Tendon and ligament anatomy

Tendon Notes Vic Cox

1. Insertions:

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

2. 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.