DEFINITION

Originally described in 1936 by Thomas18 and further defined by Thompson and Kopell19 in 1959, suprascapular nerve (SSN) entrapment is an increasingly recognized cause for shoulder pain, weakness, and atrophy of the supraspinatus and infraspinatus muscle.

SSN entrapment typically occurs at the suprascapular or spinoglenoid notch and presents with symptoms ranging from diffuse shoulder pain to weakness and atrophy of the supraspinatus and infraspinatus muscles.

Injury to the SSN is frequently caused by repetitive stretch and microtrauma in the overhead athlete, compression due to mass effect from labral cysts or other masses, and, more recently, SSN compression has been linked to massive retracted rotator cuff tears.

 

 

ANATOMY

 

The SSN arises from the upper trunk of the brachial plexus, with contributions from C5 and C6 (rarely also C4). It supplies motor innervation to the supraspinatus and infraspinatus muscles and also, provides sensory fibers to the coracoacromial (CA) ligament, acromioclavicular, and glenohumeral joints. In 15% of population, it

contains cutaneous fibers, which innervate the lateral aspect of the shoulder.8

 

The nerve traverses two potential compression points, at the suprascapular notch and spinoglenoid notch (FIG 1), and is accompanied by the suprascapular artery and vein.

 

As the nerve leaves the brachial plexus, it travels posteriorly to the clavicle to the suprascapular notch. It travels through the suprascapular notch, under the transverse scapular ligament and into the supraspinous fossa where it supplies motor innervation to the supraspinatus muscle.

 

Its accompanying artery and vein typically course above the ligament; however, in some cases, a branch of the main vessels may accompany the SSN through the suprascapular notch.

 

The suprascapular notch is a fibro-osseous canal, which lies at the medial base of the coracoid. The notch is approximately 4.5 cm medial to the posterolateral corner of the acromion and 3 cm medial to the glenoid rim

(supraglenoid tubercle).4 The anatomy of the suprascapular notch may be highly variable.

 

Continuing on its path to innervate the infraspinatus, the SSN, accompanied by the artery and vein, traverses the spinoglenoid notch to reach the infraspinous fossa.

 

The spinoglenoid notch is approximately 1.8 cm medial to the glenoid rim and 2.5 cm inferomedial to the supraglenoid tubercle.4

 

Several anatomic studies have described the presence of a spinoglenoid ligament (inferior transverse

scapular ligament) at the spinoglenoid notch. The incidence of spinoglenoid ligament has been reported to range from 16% to 100%, and its role in nerve entrapment is controversial.6,7,10

PATHOGENESIS

 

Focal nerve entrapment is most common at the suprascapular notch where the nerve is relatively confined and has limited mobility.

 

The restricted mobility of the nerve at the suprascapular notch predisposes it to various traction injuries, as can be seen with acute trauma or with repetitive overhead activities in throwing athletes, volleyball or tennis players, or with weightlifting.

 

Compression from labral ganglions can also occur, typically at the spinoglenoid notch.1,3 These cysts can develop as the result of labral tears that allow fluid extravasation but block backflow, similar to a one-way valve.

 

 

 

 

FIG 1 • Anatomy of the SSN. The SSN is accompanied by the suprascapular artery and vein, which course

over the transverse scapular ligament (TSL), while the nerve passes underneath. All three then traverse the spinoglenoid notch.

 

 

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More recently, traction injury to the nerve has been described as the result of massive, retracted tears of the posterosuperior rotator cuff.2,11

 

Direct or indirect trauma leading to SSN neuropathy has been described as the result of shoulder dislocation, proximal humerus fracture, or scapular fracture.

 

Iatrogenic injury to the SSN can occur during distal clavicle resection, positioning during spine surgery, transglenoid drilling for instability repair, shoulder arthrodesis, or the posterior approach to the glenohumeral joint.

 

NATURAL HISTORY

 

The natural history depends on the presence or absence of a space-occupying lesion as the cause of SSN neuropathy.

 

 

Without compression by a mass, most patients will improve with time and supervised physical therapy.12

 

Conversely, the presence of a mass, such as a cyst or ganglion, usually results in failure of conservative management and will require decompressive surgery.

 

The natural history of periarticular ganglion cysts in the shoulder is controversial, but they are thought to

persist and enlarge with time.15 In rare instances, spontaneous resolution of ganglion cysts has been documented.

