Pemajanan Pulsed Electromagnetic Field Pada Penyembuhan Fraktur Femur Tikus Model Delayed Union: Tinjauan Ekspresi RANK, RANKL, dan OPG Pada Kalus = Exposure of Pulsed Electromagnetic Field on Rat Femoral Fracture Healing Process in Delayed Union Model: A Study on RANK, RANKL and OPG Expression in Callus