Horse Musculoskeletal Diseases – An Introduction

The bodies of all animals allow movement to find food and to reproduce through a system of bones and connective tissue. Plants do not have this ability because their roots are stuck in the ground. They can grow and spread or they can turn to face the sun or open and close their flowers but they cannot pack their bags and leave. This is where the connective tissue comes in. It is made of a skeleton or frame of bones that is connected by ligaments (bone to bone) and the bones are articulated through joints made of a fluid filled sack (joint capsule), Each articulating bone are 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 that are attached to the bones with tendons. I prefer to call these “bone-tendon-muscle units” as each is inextricably connected as a unit, although each can have individual problems (broken bone, muscle cramp, pulled tendon). The innervation of these muscles are covered under nerves and brain. The cellular make up and nutrition of these are covered under nutrition.

Bones are divided into 2 groups: cortical and cancellous. Cortical bones join together at sutures but they don’t articulate with each other. They can articulate with cortical bones (the temporomandibular joints, vertebrae) or with a cancellous bone (hip joint). Cortical bones do not have a hollow center and are primarily made for protection of the head and they also 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 as well as provide movement through support of the body weight. Mineral storage is regulated and controlled by hormones that keep the level of calcium 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. It’s very important this system works and vitamin D (which is no longer considered a vitamin but is now considered a hormone) is essential to this. Unfortunately there is little known about vitamin D in horses including where they get it from. In humans, sunlight is essential in Vitamin D formation with some foods being a source. But horses do not consume these foods and their bodies are covered in hair and most horses have black skin preventing overexposure to sunlight. Horses fed a lot of whole grains suffer from calcium deficiency (rickets or soft bones) so dicalcium phosphate is added to every commercial feed to prevent this.

The problems that can occur are often connected with trauma either from a direct force (break or traumatic bowed tendon) or from repetitive strain (exercised induced bowed tendon). However it is my opinion that the root of most problems with connective tissue is from poor nutrition and is discussed heavily in that subject. I believe this because of the increased in soundness issues seen today over 30 to 50 years ago. If horse survival required movement and speed, how was their survival assured if they were continuously straining ligaments and tendons? Today I see soundness issues that were not in the text books in the 1980’s such as DSLD (dropped hind patterns) and kissing spine. The topics here will discuss some of these issues.