Original Research
Congenital Malformations of the First Sternal Rib

https://doi.org/10.1016/j.jevs.2016.09.013Get rights and content

Abstract

During the dissection and skeletal examination of 151 horses, a congenital malformation (CM) of the first sternal rib that influenced the aperture of the Thoracic inlet was noted in six horses. The presentation of this CM was variable between horses in gross anatomic appearance; notably, an absent first sternal rib, bifid tuberculum costae, bifid sternochondral articulation onto the sternum, flared shaft, normal first sternal rib inserting onto the cranial branch of a bifid sternochondral articulating second sternal rib, straight costal shaft, and an articulating rudimentary tuberculum costae with a ligamentous extension replacing the bony shaft and attaching to a rudimentary sternochondral articulation onto the sternum. Of the 151 horses examined, the CM of the first sternal rib was restricted to 6/60 Thoroughbred horses, and only in those that were affected by either the unilateral or bilateral transposition of the caudal ventral tubercle from C6 onto the ventral surface of C7. The normal anatomic presentation of the thoracic inlet was altered, along with associative musculature including neurological pathways. These CMs are likely to produce clinical and functional ramifications of the thoracic inlet, thoracic limb, and thoracic viscera, with the probability of altering postural and locomotive function as noted in four horses demonstrating the CM.

Introduction

In the horse, there are supposedly eight paired sternal ribs that are often referred to as “true” ribs, this is due to their distal attachment onto the sternum via a costal cartilage [1], [2]. A typical sternal rib articulates proximally with two thoracic vertebrae; however, this does not apply to the first sternal rib, it articulates cranially with the seventh cervical vertebra (C7) and caudally with the first thoracic vertebra (T1) [2]. In comparative anatomic terms, the first sternal rib is the shortest, it displays less convexity in the shaft, it has a smooth impression on the cranial border where the axillary vessels pass, its ventral extremity is the largest, and the first sternal rib is the only rib that displays a cranial deviation in its distal extremity (Fig. 1) [2], [3]. Functionally, the sternal ribs are designed to protect the heart and lungs, although the first sternal rib is cranial to the heart, and therefore, its protective role relates to the lungs [1], [3]. In addition, it provides rigidity and form; anchor points for muscles such as the scalenes and along with its articulation to the corresponding costal cartilage, limited movement during respiration [1], [2], [3], [4].

The articulation of the first sternal rib to C7/T1 and to the manubrium of the sternum via the costal cartilage creates the cranial aperture to the thorax known as the thoracic inlet (Fig. 2). This aperture is ovoid in outline and is occupied by the Longus colli muscle, trachea, esophagus, nerves such as the Phrenic, large blood vessels, for example, carotid artery and jugular vein, lymphatic vessels, lymph nodes, and in the young horse, the thymus [1], [2], [3]. Furthermore, Bradley [3] notes that the pleural sac indirectly ends blindly on the left first sternal rib and on the right can extend beyond the first sternal rib and come into contact with the scalene muscles. The role of the thoracic inlet is not clearly defined in current literature; however, its shape helps guide important structures to and from their respective organs, provides a cranial restriction that aids in retaining organs within the thoracic cavity and due to the size and shape of the first sternal rib, it provides a protective barrier for those structures. In addition, the thoracic inlet is likely to contribute in respiration by means of providing a resistance against the compressive visceral forces applied to it during expiration, especially in the galloping horse [5].

Subsequently, the first sternal rib plays an important role in the cranial thorax, and CMs have not been clearly defined in equine anatomy and to date, their existence rarely reported or understood. Even when Bradley [6] reported a case where an aged mare exhibited a bilateral rudimentary first sternal rib with a ligamentous shaft in 1901 (Fig. 3), has there been any significant research found by the current author. However, this cannot be said for other species whereby numerous CMs of the first sternal rib have been reported in dogs and humans since the 1800s [6], [7], [8] and in Holstein calves since 1999 [9], [10], [11]. Moreover, associative symptoms reported in humans included neurological impingement, visceral displacement, and the relocation of musculature, such as the scalenes [6], [7], [8]. In addition, severe CMs of the first sternal rib reported in Holstein calves simultaneously displayed complex vertebral malformation (CVM) that was inclusive of severe CMs of the cervicothoracic junction [9]. In the affected calves, there were multiple axial skeletal CMs including proximal fusion of the first sternal rib to the second sternal rib and nonparallel intercostal spacing [9], [10]. CVM is a congenital condition resulting in multiple malformations of the caudal cervical and anterior thoracic vertebrae including the first sternal rib [10]. It predominantly involves the cervicothoracic junction (C5–T2) but can also affect the lumbar vertebrae and appendicular skeleton; furthermore, these malformations have been linked to a lethal recessive gene [9], [10], [11].

