In a retrospective study, Cleveland Clinic researchers set out to test the association between cup positioning and patient-reported functional and general health outcomes, as well as dislocation rate. Going in, the team hypothesized that cup positioning with reference to the native acetabular rim, rather than the reported safe zone, would affect dislocation risk and patient-reported outcomes. All participants received CT imaging when undergoing primary, unilateral total hip arthroplasty (THA) at a single academic center by six surgeons from March 2011-January 2015.
OK to Place Acetabular Cup Outside “Safe Zone?”
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Carlos A. Higuera Rueda, M.D. is an orthopedic surgeon with Cleveland Clinic, Ohio, who co-authored the study. Asked how certain cup positioning might affect patient-reported functional and general health outcomes, he told OTW, The positioning of the components, both acetabular and femoral, would dictate the hip range of motion free of impingement and potentially dictate the mechanics that rule the muscle function around the hip girdle. When the range of motion and the mechanics of the hip are affected, the gait and activities of daily living will be affected. The spine and other joint like the knee and ankle will be affected as well.”
“It was surprising to learn that there is no relationship between positioning the acetabular component on what historically has been defined as a ‘safe zone’ (using the anterior pelvic plane) to prevent instability and have better patient-reported outcomes. Alternatively, we found that the superior and anterior rim of the acetabulum can be used to define version and inclination of the acetabular component in relation to the native acetabulum (and indirectly to the pelvis) and this position correlated better with patient-reported outcomes. This may be explained by the relationship between pelvic tilt, spine anatomy with the hip and other variables that at least in this study are merely assumptions. Future studies should be done to define such relationships with the proposed definition of acetabular component placement.”
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This is a fascinating development. In my practice we've seen similar outcomes with the revised protocol. The key differentiator seems to be patient selection criteria. Has anyone else noticed the correlation with BMI thresholds?
Great point. I'd push back slightly on the conclusion, the sample size in the cited study is too small to draw population-level inferences. That said, the directional signal is compelling and worth a larger RCT.
We implemented a similar approach last year. Early results are promising but we're still gathering 12-month follow-up data. Happy to share our protocol if anyone is interested.
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