Magnetic Resonance Imaging Findings in Dogs with

Veterinary Surgery
37:631–638, 2008
Magnetic Resonance Imaging Findings in Dogs with Confirmed
Shoulder Pathology
SEAN E. MURPHY,
DVM, Diplomate ACVS,
ELIZABETH A. BALLEGEER, DVM, LISA J. FORREST,
and SUSAN L. SCHAEFER, MS, DVM, Diplomate ACVS
VMD, Diplomate ACVR,
Objective—To evaluate the diagnostic potential of magnetic resonance imaging (MRI) compared
with a reference standard, arthroscopic and/or open surgery, in dogs with soft tissue shoulder
pathology.
Study Design—Retrospective study.
Animals—Dogs (n ¼ 21).
Methods—Magnetic resonance (MR) images were retrospectively evaluated in 21 dogs that had
surgically identified soft tissue shoulder pathology. The musculotendinous units of the biceps,
infraspinatus, teres minor, supraspinatus, subscapularis, and the medial and lateral glenohumeral
ligaments (MGHL and LGHL) were graded as either normal or abnormal. Abnormal structures
were further classified as being either inflamed, partially torn, or fully torn. Impingement of the
biceps tendon was also evaluated. Results were reported in terms of agreement and concordance
between MRI findings and surgical findings. Agreement was defined as the percentage of times MRI
findings concurred with surgical findings with respect to a structure being either normal or
abnormal. Concordance was defined as the percentage of times MRI concurred with the exact
surgically assessed pathology when abnormality was identified.
Results—The findings were biceps tendon: 90% agreement with 100% concordance; subscapularis:
95% agreement with 62% concordance; MGHL: 84% agreement with 83% concordance; LGHL:
88% agreement with 100% concordance; infraspinatus: 100% both agreement and concordance;
biceps tendon impingement: 90% agreement with 100% concordance.
Conclusions—Soft tissue abnormalities of the canine shoulder were readily identified on preoperative MR images.
Clinical Relevance—MRI shows great potential as a diagnostic tool in the evaluation of canine
shoulder disease.
r Copyright 2008 by The American College of Veterinary Surgeons
active stabilizers: the supraspinatus, infraspinatus, biceps, teres minor muscles, and the subscapularis muscle
with associated MGHL pathology.3,5–8 Although the
incidence of shoulder injury has not been fully reported, a recent abstract described 385 dogs that had
422 shoulder arthroscopies where the major cause of
lameness was intra-articular in 421 (99.8%) with the
top 3 diagnoses: instability (48%), osteochondritis dessicans (25.4%), and biceps tendon/sheath disease (9.4%).9
INTRODUCTION
S
HOULDER PATHOLOGY is a common cause of
forelimb lameness in dogs. Normal shoulder joint
function is dependent on both passive and active stabilizing structures.1,2 Specific reports have associated lameness with pathology of both the passive stabilizers: the
lateral glenohumeral ligament (LGHL), medial glenohumeral ligament (MGHL), the joint capsule,1,3,4 and the
From the Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, WI.
This study was presented in part at the Advanced Canine Arthroscopy Symposium, August 2007, Naples, FL.
Supported by a grant from the Companion Animal Fund, School of Veterinary Medicine, University of Wisconsin, Madison, WI.
Address reprint requests to Susan L. Schaefer, MS, DVM, Diplomate ACVS, Department of Surgical Sciences, School of Veterinary
Medicine, University of Wisconsin, 2015 Linden Drive, Madison, WI 53706-1102. E-mail: schaefer@svm.vetmed.wisc.edu.
