Page 81 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

56 CHAPTER 1

VetBooks.ir 1.101 condyle. They are also found less frequently at the
distal epiphyses of the proximal phalanx, third meta-
carpus/metatarsus, proximal epiphyses of the middle
phalanx, radius and tibia, glenoid of the scapula,
articular margin of the third phalanx and, occasion-
ally, the carpal and tarsal bones. The affected carti-
lage becomes infolded, creating an ‘osseous cyst-like
lesion’.

Aetiology/pathophysiology
The aetiology is not completely understood. Two
main theories predominate:

• OCD. In the young horse a failure of
endochondral ossification occurs focally and
the thickened plug of cartilage invaginates to
form a bone cyst. Necrosis and collapse with a
secondary inflammatory response coupled with
trauma are important factors in cyst formation.
• Traumatic. It is a more widely held view that
subchondral bone cysts occur at the weight-
Fig. 1.101 A fetlock brace shoe individually created bearing point of a joint as a consequence of
to fit the yearling in 1.97. Note it is in place, with the microfissure formation in the articular surface
strap that forms the brace part of the shoe tightened and subsequent ingress of synovial fluid. The
around the fetlock, to stop it knuckling forward development of a cystic lining and production
when the animal is weight bearing. (Photo courtesy of inflammatory mediators results in a synovitis
Graham Munroe) and, typically, an intermittent exercise-induced
lameness. Subchondral cysts may be seen in
older horses associated with osteoarthritis or
Prognosis articular or periarticular injury.
The prognosis is good for Stage I and guarded for
Stage II DIP joint flexural deformities as long as the Clinical presentation
cases are treated before 24 months of age. MCP flex- The condition is more common in younger horses
ural deformities carry a guarded to very poor progno- (<4 years), although it can present well into adult-
sis, depending on presenting severity. Other factors hood. Mainly fast growing larger breeds are affected,
may worsen the prognosis (e.g. delay in appropriate especially Thoroughbreds and Warmbloods,
treatment and any concurrent orthopaedic problems although the condition occasionally occurs in ponies.
in the affected limb). Most horses with acquired Lameness is usually seen when athletic work begins.
carpal contracture have a chronic unresponsive or Clinical signs are usually a subtle, insidious or inter-
untreated lameness, and it is uncommon to be able to mittent lameness that improves with rest only to
restore such horses to athletic soundness. recur once back in work. Acute-onset lameness is
less common, but can occur. Mild joint distension
SUBCHONDRAL BONE CYSTS may be present, but often is not, and flexion tests
may increase lameness only slightly. Intra-articular
Definition/overview analgesia is often required to identify the site of pain.
Subchondral bone cysts are most commonly seen Note that sufficient time should be allowed for the
at the weight-bearing surface of the medial femoral local anaesthetic to diffuse into the cyst (e.g. up to