 

PATIENT HISTORY AND PHYSICAL FINDINGS

 

SSN neuropathy secondary to compression at the suprascapular notch typically presents as a dull pain in the posterior and lateral shoulder, but the pain can also be referred to the anterior chest wall, lateral arm, and ipsilateral neck. Compression at the spinoglenoid notch is often comparatively pain-free and presents with isolated infraspinatus atrophy (FIG 2).

 

The patient often provides a history of acute or repetitive trauma to the shoulder, such as a fall on the outstretched hand, or repetitive overhead activities, such as throwing, volleyball, tennis, or weightlifting.

 

There appears to be an increased incidence of isolated infraspinatus atrophy in asymptomatic volleyball players. This typically responds well to conservative measures.

 

 

 

FIG 2 • Posterior photograph of a patient with right infraspinatus muscle wasting secondary to SSN entrapment at the spinoglenoid notch.

 

 

Depending on the chronicity and degree of compression, varying amounts of weakness in abduction and external rotation can be detected on physical examination.

 

 

In long-standing compression, atrophy of the supraspinatus and infraspinatus can be observed.

 

Atrophy, if present, may assist in differentiating compression at the suprascapular notch from that at the spinoglenoid level because supraspinatus atrophy occurs only with the former.

 

 

Palpation of the spinoglenoid notch and cross-body adduction may reproduce the patient's symptoms. The SSN stretch test may reproduce pain posteriorly. In this test, the clinician laterally rotates the patient's head away from the affected shoulder while simultaneously gently retracting the affected shoulder.9

 

It is important to exclude other potential sources of pain, such as the cervical spine, acromioclavicular joint, or rotator cuff.14

 

History of spinal or shoulder surgery and presence of surgical incisions should raise suspicion for possible iatrogenic injuries of the SSN.

IMAGING AND OTHER DIAGNOSTIC STUDIES

 

Stryker notch views, or anteroposterior radiographs of the scapula, with a 15- to 30-degrees caudally directed beam, provide visualization of the suprascapular notch. Alternatively, a computed tomography (CT) scan can provide good osseous detail in cases of posttraumatic deformity or ossification of the transverse scapular ligament.

 

Magnetic resonance imaging (MRI) can reveal labral tears and the presence of a perilabral ganglion in the area of the suprascapular or spinoglenoid notch (FIG 3). Ganglion cysts present as homogeneous masses with low signal intensity

 

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on T1-weighted images and high signal intensity on T2-weighted images.

 

 

 

 

FIG 3 • T2-weighted magnetic resonance (MR) images depicting axial (A) and oblique (B) sagittal views showing a cyst in the area of the spinoglenoid notch.

 

 

Electromyography (EMG )and nerve conduction velocity (NCV) studies can often provide a conclusive

diagnosis by showing denervation potentials, fibrillations, spontaneous activity, and prolonged motor latencies

in the supraspinatus or infraspinatus, depending on the level of entrapment.17 However, sensitivity and specificity of EMG and NCV are dependent on the operator and interpreter.

 

An anesthetic injection into the suprascapular notch can be diagnostic if it results in complete but transient pain relief.

 

DIFFERENTIAL DIAGNOSIS

Cervical radiculopathy Glenohumeral instability Rotator cuff pathology Acromioclavicular joint arthrosis Parsonage-Turner syndrome

 

 

NONOPERATIVE MANAGEMENT

 

Initial treatment for SSN neuropathy in the absence of a space-occupying lesion is conservative and will lead to nearcomplete resolution of symptoms in most cases.

 

Supervised physical therapy, followed by a self-directed home exercise program, should consist of range-of-motion exercises as well as strengthening of the rotator cuff muscles, the deltoid, and the periscapular musculature, including the trapezius, rhomboids, and serratus musculature. Restoring proper scapular function is beneficial in recovery and may prevent recurrence of the injury.

 

Length of nonoperative management should be individualized and dependent on duration and severity of symptoms. Complete resolution of pain and weakness can often take up to more than 1 year.

 

In those with compression due to perilabral cysts, CT-or ultrasound-guided cyst aspiration has shown success in about half of patients, with persistence or recurrence in the other half.15,20

SURGICAL MANAGEMENT

 

Surgical treatment is indicated in patients who have failed to respond to 6 months of nonoperative measures and continue to have significant pain and dysfunction. Earlier intervention may be considered in younger, more active patients, or those with significant muscle atrophy or weakness.

