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Stay Apparatus - Horse Anatomy (view source)
Revision as of 21:54, 14 October 2012
, 21:54, 14 October 2012→Pelvic Limb
==Pelvic Limb==
==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 it's 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.
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====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.