MSK radiologists and physician with interest in sports-related injuries and MR imaging

- The pitchers need to balance adequate laxity to allow extreme range of motion while maintaining stability.

- The anterior capsular laxity combined with posterior capsular tightness causes a relative shift of the humeral head contact point on the glenoid in the ABER position.

- The MR findings suggestive of posterosuperior impingement are undersurface abnormality of posterior supraspinatus or infraspinatus, abnormal signal in the posterior aspect of superior labrum, and cystic changes in the posterior humeral head.

- Learn the functional anatomy of the rotator cuff and labroligamentous structures frequently injured in overhead athletes

- Understand the kinematic of the throwing and related injury patterns in shoulder and elbow joints

- Recognize how the specific injury patterns appear in MR imaging

Athletes who engage in repetitive overhead motions are predisposed to distinct subset of shoulder injuries. Baseball, volleyball, handball, water polo, swimming, tennis, and cricket are typical activities with frequent overhead motions. Among them, throwing a fastball by baseball pitchers is one of the most violent movements. To maximize the force that can be generated and transferred to the ball, the pitchers need to balance adequate laxity to allow extreme range of motion while maintaining stability. The delicate equilibrium between laxity and stability has been referred to as “thrower’s paradox. However, extreme laxity places the stresses on the shoulder, specifically to the anatomy that maintains the shoulder stability. As these stresses are repeated, a wide range of overuse injuries can be developed.

Functional anatomy of shoulder

Rotator cuff
Rotator cuff consists of 4 muscles, which arise from scapula and inserts on the humeral heads. These tendons coalesce to form the inserting common tendon. The conjoined tendon of supraspinatus and infraspinatus at the posterosuperior aspect of glenohumeral joint is the most commonly injured tendon in overhead athlete. The rotator cuffs work together as a dynamic stabilizer of the glenohumeral joint to center the humeral head in the glenoid cavity through balanced force couple.

Joint capsule and glenohumeral ligaments
The joint capsule consists of a circularly and radially arranged collagen fibers and is reinforced by multiple ligaments: coracohumeral ligament (CHL), superior glenohumeral ligament (SGHL), middle glenohumeral ligament (MGHL), and inferior glenohumeral ligament (IGHL). CHL radiates from coracoid process and inserts at the greater and lesser tuberosity. SGHL runs from supraglenoid tubercle to lesser tuberosity, and forms biceps pulley with CHL. MGHL has the most variable anatomy in its thickness and insertion. It functions as an important anterior stabilizer during abduction/external rotation. IGHL consists of anterior and posterior bands. These ligaments stabilize the humeral head in the vulnerable abduction and external rotation movements.

Glenoid labrum and long head of biceps tendon
The glenoid labrum attaches to the glenoid rim in a circular manner and forms a functional units together with joint capsule, glenohumeral ligaments and long head of biceps tendon. The glenoid labrum increases bony contact area by about one third, and interact with the capsule and glenohumeral ligaments to contribute stability of the glenohumeral joint. The long head of biceps tendon (LHBT) may arise from the posterosuperior labrum, supraglenoid tubercle or a combination of both.

Rotator interval
The rotator interval is the triangular space which houses LHBT, SGHL and CHL between supraspinatus and subscapularis tendon. The LHBT passes through the rotator interval, and sharply turns at the proximal entrance of intertubercular groove. SGHL and CHL were the structural component of rotator intervals, and they unite to form a biceps pulley.

Kinematics of throwing

The baseball pitch is the most heavily investigated throwing model. The throwing motion has been divided into six phases: wind-up, early cocking, late cocking, acceleration, deceleration, and follow-through. The late cocking and follow-through phases are of particular importance. In late cocking, the anterior capsule is under significant strain to prevent anterior translation of the humerus because of the increased external rotation in the abducted and externally rotated (ABER) position. During the follow-through phase, the posterior capsule and posterior cuff undergo tremendous eccentric loads to decelerate the rapidly internal rotating arm and to restrain the significant distractive forces at the posterior shoulder joint.
Scapula is a major link of the kinetic chain. Throwers generate great velocities by delivering the force generated by the lower extremity and core muscles to shoulder and arm via scapula. The scapula not only allows for coordinated glenohumeral movement, it is also a muscle attachment site for the primary muscles controlling glenohumeral and scapulothoracic movement. If the scapula is not functioning harmoniously with the humerus, the range of motion is compromised.

