Dr. Kevin Yip

Dr Kevin Yip
Orthopaedic Surgeon
MBBS(UK), FRCS(EDIN), FAM(SING), FHKCOS(ORTHO)

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Biomechanics

During sports, repetitive microtrauma can damage the nerve in several ways, including entrapment and compression, traction and friction, direct trauma, as well as conditions affecting the brachial plexus in general. Besides direct effects on the nerve, intimal damage to the axillary or suprascapular artery has been proposed to result from direct trauma or friction that, in turn, can result in ischemic injury to the nerve, particularly at the suprascapular notch.

Because the suprascapular nerve is relatively fixed regarding its position under the rotator cuff and ligaments, it is susceptible to compression by ganglion cysts, lipomas, the transverse scapular ligament, enlarged spinoglenoid notch veins, or the spinoglenoid ligament.The more frequent sites of compression are the suprascapular notch and the spinoglenoid notch.

Between these two, the more common area of entrapment is at the spinoglenoid notch, especially in elite athletes who are involved in overhead sports. The spinoglenoid ligament inserts into the posterior glenohumeral capsule. The ligament has been demonstrated to tighten with cross-body adduction and internal rotation, which can compress the nerve (e.g., during the follow-through phase of throwing a ball or serving in tennis).

Also, with extreme abduction and full external rotation, the medial tendinous margin of the infraspinatus and supraspinatus muscles has been shown to be able to impinge strongly against the lateral edge of the scapular spine, compressing the infraspinatus branch of the suprascapular nerve . Because the injury to the nerve at the spinoglenoid notch occurs more distally, the supraspinatus muscle and approximately 30% to 40% of the infraspinatus function are left intact.

Only the innervation to the infraspinatus is affected, so asymptomatic sports activity may still be possible, because partial function of the muscle may remain and the teres minor can help to compensate for the infraspinatus dysfunction.

A second mechanism of injury to the suprascapular nerve involves traction or mechanical stretching of the nerve, with or without friction, because it travels sharply around critical points. The nerve can be fixed in several areas:

(a) the origin from the upper trunk at the Erb point,

(b) the suprascapular notch (under the transverse scapular ligament),

(c) the insertion into the infraspinatus muscle, and

(d) the spinoglenoid notch or lateral spine of the scapula.

Studies have shown that altered conduction can be identified within a nerve when it is stretched 6% beyond its resting length. Stretching the nerve more than 15% beyond its resting length leads to irreversible nerve damage.

With overhead activities in sports, such as the cocking, acceleration, and follow-through phases of serving in volleyball and tennis, the extreme torques and angular velocity that are placed on the shoulder can result in repetitive, undue tensile stresses to the nerve.

The suprascapular nerve also is susceptible to direct injury. Cases with direct nerve injury have been described following dislocation of the glenohumeral joint, fracture of the proximal humerus or scapula, and penetrating injury. Injury to the suprascapular nerve also has been a reported complication following surgical procedures, such as distal clavicle resection, shoulder stabilization, or open rotator cuff repair.

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