Specific Aim #1: To quantify the relationship between the change in pelvic tilt and functional acetabular cup anteversion between the standing and sitting positions in patients who have undergone primary THA. The investigators hypothesize that there will be a directly proportional relationship between the increase in pelvic tilt and increase in functional anteversion. The investigators anticipate that Aim #1 will identify the importance of functional anteversion in the seated position and support its consideration when defining a patient-specific cup safe zone. This data will serve as the foundation for development of a clinical tool used in pre-operative planning and post- operative rehabilitation that accounts for spinopelvic mechanics to maintain safe cup orientation when in the vulnerable seated position.