Chapter 17 Diagnosis of and Treatment Options for Disorders of the Spine 439with life as they can catastrophically affect
respiration.
The biomechanical parameters that deter
mine the likelihood of a spinal fracture are
well known and a great deal of the work
defining these has been done in rodents
(Kranenburg et al., 2010). The ultimate break
ing force of a bony structure is a simple meas
ure of its strength. The ultimate displacement
of a bony structure is the reciprocal of its brit
tleness. The work‐to‐failure measurement
defines the ability of a bony structure to
absorb energy. All three of these biomechani
cal modalities can be improved upon, but
bone remodeling must occur at a rate that
keeps up with the forces placed upon it. The
ability to resorb and replace bone in response
to stress‐related injuries is well documented
in younger dogs but less well studied in adult
dogs. The potential for bone repair in relation
to age in dogs is not known.
Genetic factors may have significance in
determining susceptibility to bony fractures.
There seems to be a strong correlation between
bone mineral density (BMD) and bone strength.
Certain strains of rats have a higher BMD and
are less prone to fractures (Turner, 2002). A
study of twins found that BMD was genetically
based, and the genetic association was particu
larly strong within the spine for BMD specifi
cally (Pocock et al., 1987).
Nutrition is also a significant factor in main
taining normal BMD and therefore preventing
potential fractures. Dogs maintained on a low
calcium diet had a decrease in their BMD. Bone
strength measured in the L2 vertebra, femoral
neck, and mid‐femur was reduced in dogs fed
calcium‐restricted diets (Motoie et al., 1995).
In 1946, E. J. Slijper in his seminal work
defined the moment of resistance (MoR) as a way
to determine the spine’s ability to resist bending.
The MoR is calculated by multiplying the trans
verse diameter of the vertebra by the square
of the height of the caudal surface of each ver
tebra (Figure 17.21). Zotti and colleagues (2011)Figure 17.20 Lateral radiograph of a pathological fracture of the thoracic spine. Although there is significant
destruction of the entire vertebra, many of these dogs present with acute onset of symptoms rather than having a slow,
progressive course as we frequently expect with neoplasia.
Figure 17.21 Lumbar vertebra. Arrows demonstrate the
measurements used by Slijper to determine the moment
of resistance.