Equine Neck and Back Pathology E-Book
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A unique reference dedicated to the diagnosis and treatment of problems of the equine neck and back
Building on the strength of the first edition, Equine Neck and Back Pathology: Diagnosis and Treatment, Second Edition explores conditions and problems of the horse’s back and pelvis, and has been expanded to include coverage of the neck as well. This book is a vital tool for all those engaged in improving the diagnosis and management of horses with neck or back problems.
The only book devoted to the conditions and problems of the equine neck, back and pelvis, it provides comprehensive coverage by international specialists on how to diagnose and treat problems in these areas. This updated and revised edition covers normal anatomy and kinematics, neck and back pathology, diagnosis and treatment of specific conditions, and complementary therapy and rehabilitation.
Equine Neck and Back Pathology: Diagnosis and Treatment, Second Edition is a valuable working resource for equine practitioners, specialists in equine surgery, veterinary nurses and allied professionals involved in treating horses. It is also an excellent supplementary text for veterinary students with a keen interest in horses.
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Contents
Cover
Title Page
Copyright
Dedication
List of Contributors
Chapter 1: The Normal Anatomy of the Neck
Introduction
Cervical Vertebrae 3–7
Atlas and Axis, C1 and C2
Joints of the Neck
Ligaments of the Neck
Muscles of the Neck
References
Chapter 2: The Normal Anatomy of the Osseous and Soft Tissue Structures of the Back and Pelvis
Introduction
2.1 Normal Anatomy of the Osseous Structures
Vertebral Numbering System
Vertebral Formula
Ossification Centres and Growth Plate Closure Times
Structure of the Thoracic and Lumbar Vertebrae
Vertebral Bodies
Intervertebral Discs
Vertebral Arch
Intervertebral Foramina
Lateral Foramina
Sacral Foramina
Vertebral Canal Contents
Transitional Vertebrae
Dorsal Spinous Processes
Articular Processes (Facets)
Transverse Processes
Lumbosacral Junction
Sacrum
Sacroiliac Joint
Articulations of the Vertebral Column
The Pelvis
The Ilium
Coccygeal Vertebrae
2.2 Normal Anatomy of the Soft Tissue Structures of the Back
Introduction
Musculature
Ligaments
2.3 Normal Anatomy of the Soft Tissue Structures of the Pelvis
Introduction
Sacroiliac Joint
The Dorsolateral and Caudal Walls
The Ventral Fibrous Structures
References
Chapter 3: The Normal Anatomy of the Nervous System
Introduction
The Spinal Cord Segments
The Spinal Nerves
The Sensory (Afferent) System
The Motor System
Spinal Nerve Distribution
Innervation of the Limbs
Summary
References
Chapter 4: Kinematics
Introduction
Historical Perspective
Biomechanical Models of How the Equine Back Works
Kinematics of the Equine Back
Kinematics of the Equine Back – in vivo Research
Further Modification of the Skin Marker Measurement Technique
Applied Kinematics of the Equine Back
Physiological Factors
Therapeutic or Diagnostic Interventions
Conclusions and Possible Future Developments
References
Chapter 5: Neurological Examination of the Back and Pelvis
Introduction
History
Examination at Rest
Thoracic Limbs and Trunk Reflexes
References
Chapter 6: Clinical Examination
Examination of the Neck
Examination of the Back and Pelvis
References
Chapter 7: Radiography of the Cervical Spine
Indications for Radiography of the Cervical Spine
Radiographic Technique
Normal Radiographic Anatomy and Incidental Findings
Radiographic Diagnosis of some Conditions Affecting the Cervical Spine
Soft Tissue Injuries
Myelography
References
Chapter 8: Radiography of the Back
Introduction
Indications for Radiography of the Back and/or Pelvis
Technical Difficulties with Radiography of the Equine Back and Pelvis
Radiographic Technique
Positioning and Radiograph Acquisition
Normal Radiographic Appearance of the Back
An Introduction to Radiographic Abnormalities of the Back and Pelvis
References
Chapter 9: Nuclear Scintigraphy and Computed Tomography of the Neck, Back and Pelvis
Introduction
Nuclear Scintigraphy
Computed Tomography
References
Chapter 10: Ultrasonography
Introduction
10.1 Ultrasonography of the Thoracic Spine
General Considerations
Supraspinous Ligament and Interspinous Ligament Ultrasonography
Ultrasonographic Technique
10.2 Ultrasonography of the Pelvis, Lumbar Spine and SacroIliac Region
Introduction
Lumbar Spine and Sacroiliac Region
Transcutaneous Evaluation
Transrectal Ultrasound
References
Chapter 11: Thermography
Introduction
Instrumentation
Principles of Use
Use in Veterinary Medicine
Clinical Thermography
Conclusions
References
Chapter 12: Neck Pathology
Introduction
Functional Anatomy
Neck Pain
Cervical Neurological Disease
Neurological Consequences of Spinal Compression
Radiographic Considerations with Cervical Spinal Compression
Clinical Conditions
Cervical APJ Osteoarthritis
Occipitoatlantoaxial Malformation
Discospondylitis
Cervical Vertebral Fractures