Submitted December 2007; Accepted March 2008
r Copyright 2008 by The American College of Veterinary Surgeons
0161-3499/08
doi:10.1111/j.1532-950X.2008.00429.x
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MRI FINDINGS IN DOGS WITH SHOULDER PATHOLOGY
Orthopedic examination, radiographs, and arthroscopy
were used as diagnostic tools. A multi-institutional abstract on shoulder pathology, which also incorporated
diagnostic ultrasound, reported the same top 3 diagnoses
with a slightly lower incidence of intra-articular pathology as the major cause for lameness, 90 of 96 reported
cases (94%).10
Determining the exact causes of shoulder lameness is
often challenging. Both an intra-articular and extraarticular assessment may be required. Preoperative diagnostic tests such as a refined shoulder examination,
radiographs, standard arthrography, ultrasound (US),
magnetic resonance imaging (MRI), and magnetic resonance (MR) arthrography may improve diagnostic competence and effect subsequent treatment options with
the ultimate goal of improving patient outcomes. MRI,
a powerful diagnostic modality, is used commonly for
diagnosis of human musculoskeletal disease providing information on intra- and extra-articular structures simultaneously with high soft tissue contrast, high-resolution
and multiplanar imaging capabilities.11
Normal musculotendinous and capsuloligamentous
structures of the canine shoulder are readily identified
by MRI in cadavers.12 The only reported clinical use of
MRI for evaluation of canine shoulder pathology has
been limited to osteochondrosis assessment and a single
case report of supraspinatus tendonopathy.13–15 Our purpose was to identify whether MRI has the potential to
play a greater role in the diagnosis of canine shoulder
disease. Our first aim was to determine whether soft tissue
abnormalities identified at surgery in clinically affected
dogs could be retrospectively identified as abnormalities
on preoperative MR images. Our second aim was to determine whether the specific types of abnormalities noted
could be distinguished from one another on MRI in these
same cases.
MATERIALS AND METHODS
Inclusion Criteria
All dogs admitted (January 2004–June 2007) for arthroscopic examination of the shoulder because of joint
pain and forelimb lameness had a preoperative MRI performed. Dogs that had a radiographic diagnosis of shoulder
osteochondritis dissecans were excluded from the study. Medical records, including all diagnostic images, were reviewed
retrospectively.
MRI Examination
Examinations were performed with the dog in lateral recumbency with the affected shoulder up. Shoulders were positioned in 100–1101 flexion and the distal aspect of the limbs
supported with a foam block to maintain the limb in a normal
stance position. A dedicated shoulder coil was placed either
around or on top of the shoulder joint depending on the
individual dog size. A 1.0 T superconducting magnet (GE
Medical Systems, Milwaukee, WI) was used for all scans.
Nineteen shoulders were scanned in 3 planes: dorsal, transverse, and sagittal.12 Two shoulders were scanned in the
transverse and sagittal planes only. A minimum of 2 sequences
were collected in each plane. Sequences used were T1-weighted
(T1), T2-weighted gradient echo (GE) and proton density fat
saturation (PD).
Direct magnetic resonance arthrography was performed
in 11 dogs using 1 mL/4.5 kg bodyweight of intra-articular
gadodiamide (Gd-DTPA-BMA; Omniscan, GE Healthcare,
Princeton, NJ). The gadodiamide was diluted before intraarticular injection with saline (0.9% NaCl) solution to 1:800 in
the first 5 dogs and 1:1200 in the final 6 dogs. After gadodiamide injection, all shoulders were re-scanned in all 3 planes
with either a T1-weighted fat saturation sequence or a PD
sequence.
Arthroscopy
Arthroscopy was performed using either a 2.7 or 2.5 mm
301 oblique arthroscope (Linvatec, Utica, NY). All procedures
were initially performed in lateral recumbency using a caudal/
lateral telescope portal with a cranial/lateral instrument portal.16 In 2 dogs, an additional arthroscopic examination from
a cranial/medial telescope portal was required. These dogs
were repositioned into dorsal recumbency and a 2.7 mm
switching stick was used to accurately transfer the arthroscope
to the cranial/medial position.
Retrieved Data
Reviewed surgical data included a detailed surgery report
(21 dogs), multiple arthroscopic still images (21), and arthroscopic video (19). Each surgery report included a completed modified shoulder assessment form evaluating biceps
tendon, intertubercular groove, subscapularis, MGHL, LGHL,
and the caudal/medial aspect of the supraspinatus insertion.10
Extra-articular structures were evaluated in dogs that had
open surgical procedures after arthroscopy. All surgical data
were reviewed and graded by the same individual (S.M.).
MRI scans were retrospectively evaluated by a radiologist
(L.F.), a senior radiology resident (E.B.), and a surgeon (S.S.).
Given that MR imaging of canine shoulder pathology is not
well described, evaluators were not blinded to the case data.
