Precision of Limb Length Measurement in Imageless Navigation THA with Modified Registration Technique in Semilateral Decubitus Position.

Background: Limb length discrepancy (LLD) following total hip arthroplasty (THA) can be a significant problem and leading to patient dissatisfaction. Several researchers have reported that imageless navigation is a reliable technique and results in more precise cup placement compared to conventional freehand techniques, however, few studies have been reported about the accuracy of the femoral stem placement.

Objective: (1) To evaluate the precision of an imageless navigation system in measuring the limb length change and (2) to evaluate LLD following imageless navigation THA with modified registration technique in semilateral decubitus position.

Material and Method: The authors reviewed 66 cases receiving cementless THA with imageless navigation from September 2013 to December 2014. The radiographic limb length change measured from pre-operative and post-operative digital x-ray was compared with the intraoperative calculation by the navigation system. Postoperative LLD in unilateral cases and second operation of staged bilateral cases were also recorded

Results: The mean radiographic limb length change measured on digital x-ray was 17.4 mm (5 to 29.3, SD 5.7). The mean limb length change calculated by navigation system was 16.8 mm (3 to 28,SD 5.9). The mean paired difference was 2.27 mm (-6 to 8, SD 0.9). This difference was significant (p = 0.01). There was significant correlation between LL change measured on digital x-ray and which were calculated by navigation system (r = 0.95, p<0.001). The navigation system had an accuracy of within 1 mm of the radiographic measurement in 7.6% of cases, within 2 mm in 39.4% of cases and within 5 mm in 93.9% of cases. The mean postoperative LLD was 2 mm (0 to 7, SD 1.9), 92% were within 5 mm.

Conclusion: The present study showed that the imageless navigation THA with modified registration technique in semilateral decubitus position offered a precise limb length measurement and the results encouraged us for clinical use to minimize LLD in THA.