Cervical Vertebral Epidural Haematomas
Vertebral Subluxation
Vertebral Neoplasia
Vertebral Osteomyelitis
Apparent Muscle Pain
Nuchal Ligament Insertional Desmopathy
Nuchal Bursitis
References
Chapter 13: Back Pathology
13.1 Traumatic damage
General Principles
Fracture of the Dorsal Spinous Processes (DSP) of the Thoracic Vertebrae (Fractured Withers)
Fractures of the Thoracolumbar Vertebral Column
Pelvic Factures
13.2 Over Riding Dorsal Spinous Processes (‘Kissing Spines’)
Anatomy of the Dorsal Spinous Processes
Incidence
Pathogenesis
Site of Pathology
History
Clinical Signs
Clinical Examination
Diagnosis
Diagnosis of Anatomical Abnormalities
Diagnostic Local Analgesic
Treatment
Surgical Techniques
Surgical Outcome
Conclusion
13.3 Miscellaneous Osseous Pathology
Introduction
Anatomical Abnormalities
Degenerative Conditions
Infections of the Back and Pelvis
Stress Fractures
Neoplasia
Miscellaneous Conditions of the Vertebral Bodies
13.4 Pathology of the Supraspinous and Dorsal Sacroiliac Ligaments
Introduction
Supraspinous Ligament
Dorsal Sacroiliac Ligament Pathology
References
Chapter 14: Sacroiliac Dysfunction
Introduction
Anatomy
Incidence
Clinical Signs
SIJ Provocation Tests
Diagnosis of SID
Management of SID
Conclusion
References
Chapter 15: Muscular Disorders
Introduction
Diagnostic Procedures
Biochemistry
DNA Testing
Biopsy
Ultrasonography
Scintigraphy
Electromyography
Disorders: Aetiopathogenesis and Treatment
References
Chapter 16: Integrative and Physical Therapies
16.1 Integrative Therapies
Introduction
Integrative Approach to Diagnosis of Back Pain
History of the Horse with Back Pain
Observation of the Horse
Palpation of the Back
Acupuncture Palpation and Diagnosis
Chiropractic Examination
The Use of Diagnostic Imaging
Saddle Fit
Rider Variables
The Use of Therapeutic Pads
Teeth, Mouth Pain
Additional Management Factors Influencing Back Pain
Treatments for Back Pain
16.2 Physical Therapies
The Role of the Equine Physical Therapist
Therapies
Conclusion
References
Chapter 17: Rehabilitation
Introduction
The Back
Anatomy
Vertebral Ligaments
Ligaments Influencing the Dorsal Section of the Body
Muscles Influencing the Dorsal and Ventral Sections of the Body
Back Pain
Rehabilitation Programmes
Rebuilding Muscle
Long Reins
Examples of Inefficient Patterns
Introduction of Active Rehabilitation
Cardiovascular and Muscular Effects of Water Therapy
Conclusion
References
Index
End User License Agreement
List of Tables
Table 2.1
Table 2.2
Table 2.3
Table 2.4
Table 2.5
Table 5.1
Table 7.1
Table 8.1
Table 8.2
Table 9.1
Table 9.2
Table 13.1
Table 13.2
Table 13.3
Table 14.1
Table 16.1
Table 16.2
Table 16.3
Table 16.4
Table 17.1
Table 17.2
List of Illustrations
Figure 1.1
Figure 1.2
Figure 1.3
Figure 1.4
Figure 1.5
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Figure 2.7
Figure 2.8
Figure 2.9
Figure 2.10
Figure 2.11
Figure 2.12
Figure 2.13
Figure 2.14
Figure 2.15
Figure 2.16
Figure 2.17
Figure 3.1
Figure 3.2
Figure 3.3
Figure 3.4
Figure 3.5
Figure 3.6
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
Figure 4.7
Figure 4.8
Figure 4.9
Figure 4.10
Figure 4.11
Figure 4.12
Figure 6.1
Figure 6.2
Figure 6.3
Figure 7.1
Figure 7.2
Figure 7.3
Figure 7.4
Figure 7.5
Figure 8.1
Figure 8.2
Figure 8.3
Figure 8.4
Figure 8.5
Figure 8.6
Figure 8.7
Figure 8.8
Figure 8.9
Figure 8.10
Figure 8.11
Figure 8.12
Figure 8.13
Figure 8.14
Figure 9.1
Figure 9.2
Figure 9.3
Figure 9.4
Figure 9.5
Figure 9.6
Figure 9.7
Figure 9.8
Figure 9.9
Figure 9.10
Figure 9.11
Figure 9.12
Figure 9.13
Figure 9.14
Figure 9.15
Figure 9.16
Figure 9.17
Figure 9.18
Figure 9.19
Figure 9.20
Figure 9.21
Figure 10.1
Figure 10.2
Figure 10.3
Figure 10.4
Figure 10.5
Figure 10.6
Figure 10.7
Figure 10.8
Figure 10.9
Figure 10.10
Figure 10.11
Figure 10.12
Figure 10.13
Figure 10.14
Figure 10.15
Figure 10.16
Figure 10.17
Figure 10.18
Figure 10.19
Figure 10.20
Figure 10.21
Figure 11.1
Figure 11.2
Figure 11.3
Figure 11.4
Figure 11.5
Figure 12.1
Figure 12.2
Figure 12.3
Figure 12.4
Figure 12.5
Figure 12.6
Figure 12.7
Figure 12.8
Figure 12.9
Figure 12.10
Figure 12.11
Figure 12.12
Figure 12.13
Figure 13.1
Figure 13.2
Figure 13.3
Figure 13.4
Figure 13.5
Figure 13.6
Figure 13.7
Figure 13.8
Figure 13.9
Figure 13.10
Figure 13.11
Figure 13.12
Figure 13.13
Figure 14.1
Figure 14.2
Figure 15.1
Figure 15.2
Figure 15.3
Figure 15.4
Figure 15.5
Figure 15.6
Figure 17.1
Guide
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Equine Neck and Back Pathology
Diagnosis and Treatment
Second Edition
Edited by Frances M.D. HensonDepartment of Veterinary MedicineUniversity of CambridgeUK
This edition first published 2018© 2018 John Wiley & Sons Ltd
Edition HistoryJohn Wiley & Sons (1e, 2009)
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
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Library of Congress Cataloging-in-Publication Data
Names: Henson, Frances M. D., editor.
Title: Equine neck and back pathology : diagnosis and treatment / edited by Frances M.D. Henson.
Other titles: Equine back pathology.