 

SSN neuropathy secondary to a mass is best treated with decompression and evaluation and potential repair of the glenoid labrum. In these cases, some surgeons advocate early surgical intervention to prevent further injury to the nerve.

 

Other sources for shoulder pain and dysfunction should be ruled out if the mass is smaller than 1 cm in diameter or is not directly compressing the neurovascular bundle.

 

Preoperative Planning

 

Oblique sagittal MRI allows visualization of the SSN in the supraspinatus fossa, the spinoglenoid notch, and the infraspinatus fossa.

 

If a space-occupying lesion is present, this imaging will assist in preoperative planning by delineating the exact position of the mass and determining whether it is confined to the supraspinatus or the infraspinatus fossa or

involves both areas.

 

A paralabral ganglion or cyst that is confined to one area, especially when associated with a labral tear or other intra-articular pathology, is often amenable to arthroscopic decompression.

 

It is useful to initially perform a diagnostic shoulder arthroscopy and treat intra-articular pathology, followed by arthroscopic or open decompression of the SSN.

 

Arthroscopic decompression has the potential advantages of treating associated intra-articular lesions, such as labral tears and avoiding the morbidity associated with open procedures.

 

Positioning

 

Either the beach chair or the lateral decubitus position can be used.

 

TECHNIQUE

• Arthroscopic Decompression

Approach to the Suprascapular Notch

 

Routine diagnostic shoulder arthroscopy is performed to assess concomitant pathology. The superior labrum is assessed with particular consideration to look for tears, which are frequently associated with spinoglenoid notch cysts.

 

The subacromial bursa is resected with a shaver, extending more medially than what is usual for subacromial decompression.

 

The bursectomy should allow adequate visualization from the acromioclavicular joint and coracoid anteriorly to the scapular spine posteriorly. The CA ligament is identified and followed medially to the base of the coracoid.

 

Viewing from the lateral portal, the coracoid is palpated with a probe or switching stick through the anterior portal, which can also be used to bluntly dissect the surrounding soft tissues to expose the coracoclavicular ligaments. Posteriorly, the muscle belly of the supraspinatus is visualized. Careful dissection medially will allow exposure of the conoid ligament. The transverse scapular ligament can be palpated at the posterior base of the conoid ligament.

 

Alternatively, the ligaments can be found approximately 15 mm medial to the acromioclavicular joint and then followed inferiorly to their insertion on the coracoid.

 

The conoid ligament is followed inferiorly to the base of the coracoid to find the suprascapular notch medially. The fibers of the conoid ligament are in continuity with the transverse scapular ligament.

 

The suprascapular notch is typically covered by the supraspinatus muscle and fat, complicating visualization of the neurovascular bundle (TECH FIG 1A).

 

An accessory portal is created approximately 2 cm medial to the standard Neviaser portal along a line that bisects the angle formed between the clavicle and spine of the scapula. An 18-gauge spinal needle helps with correct positioning of the portal.

 

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TECH FIG 1 • Schematic and arthroscopic images showing arthroscopic SSN release at the suprascapular notch. A. After soft tissue removal, the nerve (N) can be visualized underneath the superior transverse scapular ligament (STSL). A blunt trocar is retracting the overlying vessel. B. The arthroscope is positioned in the lateral portal, and the switching stick, which is introduced through the accessory Neviaser portal, is used to retract the suprascapular vessels. Using a spinal needle for localization, a working portal is established in the standard Neviaser position. C. Arthroscopic scissors positioned to cut the STSL. D. After confirming that the nerve is well protected, the overlying ligament is cut with the arthroscopic scissors introduced through the standard Neviaser working portal, as far lateral as possible. E. Residual compression from bony structures can be removed with arthroscopic shaver or burr. F,G. The ligament has been released. A, artery. (A,C,F: Courtesy of Dr. Laurence Higgins.)

 

 

Use of a switching stick or smooth trocar through the accessory Neviaser portal allows careful, blunt dissection of the fat to expose the suprascapular vessels coursing over the transverse scapular ligament, which presents as glistening white fibers.

