Abstract
In recent years the majority of X-ray departments have moved to a digital format of recording and archiving radiographs. These digital images (as with previous ‘films’) have a built in magnification factor (variable with each patient), which, may cause errors in templating for joint replacement surgery. Placing a marker of known size at the same level as the joint in question allows calculation of the magnification. This may help to restore hip offset in total hip replacement.
To establish the magnification factor for digital radiographs taken in our unit.
To assess the usefulness of marker images in accurate preoperative templating. Preoperative marker radiographs were identified retrospectively. The apparent size of the marker was measured on digital image. This value was used to calculate the magnification of the image. The scaled X-ray was up loaded to a digital templating software programme. This software uses a ‘scaling tool’ to calculate the magnification of the image. The hip joint templating tool was the used to calculate the offset of the proximal femur, this was performed with the calculated magnification and also an assumed magnification of 120%. The recommended offset of Exeter V40 stem was noted for both values.
Images were identified for 40 patients with markers. The average magnification was 122% for both PACS and Orthoview with a range 113% – 129% and a standard deviation of 4%. The median value for magnification was 120%. The average change in offset between calculated and estimated magnification was 1.275mm with a maximum change of 3mm. In two cases this difference resulted in a change in the recommended offset (5%).
The use of marker radiographs is widely described. In this small series the magnification is the same as previously reported in other studies. The difference in offset between calculated and estimated magnification was relatively small and caused a change in the recommended offset in only two patients. Variation in the use of the templating tool in our software can produce a much greater change in offset. Marker radiographs will only be useful as part of a standardised method of pre-operative templating.