The following soft tissue structures were initially identified as
either normal or abnormal on each MRI: musculotendinous
units of the biceps, infraspinatus, teres minor, supraspinatus,
subscapularis muscles, and capsuloligamentous structures including the MGHL and LGHL. If structures were identified
as abnormal, then they were further classified as inflamed,
partially torn, or fully torn. Structures were considered inflamed when enlargement was identified, often with a heterogeneous signal intensity or generalized hyperintensity. Partial
tears were identified by minor disruptions of the tendon/
ligament silhouette. More complete tearing was identified by
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MURPHY ET AL
major disruptions of the tendon/ligament silhouette. Additionally, the relationship between the biceps tendon and
supraspinatus tendon was evaluated for impingement syndrome.15 Cases of biceps impingement were further classified
subjectively as mild, moderate, or severe based on degree of
tendon compression.
Data Analysis
Results are reported in terms of agreement and concordance between MRI findings and surgical findings. Agreement
was defined as the percentage of times MRI findings concurred with surgical findings with respect to a structure being
normal or abnormal. In structures where there was agreement
that pathology was present, concordance was defined as the
percentage of times MRI findings concurred with the exact
surgically assessed pathology.
RESULTS
Twenty dogs met the inclusion criteria. Mean ( SD)
age at admission was 4.9 2.8 years. Breeds were
Labrador Retriever (n ¼ 7), mixed breed (5), Vizsla (2),
German Shorthair Pointer (2), and 1 each of Rottweiler,
Flat Coated Retriever, American Bulldog, King Charles
Cavalier Spaniel, and Miniature Poodle. Shoulder radiographs were available for review in 20 dogs. Radiographic
findings were normal in 7 dogs. Abnormal findings were
mild degenerative disease (n ¼ 7), mineralization within
the supraspinatus tendon (3), osteophytosis of the
bicipital intertubercular groove (2), mineralization of the
infraspinatus tendon insertion (1), ununited infraglenoid
tubercle (1), lateral luxation (1), and ectopic calcification
in distal biceps tendon sheath (1).
MRI examination (21 dogs) and positive contrast MR
arthrography (11 dogs) were performed without complications. MR arthrography was considered essential in
defining pathology in 3 dogs and substantially enhanced
the pathologic lesions in 3 other dogs. After arthroscopic
surgery, 10 dogs also had open surgery for therapeutic
reasons, allowing for evaluation of additional extraarticular structures. Findings from the physical examination, shoulder arthroscopy, and/or an open surgical
procedures and biopsy yielded the following overall diagnoses: primary biceps brachii tendon pathology (n ¼ 3),
biceps brachii tendon pathology with concurrent supraspinatus tendinopathy (5), medial instability (6), medial
compartment pathology without instability (1),
osteochondroma with biceps tendon impingement (1),
lateral instability (3), infraspinatus muscle contracture
(1), and osteonecrosis of the humeral head (1). Values for
agreement and concordance for each structure are listed
in Table 1. More specific descriptions of the findings are
given in the following sections.
Table 1. Agreement and Concordance Between MRI and Surgical
Findings in 21 Dogs
Structure
No. in
No. in
Agreement/No. % Agree- Concordance/ % ConcorExamined
ment
No. Examined
dance
Biceps
Subscapularis
MGHL
LGHL
Infraspinatus
Impingement
19/21
20/21
16/19
7/8
4/4
19/21
90
95
84
88
100
90
16/16
8/13
10/12
3/3
4/4
5/5
100
62
83
100
100
100
Total
85/94
90
46/53
87
All structures listed had both an MRI evaluation and a surgical assessment conducted. The number examined varied between structures as
not all tendons and ligaments were seen at every surgery.
MRI, magnetic resonance imaging; MGHL, medial glenohumeral
ligaments; LGHL, lateral glenohumeral ligaments.