 

A working portal is then established in the standard Neviaser position using a spinal needle for

localization. Additional blunt dissection is carried out until adequate visualization is achieved. Care should be taken to avoid bleeding or injury to branches of the suprascapular artery (TECH FIG 1B).

 

After confirming that the nerve is well protected using a probe or small trocar, the overlying ligament is cut with the arthroscopic scissors introduced through the standard Neviaser

 

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working portal, as far lateral as possible (TECH FIG 1C,D). If ossification of the ligament is present, or if the roof of the notch is bridged by bone, a small osteotome or Kerrison rongeur may be used.

 

The SSN is probed to ensure adequate decompression; any residual compression from the bony structures can be removed with the arthroscopic burr (TECH FIG 1E-G).

Approach to the Spinoglenoid Notch

 

As part of preoperative planning, it is important to be aware of the ganglion cyst's location relative to the glenoid on the MRI. Cysts associated with labral tears are most commonly located at the spinoglenoid notch and extend into the infraspinatus fossa.

 

With an intact labrum, a small capsulotomy is made just above the superoposterior labrum, beginning posteriorly to the biceps root and extending posteriorly for 2 to 3 cm.

 

After incising the capsule, the fibrous raphe between the supraspinatus and infraspinatus seen laterally to the spinoglenoid notch provides a useful landmark.

 

The spinoglenoid notch can be palpated with an arthroscopic instrument, providing a bony landmark that can be correlated with the cyst position as seen on preoperative MRI.

 

An accessory anterolateral portal is placed after first establishing correct orientation with an 18-gauge spinal needle, and a switching stick is introduced (TECH FIG 2A).

 

 

 

TECH FIG 2 • Intraoperative photograph demonstrating approach to spinoglenoid notch. A. An accessory anterolateral portal is placed, and a switching stick is introduced. B. Decompression of the cyst can be visually confirmed by egress of cloudy fluid.

 

 

The spinoglenoid notch and the cyst can be palpated with a switching stick. The SSN is positioned medially, in direct contact with the bone of the spinoglenoid notch; the vascular structures are positioned laterally and closer to the glenoid.

 

Ganglion cysts are typically located posteriorly to the nerve and should be removed completely, including the lining.

 

To inspect the capsule from another angle and for further decompression, the arthroscope is switched to

the anterolateral portal, and the shaver and switching stick is introduced through posterior portal.

 

Decompression of the cyst can be visually confirmed by egress of cloudy fluid that is typically encountered (TECH FIG 2B).

 

Unlike open release of the transverse scapular ligament, arthroscopic decompression of the spinoglenoid notch does not necessarily require direct visualization of the SSN. However, when using the shaver, it is important to remember that the SSN lies 1.5 cm medial to the glenoid rim—in direct contact with the bone of the spinoglenoid notch—and can be injured if débridement is carried too medially.

 

Similar to open decompression, fibrovascular tissue covers the neurovascular bundle and has to be bluntly dissected with a switching stick or similar tool through the accessory portal before the nerve can be visualized.

 

 

After cyst removal, the nerve should be inspected for any additional sites of compression. Labral pathology is addressed after complete decompression.

• Open Decompression

Approach to the Suprascapular Notch

 

Decompression of the SSN at the suprascapular notch is best achieved through a trapezius-splitting approach.

 

The anterior approach requires a more complex dissection and therefore carries a higher risk of neurovascular complications. It also offers incomplete visualization of the SSN posterior to the notch and is generally not recommended.

 

A saber-type skin incision following the Langer lines is performed over the top of the shoulder. The incision begins posteriorly at the distal third of the scapular spine and extends anteriorly to a point 2 cm medially off the acromioclavicular joint (TECH FIG 3A).

 

Alternatively, a transverse skin incision parallel to the scapular spine can be chosen instead but produces a less cosmetic scar.

 

The trapezius fascia is incised with electrocautery, and the muscle is bluntly divided in line with its fibers for a distance of 5 cm until the supraspinatus muscle is encountered.

 

Abduction of the arm decreases tension on the trapezius, which if necessary can be elevated off the scapular spine for an extensile exposure.

 

The supraspinatus muscle is bluntly dissected off the anterior aspect of the suprascapular fossa and retracted posteriorly to provide access to the suprascapular notch (TECH FIG 3B).

