Muscle:Pectoralis Minor

Pectoralis minor is a deep anterior chest muscle whose trigger points (TrPs) are tightly linked to those of the overlying pectoralis major and are almost never found active in isolation. Its referred pain closely mimics the pain of cardiac ischaemia. Its taut fibres are the landmark anatomical divider of the axillary artery and constitute a significant mechanism for neurovascular entrapment of the brachial plexus and axillary artery.

Anatomy

The pectoralis minor attaches above to the medial aspect of the tip of the coracoid process of the scapula and below to the third, fourth, and fifth ribs near their costal cartilages. It may also attach as low as the sixth rib, or as high as the first rib. A slip of the pectoralis minor may extend beyond the coracoid process in about 15% of bodies to attach to the tendons of adjacent muscles or to the greater tuberosity of the humerus.

Two other, relatively infrequent, anatomical variations: the pectoralis minimus connects the first rib cartilage to the coracoid process; the pectoralis intermedius may attach more medially onto the third, fourth, and fifth rib cartilages and attach above to the fascia covering the coracobrachialis and biceps brachii muscles.

Approximately 40% of pectoralis minor fibres are type II, decreasing slightly after age 60.

Innervation: The pectoralis minor is innervated by the medial pectoral nerve from the medial cord, and by fibres of roots C8 and T1.

Primary actions: The pectoralis minor draws the scapula forward, downward, and inward at nearly equal angles. Depression of the shoulder by this muscle stabilises the scapula when the arm exerts downward pressure against resistance. The coracoid depression is used to pull the shoulders forward. It also assists the latissimus dorsi in depression of the shoulder. When the scapula is fixed in elevation by the upper trapezius and levator scapulae muscles, the pectoralis minor becomes active during strong inhalation efforts that involve the upper chest, functioning as an accessory respiratory muscle.

Functional unit: The pectoralis minor forms a synergistic functional unit for additional support for vigorous inhalation from the levator scapulae, upper trapezius, and sternocleidomastoid in addition to the parasternal internal intercostals, lateral external intercostals, the diaphragm, and the scalene muscles. The lower trapezius muscle acts as an antagonist to the pectoralis minor in scapular rotation and protraction. Weakness of the lower trapezius can allow the scapula to ride up and tilt down anteriorly, leading to adaptive shortening of the pectoralis minor.

Referred Pain Patterns

TrPs in the pectoralis minor refer pain most strongly over the anterior deltoid area. With very active TrPs, the pain may extend upward over the subclavicular area, and sometimes covers the entire pectoral region on the same side. Spillover referred pain extends along the ulnar side of the arm, elbow, forearm, and palmar hand to include the last three fingers.

The pattern is essentially the same as that referred from the adjacent clavicular division TrPs of the pectoralis major. Pain from either pectoral muscle, and specifically the pectoralis minor, can closely mimic the pain of cardiac ischaemia.

Activation and Perpetuating Factors

Pectoralis minor TrPs may be activated as satellite TrPs due to their presence within the zone of pain induced by myocardial ischaemia, as satellites of scalene or pectoralis major TrPs, by trauma (a gunshot wound through the upper chest, or fracture of upper ribs), by a whiplash-type motor vehicle accident, by strain through overuse as a shoulder depressor (unaccustomed crutch-walking), as an accessory muscle of inspiration (during paroxysms of severe coughing, or to assist paradoxical breathing), by poor seated posture (keeping the muscle chronically shortened because of a poorly designed chair or work environment), or by prolonged compression of the muscle (knapsack with a tight strap over the front of the shoulder).

Clinical Examination

A patient with significant TrP shortening of the pectoralis minor will usually demonstrate forward (rounded) shoulders because of the forward and downward tilt of the coracoid process by the pectoralis minor.

