Magnification of digital hip radiographs differs between clinical workplaces

Abstract

Preoperative planning for total hip arthroplasty includes templating on anteroposterior radiographs. It is necessary to consider radiographic magnification in order to scale templates accurately. Studies dealing with hip templating report different values of radiographic magnification. It is not clear if the observed difference in magnification between the studies is caused by variability in studied groups, methodology or instrumentation. We hypothesize that there is a difference in magnification between clinical workplaces. Within this study, radiographic magnification was estimated on 337 radiographs of patients after total hip surgery from five orthopaedic departments in the Czech Republic. Magnification was determined for each patient as a ratio between diameter of implanted femoral head measured on radiogram and its true size. One-way ANOVA revealed significant differences in magnification between workplaces (F(4,332) = 132, p≤0.001). These results suggest that radiographic magnification depends on the workplace where it is taken or more precisely on radiographic device. It indicates potential limits in generalizability of results of studies dealing with preoperative planning accuracy to other institutions.

Pre‐operative templating for total hip arthroplasty: How does radiographic technique and calibration marker placement affect image magnification?

“…Within the literature there is a preference for the use of fixed MF. 6,10,12,[17][18][19] However our results suggest that fixed MF values of 110-120% could be too low for accurate templating. To our knowledge, studies that prefer the use of a fixed MF have neglected to mention the effect of patient body habitus.…”

“…5,6,11,18,20,22 As shown in our results the use of a SID of 100 cm concurred the greatest degree of magnification on the image and caused the greatest shift from the typical fixed MF value of 110% to 120%. Hornová et al 12 and Olmedo-Garcia et al 20 suggested that standardised MF should be calibrated for each clinical site based upon individual protocols to account for this discrepancy in SID. Kwok et al, 14 calculated the degree of magnification in a series of images, ensuring that SID and SOD were included on the initial radiographs by the radiographer as part of their inclusion criteria.…”

“…The primary goal of total hip arthroplasty (THA) is pain reduction and restoration of normal hip function, whilst limiting the rate of intra‐operative and post‐operative complications. 3 , 10 , 11 , 12 This can be achieved via accurate pre‐operative radiographic pelvic templating prior to surgical intervention. Pelvic templating relies on an understanding of the theory behind divergence of ionising radiation from a point source, and more specifically the factors, which can affect the magnification of a radiographic image: source‐to‐image distance (SID), source‐to‐object distance (SOD) and lateral positioning (See Fig.…”

“…There is still debate as to which digital templating technique is most reliable and accurate in clinical practice. Some clinical centres prefer the use of a fixed MF, 6 , 10 , 12 , 17 , 18 as it can provide consistent templating results that are less dependent on radiographic positioning. This method was also used in traditional acetate templating, with a typical assumed MF of 110 to 120%.…”

Digital magnification error when templating total hip replacement?

“…However, two other similar studies using the implanted femoral head have both shown magnification error to consistently near 6%, which suggests despite a standardized radiological protocol, scaling using an ECM has inherent consistent magnification error of 6% [13,19]. Magnification error can also differ between different radiological departments which use the same standardized protocol [20].…”