Ultrasonography of the Equine Forelimb
AMEVEQ Ultrasound Seminar – Bogota, Colombia
June 19-21, 2008
Mary Beth Whitcomb, DVM
Large Animal Ultrasound Service – Section Head
Assistant Professor, School of Veterinary Medicine
University of California, Davis
Many practitioners hope that purchasing a new ultrasound machine will eliminate all problems
associated with musculoskeletal imaging. Unfortunately this is not the case, and knowledge of basic
ultrasound principles is still required no matter what type of machine is purchased. While newer
machines usually offer better image quality than older equipment, it is not uncommon for new buyers
to be disappointed that their machine does not scan as well as they anticipated. This is usually due to
reluctance to learn and use basic instrumentation. There are many controls and buttons on the
keyboards of ultrasound machines. Many are often labeled poorly or with symbols that are difficult to
understand, but it is well worth spending some quality time with an instruction manual to learn how to
find and adjust a few key controls. These include depth, focal zones, program selection, frequency
and time-gain-compensation (TGC). No matter what machine is owned or purchased, knowledge of
these controls will improve image quality dramatically.
The majority of images discussed in this lecture can be obtained with a standard high frequency (7-14
MHz) tendon format transducer at a scanning depth of 3-6cm. Diagnostic images can also be
obtained with a rectal format transducer, but in general, these transducers are of midrange frequency
(5-7 MHz) and therefore produce images with poorer resolution compared to the higher frequency
tendon format transducers. In addition, rectal format transducers tend to be less ergonomic for long
term musculoskeletal imaging. For this reason, the purchase of a tendon format transducer is highly
recommended if significant musculoskeletal imaging is performed. The use of microconvex
transducers (4-8 MHz) will be discussed individually when their curvilinear format and midrange
frequency provides advantages over straight linear transducers.
Machine Controls & Settings
Most equine musculoskeletal imaging is performed at a depth of 4-5cm. For metacarpal (MC) or
metatarsal (MT) imaging, it is ideal to have all four structures visible on the screen in order to compare
relative sizes. If the depth is set too deep, fine detail and small lesions may be missed. If the depth is
set too shallow, the pixels may become too large and relative size of structures cannot be assessed.
Nearly all of today’s machines have movable focal zones which should always be placed in the region
of interest. For example, in order to evaluate the inferior check ligament (ICL) and suspensory
ligament (SL), the focal zones should be moved deeper into the far field to enhance image quality of
those structures. The correct program should be selected for the musculoskeletal examination. Most
machines have preset programs designed by the manufacturer to optimize image quality for different
types of exams. There are often several possible programs that can be chosen for each transducer.
Operators should learn how to check the program being used and how to change the program
depending on the type of exam being performed, i.e. reproductive versus tendon. Transducer
frequency can also be adjusted to improve image quality. Most transducers sold today are variable
frequency transducers, including blended frequency transducers. Variable frequency transducers
allow the operator to change to a higher or lower frequency or frequency range to improve
visualization of structures. Altering frequency can be beneficial, even in distal limb exams. Choosing
a lower frequency can often provide the penetration needed in such circumstances as heavily haired
horses, especially when they cannot be clipped, or in older or thick skinned horses that tend to image
poorly. TGC controls are not found on all machines but are relatively common in higher end
ambulatory machines. They are usually recognized as multiple rows of slide pods on the keyboard or
near the screen. Basically, TGC controls allow the operator to alter gain at multiple depths throughout
the image. Moving a slide pod to the right will increase the gain in that portion of the image and
moving the slide pod to the left will decrease the gain. For example, if a specific portion of the image
is too dark, moving the corresponding slide pod(s) to the right should improve visualization of that
region. This is most helpful when scanning the SL body. The far field slide pods should be moved
completely or almost completely to the right. It is also helpful to move the near field slide pods to the
left when imaging the SL.
Good patient preparation will also dramatically improve musculoskeletal imaging. Whenever possible,
the hair should be clipped with at least #40 clipper blades. The hair should be washed briefly with
soap and water and ultrasound gel applied. The time spent adequately prepping a horse for
ultrasound will be gained back by obtaining better quality images more easily and efficiently. A
standoff pad should be used for the majority of musculoskeletal imaging. The major benefit of a
standoff pad is increasing contact when scanning curved structures.
