Planning and measurement of the intended position of the acetabular component in the supine position may fail to predict clinically significant changes in its orientation during functional activities, as a consequence of individual pelvic kinematics. Optimal orientation is patient-specific and requires an evaluation of functional pelvic tilt pre-operatively. Cite this article: Bone Joint J 2017;99-B:184-91.

Patients found to have unfavourable pelvic mobility had limited lumbar flexion and more posterior standing pelvic tilt in both men and women, as well as increasing age in women. We recommend that patients undergo preoperative functional radiographic screening to determine specific parameters that can affect the functional orientation of the acetabular component. Cite this article: Bone Joint J 2018;100-B:845-52.

Accurate placement of the acetabular component can be achieved using patient-specific guides and is superior to free hand techniques and comparable to navigated and robotic techniques. Cite this article: Bone Joint J 2016;98-B:1342-6.

The functional orientation of the acetabular component during activities associated with posterior edge-loading are different from those measured when supine due to patient-specific pelvic kinematics. Individuals with a large anterior pelvic tilt during deep flexion might be more susceptible to posterior edge-loading and squeaking as a consequence of a significant decrease in the functional anteversion of the acetabular component. Cite this article: Bone Joint J 2016;98-B:910-16.

Appropriate component alignment is critical for improving stability, maximising bearing performance and restoring native anatomy after Total Hip Arthroplasty (THA). Due to the large variation in patient kinematics between functional activities, current technologies lack definition of what constitutes correct target alignment. Analysis of a large series of symptomatic THA patients confirms that apparently wellorientated components on standard radiographs can still fail due to functional component malalignment. Evidently, previously defined "safe zones" are not appropriate for all patients as they do not consider the dynamic behaviour of the hip joint.The Optimized Positioning System™ (OPS™) comprises preoperative planning based on a patientspecific dynamic analysis, and patient-specific instrumentation for delivery of the target component alignment. This paper presents the application of OPS™ in three case studies.