What Is the Optimum Acetabular Component Position and Size in Direct Anterior Approach Total Hip Arthroplasty? Influence of Stability Assessment and Impingement

Introduction Acetabular component position is an important determinant of stability, wear and impingement following total hip arthroplasty (THA). Its optimum position and size in direct anterior approach (DAA) THA has not been clearly described in previous studies. Our aim was to study the evolution of the same with reference to stability and impingement as a part of a single surgeon9s learning curve. Methods Clinical and radiographic records of first 300 consecutive DAA THAs performed by a single surgeon from April 2009 to April 2011 were reviewed from a prospective database at a single center. Radiographic analysis was done by two observers to determine acetabular inclination and anteversion on 6 week postoperative standing radiographs. Native femoral head size, measured on preoperative radiographs after adjusting for magnification, was used to calculate the native acetabular cup size. The study population was divided into three groups; Group A– 1 st 100 DAA THA cases, Group B – 2 nd 100 and Group C – 3 rd 100 corresponding to the use of intraoperative anterior stability assessment (Group B and C) and change in the cup size strategy (Group C). The incidence of instability and psoas impingement (PI) –related groin pain at 2 year follow-up was determined for the three groups. Statistical analysis was done to see if there were differences in these clinical and radiographic outcome measures in the three groups. Results Mean values for abduction were similar in all the groups. Mean anteversion was significantly lower in Group B [12.5° (± 3.3°)] and C [13.6 °(± 2.3°)] as compared to group A [24.3°(± 7.5°)]. The difference between the implanted cup size and calculated native cup size was significantly higher in Group A [5.2(± 2.1) mm] and group B [5.8 (± 2.60 mm] as compared to group C [1.4 (± 1.4) mm] (Figure 1). There were 2 anterior dislocations in group A with none in the other groups. The incidence of PI-related groin pain was higher in group B (12%) as compared to group A (2%) and group C (2%). For PI related groin pain, an arthroscopic psoas release was performed in 3 patients and cup revision in 1 patient. Discussion For optimum anterior stability, the target cup anteversion is lower in the DAA. Increasing the cup size makes the use of large heads possible. However, the implanted cup size should be as close to the native cup size with DAA THA to avoid anterior overhang (psoas impingement) or posterior overhang (cup-neck impingement and anterior instability).