Keywords horse - digital flexor tendon sheath - ultrasound-guided injection - landmark-guided
injection - interventional ultrasound
Introduction
Injection of the digital flexor tendon sheath (DFTS) is becoming a more commonly utilized
technique in the diagnosis and management of distal limb injuries in the horse. The
use of intrathecal contrast material with either radiography or computed tomography
has recently been described to diagnose deep digital flexor tendon (DDFT) and manica
flexoria (MF) tears.[1 ]
[2 ] Magnetic resonance imaging (MRI) has also confirmed a variety of injuries to structures
associated within the DFTS some of which may manifest few localizing signs. To determine
the clinical relevance of some of these injuries requires the placement of local anaesthetic
into the DFTS. However, in cases without significant effusion within the DFTS, accurate
needle placement into the synovial space can be difficult.[1 ]
[2 ]
[3 ] In addition, treatment of these injuries, such as with regenerative therapies,[4 ]
[5 ]
[6 ] often requires accurate placement of the appropriate elected treatment into the
synovial space.
Ultrasonography is now routinely performed in equine practice, both in the hospital
and ambulatory settings. In human medicine, the use of interventional ultrasound to
guide needle placement into or around a target has increased dramatically over the
past decade. These injection techniques are now considered the most reliable and repeatable
way to accurately place medication into a synovial space. A position statement from
the American Medical Society of Sports Medicine (2015) concluded that there was strong
evidence in the literature that ultrasound-guided injections are more accurate than
landmark-guided injections.[7 ] In humans, the accuracy of ultrasound-guided tendon sheath injections was significantly
increased at 87 to 100% compared with landmark-guided tendon sheath injections at
27 to 60%.[7 ] In the horse, equine practitioners have predominantly utilized landmark-guided injections
of the DFTS to date as an ultrasound-guided approach to the DFTS has yet to be described.
Ultrasonography has been shown to improve the accuracy of needle placement into many
other synovial structures in the horse including the cervical and thoracolumbar facet
joints,[8 ]
[9 ]
[10 ]
[11 ] the navicular bursa,[12 ] the sacroiliac region,[13 ] and the shoulder joint/area.[14 ]
A study evaluating the effect of different landmark-guided approaches to the DFTS
on distal limb desensitization has been previously performed in the horse,[3 ] as have comparative studies to determine the most accurate landmark-guided approach
for synoviocentesis of the DFTS.[15 ]
[16 ] Both the proximolateral approach (PLA) and the basilar sesamoidean approach (BSA)
were initially shown to be less accurate (56 and 63% successful respectively) than
other landmark-guided injections such as the axial sesamoidean approach (ASA) and
the palmar-plantar pastern approach (96 and 83% successful respectively).[15 ]
[17 ] In a later study, however, where the BSA was compared directly to the ASA, it was
found that they had comparable high success rates (100 and 96% respectively).[16 ] The BSA was also reported to be easier to perform than the ASA with an increased
chance of yielding a synovial fluid sample, particularly in limbs with minimal or
no DFTS effusion.[16 ]
In considering different ultrasound-guided approaches to the DFTS, the authors strongly
considered an ultrasound-guided BSA for direct comparison to the highly accurate landmark-guided
BSA. The contour of the limb at the site used for the BSA, however, makes ultrasound
probe placement and needle visualization technically challenging. As the purpose of
this study was to describe an ultrasound-guided approach to the DFTS that could be
readily used by equine practitioners of all skill levels, the authors chose instead
to investigate an ultrasound-guided PLA. The PLA provides the greatest flexibility
for ultrasound probe placement to visualize needle placement and manipulation relative
to all other approaches. The authors have also been utilizing the ultrasound-guided
PLA for injection of the DFTS in their hospital for cases in which there is little
to no DFTS effusion and/or in which synoviocentesis of the proximal pouch of the DFTS
is preferred due to the location of wounds or surgical portals. Our positive experience
with this approach, both in terms of real-time confirmation of accuracy and ease of
use, led us to design the current prospective cadaveric experimental study to validate
our clinical observations. We hypothesized that the ultrasound-guided PLA injection
of the DFTS would be just as reliable and accurate as the landmark-guided BSA in horses
with minimal DFTS effusion, thereby offering the clinician a safe and useful ultrasound-guided
approach to the DFTS.
