Background: Total hip arthroplasty using imageless computer navigation and the method of registering the anterior pelvic
plane [APP], an external bony landmark of the acetabulum, has been found to be unreliable in several studies. An alternative method is the acetabular center axis [ACA], which hip navigation software which uses registered anatomical landmarks of the acetabulum.

Objective: To evaluate the accuracy of acetabular cup position, size, and alignment (inclination and anteversion) after total hip replacement by means of navigation using ACA software.

Materials and Methods: This prospective observational study included 43 patients (50 hips) who had primary total hip replacement [THR] with navigation using ACA software. Data on cup size (real size and navigated size), inclination and anteversion (compared to native data), and final cup position
in three dimensions were collected for analysis. CT scans were obtained at two months post-operative for cup anteversion and a standard plain film of both hips in AP for cup inclination. Both radiographic cup anteversion and inclination were applied to the Lewinnek safe zone to evaluate the placement accuracy and to compare anatomical anteversion and inclination of the cup measured from CT scans to that from navigation. Comparison was done using the paired t-test.

Results: Of the 50 hips, 45 (90%) were within the Lewinnek safe zone and it was statistically significant (p<0.05). Comparison of anatomical data on acetabular cup alignment from the CT scans and from intraoperative data from ACA found the mean inclination (cup/anatomy) with ACA was -17.720 (SD 7.431), the mean inclination (cup/anatomy) with CT scans was -10.700 (SD 6.008), the mean anteversion (cup/anatomy) with ACA was -0.560 (SD 6.929), the mean anteversion (cup/ anatomy) with CT was 3.460 (SD 6.658), and the differences were statistically significant (p<0.05). The mean real acetabular cup size was 51.84 mm (SD 3.099, range 48 to 62) and the mean navigated cup size was 50.08 mm (SD 4.145, range 42 to 61). This difference was statistically significant (p<0.05).

Conclusion: The ACA hip navigation software is accurate in the placement of acetabular cups within the Lewinnek safe zone and in setting cup size and cup position in three dimensions.

Keywords: Hip Arthroplasty, Hip navigation, Acetabular cup alignment