Abstract
Digital radiographs are routinely used for preoperative planning, both in trauma and elective patients; particularly in preoperative templating for total hip replacement. Traditional wisdom holds that radiographs are oversized, though the degree to which this occurs is unclear. Although digital templating systems offer the use of calibration markers, this option is not always availed. We aimed to ascertain the typical magnification in departmental xrays of the hip, both to determine the typical degree of magnification as well as ascertain its consistency.
All patients undergoing dynamic hip screw fixation (DHS) in our unit over the past 12 months were identified. Using the PACS system, subsequent xrays of the patient with the implant in situ were identified; both anteroposterior abdominal and pelvic films were used. The width of a standard DHS screw (12.5 mm) was compared with the width measured on the xrays to determine a magnification factor. 164 patients were identified, of these 39 had undergone DHS fixation with subsequent xrays. 3 films were focused on the abdomen but provided good coverage of the hip also. 2 xrays were excluded – both due to limited quality. The average magnification was 26.4% (range 15.5%–42%). There was limited consistency between images.
Radiographs are a core investigation in the assessment of the orthopaedic patient. The advent of picture archiving and communications systems (PACS) has allowed the enterprising surgeon to pre-emptively plan their surgical technique and implant use. However, the utility of non-calibrated images in planning implant size is limited by variation in magnification. Surgeons should be cautious in using such images to guide their implant usage.