Materials and Methods
Specimens
A cadaveric study was performed using 40 limbs from 10 horses that were euthanatized
for reasons unrelated to this study. This number of limbs was used to achieve a power
of 0.80 as calculated with a correlation value of 0 and a significance level of 0.05
using data from the previously published PLA and BSA studies.[15 ]
[16 ] The horses were of different ages, breeds and sizes with no palpable effusion or
abnormalities within the DFTS. All limbs were used fresh.
Experimental Design
A schematic of the experimental design is shown in [Fig. 1 ]. Limbs were randomly and evenly distributed between two clinicians, one board certified
American College of Veterinary Surgeons (ACVS) and American College of Veterinary
Sports Medicine and Rehabilitation (ACVSMR) senior clinician (WRR), and one second-year
resident (CRH). This resulted in 20 limbs per clinician (10 for ultrasound-guided
injection and 10 for landmark-guided injection). Based on the unrelated study from
which these horse limbs were obtained, horses were euthanatized in groups on 3 separate
days. For each day, the order of the horse and limb was randomly assigned; however,
all 20 ultrasound-guided injections were performed in a row before all 20 landmark-guided
injections were performed in a row based on the different equipment and set-up needed
for the different approaches.
Fig. 1 Schematic overview of the experimental design. DFTS, digital flexor tendon sheath.
The fetlock region of each limb was clipped laterally and palmar/plantarly with a
#40 blade. The clipped region was then cleaned and rinsed with 70% alcohol solution.
All injections were performed using a 22-gauge 1.5-inch hypodermic needle and a slip
tip 10cc syringe with 5 mL of Ioxhexol diluted with 5 mL of sterile saline. Upon completion
of the injection, the syringe and needle were removed and the limb was flexed and
extended five times to distribute the contrast medium throughout the DFTS. Two radiographic
projections were performed of each limb, the lateromedial and the dorsopalmar/plantar,
using a large plate to include the proximal and distal extent of the DFTS. The radiographs
were reviewed by both clinicians independently at the completion of the study to confirm
if the contrast was or was not within the DFTS ([Fig. 2A ] and [B ]) and/or to report if the contrast was within any other structure such as the metacarpo/metatarsophalangeal
joint or subcutaneous tissues ([Fig. 2C ]).[1 ] All limbs were dissected after completion of the radiographs to determine if there
was inadvertent needle penetration of the surrounding soft tissue structures. For
each injection, it was recorded who performed the injection, the number of the injection
within the series for that clinician, the number of attempts needed including any
redirections of the needle, if the injection was successfully within the DFTS and
if a structure other than the DFTS was penetrated.
Fig. 2 Lateromedial (A ) and the dorsopalmar/plantar (B ) radiographic views of a contrast tenogram showing a successful ultrasound-guided
digital flexor tendon sheath (DFTS) injection with diffuse accumulation of contrast
extending throughout the proximal and distal recesses of the DFTS and no identifiable
contrast outside of the DFTS. (C ) Lateromedial radiographic view of a contrast tenogram showing an unsuccessful landmark-guided
injection with inadvertent penetration of the metacarpophalangeal joint and resultant
intra-articular contrast.
Injection Techniques
Ultrasound-guided technique—PLA: To replicate clinical application in which this approach
would be used, each limb was placed in a stand to mimic a weight-bearing stance with
the limb secured proximally at the carpus/tarsus and distally at the level of the
foot/pastern. The limb was specifically positioned to allow free access to the distolateral
aspect of the metacarpus/tarsus ([Fig. 3A ]). All ultrasound-guided injections were performed with the MyLab Alpha (Esoate North
America, Inc., Fishers, Indiana, United States) using an 18 MHz linear probe. The
ultrasound probe was placed on the palmar/plantar metacarpus/metatarsus at the level
of the proximal sesamoid bones within the fetlock canal. The ultrasound probe was
then moved proximally to identify the MF and define its proximal extent which was
considered the best location for the injection. This location was previously determined
to be ideal as it is located within the proximal pouch of the DFTS and should avoid
injury to the MF upon insertion of the needle. The DDFT was then palpated laterally
in this location and the needle was inserted palmar/plantar to the neurovascular bundle
at the dorsal aspect of the DDFT parallel to the ground, or horizontal plane, at a
45° angle off perpendicular in a dorsal to palmar/plantar direction. The horizontal
plane of the needle was critical as this allowed the needle to be visualized on the
long axis view with the ultrasound transducer in the transverse plane on the palmar/plantar
aspect of the metacarpus/metatarsus ([Fig. 4 ]). The needle was visualized within the DFTS and then advanced under ultrasound guidance
until comfortably seeded into the DFTS, ideally before midline. At this time, the
syringe was connected to the needle and 5 mL of saline and 5 mL of Ioxhexol were injected
into the DFTS. The injectate was visualized real-time using ultrasound as it was distending
the DFTS with hypoechoic fluid.
