Keywords
supraspinatus muscle - suprascapular nerve - ultrasonography - MRI
Introduction
Supraspinous fossa is an important location in the periscapular region, which houses
important structures such as the supraspinatus muscle and the suprascapular nerve.
Supraspinatus muscle atrophy, due to chronic tendon tear or any other reason, will
lead to concavity or depression in this region. The supraspinous fossa can be affected
by pathologies involving its contents, osseous boundary, or superficial soft tissue
covering it. In this pictorial review, we describe the detailed anatomy of the supraspinous
fossa with imaging of various pathologies affecting it. To the best of our knowledge,
an imaging review of supraspinous fossa has not been published to date.
Imaging Anatomy
The supraspinous fossa, also known as the supraspinatous fossa, is a concave bony
cavity in the posterior aspect of the scapula bordered by the spine of the scapula
inferiorly, acromion process laterally, and superior angle of the scapula cranially.
It is broader medially and narrower laterally. The medial two-thirds of this fossa
give origin to the supraspinatus muscle ([Fig. 1]). The suprascapular notch, located on the superior border of scapula, separates
it from the coracoid process. The superior transverse scapular ligament covers the
notch and attaches to the base of the coracoid process, converting the notch into
a foramen. The suprascapular notch has been classified into six types by Rengachary
et al with a completely ossified superior transverse scapular ligament noted in type
VI ([Fig. 2]).[1] The spinoglenoid notch is located between the lateral border of the scapula spine
and the glenoid and is a connection between the supraspinous fossa and the infraspinous
fossa. The spinoglenoid ligament covers the notch and converts it into foramen.[2]
Fig. 1 Graphic showing (A) supraspinous fossa anatomy and (B) the course of suprascapular nerve.
Fig. 2 Graphic showing Rengachary classification.
The suprascapular nerve passes through the suprascapular notch while coursing over
the scapula to reach the supraspinous fossa. It is accompanied by the suprascapular
artery and vein. The suprascapular nerve passes through the foramen, while the vessels
pass superficial to the ligament. The nerve then passes deep to the supraspinatus
muscle and gives off two branches to it before passing through the spinoglenoid notch
to pass into the infraspinous fossa, where it provides two terminal branches to the
infraspinatus muscle. A superior labral tear can give rise to paralabral cyst formation,
which can extend into suprascapular notch or spinoglenoid notch and can cause compression
of the suprascapular nerve. A predictable pattern of muscle denervation can be seen
depending on the location of the compression: supraspinatus and infraspinatus muscles
with compression at the suprascapular notch, while only the infraspinatus is affected
with compression at the spinoglenoid notch.[3]
The supraspinous fossa can be imaged with the help of conventional plain radiography,
ultrasonography, computed tomography (CT), or magnetic resonance imaging (MRI). Conventional
radiography can only give information about the bony outline, presence of calcification
or ossification. Ultrasonography is useful in evaluating the supraspinous fossa because
of its superficial location. It is a quick and practical modality with no radiation
exposure. CT is rarely needed and involves radiation. MRI is the best modality for
the evaluation of the musculoskeletal system because of its three-dimensional capability
and high spatial resolution, offering unparalleled tissue contrast.
Supraspinatus Muscle
The supraspinatus muscle is one of the rotator cuff muscles. It originates from the
supraspinous fossa of the scapula and inserts over the superior facet of the greater
tubercle of the humerus. Its tendon passes underneath the acromion process and blends
with the glenohumeral joint capsule. It is innervated by the suprascapular nerve.
Its functions include abduction of the arm, pulling the head of the humerus medially
toward the glenoid cavity, and preventing the head of the humerus from slipping inferiorly.
