Equine tendons and ligaments
Tendon Notes Vic Cox
Superficial digital flexor tendon (SDFT) -> proximal on short pastern (P2).
Deep digital flexor tendon (DDFT) -> sole surface of coffin bone = P3
The DDFT will flex the entire digit and the fetlock joint
The SDFT will flex the fetlock and the pastern (PIP) joint but not the coffin (DIP) joint
- Downward translation of the fetlock is prevented by 3 parallel tendons
- suspensory lig. (interosseous tendon) + distal sesamoidean ligaments = suspensory apparatus
- DDFT and its check ligament
- SDFT and its check ligament
But ….. remember that only the suspensory ligament attaches to the sesamoid bones while the flexor tendons slide over them, so the suspensory lig. is the main structure supporting the fetlock. Excessive stretching of these tendons -> inflammation = swelling = bowed tendon. Excessive downward movement of the fetlock (overextension) can cause chip fracture of the proximal dorsal edge of P1 due to being rammed against the distal end of the cannon bone.
3. Check ligaments act to check (prevent) downward translation of the fetlock:
- proximal = ligamentous radial head of SDF, from medial edge of distal radius
- distal check = accessory ligament of DDFT, a continuation of the palmar carpal ligament.
The distal check ligament is the one most often discussed, hence when just referred to as the check ligament the distal check is what is being referred to. The hind limb lacks significant check ligaments.
4. Distal (inferior) check ligament is cut (check ligament desmotomy) for:
- flexor deformity (contracted tendons) in foals (most common)
- early laminitis case to prevent rotation of P3 (uncommon), DDFT cut more commonly
- to reduce navicular pressure and hence pain (even more uncommon)
5. The suspensory ligament mainly attaches on the abaxial parts of the sesamoid bones and will pull up on them. Therefore, the distal sesamoidean ligaments are needed to pull “down” (distal) on the sesamoid bones. The 2 main distal sesamoidean ligaments are:
- straight = superficial ->short pastern bone (P2) proximal end between insertions of SDFT
- oblique = middle ->triangular area on palmar surface of long pastern bone (P1)
The sesamoid bones are bound to the distal end of the cannon bone by collateral sesamoidean ligaments.
6. Sesamoid fractures are caused by the upward and downward pull on the sesamoid bones may cause them to fracture (fx) as in a “tug of war”. Sesamoid fx can be basal, apical or sagittal.
7. A fibrocartilagenous intersesamoidean ligament binds the sesamoid bones together so that the proximal sesamoids form a groove for the flexor tendons. The flexor tendons are held in this groove by the palmar annular ligament.
8. At the fetlock the SDFT forms a thin sleeve (manica flexorum) around the DDFT. Mainly the SDFT is superficial to the DDFT above the fetlock but distally it inserts on P2 abaxial to the DDFT.
9. The flexor tendons also have synovial tendon sheaths (paratendon) that surround them in the fetlock and digital region.
10. In the digital region the flexor tendons are bound down by proximal and distal digital annular ligaments. The distal digital annular ligament is beyond the SDFT and therefore only covers the DDFT.
11. Tendons have elastic properties and can stretch to a certain degree but over stretching will cause damage. Stretching and rebound is a way of storing energy and then releasing it for propulsion. Transducers implanted on tendons of live horses indicate that the degree of elongation is 3% at the walk, 6-8% at the trot, and 12-16% at the gallop. Laboratory testing of isolated tendons indicates that they will rupture in the 12-16% elongation range indicating that the galloping horse is in the danger region but the duration of elongation during locomotion is less than a second which is much less than the time for laboratory strain testing.
12. The tendons most often damaged in performance horses are the superficial digital flexor (SDFT) and the suspensory ligament (SL) which is the same as the interosseous tendon. In contrast, the deep digital flexor tendon (DDFT) has a lower incidence of damage. Swelling of the SDFT results in a curvature of the flexor tendons in the cannon region. This swelling is referred to as a bowed tendon because the palmar surface of the tendon “bows” out. SL (interosseous tendon) damage is less obvious but lesions in both sites are best evaluated with ultrasound.
13. Tendons consist of regularly arranged fiber bundles that consist of living cells and an extracellular collagen matrix produced by the cells. The collagen fibers, not the cells, are responsible for the strength and elasticity of the tendon. The collagen fibers have a natural wave or crimp that allows them to stretch as the crimp is straightened out. Flattening of the crimp is not a problem but beyond that other changes due to elongation can be harmful. Mechanical stress that causes cellular or vascular damage will lead to inflammation.
14. Cyclical loading (stretching) and unloading of digital tendons results in recovery of about 90% of the energy that is put into the tendon to stretch it. As mentioned above, this is a process of storing and release of energy. The part of this energy that is lost (10%) is dissipated as heat in the tendon. When heat builds up in the tendon faster than it can be removed by radiation and blood circulation, the increase in heat can cause damage to the tendon. Therefore, heat, as well as mechanical strain can lead to tendon injury. During 7-10 minutes of galloping the core temperature of the SDFT can rise to 45-47 degrees Centigrade. This heat would kill fibroblast cells of dermis, but those of tendon are thermal resistant but, in some cases, their limit is exceeded. SDFT lesions often affect the core of the tendon more than the periphery suggesting that heat damage is a factor in the pathogenesis.