Characterization of the Caudal Ventral Tubercle in the Sixth Cervical Vertebra in Modern Equus ferus caballus

SIMPLE SUMMARY: Specialized bony structures in the cervical spine are conservative traits in Equus ferus caballus with the ventral process of C6 being deemed one of the most significant structures relative to posture and locomotion. However, studies have identified an anomalous variation in the ventral process of C6 where its caudal ventral tubercle (CVT) is either unilaterally or bilaterally absent (aCVT). Consequently, the aim of this study is to describe the morphology and provide a grading system that identifies the extent of the absence in equal quarterly increments. Only osseous specimens with congenital variants of the ventral process were examined, totaling 76 (unilateral n = 47; bilateral n = 29). Observational grading identified four levels of absence—1/4, 2/4, and 3/4, with 4/4 representing complete absence of the CVT. Fifty-six osseous specimens with aCVTs had a grade 4/4 with 41/56 extending the absent morphology cranial, thus involving the caudal aspect of the cranial ventral tubercle (CrVT). This combined morphology effectively diminished the attachment sites for the cranial, and thoracal portions of the longus colli muscle, a primary fixator, flexor, rotator, and stabilizer of the cervical vertebrae. A study comparing radiographic images with gross specimens may provide information to facilitate accurate radiological descriptions of C6 variants. ABSTRACT: This study examined the anomalous variations of the ventral process of C6 in modern E. ferus caballus. The aim was to provide an incremental grading protocol measuring the absence of the caudal ventral tubercle (CVT) in this ventral process. The findings revealed the most prevalent absent CVT (aCVT) was left unilateral (n = 35), with bilateral (n = 29) and right unilateral (n = 12). Grading was determined in equal increments of absence 1/4, 2/4, 3/4, with 4/4 representing a complete aCVT in 56/76, with a significance of p = 0.0013. This also applied to bilateral specimens. In those C6 osseous specimens displaying a 4/4 grade aCVT, 41/56 had a partial absence of the caudal aspect of the cranial ventral tubercle (CrVT). Here, grading absent CrVTs (aCrVT) followed similarly to aCVTs, though 4/4 was not observed. The significance between 4/4 grade aCVTs and the presentation of an aCrVT was left p = 0.00001 and right p = 0.00018. In bilateral specimens, C6 morphologically resembled C5, implying a homeotic transformation that limited the attachment sites for the cranial and thoracal longus colli muscle. This potentially diminishes function and caudal cervical stability. Therefore, it is recommended that further studies examine the morphological extent of this equine complex vertebral malformation (ECVM) as well as its interrelationships and genetic code/blueprint.