An artificial intelligence study analyzing x-rays of thousands of patients after hip replacement surgery is redefining the “safe zone” of the acetabulum to avoid the risk of dislocation, reports the Journal of Bone & Joint Surgery. The magazine is published in collaboration with Wolters Kluwer in the Lippincott portfolio.
Two key angles – anteversion and inclination – have been measured in the past to assess the risk of dislocation after total hip arthroplasty (THA) and to define an acetabular “safe zone” for implant placement. In contrast to the results of an influential study 40 years ago, the acetabulum’s safe zone is likely different than previously reported, according to new research by Mario Hevesi, MD, PhD, Daniel J. Berry, MD, and colleagues at the Mayo Clinic, Rochester, Minn. For orthopedic surgeons, ensuring that these angles are accurately reproduced during hip replacement can be an easy step to take to reduce the risk of postoperative dislocation.
Updated analysis suggests new safe zone to reduce risk of THA dislocations
Placement of implants within the safe zone of the acetabulum is associated with a reduced risk of postoperative hip dislocation, which is one of the most common complications after THA and has a high potential impact on the patient’s quality of life. The acetabulum is the socket of the hip joint into which the ball at the top of the femur (the femoral head) fits. A landmark 1978 paper published in JBJS defined the “ideal” angles for hip implant placement to create a “safe zone” and reduce the risk of dislocation.
Although innovative in its day, this study had important limitations, including the fact that it was based on only 300 THAs performed by five surgeons. Subsequent research has shown that many THAs that are displacing have measurements within what is known as the historical safe zone. Meanwhile, there have been many advances in surgical techniques over the decades, including capsule and soft tissue repair, and the popularization of the direct anterior approach.
To establish a more current and scientifically stricter safety zone, Drs. Hevesi, Berry and colleagues analyzed x-rays from 9,907 primary THA procedures in 8,081 patients. The interventions were performed by 35 different orthopedic surgeons at the Mayo Clinic between 2000 and 2017. To handle this large amount of X-ray measurements, the researchers used a previously validated, high-precision neural network approach with artificial intelligence.
A dislocation occurred in three percent of the cases an average of two years postoperatively. Analysis of the x-ray images showed clinically important findings for the two angles used to measure acetabular position and determine the safe zone – anteversion and inclination. The average anteversion angle of the acetabulum was 32 degrees in patients who did not have dislocation, which was significantly higher than the “ideal” anteversion angle of 15 degrees suggested in the previous 1978 study. The researchers conducted further analysis to optimize the acetabulum’s safe zone and found that the risk of dislocation was lowest at an anteversion angle between 18 and 38 degrees, which in turn was well above the historical range of 5 to 25 degrees.
For the acetabular cup angle, the newly estimated safe zone was 27 to 47 degrees – similar to the range of 30 to 50 degrees suggested in the 1978 paper. When the two angles were considered together, the anteversion had a greater impact on the risk of dislocation than the inclination, especially when a satisfactory anteversion was achieved.
While the location of the safe zone scores was similar for men and women, the surgical approach was found to affect the safe zone topology, with anterior-based hip replacement approaches less penalizing low anteversion scores. Overall, the risk of THA dislocation was lower in men, in patients with larger implanted femoral head sizes (36 millimeters or larger), and when using the direct anterior approach.
While the study is not the first to raise questions about historical safety zone measurements, it provides a new and robust set of safety zone angles based on nearly ten thousand patients and reflecting modern THA techniques and surgical experience.
These results are clinically relevant insofar as they guide acetabular cup positioning and thereby reduce the risk of dislocation in an easy-to-use manner. “
Daniel J. Berry, MD, Mayo Clinic
They suggest that current robotic or surgical navigation techniques can facilitate more precise placement of implants within the safe zone of the acetabulum during THA procedures.
Source:
Journal reference:
Mario, H., et al. (2021) Redefinition of the 3D topography of the acetabular safety zone. A multivariable study to evaluate the hip stability of prostheses. The journal for bone and joint surgery. doi.org/10.2106/JBJS.21.00406.
