Slipped tendon is a common skeletal 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. Slipped tendon 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. Slipped tendon 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.
The condition can be the result of a nutrient deficiency but it is mainly the result of their bodies being far too big for their legs to support. It involves the growth plate of the long bones, resulting in the development of short, thickened, misshapen long bones that are frequently accompanied by an enlarged hock joint and relocation of the tibiotarsal end of the bone. Slipped tendon is a significant cause of pain and lameness in these birds. Most affected birds are reluctant to stand and may spend a great deal of their time off of their feet, due to the pain. In severe cases, the chicken's Achilles tendon slips from the groove formed by the condyles of the tibia, pulling the leg out of shape to appear very crooked (valgus deformity
). When both legs are affected, it results in bowing of the legs (varus deformity