Serratus Anterior as the cause of scapula dysrhythmia

As Jon JP Warner et al (1998) explain: "The serratus anterior functions to maintain scapular stability during arm elevation. It does this by causing upward rotation and protraction of the scapula. Dysfunction of this muscle will cause winging of the scapula as the patient attempts to elevate the arm".

The long thoracic nerve is formed from the anterior primary rami of the 5th and 6th cervical vertebrae (C5 and C6). A third, or even a fourth branch, can originate from C4 and C7. It is purely a motor nerve and is the sole innervation to the serratus anterior muscle. The 5th and 6th cervical roots, along with the dorsal scapular nerve, pass through the substance of scalenus medius, a muscle in the side of your neck, whereas the seventh root passes anterior to it. The nerve passes downwards, beneath the brachial plexus and collar bone, over the first rib and on to the digitations of the serratus anterior along the outer side of the chest wall. Each digitation of the serratus anterior in innervated by an individual branch of the long thoracic nerve. the nerve can extend down to the 8th or 9th rib. Because the nerve is long and superficial it is very vulnerable to injury!

The serratus anterior has three functional components (Gregg et al 1979, Jobe 1998). The uppermost originates from the first two ribs and inserts on the superior angle of the scapula. The middle component originates from the 2nd, 3rd and 4th ribs, and inserts along the anterior aspect of the medial scapular border. The lowermost, and largest, originates from the 5th to 9th ribs. These digitations converge to insert on the inferior angle of the scapula.

During normal movement, particularly movements involving pushing, the scapula slides over the rib cage, and is held in place by the serratus anterior. If weakness or paralysis of the serratus is present, the scapula stands prominent from the rib cage when the arm is protracted against resistance. The muscle is usually tested by elevating both arms and pressing forwards against a wall (standing press-up). In synergy with the trapezius, the serratus anterior acts to provide a strong, mobile base of support to position the scapula optimally for maximum efficiency of the upper limb. As has already been stated, without the upward rotation and protraction of the scapula by serratus anterior, the arm can not be elevated fully. Gregg et al (1979) report that abduction is limited to110 degrees.

In the January/February 2000 edition of the Journal of Shoulder and Elbow Surgery, Hester et al describe a series of dissections undertaken to evaluate the course of the nerve. They make the following important observations: "In all specimens a tight fascial band of tissue arose from the inferior aspect of the brachial plexus, extended just superior to the the middle scalene muscle insertion on the first rib, and presented a digitation that extended to the proximal aspect of the serratus anterior muscle". As the next section reveals, this fascial band may be a cause of long thoracic nerve dysfunction!