Description: Second edition. | Hoboken, NJ : Wiley, 2018. | Preceded by Equine back pathology / edited by Frances M.D. Henson. 2009. | Includes bibliographical references and index. |
Identifiers: LCCN 2017030342 (print) | LCCN 2017030986 (ebook) | ISBN 9781118974506 (pdf) | ISBN 9781118974575 (epub) | ISBN 9781118974445 (cloth)
Subjects: | MESH: Horse Diseases | Spinal Diseases–veterinary | Spinal Diseases–pathology | Back Pain–veterinary | Neck Pain–veterinary | Spine–anatomy & histology
Classification: LCC SF951 (ebook) | LCC SF951 (print) | NLM SF 951 | DDC 636.1/0896–dc23
LC record available at https://lccn.loc.gov/2017030342
Cover images: (Background) © DavidMSchrader/Gettyimages; (Image on left) © Sarah Powell; (Images on right)© Frances M.D. HensonCover design by Wiley
For Joan
List of Contributors
David Bainbridge
Department of Physiology, Development and Neuroscience University of Cambridge
Cambridge
UK
Marianna Biggi
Veterinary Clinical Sciences
The Royal Veterinary College
Hawkshead Lane
North Mimms
Hatfield
UK
Mary Bromiley
Downs House Equine
Combeleight Farm
Wheddon Cross
Minehead
Somerset
UK
Adam Driver
Global Equine Group Ltd
5 King's Court
Willie Shaith Road
Newmarket
Suffolk
UK
Constanze Fintl
Department of Companion Animal Clinical Sciences
Norwegian School of Veterinary Science
Oslo
Norway
Joyce Harman
Harmany Equine Clinic Ltd
Flint Hill
Virginia
USA
Marcus Head
Rossdale's Diagnostic Centre
Cotton End Road
Exning
Newmarket
Suffolk
UK
Frances M.D. Henson
Department of Veterinary Medicine
University of Cambridge
Cambridge
UK
Richard Hepburn
B&W Equine Vets
Breadstone
Berkeley
Gloucestershire
UK
Leo B. Jeffcott
Emeritus Professor of Veterinary Science
Faculty of Veterinary Science
JD Stewart Building
University of Sydney
Sydney
New South Wales
Australia
Jessica A. Kidd
The Valley Equine Hospital
Lambourn
Berkshire
UK
Luis P. Lamas
Faculdade de Medicina Veterinaria de Lisboa
Lisboa
Portugal
Gabriel Manso-Díaz
Complutense University of Madrid
Madrid
Spain
Graham A. Munroe
Flanders Veterinary Services Cowrig Cottage
Greenlaw
Duns
Berwickshire
UK
Richard J. Piercy
Comparative Neuromuscular Diseases Laboratory
Royal Veterinary College
Hawkshead Lane
North Mimms
Hatfield
UK
Rob Pilsworth
Newmarket Equine Hospital
Newmarket
Suffolk
UK
Mimi Porter
Equine Therapy Inc.
4350 Harrodsburg Road
Lexington
Kentucky
USA
Sarah Powell
Rossdale's Diagnostic Centre
Cotton End Road
Exning
Newmarket
Suffolk
UK
Tracy Turner
Anoka Equine Veterinary Services
Elk River
Minnesota
USA
Rene van Weeren
Department of Equine Sciences
Faculty of Veterinary Medicine
Utrecht University
The Netherlands
Renate Weller
Department of Veterinary Clinical Sciences
The Royal Veterinary College
Hawkshead Lane
North Mimms
Hatfield
UK
Mary Beth Whitcomb
Department of Surgical and Radiological Sciences
School of Veterinary Medicine
University of California
Davis
California
USA
1The Normal Anatomy of the Neck
David Bainbridge
Introduction
The neck is a common derived characteristic of land vertebrates, not shared by their aquatic ancestors. In fish, the thoracic fin girdle, the precursor of the scapula, coracoid and clavicle, is frequently fused to the caudal aspect of the skull. In contrast, as vertebrates emerged on to the dry land, the forelimb separated from the head and the intervening vertebrae specialised to form a relatively mobile region – the neck – to allow the head to be freely steered in many directions.
With the exception of the tail, the neck remains the most mobile region of the spinal column in modern-day horses. It permits a wide range of sagittal plane flexion and extension to allow alternating periods of grazing and predator surveillance, as well as frontal plane flexion to allow the horizon to be scanned, and rotational movement to allow nuisance insects to be flicked off. Among domestic animals the equine neck is relatively long and the head relatively heavy, and so the neck has become strong, muscular and massive. This is enhanced by the fact that regular, forceful movements of this region must also occur to maintain balance when horses are running [1]. However, the length and flexible nature of the neck may also cause problems in the passage of foals through the birth canal.
In this chapter I will briefly review the anatomy of bones, joints, ligaments and muscles of the equine neck. The ‘locomotor component’ of the neck is a common site of pathology, and the diverse forms of neck disease reflect the sometimes complex and conflicting regional variations and functional constraints so evident in this region [2].
Unlike the abdomen and thorax, there is no coelomic cavity in the neck, yet its ventral part is taken up by a relatively small ‘visceral compartment’, containing the larynx, trachea, oesophagus and many important vessels, nerves and endocrine glands. However, I will not review these structures, as they do not represent an extension of the equine ‘back’ in the same way that the more dorsal locomotor region does.
Cervical Vertebrae 3–7
Almost all mammals, including the horse, possess seven cervical vertebrae, C1 to C7 (Figure 1.1). While C1 and C2 are extremely modified for their particular functions, C3 to C7 are more homogenous in structure. C3, C4 and C5 in particular are usually thought of as the ‘typical’ cervical vertebrae (Figure 1.2).
Figure 1.1 Lateral view of an articulated osteological preparation of the neck of a young horse.
Figure 1.2 (A) Lateral view of equine C4 vertebra and (B) cranial view of C5.
Vertebrae C3–C7 consist of an approximately cylindrical body or centrum, a structure present in all jawed vertebrates to resist longitudinal compression of the spinal column. The centra of the equine neck are the longest in the body, but become progressively smaller caudally. Those of C3–C7 possess a distinctively convex cranial surface, the head, and a correspondingly concave caudal surface, the fossa. Thus the intervertebral joints, which are far more mobile than in the trunk, may be thought of functionally as ball-and-socket joints, although their constituent parts are very structurally different from those of synovial ball-and-socket joints.
Dorsal to the centrum is the neural arch, formed from bilateral bony laminae, which surrounds and protects the spinal cord and its associated structures. The vertebral canal formed by successive arches is relatively wide in the neck, especially cranially, to allow the spinal cord, which is wide in this region, to flex freely. The vertebrae C3–C7 each develop from three primary centres of ossification – one in the centrum and one in each of the two laminae. Formation of cervical neural arches, which are either statically or dynamically stenotic, is thought to be a cause of equine cervical ‘wobbler syndrome’ [3].
The centrum and arch are adorned with a variety of bony processes for the attachment of ligaments and muscles, and which often develop as secondary centres of ossification. These vertebral processes are a feature evolved by land vertebrates to permit complex movements in three dimensions and resist torsional forces.
The single dorsal midline
spinous process
is distinctively short in equine C3–C5.
In contrast, all equine cervical vertebrae bear a characteristically large
ventral crest
, often with a pronounced
caudal tubercle
.
The bilateral
transverse processes
are large but squat, and thought to incorporate vestigial ribs, sometimes yielding the name ‘costotransverse processes’. In C3–6, the processes are bifid and slanted, with a cranial
ventral tubercle
and caudal
dorsal tubercle
. The transverse processes of C1–6 are perforated by a large
transverse foramen
, which conveys the vertebral artery and vein.
Lateral to the neural arch lie the large, irregular
articular processes
, with their smooth ovoid articular surfaces. The caudal facets are directed ventrolaterally, and the complementary cranial facets dorsomedially. Ventral to the caudal process lies a notch for passage of the laterally coursing spinal nerve.
