title: Influence of body mass index and thickness of soft tissue on accuracy of ultrasound and pointer based registration in navigation of cup in hip arthroplasty.
abstract: 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.
authors.affiliation: Orthopaedic Department, Center for Musculoskeletal Surgery, Charité, Berlin, Germany. olaf.hasart@charite.de
authors.forename: Olaf
authors.initials: O
authors.lastname: Hasart
authors.forename: Carsten
authors.initials: C
authors.lastname: Perka
authors.forename: Koenig
authors.initials: K
authors.lastname: Christian
authors.forename: Patrick
authors.initials: P
authors.lastname: Asbach
authors.forename: Viktor
authors.initials: V
authors.lastname: Janz
authors.forename: Georgi I
authors.initials: GI
authors.lastname: Wassilew
mesh_terms: D000077:Acetabulum / Q000000981:diagnostic imaging*; D019644:Arthroplasty, Replacement, Hip / Q000295:instrumentation*; D015992:Body Mass Index; D015331:Cohort Studies; D006801:Humans; D012189:Retrospective Studies; D012885:Skinfold Thickness; D014463:Ultrasonography