MR imaging of overhead athletes

MR imaging and MR arthrography can reveal the predictable injury pattern of the overhead athlete. MR arthrography still remains the mainstay for imaging in athletic patients, especially for those who suspiciously have labroligamentous abnormality. MR arthrography with ABER position enhances the detection of anterior labral tear and anterior capsular pathology by putting the anterior labrocapsular tissue under tension. In addition, ABER position places the shoulder in the position where posterosuperior impingement occurs between the humeral head and glenoid, and approximates the locations of potential sites of pathology.

Specific pathologies in throwing athletes

Glenohumeral internal rotation deficit (GIRD)
Glenohumeral internal rotation deficit is the sentinel event in the pathologic cascade of thrower’s shoulder. The extreme external rotation during late cocking phase repeatedly exerts stresses on the anterior capsule, the anterior band of the inferior glenohumeral ligament in particular, and eventually leads to anterior capsular laxity. Meanwhile, large distraction forces on posterior capsule during the follow-through phase leads to chronic attritional tear of the posterior capsule and subsequently result in a fibroblastic healing with loss of tissue compliance. Consequently, the shortened, contracted posterior capsule limits the internal rotation and also does not permit full external rotation of the humerus. The anterior capsular laxity combined with posterior capsular tightness causes a relative shift of the humeral head contact point on the glenoid in the ABER position. More recent studies, however, suggest that the posterior rotator cuff stiffness and humeral torsion also contribute the internal rotation deficit as well as the capsular contracture.

Posterosuperior impingement
Undersurface of rotator cuff to contact the posterosuperior glenoid when the arm is in the cocked position of 90° abduction and full external rotation can be a normal physiologic finding. However, the contact point shift brings the humeral head closer to the superior labrum in late cocking, promoting increased contact between the 2 structures. The MR findings suggestive of posterosuperior impingement are undersurface abnormality of posterior supraspinatus or infraspinatus, abnormal signal in the posterior aspect of superior labrum, and cystic changes in the posterior humeral head.

Anterosuperior impingement
Repetitive forceful shoulder adduction and internal rotation above the horizontal plane in follow through phase cause impingement of the biceps pulley system and superior edge of subscapularis against the anterior superior glenoid rim. Compromise of the biceps pulley system leads to long head of the biceps tendon instability, articular surface tear of subscapularis and supraspinatus disease, and eventual long head of the biceps tendon medial subluxation.

Peelback lesion
GIRD leads to a posterior shift of the vector force on LHBT, and twist at the biceps anchor in the late cocking phase. The distraction and twisting force lead to SLAP lesions. Posterior labrum is more prone to failure.

Posterior shoulder instability
Weakening and contractures in the posterior inferior capsule places the glenoid capsulolabral complex and associated stabilizers at risk for injury.

Scapular dyskinesia
Scapular dyskinesia is the abnormal positioning and motion of the scapula. In throwing shoulder, it is regarded as an adaptive response of internal rotation deficit. Shoulder pain in scapular dyskinesia commonly results in inhibition of the lower trapezius and serratus anterior, as well as tightening of the upper trapezius and pectoralis minor. The net effect of this muscular imbalance is protraction with anterior tilt of the scapula. The SICK (Scapula malposition, Inferior medial border prominence, Coracoid pain and malposition, and dysKinesis of the scapula movement) scapula syndrome is the extreme of scapula dyskinesia, caused by overuse muscle fatigue.

Neurovascular injuries
Thoracic outlet syndrome involves compression of the neurovascular contents of the thoracic outlet space. With the arm in excessive abduction, the neurovascular structures may be compressed under the pectoralis tendon and coracoid. In thowing athletes, excessive muscle bulk or scapular depression from inadequate scapular muscle stabilization can further compromise the neurovascular structures. Axillary artery thrombosis and aneurysm also have been reported in elite baseball pitchers. Quadrilateral space syndrome can occur in the presence of fibrous bands as the sequelae of repeated overhead throwing movement. Suprascapular nerve compression at suprascapular notch can occurs in the throwers because adduction and internal rotation tightens spinoglenid ligament. MR imaging can be helpful in elucidating alternative explanations for the pain.

Elbow injury in throwing athletes
Most elbow injuries occur as valgus forces reaches the maximum during acceleration phase. Repetitive tensile stresses create microtrauma in anterior bundle of the UCL, leading to progressive valgus instability. Continued shear stress and impingement in the posterior compartment lead to olecranon tip osteophyte, loose bodies and cartilage damage to posteromedial trochlear. This chronic injury pattern of elbow is referred to as valgus extension overload syndrome. Laxity in UCL also leads to stretch of the other medial structures including the flexor-pronator mass and ulnar nerve, and can causes flexor tendinopathy or ulnar neuropathy.