New insights into how bone forms, adapts and heals are revolutionising fracture prevention and repair, explains Patrick Pollock FRCVS
The ability to repair and prevent bone fractures in horses, which can come about as a result of a single trauma or as a result of ongoing microtrauma through inappropriate training, is a topic of great interest among the veternary community and horse owners alike, with exciting new insights available.
Bone is a substance that changes throughout life, in a process called remodelling. This allows the bones of foals and young horses to grow and change shape, and in adult horses enables the repair of damage and prevents bones from becoming brittle.
The amount of remodelling is affected by the horse’s age, the type of exercise he does and the surface on which he works. Research in thoroughbreds discovered that if horses undergo controlled exercise early in life, the bone “learns” to model in a way that protects it from injury in later life.
Interval training with short bursts of high-intensity exercise is more likely to result in healthy, strong bone, than low-intensity, endurance-type work. Exercise causes tiny areas of damage, leading to “stress remodelling”, where the bone adapts to the forces placed on it. This process, called an adaptive response, is critical to bone health.
Why bone fractures occur in horses
We now know that the die is cast for a fracture long before the bone actually fails. Inappropriate training before the adaptive bone response is complete leads to too much strain on the bone. This results in “fatigue injury”, microscopic damage to the bone.
If training continues, the osteoblasts lay down too much new bone, causing the bone in those parts of the skeleton placed under excessive strain to become more dense than normal. In horses, extra new bone tends to be laid down at the joints, such as around the condyles (rounded ends) of the fetlock and in the front part of the limb bones. This dense bone, called sclerotic bone, is actually quite brittle.
The advent of magnetic resonance imaging (MRI) has enabled us to see the true extent of pathological changes within the bone prior to fracture. Abnormally modelled bone with fatigue injury appears inflamed and contains excessive amounts of oedema (fluid). Stress fractures may even be visible before the horse is lame and before radiographs (X-rays) can detect the injury.
It appears certain humans may be predisposed to the development of stress fractures; genetic markers exist to identify these people, allowing them to be monitored. Recent work at the University of Edinburgh suggests this may also be the case in horses. And it would be exciting to be able to identify horses at risk of developing stress fractures through something as simple as a blood test.
We do still see fractures that come about as a result of trauma, such as a kick or a fall, and some performance horses do slip through the net and present with a fracture. Thanks to a better understanding of how to train horses to encourage the adaptive response, however, along with our ability to detect signs of bone microdamage before a fracture occurs, the number of sport horses suffering fractures is at an all-time low.
How healing occurs in bones
In contrast to soft tissues, such as tendon, ligament or skin, bone is one of the few body tissues able to heal without a fibrous scar.
Unlike the scar left after a skin wound heals, which never gains the same degree of strength and can be thought of as a fibrous “patch”, new bone laid down during fracture healing is just as strong as the original. Healing is so effective that it is often impossible to determine where the previous fracture was.
Bone healing is affected by two main factors: the size of the gap between the fractured ends of the bone and the strain across the fracture site. Strain relates to stability: a certain amount is good, keeping bone ends together, but too much interferes with healing.
To understand the healing processes, it can help to think about skin wounds. Where possible, we will close the wound and realign the edges of the skin to enable primary intention healing. Where the skin is lost, secondary intention healing occurs – the wound fills with a blood clot and then granulation tissue (proud flesh) and is eventually covered with new skin cells resulting in a larger scar.
When the gap between fractured bone ends is less than 0.01mm and the strain across the site is less than 2%, structures (cutting cones) develop across the fracture. These generate cavities that fill with bone produced by osteoblasts. This re-establishes bridges of osteons across the fracture line, which then remodel into normal lamellar bone.
As a result of this primary healing, there is no formation of a periosteal callus (lump) at the site. This is important in articular fractures, involving joints, which are common in horses, as the presence of a fracture gap then callus formation can potentiate the development of osteoarthritis.
Secondary bone healing occurs when the ends of the fractured bones are near enough to heal but not perfectly opposed, or when there is motion at the fracture site. This process involves the classical stages of injury: haemorrhage, inflammation and scar formation.
In bone, the “scar” is a soft callus made of cartilage; this then undergoes mineralisation and remodelling so that, unlike skin, the tissue becomes normal without any permanent scarring. Secondary bone healing is less of an issue in certain situations, such as fractures of the non-weight-bearing splint bones on either side of the cannons in the lower limb.
Because we now understand how bone is formed, how it adapts and responds to stress and activity and how it heals, we have never been so well prepared to identify and treat potential fractures before they occur – or to repair the damage when they do.
Understanding how bone forms
From the huge bones of the pelvis and femur to the tiny structures within the ear, each of the horse’s 205 bones performs a critical role – and each adapts in direct response to the forces and strains applied to it.
How does this remarkable, adaptable material form?
● Bone largely consists of a protein called collagen, which forms a framework for the mineral calcium phosphate. This combination makes bone strong yet flexible enough to withstand stress.
● Three types of cell are constantly at work within the bone: osteoclasts, which break down old bone through a process called resorption; osteoblasts, which make new bone, and mature bone cells called osteocytes, which basically tell the other two what to do.
● These groups of cells work together to form osteons, cylindrical structures surrounding a central canal that contains the bone’s blood supply. Osteons are aligned parallel to the long axis of the bone.
● The long bones of the skeleton have an outer layer of compact bone called the cortex. This is covered by the periosteum, a dense layer of vascular tissue which carries both nerves and the blood supply, and is important in bone healing. The medullary cavity in the centre of these bones stores the spongy marrow, where stem cells are produced.
Ref: 21 January 2021
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