The Normal Tendon
A tendon is a band of fibrous connective tissue that links muscle to bone. Tendons are used to move. As the horse’s muscle contracts, tension and corresponding tendon increase, thereby resulting in movement of the bone(s) to which it is attached.
A structure that is often confused with tendons, but has a totally different function is a ligament. A ligament is a band of fibrous connective tissue that binds bone to bone or ligament to bone. A ligaments function is to stabilise structures relative to one another, such as two bones on either side of a joint.
There are 2 main types of tendons:
- Flexor tendons are designed to flex joints via the relative movement of bones on either side of the articulation. They typically course along the backside of the limb.
- Extensor tendons function to extend (or straighten) joints via the relative movement of bones on either side of the articulation. They are usually located along the front side of the limb.
Tendons must run along at least one articulation (joint surface) to perform their function and require lubrication along each angled (or “bent”) juncture to allow for smooth and effortless movement. Without sufficient lubrication, the tendons would wear excessively due to the increased friction they would encounter as their fibre orientation changed while coursing along the curved surface(s).
The smallest functional unit in a tendon is called a fibril. Fibrils, in turn, are bundled lengthwise into collagen fibres, a few of these fibres form a primary bundle, together they form the fascicles, and finally, they end up forming the tendon itself.
We have to see the tendon structure as a built-up of tiny fibre structures (like tiny pieces of rope), organised very neatly in line with each other. When there is tension on the tendon, the fibres slide along each other and slide back when the tension reduces. The ability to stretch and recover to the setpoint is the essence of the functionality of the tendon.
The athletic ability is mainly determined by its ability to withstand heavy loading without getting damaged.
Injuries to Tendons
Tendon injuries, just like other soft tissue injuries, can be divided into acute injuries and overuse injuries.
- Acute injuries are caused by sudden trauma, such as a fall, twist, or blow to the body. An example of an acute injury in a horse is a tear in the front leg’s superficial flexor tendon (“a leg”).
- Overuse injuries occur gradually over time when an athletic or other activity is repeated so often, areas of the body do not have enough time to heal between occurrences. An example of an overuse injury in a horse is damage to the high suspensory area.
A tendon will get damaged when the pulling power is so large, that the fibres are pulled apart. Instead of sliding and bouncing back, the tendon’s fibrils are pulled to a point where they can’t recover the original position.
In most cases, a damaged tendon will present itself with local swelling, pain when touching, some heat and often (but not always) the horse shows a lameness.
The damage doesn’t always have to be directly visible. A horse can develop tendon tissue damage without showing any symptoms (subclinical injury).
These cases are dangerous as this stage often leads to serious injury if the horse is worked as usual.
Therefore, it is vital to take any sign of pain, swelling or heat in a tendon severed. A damaged tendon will go through different stages to heal. During each stage, we have the ability to influence the outcome of healing.
Tendon Healing
The first stage is the inflammatory phase. This begins immediately after injury and will last the first few days after injury. Blood flow to the area is increased. Platelets release growth factors and pro-inflammatory signals, and a fibrin clot forms a temporary framework for the tissue to stabilise the injury.
In this stage, rest, support and anti-inflammatory treatment are important. It is known that initial injury size can get bigger in the first 1-2 weeks due to mechanical “pulling” on the affected area. With this in mind, horses are rested and often supported by bandages during this stage.
The problem is that even when just standing the tendons will still be pulled and the injury can/will get worse. Bandages are not able to give enough support to avoid this “pulling”.
With this in mind, we advise using our Dynamic Support System (DSS) from an early stage (high support setting) so that excessive pulling can be avoided. (read here more about DSS)
Regular use of ice and other anti-inflammatory treatments is advisable to avoid excessive inflammation and avoidable tissue damage.
The proliferative stage starts in the first week after injury and will lead to blood clots forming and filing of the damaged area with a scar-like tissue. We must realise and accept that tendon cells don’t grow back. This means that the scar tissue needs to be “trained” to act like tendon tissue later on.
Towards the end of this stage, the active trigger to healing is reduced by the body, and the availability of natural stem cell activity and other growth-promoting factors are getting less. In this stage, it is often advisable to reactivate the healing response by introducing substances like PRP, Stem-cells, and other biological regenerative treatments.
A low amount of loading on the tendon in this stage is advisable, but can only be done safely if the tendon’s peak load is controlled. Traditional protocols leave horses during this stage on complete rest, but this will further reduce healing ability and increase the forming of inferior scar tissue.
During the last stage, remodelling of scar tissue takes place. During this stage, the first fibrils are formed (particularly type I collagen fibres) which become aligned in the direction of strain on the tendon. During maturation, the scar tissue gradually changes to scar-like tendon tissue and depending on the protocol followed this stage can take up to 12 months. Traditionally the focus is on “filling the hole in the tendon”, but filling with poor quality scar tissue leaves to abysmal results (industry average shows that only about 20% of horses return to the previous level of exercise)
Several studies (P. Eliasson 2011, M. Hammerman, et al. 2018) have shown that ruptured tendons heal faster if exposed to mechanical loading.
Early loading creates an organisation of the matrix and will lead to early fibre forming and less “unorganised scar tissue”. Early loading will lead to a better functioning tendon with a better long term outcome.
Appropriate loading is needed to increase the healing tendon’s quality without inducing microdamage and alter the inflammation in the tissue. This is hard enough to achieve in human patients, but how can we do this in equine patients?
The first step is to be able to assess the fibre content and composition in the tendon. Conventional ultrasound is very limited in producing this information. The use of Tendon Tissue Characterisation technology (UTC) makes it possible to collect the tendon’s required information to establish the best possible loading of the tendon.
UTC will show us whether the exercise (loading) of the tendon is not enough (leading to inactive scar tissue), too much (leading to micro-trauma) or appropriate for achieving optimal healing.
Managing peak load on a tendon in a horse
It is a big challenge to achieve required exercise (tendon loading) with a horse in hand or under saddle. A sudden jump away, a slip, or just being too “fresh” can lead to a sudden high peak load and diminish all the good work done in the days (weeks) before.
We acknowledge this challenge and have implemented our Dynamic Support System (DSS) in our tendon rehabilitation protocols. This device is designed to exercise horses with a set “peak load” by regulating fetlock extension. The boot has 4 settings, and depending on the stage of healing, Tendon Tissue Assessment results in the fetlock extension is gradually increased. Using this boot allows the safe exercise of the horse within the required range of motion.
Packages combining load adjustment and Tendon Tissue Assessment technology allows us to customise every rehabilitation programme, avoid setbacks and achieve optimal results.