Page 752 - Adams and Stashak's Lameness in Horses, 7th Edition

718 Chapter 5

likely that the tendency for UFP could be congenital. affected horses do not develop radiographic abnormali-
Shetland ponies are particularly affected, and UFP pre- ties from intermittent UFP. 26
VetBooks.ir long enough to reach over the medial trochlear ridge in Treatment
disposes them to develop coxofemoral luxation.
17
The condition also appears when the MPL becomes
spite of normal conformation. Examples of this include For a persistently fixed patella, a sideline may be
loss of quadriceps muscle tone and traumatic hyperex- applied to the affected limb so that as the limb is drawn
tension of the hindlimb. The ligaments may become forward, the patella is pushed medially to unhook the
stretched once upward fixation occurs, so recurrence is MPL, or downward, it often disengages the fixed patella.
common. The MPL is thought to be weaker than the Backing the horse may also dislodge the patellar liga-
other two patellar ligaments, predisposing it to elonga- ment. Personnel should take care to be out of the range
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tion. Furthermore, younger horses, when they begin of the “jerk” when the MPL releases. If the UFP cannot
training, often lack the muscle tone they will acquire as be reduced by manipulation, then a medial patellar liga-
they work. Upward fixation also has been observed in ment desmotomy is indicated.
horses abruptly taken out of training and confined to a Many horses respond to controlled conditioning to
stall. increase quadriceps strength and tone, which serves to
A recent report of 76 horses affected with UFP cites tighten the MPL. However, if the horse’s fitness level
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hoof balance and angle as predisposing factors, along drops below optimal, the UFP can recur. Obviously, rest
with reduced muscle tone from seasonally reduced or confinement is contraindicated. Conditioning, includ-
work. Higher medial hoof wall and elongated toes ing going up hills without coming back down the incline,
26
were reported to cause hyperextension of the stifle and which exacerbates the hyperextension of the stifle,
outward rotation of the limb, contributing to UFP. strengthens and tones the quadriceps.
Corrective trimming and shoeing alleviated UFP in half Dumoulin et al. reported higher medial hoof walls
of the horses and reduced the severity in an additional and elongated toes along with reduced muscle tone from
20% of the horses (see “Treatment”). seasonally reduced work as predisposing factors in
26
horses with UFP. Shortening the toe and lowering the
medial hoof wall sufficient to move break‐over medial
Clinical Signs and Diagnosis
to the toe alleviated UFP in half of the horses and
In acute UFP, the hindlimb is locked in extension, as reduced the incidence in an additional 20%. Anecdotally,
described above. The condition may relieve itself, or it other farriers share this opinion.
may remain locked for several hours or even days. In Estrogen therapy has been used to treat intermittent
29
other cases, there is only a “catching” of the patella as UFP in horses. The rationale is that estrogens can cause
the horse walks. Hyperextending the stifle by walking tendon and ligament relaxation, but it is unclear whether
down an incline may cause a jerking gait from intermit- horses with upward fixation have overly tense patellar
tent catching of the MPL, and the horse may assume a ligaments and if estrogen has any effect on patellar liga-
crouched position, presumably to prevent stifle exten- ments and tendons. It has been hypothesized that estro-
sion. Backing or moving in a tight circle also exacerbates gen affects muscle cell metabolism and muscle tone
the signs. When the MPL releases, the hindlimb usually leading to the anecdotal benefit sometimes reported.
jerks up quickly, mimicking stringhalt. Both hindlimbs However, no studies currently exist examining whether
usually are predisposed to the condition; however, truly this therapy is beneficial as a medical treatment.
unilateral cases often have a history of an inciting cause Recommended therapy is 1 mg of estradiol cypionate IM
in the affected limb. for every 45 kg of body weight (i.e. 11 mg/500 kg) once
Palpation when the limb is locked in extension reveals weekly for 3–5 weeks. Concurrent anti‐inflammatories
36
tense patellar ligaments and that the patella is locked along with exercise to strengthen the quadriceps mus-
above the medial trochlear ridge of the femur. The horse cles are usually recommended.
drags the front of the hoof on the ground when it is If conditioning and shoeing or medical therapy fail to
forced to move forward with the limb locked. When the halt the incidence of UFP, the MPL can be tightened
limb is in a normal position, the predisposition can be with scar tissue by creating a series of longitudinal inci-
evaluated by forcing the patella upward and outward sions in the MPL. This procedure can be performed
with the hand. If the limb can be manually locked in using a scalpel blade or 14‐gauge needle with the horse
extension for one or more steps, it is predisposed to either standing or under anesthesia. The incisions are
UFP; manual induction of UFP should not be possible in usually confined to the proximal third of the MPL. 49,82,97
a normal horse. This substantially thickens the MPL and presumably
Lameness usually is not severe or constant, but femo- causes a functional shortening and tightening of the
ropatellar synovitis and distension may occur with MPL. In one study, UFP was eliminated in all seven
82
97
repeated episodes. In some horses, UFP can be extremely reported cases. A large advantage of this procedure
difficult to manually reduce or correct by backing the compared with MPD is that the patella remains stable.
horse. Coxofemoral strain may be associated with An older treatment that has been commonly used for
persistent UFP, and ponies are prone to secondary horses with intermittent upward fixation and no palpa-
coxofemoral luxation. 17 ble swelling of the FP joint capsule is the injection of
Radiographs of the stifle should be taken to eliminate counterirritants into the middle (MidPL) and MPL.
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conditions that predispose to UFP. Although uncom- The injections are usually performed on the standing
mon, hypoplasia of the medial trochlear ridge, such as horse using mild sedation and nose twitch. Commonly
occurs with OCD, facilitates the displacement. Most used irritants contain 2% iodine; 1–2 mL is injected in