Original ResearchCongenital Malformations of the First Sternal Rib
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
References (25)
- et al.
Transverse and dorso-ventral changes in thoracic dimension during equine locomotion
Equine Vet J
(2009) - et al.
Management of cervical ribs and anomalous first ribs causing neurogenic thoracic outlet syndrome
J Vasc Surg
(2002) The occurrence of a congenital malformation in the sixth and seventh cervical vertebrae predominantly observed in Thoroughbred horses
J Equine Vet Sci
(2014)- et al.
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
J Equine Vet Sci
(2015) - et al.
Anatomy and physiology of farm animals
(1992) - et al.
The anatomy of the domestic animals
(1975) The topical anatomy of the head and neck of the horse
(1947)- et al.
The athletic horse
(1994) A case of rudimentary first thoracic rib in the horse
J Anat Physiol
(1901)- et al.
Cervical ribs and rudimentary first thoracic ribs considered from the clinical and etiological standpoints
J Anat
(1932)
Complex vertebral malformation in Holstein calves
J Vet Diagn Invest
Complex vertebral malformation in a stillborn Holstein calf in Japan
J Vet Med Sci
Cited by (12)
A Systematic Review of Clinical Signs Associated With Degenerative Conditions and Morphological Variations of the Equine Caudal Neck
2022, Journal of Equine Veterinary ScienceCitation Excerpt :Ataxia, neck pain or stiffness, and neurological deficits have been associated with ECCMV [8,14,23,24], APJ arthropathy [14,20], and CVSM [6,17,21]. A base-wide forelimb stance, instability, and stumbling have been identified in horse diagnosed with ECCMV [23,24]. Gough et al [20] identified neck muscle atrophy, lameness and weakness in horses diagnosed with APJ arthropathy.
Diseases of the Nervous System
2019, Large Animal Internal Medicine