The skeletal disorders causing the most severe welfare problems in different classes of poultry are bone growth disorders in young birds and bone fragility in older laying hens.

Angular and rotational deformities of leg bones are widespread in young birds, especially in broilers but also in turkeys. Dyschondroplasia is a widely observed defect in the growth plates of leg bones. It occurs most noticeably in the growth plate of the proximal tibia, the fastest growing bone in the young broiler, and is frequently referred to as tibial dyschondroplasia (TD). Resulting from a failure of growth plate chondrocytes to differentiate fully, it leads to a build up of a mass of prehypertrophic chondrocytes. The lesions develop between the ages of 2 and 5 weeks and can regress thereafter.

The importance of TD in causing bone deformity was assessed by carrying out sequential radiography of the proximal tibias of a group of 20 broilers growing up to 16 weeks of age. At this time the tibias were dissected and the angulation of the proximal end (tibial plateau angle, TP°) was measured. The severities and durations of the TD lesions, assessed by radiography, were found to be highly correlated with the TP°. Birds with abnormally large TP° were obviously lame. These observations confirm that TD can lead to distortion of bone growth and that this can contribute to lameness in broilers (Lynch et al., 1992).