The sixth cervical vertebra (Figure 1.3) differs from its cranial neighbours in that it bears pronounced paired bony sheets, the ventral laminae, which act as a site of attachment and force redirection of muscles, especially longus colli. In the horse these laminae are elaborated into cranial and caudal tubercles. C6 also possesses a longer spinous process than C3, 4 and 5 – a reflection of a gradual transition to a more ‘thoracic’ morphology.
Figure 1.3 (A) Lateral view of equine C6 vertebra and (B) cranial view of C7.
This trend continues in C7 (Figure 1.3), which has an even longer spinous process, non-bifid transverse processes, and no transverse foramen – the vertebral arteries arise too far cranially to pass through C7. However, C7 does possess a caudal notch for the passage of a spinal nerve, but it should be emphasised that the nomenclature of the spinal nerves in inconsistent. Unlike the rest of the body, cervical spinal nerves emerge cranial to the vertebra of the same number, and the nerves emerging caudal to vertebra C7 are named C8, even though there is no corresponding C8 vertebra. Finally, the centra of C7 caudally bear unconvincing costal facets for the articulation of the cranial extremities of the capitula of the first ribs [4].
Atlas and Axis, C1 and C2
The anatomy of the caudal part of the axis, C2 (Figure 1.4), is similar to that of the more caudal cervical vertebrae – with centrum and neural arch formed from the same three centres of ossification, as well as the spinous process, ventral crest and tubercle, caudal articular facets and dorsal tubercle of the transverse process. However, the cranial part of the bone is markedly aberrant to allow the unique rotational, trochoid, ‘head-shaking’ movement of the atlanto-axial joint. Its unusual shape results from the incorporation of embryonic elements of C1.
Figure 1.4 (A) Lateral view of equine C2 vertebra, and (B) ventral and (C) cranial views of C1.
A fourth primary centre of ossification, actually the annexed centrum of C1, forms the dens (‘tooth’) or odontoid process of C2. This cranially directed process is attached ventrally to the main centrum of C2 by a base formed from a further, secondary centre, which represents the cranial epiphysis of C2. The dens articulates closely with the ventral part of C1, and thus is smooth on its ventral surface, but is roughened dorsally with a midline gutter to allow attachment of stabilising ligaments. The smooth articular region of the dens is continuous with the large bilateral saddle-shaped cranial articular surfaces, which slide across reciprocal surfaces on C1 to allow rotation of the joint. These surfaces also develop from their own secondary ossification centres.
The axis contains a relatively large amount of trabecular bone compared to the other cervical vertebrae, and is also characterised by a large spinous process. Equine C2 is also distinctive in possessing bilateral foramina for the passage of the second pair of spinal nerves, which do not emerge between adjacent vertebrae, as all the more caudal spinal nerves do. In some texts these are called ‘intervertebral foramina’, which seems illogical, so the name lateral foramina is perhaps preferable.
The atlas, C1 (Figure 1.4), is the most bizarre of all the vertebrae, due to it performing specialised movements with both the skull (sagittal plane flexion and extension/‘nodding’/‘yes’ movement) and the axis (rotation about the long axis of the spine/‘shaking head’/‘no’ movement).
The atlas has no centrum and no neural spine, but is instead constituted by one large hollow cylinder formed by dorsal and ventral arches of bone. The dorsal arch is equivalent to the neural arches of other vertebrae, but the thicker ventral arch is a unique structure probably derived from paired cranial epiphyseal developmental elements. The absence of a centrum means that, unlike C2, C1 contains an unusually low proportion of trabecular bone. The caudal part of the dorsal, internal surface of the ventral arch is the smooth fovea dentis, which articulates with the dens of the axis, whereas the more cranial ‘floor’ of the atlas is much rougher.
The equine atlas is relatively large compared to that of humans, but small compared to that of dogs. Attached laterally to the arches are the irregular lateral masses, which support the caudal articular surfaces as well as the wide, ovoid and profoundly concave cranial articular surfaces. Also attached laterally are the large modified transverse processes termed wings or alae. The wings, distinctively concave ventrally in the horse, form the attachment of several of the long and short muscles of the cranial neck, and also contain transverse foramina similar to those in C2–C6. The alar notches present on the cranial edge of the wings of the atlas in some species are entirely enclosed into alar foramina in the horse, and mark the tortuous course of the vertebral arteries [5,6]. A further, slightly medial foramen permits the entry of the artery into the vertebral canal as well as the exit of spinal nerve C1 and, as with the equivalent ‘non-intervertebral’ route taken by nerve C2 through the bony laminae of the axis, this is best termed the lateral foramen.
Joints of the Neck
The articulations between C2, 3, 4, 5, 6 and 7 are similar to those found in the trunk region. Once labelled with the somewhat confusing name ‘amphiarthroses’, these intervertebral joints are best considered as compound joints each comprising two completely different forms of articulation.
First, the adjacent centra are bonded together by a single intercentral joint, a unique form of symphysis more often called an intervertebral disc. Each disc is interposed between two centra and contains two parts. An outer ring, the annulus fibrosus, consists of multiple layers of interwoven fibrous tissue with alternating diagonally oriented collagenous fibres that resist torsion and extension of the spine. The annulus becomes less fibrous centrally. Encircled by the annulus is the nucleus pulposus, an unusual admixture of fibrous tissue, gel-like matrix, and swollen cells derived evolutionarily and developmentally from the notochord. Indeed, small foveae in the centra often hint at the fact that the notochord once passed all along the body, piercing through longitudinal foramina in every centrum. However, the nucleus still retains its ancestral function, which is to resist compression. Disc disease may be rare in horses, although the age-related degeneration of their intervertebral discs has been little studied [7].
The second type of intervertebral articulation is formed by the bilateral interneural joints (also known as articular process joints, or APJs), synovial diarthroses between the facets on the articular processes that lie lateral to the neural arch. These joints are more mobile in the neck than in the trunk, although their joint capsules are strong and fibrous.
The C1–C2 atlanto-axial joint is, as mentioned previously, an unusual trochoid or pivot joint. The dens and cranial articular surfaces of C2 articulate with caudal articular surfaces and fovea dentis of C1 by means of what is, in horses, a large single synovial joint capsule – there is no disc. The congruence of the articular surfaces is poor, and the centre of rotation is maintained just above the axis of the dens by a series of ligaments, discussed later.
