A Closer Look at the
Angulation of the hindquarters (good turn of stifle) is required for extra speed. Increasing the turn of stifle proportionately increases the length of both the thigh and second thigh, thereby also lengthening the rearing muscles. These muscles, particularly of the second thigh, provide the drive. Weak hind muscles lead to poor hind action, hence 'Muscular' in the standard.
The natural result of lengthening the second thigh, by developing a good turn of stifle, is that the hind pastern shortens, giving hocks that are 'well let down' - i.e. close to the ground. A shorter hind pastern gives greater endurance. Ideally the hind pastern should incline slightly forward - foot slightly behind the hock, bur a vertical hock is the norm in the showing. The hind pastern should not incline backwards, foot in front of the hock, since these 'sickle hocks' imply that the Achilles tendon is unable to fully extend the leg. In more severe cases the dog may look as though it bounces up and down on its hind feet. Rather than being directly under the pelvis, the hind foot should stand so that the hind pastern is beneath the point of buttock.
The stifle joint is most unusual in construction. The patella runs between grooves on the end of the femur, to prevent overstraightening and act as a pulley for the rearing muscles. If the muscles and ligaments are slack the patella may jump the groove. In a dwarfed breed like the chihuahua it is more likely that the groove may not develop in proportion to the joint, shallow grooves therefore tend to be more common. This fragile joint is particularly susceptible to external damage. Enthusiastic pulling and tweaking of the hind leg can have serious results. The judge may find this patella luxation (slipping stifle) evidenced by hopping, locking the leg straight when standing, or by feeling (and sometimes hearing) the patella move when gently handled. The problem is another that age tends to amplify, and the unnatural wear on the joint may lead to arthritis.
Hip dysplasia also regularly leads to arthritis, and is not confined to big dogs, it is known in chihuahuas. HD can be debilitating in its severest form. HD occurs where the ball joint of the femur does not fit snugly in the socket of the pelvis. The result is uneven wear on both surfaces, and strain on the muscles. Subclinical HD (showing no external symptoms) is tested for by X-Rays in the most susceptible breeds, which tend to be the larger and heavier ones. Like all other structural faults HD must usually be assumed to be hereditary and breeding programmes planned accordingly to discard, rather than store up problems for future generations.
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Reproduced from the British Chihuahua Club Handbook 1987