It works in coordination with the deltoid muscle during abduction of the arm.[2] Its tendon is the most frequently damaged tendon of the rotator cuff, including
acute injury or chronic degeneration. Impingement from the acromion process is often
a contributing factor. Both ultrasound and MRI can be used to evaluate the supraspinatus
muscle and tendon. Normal muscle on ultrasonography appears as a hypoechoic structure
with echogenic fibrous tissue, giving a starry sky appearance, whereas normal tendon
appears as a fibrillar echogenic structure.[4] As muscle undergoes atrophy, it becomes more echogenic with loss of pennate pattern,
and the volume also decreases. Muscle edema is relatively difficult to evaluate on
ultrasound. MRI is an excellent modality for evaluating the supraspinatus muscle and
tendon. Normal muscle on proton density-weighted sequence appears gray (intermediate
signal intensity). A T2 hyperintense signal will be seen if there is muscle edema,
whereas atrophy will give rise to a signal pattern equaling fat on the T1-weighted
sequence along with a decrease in volume ([Fig. 3]). Normal tendon appears as a hypointense structure because of tightly packed fibers.[5]
Fig. 3 Muscle magnetic resonance imaging showing (A) normal (orange star) muscle with intermediate signal intensity on proton density-weighted
sagittal sequence. Note that supraspinatus and infraspinatus muscles show increase
signal intensity suggesting edema along with volume loss (red arrows in A), and (B) T1-weighted sagittal sequence showing fatty atrophy of supraspinatus muscle (blue
arrow).
Suprascapular Nerve
The suprascapular nerve originates from the ventral rami of the C5 and C6 and courses
through the superior border of the scapula into the suprascapular notch and then spinoglenoid
notch. The suprascapular nerve provides motor innervation to the supraspinatus and
infraspinatus muscles and receives sensory innervation from the shoulder.[2] For evaluating suprascapular nerve pathologies, electromyography and nerve conduction
studies are useful. However, ultrasonography and MRI can be used to visualize pathological
entities in the soft and bony tissues surrounding the nerve, which can be useful to
surgeons. On ultrasonography, a normal nerve appears as a fascicular structure that
can be traced along its length. The suprascapular nerve can be easily identified in
the suprascapular notch and spinoglenoid notch, and as it is accompanied by the suprascapular
artery and vein, Doppler can help in localizing the nerve. On MRI, the suprascapular
nerve can be assessed on T1 or proton density or heavily T2-weighted sequence. T1-weighted
sequences are good for visualizing normal anatomy and the course of the nerve. Fatty
infiltration of muscles is also best evaluated on the T1-weighted sequence. MR neurography
involves typically heavy T2-weighted sequences, allowing the high-signal nerves to
stand out from the darker fat-suppressed background soft tissues. With a wider field
of view, MR neurography can help in evaluating the nerve along its complete course
right from its origin at the cervical spinal nerve roots ([Fig. 4]).[6]
[7]
Fig. 4 Magnetic resonance (MR) imaging showing normal suprascapular nerve in suprascapular
notch (circle in A) and spinoglenoid notch (arrow in B), and MR neurography showing
high-signal nerves (arrow in C) standing out from the darker fat-suppressed background
soft tissues.
Pathologies
The supraspinous fossa can be affected by pathologies affecting its contents as well
as its osseous boundary and superficial soft tissue covering it. [Table 1] shows the list of pathologies affecting supraspinous fossa.
