What safe zone? The great majority of dislocated total hip arthroplasties lay within the limits of the Lewinnek safe zone for acetabular component position

Numerous factors can affect the stability of total hip arthroplasty, including the surgical approach and soft-tissue tension, patient compliance and the position of the implants. A traditional doctrine on implant position is that the inclination and the anteversion of the acetabular cup should lie within 40° ± 10° and 15° ± 10° respectively, representing the "safe zone" as defined by Lewinnek, which minimises dislocation after primary total hip arthroplasty. It is clear, however, that implants placed within this zone can dislocate, and do dislocate.
In an article published in the respected American journal “Clinical Orthopaedics and Related Research”, the investigators set out to determine whether these classic radiographic targets for acetabular inclination and anteversion accurately predict a safe zone that limits dislocation in everyday total hip arthroplasty practice.
They identified 206 total hip arthroplasties that dislocated within a cohort of 9,784 primary total hip arthroplasties performed between 2003 and 2012. Radiographic factors relating to inclination, anteversion, the centre of rotation and leg length discrepancy were analysed. The mean follow-up was 27 months (0 to 133).
The majority of those that dislocated (58%) had the acetabular cup within the Lewinnek safe zone. The mean acetabular inclination was 44° ± 8°, with 84% lying within the safe zone for inclination. The mean anteversion was 15° ± 9°, with 69% of these lying within the safe zone for anteversion. 67% of the dislocated total hip arthroplasties that had been performed through a posterior approach had the acetabular component within the combined acetabular safe zones, while this was true for only 33% of those performed through an anterolateral approach. A total hip arthroplasty performed through a posterior approach was three times more likely to lie within the combined safe zone for the acetabulum. In contrast, when implants were placed through a posterior approach they showed an increased rate of dislocation compared with those performed through the anterolateral approach.
The authors concluded that the historical target values for acetabular inclination and anteversion may be useful but should not be regarded as a safe zone, given that the majority of these routine total hip arthroplasties that dislocated lay within those target values. Stability is a multifactorial characteristic, and the ideal acetabular position for some patients may lie outside the Lewinnek safe zone; more advanced analysis is required in order to identify the correct target in that subgroup.
Source: “Clinical Orthopaedics and Related Research”, February 2016