Scapular dyskinesis (SD), also called scapulothoracic dyskinesis, is the abnormal movement or positioning of the scapula against the thorax. As this condition can be observed in healthy individuals, it may be classified as a deficiency in ideal shoulder kinematics rather than a pathology.

Causes / typical injury mechanism: SD has no single etiology. Causes fall under several categories.

• Musculoskeletal:
  • Muscular abnormalities and imbalances are the most common contributors to SD.
    • Tightness of the anterior muscles, such as the pectoralis minor and biceps short head, promote SD by overloading the posterior muscles that stabilize the scapula.
    • Fatigue of periscapular stabilizers may contribute to SD in individuals who engage in repetitive overhead movements.
  • Abnormal motion at the glenohumeral and acromioclavicular (AC) joints has been linked with SD.
    • Stiffness of the joint capsule or arthritis.
    • Instability of the joint from prior dislocation, capsular laxity, or labral tear.
  • Bony malalignment, as from a clavicle malunion, thoracic kyphosis, and cervical lordosis, can predispose to the development of SD.

• Neurologic:
  • SD may arise secondarily to radiculopathy or a nerve palsy that inhibits the innervated muscles.
    • Spinal accessory (cranial nerve XI): trapezius
    • Long thoracic nerve palsy (roots C4-C7): serratus anterior
    • Dorsal scapular nerve (C5 root): rhomboid

Classic history and presentation:

• SD may be observed in asymptomatic individuals. Pain or stiffness can evolve with time or overuse.
• Neck and shoulder pain are a common complaint in the general population.
• SD patients often present with lateral neck and shoulder pain with concomitant muscle weakness and shoulder instability, usually after recently increased frequency or intensity of activity.
• Symptoms are worsened by activity (eg, throwing or prolonged reaching) and improve with rest.
• Up to 100% of SD patients have other, coexisting shoulder pathologies.

Prevalence:

• Age – Increased prevalence in elderly individuals.
• Sex / gender – Equal prevalence.

Risk factors:

• Symptomatic SD is associated with pathology elsewhere in the shoulder, such as rotator cuff tendinopathy, labral tears, AC joint instability, and glenohumeral arthritis. It is unclear if SD predisposes patients to other shoulder pathologies or vice versa.
• SD is most frequently observed in elderly patients due to coexistent degenerative changes of the shoulder.
• Individuals who spend prolonged periods at a computer with poor posture, or who perform repetitive overhead motions (throwing, swimming, reaching, etc), are also at high risk.
• Overhead-throwing athletes are more vulnerable to SD than their non-overhead-throwing counterparts. About 94% of athletes with an injured throwing shoulder have SD.

Pathophysiology: There is no bony attachment between the scapula and thorax; this articulation is comprised of soft tissues that allow rotation and translation of the scapula across the posterior chest wall. The scapula serves as the attachment site for muscles that provide movement and stability to the upper extremity.

SD is the aberrant motion of the scapula against the thorax. Stiff, weak muscles provide a poor foundation for an upper extremity engaged in overhead work, flaring pain and reducing function.

Grade / classification system: SD is categorized using the 4-type Kibler classification system, based on the position of the scapula's medial border while the arm is at rest or in forward flexion. Multiple types of SD can be simultaneously observed in the same individual.

• Type I – Protrusion of the scapula's inferomedial angle.
• Type II – Projection of the scapula's medial border.
• Type III – Protrusion of the scapula's superomedial angle; often seen in patients with rotator cuff injuries and/or subacromial impingement.
• Type IV – Both the position and motion of the scapula are normal and symmetrical.