Page 222 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Musculoskeletal system: 1.5 The hindlimb 197

VetBooks.ir STIFLE

PATELLA
must overcome the much larger medial trochlear
ridge and is usually induced by trauma to the region.
Definition/overview
Conditions associated with the patella include Clinical presentation
upward fixation of the patella, fractures, fragmenta- Generally, the degree of lameness varies between
tion of the apex and luxation. mild and severe depending on the condition affect-
ing the patella. Hindlimb flexion commonly exacer-
Aetiology/pathophysiology bates the degree of lameness, and some horses may
The medial patellar ligament locks the patella and resent this manipulation. Upward fixation of the
its medial cartilaginous extension above the medial patella causes typical gait abnormalities, and the
trochlear ridge of the femur as part of the stay appara- signs are usually evident during rest or at the start
tus of the hindlimb. Upward fixation of the patella is of movement (Fig. 1.378). The affected hindlimb is
caused by failure to unlock this mechanism and is more extended caudally and dragged while walking; this
common in young horses and ponies. It is thought that may be temporarily relieved after a few strides or
straight hindlimb conformation predisposes horses remain for a prolonged duration. The release after
to the condition. Other factors include poor body an intermittent locking involves a snap followed by
and muscle condition and weak thigh musculature. rapid hyperflexion of the stifle and hock. The con-
Upward fixation of the patella can also occur follow- dition can also be manifested as ‘delayed release’ or
ing coxofemoral luxation, trauma to the stifle region ‘catching’ of the patella, with a sudden jerk observed
or sudden box rest in older and/or fit horses. In certain during flexion of the stifle as the horse moves.
pony breeds the condition is considered hereditary.
Acute and permanent fixation is usually unilateral,
whereas intermittent fixation can be bilateral. Some 1.378
horses also appear to suffer from a milder condition in
which there is delayed release of the patella, leading to
milder lameness and poor performance.
External injury such as a kick or hitting a fixed
object while jumping is the most common cause of
fractures of the patella. The proximity of the patella
to the trochlear ridges of the femur while the stifle
is flexed may predispose the patella to fracture if an
external blow is sustained. The medial pole of the
patella is most commonly affected, but a variety of
other fracture configurations can occur.
Fragmentation of the apex of the patella is closely
correlated to desmotomy of the medial patellar liga-
ment. It is suggested that the surgical procedure
places the patella in a different position and leads to
maltracking.
Lateral luxation of the patella is usually inherited
and occurs most commonly in miniature breed foals,
but it has been reported in other breeds as well, pos- Fig. 1.378 This Dale pony has an upwardly fixated
sibly due to trauma. Luxation is usually accompanied patella in the right hindlimb, which extends the hock
by hypoplasia of the lateral trochlear ridge, but it can and stifle, causing it to stand with the distal limb
occur with normal conformation. Medial luxation joints flexed. (Photo courtesy Graham Munroe)