title: Potential pathogenic mechanism for stress fractures of the bowed femoral shaft in the elderly: Mechanical analysis by the CT-based finite element method.
abstract: BACKGROUND
  Stress fractures of the bowed femoral shaft (SBFs) may be one of the causes of atypical femoral fractures (AFFs). The CT-based finite element method (CT/FEM) can be used to structurally evaluate bone morphology and bone density based on patient DICOM data, thereby quantitatively and macroscopically assessing bone strength. Here, we clarify the pathogenic mechanism of SBFs and demonstrate this new understanding of AFFs through mechanical analysis by CT/FEM.
  
  
  METHODS
  A prospective clinical study was performed from April 2012 to February 2014. We assembled two study groups, the bowed AFF group (n=4 patients; mean age, 78.0 years) including those with a prior history of AFF associated with bowing deformity and the thigh pain group (n=14 patients; mean age, 78.6 years) comprising outpatients with complaints of thigh pain and tenderness. Stress concentration in the femoral shaft was analysed by CT/FEM, and the visual findings and extracted data were assessed to determine the maximum principal stress (MPS) and tensile stress-strength ratio (TSSR). In addition, we assessed femoral bowing, bone density, and bone metabolic markers. Wilcoxon's rank sum test was used for statistical analysis.
  
  
  RESULTS
  All patients in the bowed AFF group showed a marked concentration of diffuse stress on the anterolateral surface. Thirteen patients in the thigh pain group had no significant findings. However, the remaining 1 patient had a finding similar to that observed in the bowed AFF group, with radiographic evidence of bowing deformity and a focally thickened lateral cortex. Patients were reclassified as having SBF (n=5) or non-SBF (n=13). Statistical analysis revealed significant differences in MPS (p=0.0031), TSSR (p=0.0022), and femoral bowing (lateral, p=0.0015; anterior, p=0.0022) between the SBF and non-SBF groups, with no significant differences in bone density or bone metabolic markers.
  
  
  CONCLUSIONS
  Significant tensile stress due to bowing deformity can induce AFFs. SBFs should be considered a novel subtype of AFF, and patients with complaints of thigh pain and femoral shaft bowing deformity must be considered at high risk for AFFs. This project (Ref: AOTAP 13-13) was supported by AOTrauma Asia Pacific.
authors.affiliation: Department of Orthopaedic and Spinal Surgery, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Japan; Department of Orthopaedic Surgery, Saku Central Hospital, Japan. Electronic address: ohorth@tmd.ac.jp.
authors.forename: Yoto
authors.initials: Y
authors.lastname: Oh
authors.affiliation: Department of Orthopaedic and Spinal Surgery, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Japan.
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Wakabayashi
authors.affiliation: Department of Orthopaedic Surgery, Saku Central Hospital, Japan.
authors.forename: Yoshiro
authors.initials: Y
authors.lastname: Kurosa
authors.affiliation: Department of Orthopaedic and Spinal Surgery, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Japan.
authors.forename: Koji
authors.initials: K
authors.lastname: Fujita
authors.affiliation: Department of Orthopaedic and Spinal Surgery, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Japan.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Okawa
mesh_terms: D000368:Aged; D015519:Bone Density; D050071:Bone Density Conservation Agents / Q000627:therapeutic use; D005260:Female; D005264:Femoral Fractures / Q000209:etiology / Q000378:metabolism / Q000473:pathology*; D020342:Finite Element Analysis; D015775:Fractures, Stress / Q000209:etiology / Q000378:metabolism / Q000473:pathology*; D006801:Humans; D007564:Japan / Q000453:epidemiology; D008297:Male; D010024:Osteoporosis / Q000150:complications / Q000378:metabolism / Q000473:pathology*; D011446:Prospective Studies; D012424:Rural Population; D014057:Tomography, X-Ray Computed
chemical_list: D050071:Bone Density Conservation Agents
keywords: Atypical femoral fracture; Biomechanics; CT/FEM; Femoral shaft; Finite element method; Insufficiency fracture; Osteoporosis; Stress fracture