Arm Position with Increased Risk of Partial Subscapularis Tear Progression Owing to Subluxation of the Long Head of Biceps Tendon: Cadaveric Biomechanical Study

Abstract

Backgroud: This study aimed to evaluate the differences in long head of biceps (LHB) motion between the normal and subscapularis intrasubstance partial tear conditions and identify the arm positions that exhibit the most significant LHB motion differences using a cadaveric biomechanical study.

Methods: The LHB tendons of 6 fresh-frozen cadaveric shoulders (2 men and 4 women; mean age, 68.4 ± 2.3 years; range, 65-71 years) were marked with metal beads and mounted in a custom-made shoulder testing system. Data for arm positions at 20° or 60° of forward flexion or abduction, with neutral rotation and maximum internal and external rotation with a torque of 1.5 N·m, were collected. Considering the scapulohumeral rhythm, 20° or 60° forward flexion or abduction in a cadaveric shoulder corresponds to 30° or 90° shoulder elevation in vivo . Mediolateral (subluxation) and inferosuperior (excursion) LHB motions were measured using a 3-dimensional digitizer, and the differences between normal and subscapularis partial tear conditions were analyzed.

Results: While the LHB mediolateral motion difference was the highest during 60° forward flexion with neutral rotation (1.2 ± 0.4, p = 0.042), 20° forward flexion with neutral rotation (0.9 ± 0.3, p = 0.024) and 60° abduction with maximum external rotation (0.9 ± 0.3, p = 0.036) also demonstrated high mediolateral LHB motion difference between the normal and subscapularis partial tear conditions. In contrast, the LHB inferosuperior motion difference was the highest during 20° forward flexion with neutral rotation (0.7 ± 0.3, p = 0.045) between the normal and subscapularis partial tear conditions.

Conclusions: Upon comparing normal and subscapularis partial tear conditions in this cadaveric study, high pathological movements of the LHB were observed during arm forward flexion with neutral rotation and abduction with external rotation. Repetitive activity in these arm positions could aggravate the condition in a partial subscapularis tear.

Keywords: Biomechanic; Cadaver; Rotator cuff Injuries; Shoulder Impingement syndrome.

Fig. 1. Left shoulder specimen mounted on a custom-made shoulder testing system. The scapula is fixed with an anterior tilt of 20°, and the humeral shaft is transected 2 cm distal to the deltoid tubercle, where an intramedullary rod and cylinder are attached. The humeral attachment of the rotator cuff tendinous portion and the long head of the biceps (LHB) are preserved. The LHB is severed just distal to the musculotendinous junction, and a muscle load of 10 N (2.2 lbs) has been applied. To enhance visualization of the proximal LHB, the acromion, distal clavicle (arrow), coracoid, and coracoacromial ligament (arrowhead) have been removed. SSP: supraspinatus, SSC: subscapularis.

Fig. 2. Three-dimensional reconstruction of the left humerus and the definition of long head of the biceps (LHB) motion. Subluxation refers to the mediolateral movement of the LHB, with medial motion defined as a positive value. Excursion refers to the inferosuperior movement of the LHB, with inferior motion defined as a positive value.

Fig. 3. Creation of a partial subscapularis intrasubstance tear in a shoulder with an intact subscapularis tendon. (A) Shoulder specimen with the acromion (arrowhead) and coracoid process (arrow) removed and mounted on the shoulder testing system. Six metal beads are sutured onto the center of the long head of the biceps (LHB), and the transverse humeral ligament is preserved (asterisk). (B) To create a subscapularis intrasubstance partial tear, a triangular osteotomy lateral to the LHB has been performed with the transverse humeral ligament preserved (asterisk). (C) The partial intrasubstance tear of the subscapularis has been created using a No. 11 blade (asterisk). (D) The bone fragment is secured using Kirschner wires, establishing the partial subscapularis intrasubstance tear condition, while the transverse humeral ligament (asterisk) is preserved. SSP: supraspinatus, SSC: subscapularis.