Creating a "Box View" of Periprosthetic Distal Femur Fractures Using Three-Dimensional Image Processing

Retrograde intramedullary nailing of periprosthetic distal femur fractures requires an open box femoral component. However, presence of an open box is difficult to determine using standard imaging. The objective of this study is to present a novel three-dimensional (3D) image processing and analysis technique, called the "Box View," which addresses this problem. Twelve patients undergoing revision total knee arthroplasty's pre-operative computed tomography scans were reformatted to subtract the tibia, and a 3D Box View movie was generated. The method of this study included 4 attending surgeons and 6 residents who reviewed radiographs, computed tomography (CT), and Box View images, then interpreted presence of an open box. Inter-observer reliability was assessed with 2-factor analysis of variance (ANOVA) and sensitivity/specificity of the Box View versus radiograph and CT was performed with a Stuart-Maxwell test. The results of this study showed that the Box View was 81% sensitive and 97% specific for identification of an open box, with a positive predictive value of 100% and a negative predictive value of 85%. The Box View's specificity was significantly better than radiographs (53%, P=.003) and CT (50%, P=.002), and sensitivity (81%) was significantly better than CT (34%, P<.001) but not radiographs (77%, P=.63). Inter-observer agreement with the Box View was 0.88, radiographs was 0.65 and CT was 0.09. The conclusion was that the Box View is a novel and useful pre-operative planning tool for treatment of distal femur fractures with high sensitivity and specificity, positive and negative predictive value, and inter-observer reliability.