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. 2024 Jul 11;9(3):e23.00120.
doi: 10.2106/JBJS.OA.23.00120. eCollection 2024 Jul-Sep.

The Relationship of Cup Inclination and Anteversion in the Coronal Plane with Ante-Inclination in the Sagittal Plane: Exposing the Fallacy of Cup Safe Zones

Moritz M Innmann  1   2 , Christian Merle  3 , Akaash Ratra  1 , Andrew Speirs  4 , Andrew Adamczyk  5   6 , David Murray  7 , Harinderjit S Gill  8 , George Grammatopoulos  1
Affiliations

The Relationship of Cup Inclination and Anteversion in the Coronal Plane with Ante-Inclination in the Sagittal Plane: Exposing the Fallacy of Cup Safe Zones

Moritz M Innmann et al. JB JS Open Access. .

Abstract

Background: This study aimed to establish an equation for calculating cup ante-inclination (AI) from radiographic cup inclination and anteversion, to validate this equation in a total hip arthroplasty (THA) cohort, and to test whether achieving previously described radiographic cup inclination and anteversion targets would also satisfy sagittal cup AI targets.

Methods: A mathematical equation linking cup AI, radiographic inclination (RI), and anteversion (RA) was determined: tan(AI) = tan(RA)/cos(RI). Supine and standing anteroposterior and lateral radiographs of 440 consecutive THAs were assessed to measure cup RI and RA and spinopelvic parameters, including cup AI, using a validated software tool. Whether orientation within previously defined RI and RA targets was associated with achieving the AI target and satisfying the sagittal component orientation (combined sagittal index, 205° to 245°) was tested.

Results: The cups in the THA cohort had a measured mean inclination (and standard deviation) of 43° ± 7°, anteversion of 26° ± 9°, and AI of 34° ± 10°. The calculated cup AI was 34° ± 12°. A strong correlation existed between measured and calculated AI (r = 0.75; p < 0.001), with a mean error of 0° ± 8°. The inclination and anteversion targets were both satisfied in 194 (44.1%) to 330 (75.0%) of the cases, depending on the safe zone targets that were used, and 311 cases (70.7%) satisfied the AI target. Only 125 (28.4%) to 233 (53.0%) of the cases satisfied the AI target as well as the inclination and anteversion targets. Satisfying inclination and anteversion targets was not associated with increased chances of satisfying the AI target.

Conclusions: Achieving optimal cup inclination and anteversion does not ensure optimal orientation in the sagittal plane. The equation and nomograms provided can be used to determine and visualize how the 2 planes used for evaluating the cup orientation and the pertinent angles relate, potentially aiding in preoperative planning.

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Conflict of interest statement

Disclosure: The authors received non-commercial research funds from Stiftung Endoprothetik (Hamburg, Germany) and the Canadian Institute of Health Research. The Article Processing Charge for open access publication was funded by personal research funds. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article (http://links.lww.com/JBJSOA/A652).

Figures

Fig. 1
Fig. 1
Nomograms illustrating the interaction between radiographic cup inclination and anteversion in the coronal plane (left) and cup AI in the sagittal plane (right).
Fig. 2
Fig. 2
Radiographic measurements of cup inclination and anteversion in the coronal plane (left) and AI in the sagittal plane (right).
Fig. 3
Fig. 3
Comparison of the AI_model and AI_eqn values for various combinations of RI (from 20° to 60°) and RA (from −20° to 20°).
Fig. 4
Fig. 4
Scatterplot illustrating the correlation between the measured and calculated AI.
Fig. 5
Fig. 5
Bland-Altman plot of agreement between measured and calculated AI, in degrees. SD = standard deviation.
Fig. 6
Fig. 6
Diagram for determination of the desired radiographic cup inclination and anteversion in the coronal plane, based on cup AI (colored lines) in the sagittal plane, in degrees.
See this image and copyright information in PMC

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