The long head of the biceps tendon is a common cause for anterior shoulder pain as the proximal third of the tendon has a high degree of innervation. Inflammation, fraying, partial tears, or instability of the long head of the biceps tendon are some of the more common clinically reported causes for anterior shoulder pain.
There is controversy about the function of the long head of the biceps tendon, especially for anterosuperior stability of the humeral head. The biceps tendon may play a substantial role in reducing anterosuperior translation of the humeral head, particularly in positions of forward flexion. Imprints on the chondral surface of the humeral head have been reported in cases of biceps tendon dislocation, and this implicates high forces on the long head of the biceps tendon. The mechanical properties of the long head of the biceps tendon, with its high stiffness, indicate its ability to transmit the large load of the muscle. Habermeyer described a 30° to 40° turn of the biceps tendon as it exits the joint and stabilizaton of the tendon by a pulley sling. Figure 1 shows a dissected cadaveric specimen and visualizes the biceps reflection pulley sling after taking down the deltoid muscle and opening the rotator cuff interval. The biceps reflection pulley sling is made up of fibers of the superior glenohumeral ligament (SGHL), the coracohumeral ligament (CHL), and partially by fibers blending in from the subscapularis and supraspinatus tendons. Figure 2 is a schematic drawing of the anatomical structures and course of fibers from the work of Werner The orientation of the fibers in the biceps reflection pulley also suggests that substantial anteromedial shear forces are absorbed by the biceps reflection pulley system. More distally, the transverse humeral ligament does not appear to play a key role in stabilizing the long head of the biceps tendon, and additionally, it is not present consistently in all individuals. The shape of the bony bicipital groove has an additional effect on the long head of the biceps tendon’s stability.
There is speculation that the angular orientation of the long head of the biceps tendon relative to the humeral head changes with joint positions and may place the biceps at risk for instability. Repetitive wear and trauma to the restraining structures of the long head of the biceps tendon may result in medial or lateral subluxation or dislocation of the tendon, which in many cases is related to tears of the subscapularis or supraspinatus tendon (S. Braun, unpublished data, 2009). Anteromedial instability of the long head of the biceps tendon has been described by various authors (W. F. Bennett, unpublished data, 2008). Lafosse et al described a series of patients with posterolateral instability of the tendon. Different classifications of subluxation and dislocation patterns of the long head of the biceps tendon with or without associated lesions of the subscapularis or supraspinatus tendon have been published.
However, there is little information on the pathomechanisms that lead to such tears of the biceps reflection pulley. Various studies have described the anatomy of the biceps reflection pulley and clinical and arthroscopic appearance of injury, but no laboratory study has investigated the biomechanics of the biceps reflection pulley system, the long head of the biceps tendon, and possible pathomechanisms of injury (W. F. Bennett, unpublished data, 2008).
To better understand the possible mechanisms of injury of the biceps reflection pulley system, we need to know how the course of the long head of the biceps tendon changes with movement as a sharp angle between the biceps anchor, the pulley sling, and the distal insertion may create high shear forces on the soft tissue sling. Moreover, in certain glenohumeral positions, the resultant force vector may point directly into the bicipital groove, stabilizing the tendon, whereas in other positions, the tendon may directly load the soft tissue sling, potentially contributing to instability or injury. We hypothesize that lesions of the biceps reflection pulley occur in a position of forward flex- ion and internal rotation because anatomically this would create a sharp angle in the tendon at the biceps reflection pulley. This tendon position in conjunction with significant sliding of the tendon in and out of the joint would create a sawing mechanism that could lead to erosion of the biceps pulley sling and the tendon over time.
The purpose of this study, therefore, was to measure the course of the long head of the biceps tendon using biplane fluoroscopy in 3 common arm positions in internal, external, and neutral rotation and to determine the shear force vector in those positions as well as the amount of excursion of the long head of the biceps tendon.
Full Article: Biomechanical Evaluation of Shear Force Vectors Leading to Injury of the Biceps Reflection Pulley
