Page 474 - Adams and Stashak's Lameness in Horses, 7th Edition

440 Chapter 4

ling/modeling and vascularization have been demon-
Box 4.1 Abnormalities that may exist in horses strated in horses with navicular disease. 22,82,136 The
classified as having navicular disease, navicular
VetBooks.ir syndrome, palmar foot syndrome, or palmar heel pain increased vascularization was shown to be a combina-
tion of active arterial hyperemia and passive venous
congestion. Obstruction of the venous outflow was
1. Navicular disease: radiographic, CT, or MRI
abnormalities within the navicular bone thought to result in venous congestion, increased bone
2. Desmitis/trauma of the podotrochlear apparatus marrow pressure, and pain. 86,88,117
a. Collateral suspensory ligaments (CSL) of the There is increasing evidence that abnormal non-
navicular bone physiological biomechanical forces leading to tissue
b. Desmitis of the distal sesamoidean impar liga- degeneration is the most likely cause of navicular dis-
ment (DSIL) ease. 6,16,45,70,88,97 Forces exerted on, or experienced by, the
c. Desmitis of the distal digital annular ligament navicular bone and the podotrochlear apparatus are
3. Tendonitis of the DDFT: usually at three locations central to these biomechanical studies. Abnormal forces
a. The insertion on the navicular bone could arise from either excessive
b. Palmar to the navicular bone physiological loads applied to a foot with normal con-
c. Proximal to the navicular bone formation or normal loads applied to a foot with abnor-
4. Desmitis of the collateral ligaments (CLs) of the mal conformation. 61,119 Poor hoof conformation and
DIP joint balance, particularly the long‐toe, low‐heel hoof confor-
5. Navicular bursitis mation accompanied by the broken‐back hoof‐pastern
6. Synovitis/capsulitis/OA of the DIP joint axis, have historically been considered major risk factors
7. Primary hoof imbalances (improper trimming or for the development of navicular disease. This concept
shoeing) has withstood the test of time, supporting the theory
8. Hoof capsule and/or heel distortions that excessive and repetitive forces applied to the distal
third of the navicular bone by the DDFT are a major
contributor to the disease. Further supporting this is the
finding that the force exerted on the navicular bone is
navicular apparatus. 46,49,96 However, documenting many negatively correlated to both the angle between the third
of these abnormalities in horses that do not have phalanx (P3) and the ground and the ratio between heel
radiographic abnormalities requires advanced imaging and toe height. 32,47,51,61 Extreme cases of collapsed or
such as computed tomography (CT) or magnetic reso- underrun heels can result in a negative palmar/plantar
nance imaging (MRI). In addition, hoof imbalances, angle where the palmar/plantar margin is lower than the
101
hoof capsule distortion, contracted or sheared heels, etc. dorsal margin of the distal phalanx (Figure 4.1). This
may be the sole cause of lameness without abnormalities type of conformation greatly increases the contact stress
within the deeper structures of the foot. 4,25,115 on the navicular bone by the DDFT. In contrast, heel
The bottom line is that not all horses with lameness elevation is used to treat horses with navicular disease
conditions associated with the palmar aspect of the foot and is thought to decrease the tension on the DDFT, which
should be labeled as having navicular disease or syndrome. reduces the forces applied to the navicular bone. 51,96,132
Some clinicians feel that navicular disease/syndrome should The location of the navicular bone between P3, the
be reserved for horses with chronic bilateral forelimb second phalanx (P2), and the DDFT suggests that it
115
lameness that fit a very specific set of diagnostic criteria. functions primarily as a buffer to spread forces among the
Most horses with true navicular disease have a bilateral anatomical structures. It ensures that the DDFT main-
96
forelimb lameness that switches to the opposite limb after tains a constant angle of insertion, and the distal aspect of
a low PD block, pain on hoof testers across the central or the bone must withstand great forces during the propul-
cranial aspect of the frog, and some evidence of radio- sive phase of the stride. 13,17,28 A study in isolated limbs
graphic abnormalities within the navicular bone. However, using pressure‐sensitive film documented that the contact
hoof tester sensitivity may be variable, and based on recent load on the navicular bone and associated joints was
MRI studies, the lack of radiographic abnormalities does highest during dorsiflexion (extension) of the limb, which
not rule out navicular bone pathology. 2,35,100 corresponds to the end of the stance phase or the begin-
ning of the propulsion phase. However, in another
17
Etiology study, both force and stress on the navicular bone in
horses with navicular disease were approximately double
The two proposed causes of navicular disease are vas- those of normal horses during the early stance phase of
cular compromise and biomechanical abnormalities the stride. This was associated with greater tension
133
leading to tissue degeneration. 6,22,23,43,71,82,88,96,97,115,119,136 on the DDFT, which the authors attributed to contraction
With the vascular theory, thrombosis of the navicular of the deep digital flexor muscle in the early stance phase.
arteries within the navicular bone, partial or complete Contraction of the deep digital flexor muscle was thought
occlusion of the digital arteries at the level of the pastern to help avoid heel‐first landing in an attempt to unload
and fetlock, and a reduction in the distal arterial blood the heels, but this appears to simultaneously increase the
supply due to atherosclerosis was thought to result in forces on the navicular bone. 96,133
ischemia of the navicular bone. 22,23,97 The vascular theory It has been estimated that the peak forces on the
has not withstood scientific scrutiny because of failure navicular bone approximate 0.67× body weight during
either to reproduce the disease by altering blood flow or the walk and 0.77× body weight at a slow trot. 88,115
to identify histological tissue changes compatible with However, the forces applied to the navicular region are
the theory. 96,97,119 In addition, increased bone remode- influenced not only by body weight but also by limb and