Many practitioners feel comfortable imaging the superficial digital flexor tendon (SDFT), deep digital
flexor tendon (DDFT) and inferior check ligament (ICL) in the front and hind limbs. The most common
misinterpretation of these structures involves the DDFT in the distal MC or MT region. The DDFT
demonstrates a large hypoechoic area on its dorsal border that is created by the ICL inserting into the
DDFT. This hypoechoic area is often more apparent in the hindlimb than the forelimb and should not
be mistaken for injury. A common missed diagnosis also involves the ICL. The lateral edge of the
ICL is the most common location for small ICL tears. This portion of the ICL is not well imaged during
traditional palmar midline imaging and therefore small tears often go undiagnosed until they become
much larger lesions. The lateral border of the ICL can be easily visualized by sliding the transducer
laterally from the traditional midline approach. Small hypoechoic areas of the lateral border are
indicative of injury to the ICL.
The suspensory ligament (SL) body is the most difficult structure to image and interpret compared to
the other MC/MT soft tissue structures. This is partially secondary to its deep location relative to the
SDFT, DDFT and ICL. It is also secondary to the variable presence of muscle fibers throughout the
length of the body. Muscle fibers tend to be more prominent in the proximal body region but
commonly extend into the distal SL body region. Muscle fibers appear as hypoechoic areas and often
create a diffusely mottled appearance, depending on muscle distribution within the SL body. These
hypoechoic areas can be easily confused with injury. The distribution of muscle within the SL body is
often bilaterally symmetrical, and therefore the contralateral limb can be scanned for comparision.
Also, due to the fact that injured ligaments tend to demonstrate enlarged cross-sectional areas (CSA),
obtaining a normal CSA (also in comparison to the contralateral limb) is supportive of muscle fibers
rather than injury.
Perhaps the biggest dilemma of MC/MT imaging is the horse whose lameness is localized to this
region by regional nerve blocks, and radiographs do not reveal the source of lameness. There is
often considerable pressure on the ultrasonographer to reveal a source of lameness. When the
ultrasound examination fails to do so, it is important to remember that proximal MC or MT nerve
blocks can often enter synovial structures such as the carpal sheath and carpometacarpal joint in the
forelimb and the tarsometatarsal joint in the hindlimb. If ultrasound is negative in such cases, these
regions should be investigated as possible sources of lameness.
Joint Imaging & Collateral Ligament Injuries
With the exception of the shoulder and pelvis, all joints have paired (medial and lateral) collateral
ligaments (CL) that function as their primary medial and lateral stabilizers. Ultrasonographic
evaluation of any CL should begin by reviewing the anatomy of the joint in question. Many joints
(carpus, tarsus, elbow, fetlock) have greater than one component (superficial and deep) to each
collateral ligament. In addition, the CL anatomy may differ from medial to lateral within the same joint
(tarsus, elbow, carpus). In general, CLs are relatively short ligaments that span the major bones
comprising each joint. Although CLs are generally oriented vertically on the medial and lateral
aspects of a joint, most CLs tend to be located slightly more dorsal at their origin (proximal
attachment) than at their insertions (distal attachment). This slightly oblique orientation is important to
remember when attempting to find their linear fibers with longitudinal imaging. The relatively small
size and flattened shape of most CLs makes them somewhat difficult to evaluate on transverse views,
therefore longitudinal imaging is preferred over transverse imaging.
Prior to the use of ultrasound, radiography formed the mainstay of CL injury diagnosis. Stressed
radiographic projections (medial-to-lateral or lateral-to-medial) often provide a positive diagnosis in
cases of CL rupture; however, characteristic joint widening may not be seen in horses with single
component ruptures or in horses with partial tearing. The presence of avulsion fractures can indicate
CL injury and should prompt an ultrasound exam to confirm associated CL injury. Many fragments
that appear to be avulsion fractures on radiographs are later found to be unassociated with an
Although CL injuries have been documented in the literature, reports are somewhat sporadic. With
the exception of the coffin joint, relatively little is known about outcome in horses with CL injury. It has
been the author’s experience that if diagnosed early and appropriate joint stabilization instituted,
horses with CL injury and/or rupture tend to recover better than would be expected compared to
equally severe injuries of other soft tissue structures. Although such experience is somewhat
anecdotal, it should be considered when presented with a horse with CL injury or rupture.
