Important tips and numbers on using the cortical step and diameter difference sign in assessing femoral rotation--should we abandon the technique?
Rotational malalignment during femoral nailing is common despite having various intraoperative assessment methods. The cortical step sign and diameter difference sign (CSSDDS) is commonly used because of convenience, yet it lack proper scientific scrutiny and is thought to be error prone. Using a software algorithm, cross-sectional dimensions were obtained from CT scans of 22 intact adult femurs at the proximal, mid and distal diaphysis. With multiple simulated scenarios the sensitivity of CSSDDS was comprehensively determined at all possible C-arm positions. At rotation, cortical width changed most significantly around the thick linea aspera and femoral diameter changed most significantly at the sagittal plane. At 15 degrees of rotation and with the linea aspera in view, CSSDDS thresholds of 0.3mm, 0.6mm and 1mm had sensitivities of 98.8%, 93.1% and 73.8%. With the linea aspera masked behind the femur and out of view, the sensitivities significantly deteriorated to 96.4%, 77.1% and 44.1% respectively. CSSDDS is sufficiently sensitive only when strict rules are followed. It is imperative that the operator position the image intensifier in lateral view under proper magnification so that steps of less than 0.6mm around the linea aspera may be appreciated.
Department of Orthopaedics and Traumatology, The University of Hong Kong, Queen Mary Hospital, China. Electronic address: fangcx@gmail.com.
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Fang
The Medical School, The University of Sheffield, Sheffield, United Kingdom. Electronic address: drwggibson@gmail.com.
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Gibson
Department of Orthopaedics and Traumatology, The University of Hong Kong, Queen Mary Hospital, China. Electronic address: catcherlau@yahoo.com.hk.
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Lau
Department of Radiology, Queen Mary Hospital, Hong Kong, China. Electronic address: bxhfang@gmail.com.
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Fang
Department of Orthopaedics and Traumatology, The University of Hong Kong, Queen Mary Hospital, China; Shenzhen Key Laboratory for Innovative Technology in Orthopaedic Trauma, The University of Hong Kong-Shenzhen Hospital, Shenzhen, China. Electronic address: wongtakman@netvigator.com.
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Wong
Department of Orthopaedics and Traumatology, The University of Hong Kong, Queen Mary Hospital, China; Shenzhen Key Laboratory for Innovative Technology in Orthopaedic Trauma, The University of Hong Kong-Shenzhen Hospital, Shenzhen, China. Electronic address: klleunga@hku.hk.
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Leung
D000465:Algorithms; D002681:China; D018483:Diaphyses / Q000033:anatomy & histology / Q000000981:diagnostic imaging* / Q000473:pathology; D005264:Femoral Fractures / Q000000981:diagnostic imaging* / Q000473:pathology / Q000601:surgery; D005269:Femur / Q000033:anatomy & histology / Q000000981:diagnostic imaging* / Q000473:pathology; D005593:Fracture Fixation, Internal / Q000379:methods*; D017759:Fractures, Malunited / Q000000981:diagnostic imaging* / Q000473:pathology; D006801:Humans; D010808:Physical Examination; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012399:Rotation; D012984:Software; D014057:Tomography, X-Ray Computed
Cortical step; Femur shaft fractures; Fluoroscopy; Intramedullary nailing; Rotation; Sensitivity