Reliability of cup position in navigated THA in the lateral decubitus position using the 'flip technique'

Malpositioning of the acetabular component in total hip arthroplasty (THA) increases the risk of dislocation, reduces the range of motion and may contribute to bearing surface wear. During computer assisted navigation, the anterior pelvic plane is registered intraoperatively by percutaneous palpation, but this may be unreliable. The aim of our study was to evaluate the reliability of imageless navigation in acetabular positioning employing data acquisition in the supine position and surgery in the lateral position ('flip technique'). We report 24 patients affected by primary osteoarthritis undergoing THA in which implants were placed with a conventional free-hand technique using the acetabular transverse ligament for cup orientation. For imageless navigation we used Orthopilot-Aesculap software. All patients had a postoperative computed tomography (CT) scan at three months, using previously validated dedicated software for cup orientation. Data collected using navigation software were compared with CT measurements. The mean acetabular inclination and anteversion recorded intra-operatively using navigation software were respectively 41°5' (SD: 9.61) and 9°5' (SD: 4.01) respectively. The mean inclination and anteversion calculated post-operatively by the CT based image software were 44°2' (SD 5.83) and 14°4' (SD 6.42) respectively. There was a statistically significant difference between the anteversion values (p=0.04). Therefore, the acquisition of parameters in the supine position with surgery performed in the lateral decubitus position creates unreliable data concerning cup anteversion using an imageless navigation system, and therefore the 'flip technique' cannot be recommended.