Serratus Anterior Weakness: One of Many Reasons for Shoulder Blade Winging

We are all familiar with the shape of our shoulder blade. It lies flat on our chest, and moves along with the arm as we position our hand in space. But in some shoulder conditions, the shoulder blade does not lie flat on the chest any more; it protrudes like the wing of a bird. The shoulder blade is also known as scapula, and the term scapular winging is used to describe this abnormal positioning of the shoulder blade, with various degrees of visible prominence. Apparently, the first surgeon who wrote about scapular winging was Dr. Velpeau, from France, in 1825 (almost 200 hundred years ago!).

Individuals who suffer from scapular winging oftentimes complain of deep-seated pain around the shoulder blade, shoulder region, and neck. In addition, raising the arm oftentimes becomes increasingly difficult. There are many reasons for scapular winging, and identifying the culprit for each particular individual can be tricky. Weakness of one particular muscle, the serratus anterior, is one of the leading causes for scapular winging. Interestingly, many health care providers will miss this condition all the time…

Serratus Anterior, Long Thoracic Nerve,… All These Names!

Unless you go to the gym to build up your muscles, you may not recognize the name serratus anterior. Serratus comes from the Latin term serrare (saw”), and it does look like a saw to some extent, since it is formed by 9 or 10 separate slips connecting the undersurface of the inner rim of the shoulder blade to each of the ribs one to eight (or one to nine) to (the second rib usually serves as the take off point for two slips).

Anatomy of the serratus anterior
The serratus anterior connects the undersurface of the inner rim of the shoulder blade to the rib cage

The serratus anterior simultaneously moves and tilts the shoulder blade forward and up when we raise our arm. This movement is critical for us to reach as high as possible; otherwise the bone in the arm (humerus) cannot clear the front of the shoulder blade. In addition, the serratus anterior propels the arm forward, as when punching a boxing bag or swimming crawl or butterfly style.

Boxing and swimming
The serratus anterior facilitates shoulder elevation and also propels the arm when punching or swimming

Do you know about Dr. Charles Bell (1774 – 1842)? He was a Scottish surgeon who developed a special interest for anatomy, in particular of the nerves. He was also a good artist! His name is most commonly associated to spontaneous palsy of half the face (Bell’s palsy). It turns out that Dr. Bell was the first person to describe the nerve that activates the serratus anterior. He named this nerve the “external respiratory nerve”, but today we call this nerve the long thoracic nerve.

Long thoracic nerve is a good name, since this nerve is particularly long (it can measure up to 25 cm – 10 inches) and runs across the chest wall or thorax. It originates from the cervical roots 5, 6 and 7 (occasionally 8), forming part of the so-called brachial plexus. After passing under the collarbone, it lies on the surface of the serratus anterior and innervates each of its muscular slips. This superficial position on the side of the chest wall makes the long thoracic nerve particularly vulnerable to injury.

Long thoracic nerve
The long thoracic nerve descends the chest wall and innervates the serratus anterior

Why Would People Develop Serratus Anterior Weakness?

You must already be guessing the most common reason: if you are thinking of injuries to the long thoracic nerve, you got it! Rarely, the serratus anterior muscle itself may be damaged if torn off from the shoulder blade in an accident. Also, some individuals will develop milder degrees of serratus weakness out of deconditioning of the muscle, potentially contributing to what we call functional scapular diskynesis (diskynesis is Greek for “poor movement”). But the majority of individuals with substantial isolated serratus anterior weakness have dysfunction of the long thoracic nerve. Of course, patients that suffer an injury to the brachial plexus may experience involvement of the long thoracic nerve. This can also happen as part of inflammatory dysfunction of the brachial plexus (Parsonage-Turner syndrome).

But why does this nerve stop working? In some cases (10 to 20%), the reason is never found, but in many others this nerve dysfunction can be traced to an injury to the chest or shoulder, strenuous exertion (overhead work or sports or heavy lifting), a recent infection (typically viral), or a surgical procedure in the vicinity of the nerve, or in a position where the patient was laying on the chest under anesthesia for a long period of time. Is the reason important? Yes! Recovery seems to be more common and faster after infection, and worst when caused by surgery.

