Page 877 - Adams and Stashak's Lameness in Horses, 7th Edition

Principles of Musculoskeletal Disease 843

support is suddenly removed, however, a pathologic accompanies this condition. Osteopetrosis has been
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fracture may result. To prevent this, the rigidity of the reported in a Peruvian Paso foal, and it may be an
VetBooks.ir time. For example, following cast removal, a cotton by Singer and Whitenack, metaphyseal diameters of
inherited condition in this breed. In the case described
external support should be diminished gradually over
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bandage with a PVC splint incorporated into the band­
long bones were wider than normal, and the cortices
age should be used to provide less stability than a cast were abnormally thick with complete obliteration of the
but more support than a bandage alone. Following medullary canal. Trabecular bone was soft and could be
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removal of the splint, a lighter, less rigid bandage should easily crushed, and the bones were not as strong as nor­
be used until eventually no external support is applied mal bones in an animal of this age. Large calluses were
to the limb. It may take several months before the bone formed at fracture sites, suggesting that such fractures
regains normal density and strength following external had occurred in utero. Because of the inherited nature of
immobilization. Occasionally, the original BMD is never the disease in other species, owners should be advised
reached. against future matings. 104
Stress protection is another example of localized
osteopenia seen in fractured bones that have been Hereditary Multiple Exostosis (Multiple Cartilaginous
repaired with rigid internal fixation devices such as Exostosis, Osteochondroma)
stainless steel bone plates. Initially, rigid internal fixa­
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tion enhances healing and permits mobility of neighbor­ Hereditary multiple exostosis is an inherited skeletal
ing joints. Over a period of months, however, the bone disorder characterized by numerous abnormal projec­
under these plates becomes lytic. This produces a weak­ tions from growing bones that result in an abnormal
ened bone with the possibility of a fatigue fracture when bone contour. 75,102,103 The condition affects most of the
the plate is removed. Theoretically, plates should be long bones as well as the ribs, scapula, and pelvis in
removed as soon as healing has occurred. Plates with a horses and is used as a model for the condition in man.
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lower modulus of elasticity have been designed to mini­ The characteristic swellings of hereditary multiple exos­
mize stress protection, especially in people and small tosis are usually present at birth, and the lesions are
animals. In horses, however, osteopenia generally does probably initiated during fetal osteogenesis. They are
not occur to the same degree because a much larger ani­ usually bilaterally symmetrical and consist of multiple
mal places a greater force on the healing bone. Therefore, firm bony enlargements of various shapes and sizes that
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stress protection due to internal fixation is of minimal are firmly attached to bone. Swellings on the limbs do
clinical significance in horses. In general, bone plates not appear to enlarge as the animal matures, but others
and other forms of internal fixation are not removed in such as those located on the ribs and scapulae usually
horses unless they contribute to infection or cause lame­ enlarge until maturity is reached (about 4 years of age).
ness in performance horses, and when they are removed, Lameness, if present, is usually due to impingement of
they are usually done so in a staged fashion. various tendons and muscle groups by the bony masses.
Some horses may be presented for various joint and
tendon sheath swellings. Grossly the tumors adopt a
75
Bone Fragility Disorder variety of shapes, ranging from conical, rounded, pedun­
Bone fragility disorder has been recently characterized culated, multilobulated, or spur‐like. Histologically such
by Anderson et al. These horses typically present with tumors appear as osteochondromas and do not appear
1
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variable degrees of lameness that is difficult to regional­ to undergo malignant transformation. They usually
ize on physical examination. The key diagnostic finding have a small cartilage cap covering underlying spongy
is that the horses have multiple sites of radioisotope cancellous bone. There is no known treatment for this
uptake, mostly in the axial and proximal appendicular condition.
skeleton, and occasionally radiographic and ultrasono­ Solitary osteochondromas occur more commonly in
graphic signs of bone remodeling at various sites. Rest horses and usually develop on the caudomedial aspect
appeared to help some horses, but most were euthanized of the distal radial metaphysis. 56,107 Solitary osteochon­
for humane reasons in the report. Bisphosphonate ther­ dromas are not considered to be an inherited condition
1
apy, namely, zoledronate, has shown some efficacy in like hereditary multiple exostosis. Horses with osteo­
treating signs of disease. 51 chondromas on the caudal aspect of the radius usually
present for lameness and swelling of the carpal canal.
The masses resemble hereditary multiple exostosis, but
Osteopetrosis the lesions are usually not symmetrical. 60,107 Radiographic
Osteopetrosis is a rare skeletal disease of horses char­ examination usually reveals an osteocartilaginous exos­
acterized by an imbalance of bone apposition and tosis protruding from the caudal aspect of the radius.
resorption. It is an inherited disease of people, rabbits, Large exostoses may cause lameness by interfering with
mice, and cattle and may be an inherited disease in muscle movement such as the humeral head of the deep
horses. The underlying problem is a failure of bone digital flexor muscle or pain from carpal canal effu­
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resorption by osteoclasts. There is complete closure of sion. 60,107 Carpal sheath effusion is usually due to the
the medullary canal at the mid‐diaphysis of the bone exostosis and usually will not resolve unless the bone is
because the canal has not been remodeled by osteoclasia removed. Surgical removal of the lesion via tenoscopy of
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during development from its embryologic state. The dis­ the carpal canal is now commonly performed. Solitary
ease is also characterized by fractures due to a lack of a osteochondromas may also develop in joints associated
normal bone structure. Since there is no evidence of with previous trauma or surgery. Small dislodged pieces
bone marrow in such bones, an anemia commonly of cartilage may become trapped in the synovium,