Fig. 3 Cadaveric limbs illustrating the limb and needle position for the two different injection
techniques. (A ) Weight-bearing limb position for the ultrasound-guided proximolateral approach.
(B ) Mildly flexed non–weight-bearing limb position for the landmark-guided basilar sesamoidean
approach.
Fig. 4 Sagittal (A ) and inset transverse (B ) magnetic resonance images unrelated to this study but shown here to demonstrate
the trajectory of the needle (red arrow) into the digital flexor tendon sheath (DFTS;
red asterisk) using the ultrasound-guided proximolateral approach based on the anatomy
in this location. (C ) Long axis view with the ultrasound transducer in the transverse plane on the palmar/plantar
aspect of the metacarpus/metatarsus. The needle (red arrow) is inserted at the dorsal
aspect of the deep digital flexor tendon (DDFT) and palmar/plantar to the neurovascular
bundle. The 45° angle is used to decrease chances of trauma to the DDFT while staying
within the DFTS (red asterisk). SDFT, superficial digital flexor tendon.
Landmark-guided technique—BSA: The previously described BSA technique[16 ] was performed with the limb secured proximally at the carpus/tarsus to a stand allowing
a non-weight-bearing mildly flexed position with the lateral aspect accessible ([Fig. 3B ]). The needle was placed in the palpable depression under the lateral proximal sesamoid
and abaxial to the lateral border of the superficial digital flexor tendon at an angle
45° to the transverse plane in the lateromedial direction and 45° to the dorsal plane
in a distoproximal direction[16 ] and 5 mL of saline and 5 mL of Ioxhexol were injected into the DFTS.
Statistical Analysis
Associations between input factors and outcome parameters were examined using Fisher's
exact tests for all input factors with two levels and using chi-squared tests for
input factors with greater than two levels such as injection order. The number of
attempts needed for successful DFTS injection was compared between the approach groups
using a Wilcoxon rank-sum test. Input factors were then entered into a logistic regression
model to examine for the effect of a single input factor while controlling for the
other factors. All analyses were performed with SAS 9.4 (SAS Institute, Cary, North
Carolina, United States) and a p -value of ≤ 0.05 was considered significant.
Results
[Table 1 ] summarizes the results of this study for each outcome parameter assessed. The ultrasound-guided
PLA resulted in a greater number of successful injections into the DFTS than the landmark-guided
BSA (19/20 vs. 16/20 respectively) with significantly fewer attempts (median of 1
attempt vs. median of 2 attempts respectively; p = 0.03). In addition, none of the ultrasound-guided PLA injections resulted in inadvertent
penetration of the metarcarpo/metatarsophalangeal joint, while two of the landmark-guided
BSA injections did. Importantly, the ultrasound-guided PLA also resulted in significantly
less penetrations of the surrounding soft tissue structures during injection than
the landmark-guided BSA (p = 0.02). The most common structure to be penetrated in the ultrasound-guided PLA
was the DDFT followed by the proximal scutum. The most common structure to be penetrated
in the landmark-guided BSA was a sesamoidean ligament (oblique or straight) followed
by the superficial digital flexor tendon.
Table 1
Comparison of the ultrasound-guided proximolateral approach and landmark-guided basilar
sesamoidean approach for digital flexor tendon sheath injection in terms of accuracy,
ease, and inadvertent penetration of other structures
Technique
Number of successful DFTS injections
Number of attempts median (range)
Penetration of MP joint
Penetration of soft tissue structure
Ultrasound-guided PLA
19/20 (95%)
1 (1–3)
0/20 (0%)
3/20 (15%)
Landmark-guided BSA
16/20 (80%)
2 (1–4)
2/20 (10%)
11/20 (55%)
p -Value
0.34
0.03[a ]
0.49
0.02[b ]
Abbreviation: DFTS, digital flexor tendon sheath; PLA, proximolateral approach; MP,
metacarpo/metatarsophalangeal.
a Indicates significant differences between the two techniques as determined using
the Wilcoxon rank-sum test.
b Indicates significant differences between the two techniques as determined using
the Fisher's exact test.