The increased tension due to TrPs in the pectoralis minor prevents the patient from reaching fully behind the back at shoulder level. The anterior depression of the caracoid and downward rotation of the glenoid fossa that are caused by pectoralis minor tension limits full flexion of the arm at the shoulder joint. Shortening of this muscle is observable as elevation (forward position) of the involved shoulder away from the table in the supine patient.

When the pectoralis minor and subscapularis muscles are both shortened by TrPs, they restrict the combined movement of abduction and lateral rotation at the shoulder. However, subscapularis TrPs restrict only glenohumeral motion, whereas pectoralis minor TrPs restrict only scapular mobility on the chest wall. When abduction of the arm at the shoulder is restricted by pectoralis minor tautness, the patient may be aware of pulling on the ribs at the limit of abduction.

The pectoralis major should be examined first for active TrPs that might obscure and confuse the localisation of TrPs in the underlying pectoralis minor.

Trigger Point Examination

In both the supine and seated positions, pectoralis minor TrPs can be localised either by flat palpation through the pectoralis major against the chest wall, or by pincer palpation around the pectoralis major. With either approach, the pectoralis major is slackened by keeping the patient's arm toward the front of the body and the forearm on the abdomen, and the pectoralis minor may be placed on the desired degree of stretch by adducting the scapula toward the military-brace position.

In non-obese patients with relatively loose skin, the pectoralis minor can usually be palpated directly by pincer palpation. The operator places the thumb (with a well-trimmed fingernail) in the apex of the axilla and slides it against the chest wall beneath the pectoralis major toward the midline, until it encounters the muscle mass of the pectoralis minor. That muscle (and the pectoralis major above it) are then encompassed by a pincer grasp between the thumb and fingers. The fibres of the pectoralis minor can then be palpated directly through the skin for a tender nodule in a taut band. Identification of TrPs in the pectoralis minor may be enhanced by elevating the shoulder cephalad to tauten the pectoralis minor, which increases the sensitivity of its TrPs without tightening the pectoralis major.

Weakness of the pectoralis minor is tested by resisting forward thrust of the shoulder with the patient supine, and with the subject elevating the hand and elbow off the table to avoid assisting the motion by downward thrust against the table.

Entrapment

The pectoralis minor is the landmark for anatomically dividing the axillary artery into three parts; the second part of the artery lies deep to the muscle. Likewise, the distal portion of the brachial plexus passes deep to the pectoralis minor muscle where it attaches to the coracoid process. When the arm is abducted and laterally rotated at the shoulder, the artery, vein, and nerves are bent and stretched around the pectoralis minor muscle close to its attachment, and are likely to be compressed if the muscle is firm and tightened by myofascial TrPs.

Entrapment of the medial cord (Fig. 43.4B) occurs in two places with this arm position: as the nerve hooks under the pectoralis minor tendon and again as its fibres hook over the first rib. The medial cord connects the lower trunk to the ulnar nerve. This entrapment causes numbness and paraesthesias of the fourth and fifth digits, but usually not of the thumb and other fingers.

The lateral cord is more directly compressed than the medial cord and connects the upper and middle trunks proximally, and the musculocutaneous and median nerves distally. This entrapment disturbs sensation over the dorsum and radial aspects of the forearm and over the palmar side of the first three and one-half digits.

Arterial entrapment is detected by loss of the radial pulse at the wrist or by a reduction of arterial blood flow. When arterial compression in abduction or hyperabduction is primarily due to TrP activity of the pectoralis minor, the radial pulse may be restored in the test position by eliminating the hyperirritability of the TrPs. When patients with active pectoralis minor TrPs were placed in the hyperabducted position to the point of just obliterating the radial pulse, pulsation returned immediately while vapocooling the skin over the stretched pectoralis minor muscle, without changing the arm position.

Entrapment by the taut pectoralis minor does not produce the hand oedema and stiffness of the fingers so characteristic of entrapment by the scalenus anterior.

The Wright manoeuvre (arm in lateral rotation and abduction at the shoulder) demonstrates the effect of severe pectoralis minor shortening due to TrP involvement.

