Tendon injuries with Dr Luke Wells-Smith

Tendon injuries in the horse can be a common cause of lameness, particularly in racing and performance horses. The horse has evolved to minimise weight of the limb and to enhance energy storage and release for fast movement. These properties allow the horse to escape from predators, however, it can contribute to the rapid and severe breakdown of the tendons. Tendons transfer the force generated by muscle to move joints and the limb. The forelimb tendons are most commonly affected, potentially because the forelimbs share 60% of the horses weight, whereas the hind limbs share only 40%. Tendons can be injured in two ways; overstrain and direct penetration (e.g. wounds). This article will discuss two major tendons and their overstrain injuries, and explain the clinical signs, diagnosis and treatment of each.

Anatomy
All tendons have the same basic structure, made from many fibres that join together to form an elastic strap of tissue capable of storing energy and rapidly releasing it. Both the Superficial Digital Flexor Tendon (SDFT) and the Deep Digital Flexor Tendon (DDFT) are capable of storing large amounts of energy and are responsible for the majority of movement during locomotion. The extensor tendons are stiffer, with little energy storage capacity. The extensor tendons maintain limb and joint position rather than provide locomotion.

Predisposing factors
Overstrain injuries occur when the horse is working at high speed/intensity (e.g. fast gallop or fast turns) and the tendons are close to their breaking point. Because there is very little leeway when it comes to damaging a tendon, the below factors have been found to contribute to an increase risk of injury;

Hoof conformation
- A long toe and low heel hoof conformation can result in a greater chance of developing soft tissue injuries

Ground surface
- Hard ground surfaces result in a faster moving horse which in turn increases the likelihood of overstrain injuries
- Slippery or uneven surfaces can also play a role

Fatigue
- Depending on the horse’s current level of fitness and the work intensity required for competition, can determine how rapidly the horse will fatigue
- Once the muscles fatigue, limb movement is altered which may predispose to tendon injuries

Overweight
- The heavier a horse is, the more force that will be generated when the limb strikes the ground and the greater the likelihood of overstrain

Lameness in other limbs
- Lameness results in an alteration in locomotion, which in turn overloads other limbs and tendons

Clinical signs
Injuries to the SDFT can manifest as the classical ‘bowed tendon'. Swelling may extend the majority of the length of the tendon. Lameness can range from subtle to obvious, particularly if there is a large degree of swelling.

Injuries to the DDFT can be clinically more subtle when compared to the SDFT. Injuries typically occur where the DDFT travels around the back of the fetlock and pastern. In the fetlock and pastern region the DDFT is enclosed by a tendon sheath, which can become full of fluid in the case of DDFT injury. Again, the lameness can range from mild to obvious depending on the degree of damage.

Diagnosis
The most important aspect of diagnosis is to perform a thorough musculoskeletal and lameness examination. Nerve blocks (use of local anaesthetic around nerves), can be used to help localise where the lameness is originating from. Ultrasound can be used to identify damaged areas of the SDFT and DDFT. It is also important to use the ultrasound throughout the rehabilitation process to ensure the tendon injury is healing. In more difficult cases, MRI can be used to image the soft tissue structures of the limb, particularly in the foot and pastern region.

Treatment
Tendon healing can be grouped into three phases with different treatments targeted at each;

Inflammatory
- First 7 days after the initial injury
- Aim is to limit inflammation.

This can be achieved in three ways:
1. Cold hosing of limb
2. Support wrap to reduce swelling from accumulating in the lower limb
3. Anti-inflammatories (e.g. Bute paste), when prescribed by your veterinarian, will reduce inflammation, swelling and pain associated with tendon injuries

Repair
- Overlaps with the inflammatory phase at 2 to 30 days post-injury
- Treatment aim is to limit further damage and influence early tendon healing.
This can be achieved in three ways:
1. Confinement in a stall will reduce further damage in the first 7-14 days after the initial injury
2. After 7-14 days, the aim is to influence tendon healing by providing low intensity exercise such as hand walking
3. Regenerative medicine products
- Branch of human/veterinary
medicine whereby natural products are harvested from the patient or ‘donors’ of the same or different species to treat medical and orthopaedic conditions. Below are two products that are commonly used to treat tendon injuries in
the horse
(1) Platelet Rich Plasma (PRP)
- Blood contains platelets, therefore they can be easily collected from the horse’s jugular vein
- The blood is then processed to concentrate the platelets
- The concentrate is then injected into the lesion under ultrasound guidance
- This concentrate contains growth factors and anti-inflammatory proteins that aid in the healing process
(2) Stem Cells
- Stem cells can be harvested from bone marrow, fat or blood of the affected horse or a donor
- Once the stem cells are harvested they are cultured, concentrated and injected into the lesion under ultrasound guidance
- How stem cells actually work is still under investigation, however the results have been very promising, particularly when treating tendon injuries in the horse

Remodelling
- Greater than 30 days after the initial injury
- Incremental increase in exercise intensity and size of yard/paddock
- Ultrasound every 1-3 months depending on the initial injury and progression through the rehabilitation period

Therapeutic Farriery
Your farrier has an important role to play in the rehabilitation and prevention of tendon injuries in the horse. A trimming/shoeing interval of 4-6 weeks is essential to ensure the toe doesn’t become long and so that the mechanics of the foot are optimised. It is important to achieve a straight hoof pastern alignment when dealing with a SDFT injury to ensure less strain on the tendon. By raising the heel of a horse standing on a hard surface, the strain on the DDFT is reduced, while the strain is increased in the SDFT. Therefore, it is important to shoe a horse with a DDFT injury with a raised heel shoe and house on a firm surface to enable the mechanics of the shoe to work effectively.

Prognosis
Factors that influence the likelihood your horse will return to work at the same level of performance include;

Time from initial injury to diagnosis
- The longer the horse is left without diagnosis of a tendon injury, further damage can be occurring

Degree of lameness on presentation
- Horses that present with a severe lameness are more likely to have a serious tendon injury that will be more difficult to treat and require a longer rehabilitation program

Size of the tendon lesion
- The larger the lesion (in diameter and length), the longer the rehabilitation period that will be required and the poorer the prognosis

Age of the horse
- As the horse ages, the tendons become less elastic and prolonged periods of time are required for the healing process to be completed

History of a previous tendon injury
- Scarring from previous injuries are less elastic and tendons tend to re-injure either side of these areas of scarring

Finance and facilities
- Treatment, such as stem cells and PRP can be expensive
- Stalls and yards of various sizes are required to house horses with tendon injuries
- A mechanical walker or time available to exercise the horse are required to implement a graded exercise program

Conclusion
Tendon injuries in the horse can be a serious cause of lameness, however early detection, a good relationship between your farrier and veterinarian, and commitment to a rehabilitation program can result in a sound horse. The use of regenerative medicine products (stem cells and PRP) have shown promising outcomes as an adjunct to a rehabilitation program.

Luke is a qualified farrier and Veterinarian. He specialises in the biomechanics of therapeutic horse shoes, hoof conformation of Thoroughbred yearlings and laminitis in broodmares.

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Story from Horse Deals Magazine February 2017

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