The C1–skull atlanto-occipital joint is a specialised sagittal-plane ginglymus or hinge joint. The paired convex occipital condyles of the bilateral exoccipital skull bones form a good fit with the large concave cranial articular facets of the atlas, allowing rotation about a transverse axis – again, there is no disc. In all horses their joint initially consists of paired bilateral synovial spaces, although these may form a ventral interconnection in later life in some individuals [6]. Genetic congenital malformation of this joint has been reported in Arab foals [8].
Ligaments of the Neck
The equine neck ligaments represent a modification, sometimes dramatic, of the ligaments present more caudally in the spine. This is due to not only the greater mobility in the region but also the constraints of supporting the large head.
The ‘yellow’, interlamellar or interarcuate ligaments, or ligamenta flava, are sheets of elastic tissue that span the space between adjacent vertebral neural arches. They contact the epidural space medially, the neck musculature laterally, blend with nearby synovial capsules, and each contain a gap via which spinal nerves may exit the vertebral canal. In the equine neck they are unusually extensive and flexible to allow movement.
The dorsal longitudinal ligament runs along the dorsal surface of the centra of all the cervical vertebrae, and thus along the ‘floor’ of the vertebral canal. It is narrower over the body of each centrum and then fans out to a wide attachment on the dorsal edge of each intervertebral disc. Notably, the ventral longitudinal ligament does not extend into the cervical region in the horse.
The interspinous ligaments are elastic in the equine neck to permit movement, but their small size reflects the diminutive nature of the spinous processes in this species. Intertransverse ligaments are not clearly apparent in the equine neck.
The supraspinous ligament is greatly elaborated in the equine neck into the extensive, strong, elastic nuchal ligament (Figure 1.5). The function of this ligament is to support the weight of the massive head and neck, and to store elastic potential energy when the head is lowered to the ground to graze, so that energy may then be retrieved to lift it. The ligament is elaborated into two parts. The funicular part is a strong cord connecting the withers to the nuchal region of the skull, laterally compressed at its cranial end. The funicular part is equivalent to the nuchal ligament of the dog, although in that species it terminates on the axis. The nuchal ligament is actually constituted by paired bilateral components, and through much of the length of the funicular part these are separated by an almost invisible midline fibrous seam. Cranially, there may be a slight divergence of the two sides of the ligament. The lamellar part, which has no equivalent in the carnivores, consists of fibres radiating from the withers and funicular part to the spinous processes of C2–C7. Some consider there to be two subdivisions of the lamellar part: a thinner caudal division inserting on C7, C6 and C5, and a thicker cranial division inserting on C4, C3 and C2. The lamellar part does not constitute a complete sheet filling the space between the funicular part and the neck vertebrae, but leaves spaces, as well as areas where the ligaments may potentially abrade the spinous processes. At these points, there are often bursae interposed between bone and ligament. The most constant are those overlying C1 (atlantal bursa), vertebra T2 (supraspinous bursa), but less constant ones may also be found dorsal to C2 (axial bursa) and C3. These bursae may occasionally be sites of inflammation [9].
Figure 1.5 Schematic midcervical transverse section of the equine neck, showing position of muscles.
The specialised function of the C1–C2 or atlanto-axial joint has led to the development of an unusual array of ligaments. The elastic dorsal atlanto-axial ligament from the spinous process of C2 to the dorsal tuberosity of C1 may be seen as a localised adjunct to the nuchal ligament, whereas the more fibrous ventral atlanto-axial ligament presumably prevents extension of the joint. The transverse, alar and apical ligaments, which bind the dens to the atlas in the dog, are almost absent in the horse, and instead a thick odontoid ligament, a continuation of the dorsal longitudinal ligament, radiates cranially from the dens to attach on the cranial roughened area of the floor of the vertebral canal of the atlas. Finally, the atlanto-axial joint is also spanned by the strong membrane tectoria, a fibrous sheet running from the region of the dens to the internal surface of the axis, as well as the ventral rim of the foramen magnum of the skull itself.
The C1-skull or atlanto-occipital joint is also specialised. Its main ligamentar support is from the strong paired lateral atlanto-occipital ligaments, which pass from the wings of the atlas to the adjacent paramastoid surface of the skull. In addition the joint is enclosed by the thin ventral atlanto-occipital membrane and the stronger dorsal atlanto-occipital membrane, which contains thick fibrous strands in a cruciate arrangement [6]. Clinical access may be gained to the underlying cisterna magna of the subarachnoid space by passing a needle through this latter membrane and the underlying meninges [10].
Muscles of the Neck
Most of the mass of the neck is made up of muscles that act to move the head and neck, the hyoid apparatus, the forelimbs or a combination of these structures. These muscles may be divided into two groups – the larger dorsal epaxial muscles, which act to extend the spine in the sagittal plane or flex it in the frontal plane if contracted asymmetrically, and the smaller ventral hypaxial muscles, which usually act to flex the spine sagittally [11]. A schematic midlevel cross-section of the neck muscles is given in Figure 1.5.
Some of the more dorsally and laterally positioned muscles probably act on the forelimb more than on the neck and head, but are mentioned here because of their origins in that region. These include the cranial portions of the trapezii and rhomboids, which insert on the scapula, the brachiocephalicus, which inserts primarily on the humerus, and the fibrous serratus ventralis, which slings the weight of the body on the forelimbs. The epaxial spinal muscles become larger and elaborated in the cervical region and take up most of the upper half of the horse's neck. The transversospinalis group continues into the neck as the complexus and multifidus muscles, as do the complex interleavings of the longissimus group, reaching as far as the skull. Splenius is also present in the horse, originating on vertebrae T3–T5 and inserting on the transverse processes of C1, C3, C4 and C5 and the caudal aspect of the skull. There are also short, specialised muscles in the cranial neck. The extensors are obliquus capitis cranialis (ventral wing of atlas to caudal skull), rectus capitis dorsalis major (spinous process of axis to caudal skull) and rectus capitis dorsalis minor (dorsal atlas to caudal skull), while obliquus capitis caudalis (spinous process of axis to caudal skull) is well aligned to act as a rotator. This profusion of smaller muscles attached to the atlas and axis explain why these vertebrae bear such expansive bony processes.
