What Safe Zone? The Vast Majority of Dislocated THAs Are Within the Lewinnek Safe Zone for Acetabular Component Position

Abstract

Background Numerous factors influence total hip arthroplasty (THA) stability including surgical approach and soft tissue tension, patient compliance, and component position. One long-held tenet regarding component position is that cup inclination and anteversion of 40°± 10°a nd 15°± 10°, respectively, represent a ''safe zone'' as defined by Lewinnek that minimizes dislocation after primary THA; however, it is clear that components positioned in this zone can and do dislocate. Questions/purposes We sought to determine if these classic radiographic targets for cup inclination and anteversion accurately predicted a safe zone limiting dislocation in a contemporary THA practice.Methods From a cohort of 9784 primary THAs performed between 2003 and 2012 at one institution, we retrospectively identified 206 THAs (2%) that subsequently dislocated. Radiographic parameters including inclination, anteversion, center of rotation, and limb length discrepancy were analyzed. Mean followup was 27 months (range, 0-133 months). Results The majority (58% [120 of 206]) of dislocated THAs had a socket within the Lewinnek safe zone. Mean cup inclination was 44°± 8°with 84% within the safe zone for inclination. Mean anteversion was 15°± 9°with 69% within the safe zone for anteversion. Sixty-five percent of dislocated THAs that were performed through a posterior approach had an acetabular component within the combined acetabular safe zones, whereas this was true for only 33% performed through an anterolateral approach. An acetabular component performed through a posterior approach was three times as likely to be within the combined acetabular safe zones (odds ratio [OR], 1.3; 95% confidence interval [CI], 1.1-1.6) than after an anterolateral approach (OR, 0.4; 95% CI, 0.2-0.7; p \ 0.0001). In contrast, acetabular components performed through a posterior approach (OR, 1.6; 95% CI, 1.2-1.9) had an increased risk of dislocation compared with those performed through an anterolateral approach (OR, 0.8; 95% CI, 0.7-0.

No Benefit After THA Performed With Computer-assisted Cup Placement: 10-year Results of a Randomized Controlled Study

“…The method used in our study has, however, been previously validated [7]. Third, the target used in this study was based on the Lewinnek et al [12] study and a recent paper showed that this target may not be appropriate [1], although most of the available systems today still use this target [6,23]. Concerning the surgical technique, the system was strictly a cup positioning software with no integration of the combined cup-stem anteversion.…”

“…No paper to our knowledge has demonstrated any difference between CAS and the standard technique concerning the risk of dislocation. A recent paper [1] showed that the actual targeted values for cup inclination and anteversion based on the original Lewinnek et al [12] paper may be useful but should not be considered as a safe zone in the sense that positioning the acetabular component within these parameters does not preclude dislocation. In fact, the majority of dislocated THAs were within the targeted values.…”

“…No difference was observed at 10 years in this prospective randomized study comparing the functional outcomes, the wear rate, and the survivorship of CAS for cup positioning in THA versus the standard technique at 10 years. Although CAS using either navigation or robotics is an accurate tool to position the cup within targets as historically defined, questions remain concerning the relevance of these targets [1,19]. The robotic era of THA should take into account these actual limitations and further studies are required to integrate the static and dynamic parameters to define an individual optimal component positioning in THA.…”

“…There have been numerous reports regarding the optimal orientation of the acetabular component in THA [1][2][3][4][5]8]. Although debate remains on this topic, the classical target for cup orientation in THA is the so-called ''Lewinnek safe zone'' with an abduction angle of 40°± 10°and an anteversion angle of 15°± 10° [1][2][3][4][5]8].…”

CORR Insights®: Can a Conical Implant Successfully Address Complex Anatomy in Primary THA? Radiographs and Hip Scores at Early Followup

“…This is a particular concern since the Wagner Cone is a monoblock implant, and so it may offer less intraoperative flexibility to modify limb lengths than do modular stems [2]. Second, the lack of modularity narrows options during revision procedures if complications such as instability or deep periprosthetic infection arise [3]; future studies might evaluate whether or not this apparent limitation arising from the implant's design is clinically important. Finally, as with all new prostheses or novel indications, additional investigations in the mid-and long-term are essential.…”