Prevalence of Anatomical Variation of the Sixth Cervical Vertebra and Association With Vertebral Canal Stenosis and Articular Process Osteoarthritis in the Horse

Abstract

The sixth cervical vertebra (C6) has unique morphology due to a ventral extension from the transverse process known as the ventral lamina. Little information was found regarding the prevalence and clinical relevance of morphologic variations. Aims of this observational, retrospective study were to characterize C6 morphologic variations in a large sample of horses. Cervical radiographic studies of 100 horses were retrieved. Data recorded were signalment, clinical history, morphology of the C6 ventral lamina, presence of articular process osteoarthritis, and presence of static vertebral canal stenosis. Morphologic variations were found in C6 vertebrae for 24/100 horses, with symmetric absence of the ventral lamina in nine horses and asymmetric absence in 15. Anomalous C6 vertebrae were more common in Warmbloods, with 19/55 Warmbloods in the population being affected (P = 0.006). No association was found with sex. There was no significant difference in the mean of the intravertebral sagittal ratios between horses with normal or anomalous C6 vertebrae; however there was a significantly greater proportion of horses with anomalous C6 vertebrae that had an intravertebral sagittal ratio of less than 0.5 at C6 (P = 0.047). There was no association between the morphology of C6 and articular process osteoarthritis. Anomalous C6 vertebrae in our population were associated with a higher likelihood of cervical pain (P = 0.013). Authors propose that morphologic variations in the C6 ventral laminae could be linked to other developmental abnormalities such as vertebral canal stenosis, might affect regional biomechanics and should therefore be considered clinically relevant in horses. Future, controlled prospective studies are needed to test this theory.

Caudal cervical vertebral morphological variation is not associated with clinical signs in Warmblood horses

“…A radiographic study found these morphologic variations in 13.3% of a random study population and in 15% (21/138) in a group of Warmblood horses with a higher prevalence in mares. Another cross‐sectional radiographic study described a frequency of 33.5% (19/55) in Warmblood horses without sex association , which is again close to our observations. It is possible that geographic, demographic and breed‐related differences can explain the minor differences in frequencies between studies.…”

“…The retrospective aspect of this study and therefore lack of a standardised diagnostic work‐up might also have influenced the outcome. No radiographic association for presence of other vertebral pathologies such as arthrosis was found in cases nor controls, confirming a lack of association as previously reported . The multivariable analysis was performed in a subset of horses to equalise groups as it was noted that the case group contained some older horses while the control group contained small numbers of different Warmblood breeds.…”

“…In their retrospective study DeRouen et al . concluded that anomalous C6 vertebrae were more common in the Warmblood horses (n = 55, 79.2%), than in other breeds (n = 45, 47.4%) and possible relationships with cervical pain due to altered regional biomechanics were suggested. Vertebral morphologic variation has been reported as unilateral or bilateral and always in two consecutive (mostly C6 and C7) vertebrae .…”

“…Existence of vertebral morphologic variation in the caudal cervical spine has been reported in one‐third of both Thoroughbred and Warmblood populations . This variation has been evaluated in more detail in radiographic studies in larger mixed groups of horses . In their retrospective study DeRouen et al .…”

Biochemical and biomechanical characterisation of equine cervical facet joint cartilage

“…Although robust epidemiological data are lacking, it has been reported that approximately 50% of horses exhibit degenerative changes of the facet joint at spinal level C6/C7 . Supporting this estimation, in a recent study of 100 skeletally mature horses, radiographic changes consistent with osteoarthritis were found in 38 and 50% of horses at spinal levels C5/C6 and C6/C7 respectively .…”