Biomechanical Evaluation of Shear Force Vectors Leading to Injury of the Biceps Reflection Pulley

Abstract

Background The clinical importance of the biceps reflection pulley (BRP), which stabilizes the long head of the biceps tendon (LHB) as it exits the joint, has been shown. However, there is controversy on the pathomechanism of injury to the BRP. The angular orientation of the LHB relative to its origin and distal course changes with joint positions and may place the BRP at risk for injury. Purpose To measure the course of the LHB in common arm positions and to determine the shear and normal (stabilizing) force vectors as well as the excursion of the LHB. Study Design Descriptive laboratory study. Methods The LHBs of 8 fresh-frozen cadaveric shoulders were marked with arthroscopically injected microbeads and mounted in a custom-built shoulder rig. Data for neutral arm position, forward flexion, and abduction were collected in internal, neutral, and external rotation using biplane fluoroscopy. Bone and LHB position were reconstructed in 3 dimensions. Results The shear component of the resulting vector was significantly higher during internal (28.4% ± 18.1%) compared with external rotation (18.9% ± 9.7%; P = .0157) and was highest in neutral arm position with internal rotation (39.2% ± 12.7%) and forward flexion with neutral rotation (36.2% ± 10.7%). The normal force vector, stabilizing the LHB, was significantly higher in abduction (55.2% ± 9.6%) compared with forward flexion (39.1% ± 12.4%; P < .0001) and neutral positions (39.1% ± 11.4%; P < .0001). The LHB excursion was significantly lower for neutral arm positions (0.7 ± 6.0 mm) compared with forward flexion (12.6 ± 8.3 mm; P < .0001) and abduction (12.0 ± 6.5 mm; P < .0001). Conclusion Increased shear load at forward flexion with internal or neutral arm rotation and internal rotation at neutral arm position may cause injury to the BRP. Additionally, a sawing mechanism caused by the 12-mm linear excursion combined with a load of the LHB through the BRP during elevation may also lead to lesions. Clinical Relevance Knowledge of the pathomechanisms of BRP injury may help in developing specific treatment and rehabilitation strategies as well as tests for physical examination.