Variations and Implications of the Gross Morphology in the Longus colli Muscle in Thoroughbred and Thoroughbred Derivative Horses Presenting With a Congenital Malformation of the Sixth and Seventh Cervical Vertebrae

• Longus colli relocates in malformed C6 and C7 Thoroughbred and derivative horses.
• Asymmetrical biomechanical forces alter articular process joints.
• As a cybernetic muscle, its afferent or efferent neural signals may influence posture.
• Reported postural changes—forelimbs: base wide (bilateral), one forward (unilateral).
• Reported forelimb ataxia, paresis, limb flaccidity, decreased limb tone, and reflex.

During the dissection of seven Thoroughbred (Tb) and two Thoroughbred derivative (TbD) horses (9) displaying the congenital malformation of C6 and/or C6 and C7; variations in the gross morphology of the Longus colli muscle were noted. In the absence of the caudal ventral tubercle (CVT) on C6 only, the insertion of the medial and ventral layers and thoracic portion of the L. colli muscle attached to the cranial ventral tubercle (CrVT) on C6. However, on transposition of the CVT from C6 to the ventral surface of C7, the medial and ventral layers, a single deep bundle, and thoracic portion of the L. colli muscle attached to the CrVT on C6 and the transposed CVT on C7. In the unilateral malformation, this placed a distinct asymmetry in the paired left-to-right longitudinal presentation and cross-sectional samples of the L. colli muscle. In the bilateral malformation, the CrVTs were longitudinally malaligned and the L. colli replicated the unilateral presentation to a lesser extent. In this presentation, asymmetry was noted in entheses patterns and articular process joints, implying abnormal mechanical load, as was confirmed in the cross sections. As the L. colli muscle has specific cybernetic roles linked to posture and locomotion, these anatomic variations imply dysfunction. Premortem examinations confirmed eight of the nine horses exhibited proprioceptive and neurologic dysfunction (stillborn not included). This raises questions as to the equilibrium of affected horses and therefore the safety in handling and riding such horses, as was found in this study.