Short-term variability of proximal femur shape in anteroposterior pelvic radiographs

Anteroposterior (AP) pelvic radiographs only give a 2D projection of what is a 3D shape. Depending on the position of the patient while the radiograph is taken, the 2D projection of the shape may vary. This paper investigates the short-term variability of projected proximal femur shape using AP pelvic radiographs. We train a statistical shape model of the proximal femur based on a set of 87 radiographs–42 of which represent 21 pairs of two radiographs per person taken at two different sessions on the same day. These 21 pairs allow us to use the parameters of the shape model to analyse the within-person variation of projected proximal femur shape. We demonstrate that the within-person variation is small compared to the overall shape variation of the proximal femur given by all 87 radiographs. This work underlines the suitability of AP pelvic radiographs–even though only giving a 2D projection–for the analysis of the relationship between proximal femur shape and diseases such as osteoarthritis or osteoporosis.