Front Matter

Chapter 17 Diagnosis of and Treatment Options for Disorders of the Spine 433

Newtons and megapascals, the relationships
give us starting points to consider improving our
treatment and training outcomes and considera­
tions for the causes of spinal degeneration.
In the study by Butterman and colleagues
(1992), there was considerable variability
between individuals. The authors believe this
was due to differences in the anatomy, in muscle
mass and distribution, in motion patterns, and in
the individual animal’s motivation to perform
the tasks. It is interesting to note that the varia­
tions between individual dogs, which affected
the results of this study, are reflected frequently
in the training of dogs to complete tasks and to
successfully compete. These individual varia­
tions require our attention if we are going to
train the successful athlete or competitor.
Several other interesting points were noted.
The force on the joint surface remained virtually
constant regardless of the speed with which the
subject was walking. The results were similar to
the findings of in vitro studies. The caudal por­
tion of the joint surface was rarely loaded unless
there was extreme extension of the spine or the
intervertebral disc had been compromised.
Breit followed up on this study by comparing
the facet geometry of the canine thoracolumbar
spine using three distinct groups of dogs (Breit,
2002). He divided 140 dogs into three groups:
large, chondrodystrophic, and small breeds.
His findings enhance the concerns we must
share about the variations between individuals.
Torsional strain between vertebrae is deter­
mined by the transverse distance between the
articular surfaces (Figure 17.11). In small‐breed
and chondrodystrophic dogs the transverse
distance was very consistent when adjustments
were made for size. In these cases, the facets
consisted of lateral and ventral components
that made the joint surfaces rather flat. In all
large breeds the transverse distance was con­
siderably less, adjusted for size, and these joints
were completely different in conformation. All
of the large breeds had a caudal facet compo­
nent that made their articular joints virtually
ball and socket joints. This design is able to han­
dle the much greater forces that their greater
mass absorbs. Interestingly, none of the small
breeds had caudal facet components but some
chondrodystrophic breeds did. The increased
stability of this conformation should be a desir­
able trait that could be heritable.

Lastly, there are several studies using the

dog as a model evaluating the mechanical

properties of the intervertebral disc. Most of

these have been in vitro studies and most have

looked at the mid‐lumbar vertebrae. However,

routine intervertebral disc failure leading to

spinal cord compression is not nearly as com­

mon in the mid‐lumbar spine as in the thora­

columbar region.

Zimmerman and colleagues (1992) deter­

mined mechanical properties of the canine

intervertebral disc at two sites, L2–L3 and L5–

L6. Compressive stiffness varied at each site,

from 717.8 N/mm at the L2–L3 interspace to

949.0 N/mm at the L5–L6 interspace. Torsional

stiffness also varied in each case. At the L2–

L3 interspace it was 1.04 Nm/degree, and

1.72 Nm/degree at L5–L6. Axial or compressive

stress on the intervertebral disc at L2–L3 was

14.03 MPa, and 16.30 MPa at L5–L6. Adjusting

for size, these are similar to those seen in people

and the differences reflect the fact that the cau­

dal lumbar spine plays a greater role in spinal

stabilization. Interestingly, the torsional stress

measured in the dog at both sites was consider­

ably higher than in people, at 30.8 MPa for L2–

L3 and 26.17 MPa for L5–L6.

Unfortunately, information from studies

using the dog spine as a model for understand­

ing and treating human spinal conditions has

Figure 17.11 Third lumbar vertebra. Arrow indicates the

transverse distance between the joint surfaces, which

plays a major role in many of the force measurements

referenced.