Page 755 - Adams and Stashak's Lameness in Horses, 7th Edition

Lameness of the Proximal Limb 721

PATELLAR LUXATION/SUBLUXATION Periods of normal ambulation occur if the displacement
is intermittent. On palpation, there are varying degrees
Although patellar subluxation and luxation have
VetBooks.ir been well documented in the literature, they are still The intermittent patellar subluxations are usually read-
of patellar displacement and joint capsule distension.
The condition
33,43,52,55,57,58
considered to be uncommon.
ily replaced, whereas complete patellar luxation can be
is usually recognized shortly after birth, but may be
delayed in acquired cases. Acquired patellar luxation difficult to replace.
Persistent lateral subluxation of the patella presents
also occurs in older horses but even less commonly than much like a horse with UFP. The limb is held in exten-
52
congenitally. Lateral 14,33,37,55,58 medial, 5,42,75 distal, uni- sion, causing the patella to be placed caudolaterad. When
58
lateral, and bilateral patella luxations have all been the limb is advanced, an awkward swinging toe‐dragging
described. Lateral luxation is most common. Severity gait occurs. Because some of these animals have a fairly
can range from a mild intermittent subluxation to a normal gait, the condition may be missed until serious
complete luxation that is difficult to manually replace. degenerative changes within the joint occur. 58
Radiographs should be taken to confirm the diagnosis
Etiology and identify the amount of degenerative changes within
the joint. The flexed cranioproximal to craniodistal tan-
Hypoplasia of the trochlear ridges (primary bone gential (skyline) view is particularly beneficial to assess
deformity) and hypoplasia of the ligamentous support the degree of displacement and the depth of the trochlea.
structures have been suggested as causes. 85,95 The MPL is In the neonate, the normal irregular borders of subchon-
not as large or substantial as the MidPL or LPL. dral ossification in the proximal trochlea should not be
In ponies and miniature horses, lateral patellar luxa- confused with degenerative changes (Figure 5.115).
tion is considered heritable. The lack of adequate
43
trochlear groove and/or lateral trochlear ridge is major
factor. Congenital trochlear inadequacy is not always Treatment
bilateral, at least in the pony. Lateral patellar luxation In one reported case of bilateral congenital lateral
42
can also occur in association with defective develop- patellar luxation in a miniature horse, the foal overcame
ment of the lateral trochlear ridge caused by OCD. the crouched stance and began to walk normally by 6
94
When the condition occurs in the mature horse, trauma months of age, although the patellae remained luxated.
is thought to be the cause. Partial tearing of the insertion Most are treated surgically to minimize permanent
of the quadriceps femoris muscles on the proximal deformity of the soft tissues and the development of OA.
patella has been cited as the cause for distal patellar When a sufficient femoral trochlea is present, stabili-
luxation between the femoral trochlear. 95 zation of the patella in the correct position by imbrica-
tion can be performed. Release of the opposite soft
Clinical Signs and Diagnosis tissue, including the contralateral patellar ligament, is
often required to reduce tension on the repair. 33,52,58,75
Foals with bilateral lateral luxation may not be able Horses with hypoplasia of the lateral trochlear ridge
to stand or may stand in a crouched position because resulting in an insufficient trochlear groove to stabilize
the displaced patella causes the quadriceps muscle group the patella require reconstruction by sulcoplasty and/or
to become a flexor rather than function as an extensor tibial tuberosity transposition. 5,32,37,55 Significant mis-
(Figure 5.126). Signs vary according to the frequency, alignment of the patellar tendons also requires transpo-
position, and degree of displacement of the patella. sition of the tibial tuberosity. When OCD is the cause of
5
the hypoplastic trochlear ridge, the cartilage surface and
underlying bone are usually insufficient to sustain
correction. 94,95

Prognosis
The outlook for soundness without surgical correc-
tion is poor, although pasture soundness may be pos-
58
sible, at least in miniature horses. The outcome is better
for horses undergoing stable correction before the soft
tissues contract and OA develops. Although successful
cases have been reported in horses, they are rare, and
miniature horses and ponies have fewer athletic expec-
tations. In general, it appears that horses have more
difficulty recovering from the repair than ponies or min-
iature horses in most situations.
In one study, a miniature horse requiring medial release,
sulcoplasty, and tibial transposition became sound for its
5
intended use. In another report, 4 miniature foals under-
going imbrication alone to stabilize bilateral lateral patel-
Figure 5.126. This foal has bilateral patellar luxation. The lar luxations survived to be functional, although one
33
quadriceps muscles cannot extend the stifle without the pulley of experienced a recurrence of unilateral lateral luxation.
the patella on the trochlea. Source: Courtesy of Dr. Cliff Honnas. Three of five horses that were surgically treated by release