The use of reverse total shoulder arthroplasty (RTSA) has increased dramatically owing to predictable improvements in active forward elevation, pain relief, and function . Traditional RTSA designs have placed the glenohumeral center of rotation more medial and inferior compared with the native shoulder . While this configuration may serve to improve deltoid recruitment, unique complications have been seen, including scapular notching and the loss of normal shoulder contour [1, 7, 24]. In addition, a more-medialized center of rotation can decrease the rotator cuff moment arm, which may limit internal and external rotation of the shoulder . Available techniques to decrease common complications include glenosphere lateralization, inferior glenoid tilt, and avoiding superior positioning of the glenoid baseplate [9, 10, 16-18, 23, 25, 30].
Currently, design improvements are progressing for RTSA. Unlike an anatomic TSA, which strives for the anatomic reconstruction of the glenohumeral joint with restoration of version, size, and joint line, the selection of implant parameters in RTSA is not as straightforward. Design changes must be tempered against incurring negative consequences. As such, individual RTSA design characteristics should be examined to determine their influences on shoulder biomechanics so that implant manufacturers may design more-effective implants in the future . Humeral component lateral offset is one design characteristic that requires further assessment to ascertain its biomechanical effects. To date, humeral component lateralization has been reported to increase the deltoid's ability to abduct the arm and to decrease overall joint reaction forces during abduction . However, the effect of humeral component lateralization on rotator cuff function remains largely unknown.
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. We theorized that increasing humeral component lateralization would result in greater torque produced by the anterior and posterior rotator cuffs.
Materials and Methods
Custom Adjustable RTSA
To vary the amount of humeral lateralization, a custom modular humeral component was used. The humeral component was based on a traditional Grammont-style with a 155° head-neck angle  and an adjustable lateral offset between the center of rotation of the humeral cup and the humeral stem by means of a modular connection between the epiphyseal to metaphyseal junction. This custom modular junction allowed the epiphysis of the humeral component to be translated medially and laterally in 5-mm increments (Fig. 1). As such, humeral offset was isolated and adjustable without changing other implant variables, such as length or head-neck angle. In total, we investigated humeral offsets of 15 mm, 20 mm, 25 mm, 30 mm, and 35 mm. The humeral component was mated to a custom 38-mm diameter glenosphere. The center of rotation of the RTSA was placed at the articular surface of the glenoid. A standard +3-mm humeral polyethylene cup was used (Delta XTENDTM; DePuy, Warsaw, IN, USA). The overarching theme of this project was not to relate the results obtained to a particular implant type or company, but rather to understand the foundations of RTSA to improve implants in the future.
Specimen Preparation and Shoulder Simulation
Eight fresh-frozen human cadaveric shoulders were used in this study (74 ± 8 years; six males, two females) from eight separate donors. Specimens were excluded if there were significant preexisting shoulder disorders that might affect study results, including glenohumeral arthritis as evidenced by cartilage defects, bone loss, or partial- or full-thickness rotator cuff tears. Each cadaver was thawed to room temperature overnight, then prepared and tested in 1 day to standardize the quality of the specimen. The shoulders were transected at the mid-humerus, leaving the entire glenohumeral joint and scapula intact with overlying skin and soft tissues. Dissection then was performed, leaving only the deltoid, rotator cuff muscles, and the glenohumeral joint. The subscapularis was mobilized off its insertion on the lesser tuberosity and reflected laterally to allow access to the glenohumeral joint. A cuff-deficient shoulder model was created by releasing the supraspinatus and the upper infraspinatus. The glenoid labrum was excised and the articular surface was reamed so that the baseplate would be placed at the level of the inferior glenoid rim. An anatomic humeral neck cut was performed and the custom adjustable humeral component was cemented in 20° retroversion.
The specimens were mounted on an in vitro shoulder simulator via a clamp that was secured along the medial aspect of the scapula (Fig. 2) . The remaining infraspinatus and teres minor muscles were sutured together as one unit, and a separate suture was placed in the subscapularis tendon. All sutures then were attached to actuators via high-tension braided lines, which were routed along anatomically accurate lines of action to generate the desired muscle force vectors. The computer-controlled pneumatic actuators applied forces to the muscles using custom software for each arm position investigated. The distal end of the humerus was secured to a 6 degree-of-freedom load cell (Nano25; ATI Industrial Automation, Apex, NC, USA) to measure torques using a custom mechanical fixture which permitted the constraint of internal and external rotation angles, plane of elevation, and abduction angle, while permitting free translation and rotation of the other degrees of freedom.
Experimental Protocol and Statistics
Five implant configurations were tested with varying amounts of humeral lateralization in 5 mm increments (15 to 35 mm). All other parameters of the RTSA were kept constant. Each specimen was evaluated in a total of 15 shoulder positions in a randomized order, based on combinations of two parameters: (1) scapular plane abduction in 0°, 45°, and 90°, and (2) humeral rotation at neutral (0°), −30° and −60° internal rotation, and 30° and 60° external rotation (Fig. 3). Shoulder positions were marked with a long-arm goniometer before testing. Forces were applied to the posterior and anterior rotator cuff muscles to generate external and internal humeral torques, respectively. The infraspinatus-teres minor unit and the subscapularis were loaded with 24 N each, as reported by Omid et al. . These forces were selected based on the cross-sectional area ratios of the respective muscles . The deltoid was loaded at 10 N to maintain glenohumeral joint reduction .
For every specimen, three trials at each configuration were performed and the mean and SD torque values were used in the statistical analysis. A three-way repeated-measures ANOVA (abduction angle, humeral lateralization, internal and external rotation angles) was used for the statistical analysis with a significance level of α = 0.05. Eight specimens were found to be sufficient to provide a power greater than 80%.
Humeral component lateralization had varying effects on rotator cuff torque, which were dependent on the internal and external rotation and abduction angles of the shoulder (Fig. 4). Posterior rotator cuff torque was affected only by humeral lateralization at 0° abduction. In this position and at 60° internal rotation, increasing the humeral lateralization from 15 to 35 mm decreased the posterior cuff torque by 1.6 ± 0.4 Nm (95% CI, −0.071 to 1.56 Nm; p = 0.06) from 4.0 ± 0.3 Nm to 2.4 ± 0.6 Nm respectively. At 60° external rotation, medializing the humeral component from 20 mm to 15 mm decreased posterior cuff torque by 1.0 ± 0.2 Nm (95% CI, −1.914 to −0.53 Nm; p = 0.038) from 7.1 ± 0.6 Nm to 6.1 ± 0.5 Nm respectively, but lateralizing from 15 mm to 30 and 35 mm, respectively, increased posterior cuff torque by 1.9 ± 0.5 Nm (95% CI, 0.08-3.8 Nm; p = 0.041) from 6.2 ± 0.5 Nm to 8.1 ± 0.7 Nm respectively and 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 (Fig. 4).
Anterior rotator cuff torque generally was more sensitive to humeral component lateralization compared with the posterior cuff, and served to increase anterior cuff torque at all three abduction angles when internally rotated, and at 45° and 90° abduction angles in neutral internal and external rotation. At 0° abduction and 60° internal rotation, increasing humeral lateralization from 15 mm to 25, 30, and 35 mm increased anterior cuff torque by 1.5 ± 0.3 Nm (95% CI, 0.46-2.5 Nm; p = 0.006) from 6.6 ± 0.6 Nm to 8.1 ± 0.5 Nm, 2.1 ± 0.4 Nm (95% CI, 0.7-3.5 Nm; p = 0.006) from 6.6 ± 0.6 Nm to 8.7 ± 0.7 Nm, and 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, respectively. At 45° abduction and 60° internal rotation, increasing humeral lateralization from 20 mm to 30 and 35 mm increased anterior cuff torque by 1.6 ± 0.4 Nm (95% CI, 0.06-3.2, p = 0.041) from 3.0 ± 1.2 Nm to 4.6 ± 0.9 Nm and 2.6 ± 0.4 Nm (95% CI, 1.0-4.2 Nm; p = 0.003) from 3.0 ± 1.2 Nm to 5.6 ± 0.9 Nm, respectively. At 30° internal rotation, increasing humeral lateralization from 20 mm to 25, 30, and 35 mm increased anterior cuff torque by 1.1 ± 0.2 Nm (95% CI, 0.3-1.9 Nm; p = 0.007) from 5.8 ± 0.6 Nm to 6.9 ± 0.4 Nm, 2.2 ± 0.3 Nm (95% CI, 0.9-3.6 Nm; p = 0.003) from 5.8 ± 0.6 Nm to 8.1 ± 0.5 Nm, and 2.8 ± 0.3 Nm (95% CI, 1.4-4.2 Nm; p = 0.001) from 5.8 ± 0.6 Nm to 8.6 ± 0.4 Nm respectively.
In neutral internal and external rotation, increasing humeral lateralization from 20 mm to 30 and 35 mm increased anterior cuff torque by 1.4 ± 0.3 Nm (95% CI, 0.4-2.5 Nm2; p = 0.01) from 6.5 ± 0.2 Nm to 8.0 ± 0.3 Nm, and 1.7 ± 0.4 Nm (95% CI, 0.05-3.3 Nm; p = 0.043) from 6.5 ± 0.2 Nm to 8.2 ± 0.4 Nm respectively, and increasing humeral lateralization from 15 mm to 30 and 35 increased anterior cuff torque by 1.2 ± 0.2 Nm (95% CI, 0.3-2.2 Nm; p = 0.012) from 6.8 ± 0.2 Nm to 8.0 ± 0.3 Nm, and 1.5 ± 0.3 Nm (95% CI, 0.2-2.7 Nm; p = 0.02) from 6.8 ± 0.2 Nm to 8.2 ± 0.4 Nm respectively. At 90° abduction and 60° internal rotation, increasing humeral lateralization from 15 mm to 35 mm in 5-mm increments increased anterior cuff torque by 0.4 ± 0.1 Nm (95% CI, 0.2-0.7 Nm; p = 0.002) from 0.6 ± 0.6 Nm to 1.0 ± 0.6 Nm, 0.7 ± 0.2 Nm (95% CI, 0.06-1.3 Nm; p = 0.03) from 1.0 ± 0.6 Nm to 1.7 ± 0.5 Nm, 0.5 ± 0.1 Nm (95% CI, 0.2-0.8 Nm; p = 0.006) from 1.7 ± 0.5 Nm to 2.2 ± 0.6 Nm, and 0.6 ± 0.1 Nm (95% CI, 0.2-0.9 Nm; p = 0.002) from 2.2 ± 0.6 Nm to 2.8 ± 0.6 Nm respectively. At 30° internal rotation, increasing humeral lateralization from 20 mm to 35 mm increased anterior cuff torque by 1.7 ± 0.2 Nm (95% CI, 1.1-2.4 Nm; p < 0.001) from 3.0 ± 0.3 Nm to 4.7 ± 0.4 Nm.
Traditionally, RTSA improves the deltoid moment arm by shifting the center of rotation in a medial and distal position compared with the native shoulder . However, this places the rotator cuff at a biomechanical disadvantage, which may limit shoulder rotation . Humeral lateralization may improve the moment arm of the rotator cuff , but available in vitro data are lacking. The results of our study suggest that the effects of humeral component lateralization on rotator cuff torque were largely dependent on the position of the arm. When the arm was not elevated, humeral lateralization tended to increase internal and external rotation torque in the direction of motion, and weaken internal and external rotation torque in the direction opposite motion. When the arm was abducted at 45° and 90°, humeral lateralization did not have an effect on external rotation, but internal rotation torque was increased by lateralization at the neutral and internally rotated shoulder positions. Overall, humeral lateralization after RTSA had an effect on rotator cuff torque. In certain shoulder positions, humeral lateralization improved the moment arms of the anterior and posterior rotator cuff, leading to increased torque (Fig. 5).
