The stifle should be free from enlargement or from any hereditary or acquired impediment to its perfect action.
With reference to the former an increase of size is sometimes found to result from an overfulness of the joint capsule with synovia, which presents itself as a fluctuating swelling, or a more firm swelling may exist in one or another part of the joint or over its general circumference, as a consequence of a past injury, or there may be sudden displacement, luxation, or "slipping out" of the patella or knee-cap, followed or not by an equally sudden return to its natural position. Some animals inherit a form of development of the stifle which permits this dislocation and return of the bone at uncertain periods. It is possible, therefore, that, however careful an examination may be conducted, this serious defect may not be detected. In some cases it is brought on by weakness, following upon influenza and other debilitating ailments, and may not show itself while the animal is under inspection. From whatever cause it may arise it impairs the usefulness of the animal and renders him unsound.