bone compliant and large variations in architecture leads to high heterogeneity. The modulus and strength vary inversely with porosity and are highly dependent on the porosity structure. The effects of aging and small cracking of trabecular bone on its mechanical properties are a source of further study. Clinical significance Studies have shown that once a human reaches adulthood, bone density steadily decreases with age, to which loss of trabecular bone mass is a partial contributor. Loss of bone mass is defined by the World Health Organization as osteopenia if bone mineral density (BMD) is one standard deviation below mean BMD in young adults, and is defined as osteoporosis if it is more than 2.5 standard deviations below the mean. A low bone density greatly increases risk for stress fracture, which can occur without warning. The resulting low-impact fractures from osteoporosis most commonly occur in the upper femur, which consists of 25-50% trabecular bone depending on the region, in the vertebrae, which are about 90% trabecular, or in the wrist. When trabecular bone volume decreases, its original plate-and-rod structure is disturbed; plate-like structures are converted to rod-like structures and pre-existing rod-like structures thin until they disconnect and resorb into the body. Changes in trabecular bone are typically gender-specific, with the most notable differences in bone mass and trabecular microstructure occurring within the age range for menopause. Trabeculae degradation over time causes a decrease in bone strength that is disproportionately large in comparison to volume of trabecular bone loss, leaving the remaining bone vulnerable to fracture. With osteoporosis there are often also symptoms of osteoarthritis, which occurs when cartilage in joints is under excessive stress and degrades over time, causing stiffness, pain, and loss of movement. With osteoarthritis, the underlying bone plays a significant role in cartilage degradation. Thus any trabecular degradation can significantly affect stress distribution and adversely affect the cartilage in question. Due to its strong effect on overall bone strength, there is currently strong speculation that analysis in patterns of trabeculae degradation may be useful in the near future in tracking the progression of osteoporosis. Birds The hollow design of bird bones is multifunctional. It establishes high specific strength and supplements open airways to accommodate the skeletal pneumaticity common to many birds. The specific strength and resistance to buckling is optimized through a bone design that combines a thin, hard shell that encases a spongy core of trabeculae. The