Functional Safe Zone for Cup Orientation in THA

Abstract

The usual safe zone for cup orientation in THA is not suitable for all patients, as the pelvic tilt varies with the movements of daily activities. A new Functional Safe Zone (FSZ) is proposed that considers the pelvic tilt in different positions. The aims of this study were to validate the proposed FSZ and to evaluate how the pelvic mobility impact it.We measured the pelvic tilts of 30 patients when standing, sitting and supine, using our ultrasound-based device and computed their FSZs. The FSZs accuracy was assessed using a Computer-Aided-Design (CAD) software. The pelvic mobility influence onto the FSZ was assessed by jointly analysing the patients’ FSZs and their pelvic tilt difference between positions.The true FSZ provided by the CAD software and the estimated FSZ were similar by 92% and differed by less than 0.5◦ at borders and at the mean orientation. Patients with stiff pelvic mobility obtained small FSZs, and conversely, patients with large pelvic tilt variations between positions obtained large FSZs.The proposed method allows the computation of a patient-specific FSZ without requir- ing additional X-ray or CT images. Patients having a low pelvic mobility with a higher risk of postoperative instability could be better managed using this FSZ.