In the mid-20th century, after laying hens started being housed in cages, cage layer fatigue began appearing. It was discovered that cage layer fatigue was due to excessive bone loss, causing osteoporosis and bone brittleness. The ribs, keel and thoracic vertebrae are common sites for bone fractures to occur. Sometimes paralysis also occurs due to vertebral fracture damaging the spinal cord. Highly productive caged laying hens inevitably develop osteoporosis. Metabolic deficiencies of calcium, phosphorous, or vitamin D can worsen the condition. Increased physical activity among laying hens has been shown to reduce the incidence of osteoporosis and cage layer fatigue. Bone weakness may have a genetic component that can be selected against and therefore reduce the incidence of this problem. Pain, debility, and mortality are some of the welfare concerns. Also, with increased bone brittleness there are welfare concerns about catching and transporting these hens. Hunane on-farm killing methods for spent hens should be considered a high priority research agenda.