Do fluoroscopic and radiographic images underestimate pin protrusion in paediatric supracondylar humerus and distal radius fractures? A synthetic bone model analysis

PURPOSE: Fluoroscopy is commonly used to confirm acceptable position of percutaneously placed pins when treating paediatric fractures. There is a paucity of literature investigating the accuracy of fluoroscopic imaging when determining pin position relative to the far cortex of the fixated bone. The purpose of this study was to evaluate the accuracy of fluoroscopic and radiographic imaging in measuring smooth pin protrusion from the far cortex of a bone model. METHODS: Eight bone models were implanted with smooth pins and anteroposterior fluoroscopic and radiographic studies were obtained. All images were evaluated by orthopaedic attending physicians, residents and medical students. The length of pin protrusion from the model surface was estimated on fluoroscopic imaging and measured on radiographs and compared with actual lengths measured on the bone models. RESULTS: 20 evaluators took a total of 320 pin measurements on images of 8 models. There was a significant difference between fluoroscopic measurements compared to radiographic measurements and actual pin lengths. There was no significant difference between radiographic measurements and actual pin lengths. Level of training of examiner was not statistically significant. On average, fluoroscopic estimations of pin protrusion were 1.53 mm shorter than the actual measured length. CONCLUSION: Fluoroscopic images underestimate the length of smooth pins protruding from a bone model surface when compared with radiographs and actual measurements. Orthopaedic surgeons using fluoroscopy should be aware of this discrepancy when assessing intraoperative fluoroscopic images to decide on acceptable implant position. LEVEL OF EVIDENCE: Level V