Abstract:

Objective　 To compare the alpha angle value measured on different planes and to determine the prevalence of cam-type femoroacetabular impingement(FAI) morphology in a young population without symptoms using computed tomography.　Methods　A retrospective review was performed on the data of 120 patients (age range, 20–40 years) with no current or previous hip disorder who underwent CT of the abdomen and pelvis from May 2009 to November 2013. Alpha angle were measured on both oblique axial plane images parallel to the long axis of the femoral neck and radial images in four planes obtained using the center of the femoral neck as the axis of rotation.Analysis of variance was used to evaluate differences between alpha angle values on the oblique axial plane and radial plane.Alpha angle greater than 55° was considered positive for cam-type FAI morphology.　Results　The mean alpha angle value on the oblique axial plane was (39.5±6.51)º，Alpha angles greater than 55° were found in 14 joints of 12 cases（10%，12/120） on the oblique axial plane image，10 cases were unilateral and 2 cases were bilateral. The mean alpha angle values at the 12,1,2,3 o’clock position were (42.8±7.3)º,(48.8±7.6 )º,(47.3±7.2)º,(40.2±6.7)º, respectively. 106 Alpha angles greater than 55° in total were found on the radial plane images. 54 abnormal alpha angles were measured at the 1:00 position, including 35% （42 /120）of the subjects.30 cases were unilateral and 12 cases were bilateral.The differences of the alpha angles between the oblique axial and the radial 12 o'clock, 1 o'clock, 2o'clock plane reached statistical significance（P<0.05）.　Conclusion　Alpha angle measurements on the radial plane images may be more accurate. The frequency of cam-type FAI morphology was high in a young population without symptoms.

Key words: Femoroacetabular impingement, Hip, Computed tomography