Background: Optimal acetabular cup position is an important determinant of the success of total hip arthroplasty (THA), and navigation systems have been developed and applied to improve placement precision. However, the registration method touching bony landmarks through soft tissues may decrease the accuracy in obese cases and in patients with acetabular dysplasia.
Methods: Placement accuracy in 69 joints in which THA was performed with imageless navigation was calculated by comparing the placement angle in the anterior pelvic plane and the placement angle in the x-ray table plane with the patient in the supine position. We defined the difference between the placement angle in the anterior pelvic plane and the angle displayed on the navigation screen as the error and the difference between the placement angle in the plane of the x-ray table and the target angle of acetabular component position as the difference in target angle. Patients were divided into obese (BMI ≥ 25) and nonobese (BMI < 25) groups and into acetabular dysplasia and nondysplasia groups.
Results: The mean ± SD navigation errors for all cases were 2.4° ± 2.0° for inclination and 3.7° ± 2.3° for anteversion. The mean difference in target angle for all cases was 2.8° ± 2.5° for inclination and 4.2° ± 3.0° for anteversion. The error in anteversion was significantly higher in the obese group (4.8° ± 2.5°) than in the nonobese group (3.2° ± 2.6°) (P = 0.01). No significant difference was observed between the acetabular dysplasia and nondysplasia groups.
Conclusions: Imageless navigation can be performed with an error of approximately 3° at the anterior pelvic plane. However, accuracy was found to decrease in obese cases.