Background: Precise identification of bony landmarks by use of pointer based navigation systems is influenced by thickness of soft tissue. Ultrasound-based navigation systems try to overcome the problems of positional deviation associated with soft tissue. The aim of the study was to investigate the influence of the BMI and the thickness of the soft tissue on the post-operative cup position and accuracy in the application of an ultrasound-based (US CAOS) and a pointer-based navigation system (P CAOS).
Methods: 82 patients received a hip replacement in minimally invasive surgery in two cohorts: US CAOS group: using ultrasound navigation (n = 39) and P CAOS group: using a pointer-based navigation (n = 43).
Results: There was a significant difference in anteversion and anteversion error between the groups. In addition, we observed a significant correlation between the thickness of the presymphysial soft tissue and the anteversion error in both groups. We also detected a significant correlation between the anteversion error and the BMI in both groups. However, the absolute error in anteversion with increasing thickness of the soft tissue layer was slighter in the ultrasound-based group compared to pointer-based navigation.
Conclusions: The accuracy of the ultrasound-based and pointer-based navigation systems are influenced by the BMI and the thickness of the soft tissue layer above the symphysis. However, ultrasound-based navigation seems to have advantages with thicker soft tissue layers, as seen in overweight and obese patients.