title: The crossover sign overestimates acetabular retroversion.
abstract: BACKGROUND
  The crossover sign is a radiographic finding associated with cranial acetabular retroversion and has been associated with pincer-type femoroacetabular impingement (FAI) in patients with hip pain. Variable morphology, location, and size of the anterior inferior iliac spine (AIIS) may contribute to the crossover sign even in the absence of retroversion. Thus, the sign may overestimate the incidence of acetabular retroversion.
  
  
  OBJECTIVE
  We asked: Can the crossover sign appear on standardized, well-positioned AP pelvis radiographs despite the absence of acetabular retroversion? And what is the contribution of variable size and morphology of the AIIS to a crossover sign?
  
  
  METHODS
  We evaluated radiographs of 53 patients with symptomatic FAI in the absence of substantial chondral degenerative changes (< Tönnis Grade 2). Forty-one radiographs met the appropriate criteria of neutral tilt and obliquity. Three independent reviewers determined presence and location of the crossover sign. Acetabular version was defined using high-resolution three-dimensional CT. CT reconstructions were used to define three AIIS types addressing AIIS morphology.
  
  
  RESULTS
  Nineteen of 38 radiographs with a crossover sign on AP radiographs had focal or global acetabular retroversion on three-dimensional CT (positive and negative predictive values = 50%). In contrast, the AIIS extended to or below the level of the anterior superior acetabular rim and was partially or completely responsible for the appearance of a radiographic crossover sign in all 19 hips with an anteverted acetabulum. High interobserver reliability (kappa > 0.8) was found for AIIS types.
  
  
  CONCLUSIONS
  A crossover sign is frequently present on well-positioned AP pelvis radiographs in the absence of acetabular retroversion. Variable AIIS morphology may explain the appearance of this sign in the presence or absence of acetabular retroversion.
authors.affiliation: Department of Orthopedic Surgery, William Beaumont Hospital, Royal Oak, MI, USA.
authors.forename: Ira
authors.initials: I
authors.lastname: Zaltz
authors.forename: Bryan T
authors.initials: BT
authors.lastname: Kelly
authors.forename: Iftach
authors.initials: I
authors.lastname: Hetsroni
authors.forename: Asheesh
authors.initials: A
authors.lastname: Bedi
mesh_terms: D000077:Acetabulum / Q000000981:diagnostic imaging*; D060751:Bone Retroversion / Q000145:classification / Q000000981:diagnostic imaging* / Q000453:epidemiology; D005260:Female; D057925:Femoracetabular Impingement / Q000000981:diagnostic imaging* / Q000453:epidemiology; D006621:Hip Joint / Q000000981:diagnostic imaging*; D006801:Humans; D021621:Imaging, Three-Dimensional; D015994:Incidence; D008297:Male; D015588:Observer Variation; D011237:Predictive Value of Tests; D011379:Prognosis; D011857:Radiographic Image Interpretation, Computer-Assisted; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed