Does Humeral Component Lateralization in Reverse Shoulder Arthroplasty Affect Rotator Cuff Torque? Evaluation in a Cadaver Model

Abstract

Background: Humeral component lateralization in reverse total shoulder arthroplasty (RTSA) may improve the biomechanical advantage of the rotator cuff, which could improve the torque generated by the rotator cuff and increase internal and external rotation of the shoulder.

Purpose: The purpose of this in vitro biomechanical study was to evaluate the effect of humeral component lateralization (or lateral offset) on the torque of the anterior and posterior rotator cuff.

Methods: Eight fresh-frozen cadaveric shoulders from eight separate donors (74 ± 8 years; six males, two females) were tested using an in vitro simulator. All shoulders were prescreened for soft tissue deficit and/or deformity before testing. A custom RTSA prosthesis was implanted that allowed five levels of humeral component lateralization (15, 20, 25, 30, 35 mm), which avoided restrictions imposed by commercially available designs. The torques exerted by the anterior and posterior rotator cuff were measured three times and then averaged for varying humeral lateralization, abduction angle (0°, 45°, 90°), and internal and external rotation (-60°, -30°, 0°, 30°, 60°). A three-way repeated measures ANOVA (abduction angle, humeral lateralization, internal rotation and external rotation angles) with a significance level of α = 0.05 was used for statistical analysis.

Results: Humeral lateralization only affected posterior rotator cuff torque at 0° abduction, where increasing humeral lateralization from 15 to 35 mm at 60° internal rotation decreased external rotation torque by 1.6 ± 0.4 Nm (95% CI, -0.07 -1.56 Nm; p = 0.06) from 4.0 ± 0.3 Nm to 2.4 ± 0.6 Nm, respectively, but at 60° external rotation increased external rotation torque by 2.2 ± 0.5 Nm (95% CI, -4.2 to -0.2 Nm; p = 0.029) from 6.2 ± 0.5 Nm to 8.3 ± 0.5 Nm, respectively. Anterior cuff torque was affected by humeral lateralization in more arm positions than the posterior cuff, where increasing humeral lateralization from 15 to 35 mm when at 60° internal rotation increased internal rotation torque at 0°, 45°, and 90° abduction by 3.2 ± 0.5 Nm (95% CI, 1.1-5.2 Nm; p = 0.004) from 6.6 ± 0.6 Nm to 9.7 ± 0.6 Nm, 4.0 ± 0.3 Nm (95% CI, 2.8-5.0 Nm; p < 0.001) from 1.7 ± 1.0 Nm to 5.6 ± 0.9 Nm, and 2.2 ± 0.2 Nm (95% CI, 1.4-2.9 Nm; p < 0.001) from 0.6 ± 0.6 Nm to 2.8 ± 0.6 Nm, respectively. In neutral internal and external rotation, increasing humeral lateral offset from 15 to 35 mm increased the internal rotation torque at 45˚ and 90˚ abduction by 1.5 ± 0.3 Nm (95% CI, 0.2-2.7 Nm; p = 0.02) and 1.3 ± 0.2 Nm (95% CI, 0.4-2.3 Nm; p < 0.001), respectively.

Conclusions: Humeral component lateralization improves rotator cuff torque.

Clinical relevance: The results of this preliminary in vitro cadaveric study suggest that the lateral offset of the RTSA humeral component plays an important role in the torque generated by the anterior and posterior rotator cuff. However, further studies are needed before clinical application of these results. Increasing humeral offset may have adverse effects, such as the increased risk of implant modularity, increasing tension of the cuff and soft tissues, increased costs often associated with design modifications, and other possible as yet unforeseen negative consequences.

Fig. 1A–C

The (A) custom humeral component and separate (B) epiphyseal and (C) metaphyseal components are shown. Threaded holes in the metaphyseal component allow for translation of the epiphyseal component in 5-mm increments, either medially or laterally from 15 to 35 mm.

Fig. 2

Each specimen is mounted on the in vitro simulator, which allows us to position the shoulder in specific degrees of abduction and rotation. Sutures connect the subscapularis (yellow line) and the infraspinatus and teres minor muscles (green line) to computer-controlled pneumatic actuators. Sutures also connect the deltoid (red line) to pneumatic actuators to maintain joint reduction. Forces then are applied to the rotator cuff muscles to generate humeral torque. Mid Delt = mid-deltoid; Ant Delt = anterior deltoid; Post Delt = posterior deltoid.

Fig. 3

The matrix diagram shows the various combinations of humeral component offset and shoulder positions tested in the current biomechanical study. IR = internal rotation; ER = external rotation.

Fig. 4A–C

Mean (±1 SD) internal (IR) and external rotation (ER) torque at 60° internal rotation, neutral internal and external rotation, and 60° external rotation for (A) 0°, (B) 45°, and (C) 90° abduction, for all humeral component lateral offsets investigated (15, 20, 25, 30, 35 mm). *p < 0.05; ANT = anterior; POST, = posterior.

Fig. 5A–C

Muscle moment arms at (A) 60° internal rotation, (B) neutral internal-external rotation, and (C) 60° external rotation for the anterior cuff (moment arm = green; anterior cuff line of action = red) and posterior cuff (moment arm = pink; anterior cuff line of action = red) for 15 mm and 35 mm humeral lateral offsets. The humeral cup is shown in yellow