Page 579 - Adams and Stashak's Lameness in Horses, 7th Edition

Lameness of the Distal Limb 545

of age. 18,32,44,74 Fractures roughly separate the bone into During training these bones rapidly remodel, which
equal portions and invariably enter the fetlock joint initially decreases bone porosity and increases bone tra-
VetBooks.ir proximally and the DSLs distally, most of these tend to ing the bone’s ability to withstand stress. The SL also
becular width and mineralizing surface, thereby enhanc-
(Figure 4.127). Due to the distractive forces of the SL
107
increases in strength with training until it exceeds the
separate. Infrequently, they remain in apposition, but
may separate at a later time. If both sesamoid bones are strength of the bone, making bone failure the method of
fractured, they usually become distracted and the sus- suspensory breakdown in racing or heavily training
17
pensory support apparatus is lost. racehorses. The vascular pattern of sesamoid bones
may be implicated in site selection of fractures as the
Etiology orientation and distribution of vessels parallel the radio-
graphic lucencies seen in horses with sesamoiditis and
The cause of most proximal sesamoid bone fractures correspond to the configuration of apical fracture
is excessive tensile forces related to fetlock hyperexten- patterns. 94
sion that maximally loads the sesamoid bones. Fetlock
extension is greatest at the end of a race due to fatigue
of the digital flexor muscles that support the fetlock. Clinical Signs
The bone fails when the sesamoid bone can no longer The medial or lateral sesamoid bones, or both, may
withstand the distraction forces applied to it by the SL be fractured. Lameness is very pronounced in acute
and DSLs. The muscle fatigue factor is illustrated when stages. The horse is reluctant to bear weight on the limb
young foals that are placed on pasture fracture their and will not permit the fetlock to descend to a normal
sesamoid bones while running to keep up with the position during weight‐bearing. Swelling, heat, and pain
dam. 32,44 It has also been reported that 70% of Quarter are often severe in the fetlock region. Tenosynovitis or
horse fatal musculoskeletal injuries occur within 10 suspensory desmitis, which also may be present, may
yards of the finish line regardless of the race distance. 7 confuse the diagnosis if radiographs are not taken. The
Other factors such as poor conditioning, improper horse evidences pain when pressure is applied to the
trimming and shoeing, and poor conformation create affected bone or bones. Extension of the fetlock during
additional stresses on the sesamoids, as do training and weight‐bearing causes pain. Observation of the gait
racing schedules. 2,68 Although uncommon, direct blunt reveals that the fetlock is held rigid so that it cannot
trauma such as a kick or interference or a misstep lead- extend as much as the opposite normal fetlock. Lameness
ing to unequal tension on the bone may also contribute at the walk and trot may not be obvious after 1–2 weeks’
to sesamoid fractures. The sesamoid bones also undergo rest, but joint effusion persists.
marked bone resorption when the fetlock is immobi- The fracture in the bone may occur in any area of the
lized, and pathologic fracture after cast removal has sesamoids, but proximal fractures are more common
been reported in adult horses. 53 than distal fractures; proximal fractures also are more

A B B

Figure 4.127. (A) Lateral and (B) oblique radiographs of a horse with a midbody sesamoid fracture (arrow).