Some of the far-ventral hypaxial muscles do not warrant much mention in a book about the ‘back’. These include the sternocephalicus, omohyoideus and the sternothyrohyoid complex. However, there are specific, although relatively small, spinal flexors in this region. One prime neck flexor is longus colli, which passes from the ventral surfaces of T1–T6 to the ventral tubercles of all the cervical vertebrae, running ventral to a bursa at T1 and the ventral laminae of C6. Another is scalenus, which originates on the first rib and inserts on the transverse processes of C1–C7. There is also the rectus capitis ventralis major, from the transverse processes of T3–T5 to the ventral skull, the rectus capitis ventralis major from the ventral atlas to the ventral skull, and the more laterally positioned but similarly short rectus capitis lateralis [12].
It is unclear to what extent these muscles are sites of pathology, especially as many of them are not amenable to direct clinical examination.
References
1
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Stashak, E.
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Janes, J.G., Garrett, K.S., McQuerry, K.J., et al. Comparison of magnetic resonance imaging with standing cervical radiographs for evaluation of vertebral canal stenosis in equine cervical stenotic myelopathy.
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Barone, R.
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Share-Jones, J.T.
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Barone, R.
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Fölkel, I.M. Lumbar intervertebral disc disease (IVDD) in horses: a longitudinal ultrasonographic and histomorphological comparison. Master Thesis, Utrecht.
8
Watson, A.G. and Mayhew, I.G. Familial congenital occipitoatlantoaxial malformation (OAAM) in the Arabian horse.
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9
García-López, J.M., Jenei, T., Chope, K. and Bubeck, K.A. Diagnosis and management of cranial and caudal nuchal bursitis in four horses.
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Johnson, P.J. and Constantinescu, G.M. Collection of cerebrospinal fluid in horses.
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Pasquini, C., Spurgeon, T. and Pasquini, S.
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2The Normal Anatomy of the Osseous and Soft Tissue Structures of the Back and Pelvis
Leo B. Jeffcott, Jessica A. Kidd and David Bainbridge
Introduction
In order to understand the pathological conditions that affect a horse's back and pelvis it is necessary to have an excellent working knowledge of its structure. The back and pelvis are made up of osseous structures, joints, muscles, ligaments, blood vessels and nerves, all of which can be altered or affected in disease. In this chapter the osseous and soft tissue structures of the back (i.e. the thoracolumbar spine, sacrum and pelvis) are discussed.
2.1 Normal Anatomy of the Osseous Structures
Leo B. Jeffcott
The vertebral column runs from the atlanto-occipital joint to the last coccygeal vertebra (Figure 2.1). As it passes through the body the vertebral column does not form a straight-line structure; rather it descends sharply from the atlanto-occipital joint to reach its lowest point at the cervicothoracic junction. The column then ascends gently to the caudal lumbar region and descends down, via the sacrum, to the coccygeal vertebrae (Figure 2.1). The external appearance of the horse, however, presents a different picture in the cranial thoracic region. Externally the withers (corresponding approximately to T3–T7) is the highest point of the back, even though the vertebral bodies are ventral to most other vertebral bodies at this point. This is due to the external elevation provided by the long dorsal spinous processes (DSPs) in the withers region, which creates a contrary impression [1].
Figure 2.1 A photograph of the skeleton of the horse. The vertebral column runs from the atlanto-occipital joint to the last coccygeal vertebra. The vertebral column descends sharply from the atlanto-occipital joint to reach its lowest point at the cervicothoracic junction. The column then ascends gently to the caudal lumbar region and descends down, via the sacrum, to the coccygeal vertebrae.
Vertebral Numbering System
The nomenclature for the classification of different vertebral segments is fairly standardised between different texts and papers, with vertebral segments traditionally counted within spinal regions from a cranial reference point. Within each region the vertebrae are numbered sequentially from cranial to caudal, e.g. T1 (first thoracic vertebra), T2 (second thoracic vertebra). However, occasionally, some authors use modified reference systems, using caudal reference points [2]. It is important to be aware of this alternative numbering system when consulting the literature in this area to avoid confusion. In this book the standard cranial reference system will be used.
Vertebral Formula
The spine of the horse is made up of cervical, thoracic, lumbar, sacral and coccygeal vertebrae (Figure 2.2). The standard vertebral formula for the horse is 7 cervical vertebrae, 18 thoracic vertebrae, 6 lumbar vertebrae, 5 sacral vertebrae and between 15 and 21 coccygeal vertebrae [3] (Table 2.1). Although there may be some variation in the number of specific vertebrae in the axial skeleton, the total number in the formula is more constant. Anecdotally, so-called ‘short-backed’ horses, such as Arabians, have been reported as having fewer vertebrae than other horses [4]. More objective data on the numbers of vertebrae have come from studies investigating the numbers of vertebrae within a population, with a number of studies designed to investigate the frequency of occurrence of the standard six lumbar vertebrae. Haussler et al. [2], in a study on thoroughbred horses, showed that only 69% of horses had the expected six lumbar vertebrae. However, it has been suggested that variations in the number of vertebrae within one spinal region are compensated for by an alteration in number in an adjacent vertebral region, in many cases to give a constant overall total vertebral number. There has been no proven association between the numbers of vertebrae that a horse has and any pathological condition.
Figure 2.2 The bones of the vertebral column from the seventh cervical vertebra to the penultimate coccygeal vertebra. The vertebral column is divided into cervical, thoracic, lumbar, sacral and coccygeal regions. There are 7 cervical vertebrae, 18 thoracic vertebrae, 6 lumbar vertebrae, 5 sacral vertebrae and 15–21 coccygeal vertebrae in the normal horse.
Table 2.1 Average vertebral formula for the horse.
Anatomical site
Number of vertebrae
Cervical
7
Thoracic
18
Lumbar
6
Sacral
5
Coccygeal
15–21
Ossification Centres and Growth Plate Closure Times
The primary ossification centres of the vertebral bodies and neural arches (i.e. those surrounding the embryonic notochord in the centrum and lateral to the neural tube in the vertebral arch) fuse shortly after birth [5], whereas the secondary separate centres of ossification do not fuse until later on in life, if at all.
Secondary centres of ossification occur in the summits of the DSPs of the cranial thoracic vertebrae (the caudal thoracic and lumbar DSPs have fibrocartilaginous caps), the extremities of the transverse processes (TPs) of the lumbar vertebrae, the epiphyses of the vertebral bodies and the ventral crest.
The age at which these secondary centres of ossification fuse to the parent bone depends on the method of estimation of growth plate closure. Postmortem and histological growth plate closure times will always report an older age of closure than radiographic surveys because radiography is a less sensitive method of identifying the presence of an open growth plate.
