Abstract
Introduction: Accurate pre-operative templating is important in primary and revision hip replacement surgery. Most manufacturers supply templates at 120% magnification, but magnification of radiographs varies markedly and this is further complicated by the use of digital systems, where radiographs can be printed at various magnifications. We have produced a simple protocol to allow radiographs to be routinely produced at a magnification of 120%.
Methods: We positioned a marker, of known size, at various distances from the x-ray tube using both conventional and digital (PACS) machines and noted when 120% marker magnification was achieved. With digital machines, we also looked at the effect of varying the magnification of the printed radiograph. We set the film-focus distance (distance between source and plate) to the optimal distance discovered in patients undergoing pelvic and hip radiographs and used a marker, placed at the mid portion of the greater trochanter, to verify the magnification of the hip joint. We compared the known marker size with the measured size on the radiographs of 35 patients. Our protocol was separately tested on 5 different x-ray machines in our hospital.
Results: The optimal film-focus distance required to achieve 120% magnification was 100 cm. With digital systems it was important to print the radiographs at “true size” (100%), rather than “best fit” (the usual default mode). In the 25 patients tested, the mean magnification achieved was 118.8%,with a SD of 1.295. Our protocol was reproducible on all five different x-ray machines in our department.
Discussion: We consistently and reproducibly achieved radiographs magnified at 120% using our simple protocol, allowing standard templates to be used for accurate pre-operative planning.
Correspondence should be addressed to Mr John Hodgkinson, BHS, c/o BOA, The Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
