title: Biomechanical factors in planning of periacetabular osteotomy.
abstract: OBJECTIVE
  This study addresses the effects of cartilage thickness distribution and compressive properties in the context of optimal alignment planning for periacetabular osteotomy (PAO).
  
  
  BACKGROUND
  The Biomechanical Guidance System (BGS) is a computer-assisted surgical suite assisting surgeon's in determining the most beneficial new alignment of a patient's acetabulum. The BGS uses biomechanical analysis of the hip to find this optimal alignment. Articular cartilage is an essential component of this analysis and its physical properties can affect contact pressure outcomes.
  
  
  METHODS
  Patient-specific hip joint models created from CT scans of a cohort of 29 dysplastic subjects were tested with four different cartilage thickness profiles (one uniform and three non-uniform) and two sets of compressive characteristics. For each combination of thickness distribution and compressive properties, the optimal alignment of the acetabulum was found; the resultant geometric and biomechanical characterization of the hip were compared among the optimal alignments.
  
  
  RESULTS
  There was an average decrease of 49.2 ± 22.27% in peak contact pressure from the preoperative to the optimal alignment over all patients. We observed an average increase of 19 ± 7.7° in center-edge angle and an average decrease of 19.5 ± 8.4° in acetabular index angle from the preoperative case to the optimized plan. The optimal alignment increased the lateral coverage of the femoral head and decreased the obliqueness of the acetabular roof in all patients. These anatomical observations were independent of the choice for either cartilage thickness profile, or compressive properties.
  
  
  CONCLUSIONS
  While patient-specific acetabular morphology is essential for surgeons in planning PAO, the predicted optimal alignment of the acetabulum was not significantly sensitive to the choice of cartilage thickness distribution over the acetabulum. However, in all groups the biomechanically predicted optimal alignment resulted in decreased joint contact pressure and improved acetabular coverage.
authors.affiliation: Department of Biomedical Engineering, Johns Hopkins University , Baltimore, MD , USA.
authors.forename: Noushin
authors.initials: N
authors.lastname: Niknafs
authors.affiliation: Department of Research and Engineering Development, Johns Hopkins University Applied Physics Laboratory , Laurel, MD , USA ; Department of Mechanical Engineering, Johns Hopkins University , Baltimore, MD , USA.
authors.forename: Ryan J
authors.initials: RJ
authors.lastname: Murphy
authors.affiliation: Department of Research and Engineering Development, Johns Hopkins University Applied Physics Laboratory , Laurel, MD , USA.
authors.forename: Robert S
authors.initials: RS
authors.lastname: Armiger
authors.affiliation: ORTON Orthopaedic Hospital , Helsinki , Finland.
authors.forename: Jyri
authors.initials: J
authors.lastname: Lepistö
authors.affiliation: Department of Research and Engineering Development, Johns Hopkins University Applied Physics Laboratory , Laurel, MD , USA ; Department of Mechanical Engineering, Johns Hopkins University , Baltimore, MD , USA.
authors.forename: Mehran
authors.initials: M
authors.lastname: Armand
keywords: articular cartilage thickness; biomechanical analysis; cartilage compressibility; periacetabular osteotomy; preoperative planning