Intra-Articular Pathology
Biceps Brachii Tendon. Biceps brachii tendon pathology was observed arthroscopically in 16 dogs. Primary
inflammation was noted in 3 dogs. Five dogs had biceps
inflammation and/or tendon disruption because of impingement by an enlarged and/or calcified supraspinatus
tendon (Fig 1). The remaining dogs had biceps tendon
inflammation (6), or partial tearing (2) secondary to other
pathologies including instability, osteochondroma, and
humeral head osteonecrosis. All 21 biceps tendons could
be readily identified with MRI. Agreement occurred in 19
of 21 dogs. In the 2 dogs without agreement, the MRI
was interpreted as mild inflammation in the biceps tendon that could not be confirmed surgically. Concordance
occurred in 16 of 16 dogs with documented surgical
pathology. On MRI examination, inflammation of the
biceps tendon appeared as an enlarged tendon with heterogeneous signal intensity on all 3 sequences, and as an
increase in signal intensity on GE and PD images. Figure
2 illustrates an MR arthrogram and arthroscopic images
of a partially avulsed biceps tendon. The diagnostic image of partial tendon avulsion was enhanced with MR
arthrography.
Subscapularis Tendon. Subscapularis tendon pathology was identified surgically in 13 dogs, with a full tendon
avulsion in 1, partial tearing in 10, and tendon inflammation in 2 dogs. Arthroscopic examination was limited
to the tendon insertion and the distal aspect of the tendon
whereas the complete musculotendinous unit could be
evaluated on all 21 MRI examinations. Agreement
between surgical and MRI findings occurred in 20 of 21
dogs. In the 1 dog without agreement, changes thought to
be within the cranial aspect of the MGHL on MRI were,
in fact, within the overlaying subscapularis tendon at
surgery. Concordance occurred in 8 of 13 abnormal
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MRI FINDINGS IN DOGS WITH SHOULDER PATHOLOGY
Fig 1. Impingement of a biceps tendon by an enlarged supraspinatus tendon (ST). (A) Arthroscopic image of a cranial/lateral
compartment of a left shoulder joint showing an enlarged ST impinging on the biceps tendon (BT). Note the associated biceps
tendonitis at the site of contact. (B) Sagittal gradient echo (GE) magnetic resonance imaging (MRI) of the same case with a grossly
enlarged ST (blue arrowheads) pressing down on the biceps tendon (white arrow). Magic angle artifact is identified by the red arrow.
(C) Sagittal GE MRI of a normal ST (white arrowheads) and biceps tendon (white arrow). Sc, scapula; H, humeral head.
joints. In 4 abnormal joints without concordance, the
MRI diagnosis was inflammation whereas partial tearing
was identified surgically. In the fifth joint without concordance, a lesion in the subscapularis tendon was interpreted as being in the overlaying MGHL as noted earlier.
On MRI examination, subscapularis tendon pathology
appeared as heterogenous signal intensity on PD and GE
sequences with enlargement of the tendon in cases of inflammation and fiber disruption, particularly along the
tendon border in cases of tearing (Fig 3). MR arthrography further enhanced the finding of irregular tendon
borders in cases with partial tearing and helped to confirm the single case of full tendon avulsion by highlighting
the void between tendon and bone.
Medial Glenohumeral Ligament. MGHL pathology
was identified surgically in 12 dogs. Three dogs had inflammation of the capsuloligamentous region, 6 had partial tearing of the ligament, and 3 had complete or near
complete tearing of the ligament. The MGHL could be
identified on 19 MRI studies. Two early MRI studies did
not include the dorsal plane, which prevented evaluation
of MGHL. Agreement occurred in 16 of 19 dogs. In 2
dogs without agreement, MRI findings were considered
normal when mild inflammation and partial tearing were
surgically identified, respectively. The dog with the undiagnosed partial tear was a 4.5 kg Miniature Poodle. In
the third dog without agreement, MRI findings were
suggestive of inflammation but this was not observed
surgically. Concordance occurred in 10 of 12 abnormal
dogs. MRI findings consisted of capsuloligamentous
thickening in inflammatory cases with heterogeneous signal intensity on both PD and GE sequences. Partial tears
were identified by minor disruptions of the tendon silhouette. More complete tearing was seen as the major
disruption of the tendon silhouette often with an irregular, undulating pattern on positive contrast MR arthrography (Fig 4).
Lateral Glenohumeral Ligament. The LGHL was
observed arthroscopically in 8 dogs. Pathology was present in 3 dogs with partial tearing of the ligament in
2 dogs, and full tearing with lateral humeral head luxation in 1 dog. On MRI examination, the LGHL was
identified in all the 8 dogs. Agreement occurred in 7 of
8 dogs. In the 1 dog without agreement, the MRI diagnosis of inflammation could not be confirmed surgically.
