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FIGURE 2 Laterolateral radiographs of three horses with radiographic evidence of cervical vertebral malformation. (a) Flaring of the caudal epiphysis (*), caudal extension of the dorsal lamina (dotted lines) which extends to the cranial aspect of the caudal vertebral body (arrow), stenosis and funnel shape of the vertebral canal (solid lines), (b) subluxation of the intercentral joint (arrow) with the horse standing in a neutral position, with an interverterbal sagittal ratio of 0.44, (c) step formation at the intercentral joint.
Intervertebral disc disease of the cervical spine
Disease of the cervical intercentral joints is rare in horses and in- cludes
conditions such as discospondylitis and intervertebral
disc protrusions (Dyson et al., 2020 ; Foss et al., 1983 ; Sweers & Carstens, 2006 ). Discospondylitis is a disease of the interverte- bral disc and vertebral endplates, and in horses can have a septic or traumatic aetiology (Dyson et al., 2020 ; Hillyer et al., 1996 ). It most commonly occurs between C6 and T1 and is radiographically characterised by lysis of the vertebral epiphyses, with or without alteration in the width of the intercentral joint (Dyson et al., 2020 ; Geller et al., 2022 ; Sweers & Carstens, 2006 ). Reported cervical disc protrusions are limited to a few cases, which radiographically demonstrated narrowing of the intercentral joint (Foss et al., 1983 ; Nixon et al., 1984 ). Both these diseases usually result in clinical signs including neck pain, reduced range of motion and ataxia, and therefore are unlikely to be present in a clinically normal horse at PPE. Additionally, degenerative intervertebral disc disease has been described in horses and has previously been reported to be of low clinical importance (Bollwein & Hanichen, 1989 ). Degenerative in- tervertebral disc disease presents with narrowing of the interver- tebral disc space on radiographs and is not associated with osseous change (Bergmann et al., 2018 ; Foss et al., 1983 ). Recent evidence suggests it may be a relatively common finding in horses euthanised for unrelated diseases (Bergmann et al., 2018 ). It is most frequently observed in the caudal neck, between C5 and T1 and is significantly associated with increased age (Bergmann et al., 2018 ). There are a limited number of case reports which demonstrate the potential for
this condition to progress into more clinically relevant disease pro- cesses such as disc protrusion, which can then cause extradural im- pingement of the spinal cord (Foss et al., 1983 ; Speltz et al., 2006 ). Further research is needed to establish how frequently this occurs and therefore the clinical relevance of this finding. A recent paper has proposed a cervical disc width index which takes into consid- eration the width of the intercentral joint relative to the length of the vertebral body. Significant mean increases in width index were seen more caudally in the cervical spine compared to the cranial aspect; they remained static up to 18 months old and did not differ significantly between sexes (Veraa et al., 2021 ). Standardised meas- urements aid the subjective evaluation of cervical radiographs, how- ever, more research is needed to ascertain their predictive value.
Spondylosis deformans
Spondylosis deformans is a degenerative condition of the inter- central joint characterised by progressive bone formation which eventually bridges the joint resulting in ankylosis (Haussler, 1999 ). This is particularly common in the thoracolumbar region of dogs and is considered to be clinically insignificant unless there is bony impingement of the spinal cord or nerve roots, which is rare (Levine et al., 2006 ; Widmer & Thrall, 2018 ). In horses, spondylosis deformans is most common between T10 and T14 (Denoix, 2007 ), but is still rare, seen in only 3.4% of horses with back pain (Meehan et al., 2009 ). Approximately a third of these horses showed con- current increased radiopharmaceutical uptake on scintigraphy and
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