Role of enabling technologies in THA: Setting the stage Impact of component position in THA Wear/lysis Effect of edge loading, impingement Instability Together, the most common cause for revision hip arthroplasty Ideal component position: Work of Lewinneck: the “safe zone” for stability Can we achieve this? HSS study Mass General Study: 2000 THR's, only 50% within desired range Need for assistance? Maybe? Types of Guidance: Navigation: use of mechano or optical tracking system that after some registration acquisition, facilitate spatial placement. The systems can either be image based (pre-operative CT scan) or imageless where multiple points are acquired and a “best fit” is matched to a library of pelvic geometries. Robotics: combines the spatial application of navigation with the precision bone preparation afforded by robotic milling. Robotic use can either be active whereby the robotic preparation is performed by the computer driven system (ie ROBODOC™). Alternatives include surgeon controlled but robot guided (haptic) type systems. Perceived Advantages: Robotic assisted: Bone preparation: spherical shape of socket consistently “rounder” than manually controlled reaming Implant insertion: by establishing boundaries of insertion, final implant position achieves desired position Unknowns: Cost effectiveness Do we really know where the socket is best located for an individual patient? While we rely on the safe zone of Lewinneck for our desired implant position, the impact of lumbosacral disease deformity could/should impact where the socket is placed.
Instability after total hip arthroplasty is the primary cause for revision surgery and is a frequent complication following revision surgery for any reason (Bozic et al, JBJS 2009). Surgical management of the unstable hip has not been uniformly successful with the best results occurring in those hips in which an identifiable cause of instability can be determined (Daly & Morrey, JBJS 1992). It was these sobering findings that lead to the development of and increased use of constrained acetabular components. While the results of revision surgery for instability using constrained components have been encouraging (Shapiro, Padgett, Sculco, J Arthroplasty 2003) with a re-dislocation rate of less than 3%, reoperation for other reasons have noted to increase with time. The commonly used tripolar configuration has been susceptible to bearing damage at both the inner and outer bearing surface by the nature of the constrained mechanism (Shah, Padgett, Wright, J Arthroplasty 2009). In addition, we have noted instances of fixation failure directly related to the constrained acetabular device either from loss of implant fixation to the pelvis with or without cement (Yun, Padgett, Dorr, J Arthroplasty 2005). The observation of these failure modes ranging from either fixation failures to overt biomaterial failure have lead us to be extremely cautious in the “routine” use of constrained liners in revision THR. Implant instability due to poor position should be revised as appropriate to correct alignment. The use of either larger diameter heads or the emerging use of dual mobility articulations seems more appropriate at this time.
