Your thigh bone (femur) is the longest and strongest bone in your body. Because the thigh bone is so strong, it usually takes a lot of force to break it. For example, accidents are the number one cause of thigh bone fractures.Your thigh bone (femur) is the longest and strongest bone in your body. Because the thigh bone is so strong, it usually takes a lot of force to break it. For example, accidents are the number one cause of thigh bone fractures.Your thigh bone (femur) is the longest and strongest bone in your body. Because the thigh bone is so strong, it usually takes a lot of force to break it. For example, accidents are the number one cause of thigh bone fractures.Your thigh bone (femur) is the longest and strongest bone in your body. Because the thigh bone is so strong, it usually takes a lot of force to break it. For example, accidents are the number one cause of thigh bone fractures.