title: Acetabular Cup Anteversion and Inclination in Hip Range of Motion to Impingement.
abstract: It is advocated that to avoid complications associated with femoral stem impingement, acetabular positioning should be within a "safe zone." However, instability remains prevalent despite accurate cup positioning, with studies showing dislocations of cups despite positioning within safe zones. We assessed cup position angles associated with impingement in a group of subjects during (1) squatting; (2) object pick-up; and (3) low chair rise.
  
  
  
  Ten subjects (mean age, 69 years; body mass index, 28.4 kg/m(2)) performed object pick up, squatting, and low-chair rising. Femur-to-pelvis relative motions were recorded for flexion/extension, abduction/adduction, and internal/external rotation. A previously reported custom-validated hip range-of-motion 3-dimensional simulator was used, set for neutral pelvic tilt and 15(°) of stem version. Acetabular cup abduction and anteversion combinations were chosen. The software computed minimum clearances between components for any hip position. An idealized tapered wedge stem with a 132° neck angle and a 36-mm femoral head was used.
  
  
  
  Eight subjects had impingement on squatting between 21(°) and 51(°) of inclination. During object pick-up, 9 subjects had impingement with inclination and anteversion angles within the "safe zone." In low-chair rise, 8 subjects had impingement at cup inclination angles between 14.5(°) and 49.5(°).
  
  
  
  The true acetabular target for impingement-avoidance motion is much smaller than previously believed and varies considerably between patients. Certain activities, such as picking up an object, low-chair rise, and squatting reduce the size of the safe zone. This study supports the need for better individualized preoperative patient-specific planning and intraoperative execution for placement of the components.
authors.affiliation: Stryker Orthopaedics, Mahwah, New Jersey.
authors.forename: Thomas F
authors.initials: TF
authors.lastname: McCarthy
authors.affiliation: Stryker Orthopaedics, Mahwah, New Jersey.
authors.forename: Vincent
authors.initials: V
authors.lastname: Alipit
authors.affiliation: Stryker Orthopaedics, Mahwah, New Jersey.
authors.forename: Jim
authors.initials: J
authors.lastname: Nevelos
authors.affiliation: Rubin Institute for Advanced Orthopedics, Center for Joint Preservation and Replacement, Sinai Hospital of Baltimore, Baltimore, Maryland.
authors.forename: Randa K
authors.initials: RK
authors.lastname: Elmallah
authors.affiliation: Rubin Institute for Advanced Orthopedics, Center for Joint Preservation and Replacement, Sinai Hospital of Baltimore, Baltimore, Maryland.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Mont
mesh_terms: D000077:Acetabulum / Q000601:surgery*; D000368:Aged; D000369:Aged, 80 and over; D019644:Arthroplasty, Replacement, Hip / Q000379:methods*; D015992:Body Mass Index; D005269:Femur / Q000601:surgery*; D005270:Femur Head / Q000601:surgery; D006801:Humans; D004204:Joint Dislocations; D008875:Middle Aged; D011474:Prosthesis Design; D016059:Range of Motion, Articular; D012984:Software
keywords: acetabular cup; anteversion; impingement; inclination; safe zone; total hip arthroplasty