Page 751 - Adams and Stashak's Lameness in Horses, 7th Edition

Lameness of the Proximal Limb 717

fragmentation was experimentally reproduced in 11 of UPWARD FIXATION OF THE PATELLA (UFP)
38
12 horses following MPD. However, it is important to Upward fixation of the patella (UFP) occurs when the
VetBooks.ir from trauma or use‐related injuries of the FP joint. medial patellar ligament (MPL) becomes caught over
remember that fragmentation of the patella can occur
the medial trochlear ridge. In severe cases it becomes
Clinical Signs fixed in that position, the hindlimb cannot be flexed,
and the horse assumes a posture with the affected limb
The clinical signs consist of variable uni‐ or bilateral extended in a caudally abducted position with the fet-
hindlimb lameness and stiffness. FP joint effusion is lock flexed due to the reciprocal apparatus (Figures 5.123
common and palpable thickening of the joint capsule is and 2.92). More commonly, intermittent UFP occurs,
often present, particularly in horses that have under- and a delayed release of the limb from extension is seen
gone an MPD. Radiographic changes include bony at a walk.
62
fragmentation, spurring, subchondral roughening, and
subchondral lysis of the distal aspect of the patella Etiology
(Figure 5.122). 62,84,108 Cartilage fibrillation without
radiographic abnormalities also has been reported. 38 UFP generally has been considered to occur in horses
that have exceptionally straight hindlimbs; however, it
can occur in horses with any conformation. Rooney
Treatment considers most horses to have femorotibial angles
Horses are usually best treated with arthroscopic sur- approximating 135° and has stated that the problem
86
gery. The lesions observed at arthroscopy vary from occurs when the angle reaches 143°–145°. Although
flaking, fissuring, undermining, or fragmentation of the the exact angle of occurrence likely varies among horses,
articular cartilage to fragmentation or lysis of the bone it is reasonable to say that a straight‐legged horse has a
at the distal aspect of the patella. The subchondral bone greater chance of reaching its individual point of injury.
is involved in most cases that had previous MPD. Furthermore, hyperextension of the limb exacerbated by
Fragments may be buried in the synovial membrane and walking a horse downhill causes the UFP to occur more
tend to emanate from the more lateral portion of the frequently. Body type is strongly inherited; thus it is
distal patella. 62,102

Prognosis
In one study, 8 of 12 horses that had a previous MPD
became sound for their intended use after arthroscopic
treatment of the FP joint. In another study of horses
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undergoing MPD, 3 of 49 horses remained lame. One
8
of these horses had patellar fragmentation and did not
become sound following arthroscopic debridement of
the lesion. One additional horse with documented patel-
lar fragmentation in that study was able to work with
intra‐articular medication. In most cases, the patellar
misalignment was reported to correct within 5 months
after MPD and was not reported in horses 3 months
84
after experimental MPD. 38

Figure 5.123. The horse has upward fixation of the patella. The
Figure 5.122. Flexed lateromedial radiograph of the stifle limb is locked in extension and extended caudally and slightly
demonstrating fragmentation of the apex of the patella (arrow). laterally. The digit is also fixed in the flexed position.