Page 597 - Adams and Stashak's Lameness in Horses, 7th Edition
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Lameness of the Distal Limb  563

             THE METACARPUS AND METATARSUS

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             BUCKED SHIN COMPLEX AND STRESS
             FRACTURES OF THE DORSAL THIRD
             METACARPUS
               Bucked shins and stress fractures of the dorsal surface
             of the third metacarpal bone constitute a spectrum of
             diseases that are observed in young (2–3 years of age)
             fast gaited horses.  Although the greatest incidence is
             observed in young racing Thoroughbreds, it also affects
             young Quarter horses and occasionally racing
             Standardbreds (Figure 4.140). The hindlimbs are much
             less commonly involved. Overall, the incidence of
             bucked shins and stress fractures has decreased signifi-
             cantly in the last 10 years, most likely due to changes in
             training methods.


             Etiology
               Different subdivisions of this disease are described
             later; however, it appears that all clinical syndromes
             have a common pathogenesis.
               In young horses put in training, the metacarpal bone is
             exposed to new stresses. In younger horses (2‐year‐olds),
             the metacarpal bone is less stiff, and therefore, greater
             strains are measured on the dorsal cortex during high‐
             speed exercise than in older horses. This high‐strain envi-
             ronment may lead to cyclic fatigue of the bone and
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             subsequent bone pain (bucked shins, sore shins).  Dorsal
             metacarpal strain is even greater in the lead limb (the left
             limb in racing in the United States), correlating with the
             most common location of dorsal cortical stress fractures. 28
               During normal maturation of the third metacarpus,   Figure 4.140.  A racing Quarter horse with the classical
             horses aged 1–2 years resorb primary osteons and have a   metacarpal profile of a dorsal cortical stress fracture.
             greater amount of resorption cavities and incompletely
             filled  secondary  osteons  than  older  horses.  This  bone
             structure is more susceptible to fatigue microdamage   formation at the midshaft of the third metacarpal bone.
             resulting from training because of higher bone porosity,   Additionally, horses completing the full training pro-
             fewer completed secondary osteons, and a lower propor-  gram had greater mineral bone content despite a lighter
             tion of circumferentially oriented collagen fibers.  In   body weight.  Foals significantly increase metacarpal
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                                                        133
             racing Quarter horses put in training, the bone density of   bone mineral density with age, and lack of exercise
             the medial and lateral dorsal metacarpus significantly   retards the normal bone development. 25
             decreased during the first 62 days of training and then   In summary, in horses in race training, high strains
             significantly increased from day 104 to day 244 of train-  can induce cyclic fatigue of bone, resulting in microdam-
             ing. 52,90  Horses experienced fewer bone‐related injuries   age or ultimate bone failure. The body responds with
             when they had greater cortical mass in the lateral aspect   bone modeling and remodeling, but microfracture dam-
             of the third metacarpus at the commencement of train-  age may develop and cause pain. Historically, the major-
             ing.  The changing shape of the metacarpal bone during   ity (more than 80%) of 2‐year‐old racing Thoroughbreds
                                                                                                                6
                90
             maturation  and training 29,122  is consistent with lower   and many racing Quarter horses  demonstrated dorsal
                       93
                                                                                             52
             strains during high‐speed exercise in older horses.  cortical pain with an estimated 12% that went on to
               The thickening of the dorsal cortex, including diffuse   develop acute failure or dorsal cortical fracture, usually
             dorsal periosteal new bone seen particularly in horses in   within 6 months to 1 year of showing dorsal cortical
             training, is likely a natural response to these demands on   pain.  Transformational clinical and experimental stud-
                                                                     93
             the bone to withstand stress without developing microf-  ies performed during the 1990s evaluating bone mode-
             ractures or dorsal cortical fractures (Figure 4.141). This   ling and remodeling in  Thoroughbred racehorses
             has been further confirmed by experimental exercise   demonstrated that galloping long distances increased
             conditions that induced marked modeling and remode-  the likelihood of bucked shins by 36.4%, while breezing
             ling of the dorsal cortex, particularly subperiosteal bone   short distances reduced the likelihood of bucked shins
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