Bone and Joint

Bones have two main sections, the epiphysis and the diaphysis. The epiphysis is the end of a bone. It is covered by articular cartilage. The diaphysis is the shaft, the middle of which contains bone marrow (where red blood cells are produced).

A joint is made up of two bones, articular cartilage, and a joint capsule. The joint capsule contains the synovial membrane which produces synovial fluid. The cartilage and the synovial fluid provide cushion when bones are rubbed together during movement.

Remodeling

When a horse is young the bones are not fully grown. A portion of the bones is made up of epiphyseal cartilage (commonly know as the growth plate) at this point. Over the course of a few years this cartilage is turned into bone. This conversion to bone is called endochondrial ossification. This is one of the horse's first experiences with bone remodeling.

Remodeling is simply changes is the bones. This happens throughout a horse's life. It is influenced by nutrition, exercise, and stress. If a horse is deficient in calcium it is taken from the bones to whatever more vital area needs it (a horse can live with weaker bones, but it's life is compromised if the heart, an organ absolutly necessary for life, doesn't have enough of it), this causes a bone to weaken. When a horse moves stress is put on the bones which usually causes them to become stronger, but when too much stress is put on the bones they actually weaken. Stress caused by poor conformation or obesity is another thing. Poor conformation causes extra stress on the legs and obesity means that the horse always has more pressure on his bones which adds to any stress on the bones produced by too much exercise.

Leg Bones: Relationships

Weight is distributed approximatly 60% up front and 40% in the hindquarters. While this puts more stress on the front legs it frees up the hindquarters, allowing them to produce propulsion and/or impulsion. Horses do no have clavicle bones like humans to attach the scapula (shoulder blade) to the body. The is scapula is attached to the body via a muscle sling which absorbs concussion. Attaches to the scapula is the radius and ulna which are fused in the horse (unlike humans who have the two individual bones). The ulna is near the top and is the part we point to and say 'elbow.' Next come the 7 bones in the knee. Because there are 7 bones there they can sort of squash together during movement to absorb more concussion. Next come the cannon bones with the splint bones held to it by the interroseus ligament (this ligament should fuse the splints to the cannon by the time the horse is age 6.) Next come the two sesamoid bones, the long and short pastern bones (1st and 2nd phalanx), the navicular bone, and the coffin bone (pedal bone).

The hind legs are attached to the body by the pelvis (made up of the ilium, ishium, and the pubis which is near the femur socket). Next comes the femur and the tibia and fibula which are fused together. The femur, tibia, and fibular, together with the patella make up the stifle joint. Horses can lock this joint which is how they manage to sleep standing up (called the stay apparatus). Next come the 6 hock bones, the most important of which are the calcanius and the talus. Theses two bones are what allows this joint to move. After the hock the leg is made up of the same structures as the foreleg after the knee. The hind cannon tends to be longer and the splint bones may take longer to fuse.