Developmental Orthopedic Disease (DOD) is a term used for a cluster of abnormal skeletal growth conditions that affect young, growing horses. DOD is one of the main causes for future joint disease and can potentially jeopardize the worth and future career of the horse. DOD incorporates the following spectrum of conditions:
- Osteochondrosis (OC): Osteochondrosis is an umbrella term that applies to the results of abnormal endochondral (within cartilage) ossification, which is the process by which soft cartilage cells are transformed into hard bone cells. In horses, they can occur on any joint surface or in subchondral bone (referred to as osteochondritis dissecans (OCD)), deep beneath a joint's surface (referred to as subchondral bone cysts), and at the physes (growth plates) of long bones and vertebrae. OC most commonly occurs in the tarsus, stifle, and fetlock joints. It can also uncommonly affect the scapulohumeral, elbow and hip joints.
- Physitis: Physitis, formerly referred to as epiphysitis, is inflammation of the growth plate (the metaphyseal growth plate), where the structural integrity of the metaphyseal bone is weakened by the disturbance in maturation of the cartilage cells. The areas most affected include the distal (lower) metacarpus or metatarsus (cannon bones), distal radial physis (area just above the knee), and distal (lower) tibia.
- Flexural limb deformities (FLD): Flexural limb deformities, also known as contracted tendons or ballerina syndrome, is the deviation of the limb from the normal axis in the sagittal plane and can be either hyperflexion or hyperextension of a foal's joint region, usually the forelimbs. FLD can be congenital or acquired.
- Angular limb deformities: Angular limb deformities, also known as bent legs, include any deviation of the limb in the sagittal plane. It is thought to be associated with growth imbalances induced by on of the following defects: poor conformation, irregularities of endochondral ossification of the bones in the carpus or tarsus, asyndronous longitudinal growth rate, trauma, joint laxity, and excessive exercise.
- Tarsal bone collapse: Tarsal bone collapse, also known as cuboidal bone malformation, incomplete ossification of cuboidal bones, cuboidal disease and tarsal bone necrosis, is associated with collapse of the dorsal aspect of the central and third tarsal bones which causes a flexion deformity of the tarsus.
- Acquired vertebral deformities: Acquired vertebral deformities, also known as lordosis or kyphosis, is a condition that describes different degrees of lordosis and kyphosis, where the vertebral column deviates abnormally in a ventral or dorsal direction respectively.
- Cervical Vertebral Malformation (CVM): Cervical Vertebral Malformation, also known as wobblers, is characterized by malformation or compression of the spinal cord, leading to spasticity, ataxia and incoordination in affected horses.
All conditions that collectively constitute DOD are thought to the result of a multitude of different factors, however the key elements being:
- Rapid growth
- Biomechanical stress or trauma on the joint and growth cartilage
- Diet imbalances
- Genetic predisposition
- Hormonal factors (e.g. hyperinsulinemia)
In horses with DOD, clinical signs usually start to become noticeable the first two years of life. Only certain DOD conditions may produce external clinical signs, such as angular limb deformities, in which horses most frequently present with joint heat, prolonged recumbency, stiffness, reduced activity (linked to lack of movement/playing), lameness, and enlarged growth plates. Most other DOD conditions result in horses having no obvious external signs of being affected.