Second in a Series

In this series, Dr. Deb Bennett examines the equine hind limb. In this installment, Bennett explores the hock.

Farrier Takeaways

• Rapid flexion and extension of the joint during normal locomotion engenders friction between closely adjacent structures and between tendons and the underlying bone, necessitating the development of bursae.
• The compact structure of the hock means that many tendons must fit into a small area; some are even interwound or interlaced.
• Horses that are repeatedly asked to perform incorrect extensions of stride often develop cunean tendon irritation. Those with “post-legged” hindlimb conformation are also susceptible to chronic tenderness and swelling.

Anterior view of the bones and ligaments of the hock. This is a deep dissection; see other figures for more superficial structures. Note the oblique orientation of the trochleas of the tibial tarsal (astragalus). (Illustration based on the Dittrich original 1.)

Medial and lateral views of the bones and ligaments of the hock. 1 and 2 = Branches of the short part of the medial ligament that binds the tibial and fibular tarsal bones. 3 = Tarso-metatarsal ligament that binds the fibular tarsal to the fourth tarsal and top of the cannon bone. 4 = Tendon of the deep digital flexor muscle, which plays over the smooth, saddle-shaped surface of the sustentaculum. It is protected by the tarsal sheath, a hose-like thickened membrane (dark blue). 5 = Proximal tuberosity of the tibial tarsal bone. 6 = Lateral edge of the sustentaculum. 7 = Short ligament that binds the fibular and tibial tarsal bones. (Illustration based on the Dittrich original 1.)

Dissection of a right hock in lateral view (from an Equine Studies Institute wet lab session).

Closeup images of the hock in anterior view. A is “reality,” a photograph of a dissection carried out in my wet lab. B is the “road map,” an illustration from Schmaltz’ 1904 anatomy2.The interlacing of the cunean tendon, peroneus tertius yellow ligament band and main tendon of insertion of the tibialis anterior muscle are the most important features. The photograph brings out details of the annular ligament wrappings and fine blood vessels, while the illustration presents an idealized view of “what the student ought to be able to find” in terms of blood vessels and nerves which are never so plainly visible in an actual dissection.

Vertical section cut through the midline of the hock joint. In section preparations, structures that have a lot of depth or curvature will appear to pop in and out of view, for example, the fibular tarsal bone, of which both 2 and 6 are parts. Likewise, the splint bone does not show in this cut because it is too far above the midline plane of the limb. When studying sections, it is important to remember that the living anatomy is three-dimensional. Visualize the annular ligaments, tarsal sheath, synovial sheaths and joint capsule as three-dimensional structures that wrap around the bones and ligaments of the hock joint. (Illustration based on the Dittrich original1.)

The main feature to study here is the relationship between the joint capsule, the sustentaculum, the tarsal sheath and the tendon of insertion of the deep digital flexor muscle. The bursae (purple) are also important. 1 = Tibial tarsal bone (astragalus). 2 = Base of fibular tarsal bone (calcaneus). 3 = Central tarsal bone. 4 = 3rd tarsal bone. 5 = Metatarsal III (cannon bone). 6 = Tuber calcis. The perforating tarsal vein passes through a canal in the fourth tarsal bone. Other gaps between tarsal bones provide anchorage for short, strong internal ligaments that bind the small bones of the hock together, and for small vessels. The tarso-metatarsal ligament is intimately adherent to the joint capsule. See Figure 5 for the complex relationship between the medial tendon of the tibialis anterior muscle and the peroneus tertius yellow ligament. The edges of the membrane of the anterior parts of the joint capsule appear ragged because they are as thin as the wall of a balloon that was torn during the process of sawing the limb in half. The tarsal sheath is the thickened outer layer of the tendon of insertion of the deep digital flexor muscle, which plays over the sustentaculum and the thickened posterior portion of the joint capsule. See Figure 9 for 3D views of the tarsal sheath.

Cross-section through the hock joint, cut horizontally at a level near the upper surface of the central tarsal bone. This view clarifies the relationship of the tendon of the deep digital flexor muscle, tarsal sheath and thickened joint capsule. Note also the distribution of superficial and deep blood vessels and nerves and the position of the calcaneal and subcutaneous bursae (after Delling et al., 2013 3.)

The structure of a tendon sheath is shown in (B). Please note, this is a “concept” illustration to aid in understanding the structure of tendon sheaths. Imagine a cable (blue) that runs through an inner tube (pink) as in view (A). The cable is coated with Velcro and the inner surface of the inner tube is too so that if the cable is pulled, the inner tube will “roll” (curved arrow). Now in your mind’s eye, elongate the inner tube into a hot-dog shape — this is the basic shape of a real tendon sheath. The cut portion shows the double-walled structure and the filling of synovial fluid. As the tendon plays back and forth, the sheath rolls on the inner lubrication, providing for nearly friction-free movement.