Differential Diagnosis

Differential diagnosis of symptoms caused by TrPs in the pectoralis minor includes thoracic outlet syndrome, C7 and C8 radiculopathy, supraspinatus tendinitis, bicipital tendinitis, and medial epicondylitis. Articular dysfunctions likely to be associated with pectoralis minor TrPs include elevation of the third, fourth, and fifth ribs.

Treatment

Trigger Point Release

Of primary importance is the correction of faulty posture, particularly round-shouldered posture, and instructions to the patient for maintenance of correct posture and movement.

Instead of the usual spray-and-stretch technique, this chapter presents the same initial spray followed by manual release of the tense muscle. A prespray technique is applied with vapocoolant in upsweeps over the anterior chest and shoulder and down the ulnar surface of the arm, followed by application of a manual release technique.

Manual release: One hand applies pressure on the shoulder to move the upper part of the scapula posteriorly while stabilising the costal attachments of the muscle with the other. If the lower trapezius is weak, it should be strengthened in order to provide scapular stabilisation.

Postisometric relaxation and contract-relax are effective for release of central TrPs. The primary therapeutic approach to attachment TrPs is to inactivate the central TrPs causing them.

The prespray technique for the pectoralis minor is performed with the patient supine. The arm is raised diagonally overhead (slightly abducted and laterally rotated) just to the onset of resistance or discomfort. Upsweeps of spray cover the pectoralis minor muscle and its pain pattern, extending distally to include the ulnar aspect of the forearm and the three ulnar fingers. This position of the arm also lengthens the pectoralis major which should be sprayed at the same time to avoid aggravating its TrPs. Both pectoral muscles are frequently involved together.

Trigger Point Injection

Injection of pectoralis minor TrPs should be done with the patient supine, not seated, to avoid psychologically induced syncope, and only after TrPs in the pectoralis major have been inactivated to avoid recurrence. Whenever possible, the hand of the operator locates the pectoralis minor underneath the pectoralis major using pincer palpation, with the fingers (or thumb) contacting the pectoralis minor directly. The needle is directed parallel to the rib cage toward the coracoid process.

After injection of TrPs, the patient should move the arm and shoulder slowly three times through full range of motion for the pectoralis major, followed by moist heat over the pectoral region.

Corrective Actions

TrPs should be inactivated in any muscles, such as the scalene group and pectoralis major, that refer pain to the region of the pectoralis minor and thus are likely to harbour TrPs. Activity stress due to overuse must be avoided. Standing and seated posture should be improved. A weak lower trapezius should be strengthened. A heavily loaded brassiere strap that compresses the pectoralis minor should be avoided; the strap may be placed on the acromion to relieve pressure on the muscle or padded to distribute the load more widely. The patient should learn to maintain full pectoral muscle length by using the In-doorway Stretch Exercise or by doing a similar stretch in the corner of a room.

To minimise aggravation of pectoralis minor TrPs when the muscle is placed in the shortened position when sleeping, the patient avoids sleeping "curled up" on the side with the shoulder forced strongly forward.

Satellite Trigger Points

• Pectoralis major — one rarely finds active TrPs in the pectoralis minor without active TrPs in the pectoralis major
• Anterior deltoid — associated with pectoralis major involvement; shares common satellite pathway
• Scalene muscles — refer pain to the pectoral region; pectoralis minor may develop as satellite
• Sternocleidomastoid — associated with pectoralis minor involvement

Related Pages

• Muscle:Pectoralis Major — primary co-active muscle; almost always active when pectoralis minor is involved
• Muscle:Scalene — key satellite source and TOS co-contributor
• Muscle:Sternalis — overlying anomalous muscle in same pain zone
• Pain:Ear and TMJ — diagnostic algorithm for head and neck

References

• Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 1: The Upper Half of Body. 2nd ed. Baltimore: Williams & Wilkins; 1999. Chapter 43.