 

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TECH FIG 3 • Schematic and intraoperative photograph demonstrating SSN release at the suprascapular notch. A. The trapezius muscle is split in line with its fibers. B. The supraspinatus muscle is bluntly dissected off the suprascapular fossa and retracted to expose the suprascapular notch. C. The transverse scapular ligament has been released.

 

 

The overlying suprascapular artery and vein are gently retracted to expose the transverse scapular ligament.

 

A small right angle clamp can be used to bluntly dissect under the ligament and protect the underlying nerve while the ligament is transected with a scalpel.

 

Occasionally, the nerve is still tethered after release of the transverse ligament, requiring careful resection of the medial aspect of the suprascapular notch. The resected edge of the bone must be smooth at the completion of the procedure.

 

If the trapezius was detached during the approach, it should be sutured back to the bone of the scapular spine. If the muscle was only split in line with its fibers, it is reapproximated with interrupted, absorbable sutures.

Approach to the Spinoglenoid Notch

 

The posterior approach provides direct visualization of the SSN at the spinoglenoid notch.

 

A 5-cm longitudinal skin incision is made beginning 3 cm medial to the posterolateral corner of the acromion and extending toward the posterior axillary skin pouch. To provide a cosmetically acceptable scar, Langer lines are followed.

 

With the goal of having deltoid split centered over the spinoglenoid notch, the underlying fascia and deltoid muscle are split in line with its fibers beginning at the level of the scapular spine and extending 5 cm distal from the posterior acromion (TECH FIG 4A). A stay suture placed at the distal most extent of the incision protects against propagation of the split, which carries a risk of injury to the axillary nerve.

 

The fascia over the infraspinatus is then split, and the infraspinatus is identified. The muscle belly is dissected off the scapular spine and retracted inferiorly (TECH FIG 4B).

 

Commonly, a small area of vascular fibrous tissue is encountered posteriorly to the site of the spinoglenoid notch, covering the suprascapular neurovascular structures.

 

If a ganglion is present, the contents of the cyst should be removed along with the wall (TECH FIG 4C).

 

A spinoglenoid ligament, which is a fascial veil-like structure from the lateral edge of the scapular spine to the posterior capsule of the shoulder, is excised if present. Careful dissection is required to prevent injury to the SSN and vessels.

 

Using a smooth, small freer, gently palpate the nerve and confirm its mobility.

 

Decompression is complete when the SSN can be followed along its entire length from the spinoglenoid notch until it arborizes into its infraspinatus branches (TECH FIG 4D).

 

The infraspinatus muscle is allowed to return to its anatomic position.

 

The deltoid muscle and fascia is reapproximated with interrupted, absorbable sutures.

 

 

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TECH FIG 4 • Schematics and intraoperative photographs showing SSN release at the spinoglenoid notch.

A. The deltoid muscle is split in line with its fibers, beginning about 4 cm medial to the posterolateral corner of the acromion. B. The spinoglenoid notch has been exposed. The retractors displace the infraspinatus

muscles posteriorly and inferiorly. C. A multilobulated ganglion cyst. D. The SSN is now visible after the soft

tissue band has been divided.

 

 

PEARLS AND PITFALLS

 

Arthroscopic ▪ Hemostasis and visualization can be improved by increasing the fluid pressure decompression to 50 mm Hg and using an electrothermal device to cauterize bleeders.

• The decompression should be performed before treatment of any concomitant pathology to avoid further complicating this procedure owing to fluid extravasation and swelling.

• A 70-degree scope is sometimes helpful to visualize the notch.

• Visualization should be performed through the lateral portal, with posterior and accessory medial working portals.

 

 

 

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POSTOPERATIVE CARE

 

The arm is immobilized in a sling for 2 or 3 days for comfort.

 

 

In an outpatient physical therapy setting, pendulum exercises are started on postoperative day 1, and active motion is increased as tolerated.

 

Once motion is ˜80% of their contralateral shoulder, a strengthening program is initiated.

OUTCOMES

Nonoperative treatment is successful in 80% of patients without space-occupying lesions.12

Open decompression with release of the transverse scapular ligament improved pain and weakness in 73% to 87% of patients.5,21

Recent outcome studies have reported, arthroscopic decompression result in functional improvement in 75% of patients and sensory disturbance recovery in 100% of patients.13,16

 

 

COMPLICATIONS

Damage to the SSN and vessels

Damage to the spinal accessory nerve if mobilization of the trapezius muscle is carried out far medially

Incomplete decompression, especially in rare cases of compression at both suprascapular and spinoglenoid notch

 

 

REFERENCES

1. Aiello I, Serra G, Traina GC, et al. Entrapment of the suprascapular nerve at the spinoglenoid notch. Ann Neurol 1982;12(3):314-316.

