Tibial dyschondroplasia (TD) is a skeletal bone disorder that occurs commonly in modern-day "meat-type" chicken breeds (often referred to as "broilers"). Broilers have been genetically altered by the poultry industry since the 1950s to produce meat--particularly breast meat--fast. Through years and years of breeding, the commercial meat industry developed a chicken that within 6 weeks of hatching from an egg, was almost two times the size it normally should be, with 80 percent larger breast area. TD is just one of several health problems and disorders that occur as a result of this unnatural acceleration in growth rate and increase in size/proportion. TD is a significant welfare problem for chickens---for the disorder is very painful for the bird to use it's legs. According to a 2008 study of 50,000 chickens it was discovered that over 27 percent of the chickens had difficulty walking by the time they were 40 days of age; 3.3 percent of the chickens could barely walk. These numbers don't include the number of birds "culled" before the day of the inspection at day 40.
TD involves the development of an abnormal mass of cartilage at the growth plate of the long bones of the leg, usually the tibia bone. Instead of calcifying, the cartilage remains soft and will often bend backwards under pressure or heavy body weight. As a result, chickens with TD often suffer from spontaneous leg fractures
, necrosis, and severe lameness. Birds with severe lameness are often unable to stand due to the pain, and often die from starvation from being unable to reach food or water, or trampled to death by other flock members.