INTRA-OPERATIVE DIGITAL IMAGING: ASSURING COMPONENT ALIGNMENT

Total hip replacement is among the most successful interventions in medicine and has been termed “The Operation of the Century”. Most major problems have been solved including femoral fixation, acetabular fixation, and wear. With a success rate of over 95% at 10 years in both hip and knee arthroplasty in a number of studies, the question remains as to whether the current status quo is optimal or acceptable. The literature, however, reports are from centers that represent optimised results and registry data, including the Medicare database, indicates that substantial short-term problems persist. The major issue is the variability in the performance of the procedure. The inability to consistently position components, particularly the acetabular component, results in major problems including instability and limb length discrepancy. A report by Malchau, et al. reveals that even among the best surgeons, optimal acetabular component positioning is only achieved 50% of the time. The penalty for missing the target is increased incidence of instability, increased wear rate, and diminished function due to restricted motion. Complications are related to position and a major potential explanation is the impact of patient position. Traditional imaging presents a two-dimensional rather than three-dimensional view of the patient and the patient is in a supine, non-functional position at the time that imaging is performed. Adverse events attributed to malposition, however, occur in functional positions and there is evidence that the orientation of the pelvis changes from the supine position at which imaging is performed. This topic has been studied extensively on three continents and the consensus is that the pelvis shifts on the order of 30–40 degrees from the supine to standing and sitting and furthermore, the acetabular component position changes proportionally with the rotation of the pelvis that occurs. How do we incorporate this information into imaging arthroplasty patients? This would require imaging the entire body, acquiring AP and lateral images simultaneously so that 3D imaging can be performed, performing imaging in a functional position (standing or sitting) and optimally at a lower radiation dose since these patients have repeated images and therefore a cumulative radiation dose over their lifetime. This technology was FDA approved for use in the hip and knee in 2011 and pilot studies have been performed at Washington University School of Medicine in St. Louis to validate the number of the hip and knee arthroplasty applications. In conclusion, weightbearing and rotation have substantial impact on the standard measurements obtained before and after hip and knee arthroplasty. These differences in measurements between supine, sitting, and standing as well as correction for rotation may explain the lack of a stronger correlation between component position and a variety of complications that are observed such as variability in wear rates as well as instability. In knee arthroplasty, the change in mechanical axis that occurs from restoring all of patients to a neutral mechanical axis may explain some of the persistent pain and dissatisfaction that has been recently been reported at a relatively high percentage of knee arthroplasty patients. Because of the numerous potential clinical implications of three-dimensional weightbearing imaging, it is likely that the future of arthroplasty imaging will focus on functional three-dimensional imaging of the patient.