Background: One of the limitations in imageless navigation is the variability in bony landmark registration among surgeons.
If the bony landmark registration is different among surgeons, the final cup position determined by the navigation will also be
different.
Objective: To determine intra and inter-observer reliability of the bony landmark registration in three surgeons with varying
experience in imageless navigation total hip arthroplasty (THA) in semilateral decubitus position.
Material and Method: The authors reviewed 60 cases receiving cementless THA with imageless navigation between June 2014 and April 2015. All cases were registered for anterior pelvic plane (APP) by three surgeons. The first surgeon (S1-YS) is senior staff with experience in imageless navigation, the second surgeon (S2-JS) is junior staff with one year of experience
in imageless navigation (with more than 50 cases), and the third surgeon (S3-AC) is a fourth-year orthopedic resident with
no experience in imageless navigation. Using the final cup position of the experienced surgeon as the gold standard to
determine the reliability of registration.
Results: There were no significant differences in cup abduction angle between (S1) and (S2) (p = 0.27) and (S1) and (S3) (p
= 0.79). There was no significant difference in cup anteversion angle between (S1) and (S2) (p = 0.1) but there was significant
difference between (S1) and (S3) (p<0.001). For the intra-observer reliability, the ICCs of abduction angle was 0.95 for (S2) and 0.86 for (S3) and the ICCs of anteversion angle was 0.91 for (S2) and 0.86 for (S3). For the inter-observer reliability, the ICCs of abduction angle between (S1) and (S2) was 0.89 and between (S1) and (S3) was 0.87, the ICCs of anteversion angle between (S1) and (S2) was 0.8 and between (S1) and (S3) was 0.72.
Conclusion: The reliability of registration was acceptable in abduction angle among surgeons. The reliability of registration
was not acceptable only in anteversion angle between experienced (S1) and non-experienced surgeons (S3). For the surgeon with one year of experience in imageless navigation, the result of registration process was comparable to and reproducible with the experienced surgeon in both abduction and anteversion angles.

Keywords: Reliability, Imageless navigation, Total Hip Arthroplasty