title: Identification of femoral head center of bipolar hemiarthroplasy in radiostereometric analysis with elementary geometrical shape models.
abstract: Elementary geometrical shape (EGS) models are useful in radiostereometric analysis (RSA) on hip stems because tantalum markers attached to the stems can be omitted. In order to create an EGS model of a femoral stem, the center of the femoral head has to be identified. The contour of the femoral head is recommended to be used. However, the contour of the femoral head cannot be detected exclusively by computer if it is combined with a bipolar head or a metal cup. We therefore hypothesized that the contour of the outer head of bipolar hemiarthroplasty can be included in the EGS model as well as the femoral head contour. We calculated the time required for the detection of the contour, the precision of analysis and the stem micromotion at 2 years using the two different methods in the same picture set and compared the results. The detection of the bipolar head contour was 10 times faster than that of the femoral head contour. The precision for subsidence was 0.16 mm in EGS RSA with the femoral head contour, and 0.15 mm with the bipolar head contour (p=0.68). The precisions were comparable and clinically acceptable. There was no significant difference between the results of the 2-year micromotion with the two different methods. We conclude that this new method is applicable to measure stem micromotion of hemi-arthroplasty with EGS RSA and the method facilitates the Radiostereometric analysis.
authors.affiliation: Center for Implant and Radiostereometric Research Oslo, Oslo University Hospital, Ullevål, Oslo, Norway. Electronic address: masakotsukanaka@gmail.com.
authors.forename: Masako
authors.initials: M
authors.lastname: Tsukanaka
authors.affiliation: Center for Implant and Radiostereometric Research Oslo, Oslo University Hospital, Ullevål, Oslo, Norway.
authors.forename: Stephan M
authors.initials: SM
authors.lastname: Röhrl
authors.affiliation: Department of Orthopaedics and Clinical Sciences, Lund University, Skane University Hospital, Malmö, Sweden.
authors.forename: Thord
authors.initials: T
authors.lastname: von Schewelov
authors.affiliation: Center for Implant and Radiostereometric Research Oslo, Oslo University Hospital, Ullevål, Oslo, Norway.
authors.forename: Lars
authors.initials: L
authors.lastname: Nordsletten
mesh_terms: D019644:Arthroplasty, Replacement, Hip / Q000295:instrumentation* / Q000379:methods; D005269:Femur / Q000601:surgery; D005270:Femur Head / Q000601:surgery*; D062785:Hemiarthroplasty / Q000295:instrumentation* / Q000379:methods; D006801:Humans; D008962:Models, Theoretical; D009038:Motion; D020959:Polyethylene / Q000737:chemistry; D011184:Postoperative Period; D059869:Radiostereometric Analysis; D015203:Reproducibility of Results; D012984:Software; D013314:Stress, Mechanical; D013635:Tantalum
chemical_list: D013635:Tantalum; D020959:Polyethylene
keywords: Elementary geometrical shape; Femoral head center; Hemi hip arthroplasty; Radiostereometry; Stem micromotion