Changes

====Patellar Locking====

====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.

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.

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