Concordance occurred in 3 of 3 abnormal dogs. MRI
findings consisted of capsuloligamentous thickening in
inflammatory cases with concurrent ligament/joint
Fig 2. Partial avulsion of a biceps tendon. (A) Arthroscopic image of the cranial/medial compartment of a left shoulder joint
showing a partial avulsed biceps tendon (arrowhead). The remaining tendon, identified by the yellow arrow, is still attached intact.
(B) Sagittal proton density fat saturation (PD) magnetic resonance (MR) arthrogram with enlargement of the biceps tendon and a
filling defect evident along the scapular attachment site (blue arrowheads). (C) Normal sagittal PD MR arthrogram for comparison
with the white arrow at the biceps tendon insertion. Sc, scapula; H, humeral head.
MURPHY ET AL
635
Fig 3. Enlargement and partial tearing of the subscapularis tendon. (A) Arthroscopic view of the medial compartment of a left
shoulder joint showing disruption of the normal subscapularis fiber pattern (blue arrowheads). Mild fraying of the medial glenohumeral ligament (MGHL) is also evident. (B) Dorsal proton density fat saturation (PD) magnetic resonance imaging (MRI)
showing gross enlargement, heterogeneity, and loss of the finite silhouette of the subscapularis tendon (blue arrowheads). Sc,
scapula; H, humeral head. (C) Dorsal PD MRI of a normal shoulder. Note the size and homogeneity of the subscapularis tendon
(white arrows). Both (B) and (C) MR images were prearthrogram.
capsule disruption in partial or full tears usually at or
near the scapular attachment (Fig 5). With MR arthrography, joint distension further enhanced observation of
the ligament/capsular disruption.
Supraspinatus Tendon. The caudal/medial aspect of
the supraspinatus insertion was assessed arthroscopically;
although this tendon is not intra-articular, enlargement
of this tendon can be identified on arthroscopy. Primary
tendon enlargement, causing compression or impingement of the biceps tendon was identified surgically in
5 dogs; 2 dogs were considered severe, 1 moderate, and
2 mild. When evaluating solely for arthroscopically evident tendon enlargement/impingement, agreement occurred in 19 of 21 dogs. In the 2 dogs without agreement,
the MRI diagnosis of mild impingement could not be
confirmed at surgery. Concordance occurred in 5 of
5 dogs. Supraspinatus pathology other than impingement
was noted on MRI in 8 dogs. These changes were confirmed with open surgery in only 1 dog. The diagnosis in
these 8 dogs included dystrophic mineralization and
Fig 4. Severe tearing of a medial glenohumeral ligament (MGHL). Arthroscopic images of the (A) cranial/medial, (B) medial,
and (C) caudal/medial compartments of a left shoulder of a dog with moderate disruption of the cranial (Cr) aspect of the ligament
and complete tearing of the caudal (Cd) aspect of the ligament. (D) Dorsal gradient echo (GE) magnetic resonance imaging (MRI)
showing enlargement and heterogeneity of the MGHL (blue arrowhead). Also note enlargement and heterogeneity of the infraspinatus tendon (white ) and hyperintensity of a portion of the infraspinatus muscle (black ). Sc, scapula; H, humeral head.
(E) Dorsal T1 magnetic resonance (MR) arthrogram of the same shoulder showing enlargement, heterogeneity, and an undulating
pattern to the MGHL (red arrowhead). (F) Dorsal T1 MR arthrogram of a normal shoulder. The white arrow identifies a normal
MGHL.
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MRI FINDINGS IN DOGS WITH SHOULDER PATHOLOGY
Fig 5. Tearing of the lateral glenohumeral ligament (LGHL). (A) Arthroscopic image of the cranial/lateral compartment of a
right shoulder as seen from a caudal lateral portal. The arrowhead identifies frayed portions of the LGHL. (B) Dorsal proton
density fat saturation (PD) magnetic resonance imaging (MRI) showing distension of the lateral joint capsule with disruption of the
LGHL (blue arrowheads). Also note the hyperintensity surrounding the infraspinatus tendon (white ) and within the bicipital
sheath (black ). Sc, scapula; H, humeral head. (C) Dorsal PD MRI of a normal shoulder. The white arrow identifies a normal
LGHL.
tendon enlargement that was not evident arthroscopically. MRI findings for supraspinatus pathology were heterogeneous signal intensity on all 3 sequences and tendon
enlargement with intermixed hypointensity if mineralization was present (Fig 1). On positive contrast arthrography, a lack of contrast filling between the biceps and
the supraspinatus tendon on the sagittal view was observed when there was impingement along with medial
displacement of the tendon on transverse images.
