Cam type femoro-acetabular impingement: quantifying the diagnosis using three dimensional head-neck ratios.
OBJECTIVE
Cam hips are commonly quantified using the two-dimensional α angle. The accuracy of this measurement may be affected by patient position and the technician's experience. In this paper, we describe a method of measurement that provides a quantitative definition of cam hips based upon three-dimensional computed tomography (CT) images.


METHODS
CT scans of 47 (24 cam, 23 normal) femurs were segmented. A sphere was fitted to the articulating surface of the femoral head, the radius (r) recorded, and the femoral neck axis obtained. The cross sectional area at four locations spanning the head neck junction (r/4, r/2, 3r/4 and r), perpendicular to the neck axis, was measured. The ratios (Neck/Head) between the areas at each cut relative to the surface area at the head centre were calculated and aggregated.


RESULTS
Normal and cam hips were significantly different: the sum of the head-neck ratios (HNRs) of the cam hips were always smaller than normal hips (p < 0.01). A cut off point of 2.55 with no overlap was found between the two groups, with HNRs larger than this being cam hips, and smaller being normal ones.


CONCLUSIONS
Owing to its sensitivity and repeatability, the method could be used to confirm or refute the clinical diagnosis of a cam hip. Furthermore it can be used as a tool to measure the outcome of cam surgery.
MSK Laboratory, Imperial College London, Charing Cross Hospital, London, UK. m.masjedi@ic.ac.uk
Milad
M
Masjedi
Charles S
CS
Marquardt
Isabella M H
IM
Drummond
Simon J
SJ
Harris
Justin P
JP
Cobb
D005260:Female; D057925:Femoracetabular Impingement / Q000000981:diagnostic imaging*; D005270:Femur Head / Q000002:abnormalities* / Q000000981:diagnostic imaging*; D005272:Femur Neck / Q000002:abnormalities* / Q000000981:diagnostic imaging*; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008297:Male; D008875:Middle Aged; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*