Specific Aim #2: To quantify the relationship between the change in lumbar lordosis 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 decrease in lumbar lordosis and increase in functional anteversion and that patients with history of lumbar immobility will have decreased change in functional anteversion. The data obtained in Aim #2 can be used to identify those at increased risk of dislocation. It will also provide insight into, and a basis for further study of, the relationship between seated functional anteversion and specific etiologies of lumbar immobility (spinal fusion, ankylosing spondylitis, flat back syndrome, degenerative changes, etc.) that may affect THA dislocation.