Equine athletes are under a tremendous amount of stress in their limbs which often leads to various lameness issues. One of the most common injuries is tendonitis, specifically of the superior digital flexor tendon (SDFT)1. Tendons are responsible for connecting and transferring the force from muscles to bones. The SDFT specifically stretches from below the knee or hock to the pastern bone2 (see figure 1) and is involved in flexing the digit. This tendon is under considerable loading stress, especially in disciplines involving high speeds or jumping, such as Three-Day Eventing.
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Figure 1. An anatomical view of the front leg demonstrating the location of the SDFT1
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In three-day eventing, horses compete over the course of three days in dressage, cross country, and stadium jumping. All three of the events are strenuous, but cross country and stadium jumping are particularly stressful on equine forelegs due to hyperextension and excessive loading when landing from a jump. Extreme and repeated hyperextension causes strain to the SDFT. This repeated strain can cause individual fibers to tear leading to tendonitis and the rupturing of a tendon if the injury is left undiagnosed or untreated.
Consider a scenario where a seasoned three-day eventing horse competed throughout the spring and summer. At the end of the summer, the horse was completing the cross-country event at a competition. Towards the end of the course, the horse stumbled out of the jump. After the course ended, the horse was obviously favoring the front left leg. The rider checked all four of the horse’s legs for heat and swelling and noticed significant swelling and heat in the left fore leg. Hosing with cool water was initiated and a veterinarian was contacted. When the veterinarian arrived, there was obvious swelling and a “bowed” appearance to the back of the leg (see figure 2).
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| Figure 2. A bowed tendon in the left foreleg2 |
The veterinarian requested information on the horse including the horse’s competition schedule and medical history. The horse had been training since it was two years old, and is now 16 years old. There were no significant training changes, although the horse had been to several recent competitions in a short period of time. In addition, a new farrier placed shoes on the horse. The veterinarian began a thorough examination and, on the posterior side of the left front leg, observed swelling and firmness where the SDFT tendon is located. The veterinarian also checked the deep digital flexor tendons and suspensory ligaments (King and Mannsman, 2005). To assess the extent of damage, an ultrasound of the left front SDFT was conducted.
Tendons are composed of longitudinal rows of fibers that stretch the length of the tendon and are cross linked to each other. Disruption of numerous individual fibers can cause edema and swelling and eventually tendon rupture. Ultrasound is the best tool to identify the extent of the fiber damage so a veterinarian can develop the best treatment and rehabilitation plan possible. Areas of fiber disruption and legions appear as dark spots within white regions of tendons and are graded on a scale of 1 to 43 . The patient was observed to have a grade 3 lesion based on the ultrasound (see figure 3).
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| Figure 3. The circled red area indicates a grade 3 lesion within the SDFT. |
After conversing with the owner, the veterinarian recommended surgery as the best option because the owner planned to continue competing with the horse. Virginia Reef’s study in 2001 found that an aggressive surgical approach, tendon splitting combined with a check desmotomy, resulted in 81% of horses returning to competition, 68% at the same level or better. Of those horses, a large portion were three-day eventers. Both of the procedures are completed under general anesthesia and should be completed as soon as possible after the injury for the best results. Tendon splitting involves cutting multiple small incisions in the affected area to relieve the blood and fluid pressure that is building thus allowing the hole in the tendon to fill quicker. The incisions also cause inflammation which increases blood flow to the injured area which aids with healing. Check desmotomy is a preventative procedure in which the check ligament, which works to stabilize flexor tendons, knee is cut. Previously damaged tendons are more likely to overstretch again and cutting this ligament allows the tendon to stretch further during the healing process. As the tendon heals, the ligament also heals. Based on the veterinarian’s recommendation and their own research, the owner agreed to complete a check desmotomy and tendon splitting in the left front leg. After both of the procedures, non-steroidal anti-inflammatory and antibiotics were prescribed and a bandage was wrapped around the surgery site on the midportion of the front left leg. The bandage was removed after 7 days3. This treatment was an aggressive plan but gave the equine athlete the best chance of returning to competition at its previous level.
The most effective treatment for tendonitis is prolonged rest with the horse initially confined to its stall. Starting two days post-op, the horse was hand walked twice a day for 15 minutes. Hand walking increases blood flow to the region and also places slight strain on the tendon so that the new collagen fibers are placed in a longitudinal pattern to improve the tendon elasticity and strength. Normally, injured tendon fibers fill the tendon hole in a “criss-cross” pattern which is not as strong and increases the chances of re-injuring the tendon3. Since the patient in this case is an athlete, it is at a higher risk of re-injury and will benefit from stronger collagen fibers filling in the injured hole.
King and Mansmann (2005) describes the rehabilitation plan for tendonitis in the SDFT in two phases: “Phase 1 - Light Exercise” and “Phase 2 - Gradual Return to Normal Activities” (see figure 4). Phase 1 began with hand walking for 12 weeks, increasing by 5 minutes each week after week two until the horse was walked for 40 minutes each day. At four weeks post-op, the patient began swimming in a pool for 5 minutes a day. Swimming is beneficial as it strengthens the tendon without placing significant stress on it. Every two weeks, the lesion was checked via ultrasound. Around month four, when the tendon was observed to have healed (the hole is filled), Phase 2 began. Phase 2 started with work under saddle. For one month, the horse was lightly walked for 15 minutes under saddle. At month 6, this was increased to jogging for 20 minutes under saddle. Throughout the month, the duration of exercise was increased until, at month 7, the horse started cantering under saddle. Only after 8 months post-injury did the horse return to strenuous exercise which included jumping and galloping. The goal was to gradually increase the horse’s workload so the tendon could heal and strengthen without experiencing further damage. Throughout this process, the recovering left leg, as well as the right leg, was continuously checked for signs of developing lameness of injury.
Throughout the rehabilitation process, the horse also had monthly shockwave therapy on the left leg as well as weekly Magnawave treatments3. Both of these therapies improved blood flow to the injured area which aided in the healing process.
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Figure 4 shows how the transition through the rehabilitation phase over 8 months. The tendon was visibly healed at 4 months.
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After 8 months, the patient returned to full training and returned to competition a year later. To prevent reinjury, the horse was shoed consistently by the same farrier who understood his previous injury. The horses training schedule was also altered to include more endurance and strength training so the horse did not experience repeated overloading on the tendon3.
Tendonitis is one of the most common injuries in equine athletes and can be one of the hardest to recover from. The aggressive approach used in this case helped the horse return to competition, however, this approach may not always yield the same results. Time off and rest along with hand-walking are the most crucial components to ensure the horse recovers and develops strong collagen fibers in the affected area. Proper preventative measures, such as constantly monitoring for signs of heat and swelling along the cannon bone as well as properly shoeing the horse and focusing on strength training are the best ways to prevent tendonitis form affecting any horse.
Authors
Ashley Lounsberry, Animal Science and Peggy Auwerda, Equine Extension and Outreach Specialist
The report is a project for ANS313 Exercise Physiology of Animals
References
- Bradshaw H. Evaluation of equine superficial digital flexor tendon lesions. The Veterinary Nurse. 2012. Published November 12, 2012. Accessed March 12, 2021.
- Carson DM, Ricketts SW. Tendon injuries in horses. VCA. 2021. Published 2012. Accessed March 12, 2021.
- King C, Mansmann R. Tendonitis (bowed tendon). Lameness. 2005. 400-415.