Masterclass: SLAP lesions – Part I

In part one of this 2-part article, Chris Mallac explains the essential anatomy and biomechanics of the biceps anchor, how and why injury occurs, and how to diagnose an injury.

The SLAP lesion was recognised as being a definitive clinical entity after the development of shoulder arthroscopic interventions¹. SLAP is an acronym for ‘superior labral tear from anterior to posterior’, and the term ‘SLAP’ lesion refers to a particular shoulder injury that affects the long head of the biceps anchor to the glenoid labrum.

The understanding on the pathophysiology behind SLAP lesions has progressed greatly since the advent of arthroscopic shoulder surgery. Previously with open-shoulder surgery it was difficult to visualise the superior labrum and biceps attachment, resulting in unknown, missed and poorly understood SLAP lesions.

Although injuries to superior glenoid labrum are uncommon², an injury to the superior labrum and biceps anchor can be a source of deep shoulder pain and it is frequently seen as an overuse injury in overhead athletes and baseball pitchers. If left untreated, a SLAP lesion can have devastating consequences for the overhead athlete in terms of pain, dysfunction and loss of power in sports.

Figure 1: Anatomy and biomechanics of GHJ

Anatomy and biomechanics (see figure 1)

The gleno-humeral joint (GHJ) relies on both passive and active restraints to create joint stability. The passive restraints consist of the capsule-ligamentous structures, the glenoid labrum and negative intra-articular pressure. Dynamic restraints include rotator cuff muscles, periscapular muscles and the long head of the biceps muscle³, which attaches to the superior margin of the labrum and glenoid rim.

Of particular interest is the fibrocartilage labrum that surrounds the glenoid rim circumferentially, increases the depth of the glenoid socket and enhances the passive stability of the GHJ⁴. By increasing the depth of the glenoid fossa, the shear effect of the humeral head is reduced⁵. The superior part of the labrum is loose and meniscoid in structure, and attached by loose connective fibres to the glenoid rim. Up to 50% of the long head of the bicep tendon originates on the superior labrum, with the remaining 50% attached to the supraglenoid tubercle⁶.

It is well understood that the rotator cuff work as a force couple to contract and compress the humeral head into the glenoid fossa during shoulder movements. The long head of the bicep also plays a role in ensuring GHJ stability. The long head of the biceps tendon and superior labrum help to stabilise the humeral head (usually in the abducted and externally rotated arm) through a pulley action, and this is an important requirement in sports such as baseball pitching. It is reported that complete lesions of this biceps-labrum complex results in a 6mm increase in GHJ anterior translation⁷.

Figure 2: Anatomical variants of the glenoid labrum

Interestingly, there are some major anatomical variants of the glenoid labrum (see figure 2). The primary ones of interest, and which should not be confused with a genuine SLAP lesion are⁸:

1. Sublabral foramen, which is a groove between the normal anterosuperior labrum and the anterior cartilaginous border of the glenoid rim. Usually seen in the 3-o’clock position.
2. A small sublabral recess, which represents a gap located inferior to the biceps anchor and the anterosuperior portion of the labrum. It is usually seen in 12-o’clock position of the glenoid in arthroscopic surgery.
3. Cordlike middle glenohumeral ligament.
4. Buford complex, which is characterised by the absence of the anterosuperior labral tissue, with the presence of a thick cordlike middle glenohumeral ligament.

Pathomechanics

Andrews et al (1985) was the first to recognise that the superior labrum and biceps attachment could be injured in overhead throwing athletes⁹. However, the first researchers to recognise and describe the ‘SLAP’ lesion were Snyder et al (1990)¹⁰. They generated the first classification system to define SLAP lesions and established the current understanding of the pathologic anatomy of SLAP lesions¹¹.

These lesions can occur either in isolation or in a conjunction with other shoulder problems. Examples include rotator cuff tears, instability (including acute shoulder dislocation) or biceps tendon pathologies^{12 13}. Other labral lesions usually accompany traumatic shoulder injuries such as an acute dislocation such as a Bankart lesion. However, of interest is the fact that the antero-superior margin of the glenoid rim and labrum has limited vascularity, making it more vulnerable to injuries and having impaired healing potential¹⁴.

