The pelvic incidence is an anatomical and biomechanical pelvic parameter determining spine sagittal morphology and kinematics. Stiffening of the lumbo-pelvic complex, a result of degeneration, affects the functional cup positioning, putting prosthetic hip patients at risk of instability. The anti-dislocation dual mobility (DM) device may be clinically advantageous by reducing the risk of prosthetic instability for older patients with spine ageing. Our study aims to answer the following questions: (1) is there a relationship between prosthetic hip instability and the standing cup position, (2) is there a relationships between prosthetic hip instability and the pelvic incidence, (3) is there a relationships between prosthetic hip instability and the severity of the spine degeneration?, (4) is the DM cup device an effective option for reducing the risk of prosthetic instability related to spine degeneration?

There is a relationship between prosthetic hip instability and the standing cup position and pelvic parameters.

Case-control study on prospectively collected data since 2009. From 1672 conventional total hip replacements (THR–5.4% dislocation rate) and 1056 DM-THRs (1.1% dislocation rate) performed at our institute since 2009, we created three groups: 33 patients with unstable THR (group 1–case), 41 patients with stable THR (group 2–control), and 42 patients with stable DM-THR (group 3–control). The cup orientation was measured on standing pelvic radiographs and the spino-pelvic parameters were measured on standing EOS™ biplanar images or lateral full spine radiographs.

By comparing patients from group 1 with those of group 2 we found they had similar cup position (57% versus 51% fitting the safe zone, p=0.58), higher pelvic incidence (58° versus 51°, p=0.01), and more severe spine degeneration (smaller anterior pelvic plane Tilt (2° versus 7° [p=0.002]), a larger pelvic incidence–lumbar lordosis mismatch (17° versus 8° [p=0.005]), and a higher proportion of spino-sacral angle<127° (70% versus 43%, (p=0.02)). Patients from group 3 had similar cup position, pelvic incidence, and spine degeneration compared to patients from group 1.

Patients with spine-hip relation type 2C/D (high pelvic incidence and severe spine degeneration) have an increased risk of instability that is partly compensated for by the use of a DM device. Preoperative screening of patients with abnormal spine-hip relation would improve THR planning and reduce the risk of prosthetic hip instability. The use of a DM device on spine-degenerated elderly patients is probably sound.

III, case-control study.