Injury of the Long head of the biceps tendon (LHBT) is an important source of anterior shoulder pain, and its mechanism involves the interaction of multiple factors. LHBT originates from the superior glenoid tubercle and the superior labrum, and is a tendon divided into the internal and external segments of the joint. Its tendon sheath is closely connected to the rotator cuff. About 95% of LHBT injuries are related to rotator cuff tears and acromial impaction, and simple injuries are rare. Histological studies have shown that the nerve distribution to LHBT is uneven, with sensory and sympathetic nerve fibers densely distributed at the proximal end, which may be the direct neural basis of pain. The pathological mechanism of LHBT injury is complex. The main theories include: degeneration mechanism: Excessive exercise leads to the degeneration of collagen fibers and a decrease in the maximum load of tendons; Hourglass-like degeneration: Fusiform swelling of the tendon accompanied by mechanical compression, commonly seen in patients with massive rotator cuff tears; Abnormal morphology of internodule sulcus: Gleason PD et al. pointed out that an increase in groove width, a decrease in depth, and a reduction in the Angle of the inner wall would intensify the mechanical wear of LHBT during activity; The "Internal Impingement" theory: This theory holds that repeated overhead movements cause the humeral head to collide with the superior labrum, resulting in LHBT squeezing, wear and tear. Other factors: such as TNF-α and IL-1β mediating tenosynovitis; Pain-depression comorbidity mechanism: The lateral habenula mediates explosive neuronal discharges through T-type calcium channels, regulating chronic pain and mood disorders; Insufficient blood supply to the tendons, direct trauma, and injuries to the Pulley structure in the rotator cuff space (including supraspinatus tendons, subscapularis tendons, superior glenohumeral ligaments, and coracohumeral ligaments), etc. Imaging assessment is of vital importance in mechanism research: ①CT measurement: Widely used to analyze the morphology of the internodule sulcus. Abboud et al. confirmed that the width, depth of the groove, and the presence of bone spurs were associated with LHBT injury. Based on the CT data of patients with rotator cuff injury, Urita and Funakoshi et al. further found that those with LHBT injury tended to have narrower groove widths and greater groove depths. ②MRI measurement: The advantages of MRI technology in the diagnosis of LHBT injury are reflected in the following aspects: through innovative measurement indicators (coracobrachialis distance, coracoprocess overlap), precise positioning through image fusion, and AI automatic segmentation and risk prediction, the accuracy and efficiency of diagnosis have been significantly improved. Maria J. Leite et al., through the study of a large number of patients with rotator cuff injury, proposed that coracohumeral distance and coracoid overlap are effective MRI indicators for predicting LHBT injury and are helpful for clinical diagnosis; Lu Yi et al. proposed arthroscopic and MRI image fusion technology to accurately locate the injury range of LHBT, which is particularly suitable for complex cases of SLAP type Ⅳ injury combined with superior labrum barral handle tear. Artificial intelligence-assisted deep learning models (such as U-Net) automatically segment LHBT in MRI with an accuracy of 92% and can predict the tear risk area (tear risk increases by 3 times when the internodule groove Angle is >40°). LHBT, as the core source of anterior shoulder pain, its stability depends on the synergistic effect of the bony structure in the internodule sulcus and the Pulley complex (including the tendon sheath, the sickle ligament of the pectoralis major muscle, and the pulley structure). Studies have confirmed that over 90% of LHBT injuries are accompanied by rotator cuff lesions or joint instability, and are significantly associated with morphological variations in the intertuberous groove (width, depth, medial wall Angle, and bone spur formation). Although the "Internal Impingement" theory is widely accepted, traditional research has two limitations: reliance on cadaver specimens or two-dimensional image measurements, and the neglect of the dynamic biomechanical influence of the three-dimensional spatial configuration of the humeral head, coracoid process, and internodal groove. At present, clinical diagnosis is facing severe challenges. Conventional MRI has insufficient sensitivity (24.3%) for detecting LHBT injury, and the complex three-dimensional anatomical relationships of the shoulder joint make it difficult to explain the injury mechanism. It is particularly worth noting that previous studies have failed to answer the following key questions: ①How does the dynamic displacement of the humeral head affect the stress distribution of LHBT in the internodule sulcus? ② Does the spatial position variation of the coracoid process change the restraint efficiency of the pulley complex? ③ What is the synergistic effect rule of bony structure and soft tissue stability mechanism from a three-dimensional perspective?

To deeply explore the mechanism of positional changes of the scapula and humeral head in LHBT injury by constructing a three-dimensional (3D) model based on the scapular-humeral head and combining it with biomechanical principles.

The included research subjects were divided into three groups: Group A (complete rotator cuff injury group, 51 cases), Group B (incomplete rotator cuff injury group, 21 cases), and Group C (healthy control group, 24 cases), totaling 96 cases. All CT data of the shoulder joints were standardized. Three-dimensional models of the scapula and humeral head were constructed using Mimics and 3-matic. Based on the 3D model, measure the upward movement distance of the humeral head (HHUM), the position of the internodal groove (PIG), and the position of the coracoid process (PC). Using statistical analysis, the differences in general and measured parameters among groups were compared.

The average ages were as follows: Group A (58.59±8.03) years old, Group B (51.90±10.85) years old, and Group C (52.33±5.31) years old. HHUM: Group A was (8.08±2.13) mm, Group B was (7.18±1.41) mm, and Group C was (5.80±1.50) mm; PIG and PC: The PIG in group A was (48.67±9.08) °, and the innermost point, topmost point and bottommost point of PC were (21.88±3.30) mm, (29.24±4.48) mm and (19.24±4.65) mm, respectively. In group B, the PIG was (46.68±12.65) °, and the PC was (21.52±3.78) mm, (29.09±4.44) mm, and (18.85± 5.13) mm. In Group C, PIG was (52.39±10.95) °, and PC was (22.58±4.22) mm, (28.07±3.94) mm, and (18.25±4.79) mm. Statistical analysis indicated significant age differences (P<0.05) and HHUM values (P<0.05) among Group A, Group B, and Group C. In the PIG and PC measurement results, there were no statistical differences among the three groups (P>0.05) .

The distance of humeral head movement shows a positive correlation with the occurrence of rotator cuff tendon injury combined with LHBT injury. An increase in the degree of upward movement significantly increases the risk of combined injury. However, the location of the internodule groove and the anatomical landmark points of the coracoid process (the innermost, uppermost, and lowermost ends) have no significant association with LHBT injury. Furthermore, advanced age was identified as an independent risk factor for rotator cuff-LHBT compound injury, and its influence was significantly more substantial than that of gender or specific injury sites.