Figure 3: Synder’s 4 subtype classification system

Synder et al (1990) originally described and classified these tears into 4 distinct types (I-IV – see figure 3) ¹⁵. This classification system later required some modifications. According to Maffet et al (1995), only 62% of their shoulder series was accurately fitted Snyder’s classification schema so they composed a new classification system¹⁶. As a result, they described an extra 6 new subtypes (V-X – see box 1);

It is important to highlight that often, these SLAP lesions occur alongside other shoulder pathologies. It has been noticed that Type I lesions tend to occur alongside rotator cuff lesions, whereas type III and IV lesions tend to occur with traumatic shoulder instability. One study also showed that in type II lesions, older patients had rotator cuff injuries whereas younger patients presented with shoulder instabilities¹⁷.

Incidence

It has been recognised that acute SLAP tears are a relatively uncommon cause of shoulder pain and dysfunction in athletes taking part in overhead activities such as tennis players, volleyball/waterpolo players baseball pitchers^{18 19 20}. While injury may occur in a one-off traumatic event such as dislocated shoulder (where the humeral head impacts against the superior labrum and biceps anchor and creates a tear or a severe traction type force to the biceps muscle), the majority are caused by repetitive wear and tear.

There are three potential overuse mechanisms underlying the pathophysiology of superior labral tears in overhead athletes^{21 22}:

Glenohumeral internal rotation deficit (GIRD) and internal impingement

It is known that glenohumeral external rotation increases over time in throwers (particularly baseball pitchers) due to the extreme rotation needed at late cocking and early acceleration of the throwing action. However, this is usually accompanied by a corresponding loss of internal rotation capacity due t o tightness in the infraspinatus/ teres minor and posterior capsule of the shoulder seen on the throwing side of the body²³. This tight posterior shoulder musculature and posteroinferior capsule alters the GHJ kinematics so that the glenohumeral contact point shifts posterosuperiorly, especially during overhead-throwing activity²⁴.This creates an internal impingement of the articular side of the rotator cuff tendons and posterosuperior labrum between the humerus and the glenoid rim, precipitating a SLAP lesion²⁵. This internal impingement was first described by Walch et al (1992) as an intra-articular impingement of the rotator cuff in the abducted and externally rotated shoulder²⁶.

Peel-back mechanism

During the late cocking phase of throwing, the biceps tendon is twisted at its anchor, which transmits torsional forces to the postero-superior labrum, resulting in peelback of the labrum^{27 28 29}. In a throwing shoulder, repeated initiation of this mechanism can lead to failure of the labrum over time with avulsion from the bone³⁰.

Eccentric biceps load

The biceps muscle acts eccentrically in the deceleration and follow-through stages of throwing. This eccentric load can place a traction effect on the superior labrum³¹.

Injury signs and symptoms

The clinical diagnosis of a SLAP lesion can be an extremely challenging endeavour to the sports clinician because there are no unique clinical findings associated with this type of pathology. Also the condition is frequently associated with other shoulder problems such as impingement, rotator cuff tears, degenerative joint disease and other soft tissue-related injuries³². However , some common ( but non-specific) features do exist³³. These include:

• Dull, throbbing, ache in the joint.
• Difficulty sleeping due to shoulder discomfort. The SLAP lesion decreases the stability of the joint which (when combined with lying in bed) causes the shoulder to drop.
• A catching feeling during throwing and pitching. Throwing athletes may also complain of a loss of strength or significant decreased velocity in throwing.

Examination

There are numerous physical examination tests described to detect a SLAP injury. These can be sensitive but not very specific³⁴. These include:

1. Active compression / O’Briens’s Test (O’Brien et al 1998)³⁵

The shoulder is placed into approximately 90 degrees of forward flexion and 30 degrees of horizontal adduction across the midline of the body. Using an isometric hold, a downward resistance is applied in this position, with both full shoulder internal and external rotation (altering humeral rotation against the glenoid in the process). A positive test for labral involvement is when pain occurs with the shoulder in internal rotation and forearm in pronation (thumb pointing toward the floor). Symptoms are typically decreased when tested in the externally rotated position or the pain is localised at the acromioclavicular (AC) joint. O’Brien found this manoeuvre to be 100% sensitive and 95% specific as it relates to assessing the presence of labral pathology.

