Although the authors of this study have already described the morphologic and morphometric cervical MRI findings of dogs with DA-CSM, clinically normal Doberman pinschers, and clinically normal English Foxhounds, no results have yet been reported on their intervertebral disk width [13–16]. These previous studies have demonstrated that the occurrence and degree of disk degeneration and protrusion could be considered part of normal age-related spinal degeneration [13, 15]. Additionally, the present study suggests that cervical intervertebral disks in Doberman pinschers and English Foxhounds become wider with increasing age.
A recent study compared the morphometric features between 16 Doberman pinschers with clinical signs of cervical spondylomyelopathy and 16 clinically normal Doberman pinschers . Significantly wider intervertebral disks in the group of clinically affected Doberman pinschers were demonstrated. It was suggested that the wider intervertebral disks of clinically affected Doberman pinschers could potentially be at higher risk of herniation and that the volume of disk protrusion into the vertebral canal would be higher than that of clinically normal dogs . This finding, along with a relative vertebral canal stenosis, could potentially explain the development of clinical signs in Doberman pinschers with cervical spondylomyelopathy . In contrast, the study reported here did not demonstrate that clinically affected Doberman pinschers have overall wider cervical intervertebral disks compared to clinically normal dogs. Even when looked at each intervertebral disk location separately, no significant differences were found between the different groups of dogs. Therefore, the results of this study do not provide evidence that wider intervertebral disks can be considered a potential risk factor for the development of clinical signs of DA-CSM. Although the exact reason is unknown, a potential explanation for the different results between both studies can be found in the age of included dogs. The mean age of the 16 clinically affected Doberman Pinschers in the study of da Costa et al.  was 6 years, while the mean age of the 16 clinically normal dogs was 4.3 years with only 3 dogs being older than 6 years of age. Since the results of the present study suggest that cervical intervertebral disks become wider with increasing age, it is possible that the difference found by da Costa et al.  could have been influenced, at least in part, by a difference in age between both groups of studied dogs.
The effects of advanced age on the imaging, morphometric, biomechanical and histopathological features of the intervertebral disk space have been investigated in human and laboratory-animal studies [17–23]. An altered fibrecohesivity , lamellae thickening, decrease in water and proteoglycan content with fragmentation and alterations in cross-linking of the collagen fibres in aged intervertebral disks has been demonstrated . Such changes reduce the mechanical strength  and decrease the structural integrity of the aged disk . These degenerative changes allow intervertebral disk deformation . A study investigating age-related changes in the spine of laboratory-Beagles  demonstrated a decreased elastic modulus of the aged intervertebral disk along all levels of the vertebral column. The intervertebral disk spaces became less strong and less stiff with increasing age . Although not significantly different, a recent study demonstrated higher values for cervical intervertebral disk thickness in 6 geriatric compared to 4 aged rats . Unfortunately, this study was limited by a rather small population size and the morphometric variable “intervertebral disk thickness” was associated with a low statistical power. In contrast to our findings, increased age in people usually results in an age related narrowing of the intervertebral disk space . This discrepancy is potentially related to different biomechanical properties of the bipedal human compared to the quadrupedal canine vertebral column. In rats, it has been demonstrated that bipedism and upright posture can change vertebral canal dimensions in the lumbar vertebral column .
No significant difference in intervertebral disk width was demonstrated between clinically normal Doberman pinschers and clinically normal English Foxhounds. This suggests that intervertebral disk width should not be considered a strictly breed specific morphometric feature. However, it should be emphasized that only two large-dog breeds with a comparable body conformation were included in this study. This is highlighted by the results of a recent study, demonstrating different micromorphometric variables between chondrodystrophic and non-chondrodystrophic dogs . Therefore, further studies are indicated to assess intervertebral disk width in different dog breeds with a variable body conformation. Until such studies have been performed, we do not recommend extrapolating our results of intervertebral disk width to other dog breeds. Although not statistically different in our study population, there was a trend for male dogs to have wider intervertebral disks compared to female dogs. Larger body dimensions of male dogs compared to female dogs can probably explain this finding.
In this study, intervertebral disks with imaging findings suggestive for complete intervertebral disk degeneration were excluded from further analysis. This was done for several reasons. First, this was considered necessary to allow comparison of our results with the results of da Costa et al. , which currently represents the only similar study in veterinary literature. Although not strictly defined an exclusion criterion in that study,completely degenerated disks were not included for further analysis, as they could not be measured accurately . Second, it has been suggested that intervertebral disk width cannot be reliably assessed on low-field MRI in case of complete intervertebral disk degeneration . More specifically, it seems very difficult to discriminate the hypointense signals from the completely degenerated disk and the adjacent vertebral endplates . Third, it cannot be excluded that complete intervertebral disk degeneration in itself would influence intervertebral disk width and act by this way as a confounding variable when comparing different groups of dogs. Not surprisingly, the majority of excluded intervertebral disks were situated between C6-C7. We cannot exclude that this could have influenced the results of our study at this specific anatomic level. The rather high proportion of excluded disks between C6-C7 can be explained by the fact that this site is most commonly affected in both naturally occurring DA-CSM [6, 7] and age related disk degeneration in clinically normal large-breed dogs [11, 15]. The predilection of the more caudally located cervical intervertebral disk spaces to demonstrate disk degeneration in large breed dogs is most likely related to the increased amount of axial rotation possible in the caudal cervical vertebral column compared to the cranial cervical vertebral column . Axial rotation is facilitated by concave shaped articular facets and it has been demonstrated that the cervical vertebrae of large breed dogs have more pronounced concave shaped articular facets than small breed dogs . Axial rotation is considered the main force responsible for disk degeneration,more so than flexion, extension, and lateral bending .