Lower leg anatomy on horses is intricate and critical for movement. Maintaining straightness of movement is vital; improper angles may result in bowed tendons.
Most of the muscles responsible for moving a horse’s lower legs lie above its knee or hock, making his lower legs lighter and increasing his speed, but also subjecting his tendons to more force than they’re used to being exposed to.
The Digital Extensor
Horses don’t possess muscles below the knee or hock, forcing the bones of their distal limbs to carry much of the load for moving while often experiencing significant strain. A healthy horse should therefore possess sturdy, well-structured legs – although this increases their susceptibility to injuries.
The long digital extensor (LDE) tendon extends from the fetlock joint to the pastern and is protected by a sheath filled with synovial fluid similar to what we find in human knees, similar to what humans experience when moving their joints. As legs flex and extend, this sheath moves with them, providing space for natural tendon movement within. When bent, its position changes as does that within this protective sheath – with natural grooves formed on splint bones creating natural grooves where its rests within its sheath; spontaneous displacement has only ever been reported on two occasions due to conformational abnormalities or trauma contributing factors contributing to this condition.
The long digital extensor muscle combines feathery muscles into one large, powerful unit. Part of the anterior compartment of the leg (including tibialis anterior and extensor hallucis longus muscles), it works to evert and extend the foot. A horse using this muscle group has incredible reach and power when moving its leg through any angle at speed – especially over short distances.
The Digital Flexor
As horses lower themselves into motion, their deep digital flexor tendon (DDFT) serves to absorb and recoil force on weight bearing legs. This tendon runs through the carpal canal at the back of their knee, over their navicular bone, and into their foot where many injuries to tendons and ligaments commonly occur in horses.
Within the hoof capsule lies a dense digital flexor tendon (DDFT). This tendon lies behind the navicular bone separated by a fluid space known as the navicular bursa and during strenuous exercise it may become stretched beyond repair, often caused by uneven ground work, fast exercise or jumping, poor foot conformation or inappropriate shoeing. Most commonly seen on front legs and often contributing significantly to navicular disease progression, injuries to this tendon often result in sudden severe lameness with potential impactful implications that contributes significantly towards developing diseased conditions of this kind.
The navicular bone is the final bone in a horse’s leg and forms its fetlock joint, an essential joint which allows horses to adjust their angle of travel as speed and direction change, acting like an elastic spring to store energy upon movement. Furthermore, this joint is unique as it’s one of only three in their hind legs that do not feature two independent joints; others such as the stifle are located between femur and tibia (human knee joint equivalent).
The Digital Cushion
The digital cushion is a soft wedge-shaped mass that fills the gap between frog and ungual cartilages and serves to alleviate mechanical loads while supporting internal structures of the foot such as the navicular bone.
The frog and digital cushion serve as the main shock-absorbers in a horse’s leg. They also help control hoof movement during movement by maintaining position and breaking over of its hoof wall. Toe extension places undue strain on both deep digital flexor tendon (DDFT) and cushion which may not be well equipped to withstand this additional stress; especially problematic when combined with long walls at the toe.
Studies have demonstrated that the composition of horses’ digital cushions varies between and among species, including foals. Foal digital cushions contain more adipose tissue than their adult counterparts. Foals’ digital cushions also tend to contain more fatty tissue than adult ones. Foals’ pars torica digital cushion contains dense connective tissue with an abundance of hyaluronic acid-rich matrix material and elastic fibres while their pars cunealis counterpart is compactly attached by chondropulvinal ligaments anchored to cartilages by chondropulvinal ligaments.
One study involved cutting and sectioning 12 Quarter horses’ cadavers to collect samples of their digital cushions’ abaxial proximal, axial distal, lateral and medial regions stained with Masson’s Trichrome, Hematoxylin & Eosin and Weigert’s elastic stain for microanatomical comparison and better understanding their functional properties.
Hoofs are the outer covering of a horse’s foot, made of strong yet resilient material that support weight distribution while also safeguarding more sensitive structures inside.
Hoofs feature an inner structure known as the frog that serves to absorb shocks and act as grip to ensure they don’t slip on uneven terrain. This unique structure, which ranges in color from yellow-gray to whitish-yellow hues, contains delicate nerve endings.
Just below the frog lies an area of flexible cartilages known as the digital cushion that contributes to heel formation while acting as one of the primary shock absorbers.
Just above the digital cushion and between the coffin bone and long pastern bone sits a small bone called the navicular bone, which helps stabilize it while providing some tilt for uneven ground. Other bones make up other parts of the lower leg and foot; two major tendons support and move all these bones – the extensor tendon straightens coffin bone as well as straightening fetlock pastern bones; deep digital flexor tendon bends navicular bone as well as coffin bones – creating the springy pogo stick effect that allows horses to store elastic energy during locomotion.