The main reason why a femoral neck fracture can cause knee pain is because the area of innervation is affected. The nerve felt by a femoral neck fracture is the lumbar 4 nerve root, which is also the 4th nerve root emanating from the lumbar spine. This nerve is responsible for sensation not only in the hip region, but also in the knee region. Therefore, when the hip joint becomes diseased, such as a femoral neck fracture or ischemic necrosis of the femoral head, not only will you feel pain in the hip joint, but also in the knee joint. This is mainly because the same nerve root is responsible for this mutual, identical sensation called referred pain.