    

    

2. Albritton MJ, Graham RD, Richards RS II, et al. An anatomic study of the effects on the suprascapular nerve due to retraction of the supraspinatus muscle after a rotator cuff tear. J Shoulder Elbow Surg 2003;12(5):497-500.

    

    

3. Bhatia S, Chalmers PN, Yanke AB, Romeo AA, Verma NN. Arthroscopic suprascapular nerve decompression: transarticular and subacromial approach. Arthrosc Tech 2012;1(2):e187-e192.

    

    

4. Bigliani LU, Dalsey RM, McCann PD, et al. An anatomical study of the suprascapular nerve. Arthroscopy 1990;6(4):301-305.

    

    

5. Callahan JD, Scully TB, Shapiro SA, et al. Suprascapular nerve entrapment. A series of 27 cases. J Neurosurg. 1991;74(6):893-896.

    

    

6. Cummins CA, Anderson K, Bowen M, et al. Anatomy and histological characteristics of the spinoglenoid ligament. J Bone Joint Surg Am 1998;80(11):1622-1625.

    

    

7. Demaio M, Drez D Jr, Mullins RC. The inferior transverse scapular ligament as a possible cause of entrapment neuropathy of the nerve to the infraspinatus. A brief note. J Bone Joint Surg Am 1991;73(7):1061-1063.

    

    

8. Horiguchi M. The cutaneous branch of some human suprascapular nerves. J Anat. 1980;130(pt 1):191-195.

    

    

9. Lafosse L, Piper K, Lanz U. Arthroscopic suprascapular nerve release: indications and technique. J Shoulder Elbow Surg 2011;20(2 suppl): S9-S13.

    

    

10. Mall NA, Hammond JE, Lenart BA, et al. Suprascapular nerve entrapment isolated to the spinoglenoid notch: surgical technique and results of open decompression. J Shoulder Elbow Surg 2013;22(11):e1-e8.

     

     

11. Mallon WJ, Wilson RJ, Basamania CJ. The association of suprascapular neuropathy with massive rotator cuff tears: a preliminary report. J Shoulder Elbow Surg 2006;15(4):395-398.

     

     

12. Martin SD, Warren RF, Martin TL, et al. Suprascapular neuropathy. Results of non-operative treatment. J Bone Joint Surg Am 1997; 79(8):1159-1165.

     

     

13. Oizumi N, Suenaga N, Funakoshi T, et al. Recovery of sensory disturbance after arthroscopic decompression of the suprascapular nerve. J Shoulder Elbow Surg 2012;21(6):759-764.

     

     

14. Piasecki DP, Romeo AA, Bach BR Jr, et al. Suprascapular neuropathy. J Am Acad Orthop Surg 2009;17(11):665-676.

     

     

15. Piatt BE, Hawkins RJ, Fritz RC, et al. Clinical evaluation and treatment of spinoglenoid notch ganglion cysts. J Shoulder Elbow Surg 2002;11(6):600-604.

     

     

16. Shah AA, Butler RB, Sung SY, et al. Clinical outcomes of suprascapular nerve decompression. J Shoulder Elbow Surg 2011;20(6): 975-982.

     

     

17. Shi LL, Freehill MT, Yannopoulos P, et al. Suprascapular nerve: is it important in cuff pathology? Adv Orthop. 2012;2012:516985.

     

     

18. Thomas A. La paralysie du muscle sous-epineux. Presse Med 1936;64: 1283-1284.

     

     

19. Thompson WA, Kopell HP. Peripheral entrapment neuropathies of the upper extremity. N Engl J Med 1959;260(25):1261-1265.

     

     

20. Tirman PF, Feller JF, Janzen DL, et al. Association of glenoid labral cysts with labral tears and glenohumeral instability: radiologic findings and clinical significance. Radiology 1994;190(3):653-658.

     

     

21. Vastamaki M, Goransson H. Suprascapular nerve entrapment. Clin Orthop Relat Res 1993;(297):135-143.