Comparison of PACS and Bone Ninja mobile application for assessment of lower extremity limb length discrepancy and alignment

PURPOSE: There are over 500 medically related applications (apps) for mobile devices. Very few of these applications undergo testing and peer-review for accuracy. The purpose of this study is to assess the accuracy of limb deformity measurements on the Bone Ninja app compared to PACS and to determine the intra- and inter-observer variability among different orthopaedic practitioners. METHODS: Four participants (attending, senior and junior resident, and physician assistant) measured the leg length (LL), the lateral distal femoral angle (LDFA), and the medial proximal tibial angle (MPTA) of 48 limbs (24 patients), twice with both Bone Ninja and PACS. The difference between the measurements obtained with the Bone Ninja app and PACS were measured. We determined the consistency of the intra-observer intra-class correlation coefficient (ICC) for both systems. RESULTS: There were no statistical differences in leg length discrepancy (LLD), MPTA, or LDFA measurements between Bone Ninja and PACS (p = 0.96, 0.87, and 0.97, respectively). The intra-observer ICC for the LL, LDFA, and MPTA was similar between Bone Ninja and PACS (0.83, 0.89, and 0.96 vs. 0.96, 0.93, and 0.95, respectively). The inter-observer ICC was similar between Bone Ninja and PACS (0.95, 0.96, and 0.99 vs. 0.99, 0.98, and 0.98, respectively). CONCLUSIONS: This study demonstrates that Bone Ninja is an accurate educational tool for measuring LLD, LDFA, and MPTA. Both systems are reliable instruments for evaluating limb length differences and angles on standing radiographs for pre-operative deformity planning and education. This is the first study to evaluate the accuracy of Bone Ninja compared to the gold standard of PACS.