title: Primary stability of the Fitmore stem: biomechanical comparison.
abstract: OBJECTIVE
  After clinical introduction of the Fitmore stem (Zimmer), we noticed the formation of cortical hypertrophies in a few cases. We questioned whether (1) the primary stability or (2) load transfer of the Fitmore stem differs from other stems unassociated with the formation of hypertrophies. We compared the Fitmore stem to the well-established CLS stem.
  
  
  METHODS
  Four Fitmore and four CLS stems were implanted in eight synthetic femurs. A cyclic torque around the stem axis and a mediolateral cyclic torque were applied. Micromotions between stems and femurs were measured to classify the specific rotational implant stability and to analyse the bending behaviour of the stem.
  
  
  RESULTS
  No statistical differences were found between the two stem designs with respect to their rotational stability (p = 0.82). For both stems, a proximal fixation was found. However, for the mediolateral bending behavior, we observed a significantly (p < 0.01) higher flexibility of the CLS stem compared to the Fitmore stem.
  
  
  CONCLUSIONS
  Hip stem implantation may induce remodelling of the periprosthetic bone structure. Considering the proximal fixation of both stems, rotational stability of the Fitmore® stem might not be a plausible explanation for clinically observed formation of hypertrophies. However, bending results support our hypothesis that the CLS stem presumably closely follows the bending of the bone, whereas the shorter Fitmore stem acts more rigidly. Stem rigidity and flexibility needs to be considered, as they may influence the load transfer at the implant-bone interface and thus possibly affect bone remodelling processes.
authors.affiliation: Laboratory of Biomechanics and Implant Research, Department of Orthopaedics and Traumatology, University Hospital Heidelberg, Schlierbacher Landstrasse 200a, 69118, Heidelberg, Germany.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Pepke
authors.forename: Jan
authors.initials: J
authors.lastname: Nadorf
authors.forename: Volker
authors.initials: V
authors.lastname: Ewerbeck
authors.forename: Marcus R
authors.initials: MR
authors.lastname: Streit
authors.forename: Stefan
authors.initials: S
authors.lastname: Kinkel
authors.forename: Tobias
authors.initials: T
authors.lastname: Gotterbarm
authors.forename: Michael W
authors.initials: MW
authors.lastname: Maier
authors.forename: J Philippe
authors.initials: JP
authors.lastname: Kretzer
mesh_terms: D019644:Arthroplasty, Replacement, Hip / Q000295:instrumentation*; D001696:Biomechanical Phenomena; D005269:Femur / Q000000981:diagnostic imaging*; D006622:Hip Prosthesis; D006801:Humans; D015994:Incidence; D007593:Joint Instability / Q000453:epidemiology; D008422:Materials Testing / Q000379:methods*; D011474:Prosthesis Design; D011859:Radiography; D016059:Range of Motion, Articular; D019415:Torque