The current study had several limitations related to its in vitro biomechanical design. First, it does not allow for any effects of soft tissue adaptation with time. This may directly affect the true torque values generated by in vivo rotator cuff muscles. For instance, the subscapularis may be contracted before RTSA performed for posterosuperior cuff tear arthropathy. With increasing humeral component lateralization, this internal rotation contracture of the glenohumeral joint might be worsened. The current biomechanical study cannot account for such soft tissue changes. Additionally, with increased humeral lateralization, in some cases repair of the subscapularis may not be possible; and if repaired tightly, the subscapularis may function as a tether to limit external rotation. We also could not replicate the true in vivo effects of humeral lateralization as the cadavers had grossly normal rotator cuff tendons without degeneration, scarring, and/or tearing. In addition, despite our attempts to reproduce anatomic muscle vectors and physiologic forces according to the cross-sectional area ratios of the respective muscles , the true dynamic vectors are likely still different. Physiologic muscle forces are far more complex than the fixed values generated by computer-controlled pneumatic actuators. Such limitations are common to any in vitro study design, but the outcomes are preliminary and warrant further investigation.
The results obtained in our study apply to the specific conditions tested. As such, our data are specific to implants with neck-shaft angles of 155° and may not apply to other angles commercially available, such as 145° or 135° configurations. We also used 20° humeral component retroversion, but the optimal version in RTSA is still unknown [5, 20, 28]. Additionally, our experimental protocol involved testing the shoulder in various angles of rotation and abduction, some of which cannot be achieved by patients after RTSA, such as the limits of shoulder rotation at 60° or abduction at 90°. However, we tested our model in these shoulder positions because we aimed to detect any changes and trends in torque under the conditions of an in vitro biomechanical study design.
Maximizing rotator cuff muscle torque may lead to improved glenohumeral rotation after RTSA. In patients with bilateral shoulder arthroplasties with an anatomic TSA and RTSA performed on opposite sides, RTSA led to less external rotation (43° versus 12°; p < 0.0001) and internal rotation (spinal level T8 versus L1; p < 0.0001) after a minimum of 1 year followup . Bergmann et al.  suggested that the limitations in glenohumeral ROM were attributable to a lack of generated muscle force. Humeral lateralization may be a design modification that improves rotator cuff torque and glenohumeral rotation. Using a Grammont-style RTSA, Sirveaux et al.  reported only an average improvement of 11° external rotation after a mean followup of 44 months. In contrast, Frankle et al.  studied the results of RTSA using a less-medialized design and reported a mean postoperative external rotation of 36°. Unfortunately, the ability to place our observations into perspective is limited by a lack of available data indicating what absolute differences in torque are required for certain activities. However, it is clear that external and internal rotation torques are reduced after RTSA compared with those in normal healthy control subjects  and after anatomic TSA . Techniques to increase rotator cuff torque remain an important aspect to advance RTSA outcomes.
Historically, lateralizing the center of rotation in RTSA has led to increasing shear forces on the glenosphere and baseplate, which can increase the risk for glenoid loosening . With a lateralized RTSA implant configuration, Frankle et al.  reported a 12% rate of baseplate failure after an average followup of only 33 months. As it pertains to humeral lateralization, Giles et al.  showed that humeral lateralization did not increase joint loads, unlike glenosphere lateralization. Nevertheless, robust long-term clinical survivorship studies are lacking. In addition, the risks associated with implant modularity are only recently being recognized in shoulder arthroplasties . There also are substantial costs involved with increasing implant modularity. Taken together, the results of our biomechanical investigation should not be interpreted as evidence to support immediate changes to increase the lateral offset in current RTSA implants.
In this cadaver study, humeral component lateralization had an overall beneficial effect on the torque that the rotator cuff is able to generate. As such, it is a promising implant configuration that might improve RTSA biomechanics. However, our results remain preliminary and are drawn from a cadaveric biomechanical model with inherent limitations that are ultimately different from clinical in vivo situations. Further biomechanical and clinical studies are needed to correlate gains in rotator cuff torque with improved glenohumeral rotation and stability and to determine whether any substantial negative effects exist.
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