Letter to the Editor concerning "Impact of spinal alignment and stiffness on impingement after total hip arthroplasty: a radiographic study of pre‑ and post‑operative spinopelvic alignment" by Hagiwara S, et al. (Eur Spine J. 2020; doi.org/10.1007/s00586-020-06589-z).

Dislocation is one of the remaining challenges after total hip arthroplasty. The spinopelvic mobility is considered to be the key to solve this problem and is of interest both to arthroplasty and spine surgeons. The purpose of this letter is to discuss the spinopelvic mobility and spinal stiffness described in the paper titled "Impact of spinal alignment and stiffness on impingement after total hip arthroplasty: a radiographic study of pre‑ and post‑operative spinopelvic alignment." by Hagiwara S, et al. METHODS: Examining the consistency between this paper and previously published papers on spinopelvic mobility. In this article, radiographic clearance of anterior impingement was defined as adding of femoral shaft angle and sacral slope (SS), and that of posterior impingement as adding SS and femoral shaft angle subtracting 90º in the sitting position. The impingement itself and other factors for dislocation including implant design, implant orientation, extra-prosthetic impingement and their mobilities are not considered in this parameter, and it is better if the validity of this parameter is shown. The term "rigid spine" and "spinal stiffness" are used in the manuscript. When THA candidates are evaluated, they are categorized according to the flexibility and/or sagittal balance. It would be better if the definition was described in the text and the clearance for impingement was shown to be affected by spinal stiffness. The conclusions and titles are overstated from the results, but this paper is highly valuable in reminding spinal surgeons of the importance of spinopelvic alignment and mobility in THA.