CORR Insights®: Small Random Angular Variations in Pelvic Tilt and Lower Extremity Can Cause Error In Static Image-based Preoperative Hip Arthroplasty Planning: A Computer Modeling Study

Where Are We Now?

THA is a remarkably successful operation. Some of the large problems that were evident in the early years of THA, such as wear and osteolysis, have largely been solved. The Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) reported that for THAs implanted between 2013 and 2018, the most common reasons for revision were loosening, dislocation, infection, and fracture [1]. The 5-year incidence of THA revision for dislocation dropped from 1% for implants placed between 1999 and 2005 to 0.6% for implants placed between 2013 and 2018. This reduction is likely because of improvements in implant design, such as head modularity, offset options, neck geometry improvements, and increased head sizes.

The etiology of dislocation is certainly multifactorial. Soft tissue management is related to the risk of dislocation [7], and clinical experience supports a role for muscle weakness and patient selection. Implant position also influences the risk of THA dislocation, but the best femoral and acetabular component position remains elusive and may not be the same for all patients.

Recent research has identified a relationship between spine mechanics and THA dislocation, and studies have been directed toward understanding this relationship [2, 3, 6]. Ultimately, the goal of such research is to make it easy for surgeons to customize the implant position for each patient. Surgeons interested in doing this currently obtain functional radiographs such as standing and sitting radiographs of the pelvis to understand spine-pelvis mechanics [4], but Eslam Pour et al. [5] have identified a problem with this approach. The surgeon may be able to position implants to avoid impingement based on static images of the pelvis in various functional positions, but dynamic activities of daily living will not exactly reproduce these static positions. Deviations in pelvic position from the static images of as little as 2° to 3° may cause a prosthesis (positioned to avoid impingement in a static set of radiographs) to impinge during dynamic real-life situations. The current study by Eslam Pour et al. [5] thus raises the question of whether a simple static radiographic assessment of hip or spine motion is enough to define a personalized “safe zone” for THA component placement. The authors suggest that a more sophisticated analysis is needed, possibly with dynamic motion analysis or dynamic computer simulation models. I believe the findings of this study are important, but I am uncertain regarding the need for this additional analysis and am concerned that the authors’ suggested path forward would add much complexity and cost to an otherwise fairly routine, common, and successful operation.

Where Do We Need To Go?

The root problem addressed in this research [5] is THA dislocation. Although the authors have identified a problem with assuming that dynamic movements are represented adequately by static images, I would be interested to know whether implant positioning based on static functional images results in a reduced incidence of dislocation compared with traditional methods. Understanding the magnitude of benefit (if any) from this approach will give important perspective into the value of introducing even more complexity into the preoperative assessment of pelvis-spine motion during activities of daily living.

Assuming that static imaging does not lead to a substantially reduced incidence of dislocation, a direction for future research (as suggested by the authors) would be to find some way to account for variable patient movement during common activities of daily living to identify custom THA implant positions that minimize prosthesis-to-prosthesis impingement.

As I think about how this might be accomplished for the large number of THAs performed each year, broader questions come to mind. Given that the incidence of revision for dislocation at 5 years, as reported in the AOANJRR [1], is 0.6%, and that dislocation is multifactorial and does not depend only on implant position, what is the maximum effect on revision for dislocation that can be expected if we were somehow able to identify and achieve a perfect, customized implant position? Is there a scenario where such a solution could be practically executed and remain cost-effective?

How Do We Get There?

Some hip arthroplasty surgeons are now routinely incorporating an assessment of static preoperative functional images into their THA workflow. A study should be done describing exactly how these images were obtained, how information was drawn from these images to determine a custom implant position, and how (and whether) the custom implant position was achieved in the operating room. Such a study should then compare the incidence of dislocation when implants are placed with and without the use of information from preoperative static functional radiographs.

If it is determined that information from static functional images is insufficient to reduce the incidence of THA dislocation, pushing forward a technological solution that goes beyond using simple static images will be challenging. In the conclusion of their article, the authors [5] said that “suggesting an accurate personalized safe zone for THA requires dynamically collected data, over a period of time, of daily and leisure activities …” They acknowledged that resource limitations would make this challenging and suggested that computer simulations could address this, although such simulations would “need to consider random variations in pelvis and lower extremity alignment to attempt to imitate real-life motions as these motions have random variations as well.” If I understand the authors correctly, they envision that static standing and sitting images of the pelvis would continue to be obtained, and then computer simulations could be run, as done in this study, to account for variability in pelvic motions that naturally occur during activities of daily living. The authors reported running more than 5.3 million simulations for a single set of implants and positions, and admitted in the limitations section of their article that varying implant positions and attributes would make the number of simulation calculations skyrocket to impractical levels.

In my view, this is just the beginning of the difficulties. As noted by the authors in the limitations section of their article [5], they did not consider extraarticular bone-bone or bone-soft tissue-bone impingement in this model. In my experience, posterior instability is most commonly due to extraarticular impingement. To address this, custom computational models based on CT images of patients would need to be built and used to simulate implant positions and identify possible extraarticular and intraarticular impingement. To be realistic and accurate, soft tissue would need to be included to account for bone-soft tissue-bone impingement, which is difficult with computer simulation. Further, it has been reported that spino-pelvic mechanics change after THA [8] and may not be predictable, calling into question whether a preoperative analysis of spino-pelvis mobility is even relevant to postoperative THA mechanics.

My concern with this path is that it can rapidly become impractical and expensive to do such custom modeling as a routine part of primary THA. Routine use of navigation or robotics to place implants precisely would add additional cost. Although I am skeptical that routine customized preoperative computer modeling that incorporates random variations in patient movement could be broadly incorporated in preoperative planning and executed in the operating room, I feel that there could be a role in patients with recurrent dislocation of existing implants when the etiology of dislocation is not clear. Because technology-based solutions are considered to address problems such as dislocation, the potential magnitude of the treatment benefits, if any, associated with those solutions should be assessed, and I believe an economic analysis would be useful to determine how large the benefit (and how low the cost) would need to be to make these interventions cost-effective.