The hock joint in lateral and medial views. These are necessary “road maps” or idealizations. Note the retinacula, strong short bands developed within different layers of the annular ligaments. They function to keep the various long tendons in place even if the hock joint is sharply flexed under conditions of great force. Also highlighted are the tendon sheaths (green) which facilitate the up-and-down play of various long tendons of insertion. Note how the calcaneal and subcutaneous bursae are sandwiched between tendons and the skin, as is the bursa that lies under the cunean tendon. The superficial digital flexor band, like the peroneus tertius, is a yellow ligament (after the Dittrich original1).

Horses with “extremes” of hind limb conformation are more likely to suffer from hock-related lamenesses. Conformation faults give rise to altered limb swing, hoof strike and breakover. A: An aged Arabian gelding with post-legged hindlimb conformation, which predisposes to stifle locking during movement (to be discussed in our next installment), as well as to cunean tendon irritation and pathology. B: A 5-year-old Tennessee Walking Horse stallion showing the opposite extreme. Over-angulation of the hind limb, sometimes called “sickle hock,” predisposes to curby hock and various types of spavin.

Bones of left hock in lateral view. Red arrows show areas of exostosis (abnormal deposition of coarse, rough bone), any and all of which may be called “bone spavin.” This specimen is a skeletal mount of an aged Thoroughbred gelding.

Various manifestations of “bone spavin” in the hocks of living horses. Unlike horses with curbs, capped hocks or bogs, spavined horses are often sore and lame. A: Rear view of the left hind leg showing exostotic enlargement of the junction between the fourth tarsal bone and the top of the lateral splint bone. B: Front view of the right hind leg of a different horse, showing exostotic enlargement of the lower tubercle of the tibial tarsal bone and the central tarsal bone. The red arrow shows an exostotic enlargement of the collateral ligament that binds the long and short pastern bones. Below it is a contracted hoof showing ringbone and a medial wall flare. This is a chicken-and-egg situation; it would be difficult to say which came first — the hock pathologies or those of the more distal limb.

Pathological lumps and swellings. A: Lateral view of a right hind limb. B: Antero-lateral view of a right hind limb. Capped hocks (C, medial view of left hind limb) are sometimes the result of strain, as from jumping, but much more often are seen in horses that habitually kick the walls of the stall. The enlargement is the result of irritation and subsequent fluid build-up in the subcutaneous and calcaneal bursae. Self-quiz: what is the anatomical origin of each pathology? Which are likely to make the horse chronically lame? Which may feel hot, and which usually feel cool? Which feel soft and which are hard? Which are likely to resolve over time?

Curby hock is the result of rearward displacement of the small bones of the hock and the cannon bone and splints relative to the tibial and fibular tarsal bones (view A; compare this closeup of the hock bones with Figure 11). This in turn forces backward displacement of the plantar ligament (shown in blue), creating a bulge in the rear contour of the hock and a “cut out” contour in front (view B). This structural defect is never, in my experience, the result of strain due to athletic effort but rather develops during perinatal life, either just before or just after birth when the ligaments that bind the hock bones in place are still soft and rubbery. A strain of the plantar ligament itself, without bony displacement, is the result of athletic effort. It also produces a bulge on the back of the hock.

Practicum and self-quiz, part 1: the hock in postero-lateral view. Learning the topological anatomy of the hock is easier when you know the underlying bone structure. Palpate with your fingertips, then compare with Figure 16.

Hock bones in postero-lateral view. Compare with Figure 15.

Practicum and self-quiz, part 2: the hock in postero-medial view. Palpate with your fingertips, then compare with Figure 18.

Hock bones in postero-medial view. Compare with Figure 17.

Answers to Self-quiz Questions in Figure 14

Jack and “bone” spavins involve ossification and exostosis at the insertions of binding and collateral ligaments. Jack spavin is the name used when the pathology arises on the anterior aspect of the hock. Bog spavin is fluid distension of the anterior compartment of the tibiotarsal joint capsule. Thoroughpin is fluid distension of the posterior compartment of that capsule. Capped hocks arise from irritation, inflammation and fluid accumulation in the subcutaneous and/or the calcaneal bursae. The hard spavins that involve exostosis often make the horse lame and at times will feel hot. Bogs and caps, which involve fluid distension and feel soft and cold to the touch, generally do not make the horse lame. Once established, none of these conditions usually return to normal. Hard spavins involving the splint bones are the most likely to completely resolve, especially if hoof imbalances are appropriately addressed.