The radiological study of the sagittal alignment of the spine in static and dynamic positions has allowed a better understanding of the clinical results of total hip prostheses. According to the Roussouly classification, the sacral slope in a standing position characterizes the patient with a stiff spine (less than 35̊) or a flexible spine (greater than 35̊). The objectives of this study were to compare, in a population of patients operated on for femoroacetabular impingement (FAI), firstly, the kinematics of the lumbar-pelvic-femoral complex and secondly, the clinical presentation depending on whether the spine is stiff or flexible.

Patients with stiff spines (SS<35̊) were “hip users” and had less ability to compensate for their hip pathology.

This prospective and comparative multicenter study was conducted with patients operated on arthroscopically for FAI between 2020 and 2021. All patients included received preoperative EOS imaging of the lower limbs and spine, in standing and seated positions. The following parameters were measured: pelvic tilt (PT), pelvic incidence (PI), lumbar lordosis (LL), and sacral slope (SS), as well as dynamic parameters (intrinsic mobility of the hip: delta standing/sitting femoral sacral angle, and of the pelvis: delta standing/sitting sacral slope). The functional result was evaluated at 1 year.

In the stiff spine group, 62 patients were included versus 138 in the flexible spine group. The mean follow-up was 15.1±3.3 months (9.2–24.3). Nineteen patients were lost to follow-up and there were 2 revisions for iterative arthroscopy. Intrinsic pelvic mobility and intrinsic hip mobility were 16̊±13.5 (−12; 44) and 44.5̊±28.5 (−9; 99) respectively in the stiff spine group. In the flexible spine group, these same measurements were 22̊±11.5 (−30; 45) and 29.7̊±22.7 (−33; 82) respectively. In the stiff spine group, the symptoms occurred at a significantly younger age: 28.3 years±9.5 (18–51) versus 31 years±8 (18–54) (p=0.017).

The sacral slope is a determining factor in the kinematics of impingement and the appearance of symptoms. The sacral slope helps define the concept of a “hip user” in symptomatic FAI.

IV.