Unknown femur orientation during X-ray imaging may cause inaccurate radiographic measurements. The aim of this study was to assess the effect of 3D femur orientation during radiographic imaging on the measurement of greater trochanter to femoral head centre (GT-FHC) distance. Three-dimensional femoral shapes (n=100) of unknown gender were generated using a statistical shape model based on a training data of 47 CT segmented femora. Rotations in the range of 0° internal to 50° external and 50° of flexion to 0° of extension (at 10 degree increments) were applied to each femur. A ray tracing algorithm was then used to create 2D images representing radiographs of the femora in known 3D orientations. The GT-FHC distance was then measured automatically by identifying the femoral head, shaft axis and tip of greater trochanter. Uniaxial rotations had little impact on the measurement with mean absolute error of 0.6 mm and 3.1 mm for 50° for pure external rotation and 50° pure flexion, respectively. Combined flexion and external rotation yielded more significant errors with 10° around each axis introducing a mean error of 3.6 mm and 20° showing an average error of 8.8 mm (Figure 1.). In the cohort we studied, when the femur was in neutral orientation, the tip of greater trochanter was never below the femoral head centre. Greater trochanter to femoral head centre measurement was insensitive to rotations around a single axis (i.e. flexion or external rotation). Modest combined rotations caused the tip of greater trochanter to appear more distal in 2D and led to deviation from the true value. This study suggests that a radiograph with the greater trochanter appearing below femoral head centre may have been acquired with 3D rotation of the femur.
Ideal placement of the acetabular component remains
elusive both in terms of defining and achieving a target. Our aim
is to help restore original anatomy by using the transverse acetabular
ligament (TAL) to control the height, depth and version of the component.
In the normal hip the TAL and labrum extend beyond the equator of
the femoral head and therefore, if the definitive acetabular component
is positioned such that it is cradled by and just deep to the plane
of the TAL and labrum and is no more than 4mm larger than the original
femoral head, the centre of the hip should be restored. If the face
of the component is positioned parallel to the TAL and psoas groove
the patient specific version should be restored. We still use the
TAL for controlling version in the dysplastic hip because we believe
that the TAL and labrum compensate for any underlying bony abnormality. The TAL should not be used as an aid to inclination. Worldwide,
>
75% of surgeons operate with the patient in the lateral decubitus
position and we have shown that errors in post-operative radiographic
inclination (RI) of >
50° are generally caused by errors in patient positioning.
Consequently, great care needs to be taken when positioning the
patient. We also recommend 35° of apparent operative inclination
(AOI) during surgery, as opposed to the traditional 45°. Cite this article:
