title: Instability of total hip replacement: A clinical study and determination of its risk factors.
abstract: OBJECTIVE
  To determine the risk factors associated with prosthetic dislocation and simulate a finite element model to determine the safe range of movement of various inclination and anteversion cup positions.
  
  
  METHODS
  Retrospective Case Control study with 46 dislocated patients from 1994 to 2011.
  
  
  METHODS
  83 randomly selected patients. Dislocation risk factors described in the literature were collected. A prosthetic model was simulated using finite elements with 28, 32, 36 mm heads, and a 52 mm cup. Acetabular position was 25°, 40°, and 60° tilt and with 0°, 15° and 25° anteversion. In extension of 0° and flexion of 90°, internal and external rotation was applied to analyze the range of movement, maximum resisting moment, and stress distribution in the acetabulum to impingement and dislocation.
  
  
  RESULTS
  There was greater dislocation in older patients (p=0.002). Higher dislocation in fractures than in osteoarthritis (p=0.001). Less anteversion in dislocated patients (p=0.043). Longer femoral neck in dislocated patients (p=0.002). Finite element model: lower dislocation when there is more anteversion, tilt and bigger femoral heads.
  
  
  CONCLUSIONS
  Advanced age and fractures are the major risk factors for dislocation. "Safe zone" of movement for dislocation avoidance is 40°-60° tilt and 15°-25° anteversion. Both the defect and excess of soft tissue tension predispose to dislocation. Bigger femoral heads are more stable.
authors: Servicio de Cirugía Ortopédica y Traumatología, Hospital Clínico Universitario Lozano Blesa, Zaragoza, España. Electronic address: ezqlaura@gmail.com.
authors: L
authors: L
authors: Ezquerra-Herrando
authors: Servicio de Cirugía Ortopédica y Traumatología, Hospital Clínico Universitario Lozano Blesa, Zaragoza, España.
authors: B
authors: B
authors: Seral-García
authors: Instituto Universitario de Investigación en Ingeniería de Aragón (I3A), Universidad de Zaragoza, Zaragoza, España.
authors: M P
authors: MP
authors: Quilez
authors: Instituto Universitario de Investigación en Ingeniería de Aragón (I3A), Universidad de Zaragoza, Zaragoza, España.
authors: M A
authors: MA
authors: Pérez
authors: Servicio de Cirugía Ortopédica y Traumatología, Hospital Clínico Universitario Lozano Blesa, Zaragoza, España.
authors: J
authors: J
authors: Albareda-Albareda
mesh_terms: D000368:Aged; D019644:Arthroplasty, Replacement, Hip; D016022:Case-Control Studies; D005260:Female; D020342:Finite Element Analysis; D005500:Follow-Up Studies; D006621:Hip Joint / Q000503:physiopathology; D006622:Hip Prosthesis; D006801:Humans; D004204:Joint Dislocations / Q000209:etiology* / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D011474:Prosthesis Design; D011475:Prosthesis Failure / Q000209:etiology*; D016059:Range of Motion, Articular; D012189:Retrospective Studies; D012307:Risk Factors
keywords: Anteversion; Anteversión; Dislocation; Elementos finitos; Finite elements; Inestabilidad; Instability; Luxación; Prótesis total de cadera; Total hip arthroplasty