Page 129 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

104 CHAPTER 1

VetBooks.ir 1.183 apparatus). Compressive forces and stress on the
navicular bone exerted by the DDFT are certain
to play a role in the pathogenesis of classic navicu-
lar disease. These forces may be influenced by such
extraneous factors as the horse’s foot anatomy, con-
formation/balance, discipline, working routine and
trimming and shoeing. Under the influence of the
same factors, instability may develop between the
pre- existing osseous fragments of the distal border
and the parent navicular bone and cause the remod-
elling changes described above that lead to pain.
The same biomechanical factors may cause acute
or chronic repetitive strain injury in the supporting
Fig. 1.183 Fat-suppressed MR image of generalised
bone oedema in the spongiosa of the navicular bone. ligaments of the navicular bone, causing desmitis,
entheseopathy and osseous remodelling and pain.
Acute monotonic or chronic repetitive trauma may
3 Primary inflammation, contusion or necrosis also be the cause of primary medullary oedema,
of the medulla results in signs of oedema or haemorrhage or necrosis. In addition, a weak heri-
haemorrhage in the spongiosa of the navicular tability influence has been identified for navicular
bone. In rare cases, necrosis of the medullary disease, which may explain some breed predisposi-
adipose tissue may occur. These pathological tions. The previously described unifying vascular
changes may be the result of trauma rather theory leading to ischaemic necrosis is now largely
than be of degenerative origin. To distinguish forgotten as it has been contradicted by both MRI
bone oedema caused by primary inflammation evidence and pathological observations.
or contusion of the navicular bone from bone
oedema secondary to degenerative remodelling Clinical presentation
changes, sequential MRI is required. Primary Horses are usually first presented in early middle
inflammatory oedema will resolve with rest age (7–12 years old), although the disease may some-
while degenerative oedema tends to persist times be seen in young horses when they are first
regardless. Whether primary inflammation of broken in. Warmbloods and Quarter horses are
the medulla may be a precursor of degenerative most frequently affected but navicular disease can
disease is currently unknown (Fig. 1.183). be seen in any breed, although rarely in native pony
4 Primary desmitis or entheseopathy of the breeds and Arabians. Navicular disease may be seen
supporting ligaments of the navicular bone can in horses with a variety of foot shapes. The typi-
be a cause of lameness and lead to remodelling cal history includes loss of performance, stiffness,
changes of the proximal and distal borders of stumbling, shortening of the stride and mild shifting
the navicular bone. Primary injury of these bilateral forelimb lameness, especially on circles or
ligaments is rare and usually occurs secondary on hard ground. The lameness is of insidious onset
to chronic degenerative disease of the navicular and bilateral, although it may occasionally be acute,
bone or the DDFT and is associated with more severe and only involve one foot. Lameness
navicular bursitis. may become first apparent after a change in owner-
ship, shoeing routine, workload or turnout routine,
Aetiology/pathophysiology or an enforced period of rest for an unrelated injury.
Given the variety of clinical presentations it is likely Lameness may be most noticeable when the horse
that there are a number of different pathological con- first leaves the stall or at the beginning of exercise.
ditions, each with a different aetiology, that result During the early stages of the disease, horses tend to
in the same end-point (i.e. pain in the navicular warm out of the lameness. Horses with pain at rest