title: The John Charnley Award: risk factors for cup malpositioning: quality improvement through a joint registry at a tertiary hospital.
abstract: BACKGROUND
  Few studies have examined factors that affect acetabular cup positioning. Since cup positioning has been linked to dislocation and increased bearing surface wear, these factors affecting cup position are important considerations.
  
  
  OBJECTIVE
  We determined the percent of optimally positioned acetabular cups and whether patient and surgical factors affected acetabular component position.
  
  
  METHODS
  We obtained postoperative AP pelvis and cross-table lateral radiographs on 2061 consecutive patients who received a THA or hip resurfacing from 2004 to 2008. One thousand nine hundred and fifty-two hips had AP pelvic radiographs with correct position of the hip center, and 1823 had both version and abduction angles measured. The AP radiograph was measured using Hip Analysis Suite™ to calculate the cup inclination and version angles, using the lateral film to determine version direction. Acceptable ranges were defined for abduction (30°-45°) and version (5°-25°).
  
  
  RESULTS
  From the 1823 hips, 1144 (63%) acetabular cups were within the abduction range, 1441 (79%) were within the version range, and 917 (50%) were within the range for both. Surgical approach, surgeon volume, and obesity (body mass index > 30) independently predicted malpositioned cups. Comparison of low versus high volume surgeons, minimally invasive surgical versus posterolateral approach, and obesity versus all other body mass index groups showed a twofold (1.5-2.8), sixfold (3.5-10.7), and 1.3-fold (1.1-1.7) increased risk for malpositioned cups, respectively.
  
  
  CONCLUSIONS
  Factors correlated to malpositioned cups included surgical approach, surgeon volume, and body mass index with increased risk of malpositioning for minimally invasive surgical approach, low volume surgeons, and obese patients. Further analyses on patient and surgical factors' influence on cup position at a lower volume medical center would provide a valuable comparison.
  
  
  METHODS
  Level II, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.
authors: Harris Orthopaedic Laboratory, Massachusetts General Hospital, 55 Fruit Street, GRJ 1206, Boston, MA 02114, USA.
authors: Mark C
authors: MC
authors: Callanan
authors: Bryan
authors: B
authors: Jarrett
authors: Charles R
authors: CR
authors: Bragdon
authors: David
authors: D
authors: Zurakowski
authors: Harry E
authors: HE
authors: Rubash
authors: Andrew A
authors: AA
authors: Freiberg
authors: Henrik
authors: H
authors: Malchau
mesh_terms: D000077:Acetabulum / Q000601:surgery; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects*; D001363:Awards and Prizes; D005260:Female; D006622:Hip Prosthesis; D006784:Hospitals, Teaching; D006801:Humans; D008297:Male; D019300:Medical Errors; D008875:Middle Aged; D009985:Orthopedics; D011183:Postoperative Complications; D011475:Prosthesis Failure; D058996:Quality Improvement; D012042:Registries; D012307:Risk Factors; D012955:Societies, Medical; D055815:Young Adult