title: Reduced head-neck offset in nontraumatic osteonecrosis of the femoral head.
abstract: BACKGROUND
  Risk factors for nontraumatic osteonecrosis of the femoral head have in common that they trigger intravascular coagulation and thus lead to devascularization of the femoral head. In part of the patients, however, no risk factors seem to be evident. Mechanical reasons contributing to nontraumatic osteonecrosis have not been discussed so far. We hypothesized that recurrent traumatization of the vessels supplying the femoral head by a cam-type mechanism as in femoroacetabular impingement could add to intravascular coagulation. We, therefore, asked whether structural abnormalities at the femoral head-neck junction indicative of such a mechanism could be observed in radiographs of patients with osteonecrosis of the femoral head.
  
  
  METHODS
  The preoperative anteroposterior and lateral radiographs of 77 patients who underwent surgery because of osteonecrosis of the femoral head were retrospectively screened for a reduced head-neck offset by measuring the α-angle. For comparison, the α-angle was measured on anteroposterior and lateral radiographs of 339 control subjects without evident underlying hip pathology.
  
  
  RESULTS
  The mean α-angle was 62.8° (SD 18.7°) for anteroposterior and 67.6° (SD 13.2°) for lateral radiographs in patients with nontraumatic osteonecrosis of the femoral head, whereas in control subjects, the mean α-angle was 47.2° (SD 9.6°) (p < 0.0001) and 47.6° (SD 10.3°) (p < 0.0001), respectively.
  
  
  CONCLUSIONS
  A reduced head-neck offset in patients with nontraumatic osteonecrosis of the femoral head may act as a mechanical (co-)factor in developing osteonecrosis of the femoral head.
authors.affiliation: Department of Orthopaedic Surgery, University of Ulm, Oberer Eselsberg 45, 89081, Ulm, Germany. christian.fraitzl@uni-ulm.de
authors.forename: Christian R
authors.initials: CR
authors.lastname: Fraitzl
authors.forename: Thomas
authors.initials: T
authors.lastname: Kappe
authors.forename: Annina
authors.initials: A
authors.lastname: Brugger
authors.forename: Christian
authors.initials: C
authors.lastname: Billich
authors.forename: Heiko
authors.initials: H
authors.lastname: Reichel
mesh_terms: D000328:Adult; D005260:Female; D005270:Femur Head / Q000002:abnormalities* / Q000000981:diagnostic imaging*; D005271:Femur Head Necrosis / Q000000981:diagnostic imaging*; D005272:Femur Neck / Q000002:abnormalities* / Q000000981:diagnostic imaging*; D006801:Humans; D008297:Male; D011859:Radiography; D012189:Retrospective Studies