Does Individualization of Cup Positioning Improve Hip Implant Stability?
The success of Total Hip Arthroplasty (THA) in the modern world has been unparalleled with over 90% of patients reporting satisfaction1. However, dislocation due to prosthetic and bone impingement continues to pose a challenge for orthopedic surgeons. The primary goal of THA is to provide pain relief and improve hip function and movement at the hip joint. And without proper implant positioning, it is impossible to achieve these goals.
Lewinnek and colleagues published a paper in 1978 suggesting that positioning the acetabular component of a hip replacement prosthesis at 40±10° of inclination and 15±10° of anteversion has reduced the incidence of dislocation. This “safe zone” orientation was widely adopted and practiced across the board by hip arthroplasty surgeons2. Recently, however, more recent studies have shown that this is not entirely true and placement within this safe zone does not guarantee stability. A study conducted by Abdel in 2016 showed that of the 206 dislocations, 58% had the acetabular component placed in this “safe zone”3 indicating that this zone is not as safe as previously envisaged.
One of the chief reasons for the aforementioned problem is that the hip and spine usually have co-existing degenerative changes and without accounting for lumber mobility during postural changes, there will always remain a high chance of dislocations and impingements. Studies have shown that changes in lumbar mobility, such as when transitioning from sitting to standing, can affect pelvic tilt. This is very significant because the acetabulum is part of the pelvis and any changes in pelvic motion are bound to alter its orientation (anteversion and inclination). This leads to the conclusion that the position of the acetabular component achieved during total hip arthroplasty (THA) may not remain constant as patients move and mobility might make the prosthesis unstable4. Lembeck and colleagues found that for every degree of change in sagittal pelvic tilt, there is a corresponding 0.7 to 0.8° change in acetabular component anteversion5.
Moreover, studies have also shown that pre-operative planning radiographs fail to provide adequate information about the cup orientation during functional movement6. Similarly, examination of removed acetabular components has shown that more than half of them show signs of prosthetic impingement. This implies that the traditional methods used before surgery to determine the position of the acetabular component may not be sufficient7.
So, what is the way out? Are we stuck with a high risk of dislocation and joint impingement? Luckily, that is not the case. A study conducted by Dennis and colleagues in 2023 has shown promising results by individualizing cup positions for prostheses. Using simulations and 3D reconstructions for every patient individually and positioning the acetabular cup accordingly, a significant decline in prosthetic impingement was observed. The results demonstrate the prevalence of prosthetic impingement was significantly less in the patient’s individualized choice of acetabular component position (9%) when compared to the 6 preselected cup positions (18 to 61%; P<0.0001)8.
These results were inferred once the “safe zone” was considered a dynamic entity. The acetabulum is part of the pelvis, its orientation and that of the acetabular component in total hip arthroplasty (THA) can change during postural positional changes. Studies have referred to this dynamic change in position as the “functional” acetabular component orientation.
As already mentioned, the above-mentioned study used simulations and a virtual 3D planning system to analyze spinopelvic mobility using individualized dynamic sagittal spinopelvic radiographs. This rendered better results, however, there are some potential limitations to the study that should be considered. The 3D-CT impingement detection algorithm used in the study only assessed the bony and prosthetic impingement ignoring the contact mechanics, femoral component anteversion, leg length inequality, medial and anterior offset, soft-tissue laxity, surgical approach, and the size of the femoral head. Factors that are equally responsible for the instability of THA.
The results showed beyond doubt that utilization of a patient’s individualized acetabular component position based on a preoperative analysis of individual spinopelvic motion patterns reduces the risk of prosthetic impingement. The data, however, is limited and further research will hopefully cement this research.
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1. Weber, Markus, et al. “Predictors of outcome after primary total joint replacement.” The Journal of Arthroplasty 33.2 (2018): 431-435.
2. Lewinnek, George E., et al. “Dislocations after total hip-replacement arthroplasties.” JBJS 60.2 (1978): 217-220.
3. Abdel, Matthew P., et al. “What safe zone? The vast majority of dislocated THAs are within the Lewinnek safe zone for acetabular component position.” Clinical Orthopaedics and Related Research® 474 (2016): 386-391.
4. Lazennec, Jean-Yves, Adrien Brusson, and Marc-Antoine Rousseau. “Hip–spine relations and sagittal balance clinical consequences.” European spine journal 20 (2011): 686-698.
5. Lembeck, Burkhard, et al. “Pelvic tilt makes acetabular cup navigation inaccurate.” Acta orthopaedica 76.4 (2005): 517-523.
6. Vigdorchik, Jonathan M., et al. “Prevalence of risk factors for adverse spinopelvic mobility among patients undergoing total hip arthroplasty.” The Journal of Arthroplasty 36.7 (2021): 2371-2378.
7. Shon, Won Yong, et al. “Impingement in total hip arthroplasty: a study of retrieved acetabular components.” The Journal of arthroplasty 20.4 (2005): 427-435.
8. Dennis, Douglas A., et al. “Does Individualization of Cup Position Affect Prosthetic or Bone Impingement Following THA?.” The Journal of Arthroplasty (2023).