Extra-Articular Pathology
Infraspinatus. Infraspinatus pathology was identified
on MRI in 4 dogs. All dogs had an open surgical procedure. These procedures were tenotomy for contracture
(1), debulking of tendon mineralization that impinged on
the acromion with limb abduction (1), and medial and
lateral joint stabilization (2). In 3 of these dogs, the tendon was directly observed and MRI findings had 100%
agreement and concordance with the surgical findings.
Specific MRI findings for contracture were heterogeneous
signal intensity within the muscle and at the muscle–
tendinous junction with minimal extension into the tendon, best appreciated on the sagittal imaging planes.
Acute injury appeared as an enlarged tendon with
heterogeneous signal intensity and a hyperintensity
within the muscle on GE or PD sequences because of
suspected edema (Fig 4).
Teres Minor. The teres minor was identified on all 21
MRI examinations. MRI-assessed pathology included
muscle inflammation in 2 dogs, both of which had medial
instability. Inflammation appeared as an enlargement
with a hyperintensity on GE or PD sequences; however,
no open or arthroscopic procedure was used to validate
these findings. The teres minor was noted to be normal in
1 open procedure and MRI results were in agreement
with this finding.
DISCUSSION
In 21 dogs that had surgical exploration for shoulder
pain, we found a high level of agreement and concordance between MRI and surgical findings. Of the 94
structures examined at surgery, there was 90% agreement
between MRI and surgical findings, and overall concordance for 53 abnormal structures was 87%. Although we
did not examine the direct relationship between diagnostic MRI and therapeutic outcome, our results show that
preoperative MRI can define normal and abnormal soft
tissue structures in the shoulder joint of dogs with lameness attributable to the shoulder region. The effect of
preoperative MRI on treatment modification in canine
shoulder lameness is unknown. By comparison, in a
meta-analysis (265 papers) of the efficacy of MRI of the
human shoulder, MRI altered the primary diagnosis in
23–68% of cases and management plans were subsequently modified in 15–61% of cases.17
When considering the clinical application of a new
diagnostic tool, concern should be raised for the potential
to misinterpret the information obtained. For biceps tendon inflammation, MRI findings were over interpreted in
2 dogs; in both cases, dogs were considered to have mild
inflammatory changes on MRI. These changes were either incorrectly identified or so mild as to remain within
the body of the tendon and thus were not evident during
arthroscopy. Over interpretation was also noted in 1 dog
with MGHL inflammation, 1 dog with LGHL inflammation, and 2 dogs with biceps tendon impingement.
Discrepancies for the diagnosis of impingement between
MRI and arthroscopy may be a factor of the reference
standard. In our study, the ‘‘gold standard’’ for impingement classification was arthroscopic examination; however, joint distension during arthroscopy may change the
spatial relationship between these structures creating a
false negative on examination.
MURPHY ET AL
Although abnormalities in the MGHL and the subscapularis tendon were easily identified on MRI, there was
a tendency to under diagnose the severity of the lesion in
these structures. In 1 dog where a partial tear was not
identified in the subscapularis tendon, the changes seen
on MRI were attributed to pathology in the overlying
MGHL. These structures are in close proximity and a
lack of appropriate joint distension during arthrography
may lead to misinterpretation of lesion location. Evaluation of MR images can also be challenging in smaller
dogs. A partial tear of the MGHL was not identified by
MRI in a toy poodle; however, a subscapular tendon
avulsion was correctly identified in this same dog.
