title: Neck fracture of femoral stems with a sharp slot at the neck: biomechanical analysis.
abstract: BACKGROUND
  Fracture of the femoral stem in total hip arthroplasty (THA) is a rare complication. We have encountered 2 cases of neck fractures of the femoral stem occurring 9 and 12 years after THA. Morphological and biomechanical analysis were performed to investigate the mechanism of these fractures.
  
  
  METHODS
  A titanium alloy femoral stem having a slot with sharp corners (R = 0.2 mm) at the neck had been implanted in both cases. Fracture surfaces were examined by use of scanning electron microscopy (SEM). Stress concentration was simulated by using a finite element method (FEM) to compare slots with sharp (R = 0.2 mm) and smooth (R = 2 mm) corners.
  
  
  RESULTS
  Study of the retrieved stems revealed that neck fractures had occurred at the distal end of the slot in both cases. SEM revealed numerous fine fissures extending from the anterolateral edge, striations on the middle of the fracture surface, and dimples on the posteromedial surface, suggesting that the fractures had occurred from the anterolateral aspect toward the posteromedial aspect because of metallic fatigue. FEM analysis showed that mechanical stress was concentrated at the distal and anterolateral corners of the slot. Under 3500-N loading force, the stress at the sharp corner was 556 MPa, which was approximately twofold that at the smooth corner and exceeded the fatigue strength of titanium alloy.
  
  
  CONCLUSIONS
  These findings showed that the sharp corner of slot increased stress concentrations at the anterolateral aspect and led to the neck fractures.
authors.affiliation: Department of Orthopedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Kensei
authors.initials: K
authors.lastname: Yoshimoto
authors.affiliation: Department of Orthopedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan. yasunaka@ortho.med.kyushu-u.ac.jp.
authors.forename: Yasuharu
authors.initials: Y
authors.lastname: Nakashima
authors.affiliation: Kyocera Medical Corporation, 3-3-31 Miyahara, Yodogawa-ku, Osaka, 532-0003, Osaka, Japan.
authors.forename: Akihiro
authors.initials: A
authors.lastname: Nakamura
authors.affiliation: Department of Orthopedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Taro
authors.initials: T
authors.lastname: Mawatari
authors.affiliation: Division of Renewable Energy Dynamics, Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan.
authors.forename: Mitsugu
authors.initials: M
authors.lastname: Todo
authors.affiliation: Department of Orthopedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Hara
authors.affiliation: Department of Orthopedic Surgery, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan.
authors.forename: Yukihide
authors.initials: Y
authors.lastname: Iwamoto
mesh_terms: D000497:Alloys; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects*; D001696:Biomechanical Phenomena; D005265:Femoral Neck Fractures / Q000000981:diagnostic imaging / Q000503:physiopathology* / Q000601:surgery; D020342:Finite Element Analysis; D006801:Humans; D008297:Male; D008875:Middle Aged; D011475:Prosthesis Failure; D011859:Radiography; D012086:Reoperation; D013314:Stress, Mechanical
chemical_list: D000497:Alloys; C528635:titanium aluminum alloy