Inaccuracies in the Use of Magnification Markers in Digital Hip Radiographs

Abstract

Background With the ubiquity of digital radiographs, the use of digital templating for arthroplasty has become commonplace. Although improved accuracy with digital radiographs and magnification markers is assumed, it has not been shown. Questions/PurposesWe wanted to (1) evaluate the accuracy of magnification markers in estimating the magnification of the true hip and (2) determine if the use of magnification markers improves on older techniques of assuming a magnification of 20% for all patients. Methods Between April 2013 and September 2013 we collected 100 AP pelvis radiographs of patients who had a THA prosthesis in situ and a magnification marker placed per the manufacturer's instructions. Radiographs seen during our standard radiographic review process, which met our inclusion criteria (AP pelvic view that included a well-positioned and observed magnification marker, and a prior total hip replacement with a known femoral head size), were included in the analysis. We then used OrthoView TM software program to calculate magnification of the radiograph using the magnification marker (measured magnification) and the femoral head of known size (true magnification). Results The mean true magnification using the femoral head was 21% (SD, 2%). The mean magnification using the marker was 15% (SD, 5%). The 95% CI for the mean difference between the two measurements was 6% to 7% (p \ 0.001). The use of a magnification marker to estimate magnification at the level of the hip using standard radiographic techniques was shown in this study to routinely underestimate the magnification of the radiograph using an arthroplasty femoral head of known diameter as the reference. If we assume a magnification of 20%, this more closely approximated the true magnification routinely. With this assumption, we were within 2% magnification in 64 of the 100 hips and off by 4% or more in only four hips. In contrast, using the magnification marker we were within 2% of true magnification in only 20 hips and were off by 4% or more in 59 hips. Conclusion We found the use of a magnification marker with digital radiographs for preoperative templating to be

Accuracy and Reliability of Preoperative On-screen Templating Using Digital Radiographs for Total Hip Arthroplasty

“…Whiddon and Bono18) reported that the use of radiographic size markers is encouraged to mitigate potential errors, and this marker provides an easy calculation of magnification because the actual measurement will correspond with the percentage of magnification. Whereas one recent study suggested that the use of magnification marker while taking preoperative radiographs of the hip may be unnecessary, as simply setting the software to assume a 20% magnification actually was more accurate5).…”

“…Previously many authors have emphasized the importance of preoperative templating in total hip arthroplasty (THA)1234). Benefits of preoperative templating include estimation of appropriate acetabular and femoral component sizing, appropriate restoration of preoperative offset, correction of leg length discrepancy (LLD), and anticipation of special componentry or techniques5). Moreover, the survival of an implant is critically dependent on understanding the biomechanical balance and ensuring the accurate alignment of the implant6).…”

É importante saber onde posicionar o marcador esférico para o planejamento digital de artroplastia de quadril?

“…Estudos que analisam radiografias para planejamento de artroplastia utilizam diferentes métodos de posicionamento e de marcadores sem justificar a escolha, posicionando entre as coxas ou ao nível do trocânter maior. 6,11,14,15,21 Gamble et al 8 não especificam a técnica do posicionamento, apenas indicam a região. Kosashvili et al 22 não utilizaram qualquer marcador e apenas padronizaram a magnificação em 115% para fins práticos, com pouca interferência no resultado final.…”

“…Essas dificuldades podem implicar na necessidade de realização de múltiplas radiografias para adequação do exame, podendo influenciar na calibragem: descentralização do feixe radiográfico para o lado que foi posicionado o marcador ou colocação do marcador em posição acima ou abaixo do nível do trocanter maior para não ser afastado lateralmente pelo coxim gorduroso posterolateral da coxa. Alguns autores 8,[10][11][12]21 analisaram a relação do peso e/ou índice de massa corpórea (IMC) com a magnificação radiográfica, sem identificar resultados significantes, entretanto, nesse estudo houve correlação considerada ruim desses fatores com a ausência do marcador no trocânter maior, podendo ser desconsiderada na prática.…”

Magnification of digital hip radiographs differs between clinical workplaces

“…The templating procedure has potential inherent flaws resulting from unknown radiographic magnification [ 4 ]. Preoperative radiographs could be performed with a magnification marker that allows scaling of the radiograph [ 5 ].…”