[Table 2 ] summarizes the associations between the input factors examined and the main outcome
assessment of successful DFTS injection. Neither clinician experience (senior vs.
resident) nor injection order was associated with successful DFTS injection when examined
alone for either approach and for overall injections. The limb (front vs. hind) injected
was found to be significantly more successful with hindlimbs having a greater success
rate overall (p = 0.05). When these input factors were entered into a logistic regression model,
none of the factors were determined to have a significant effect on successful DFTS
injection.
Table 2
Association between input factors and successful digital flexor tendon sheath injection
Input factor
Ultrasound-guided PLA
Landmark-guided BSA
Overall
Senior clinician vs. resident clinician
10/10 (100%)
9/10 (90%)
7/10 (70%)
9/10 (90%)
17/20 (85%)
18/20 (90%)
p -Value
1.00
0.58
1.00
Forelimb vs.
Hindlimb
9/10 (90%)
10/10 (100)
6/10 (60%)
10/10 (100%)
15/20 (75%)
20/20 (100%)
p -Value
1.00
0.09
0.05[a ]
Injection order
No effect
No effect
No effect
p -Value
0.39
0.59
0.77
Abbreviations: BSA, basilar sesamoidean approach; DFTS, digital flexor tendon sheath;
PLA, proximolateral approach.
a Indicates a significant difference for an individual input factor as determined using
the Fisher's exact test. When these input factors were entered into a logistic regression
model, none of the factors were determined to have a significant effect on successful
DFTS injection.
Discussion
The purpose of this cadaver study was to describe a reliable ultrasound-guided PLA
for injection of the DFTS that has been utilized clinically by the authors and to
demonstrate that this technique would be as reliable and accurate when compared with
the landmark-guided BSA.[16 ] The results of this study revealed that in our hands the use of the ultrasound-guided
PLA led to a greater number of successful DFTS injections than the landmark-guided
BSA with significantly fewer attempts and significantly less penetration of the surrounding
soft tissue structures. The positive results for the ultrasound-guided PLA technique
compared with the landmark-guided BSA were seen both with an experienced senior clinician
and resident in training, suggesting ease of use.
Horses with active tenosynovitis of the DFTS, generally due to a peripheral or marginal
tendon injury, will typically have moderate DFTS effusion which makes injection of
the DFTS quite easy.[18 ] However, horses with tendon core lesions or other injuries within the DFTS may have
less apparent clinical signs and may also have minimal DFTS effusion.[18 ] Additionally, the growing use of MRI in clinical equine practice has increased the
diagnosis of soft tissue injuries while also finding injuries are often not in isolation,
but rather in combination with other injuries. The clinical relevance of some of these
injuries observed on MRI and the final diagnosis remains in question until the source
of pain can be confirmed with diagnostic analgesia. The use of diagnostic analgesia
can be performed by injecting anaesthetic in several different ways such as peri-neural
or intra-synovial which includes injection of joints, tendon sheaths and bursae. Peri-neural
analgesia is the simplest and most common approach but is the least specific. In addition,
interpretation of results can be confusing due to errors in technique (placement of
the regional block), diffusion of the local anaesthetic over time and individual variations
in nerve supply adversely influencing the effectiveness of the local anesthetic.[19 ] Utilizing intra-synovial analgesia can improve the accuracy of diagnostic analgesia
and further localize a lesion provided it is performed successfully.[20 ] While ultrasound-guided needle placement is sometimes perceived to be technically
more difficult, its use is warranted for this purpose as it has been shown in both
human and equine medicine to significantly improve the accuracy of synoviocentesis.[4 ]
[5 ]
[6 ]
[7 ]
[8 ]
[9 ]
[10 ]
[11 ]
In cases of minimal DFTS effusion and/or cases in which multiple structures are involved,
the use of ultrasound is recommended to improve accuracy. One of the noted risks of
blind or landmark-guided needle placement in the DFTS is inadvertent penetration of
the fetlock joint.[15 ]
[16 ] Entrance into the fetlock joint has been reported for both the PLA and ASA landmark-guided
techniques and becomes a more substantial risk when fetlock joint effusion is present,
especially in the palmar/plantar pouch.[15 ]
[16 ] In this study, 95% of the ultrasound-guided PLA injections were successfully within
the DFTS with no inadvertent penetration of the fetlock joint, while 80% of the landmark-guided
BSA injections were successfully within the DFTS and 10% (2/20) of the injections
penetrated the fetlock joint. It is important to note that this was seen with both
an experienced senior clinician and a resident in training, as deciding which technique
to use likely depends on the clinician's experience level and comfort level while
learning a new procedure. It is unclear why fetlock joint penetration occurred in
this study when using the landmark-guided BSA injection as this has not been previously
reported.[15 ]
[16 ] These conflicting results could be due to differences in fetlock joint effusion
between the limbs used or due to differences in author experience with the BSA.