Ultrasound of the Carpus
The most common indication for carpal ultrasound is regional swelling, wounds or distention of carpal
synovial structures. The extensor carpi radialis (ECR) tendon is the largest of the 3 extensor tendons
and courses over the dorsum of the carpus to its insertion onto proximal MC3. The common digital
extensor (CDE) tendon is next largest and is located dorsolaterally relative to the ECR tendon. The
lateral digital extensor (LDE) tendon is quite small and is located at the lateral aspect of the carpus
near the lateral collateral ligament (LCL). All 3 extensor tendons are encased in tendon sheaths that
extend from the distal radius to the proximal metacarpus. Due to their superficial location, all are at
risk for contamination and sepsis in horses with skin wounds or lacerations. Ultrasound is useful in
horses with distention of any of these structures to detect the relative degree of effusion versus
synovial thickening that contributes to the distended appearance of tendon sheaths. Horses with
septic synovial structures typically demonstrate severely thickened synovial membranes with a lacey
and edematous appearance. There is often little effusion in cases of sepsis. Ultrasound is
exceptionally useful in such cases to guide needles into small fluid accumulations for cytologic
analysis and culture. Ultrasound can also detect tendon injury as a cause of tenosynovitis. Abnormal
tendons can demonstrate surface defects as well as internal hypoechoic areas with corresponding
Reports of collateral ligament injury in the carpus are not common in the literature; however, the
author has seen a handful of horses with rupture of the medial collateral ligament (MCL). The MCL is
a somewhat large and broad ligament compared to the lateral collateral ligament (LCL) which has a
small flattened shape. The normal MCL tends to show a more linear fiber pattern from its origin on
the distal radius to the midligament region, but has a slightly more irregular fiber pattern in its distal
half. This is secondary to the differing orientation of fibers as they insert onto the bones of the carpus
and metacarpus. For this reason, care should be taken not to overinterpret the distal portion of the
MCL. Horses with severe carpal swelling and lameness should be evaluated for CL rupture,
especially if the swelling localizes medially or laterally. Stressed radiographic projections may
demonstrate joint widening typical for CL rupture, but the absence of radiographic joint instability
should not preclude ultrasound for CL injury. If left undiagnosed or untreated, affected horses may go
on to develop irreversible angular limb deformity. Affected horses require splinting and/or heavy
Ultrasound of the Carpal Canal
Indications for ultrasound of the carpal canal include distention of the carpal sheath, swelling of the
carpal canal region and evidence of injury to the superficial digital flexor tendon in the proximal
metacarpus. Structures evaluated include the superior check ligament (accessory ligament of the
SDFT), superficial digital flexor tendon (SDFT) and deep digital flexor tendon (DDFT) as they course
through the carpal sheath and carpal canal. The carpal sheath should also be evaluated for effusion
and/or thickening of its synovial membranes.
Evaluation of the carpal canal begins at the level of the chestnut which is approximately the level of
the musculotendinous junction of the flexor tendons. This transition from muscle to tendon is
important to consider during image interpretation, as muscle is hypoechoic to anechoic and may
easily be mistaken for injury. Injury of the SDFT is a common soft tissue cause of lameness related to
the carpal canal. In contrast to the metacarpus, core lesions are not common in this region. Affected
tendons may be enlarged with diffuse mottling or have scattered hypoechoic areas. Regardless of the
appearance, SDFT injuries in the carpal canal typically do not show sonographic evidence of healing
on repeat exams. Injury to the DDFT is infrequently visualized in the carpal canal region. Surface
tearing may be present, but is not likely to be visualized ultrasonographically.
Superior Check Ligament
Injuries of the superior check ligament (SCL) are most common in Thoroughbred racehorses but may
be seen in horses of other disciplines. Affected horses typically have generalized fullness in the
region of the chestnut. The SCL is visualized by placing the transducer palmaromedially immediately
distal to the chestnut. The SCL is located slightly medial to the flexor tendons and has a somewhat
rectangular shape on transverse views. Echogenicity is homogeneous in normal horses but fiber
pattern is often short and choppy on longitudinal views, even in normal horses. Injuries typically
create an enlarged size with a diffusely abnormal echogenicity, as opposed to core lesions.
