BICEPS TENDON–SUPERIOR LABRAL COMPLEX: SLAP LESIONS

BICEPS TENDON–SUPERIOR LABRAL COMPLEX: SLAP LESIONS

The role of the biceps long head tendon within the glenohumeral mechanism has long been neglected. Often dismissed as a minor player on the shoulder as a humeral head depressor, the long head tendon of the biceps has been recognized for its role as an elbow stabilizer and decelerator. Since the advent of arthroscopic research on the shoulder, the role of this structure has been more appreciated. Data by Snyder et al suggested that anterior to posterior superior labral injury (SLAP) occurs in a very limited number of cases in the general population and the mechanism of trauma is varied. Maffet et al, in a review of 712 surgical shoulders with significant abnormalities of the biceps tendon and upper lip, suggested that these separations are caused by various events. In the general population, injury to the labrum-biceps tendon complex is primarily a traumatic event. Two injury mechanisms have been proposed for a disabling injury to this structure in the athlete throwing overhand: the peak forces occurring in deceleration and the take-off mechanism during terminal external rotation.

Andrews et al looked at a population of 73 throwing athletes and found that 60% of this group had superior anterior labrum injuries and another 23% had superior anterior and posterior superior labrum injuries. In a subset of baseball pitchers, this injury was associated with a partial supraspinatus injury in 73% of the athletes. A smaller group than 7% showed a partial long head injury of the biceps. The researchers speculated that the incident of injury to this region of the glenoid labrum was the result of tremendous eccentric stresses applied to the biceps in an attempt to decelerate the arm during the follow-up phase of the overhead throw. A correlation with patient history revealed that 95% of patients reported pain when throwing over the head and 45% of the population reported a hitting or grabbing sensation. On physical examination, the burst was evident in the position of complete abduction and complete flexion, as the upper arm was in line with the ear in 79% of the athletes. None of the population showed significant rotator cuff or biceps tendon weakness. This injury gave the athlete a feeling of instability. In a retrospective total of 2,375 arthroscopically assessed shoulders, Snyder et al reported 140 cases with lesions of the superior glenoid labrum. These cases represented only 6% of the sample population. Involvement of the dominant shoulder versus the non-dominant shoulder was greater than 2 to 1. No radiographic findings could be correlated with the disorder. At that time, no clinical tests were considered specific for the upper lip. About half of the patients described a painful seizure or burst, a finding consistent with the previous study. Only about a third showed a positive biceps strain test. 55% of these shoulders were classified as a type II SLAP lesion consisting of the detachment of the tendon of the upper lip and biceps from the glenoid rim. Of these shoulders, only 28% were isolated from a rotator cuff tear or other labial problem.

Rodosky et al  investigated the role of the long head of the biceps and its attachment to the superior labrum in a laboratory model of the glenohumeral joint positioned in abduction and external rotation as experienced by the overhand thrower. The investigators hypothesized that the presence of the long head of the biceps acted to help limit the external rotating shoulder. The biceps compressed the humeral head

against the glenoid resisting the rotation. The long head of the biceps withstood higher external rotational forces without the inferior glenohumeral ligament’s experiencing a greater strain. This finding suggested that the biceps has a role in the provision of anterior stability. The glenohumeral joint demonstrated

heightened torsional stiffness as force was increased through the long head of the biceps. When a surgical SLAP lesion was created, torsional rigidity decreased 26%, and the strain on the inferior glenohumeral ligament was increased by 33%. This model suggested that the shoulder thus depends on the long head of the biceps to provide dynamic stability to the glenohumeral joint in the cocking, acceleration, and follow-through phases. This dynamic stability ensures a consistent stress on the inferior glenohumeral ligament. The long head of the biceps acts as a continuum provider of axial tension as a protective mechanism for the humerus and the inferior glenohumeral ligament. A cadaveric study measuring strain to the superior labrum and biceps anchor through the phases of throwing reported increased labral strain during the late cocking phase of throwing.  In another cadaveric study that compared the late cocking and a simulated deceleration phase of throwing (in-line loading), the biceps anchor was loaded to failure.  Anchor failure in the late cocking position resulted in a type II SLAP

lesion. Most deceleration failures resulted in midsubstance tears of the tendon itself. This study reported significantly higher structure strength with the in-line loading–deceleration phase. In a study incorporating a three-dimensional finite element model, the superior labrum was stressed through the long head of the biceps tendon in four phases of throwing. In addition to the throwing phases, three types of insertions

were analyzed: a mostly posterior origin, an equal anterior and posterior origin, and a mostly anterior origin. The deceleration phase provided the highest labrum stress in all orientation types, and the highest stress orientation was in the mostly anterior model.

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