The secondary centres of ossification present in the summits of the DSPs of the cranial thoracic region from T2 to around T9 are reported to fuse to the parent bone between 9 and 14 years of age [3], but in many cases, in the author's experience, they never fuse to the parent bone, even in aged horses. Thus they can be confused with fractured summits of the DSPs on radiographs if this developmental feature is not appreciated (Chapter 8).
The secondary centres of ossification at the cranial and caudal epiphyses of the vertebral bodies have reported closure times of between 3 and 3½ years of age using radiographic techniques (Chapter 8) [5]. However, gross anatomical studies suggest that the plates fully close later and asynchronously. The physes are reported to close between 4.9 and 6.7 years, with the cranial physis closing first, usually 1–2 years before the caudal physis [2]. The secondary centres of ossification of the TPs close in the first few months of life, although specific reports of this are not available.
Structure of the Thoracic and Lumbar Vertebrae
A typical thoracic vertebra is made up of a vertebral body, a vertebral arch and vertebral processes (Figure 2.3). The vertebral body provides the surface against which the intervertebral disc sits, whereas the vertebral arch provides a gap in the osseous structure through which the spinal cord runs. The vertebral processes are the sites of attachment for various ligaments and muscles and are named the DSPs, the transverse processes (TPs) and the articular processes (APs) (Figure 2.3). These processes vary subtly within each anatomical region and this variation reflects the functional and structural demands at that particular anatomical site, e.g. the length of the DSP varies from region to region, being particularly long between T3 and T7. The lumbar vertebrae, in contrast, have long TPs and medium height DSPs.
Figure 2.3 Vertebral anatomy: (a, b) line diagrams of a thoracic vertebra labelled to show the different anatomical regions of the vertebra: (a) a craniocaudal oblique diagram, (b) a lateral diagram. (c, d) Photographs of typical vertebrae: (c) thoracic vertebra, (d) lumbar vertebra; A, vertebral body; B, vertebral canal; C, dorsal spinous process; D, articular facet; E, transverse process. Note the much longer transverse processes in the lumbar vertebra compared with the thoracic vertebra.
Vertebral Bodies
The vertebral bodies of the equine thoracolumbar spine (see Figure 2.3) provide support for weight-bearing and attachment sites for soft tissues and muscles. They are convex in shape cranially and concave in shape caudally. Ventrally a ridge of bone, the ‘ventral crest’ (Figure 2.3) is observed on approximately four to eight vertebrae (mean 5.5 ± 0.8 [2]) centred around the thoracolumbar junction.
The shape of the vertebral bodies changes from a rounded shape in the thoracic region to a dorsoventrally flattened shape in the caudal lumbar and sacral regions. It has been hypothesised that this shape change limits movement laterally between these vertebrae, but not dorsoventrally. Other anatomical variations between sites include the observation that prominent ventral body ventral crests are found in the cranial thoracic area and between T15 and L3. The ventral crest at the latter site, which can vary in the number of vertebrae involved, is believed to be the site of insertion of the crura of the diaphragm.
Intervertebral Discs
The intervertebral discs (or fibrocartilages) are positioned between adjacent vertebral bodies to form fibrocartilaginous articulations. They function to aid weight bearing, axial shock absorption and the maintenance of vertebral flexibility; they have both proprioceptive and nocioceptive fibres in the outer third of the disc. The discs are made up of a gelatinous central nucleus pulposus and an outer fibrous annulus fibrosus; this annulus fibrosus is designed to provide rotational stability to the intervertebral joint by being formed of concentric layers of fibres angled relative to each other.
The width of the intervertebral discs differs between anatomical sites. In one study it was demonstrated that the intervertebral discs were thicker at T1–T2 (average 5.9 mm, [6]) than elsewhere in the thoracic spine, with the average diameter of an intervertebral disc in the midthoracic region being 2.5 mm. It was also shown that the lumbosacral junction has a wider intervertebral disc compared with elsewhere in the spine (average 3.6 mm).
In the horse, relatively few clinical problems arise from intervertebral disc pathology, particularly compared with the high frequency of pathology at this site in dogs and humans. However, discospondylitis and intervertebral disc degeneration are occasionally seen. Intervertebral disc herniation is extremely rare in the horse, possibly because of the poorly developed nucleus pulposus and thin intervertebral disc.
Vertebral Arch
The spinal cord runs through the vertebral arch, which is made up of the dorsal part of the vertebral body ventrally, the vertebral lamina dorsally and the pedicles laterally. Dorsally in the vertebral arch the ventral laminae are connected by the ligamenta flava. The vertebral arches of the spinal vertebrae together form the continuous vertebral canal housing the spinal cord and its associated structures up to the cranial sacral region where the spinal cord terminates in the cauda equina (Chapter 3). The vertebral arch is relatively large compared with the diameter of the spinal cord, ensuring no compression of the cord during movements of the spinal segments in the normal spine. However, in pathological conditions narrowing of the vertebral arch can occur (i.e. if there is displaced bone secondary to a fracture or new bone formation in osteoarthritis). In these cases spinal cord compression may result in onset of neurological signs.
Intervertebral Foramina
Between the vertebral arches of each vertebra there is a small opening on either side – the intervertebral foramina (Figure 2.4). These intervertebral foramina are formed by ventral notches in the cranial and caudal margins of the vertebral arch. The intervertebral foramina permit soft tissue structures (nerves, blood vessels and lymphatics) to exit the bony vertebral canal at each segment.
Figure 2.4 Intervertebral and lateral foramina of the thoracic spine: (A) photograph of a skeleton; (B) a cross-section from a postmortem specimen. In (A) an open intervertebral foramen is seen (black arrow) in the vertebral segment cranial to an intervertebral foramen that has spurs of bone formation protruding into it from dorsal and ventral (arrow head). In (B) this new bone formation is seen on the postmortem specimen.
Lateral Foramina
In addition to the intervertebral foramina that are observed on either side of the spine at each segmental junction between vertebrae, a second lateralised opening out of the bony canal is also present intermittently in some individuals. These are known as the lateral foramina. Thoracic vertebrae T11, T15 and T16 have been reported as showing the highest incidence of fully formed lateral foramina in the thoracolumbar spine [7].
The origin of the lateral foramina is not known; however, it has been proposed that they arise from the intervertebral foramina as a consequence of spur formation in the caudal ventral notch of the vertebral arch (Figure 2.4). Progressive calcification of the caudal ventral notch can occur, similar to that seen in ventral spondylitis (Chapter 13). In addition to a fully enclosed lateral foramen, there are a number of other variations in the anatomy of the caudal notch of the vertebral arch, including one or more spurs protruding into the notch [7]. It has been demonstrated that, where present, the lateral foramen contains spinal nerves and some vessels, and the presence of lateral foramina may possibly be associated with spinal nerve impingement during and subsequent to this calcification.