Caution should also be used with interpretation of
MRI of the supraspinatus tendon because there is a tendency for it to appear hyperintense in both normal and
abnormal cases on PD and GE images. The cause of this
hyperintensity is unknown but may be because of significantly higher collagen content at the insertion relative
to the infraspinatus and subscapularis tendons.18 The role
that supraspinatus tendon disease may play in thoracic
limb lameness is also debatable because supraspinatus
tendon calcification and tendonosis have been identified
in dogs without clinical signs.5,19 Caution should also be
used when interpreting changes in the biceps tendon as
creation of a magic angle artifact is common. The magic
angle artifact occurs as the tendon crosses the intertubercular groove and the tendon fibers approach a 551
angle to the static magnetic field. An increase in signal is
appreciated on T2-weighted images with a low echo time
and should not be misinterpreted as pathology (Fig 1).11
MR Arthrography
Direct MR arthrography is used commonly in people
to determine the integrity of intra-articular ligamentous
and fibrocartilaginous structures.20 In the only clinical
report of direct MR arthrography of the canine shoulder,
the technique was considered unrewarding for evaluation
of osteochondrosis lesions. It should be noted that the
gadopentetate dimeflumine used as a contrast agent in
that study was used undiluted. Dilution of the contrast
agent may be necessary to prevent the hyperintensity of
the agent from obscuring subtle lesions on the images. In
a cadaver study, a dilution of 1:1200 was optimal for
direct MR arthrography of the canine shoulder.21 We
found that MR arthrography was helpful for identifying
tendon avulsion (partial or full) and severe tendon/ligament tears. Arthrography was also useful in defining the
potential space between the biceps and supraspinatus
tendons when impingement was suspected. Our greatest
challenge with arthrography was achieving consistent
filling of the medial compartment.
637
All dogs in our study had more than 1 abnormality in
the shoulder joint as identified by both surgery and MRI;
many had multiple pathologic changes. For instance,
subscapularis tendon pathology occurred in 57% of
cases. Whereas 1 dog had a primary tendon avulsion,
tearing of the cranial aspect of the subscapularis tendon
was noted in all 5 dogs diagnosed with MGHL-related
medial instability. These findings were similar to a report
by Cook et al where subscapularis tendon tears were
noted in 86% of 43 dogs with medial instability.3 In our
study, subscapularis tendon pathology was also present
with biceps tendonitis (2), osteonecrosis (1), infraspinatus
contracture (1), medial compartment pathology (1), and
lateral instability (1). The relationship of subscapularis
pathology with causation of lameness or clinical outcome
has not been determined.
Extra-Articular Pathology
MRI permitted more complete evaluation of extraarticular pathology than arthroscopic surgery alone. The
primary pathologic lesion was considered extra-articular
in 15% of our dogs, which is much higher than previously
reported. When cases of joint fracture and OCD were
eliminated, Bardet identified only 0.4% cases with an
extra-articular diagnosis whereas Cook reported 9% with
extra-articular tendonopathy.9,10 It is important to note
that musculoskeletal ultrasonography was used in the
second study. Ultrasonography is useful in evaluating
extra-articular structures on the lateral and cranial/
medial aspect of the shoulder joint22; however, ultrasonography is limited in its ability to completely evaluate
the medial compartment. We identified secondary extraarticular pathology of the supraspinatus, infraspinatus,
subscapularis, or teres minor muscles in conjunction with
primary intra-articular injury in 12 of 21 dogs. Injury to
more than one structure, as seen in Fig 4, illustrates the
complexity of the shoulder joint and the challenge in
determining both the exact cause of pain/lameness and
appropriate therapeutic modality.
Study Limitations
Our study is limited by its retrospective, nonblinded
nature, and small sample size. Even though human trials
of MRI-assessed rotator cuff pathology had little change
when radiologists were unblinded and provided with
arthroscopy results,23 we chose not to calculate predictive
values as we felt our protocol was inadequate for validation. Our study was not blinded because of a lack
of publications describing MRI findings in canine shoulder disease and therefore lack of current knowledge in
interpreting canine shoulder MRI. Histopathology of
tendon and ligament pathology may have helped to more
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MRI FINDINGS IN DOGS WITH SHOULDER PATHOLOGY
accurately understand and correlate MRI findings with
the surgically documented changes.
This study demonstrates the potential of MRI in the
field of canine musculoskeletal disease. MR imaging allows for an accurate assessment of both intra- and extraarticular joint pathology. Future studies are needed to
determine how this additional diagnostic information
should influence therapeutic choices and modalities.
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