2. Biceps load test II

During this test, the shoulder is placed in 90 degrees of abduction and maximally externally rotated. At maximal external rotation and with the forearm in a supinated position, the patient is instructed to perform a biceps contraction against resistance. Deep pain within the shoulder during this contraction is indicative of a SLAP lesion. This test was further refined with the description of the ‘biceps load II manoeuvre’. The examination technique is similar, although the shoulder is placed into a position of 120 degrees of abduction rather than the originally described 90 degrees. The biceps load II test was noted to have greater sensitivity than the original test. Of these tests, only biceps load test II shows value in identifying patients with a SLAP-only lesion, with no other concomitant pathology^{36 37}.

3. Uppercut test (Kibler et al 2009)³⁸

This is a dynamic SLAP lesion test, which involves the patient rapidly bringing the hand up and toward the chin, replicating a boxing upper-cut punch. This upper-cut test had equal sensitivity (73%) and specificity (78%) in detecting a biceps tendon injury.

4. Compression rotation test (Synder et al 1990)³⁹

The compression rotation test is performed with the patient in the supine position. The glenohumeral joint is then compressed and the humerus is rotated in an attempt to trap the labrum within the joint. The presence of an uncomfortable clunk may indicate a labral tear. The arm can be abducted with an anterior-directed force, or adducted with a posterior-directed force to assess for anterior and posterior labral lesions, respectively.

5. Resisted supination external rotation test (Myers et al 2005)⁴⁰

Similar to the biceps load test, this test is performed with the patient in supine and the shoulder at 90 degrees of shoulder abduction, with 65-70 degrees of elbow flexion and the forearm in neutral position. The examiner resists against a maximal supination effort while passively externally rotating the shoulder.

6. Anterior slide test (Kibler 1995)⁴¹

This test is performed with the patient’s hands on the hips and the thumbs pointing posteriorly. One of the examiner’s hands is placed across the top of the shoulder from behind, while the other hand is placed behind the elbow. A forward and slightly superiorly directed force is applied to the elbow and upper arm, and the patient is asked to push back against this force. Pain localised to the front of the shoulder under the examiner’s hand, or a pop or click in the same area, or both, is considered a positive test. These tests form a cohort of tests that can all be used to attempt to assess for a SLAP lesion. While an individual test may not be overly sensitive, a number of positive tests would strongly hint at a SLAP lesion as being the source of the shoulder pain. Another important part of the physical examination is the evaluation of scapular kinematics. There may be scapular dyskinesis such as scapular winging, poor retraction during the cocking phase of pitching. A postural analysis may show scapula downward rotation and anterior tilting. If a periscapular muscle atrophy or severe scapular winging is noted, an associating cervical spine pathology or accessory/long thoracic nerve palsy should not be excluded⁴².

Imaging

Some authors believe that the only definitive way to detect a SLAP lesion is through arthroscopy and argue that imaging modalities will miss many of the subtle SLAP lesions as false negatives⁴³. MRI, particularly MR arthrography (MRA), is the gold standard imaging method to detect SLAP tears⁴⁴. This is because the intra-articular injected contrast medium distends the joint capsule, outlines intraarticular structures and leaks into tears^{45 46}. This in turn means a clearer delineation of the anatomic structures and SLAP lesions from anatomic variations like sublabral recess or sublabral foramen. A sublabral recess or superior sulcus is a normal variant that is present in more than 70% of individuals. In this variation, the base of superior labrum is not attached to the superior glenoid and in some cases, this recess can be up to 1.4 centimeters deep⁴⁷.

MR arthrography can also detect spinoglenoid cysts. These cysts may cause entrapment of suprascapular nerve causing shoulder pain, weakness in external rotation and infraspinatus muscle atrophy.

SLAP tears are best seen on coronal oblique sequences with the arm in the abducted and externally rotated position, as the contrast medium fills the gap between glenoid and superior labrum⁴⁸. Although MR arthrography is the best imaging technique to evaluate SLAP lesions, a high incidence of false positives is possible therefore findings need to be correlated with cl inical history and physical examination.

Summary

A SLAP lesion is an uncommon shoulder pathology, affecting primarily athletes involved in an overhead sport. Baseball pitchers seem to be the most vulnerable group due to the unique forces they place on the biceps-labral complex in the act of pitching. The purpose of part one of this Rehabilitation Masterclass on SLAP lesions was to present the relevant anatomy and pathomechanics of the shoulder in relation to SLAP lesions, and how a SLAP lesion is most often diagnosed. Part two will discuss in detail the surgical management of a SLAP lesion, and in particular how it is rehabilitated in the event of shoulder surgery to repair the biceps anchor.

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