Effect of glenohumeral abduction angle on the mechanical interaction between the supraspinatus and infraspinatus tendons for the intact, partial-thickness torn, and repaired supraspinatus tendon conditions

Abstract

Rotator cuff tears are difficult to manage because of the structural and mechanical inhomogeneity of the supraspinatus tendon. Previously, we showed that with the arm at the side, the supraspinatus and infraspinatus tendons mechanically interact such that conditions that increase supraspinatus tendon strain, such as load or full-thickness tears, also increase infraspinatus tendon strain. This suggests that the infraspinatus tendon may shield the supraspinatus tendon from further injury while becoming at increased risk of injury itself. In this study, the effect of glenohumeral abduction angle on the interaction between the two tendons was evaluated for supraspinatus tendon partial-thickness tears and two repair techniques. Principal strains were quantified in both tendons for 0 degrees , 30 degrees , and 60 degrees of glenohumeral abduction. Results showed that interaction between the two tendons is interrupted by an increase in abduction angle for all supraspinatus tendon conditions evaluated. Infraspinatus tendon strain was lower at 30 degrees and 60 degrees than at 0 degrees abduction angle. In conclusion, interaction between the supraspinatus and infraspinatus tendons is interrupted with increase in abduction angle. Additionally, 30 degrees abduction should be further evaluated for management of rotator cuff tears and repairs as it is the angle at which both supraspinatus and infraspinatus tendon strain is decreased.

Figure 1

Experimental setup allows for reproducible and repeatable changes in glenohumeral abduction angle. The humerus is moved and pinned in 1 of three positions that correspond to the three abduction angles evaluated.

Figure 2

Strain maps showing apparent maximum principal strain in the infraspinatus tendon for the intact supraspinatus tendon at 60N at (A) 0° abduction and (B) 30° abduction. Contrary to hypothesis, apparent maximum principal strain was higher at 0° than at 30° abduction.

Figure 3

For all supraspinatus tendon tears, effect of abduction angle on apparent average maximum principal strain in the infraspinatus tendon. For all supraspinatus tendon tears (A1:intact, B1:33% partial-thickness, C1:66% partial-thickness and D1:full-thickness), maximum principal strain was higher at 0° abduction than at 30° and 60° abduction.

Figure 4

For all supraspinatus tendon tears, effect of abduction angle on apparent average maximum principal strain in the supraspinatus tendon. For all supraspinatus tendon tears (A1:intact, B1:33% partial-thickness, C1:66% partial-thickness and D1:full-thickness), maximum principal strain was higher at 0° than at 30°, and at 60° abduction than at 0° and 30° abduction.

Figure 5

For all supraspinatus tendon tears, effect of abduction angle on apparent average minimum principal strain in the infraspinatus tendon. For all supraspinatus tendon tears (A1:intact, B1:33% partial-thickness, C1:66% partial-thickness and D1:full-thickness), minimum principal strain in the infraspinatus tendon was more compressive at 0° than at 30° and 60° abduction.

Figure 6

For all supraspinatus tendon tears, effect of abduction angle on apparent average minimum principal strain in the supraspinatus tendon. Minimum principal strain in the supraspinatus tendon was lower at 0° than at 30° abduction (for A1: intact, B1: 33% partial-thickness tear, and C1: 66% partial-thickness tear) and lower than 30° and 60° abduction for the 100% full-thickness tear (D1).

Figure 7

Effect of abduction on apparent average maximum principal strain in the infraspinatus tendon following (A) arthroscopic and (B) transosseous repairs. For both techniques, apparent average maximum principal strain was significantly higher at 0° than at 30° and 60° abduction.

Figure 8

Effect of abduction on apparent average minimum principal strain in the infraspinatus tendon following (A) arthroscopic and (B) transosseous repairs. For both techniques apparent average minimum principal strain was significantly more compressive at 0° than at 30° and 60° abduction.