Resolving inconsistencies in defining the target orientation for the acetabular cup angles in total hip arthroplasty

Dislocation following total hip arthroplasty is a major complication and malorientation of the acetabular cup is one of the primary factors affecting dislocation. Different conventions used to describe the cup orientation produce significant variations in the recommendations for correct positioning, which in turn make it difficult for clinicians to properly interpret and apply previously reported studies.

We examined nine articles presenting recommendations for the range of target orientations of the acetabular cup to minimize the risk of dislocation (referred to as the 'safe zone'). Those studies included five ways to define the cup orientation and two methods to define the reference frame. We converted those recommendations to a single representation based on the radiographic angles expressed in the pelvic frame reference.

After conversion, the mean recommended anteversion angle was shifted downward by 5 degrees (P<0.01). Also, the target orientation recommendations became more consistent, especially for the anteversion angles where the standard errors of the upper and lower limits were reduced by 61% (P=0.02) and 23% (P=0.04), respectively.

The choice of reference frame and the definition for acetabular cup orientation angles can have a significant effect on the target orientation for the acetabular cup. Recommendations for the target orientation should always explicitly state which reference frame and angle definition is being used. The averaged recommendation of the studies assessed here is 41 degrees inclination and 16 degrees anteversion in radiographic angles or 39 degrees inclination and 21 degrees anteversion in operative angles, both expressed in the pelvic reference frame.