The bodies of all animals allow movement to find food and reproduce through a system of bones and connective tissue. Plants do not have this ability because their roots are immovable in the ground. They can grow and spread, turn to face the sun or open and close their flowers, but they cannot pack their bags and leave. The ability to move over the ground is where the musculoskeletal system comes in. The skeleton or frame of bones are connected by ligaments (bone to bone) and articulated through joints connected by fluid-filled sacks (joint capsules). Each articulating bone is capped with a layer of cartilage (joint cartilage) to cushion concussion from movement. Causing this movement are voluntary contractions and relaxations from groups of muscles attached to the bones with tendons. I prefer to call these “bone-tendon-muscle units.” Each is inextricably connected as a unit, although each can have individual problems (broken bone, muscle cramp, pulled tendon). The discussion of the innervation of these muscles is under the neurology section. The discussion of cellular makeup and nutrition is under the nutrition section.
Two groups of bones are called cortical and cancellous. Cortical bones join together at sutures, but they don’t articulate with each other. They sometimes articulate with cortical bones (the temporomandibular joints, vertebrae) or a cancellous bone (hip joint). Cortical bones do not have a hollow center and primarily aim to protect the head and store minerals.
Cancellous bones have marrow inside the center hollow portion where stem cells (precursor cells) form much of the blood and immune system. These bones also store calcium and other minerals and provide movement through body weight support. Mineral storage is regulated and controlled by hormones that keep the calcium level in the blood at an exact point. If calcium levels increase or decrease from this point, the horse (or human) will go into a coma and die. This system must work, and vitamin D now considered a hormone and not a vitamin, is essential for regulating calcium storage in bones. Unfortunately, the formation of Vitamin D in horses is poorly understood. In humans, sunlight is necessary for Vitamin D formation, with some foods being a source. But horses, with their hair-covered bodies and black skin, prevent exposure to sunlight through the skin. In addition, horses fed excess whole grains suffer from calcium deficiency (rickets or soft bones), requiring dicalcium phosphate to be added to every commercial feed as prevention.
Problems with bones, tendons, and ligaments come from direct force trauma (break or traumatic bowed tendon) or repetitive strain (exercised-induced bowed tendon). However, I believe that the root of most problems with connective tissue is poor nutrition which is discussed heavily on this website. I see the increase in soundness issues today over 30 to 50 years ago. If horse survival required movement and speed, how was their survival assured if they continuously strained ligaments and tendons? Today I see soundness issues not in the textbooks in the 1980’s such as DSLD (dropped hind fetlocks), poor top line, and kissing spine. The topics here will discuss some of these issues.