Scapular dyskinesia is a term used to describe poor movement patterns of the shoulder blade. The shoulder blade, or scapula, moves in multiple planes and must be coordinated with the glenohumeral joint in order to allow full range of motion of the shoulder. When the length, strength or timing for firing the scapular stabilizing musculature is not optimal, scapular dyskinesia occurs. One type of scapular dyskinesia is winging of the scapulae.
Scapular winging can result from either weakness or stiffness of multiple muscle groups, including serratus anterior, trapezius, rhomboid major and minor, and/or levator scapulae. Of these muscles, serratus anterior is the most common muscle contributing to winging. It is a flat muscle that originates on the upper eight or nine ribs and inserts on the medial border of the scapula. Its primary action is to stabilize the scapula against the rib cage, then laterally rotate the inferior angle of the scapula during overhead activity. The trapezius muscle may also be involved with scapular winging. This muscle helps retract, elevate and rotate the scapula and is most often injured with surgeries in the cervical area. A third muscle group that may be involved is the rhomboid major and minor, which together retract, elevate and medially retract the inferior angle of the scapula. Injury to these muscles can be a result of entrapment of the C5 nerve under a hypertrophied scalene muscle.
The cause of scapular dyskinesia is a muscle imbalance of the scapular stabilizers that can either be neurogenic in nature or inherently muscular. With scapular winging in particular, traction or stretch injuries to the long thoracic nerve can be a primary cause. The long thoracic nerve passes between the anterior and middle scalenes then travels along the chest wall to the serratus anterior. Positions in overhead sports can easily stretch the long thoracic nerve resulting in repetitive or traumatic stretch injuries to the nerve and resulting in neuropraxia that inhibits the serratus anterior. Neuropraxia can occur with increases in nerve length of only ten per cent. Aside from long thoracic nerve injuries, spinal accessory nerve injury can also lead to scapular winging as it inhibits trapezius muscle activity.