title: Influence of stem design on the primary stability of megaprostheses of the proximal femur.
abstract: OBJECTIVE
  Extended bone defects of the proximal femur can be reconstructed by megaprostheses for which aseptic loosening constitutes one of the major failure modes. The basic requirement for long-term success of endoprostheses is primary stability. We therefore assessed whether sufficient primary stability can be achieved by four different megaprostheses in a standardised bone defect of the proximal femur and whether their different design leads to different fixation patterns.
  
  
  METHODS
  Four different designs of proximal femoral replacements were implanted into 16 Sawbones® after preparing segmental bone defects (AAOS type II). Primary rotational stability was analysed by application of a cyclic torque of ±7 Nm and measuring the relative micromotions between bone and implant at different levels. The main fixation zones and differences of fixation patterns of the stem designs were determined by an analysis of variance.
  
  
  RESULTS
  All four implants exhibited micromotions below 150 μm, indicating adequate primary stability. Lowest micromotions for all designs were located near the femoral isthmus. The extent of primary stability and the global implant fixation pattern differed considerably and could be related to the different design concepts.
  
  
  CONCLUSIONS
  All megaprostheses studied provided sufficient primary stability if the fixation conditions of the femoral isthmus were intact. The design characteristics of the different stems largely determined the extent of primary stability and fixation pattern. Understanding these different fixation types could help the surgeon to choose the most suitable implant if the fixation conditions in the isthmus are compromised.
authors: Laboratory of Biomechanics and Implant Research, Department of Orthopaedics and Traumatology, University Hospital Heidelberg, Schlierbacher Landstrasse 200a, 69118, Heidelberg, Germany, stefankinkel@gmx.de.
authors: Stefan
authors: S
authors: Kinkel
authors: Jan Dennis
authors: JD
authors: Graage
authors: Jan Philippe
authors: JP
authors: Kretzer
authors: Eike
authors: E
authors: Jakubowitz
authors: Jan
authors: J
authors: Nadorf
mesh_terms: D019544:Equipment Failure Analysis; D005269:Femur / Q000033:anatomy & histology* / Q000601:surgery; D006621:Hip Joint / Q000033:anatomy & histology* / Q000601:surgery; D006622:Hip Prosthesis / Q000145:classification*; D006801:Humans; D007593:Joint Instability / Q000209:etiology*; D008953:Models, Anatomic; D010027:Osteotomy; D011474:Prosthesis Design / Q000145:classification*; D011475:Prosthesis Failure / Q000209:etiology*; D016059:Range of Motion, Articular
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