Distances and angles in standing long-leg radiographs: comparing conventional radiography, digital radiography, and EOS

Christof Birkenmaier, Louise Levrard , Carolin Melcher , Bernd Wegener, Jens Ricke, Boris M Holzapfel, Andrea Baur-Melnyk , Dirk Mehrens 

Skeletal Radiol. 2024 Aug;53(8):1517-1528.doi: 10.1007/s00256-024-04592-9. Epub 2024 Feb 21.

Abstract

Objective: Distances and angles measured from long-leg radiographs (LLR) are important for surgical decision-making. However, projectional radiography suffers from distortion, potentially generating differences between measurement and true anatomical dimension. These phenomena are not uniform between conventional radiography (CR) digital radiography (DR) and fan-beam technology (EOS). We aimed to identify differences between these modalities in an experimental setup.

Materials and methods: A hemiskeleton was stabilized using an external fixator in neutral, valgus and varus knee alignment. Ten images were acquired for each alignment and each modality: one CR setup, two different DR systems, and an EOS. A total of 1680 measurements were acquired and analyzed.

Results: We observed great differences for dimensions and angles between the 4 modalities. Femoral head diameter measurements varied in the range of > 5 mm depending on the modality, with EOS being the closest to the true anatomical dimension. With functional leg length, a difference of 8.7% was observed between CR and EOS and with the EOS system being precise in the vertical dimension on physical-technical grounds, this demonstrates significant projectional magnification with CR-LLR. The horizontal distance between the medial malleoli varied by 20 mm between CR and DR, equating to 21% of the mean.

Conclusions: Projectional distortion resulting in variations approaching 21% of the mean indicate, that our confidence on measurements from standing LLR may not be justified. It appears likely that among the tested equipment, EOS-generated images are closest to the true anatomical situation most of the time.

Keywords: Cone-beam; Conventional radiography; Digital radiography; Distortion; EOS; Fan-beam; Lower extremity; Projectional distortion; Slot-scanner; Valgus; Varus.

https://pubmed.ncbi.nlm.nih.gov/38378861/