Making The Diagnosis

Individuals with serratus anterior weakness or paralysis will typically complain of pain in the shoulder region, around the shoulder blade, and up into their neck. When the condition is just starting, some individuals will suffer more intense nerve pain for a few days, and their pain will settle down as the paralysis progresses. With time, raising the arm fully may become more and more difficult. In addition, their family members of friends may have noted the abnormal prominence of the inner edge of the shoulder blade, or winging. Long thoracic nerve palsy seems to be more common on the dominant side.

Scapular winging
Left sided scapular winging

The shoulder specialist needs to perform a complete evaluation of the muscles around the shoulder blade looking at the back of the patient with his or her shirt off. In some individuals with serratus weakness, the scapula will lie in a winged position even at rest. To examine the serratus anterior, the shoulder specialist will try to demonstrate worse winging with certain maneuvers. In serratus anterior palsy, winging worsens if the patient attempts to raise the arm against resistance from an angle of approximately 45 degrees. Winging will also become more pronounced if the patient performs push-ups on a wall. We also ask the patient to place the arm in the punching position and resist the hand of the examiner pulling back (boxer punch test). Another maneuver may be used to determine to what degree resolution of winging will improve raising the arm. In the scapular assistance test, the shoulder specialist keeps the shoulder blade on the chest wall with his hand, and patients typically feel it is easier to raise their arm fully.

Physical exam serratus anterior

Radiographs are typically normal. The test typically used to assess nerve function is called electromyogram (EMG); it will detect nerve and muscle problems. Because the serratus muscle is somewhat flat and may be atrophied, some times EMGs will fail to detect long thoracic nerve dysfunction. Occasionally, an MRI of the chest wall may be used to identify serratus anterior atrophy, or infiltration of the muscle with fat.

Treatment Options

Luckily, some individuals with long thoracic nerve palsy will recover spontaneously without treatment. About half of the individuals diagnosed with serratus anterior palsy recover with no treatment. On average, recovery takes a little longer than one year (it may take up to two years). As mentioned before, recovery is more likely to occur when the palsy is linked to a recent infection, and much more unlikely when related to surgery.

For these reasons, for individuals that are diagnosed within the first year, the best option may be to wait for spontaneous recovery. Occasionally, a brace to support the shoulder blade will help alleviate symptoms until the nerve recovers. As soon as the muscle is recovering some strength, exercises to strengthen the serratus anterior should be instituted. Our favorite is the dynamic resisted hug. An elastic resistance band is placed behind the neck or upper thorax, and each end of the band grabbed with one fist. Both arms are then advanced forward with the elbows semiflexed, as if hugging someone in front of you. As strength improves, a weight machine may be used for one-arm split-stance cable press.

Dynamic hug
Dynamic resisted hug

For those patients that will not show signs of recovery by one year, consideration should be given to surgery, especially if there are absolutely no signs of improvement, or if the patient presented with poor prognostic factors (i.e., it happened after surgery or major trauma). The two main surgical options include (1) releasing scar tissue and freeing up the nerve (this is technically called neurolysis) or (2) transferring a healthy muscle to the shoulder blade. Transfer of half the pectoralis major has become our procedure of choice. Very rarely, consideration is given to fusing the scapula to the chest wall (scapulothoracic arthrodesis).

Split pectoralis major transfer for serratus anterior palsy

The pectoralis major muscle arises partly from the collarbone (clavicular head) and partly from the anterior chest wall (sternal head). It attach into the humerus bone in the arm.

Pectoralis Major
The two portions or heads of the pectoralis major

In patients with serratus anterior palsy, moving the sternal head of the pectoralis from the humerus to the lower tip of the shoulder blade improves pain, resolves winging, and provides power for improved active elevation. The procedure requires two incisions or skin cuts, one in the front of the arm to free up the sternal head of the pectoralis from the humerus, and a second small one over tip of the shoulder blade for reattachment of the transferred tendon.

Pectoralis major transfer
Split transfer of the sternal head of the pectoralis major from the humerus to the tip of the scapula

After surgery, the arm is placed in a sling for six weeks. Therapy starts at that point and concentrates on regaining complete motion and strengthening the muscles around the shoulder and shoulder blade.


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