Redefining the Acetabular “Safe Zone” with AI

    In 1978, Lewinnek et al [1] published what was considered at the time to be a landmark paper that would help orthopaedic surgeons reduce the risk of postoperative dislocation in their total hip arthroplasty (THA) patients.

    The key, the study authors said, was to use what they defined as the “ideal” anteversion and inclination angles – 15° ± 10° for anteversion, 40° ± 10° for inclination – when placing the acetabular cup. The “safe zone” these angles created would increase the stability of the hip to prevent dislocation, they said.

    More than 40 years of research and clinical practice has shown, however, that this isn’t the case: Placing the acetabular cup in the historic safe zone does not reliably prevent dislocation. In a study by Abdel et al, [2] for example, the acetabular cup had been positioned in the historic safe zone in more than 50% of THAs that dislocated.

    RELATED: Challenging the Lewinnek Safe Zone

    Other study authors have found that a functional safe zone [3] or a patient-specific safe zone [4] is more likely to be successful in reducing post-THA dislocations, as is the use of technology not available when Lewinnek et al conducted their study, such as computer navigation and robotic surgery. [5-8]

    Applying AI to the Issue

    Despite advances in THA techniques and technology, instability remains a common cause of revision THA. [9-10] So, although THA reliably and reproducibly relieves pain and restores functions in most patients, [11] researchers have been unable to find a way to reduce the postoperative dislocation rate.

    Researchers from Mayo Clinic may have taken the first step in a very high tech way: They’ve used artificial intelligence (AI) to analyze thousands of post-THA radiographs and then calculate anteversion and inclination angles that redefine the concept of an acetabular safe zone.

    Their study has been published online ahead of print by The Journal of Bone & Joint Surgery.

    One of the limitations of the Lewinnek study was the data used: The authors based their definition of the safe zone on only 300 radiographs from THAs performed by only 5 surgeons. The Mayo Clinic researchers wanted to establish a more up-to-date, scientifically rigorous safe zone.

    To do so, they analyzed radiographs from 9907 primary THA procedures (8081 patients; 4166 women, 3915 men) performed between 2000 and 2017 by 35 orthopaedic surgeons at Mayo Clinic. They managed this large volume of radiographic measurements by using a previously validated, highly precise AI neural network approach.

    Study Findings

    In this study, dislocation occurred in 3% of cases at an average of 2 years postoperatively. Analysis of radiographs showed clinically important findings for anteversion and inclination in the context of acetabular cup position and the acetabular safe zone:

    Anteversion angle. The average acetabular anteversion angle was observed to be 32° among patients who did not have a dislocation, which was significantly higher than the anteversion angle of 15° proposed by Lewinnek et al.

    The dislocation risk, based on multivariable modeling, was found to be lowest at an anteversion angle between 18° and 38°, which was again substantially higher than that of the historical range of 5° to 25°.

    RELATED: Cup Placement Is More than Aiming for a Safe Zone

    Inclination angle. The acetabular inclination angle for the newly estimated safe zone was 27° to 47°, similar to the 30° to 50° range proposed by Lewinnek et al.

    When the 2 angles were considered together, anteversion had a greater impact on dislocation risk than did inclination, provided that satisfactory anteversion was achieved.

    Surgical approach. Although the location of the safe zone values was similar for men and women, the surgical approach was found to affect the topology of the safe zone, with anterior-based THA approaches penalizing low values of anteversion to a lesser extent.

    Overall, the risk of THA dislocation was lower for men, for patients with larger implanted femoral head sizes (36 mm or larger), and for patients who underwent THA by the direct anterior approach.

    Final Thoughts

    Although the study is not the first to raise questions about historical safe zone measurements, it provides a new and robust set of safe zone angles based on nearly 10,000 post-THA radiographs, reflecting modern THA techniques and surgical experience.

    “These findings are clinically relevant in that they guide acetabular positioning and, in doing so, may mitigate dislocation risk in a readily implemented manner,” the researchers concluded.

    They also called for additional research “to characterize the 3D interaction between acetabular positioning and spinopelvic as well as femoral-sided parameters.”


    Hevesi M, Wyles CC, Rouzrokh, et al. Redefining the 3D topography of the acetabular safe zone: a multivariable study evaluating prosthetic hip stability. J Bone Joint Surg Am. 2021 Dec 27. doi: 10.2106/JBJS.21.00406. Online ahead of print.


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