A major advantage of ultrasound guidance is real-time visualization of the needle
moving through the soft tissues. This generally allows for very accurate placement
of the needle prior to injection. In addition, the flow of fluid and subsequent distention
of the DFTS can be visualized while actively injecting, further confirming correct
needle placement. It should also be noted that the distention that occurs following
injection into the DFTS can enhance the ultrasound examination due to the increased
volume of fluid that surrounds the soft tissue structures within the synovial space.[18 ] Fluid surrounding the flexor tendons can provide contrast and assist in identifying
tendon margin tearing and fraying. In equine practice, needle placement in the distal
limb is commonly performed free hand without the needle attached to the syringe due
to the potential of the horse to react or kick during needle placement. While using
this technique of needle placement, care must be taken to maintain the needle position
while attaching the syringe to the needle as well as while performing the injection.
Inadvertent pushing or advancing of the syringe and needle was seen commonly while
performing this study and may redirect the needle into the dorsal lining of the DFTS
or into a soft tissue structure. The use of ultrasound with real-time visualization
allows for recognition and prevention or correction of this common mistake. In this
study, ultrasound-guided PLA injections required significantly fewer attempts and
also resulted in significantly less inadvertent penetration of surrounding soft tissue
structures compared with landmark-guided BSA injections (15 vs. 55% respectively).
Neither of the previous studies evaluating the accuracy of the landmark-guided BSA
examined inadvertent penetration of surrounding soft tissue structures, but discussed
the potential for needle redirection to damage nearby soft tissue structures.[12 ]
[13 ] In the ultrasound-guided PLA to the DFTS, the goal is to place the needle proximal
to the MF, dorsal to the DDFT, and palmar/plantar to the neurovascular bundle as fully
described in the methods. Although the risk is low, the DDFT is the most likely soft
tissue structure to undergo iatrogenic trauma with this approach, particularly when
minimal to no DFTS effusion is present. In the landmark-guided BSA, the most likely
soft tissue structures to undergo iatrogenic trauma are the sesamoidean ligaments,
the superficial digital flexor tendon, and less likely the DDFT. The clinical relevance
of iatrogenic trauma to these structures is unknown but should still be considered.
A major limitation of this study was that it utilized both different techniques and
different approaches. While this was purposefully performed for the reasons stated
in the introduction, it does make interpretation of the findings difficult and in
particular relies on the previously reported inaccuracy of the landmark-guided PLA
to justify the added benefit of ultrasound guidance. Another limitation of this study
was that it was a cadaver study without the added factors of horse temperament and
possible movement. The cadaver limbs also were not fully weight-bearing despite our
best efforts to replicate a weight-bearing stance for the ultrasound-guided PLA. However,
in comparing the authors' experiences using this technique in the hospital on clinical
patients with cadaver limbs, no appreciable differences were noted. Ultimately, the
DFTS approach to be utilized is likely to be dictated by clinician comfort level,
presence or absence of DFTS effusion, and potentially the existence of a laceration,
wound or concurrent skin irritation which could compromise the sterility of a particular
approach. Therefore, having multiple accurate techniques in different locations gives
the clinician the power to make these necessary decisions. In conclusion, the ultrasound-guided
PLA is a reliable and accurate approach that should be strongly considered for synoviocentesis
of the DFTS, particularly in cases in which effusion is not present.
