Why are tendons important?
Tendons are important for transmitting forces from muscle to bone. The equine flexor tendons are important for supporting the fetlock joint during standing and locomotion, and for energy efficient movement, but at peak performance the digital flexor tendons are operating close to failure strains.
Why does injury occur?
Because of this balance between efficiency and failure, tendon injury frequently occurs as the result of repetitive overloading. There may be increased likelihood of tendon injury if there is underlying tendon degeneration – related to increasing age and/or cumulative exercise. Rest, however, is not necessarily a good thing – a 4-6 week period of box rest has been reported to lead to a deterioration in tendon strength. Generally subtle damage tends to occur before severe damage or rupture and the middle (core) part of the tendon is more likely to be affected than the edges. When severe damage occurs, there is cycle of inflammation and then repair. However, the repaired tendon is less elastic than original tendon so reinjury is frequent, either to the repaired tendon or the interface between the normal and repaired tendon.
Flexor tendon injury is more likely with greater loads, so increased weight of horse and rider, greater height of fences and faster speeds will potentially increase the risk of injury. The more times the tendon is stretched near its limit can also increase the risk of injury, for example by increased number of fences or distance galloped. In general, jumping appears to be associated with deep digital flexor tendon injury in the pastern and foot, and jumping from a high height or jumping at speed are associated with superficial digital flexor tendon injury. As the muscles become tired, uncoordinated movement can result in excessive forces on a tendon. Foot conformation can also influence tendon loading so foot balance is important in both prevention and management of injuries.
Although repetitive overloading is probably the most likely reason for tendon injury, damage to tendons can also occur as a result of direct trauma. Laceration of the entire tendon will stop the continuity between muscle and bone. For a digital extensor tendon, this is rarely a major problem, unless a tendon sheath becomes infected. However, for the digital flexor tendons, a loss of support for the palmar aspect of the distal limb is potentially extremely serious. The involvement of a tendon sheath can also affect the prognosis.
Signs of tendon damage
For a complete rupture or laceration of the deep digital flexor tendon, the toe of the foot becomes raised from the floor as the pedal bone loses support from the tendon. If there is complete laceration or rupture of the superficial digital flexor tendon, there is reduced support for the fetlock joint, which may appear to drop. However, in most cases, there is not complete laceration or rupture, so the signs of damage can be less obvious. Swelling, heat and pain on pressure over the affected tendon are the most likely signs of damage, although for tendons within the carpal canal the horse may resent flexion of the knee, and for tendons within the digital flexor tendon sheath (behind the fetlock joint) the horse may resent flexion of the fetlock. When there is injury to the deep digital flexor tendon within the foot or pastern, the horses usually shows signs of foot pain.
Diagnosis of tendon injury
Nerve blocks may be used to locate the site of damage. Injuries to the superficial and deep digital flexor tendons above the pastern are most frequently diagnosed using ultrasonography (ultrasound examination). For tendons within a sheath, tenoscopy (keyhole surgery) can often be useful to see the surface of the tendon. For injuries to the deep digital flexor tendon within the foot or pastern, pain is often improved by nerve blocks to the foot, and in the past would have been confused with ‘navicular disease’. Deep digital flexor tendon injury can occur either in conjunction with or in the absence of navicular bone abnormalities. Tendon injury may also be found in conjunction with damage to ligaments within the foot such as the impar ligament or collateral ligaments of the coffin joint. Magnetic resonance imaging (MRI) is the best method for diagnosing tendon or ligament damage within the foot, and has shown the importance of both tendon and ligament damage within the foot. As MRI is increasingly being used in veterinary practice, it is now possible to diagnose tendon and ligament problems that could not previously have been diagnosed, ensuring that affected horses are treated in the optimal way.
A horse with complete laceration of the deep digital flexor tendon after becoming entangled in wire – resulting in the toe being lifted off the floor.
MRI of a horse with damage to the deep digital flexor tendon (arrow) just above the navicular bone. On this MRI picture, it is possible to see the details of the tendons, bone, articular cartilage and other structures within the foot that are not possible to see using other types of imaging.
Rachel C Murray MA VetMB MS PhD Diplomate ACVS MRCVS
Many thanks to the Animal Health Trust for this article and images