The femur bone spans the distance between the hip and stifle (knee) joints. Large muscle groups attach to the femur to allow flexion and extension of the rear limb. The top of the femur forms the ball in the ball and socket hip joint, while the bottom of the femur articulates with the tibia in the knee joint. Problems associated with the femur occur from developmental abnormalities (e.g.: hip dysplasia), fractures, dislocation of the femur from the hip joint, bone infections (e.g.: Blastomycosis), and tumors (e.g.: osteosarcoma). Pets with mild femur pain may favor the affected leg, be slow to rise, and become exercise intolerant. Significant femur pain (e.g.: traumatic or pathologic fracture) appears as swelling and non-weight bearing lameness on the affected limb.