Sacral Foramina
In the sacrum the soft tissue structures exit the fused vertebral arches via either dorsal or ventral (pelvic) sacral foramina. The dorsal branches of the sacral spinal nerves exit via the dorsal sacral foramina. The pelvic sacral foramina communicate with the vertebral canal ventrally and contain the ventral branches of the sacral spinal nerves.
Vertebral Canal Contents
The vertebral canal contains the spinal cord and the structures surrounding the spinal cord (i.e. the cerebrospinal fluid, meninges, fat and vascular plexus). The spinal cord has segmentally paired dorsal and ventral motor roots that converge within the intervertebral foramen to form the spinal nerves (see Chapter 3).
Transitional Vertebrae
Transitional vertebrae are located between two adjacent vertebral regions and have the morphological characteristics of both these regions, i.e. they are ‘hybrid’ vertebrae. They occur, therefore, at the cervicothoracic, thoracolumbar or lumbosacral junction. A few studies have documented the incidence of transitional vertebrae. Haussler et al. [2] showed that 22% of their study population had thoracolumbar transitional vertebrae, none had lumbosacral transitional vertebrae and 36% had sacrococcygeal transitional vertebrae. Transitional vertebrae may exhibit their unusual morphology either through left-to-right asymmetry or via altered cranial-to-caudal graduation in the morphology. In large-scale studies of lumbosacral transitional vertebrae in humans and dogs, the morphological characteristics of the transitional vertebrae have been demonstrated to occur at the vertebral arches and transverse processes rather than at the vertebral body.
In clinical practice the most common transitional vertebra is transitional C7, which is often detected on lateromedial radiographs of the base of the spine and is characterised by having a short DSP, when it would normally have none at all.
Dorsal Spinous Processes
The DSPs project dorsally from the vertebral arch to rise above the vertebra into the epaxial musculature. The function of the DSPs is considered to be as levers for the muscular and ligamentous attachments of the vertebral column; bilateral contraction of the muscles that attach to the DSPs causes spinal extension, and unilateral contraction causes rotation.
The DSPs vary in their length, shape and angulation in different regions and will be considered anatomically from T1 running caudally. T1 has an extremely small DSP, rising approximately to twice the height of the vertebral body dorsally (Figure 2.5). This is the first elongated DSP in the vertebral column in most horses; however, occasionally an elongated DSP is seen on C7 (i.e. C7 has the properties of a transitional vertebra). Care must be taken not to automatically assume that the first obvious DSP on a lateromedial radiograph is therefore T1.
Figure 2.5 A photograph of vertebrae cervical 7 (C7) and thoracic 1 (T1). Note the elongated dorsal spinous process on T1 (arrow) compared with C7 (arrowhead).
Although T1 has a small spinous process, the DSPs in the cranial thoracic vertebral region are markedly elongated in the region of T2–T8 to form the withers (Figure 2.6). The apex of the withers is formed by the DSPs of T4–T7 (Figure 2.6). As noted above, the tips of the DSP of approximately T4–T7 have separate centres of ossification. From an apex at T6 or T7 the length of the DSP decreases down to approximately T12; the height of the DSP decreases slightly down to the anticlinal vertebra (the vertebra at which the angulation of the DSPs changes; see below, Figure 2.6) and then increases gently to the last lumbar vertebra (Figure 2.6).
Figure 2.6 Three photographs of the vertebral column of a horse showing the shape and angulation of the dorsal spinous processes (DSPs): (A) cranial thoracic region (withers); the DSPs are markedly elongated and angle caudally. Note the separate centres of ossification in the most cranial vertebrae. In (B) the DSPs of the mid-thoracic region are seen. At this site the DSPs are shorter than in the cranial thoracic region and have a ‘beak shape’ at their summits. The DSPs of the more cranial vertebrae angle caudally until the ‘anticlinal’ vertebra (black arrow). From this point caudally the DSPs angle cranially. In (C) the DSPs of the caudal thoracic and first four lumbar vertebrae are seen. The DSPs angle cranially.
The shape of the DSPs also depends on the anatomical site from which they arise. DSPs from vertebrae T1–T10 are narrow and tend to be quite straight (Figure 2.6). At T11–T16 they have a marked beak-shaped outline, wider at their base than at their apex and forming a cranial beak with a rounded caudal aspect at their summits. The cranial and caudal borders of the DSPs are often roughened due to new bone formation on these edges, which are the insertions of the interspinous ligaments; the dorsal summits of the DSPs are also often roughened at the sites of attachment of the supraspinous ligament (Chapter 10).
The angulation of the thoracolumbar DSPs changes from T1 caudally to the lumbosacral junction. From T1 to T14 the DSPs are angled dorsocaudally (i.e. towards the tail; Figure 2.6A). At T15, the so-called ‘anticlinal vertebra’ (Figure 2.6B), the DSP is upright and then from T17 to L6 the DSPs are angled dorsocranially (towards the head) (Figure 2.6C). The anatomical reason for this alteration in DSP angulation is suggested to be due to attached soft tissue interactions. The position of the anticlinal vertebra suggests an alteration, at this anatomical site, of the soft tissue forces acting on the spine. The cranial thoracic region transmits forces from the head, neck and forelimbs, whereas the caudal thoracic and lumbosacral regions transmit forces associated with the hindlimbs; therefore the pull of the associated soft tissue structures does indeed alter either side of the anticlinal vertebra.
A further change in DSP angulation is observed in the sacrum, which is inclined dorsocaudally (Figure 2.7). The anatomical consequence of this alteration in angulation, without the intermediary of an anticlinal vertebra as occurs in the thoracic spine, is that a wide interspinous space is formed at the lumbosacral junction (Figure 2.7). It has been suggested that this wide interspinous space allows an increased range of motion at this site without the risk of process impingement. The wide space between L6 and S1 is a relatively consistent finding; however, in one study 36% of horses had an equally wide interspinous space between L5 and L6 [2]. This has, at the current time, little clinical relevance apart from possibly making more difficult the identification of the landmarks for cerebrospinal fluid retrieval from the lumbosacral space.
Figure 2.7 A photograph of the lumbosacral region: the wing of the ilium is seen in front of the cranial part of the sacrum. The last lumbar vertebra angles cranially and the sacrum angles caudally. The lumbosacral space is wide compared with any other interspinal space in the vertebral column (black arrow).