Ultrasound of the Elbow
Overview - Draining Tracts
The most common indication for ultrasound of the elbow joint is to document the source of severe
elbow lameness, especially in horses with wounds or draining tracts. The lateral aspect of the
proximal radius is a frequent location for draining tracts in this region. Such tracts often communicate
with the underlying bony surface and may additionally communicate with the elbow joint capsule.
Involvement of the lateral collateral ligament may also be found. Ultrasound is ideally suited to follow
chronic draining tracts to their source which may include foreign material, osteomyelitis and/or bony
sequestration. Contrast radiography may be performed in such cases, but contrast material may not
extend to the source of drainage or demonstrate the presence of nonmetallic foreign bodies. A wide
area should be clipped and evaluated surrounding the wound. Evaluation of the tract should begin at
the external wound site. Granulation tissue of chronic draining tracts is typically hypoechoic and easy
to follow. Some tracts are best identified on long axis while others are most easily followed on
transverse views. Longstanding tracts often demonstrate an “onion skin” appearance on transverse
views with multiple concentric rings surrounding the lumen. Pinpoint hyperechoic gas echoes may be
seen within the lumen of the tract, especially near the skin wound. Foreign material will demonstrate
a hyperechoic appearance and typically casts strong shadows. The appearance of osteomyelitis is
described below (see scapula). Bony sequestra may sometimes be difficult to distinguish from
osteomyelitis, especially in cases without significant distraction of the fracture fragment from the
parent bone. Ultrasonographic localization of foreign bodies can help to establish a plan for surgical
removal. Foreign body removal can also be performed as an ultrasound-guided procedure.
The bony surfaces of the elbow joint are generally smooth, but may be roughened in cases of septic
arthritis. The joint capsule is often not visible in normal horses, although mild synovial thickening and
mild anechoic effusion may be present. Septic arthritis often creates severely thickened synovial
membranes that may have an edematous or lacey appearance. The presence of joint effusion in
affected horses is variable.
The lateral collateral ligament is readily visualized at the lateral aspect of the elbow joint. Longitudinal
imaging is most useful for its evaluation, but transverse views can be readily performed. As all fibers
are not aligned perfectly, some irregularity in fiber pattern will be seen on longitudinal views.
Collateral ligament injury has been seen in horses with wounds, trauma or chronic draining tracts, but
is otherwise not common. The medial collateral ligament is difficult to visualize as it located high in
the axillary region and the pectoral musculature limits accessibility to this region.
Ultrasound of the Shoulder
Indications for shoulder ultrasound include regional swelling, lameness localized to the bicipital bursa
or shoulder joint, lameness refractory to lower limb blocks or wounds to the shoulder region.
Extension of wounds to synovial structures of the shoulder (bicipital bursa, infraspinatus bursa,
shoulder joint) should be investigated ultrasonographically.
Alcohol saturation is usually adequate for shoulder imaging. While a tendon or rectal format
transducer is used for most structures, a microconvex transducer is often useful to evaluate the
shoulder joint and medial and lateral lobes of the biceps tendon. A standoff pad is rarely necessary
except if detailed evaluation of the scapular spine is desired. A complete shoulder exam should
include evaluation of the biceps tendon, bicipital bursa, infraspinatus tendon and bursa and bony
surfaces of the humeral tubercles and scapula.
Biceps Tendon, Bicipital Bursa & Humeral Tubercles
The biceps tendon and bicipital bursa are the most commonly affected structures in the shoulder. The
examiner should be aware that the biceps tendon demonstrates significant changes in size, shape
and echogenicity from its origin on the supraglenoid tubercle (small, crescent shaped on transverse
views) to its musculotendinous junction distal to the humeral tubercles (large with a marbled
appearance, characteristic of muscle). The proximal portion of the biceps tendon demonstrates a
large oval shape between the biceps origin and the humeral tubercles. Large hypoechoic areas are
often present at this location due to the presence of a variable amount of fat. The biceps tendon
demonstrates its best known appearance at the level of the humeral tubercles where it becomes
nearly bilobed into its medial and lateral lobes. The lateral lobe is slightly larger than the medial lobe
and is located within the intertubercular groove between the cranial eminence of the greater tubercle
and the intermediate tubercle. The medial lobe is located within the intertubercular groove between
the intermediate tubercle and the lesser tubercle. Each lobe should be imaged individually.
