Locally the patella is found to be outside the external lip of the trochlea, and its internal ligaments are extremely tense.
If the luxation is spontaneous and of muscular origin, or a consequence of relaxation of the tendons, it is usually noticeable immediately the animal leaves the stall. The animal cannot move without great difficulty. It grows steadily worse with the lapse of time, because the synovial membrane becomes irritated and chronic arthritis is set up.
The symptoms are identical with those of traumatic luxation, but are only temporary.
Diagnosis. The position of the limb and the displacement of the patella are sufficiently distinctive to render diagnosis fairly easy, and to allow of this accident being differentiated from luxation of the femoro-tibial joint or hooking of the patella in the ischio-tibial muscle.
Prognosis. The prognosis varies greatly. If the luxation is of traumatic origin and the accompanying symptoms are not grave, reduction may be followed by permanent recovery. On the other hand, in spontaneous luxation recurrence is almost inevitable.
Treatment. The indications for treatment may be comprised in a single phrase: reduction, with immobilisation of the parts for a sufficient time. To effect reduction, a strip of webbing is fixed around the pastern of the affected limb, passed over the withers, in front of the shoulder of the opposite side, and brought round in front of the neck or between the front limbs. By means of this an assistant exercises strong traction on the limb until the fetlock is raised as high as the elbow of the same side. The operator then applies strong inward pressure to the patella, which usually slips back on to the gliding surface of the trochlea at the first or second effort.
Fig. 22.—Bandage for luxation of the patella.
After-treatment comprises the application of a smart blister, producing œdematous infiltration of all the tissues around the joint, and thus impeding movement and recurrence of luxation. For this purpose various preparations are used—e.g., cantharides, biniodide of mercury, croton oil, etc. It is also advisable to fix the animal so that for a time it cannot lie down, and to secure the pastern to the neck by means of a side-line.
As an experiment, cases of simple fixation of the patella on the summit of the internal lip of the trochlea might be treated by Bassi’s method—i.e., subcutaneous division of the internal lateral ligament of the patella which holds the bone in its abnormal position.