Table 1
Pathologies affecting supraspinous fossa
|
1
|
Pathologies affecting supraspinatus muscles
|
(a) Muscle atrophy
(b) Delaminating tear with cyst
(c) Denervation muscle edema
(d) Parsonage–Turner's syndrome
|
|
2
|
Pathologies affecting suprascapular nerve (suprascapular notch)
|
(a) Idiopathic
(b) Space occupying lesion such as paralabral cyst
|
|
3
|
Pathologies affecting scapular body, distal clavicle, and spinous process
|
(a) Tumor
(b) Trauma
|
|
4
|
Glenohumeral pathology extending into supraspinous fossa
|
(a) Infective arthritis with extension of abscess
(b) Paralabral cyst
(c) Ganglion cyst
(d) Synovial cyst
|
|
5
|
Acromioclavicular joint pathology extending into supraspinous fossa
|
(a) Ganglion cyst
(b) Synovial cyst
|
|
6
|
Miscellaneous
|
(a) Soft tissue tumor
(b) Hemangioma
(c) Hematoma
(d) Myositis ossificans
(e) Fibromatosis
(f) Myocysticercosis
|
Supraspinatus Muscle
The supraspinatus muscle can be affected by a wide variety of pathologies ranging
from trauma, infection, and denervation to tumors. The most frequent condition affecting
the supraspinatus muscle is its involvement secondary to tendon degeneration and tear.[8] Tendon tears can range from partial thickness and partial width tears to full thickness,
complete width tears. Disuse of the affected shoulder can affect muscle bulk and health.
With chronicity, gradual atrophy of the supraspinatus muscle is a frequent finding.
Fatty atrophy of the supraspinatus muscle can be assessed using ultrasonography or
MRI. On ultrasound, the atrophic muscle will have reduced bulk and appear more echogenic
with loss of pennate pattern. Khoury et al found a good correlation between ultrasound
and MRI in the assessment of supraspinatus muscle atrophy and fatty infiltration.[9] There are different grading systems used in assessing supraspinatus atrophy on ultrasound
and MRI. On MRI, Thomazeau et al proposed a method based on calculation of occupation
ratio of supraspinatus muscle. The occupation ratio is the ratio between the cross
section of the muscle belly and that of its fossa on the Y view. A ratio between 1
and 0.6 is considered as grade I, 0.4 to 0.6 as grade II, and less than 0.4 as grade
III, with atrophy increasing from I to III.[10] Another method described by Goutallier et al is based on subjectively grading the
proportion of fat in the supraspinatus muscle on the sagittal T1-weighted images.
Using this method, grade 0 stands for normal muscle; grade 1: presence of fatty streaks;
grade 2: muscle more than fat; grade 3: fat equals muscle; and grade 4: fat more than
muscle[11] ([Fig. 5]). Zanetti et al described a tangent sign for identifying supraspinatus muscle atrophy.
On the Y view of a sagittal MRI, a normal supraspinatus muscle should cross superior
to a line drawn through the superior borders of the scapular spine and the superior
margin of the coracoid process. This finding is not present with atrophy[12] ([Fig. 6]). On ultrasound, Khoury et al proposed a method similar to the previously described
occupation ratio. Another method described by the authors on ultrasound is qualitative,
based on comparison of echogenicity of the supraspinatus with that of the trapezius
muscle.[9] Another pathology that can be secondary to tendon tear is cyst formation. It is
seen in cases of delaminating tears extending up to the myotendinous junction with
formation of intramuscular cyst.[13]
Fig. 5 Goutallier classification of fatty atrophy of muscle.
Fig. 6 Sagittal magnetic resonance imaging (Y view) showing “tangent sign” for supraspinatus
fatty atrophy; note that normal muscle (A) crosses the tangent, whereas atrophic muscle (B) does not cross it.
Denervation of the supraspinatus is generally due to involvement of suprascapular
nerve in the suprascapular notch or proximal to it. Parsonage–Turner's syndrome, an
idiopathic viral neuritis affecting the brachial plexus, can affect the suprascapular
nerve resulting in shoulder pain and weakness ([Fig. 7]).[14] Superior labral tears can give rise to paralabral cyst formation, which can extend
into the suprascapular notch causing compression of the nerve and subsequent muscle
denervation. In the acute phase, edema will be seen on T2 fat-suppressed sequences
in the denervated muscle. Chronic pathology causes muscle atrophy.
Fig. 7 Parsonage–Turner's syndrome in two different patients showing edema in supraspinatus
and infraspinatus muscles.
