Veterinary diagnostic imaging : the horse

572 Pages·2006·34.546 MB·English

by  Charles S Farrow

11830 Westline Industrial Drive
St. Louis, Missouri 63146
VETERINARYDIAGNOSTIC IMAGING: THE HORSE ISBN 13 978-0-323-01206-5
Copyright© 2006,MosbyInc. ISBN 10 0-323-01206-X
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About This Book
Successful diagnostic medical imaging, equine and siders the historical and clinical features of a particu-
otherwise, depends on 2 things: image quality anddiag- lar case, and then, based on past experience, looks first
nostic ability.It is the latter skill that I hope to enhance at one or more high-yield areas of the image for spe-
with this book. cific disease indicators. Of course the entire film will
eventually be examined thoroughly, but it is the initial
directed search that distinguishes the experienced
from the inexperienced. It is this latter expert skill, use
III DIAGNOSTIC SUCCESS BEGINS
of the where and what approach, that I hope to impart
WITH A QUALITY IMAGE
to the readers of this book.
To further augment the reference value of this text,
Image quality has been defined in a variety of ways, I have included contextual normals, numerous
but can be distilled down to two essential ingredients, anatomical specimens, a wide spectrum of disease
contrast and clarity. Contrast means that a portion of a variation and degree of involvement, and a generous
particular image differs sufficiently from its back- number of combined orientation and close-up views.
ground that it can be recognized as such. Without con-
trast a lesion remains camouflaged, defying detection.
But contrast alone is not enough. A lesion must also
III CONTEXTUAL NORMALS AND
possess sufficient clarity of size, shape, position, and
ANATOMIC SPECIMENS
density—so-called disease indicators—to be recognized
or looked up subsequently in an appropriate reference
source. Colleagues and students alike have told me repeatedly
that normal radiographs are the most useful when dis-
played next to or nearby case example. On considera-
tion, this makes perfect sense, given the subtleties of
III NOVICE VERSUS EXPERT
radiographic diagnosis, the anatomic nature of
STRATEGIES medical imaging, and the inherently comparative
nature of the diagnostic process.
Novices are rightfully taught to carefully scrutinize Over the years I have accumulated a number of
each and every medical image without bias or expec- anatomic specimens, bones and dried tendons for
tation and then to render a diagnosis based on proba- the most part, which I have used countless times
bility. Experienced medical imagists, on the other while trying to figure out one radiographic problem
hand, typically employ a more efficient where and what or another. I have photographed these specimens
approach to film reading, a form of intuitive diagnosis and added them to the most appropriate portions of
common to most medical specialties. Using the where the text, where I believe they should prove most
and what stratagem, the experienced film reader con- useful.
vii
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viii About This Book III
III FULL DISEASE SPECTRUMS (7) Abdomen. Within these sections are 44 individual
AND ORIENTATION/DETAIL chapters covering not only various body regions but
CASE EXAMPLES important related subjects as well. For example, in the
extremital section there is a chapter on maturity, imma-
turity, and dysmaturity, to include a discussion on the
As in Volume I, The Dog and Cat,I have done my utmost
limitations and necessity of radiometrics. Another
to provide as many examples of each disease as
chapter in the same section deals with fracture healing
possible, ranging from the barely perceptible to the
and bone remodeling.
obviously diseased, eschewing the more traditional
Ihave also attempted to introduce the concept of
approach of simply displaying one or two classics.
anatomic-radiologic correlation by beginning many
Although of undeniable teaching value, classic exam-
chapters with a brief list of anatomic facts. Hopefully,
ples are seen only occasionally, with less fully featured
these condensed info-packets will serve both to whet
cases being the rule.
the reader’s intellectual appetite and provide an
Likewise I incorporated a combination of orienta-
anatomic framework on which to consider the related
tion and close-up views for as many cases as possible
clinical material.
in ensure that the reader fully appreciates the nuances
of each lesion, seemingly small features that often pay
big diagnostic dividends. Credit
As with the initial volume in this series, The Dog and
Cat,I have done my utmost to fully credit those whose
III ORGANIZATION AND CREDIT
original observations comprise the fabric of this work.
Specifically, I have acknowledged these individuals
Organization both contextually and at the conclusion of each chapter
so that the reader may fully appreciate their important
This textbook on equine medical imaging is organized
contributions to the field of medical imaging. My
in a traditional anatomic fashion. There are 7 sections:
apologies if I have inadvertently omitted anyone.
(1) Extremities, (2) Skull, Face, Jaws, and Cranium, (3)
Throat and Neck, (4)Spine, (5) Thorax, (6) Hear, and Charles S. Farrow
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Acknowledgments
As with any project of this size, much effort is required by a large number of
people. However, among this company are two exceptional individuals worthy
of special recognition: Jolynn Gower, Senior Development Editor, and Rachel
Dowell, Senior Project Manager. Awriter couldn’t hope for two more competent
or understanding collaborators.
I asked Jolynn and Rachel for pictures of themselves, believing this would be
a fitting tribute, but in their modesty they declined. Instead, they suggested I use
a photograph of an animal, which I hope they find satisfactory.
Charles S. Farrow
ix
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S E C T I O N I
The Extremities
C h a p t e r 1
Skeletal Maturity, Immaturity,
and Dysmaturity
III NEWBORN VERSUS ADULT BONES produce the new bone required for axial development:
