Uncommon Injuries: Subcoracoid impingement

Chris Mallac explores the anatomy and biomechanics of subcoracoid impingement syndrome, including how clinicians can diagnose and most effectively manage this condition.

Sub-coracoid impingement (SCI) syndromes are an uncommon cause of anterior shoulder pain in the athlete; the prevalence in the general population who complain of anterior shoulder pain is approximately 5%⁽¹⁾. Although originally identified and described as a source of anterior shoulder pain in 1909⁽²⁾, Gerber et al (1985) were the first to truly describe a form of impingement between the deep surface of the coracoid process, the coracoacromial ligament and the anterior rotator cuff – postulating that this was due to a narrowing of the subcoracoid space⁽³⁾.

Anatomy

The coracoid process is an anterior extension of the scapula that varies considerably in height and length. This individual variation creates differences in the shape and the size of the space between the coracoacromial arch and the rotator cuff^(4,5), which explains the potential congenital cause of SCI^(6,7). Anatomically, the coracoid process presents an origin point for several muscles and ligaments (see figure 1). These include^(8-10):

• The pectoralis minor attaches to the medial border.
• The short head of the biceps brachii and the coracobrachialis connect as a conjoined tendon to the apex.
• The suprascapular ligament attaches to the base.
• The coracohumeral ligament attaches to the lateral border.
• The conoid and trapezoid ligaments attach to the superior coracoid.

Figure 1: Anatomy of the coracoid process and surrounding structures

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The space behind and below the coracoid process is termed the ‘subcoracoid space.’ It consists of:

• The articular capsule of the glenohumeral joint (particularly the middle glenohumeral ligament and coracohumeral ligament);
• The subacromial-subdeltoid bursa (which may extend as the subcoracoid bursa)^(11,12);
• The subscapularis tendon and its bursa;
• The long head of the biceps tendon.

The space between the coracoid process and glenoid is termed the ‘coraco-glenoid space,’and the distance between the coracoid and the humerus is termed the coracohumeral distance (CHD) or coracohumeral interval (CHI). The morphology of the coracoid process can lead to a narrowing of these spaces and predispose the shoulder to an SCI⁽⁶⁾.

Pathogenesis of SCI

Many conditions can account for anterior shoulder pain in the athlete. These include (but are not limited to) common pathologies such as:

• Subacromial impingement – subacromial bursa and supraspinatus.
• Calcific and non-calcific tendinopathies.
• Disorders of the long head of the biceps tendon, such as tenosynovitis and SLAP lesions.
• Acromioclavicular joint injuries.
• Anterior shoulder instability.

The involvement of the coracoid process, however, is an unusual source of anterior shoulder pain and pathology. The two underlying mechanisms that may cause SCI are:

1. Subcoracoid stenosis. Created by a smaller distance in the CHD that closes the space, leading to impingement of the soft tissues (bursa and tendon)⁽¹³⁾.
2. Anterior humeral head translation due to a rotator cuff dysfunction, which abuts the humeral head into the coracoid process⁽¹⁴⁾.

Gerber et al believed that the primary cause of SCI was a narrowing between the coracoid process and humeral head (the CHD), and also unique differences in shape and size of the actual coracoid process⁽³⁾. Using CT scan imaging of the bone structure, they showed that the typical distance of the CHD was approximately 8.6 mm; however, this distance was smaller in patients suspected of having coracoid impingement.

Asal et al also found a correlation between subscapularis pathology and the CHD due to morphology faults in the coracoid process⁽¹⁵⁾. The association was strong when there was a CHD of less than 6mm, a decreased coraco-glenoid angle, and an increased coracohumeral angle. They described the coracoid as being one of:

• Type A - flat coracoids (most common)
• Type B - osteophyte at the tip of the coracoids
• Type C - hooked coracoids (seen most often in those with SCI)

Differences also exist in coracoid morphology as someone ages. The subcoracoid space becomes narrower and hooked in older shoulders. These skeletal changes may make older joints more prone to SCI - a consideration in aging athletes⁽¹⁶⁾. Conversely, some cadaveric studies have suggested coracoid anatomy is not a significant cause of impingement. Instead, some researchers believe that SCI occurs due to acquired pathomechanical issues, such as anterior translation of the humeral head due to gross instability or rotator cuff imbalances, that lead to the humeral head translating forward⁽¹⁷⁾.

Sport type and SCI

The most provoking movement is repetitive and abrupt shoulder flexion, combined with internal rotation and adduction movements against resistance, with the elbow extended⁽¹⁾. This motion decreases the CHD and may compress the soft tissues between the lesser tuberosity of the humerus and the coracoid tip (the ‘roller-wringer’ effect)⁽¹⁸⁾. The repetitive movement, in turn, may lead to a slow breakdown of the subscapularis tendon^(10,19-21). Sports that repeatedly reproduce this movement include boxing (see figure 2), combat sports that involve striking, water sports such as kayaking, and the follow-through phase of throwing (eg baseball, cricket).

