. Therefore, from the perspective of rehabilitation and therapeutic intervention, the primary targets should be the thoracic spine (paraspinal muscles) and the hip (gluteal extensors), and the clinical utility of incorporating knee and ankle parameters into predictive models may be limited. Similarly, incorporating factors such as body mass index, muscle strength, and joint mobility should be carefully considered in the context of constructing simple and clinically practical models.

Regarding the statistical issues raised, effect sizes and confidence intervals were omitted with reference to prior studies, and we believe this does not compromise the reliability of our results. Measurement reproducibility was rigorously assessed, with intra- and interobserver intraclass correlation coefficients values ranging from 0.83 to 0.99, indicating almost perfect agreement. Thus, even though some statistical details were not presented, we consider our findings to be robust and trustworthy.

As noted at the outset, we ultimately believe that large-scale prospective studies will be necessary to establish the clinical utility of pelvic femoral angle (PFA)–(pelvic incidence [PI]–lumbar lordosis [LL]) more definitively. In addition, in our future research, we aim to develop a new predictive model incorporating thoracic kyphosis in addition to PFA–(PI–LL), supported by power analysis and external validation, while also applying more robust statistical methods to address the issue of multiple comparisons.