Stay Apparatus - Horse Anatomy

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Thoracic Limb

Stay apparatus of the thoracic and pelvic limbs

When a horse is standing, muscles on opposite sides of the joints delicately contract and relax so that it stays in balance. This obviously puts considerable strain and stress on the muscles. To combat this, the horse has increased the fibrous content of many muscles. These collagenous muscles have been named collectively as the stay apparatus. This is further divided into the suspensory apparatus and the check apparatus.

Suspensory Apparatus

'Interosseous medius/Suspensory ligament'

Proximally, this attaches to the distal row of carpal bones and metacarpus. It runs distally on the palmar surface of the metacarpal bone and then bifurcates proximal to the metacarpophalangeal (fetlock) joint, with a small branch connecting to each sesamoid. It then continues distally on the dorsal surface of the proximal phalanx and joins the common digital extensor tendon, which continues to insert on the middle and distal phalanges.

Associated ligaments:

Collateral sesamoid ligaments - connect the abaxial surface of the sesamoid to the metacarpus and proximal phalanx.
Palmar ligaments - connect the sesamoid bones together.
Distal sesamoidean ligaments - connect the sesamoids to the proximal and middle phalanges. This ligament can be seen as a direct continuation of the interosseous muscle with the sesamoids emdedded in it.

Thus by virtue of the ligamentous arrangement, the suspensory apparatus without any muscular action can resist extension of the fetlock and pastern through the distal sesamoidean ligaments and resist flexion of the pastern and coffin joints through the common digital extensor tendon.

Stay Apparatus

The serratus ventralis supports the cranial part of the body, and connects the costal side of the scapula to points of attachment on the caudal cervical vertebrae and cranial ribs. It contains a tendinous layer that suspends the body once the muscles relax. This will, however, cause flexion of the shoulder which needs to be balanced by the extensors of the shoulder.

The Biceps brachii fulfils this role. Its collagenous tendon extends the length of the muscle and divides near the elbow. The short tendon inserts on the radial tuberosity, the long tendon (lacertus fibrosus) continues distally and blends with the tendon of the extensor carpi radialis and forearm fascia. Combined, these insert on the proximal end of the metacarpus. Thus, the biceps have provision to be able to relax without the collapse of the shoulder, elbow and carpal joints.

Check Apparatus

The superficial digital flexor muscle, continues past the carpus as a thick tendon. The accessory or superior check ligament is its tendinous radial head that joins the muscle at this point. The tendons continue distally and bifurcate by the fetlock into slips, that attach to the medial and lateral aspect of the pastern joint. There is therefore a ligamentous connection from the distal radius to the pastern that can help prevent extension of the carpal and fetlock joint without muscle contractions.

The deep digital flexor also forms a thick tendon just proximal to the carpus. The accessory ligament or inferior check ligament, joins the tendon at the middle of the metacarpus. They then continue through the bifurcation of the superficial digital flexor tendon , through the intersesamoidean groove at the fetlock joint, over the pastern and insert on the distal phalanx. This ligamentous tissue between the metacarpal region and distal phalanx helps prevent extension of the fetlock , pastern and coffin joints.

Pelvic Limb

The equine hindlimb skeleton supports a massive amount of weight from the main body of the horse. This weight is centered on the femoral head and then continues in a vertical line down to the ground by intersecting the hoof. This means that the stifle joint lies cranially of this line and the hock, fetlock and pastern lie caudal to the line. This would result in a collapse of the leg by flexion of the stifle and hock, and overextension of the fetlock and pastern, if there wasn't some supporting mechanism. The stay apparatus is a collection of tendons and ligaments that provide this support, whilst using minimal muscular effort. The stay apparatus has three main components; patellar locking, the reciprocal mechanism and the check apparatus.

Patellar Locking

This is not strictly part of the stay apparatus as there is no increase of collagenous tissue of muscle, but it is so important in reducing muscle fatigue by immobilising the stifle joint; it is considered here. It is the specific anatomy of the equine stifle that enables the locking to occur and includes a number of anatomical structures. The trochlea groove is wide for the gliding movement of the patella and the medial trochlear ridge is the larger of the two ridges. It is wide and round proximally and surmounted by the tubercle of the trochlea. The articular aspect of the patella has a vertical ridge which sits in the trochlear groove and is flanked by two concave areas which relate to the trochlea ridges. The medial concavity is continued by the patellar cartilage. This is a strong curved plate of fibrocartilage that is adapted to the medial ridge. The horse has three patellar ligaments; the lateral and middle patellar ligaments bind the patella to the tibia, whilst the medial patellar ligament attaches the patellar cartilage to the tibia. During normal flexion and extension of the stifle the patella will glide in the trochlear groove. When the horse rests its weight on the hindlimb the stifle can extend beyond a certain point. This causes the patella to move to the proximal extremity of the trochlear groove, a medial twist then results in the medial ridge protruding between the middle and medial patellar ligaments with the patellar cartilage hooking over the trochlea tubercle. Thus the patella is locked and further flexion is not possible. This results in an immobile stifle joint and so no muscle activity is needed to maintain the leg in this position; thus allowing the horse to rest a lot of its weight on this leg. To release the patella, the horse shifts its weight to the other limb and the quadriceps contract drawing the patella proximally. It then twists laterally and is returned to the trochlear groove.

Reciprocal Mechanism

The reciprocal mechanism is a modification made of largely tendinous muscles on either side of the tibia. On the cranial aspect is the peroneus tertius. It's origin is between the trochlea and lateral condyle. It bifurcates distally, both parts inserting in the tarso-metotarsal region. This fibrous muscle ensures that flexion of the stifle is accompanied by flexion of the hock. On the caudal aspect are the superficial digital flexor and the associated fibrous band of the gastrocnemius. The band of the gastrocnemius orginates on the lateral supracondyloid tuberosity of the femur and inserts on the calcaneus. The superficial digital flexor originates as a strong tendon from the caudal aspect of the femur between the two heads of the gastrocnemius. It runs distally covering the gastrocnemius tendon and partially attaches to the point of the hock. It continues distally and inserts as in the forelimb. These two muscles ensure that when the stifle is in extension the hock is also in extension. These structures thus receive their name from the reciprocating action of when the femur moves cranially the distal limb swings caudally and visa versa. This apparatus also ensures that when the patella is locked the hock is also immobilised. These results are obtained with no muscle fatigue.

Check Apparatus

The check apparatus of the accessory ligament of the deep digital flexor tendon is very similar to that of the forelimb. The superficial digital flexor muscle is entirely tendinous and so there is little/no muscle to be fatigued, thus a 'check' ligament would be of no advantage and so there is none in the hindlimb.

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