Background and objectives
The LUMiC prosthesis reconstructs periacetabular defects after tumor resection. However, dislocation remains a substantial risk. Alterations in the center of rotation (COR), anteversion (AV) and inclination (INCL) may contribute, but their impact in LUMiC reconstructions has not been studied. This study aimed to assess whether deviations in COR, AV and INCL are associated with dislocation risk.

Methods
An international multicenter retrospective cohort study was conducted in patients who underwent internal hemipelvectomy with LUMiC reconstruction between 2008 and 2022. Changes in preoperative and postoperative COR, AV and INCL were calculated using a semi-automatic method based on 3D models generated from CT data. An univariate Cox regression model was estimated to assess associations with dislocation risk.

Results
Of 114 eligible patients, 60 had postoperative CT scans suitable for analysis. Mean postoperative changes were +9.0 ± 13.7° in AV, -14.1 ± 15.6° in INCL and COR displacement of 4.7 ± 13.7 mm (lateral), 0.7 ± 14.7 mm (anterior), and 16.0 ± 16.2 mm (superior). Dislocation occurred in 11 patients (18.3%). Medial COR displacement was significantly associated with dislocation risk (HR 1.07 per mm; p = 0.018), while AV, INCL and other COR components were not.

Conclusion
Medial displacement of the COR after LUMiC reconstruction might increase the dislocation risk. Restoring lateral offset may improve postoperative stability.