first as a living and later as dead, cartilaginous scaf-
The bones of the newborn foal differ dramatically from folding; then as an area of disorganized, roughened
those of the adult horse. Specifically, they are smaller, new bone; and finally as a structurally refined cortex
smoother, and generally rounder (Figure 1-1). Many and medulla—a process that continues until matura-
are composed of multiple parts, the result of as yet tion is complete.
unfused, secondary growth centers (Figure 1-2). Seen The metaphysis of an immature long bone can
radiographically, the joint spaces of foals—in reality appear quite rough and irregular compared with the
composed mostly of cartilage—appear disproportion- adjacent shaft (Figure 1-5), inviting misdiagnoses such
ately wide compared with those of adults (Figure 1-3). as fracture, infection, or osteochondritis. Such concern
The outer perimeter of some secondary growth centers is usually unwarranted, however, as a comparison
appears abnormally roughened and in places incom- image of the opposite metaphysis will readily reveal.
plete, falsely suggesting infection or osteochondritis This temporarily roughened area is termed the cutback
(Figure 1-4). Some bones, as yet unossified, are invisi- zone, or simply, thecutback.
ble altogether. The cutback zone is the place in the bone, situated
on the metaphyseal side of the growth plate between
the shaft and the epiphyses, where the bone changes
Growth Plates, Cutback Zones, and
from wide and rough to smooth and narrow, an orga-
Tubulation
nizational process termed tubulation (Figure 1-6).
Along bone (and some short bones) grows longitudi- Cutback zones are for the most part quite variable but
nally from either end, although not always equally. typically are most pronounced during the first few
Cartilaginous growth plates, or physes, continuously months of skeletal development.
1
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2 SECTION I III The Extremities
A B
Figure 1-1 • Bones of a young foal’s distal forelimb (A) compared with those of an adult horse (B). The bones of the foal are
smaller, smoother, and rounder than those of an adult and contain numerous unfused epiphyses and open growth plates
(simulated with black acrylic). Specimen preparation resulted in distal phalangeal splitting.
A B
Figure 1-2 • Bones of a young foal’s stifle as seen in lateral (A) and frontal (B) perspectives show separate ossification
centers for the (1) distal femoral epiphysis, (2) proximal tibial epiphysis, and (3) tibial tuberosity. Growth plates are simulated
with black acrylic.
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CHAPTER 1 III Skeletal Maturity,Immaturity,and Dysmaturity 3
A,B C
Figure 1-3 • Comparative differences in carpal cartilage spaces as a function of age: newborn foal (A), 2-month-old foal (B),
and adult horse (C).
Figure 1-4 • Close-up ventrodorsal view of the hips of a Figure 1-5 • Close-up view of the distal tibia of a foal
young foal show typically roughened femoral head and shows roughing and flaring on the metaphyseal side of the
greater trochanter bilaterally, a normal variant in immature growth plate, a normal but temporary finding termed
horses. the cutback zone.
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4 SECTION I III The Extremities
A B
Figure 1-6 • Close-up, lateral views of the proximal tibial growth plate before (A) and after (B) closure showing the process
of long bone tubulation, a process by which a roughened, structurally undifferentiated metaphysis smoothes, narrows, and
eventually develops a discrete cortex and medulla.
Separate Ossification Centers
Table 1–1•XEROGRAPHICALLY OBSERVED
(Secondary, Accessory Growth Centers)
APPEARANCE AND DISAPPEARANCE OF
Appearance and Disappearance.Most secondary ossi- DISTAL FORELIMB GROWTH PLATES IN FOALS
fication centers are radiographically evident at birth FROM BIRTH TO 6 MONTHS OF AGE
and then gradually disappear as they become incor-
First radiographic appearance of distal epiphyseal
porated into the parent bone, a process termed fusion.
ossification in metacarpal 2 and metacarpal 4
From a practical perspective, the presence (or absence) (extremely variable) 4-38 wk
of separate ossification centers enables one to estimate First radiographic appearance of the crena 4-22 wk
the age of an immature horse (assuming the precise Closure of the proximal growth plate of P2 18-30 wk
Closure of the proximal growth plate of P1 22-38 wk
date of birth is not known).
Closure of the distal growth plate of MC3 18-38 wk
Smallwood and colleagues described the xerora-
diographic appearance of the growth plates of the
distal forelimb of the foal from birth to 6 months of age
(Table 1-1).1 In a companion article, Metcalf and co-
workers described the scintigraphic appearance of the Ossification Fronts. Growth of the normal epiphyses
distal forelimb growth plates over the same period of is outward, increasing in volume while preserving
development, with the aim of establishing normal shape. Epiphyseal ossification follows suit but in a
comparisons.2 somewhat uneven, random fashion. The result is
Because hard, highly concussive-type running and that for a few weeks during early development, the
jumping can potentially injure growth plates, or more condylar-type epiphyses, such as those found on the
specifically their circulation, most trainers eschew proximal and distal humerus and distal femur, may
such training until certain sentinel growth plates, most assume a distinctive, serrated appearance that resem-
often those found in the distal radii, have fully closed bles some forms of osteochondritis and osteomyelitis
(Figure 1-7). (Figure 1-8). Adams and Thilstead illustrated this phe-
Because of their relatively weak cartilaginous nomenon in their description of the radiographic
attachment, accessory growth centers are subject to appearance of the developing equine stifle from birth
avulsion-type fractures, injuries that in some instances to 6 months of age.3
may be so subtle that only a comparison radiograph of This is a normal, transient variation, typically found
the opposite leg will confirm their existence. bilaterally, and termed an ossification front. Where