Studies showing CMs of the first sternal rib in the dog and human have noted significant variations: bifid extremities, ligamentous shafts, flared shafts, and fused first to second ribs [6], [7], [8], [12], [13]. However, in Holstein calves, the CM of the first sternal rib was always expressed in conjunction with vertebral malformations of 2 or more vertebrae in the cervicothoracic junction, and in addition, multiple other skeletal and visceral defects were noted; for example, hemi vertebra and intraventricular septum [9], [10], [11]. In the horse, significant CMs have already been reported in the cervicothoracic junction in the same region of the axial skeleton as CVM in Holstein calves [14]. Additionally, coinciding secondary conditions in these affected horses found that normal function was impeded in the axial and appendicular skeleton in direct correlation to such CMs [14], [15]. Aside from skeletally related deviations, for example, scoliosis and those studies already cited in this study, there were no other CMs noted by this author.

Although research into the birthing traumas of foals has reported the impact of sternal rib fractures and chondral displacement, fracture/s of the first sternal rib due to birth trauma are rarely noted [16], [17], [18], [19]. However, when reported in the adult horse, first sternal rib fractures may result in forelimb lameness, muscle atrophy, neurological gait deficit, or abnormal behavior when the horse is tacked up or mounted [16]. Owing to the placement of the thoracic limb, radiographic imagery of the first sternal rib is problematic, and this also applies to the restrictions placed on ultrasonography, owing to muscle mass in the region [20], [21], [22], [23]. Therefore, the purpose of this study will focus on postmortem data to establish the variability of a CM of the first sternal rib to normal presentation and where possible, correlate these findings to premortem data.

Section snippets

Normal Anatomy

At the proximal extremity of the first sternal rib, there exist two articulating facets: tuberculum costae and caput costae, the latter is further divided into a cranial and caudal Facies articularis capitis costae (Fig. 1). The tuberculum costae articulates into the transverse process of T1, the facies articularis capitis costae (cranial) into the Fovea costalis caudalis of C7, and the Facies articularis capitis costae (caudal) into the Fovea costalis cranialis of T1. The neck between the

Results

Of the 151 horses examined, six displayed a CM of the first sternal rib in either a unilateral (3) or bilateral (3) presentation. It occurred only in Thoroughbred horses displaying a CM of C6 and C7 as per May-Davis [10]. Of the 60 Thoroughbred horses in this study, 10% displayed a CM of the first sternal rib (6), 43.3% a CM of C6 (26), 18.3% a transposed caudal ventral tubercle (CVT) from C6 onto the ventral surface of C7 (11), and 55.5% of the combined C6 and C7 CM displayed a CM of the first

Discussion

In this study, the CM of the first sternal rib appeared to be breed related and its concurrent presentation with the CM of C6 and C7 [10] would suggest an embryonic mutation similar to that found in Holstein calves [7], [8], [9]. Further research identified that CVM in Holstein calves was a recessive syndrome directly linked to a transversion mutation within the SLC35A3 gene high [17] and that an incidence of high embryonic mortality and severe phenotypic abnormalities existed in nonviable

Conclusion

This study showed that of the 151 horses investigated, 6 of 60 Thoroughbred horses displayed a CM of the first sternal rib, and that this CM was breed related. Furthermore, it only appeared in the presence of a congenitally malformed C6 and C7, and this implies a genetic mutation as found in Holstein calves. In addition, correlating data and incidental findings noted embryonic mortality in four potential siblings of Tb No. 5; that Tb No. 4 could not hold a foal and one CM C6 mare delivered a

Acknowledgments

The author wishes to thank Janeen Kleine and Catherine Walker for their contributions. The Australian College of Equine Podiotherapy and the Nippon Veterinary and Life Sciences University for the use of their facilities. Also to those authors/editors/publishers of those articles, journals and books cited in this manuscript.

Author contributions: S.M.-D. wrote and reviewed this article solely.

Conflict of interest: S.M.-D. has no conflict of interest in the preparation or presentation of this

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