Suprascapular Nerve
Suprascapular nerve pathology is a rare diagnosis that is increasingly being recognized
among the conditions that cause shoulder pain and dysfunction. Along its course, a
wide variety of mechanisms can cause injury or compression. The nerve can be compressed
or injured in the suprascapular notch and the spinoglenoid notch. As described previously,
several anatomical variations of the suprascapular notch can cause compression of
the nerve. Fractures of the suprascapular notch, or scar tissue around healing distal
clavicle fracture can cause nerve entrapment symptoms. A hypertrophied inferior transverse
scapular ligament and enlarged veins have been found as predisposing factors for nerve
compression in the spinoglenoid notch leading to selective atrophy of the infraspinatus
muscle. Al-Redouan et al performed a detailed topographical study of the suprascapular
canal and organized the different types of suprascapular nerve entrapment according
to the anatomical localization within the canal. They divided suprascapular canal
as an osteofibrous canal composed of three segments: an entrance, a passage, and an
exit. They further classified the suprascapular nerve entrapment into five subtypes
(pre-entrance, entrance, passage, exit, and postexit) depending on the level.[15]
Superior labral tears can give rise to paralabral cyst formation, which can extend
into the suprascapular notch or spinoglenoid notch, causing compression of the suprascapular
nerve. If the nerve is compressed in the suprascapular notch, denervation of both
the supraspinatus and infraspinatus is seen, whereas if the compression is in the
spinoglenoid notch, selective denervation of infraspinatus is resulted ([Fig. 8]). Soft tissue and bony masses, such as lipomas and ganglion cysts, have been known
to cause nerve compression. Recently, it has been documented that there is a possible
association between suprascapular neuropathy and retracted rotator cuff tears, with
an improvement in nerve function following repair of the tear. However, other studies
have found that overlateralization of the supraspinatus and infraspinatus during a
repair can place the motor branches of the suprascapular nerve on tension. Hematoma
in the scapular region causing nerve compression has been reported.[16]
Fig. 8 Magnetic resonance imaging of the shoulder showing superior labral tear (arrow in
A) with paralabral cyst (star in A) and (B) formation extending into spinoglenoid notch causing compression of suprascapular
nerve.
Overhead activities put the shoulder in external rotation and abduction. This can
cause the supraspinatus and infraspinatus muscles to impinge on the scapular spine
compressing the suprascapular nerve in the spinoglenoid notch resulting in isolated
infraspinatus muscle weakness. Iatrogenic injury to the suprascapular nerve can also
occur in cases of distal clavicle excision and posterior shoulder surgeries. Making
a clinical diagnosis of suprascapular nerve pathology is very difficult as there are
numerous conditions around the shoulder that can mimic symptoms of suprascapular neuropathy.
A high degree of suspicion is required in overhead athletes who are at risk of suprascapular
neuropathy. Imaging can help in identifying organic causes of suprascapular neuropathy
which can help in taking management decisions. MRI is the best imaging modality and
can visualize the degree of muscle edema/atrophy, labral tear, paralabral cyst, rotator
cuff pathologies, and soft tissue masses around the shoulder joint. Nonsurgical management
generally suits patients who suffer from neuropathy due to overuse or overhead athletic
activities and when no focal compression of the nerve is identified. However, patients
with massive retracted rotator cuff tears and/or space-occupying lesions causing nerve
compressions benefit from open/arthroscopic surgeries.[17]
[18]
Pathologies Affecting Scapular Body, Distal Clavicle, and Spinous Process
Neoplastic involvement of the scapular body, distal clavicle, and spinous process
can be primary or metastatic in origin, with the latter being more common. As the
scapula is a flat bone, chondrosarcoma is the most common primary malignant tumor
in middle-aged and elderly individuals, whereas Ewing sarcoma is seen in the pediatric
age group.[19]
[20] Osteochondroma can be seen in adolescent and young adults, more frequently in diaphyseal
aclasis.[21] These tumors can extend into the supraspinous fossa ([Figs. 9] and [10]).