Figure 2: Shoulder horizontal flexion, internal rotation, and adduction

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SCI may also be prevalent in overhead sports. In a cadaveric study, Colas et al demonstrated how friction occurs between the coracoid process and the subscapularis muscle during movement⁽²²⁾. The thick superior band of the subscapularis undergoes a shear against the coracoid process, and coils around the process during elevation with external rotation. The sheet-like subscapular bursa moves with the muscle and protects it from friction against the coracoid process. Repetitive shear and compression of these soft tissues with overhead movements (such as baseball pitching and tennis serving) may predispose the athlete to an SCI.

Other pathology may mimic SCI, including a condition known as ‘bench presser’s shoulder’, which often affects male weightlifters and bodybuilders. However, this condition, insertional tendinopathy of the pectoralis minor muscle on the coracoid process, is distinctly different. The key feature of this injury is pain on palpation of the medial side of the coracoid process (not on the lateral coracoid) and sharp pain with the bench press movement⁽²³⁾.

Diagnosing SCI

The typical signs and symptoms of SCI are as follows^(1,3,19):

• Dull pain in the anterior region of the shoulder, which may (rarely) radiate to the arm and forearm.
• Pain induced and worsened by elevation and internal rotation, abduction, or simple internal rotation of the arm. It is greatest at between 120 - 130° of flexion, rather than between 60 and 120° (as seen in the more common subacromial impingement syndrome).
• Pain on palpation of the lateral coracoid process at the common tendon origin of the biceps brachii and coracobrachialis muscles (compare this to the medial pain experienced with ‘bench presser’s shoulder above).
• Positive coracoid impingement test. This test is similar to the Kennedy-Hawkins impingement sign, except that the patient’s shoulder is placed passively in a position of cross-arm adduction, forward elevation, and internal rotation to bring the lesser tuberosity in contact with the coracoids⁽²⁴⁾. A positive test produces a painful clicking in the anterior shoulder.
• Positive Yokum test (see figure 3). The patient places the hand of the affected side on the opposite shoulder. Apply a downward force to the arm with the patient resisting isometrically. This is positive if pain is reproduced⁽²⁵⁾.
• Infiltration of local anesthetic into the subcoracoid space temporarily relieves the pain.

Figure 3: The Yokum test

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 Imaging

Possible imaging modalities are:

1. Standard radiographs- to detect bony abnormalities in the coracoid and to measure the CHD/CHI.
2. MRI- to determine the structures that are damaged and impinged, such as the subscapularis tendon and associated bursa. Also, cine MRI can be used to evaluate the dynamic circulatory aspects of SCI with the arm held in internal rotation⁽²⁶⁾.
3. CT Scans- to assess the specific anatomy of the coracoid process, such as the lateral projection measurement and the CHD⁽¹⁰⁾.

Management of SCI

Management of SCI starts with a thorough examination of scapular function and dysfunction. Muscle imbalance in the form of overactive pectoralis minor with serratus anterior weakness will lead to both poor scapular positioning and scapular muscle fatigue. Observation of movement may detect aberrant form in frontal plane pushing movements such as throwing a punch and overhead movements such as the tennis stroke.

Address imbalances in the rotator cuff, which lead to humeral head anterior translation and likely impingement against the coracoid process. The thoracic spine position also has a direct influence on scapular posture. Exaggerated thoracic kyphosis leads to more anterior tilt of the scapula. This posture brings the coracoid process closer to the humeral head and may then decrease the CHD and cause impingement.

A conservative way to manage the ‘compression’ in the subcoracoid area is to ‘decompress’ the space by retracting the scapula and externally rotating the humerus to increase the CHD. The stretch shown in figure 4 does this and also stretches the pectoralis minor muscle. This stretch can be held for 10-15 minutes to allow gravity to open up the chest and decompress the subcoracoid space slowly.

Figure 4: Subcoracoid decompression stretch

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More direct medical interventions include injections and surgery. Gigante et al (2016)found that a single infiltration injection of corticosteroid was a straightforward and effective way to relieve pain in this condition⁽¹⁾. The types of surgery available to correct this condition include:

• Rectifying any anatomical changes that have reduced the subcoracoid space. These include the removal of any subscapularis tendon lesions (ossification or calcification), ganglion cysts, previous malunions of humeral head fractures, or scapular fractures^(24,27).
• Coracoplasty to reshape or remove part (osteotomy) of the coracoid process. An osteotomy will involve reinserting the conjoined tendon of the short head of biceps/coracobrachialis.
• Resection of the coracoacromial ligament and conjoined tendon⁽³⁾.
• Subcoracoid decompression procedures, and debridement of the subscapularis tendon, the short head of biceps tendon, and anterior supraspinatus lesions⁽¹⁸⁾.

Summary

SCI is an uncommon condition in athletes, usually affecting those involved in repetitive adduction and internal rotation movements that may decrease the CHD, or in overhead sports that require abduction and external rotation. Specific tests help distinguish SCI from other shoulder pathologies. If these tests are positive, the athlete must refrain from performing the offending movements for a period of time. Correct the compression effect of scapula position and any humeral head anterior translation caused by rotator cuff imbalances through strengthening and neuromuscular re-education. If these treatments fail, then the medical interventions include injections and surgery.

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