Tendon injuries in horses can signal the end of an equine’s athletic career, although there are many horses that successfully return to riding, and competition, albeit sometimes at a lower level, after appropriate treatment and a significant recovery period.
In order to understand why and how these injuries occur, it is helpful to be familiar with the structures involved.
Tendons are the straps of connective tissues that attach a muscle to other body parts, usually bones. They transfer the force of muscle contraction to the bones enabling movement; the tendon is firmly connected to muscle fibres at one end and to components of the bone at the other end.
While tendon injuries are a common problem in horses, tendons are actually remarkably strong, with relatively high tensile strengths for a soft tissue. Their great strength is due to the impressive anatomical structure and tissue composition of the tendon fibres. They contain many collagen fibres running in parallel orientations.
The tendons in the horse’s lower leg include:
- the extensor tendon at the front of the leg
- the superficial digital flexor tendon at the back of the leg
- the deep digital flexor tendon located at the back of the leg between the cannon bone and suspensory ligament, and the superficial digital flexor tendon
Causes of tendon injuries
A damaged tendon occurs when the tendon is strained, which is when the fibres within the tendons in the lower leg are overloaded (internal damage) or when they suffer an impact injury (external damage). Typically the individual tendon fibres are broken, similar to an old rope fraying. When this happens there is a large inflammatory response with swelling around the injury and the horse is likely to be lame.
Tendonitis is the term that is use to describe a tendon strain and this can vary from a minor tweak to a few tendon fibres to a major disruption of the whole structure.It is important to be aware that the degree of lameness is not necessarily related to the amount of damage to the tendon. Confusingly, some horses can have quite extensive damage without being very lame, or the affected limb being very swollen. Equally, subtle thickening may be related to substantial tendon damage.The superficial digital flexor tendon in the foreleg is where most tendon injuries occur, ranging from a mild strain to a complete rupture of the tendon. The deep digital flexor tendon is also at risk of injury, while injuries to the extensor tendon are far less frequent and less severe when they do occur. The advent of MRI has meant that tendinitis of the deep digital flexor tendon is now a much more commonly diagnosed problem at the lower end of the tendon to within the horse’s foot.Vets generally describe tendon injuries by three levels of severity — mild, moderate and severe — and generally the more severe the damage, the poorer the prognosis.
Signs of a tendon injury
- Heat at the site of the injury
- Swelling around the site of the injury
- Lameness of varying severity
- If the deep or superficial flexor tendon is completely ruptured, the fetlock will drop towards the ground
When diagnosing a tendon injury, your vet will begin by palpating the swollen soft tissues around the cannon bone to identify which structure is involved.
To confirm the extent of the damage, ultrasound imaging will be used so the vet can visualise the extent of the damage within the tendon. The initial swelling will need to have subsided to get a useful scan of the injury so this normally takes place around one week after the injury occurred.
While waiting for the scan, the standard recommendation is that horse should be kept bandaged on box rest, with frequent cold therapy in the form of cold water or ice boots applied to the injury to help reduce the swelling. Your vet is likely to also prescribe anti-inflammatories such as phenylbutazone (bute), for both pain relief and to help reduce the swelling.
It is important to inhibit the natural inflammation that occurs with an injury as far as possible during the acute phase as this can force the damaged tendon fibres to separate further, worsening the injury.
Once the severity of the injury has been confirmed via the ultrasound scan, your vet will be able to prescribe suitable treatment and a rehabilitation program to follow to slowly bring your horse back into work.
Cases where there is a clear hole (core lesion) showing on the scan may benefit from the injection of either platelet rich plasma (PRP)1 or stem cells2 into the injured area to help kick start the healing process by releasing growth factors into the site of the tissue injury.
PRP is produced from a sample of blood taken from the injured horse. The blood is either passed through a filter (which can be done on the horse’s own stable yard, removing the risk to transporting a horse with an injured tendon to a specialist veterinary clinic) or placed in a centrifuge (if the horse is located at a vet clinic with this equipment), which increases the number of platelets and growth factors the sample contains.
The PRP sample can be ready to inject into the horse within 15-30 minutes after taking the blood sample. The injection site is clipped and cleaned thoroughly to reduce any risk of infection. Then the injections are directed into the site of the injury under the guidance of an ultrasound scanner while the horse is under standing sedation and after having local anaesthetic applied to the area. The leg is then bandaged for 24/48hrs while the horse remains on box rest.
Stem cells are also taken from the injured horse and then cultured in the lab before being injured into the site of the injury, similar to PRP. The bone marrow harvesting process is invasive for the animal, and must take place at a specialist clinic. Once the harvested cells are in the lab, it takes several weeks for them to be cultured and prepared for use.
Recent research has reviewed other forms of regenerative therapy, including using stem cells sourced from embryos. These embryonic stem cells grow forever in the lab so can be ready to use immediately and scientists believe they work by helping to rebuild the tissue directly, reducing the amount of scar tissue laid down.3
There are also surgical options that are more commonly used to repair marginal tears or defects of the deep digital flexor tendons within the digital flexor tendon sheath by tenoscopy. This may be combined with regenerative therapies, such as stem cell or PRP injection.
In all tendon injuries, controlled exercise from the early days is considered beneficial, but turning the horse out is not. Walking the horse in-hand on a firm non-slip surface over gradually increasing periods of time followed by several months of walking under saddle, then ridden walking and trotting would be a normal exercise programme while rehabbing a damaged tendon.
Some horses benefit from light sedation during the walking stages to reduce the likelihood of them suffering a set back for the injury caused by leaping around.
While traditional exercise boots or bandages cannot help support an injured tendon during exercise, there are now orthopaedic boots available4 which are designed to stop the fetlock joint from over extending, thereby preventing additional injury to the soft tissue at the rear of the leg during rehab. These boots are only suitable for use during the horse’s recovery and can be hired on a veterinary referral basis.
By monitoring the tendon’s healing process with regular ultrasound examinations your vet will be able to tailor a convalescent work programme for your horse and advise as to when paddock turnout can be introduced.
As a general rule, an injured horse should have an ultrasound examination of the injury before each increase in the level or type of his work.
Prognosis after a tendon injury
The prognosis following injury depends primarily on the level of damage and the horse’s job. The tissue that is laid down in the tendon after an injury is never as effective as the original tissue and therefore will always be at risk of reinjury.
Many horses do successfully return to work and competition after tendon injuries after a suitable period of rehabilitation. The owner/rider must decide what level of work/competition is appropriate for the horse following an injury, in conjunction with their vet. In some cases it may be wise to reduce the risk of reinjury by adjusting the level or type of work required from the horse.
1, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5015224/ – Effect of intralesional platelet-rich plasma (PRP) treatment on clinical and ultrasonographic parameters in equine naturally occurring superficial digital flexor tendinopathies – a randomized prospective controlled clinical trial – September 2016
2, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6582602/ – Strategies of tenogenic differentiation of equine stem cells for tendon repair: current status and challenges – June 2019
3, https://www.horseandhound.co.uk/news/animal-health-trust-embryonic-stem-cell-research-tendon-injuries-682011 – Top scientists make breakthrough in stem cell research for tendon injuries – March 2019
5, https://www.ncbi.nlm.nih.gov/pubmed/14649360 – Lameness in 46 horses associated with deep digital flexor tendonitis in the digit: diagnosis confirmed with magnetic resonance imaging – November 2003
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