Fig. 9 Conventional radiograph (A) and magnetic resonance imaging (B–D) of a 66-year-old man with chondrosarcoma affecting scapular spine with extension
into supraspinous fossa.
Fig. 10 Magnetic resonance imaging in different patient showing osteochondroma (arrow in
A), nerve sheath tumor (arrow in B and C), metastasis (arrow in D and E), and lymphoma (arrow in F and G) affecting supraspinous fossa.
Scapular body fracture is rare but can be seen in high-energy trauma, such as road
traffic accidents. Scapular fractures are frequently associated with fractures in
other parts of the body. The majority of scapular fractures are extra-articular affecting
the body (most common), neck, and spine. Hematoma secondary to fracture can be seen
in the supraspinous fossa.[22]
Glenohumeral Pathology Extending into Supraspinous Fossa
Acute trauma, dislocation, or chronic repetitive trauma to the glenohumeral joint
can affect the labrum with formation of a paralabral cyst, which can extend into the
suprascapular notch, supraspinous fossa, and spinoglenoid notch with secondary effects
as already described in the preceding paragraphs. Ganglion cyst or synovial cyst can
also arise from the arthritic glenohumeral joint and extend into the supraspinous
fossa.[23] Infective arthritis affecting the glenohumeral joint can lead to periarticular abscess
formation, which can extend into the supraspinous fossa.
Acromioclavicular Joint Pathology Extending into Supraspinous Fossa
The acromioclavicular joint is frequently involved by osteoarthritis. Ganglion cysts
or synovial cysts can arise from the arthritic acromioclavicular joint and extend
into the supraspinous fossa.[24]
Miscellaneous
Benign and malignant soft tissue tumors can arise in the region of the supraspinous
fossa, similar to anywhere else in the body. The space can be affected by hemangioma,
hematoma, fibromatosis, myocysticercosis, and myositis ossificans. The most common
benign soft tissue tumor in this region is a lipoma, which is generally a well-defined
and localized lesion with signal intensity equal to fat on all the sequences ([Fig. 11]). Large size, septa, nodularity, and enhancing soft tissue component point toward
an atypical lipomatous lesion or liposarcoma. Another soft tissue neoplastic lesion
that can be seen in this region is fibromatosis, which is a locally aggressive, nonmetastasizing
fibrous lesion that shows predominantly hypointense signal on T2-weighted sequence
and, depending on cellularity, hyperintense signal and enhancement ([Fig. 12]).
Fig. 11 Magnetic resonance imaging showing lipoma (star) in supraspinous fossa following
fat signal intensity.
Fig. 12 Magnetic resonance imaging showing fibromatosis (star) in supraspinous fossa showing
heterogeneous signal intensity with areas of low signal intensity in it.
Myocysticercosis is prevalent in developing countries. The lesions are predominantly
cystic with an eccentric nodule, which can be calcified at times. Perilesional edema
or fluid can be seen depending on the stage. Myositis ossificans is another important
entity that can be mistaken for malignancy on imaging. It is secondary to trauma and,
contrary to its name, inflammation is not a feature. It is one of the skeletal “don't
touch” lesions. Typical radiographic appearances include peripheral calcification
with a lucent center. It is often separated from the adjacent bone by a radiolucent
cleft ([Fig. 13]).[25]
Fig. 13 Conventional radiograph (A) and magnetic resonance imaging (B–D) showing myositis ossificans (arrow) in supraspinous fossa.
Conclusion
The supraspinous fossa is an important location that can be affected by various pathologies.
In this pictorial review, we have discussed the detailed anatomy of the supraspinous
fossa along with imaging findings of common and uncommon pathological processes affecting
it. An awareness of the imaging findings of these entities is essential for a radiologist
to avoid misinterpretation and can aid a timely diagnosis.
