Unfortunately tendon injuries in horses are all too common. They account for 30 % of the wastage of young thoroughbreds in training, and treatment can be both long and difficult.
The picture above illustrates the typical appearance of a bowed tendon. In almost all tendon injuries it is the Superficial Digital Flexor Tendon (S.D.F.T.) that is affected. Rarely the check ligament or the Deep Digital Flexor (D.D.F.T.) will be involved. In trotting horses the suspensory ligament is more often the problem.
A bowed tendon.
Tendons are composed of Collagen type I fibres. These are long fibres which run longtitudinally up and down the tendon. They are slightly elastic and absorb some of the mechanical force exerted on the stay apparatus during exercise. The S.D.F.T. is thinner than the D.D.F.T. and also takes the entire load on the leg before the D.D.F.T. starts to act. This is the reason that the S.D.F.T. is more frequently injured than the D.D.F.T. The most common injury seen is the 'core lesion'. This is literrally a blood filled hole in the centre of the tendon caused by the rupture of the tendon fibres. Ultrasound scanning can detect these cores at an early stage.
The images below were obtained recently from one of our patients at Pool House Veterinary Group. A four year old National Hunt Horse which had recently started fast work.
Note the black (hypoechoic) hole in the centre of the S.D.F.T. This represents blood/serum in the centre of the tendon. This is the classical appearance of a core lesion.
The scans illustrate a core lesion in cross section sequentially from just below the knee to just above the fetlock. The last two scans are longtitudinal scans, showing the severe fibre disruption in the S.D.F.T.
Healing will take a long time. Use of modern agents such as polysulphated Glycosaminoglycans (Adequan) can help the healing process by inhibiting the collagenase enzymes, encouraging macrophages to remove necrotic (dead) tissue, and stimulating the formation of type I collagen. Left to natural processes the tendon would heal by the laying down of scar tissue (type III collagen) this is inelastic, and hence cannot usually withstand the extreme forces a tendon must be able tolerate. A tendon injury such as the one above will need between 12 and 14 months to heal. A careful controlled exercise program is essential to enable good healing.
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