Hypoechoic areas are created by the normal curvature of the biceps tendon over the humeral
tubercles, making mild injuries difficult to differentiate from its normal appearance. Comparison to the
contralateral limb is often required to confirm mild or even moderate injury. Severe biceps tendonitis,
however, usually results in severe distortion of its normal architecture and shape.
The bicipital bursa is a relatively large structure that extends proximally to the supragenoid tubercle
and distally to the proximal humerus where it has a large synovial reflection. The bursa nearly
completely surrounds the biceps tendon at the level of the humeral tubercles. The bursa is most
susceptible to wound penetration at this location. Horses with septic bicipital bursitis demonstrate
severe thickening of bursal synovial membranes with or without significant effusion. Synovial
thickening is usually most apparent superficial to the biceps tendon. Mild bicipital bursitis is most
reliably imaged between the lateral lobe of the biceps tendon and the cranial eminence of the greater
tubercle. This location can also be used to access the bursa for ultrasound guided procedures.
Severe nonseptic effusion is uncommon but can be seen.
The humeral tubercles should be evaluated in all horses during imaging of the lobes of the biceps
tendon. Normal humeral tubercles demonstrate a smooth undulating appearance. Visibility of step
defects of any of the humeral tubercles is consistent with fracture. Ultrasound often assists with the
diagnosis of humeral tubercle fractures, as radiography of this area is often difficult to interpret.
Infraspinatus Tendon & Bursa
Although infrequently affected, abnormalities can result in significant lameness in the horse.
Evaluation of infraspinatus (IS) structures should therefore be included as part of a standard
examination of the shoulder. The IS muscle belly can be located via transverse imaging of the
scapular spine and subsequently sliding the transducer caudally until the concave surface of the
infraspinous fossa can be seen. The IS tendon arises from the center of the IS muscle belly at the
level of the mid scapula and is followed distally towards the caudal eminence of the greater tubercle
where the tendon demonstrates a trilayered appearance. The tendon then continues distally towards
its insertion onto the craniolateral aspect of the proximal humerus, slightly proximal to the deltoid
tuberosity. The IS bursa is located deep to the IS tendon at the level of the caudal eminence of the
greater tubercle but is typically not visible in normal horses. Wounds may communicate with the IS
bursa, resulting in septic bursitis. Similar to the bicipital bursa, the septic IS bursa demonstrates
markedly thickened and edematous synovium with varying amounts of synovial fluid accumulation.
Ultrasound guidance is useful for synovial fluid sampling. IS tendon abnormalities have been seen in
horses with bony abnormalities (fracture and/or osteomyelitis) of the caudal eminence of the greater
The scapular spine and infraspinous and supraspinous fossae are superficially located structures and
require a relatively shallow scanning depth of 4-6cm. All bony surfaces should be evaluated for
evidence of step defects that would indicate fracture. Scapular fractures involving the glenoid cavity
and supraglenoid tubercle are best identified radiographically; however, midbody and scapular spine
fractures are difficult to identify on radiographs due to superimposition of the thorax and contralateral
scapula. Ultrasound is ideally suited to document fractures of this portion of the scapula. Hematoma
formation and/or muscle tearing may be present in association with the fracture.
Ultrasound is also useful to assist in the diagnosis of osteomyelitis of the scapula, which may also be
difficult to diagnose radiographically. Communicating wounds and draining tracts may be absent in
affected horses. Radiographic evidence of osteomyelitis can be detected within 7-10 days, whereas
ultrasound can detect periosteal changes within a few days and even sooner if direct communication
of a wound to an underlying bony surface is present. Sonographically, early evidence of osteomyelitis
consists of a “fluffy” or thickened appearance to the periosteal surface of the affected bone. A thin
overlying fluid layer is often present. Destructive lesions may also create small to large defects in the
normally smooth contour of bone. Ultrasonographic guidance can be used to obtain fluid samples
from affected bone, especially if overlying abscessation is visible.
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