title: Assessment of the primary rotational stability of uncemented hip stems using an analytical model: comparison with finite element analyses.
abstract: BACKGROUND
  Sufficient primary stability is a prerequisite for the clinical success of cementless implants. Therefore, it is important to have an estimation of the primary stability that can be achieved with new stem designs in a pre-clinical trial. Fast assessment of the primary stability is also useful in the preoperative planning of total hip replacements, and to an even larger extent in intraoperatively custom-made prosthesis systems, which result in a wide variety of stem geometries.
  
  
  METHODS
  An analytical model is proposed to numerically predict the relative primary stability of cementless hip stems. This analytical approach is based upon the principle of virtual work and a straightforward mechanical model. For five custom-made implant designs, the resistance against axial rotation was assessed through the analytical model as well as through finite element modelling (FEM).
  
  
  RESULTS
  The analytical approach can be considered as a first attempt to theoretically evaluate the primary stability of hip stems without using FEM, which makes it fast and inexpensive compared to other methods. A reasonable agreement was found in the stability ranking of the stems obtained with both methods. However, due to the simplifying assumptions underlying the analytical model it predicts very rigid stability behaviour: estimated stem rotation was two to three orders of magnitude smaller, compared with the FEM results.
  
  
  CONCLUSIONS
  Based on the results of this study, the analytical model might be useful as a comparative tool for the assessment of the primary stability of cementless hip stems.
authors.affiliation: Katholieke Universiteit Leuven (K,U,Leuven), Division of Biomechanics and Engineering Design (BMGO), Celestijnenlaan 300C, 3001 Heverlee, Belgium.
authors.forename: Maria E
authors.initials: ME
authors.lastname: Zeman
authors.forename: Nicolas
authors.initials: N
authors.lastname: Sauwen
authors.forename: Luc
authors.initials: L
authors.lastname: Labey
authors.forename: Michiel
authors.initials: M
authors.lastname: Mulier
authors.forename: Georges
authors.initials: G
authors.lastname: Van der Perre
authors.forename: Siegfried V N
authors.initials: SV
authors.lastname: Jaecques