Feasibility study of custom hip stem design based on X-ray films.

The hip stem design based upon X-ray films requires sufficient information in order to customize for the patient. If the errors are resulted from rotating angles, more X-ray films may carry out the solution. The present study aimed to realize the improvement of multiple X-ray films for hip stem design. Eight X-ray films were taken from a volunteer. The characteristic length of X-ray film was input into cosine function to derive the relative rotating angle of each X-ray film. The combinations of 2, 4, 6, and 8 X-ray films that included true AP view and lateral view were obtained to construct the corresponding stem. CT images were used to construct femur and stem model to verify the rotating angle of femur in each X-ray film. A quantitative comparison was made between the hip stems for the fill ratio, fit gap and contact area. The results showed that the markers on the body surface would result in error and were unable to provide reference to the rotating angle of femur. The contact area and fill ratio of the stems designed by 6 or 8 X-ray films would get closer to that by CT images. It is concluded that the rotation of the femur could be determined from the length of femoral neck as well as the distance between the tips of medial and lateral condyles. The use of 6 or more X-ray films for custom-made hip stem as a source of design is recommended in this study.