While early clinical studies have demonstrated favorable outcomes following bilateral RSA, there is an evident discrepancy in the literature regarding the restoration of internal rotation. The first known report on bilateral RSA outcomes was a case series by Wiater et al. [16] involving 16 patients with a minimum follow-up of 2 years. These authors reported significant improvements postoperatively in the Constant-Murley (Constant) score, American Shoulder and Elbow Surgeons (ASES) score, Subjective Shoulder Value (SSV), visual analog scale (VAS) for pain, and active forward elevation in both shoulders [16]. Notably, the achieved results were comparable to those of matched unilateral RSA patients [16]. However, patients were found to have unreliable improvements in rotation, with 8 (50%) and 2 (12.5%) patients demonstrating decreased active external rotation and internal rotation, respectively, in at least one shoulder. Nevertheless, 94% of patients reported high satisfaction [16]. Wirth et al. [7] similarly highlighted the risk of insufficient internal rotation following bilateral RSA. In their retrospective study including 57 patients with a minimum follow-up of 1 year, 21% and 33% of patients were unable to reach the lumbosacral junction after the first and second RSA, respectively, and 5% had insufficient internal rotation bilaterally at 2 years [7].

On the other hand, several studies have shown optimal outcomes with postoperative improvements in both external and internal rotation. In a study by Levy et al. [6], a prospective case series analysis of 19 patients with a mean follow-up of 48.4 months (24–75 months) and a mean duration between staged operations of 18.2 months (3–46 months) was performed. According to the authors, there was significant improvement in clinical function, as the average Constant score improved from 18.7 to 65.1 points, the average SSV improved from 2.1 to 9.2 points, and active ROM improved in all aspects (forward elevation, abduction, internal rotation, external rotation) [6]. Moreover, in contrast to previous studies, patients’ mean active internal rotation significantly improved from 9° to 81°, and 30 shoulders (79%) could reach above the sacroiliac joint [6]. Mean active external rotation also significantly improved from 20° to 32°, with 35 shoulders (92%) achieving >20° of external rotation in adduction and 31 shoulders (82%) demonstrating full external rotation in elevation [6]. Moreover, Mellano et al. [4] analyzed 50 patients (100 shoulders) with a mean follow-up of 61 months and a minimum 2-year follow-up after the second procedure, determining that significant improvements in both clinical outcomes and pain were attained as demonstrated by changes in the mean ASES score (from 35.8 to 76.5 points), mean VAS pain score (5.7 to 1.0 points), mean Simple Shoulder Test (SST) score (2.4 to 8.0 points), and mean 12-item short form survey (SF-12) physical component subscore (30.5–39.7 points) [4]. The mean postoperative ROM also significantly improved from 72° to 136° of forward elevation, 84° to 121° of abduction, 26° to 45° of external rotation in adduction, and 39° to 58° of internal rotation in abduction, respectively [4]. A summary of studies assessing clinical outcomes and ROM is presented in Table 1 [4,6-8,16-18].

It is important to note, however, that internal rotation is not measured uniformly across studies in the literature. Though some measure active internal rotation in degrees, with the arm at the side or abducted to 90° [4,6], using a goniometer to measure internal rotation can be difficult and may not be possible or convenient during a routine clinical examination [17]. As a result, many physicians use the scratch test, which assesses the highest vertebral level one can reach behind their back, to measure functional internal rotation [6-8,16-18]. Although this approach is the most common method for such measurement, one limitation of the scratch test is that it does not measure internal rotation in isolation, but rather assesses a combination of shoulder extension, internal rotation, and elbow flexion. As such, one of the biggest compensations for reaching behind the back above the sacroiliac joint is elbow flexion.

Concerns have been raised about performing bilateral RSA due to the potential deficits in rotation, which could lead to difficulties with ADLs and impact the quality of life of patients. While the results of restoring internal rotation have varied in the literature, the evident improvements in clinical outcomes, pain relief, and various ROMs raise the question of whether deficits in internal rotation movements indeed affect ADLs and outweigh the clinical benefit of RSA. Several studies have sought to explore the ability of patients to complete ADLs and compensate for any loss in rotation following bilateral RSA.

In one study by Mellano et al. [4], the majority of patients retained independence with personal hygiene and ADLs following bilateral RSA. Among the 50 patients these authors studied, 94% used the same hand for toileting without difficulty, 97% did not require an assistive device for toileting, 66% were able to wash the opposite shoulder with the contralateral hand, and 83% could tuck a shirt-tail into the back of their pants [4]. However, it was found that washing the back and securing a bra were the most-impacted ADLs, with 17% of patients reporting these actions were “somewhat difficult,” 16% noting they were “very difficult,” and 39% noting they were “unable to perform” them. Additionally, 50% of patients required an assistive device in the shower [4]. Nevertheless, the patients who had to adapt and modify their hygiene habits reported the use of different strategies allowing them to compensate successfully. For patients who experienced difficulty in the shower or when washing their back, the most common strategies used included employing a long scrubber, using a long washcloth, and switching to baths instead of showers [4]. For female patients who had difficulty securing a bra, the most common strategies included fastening the bra in the front, switching to a sports bra, and fastening the bra and subsequently stepping into it and bringing it up the body [4].

Similarly, Levy et al. [6] reported that all 19 patients of theirs retained independence with personal hygiene and ADLs after bilateral RSA, with a mean activities of daily living external and internal rotations (ADLEIR) score of 33 out of a possible 36 points. Most patients were also able to resume their leisure activities, such as gardening, golf, swimming, and bowling, without limitations [6]. Morris et al. [18] evaluated 11 patients with bilateral RSA with a minimum 2-year follow-up, and all of these patients were able to use at least one arm to comb their hair, reach their back pocket, eat with a utensil, wash the opposite shoulder, and manage toileting. Only one patient (9%) reported being unable to perform perineal hygiene and to dress alone postoperatively [18].

Moreover, Assenmacher et al. [8] conducted a survey to assess how patients with bilateral RSA perform various ADLs. Of 31 patients with a minimum 1-year follow-up, 100% were able to manage toileting without difficulty and 87% could reach their back pocket [8]. None of their patients described themselves as permanently or temporarily disabled because of their shoulders [8]. Similar to in the previous studies, when washing their back or securing a bra, 29% of patient found it very difficult and 39% were unable to perform these tasks, respectively [8].

Several studies have compared the effectiveness of treatment for various combinations of TSA procedures [5,9,19-22]. Cox et al. [20] compared outcomes of 26 bilateral anatomic TSA and 13 bilateral RSA matched patients with a minimum 2-year follow-up and recorded significantly greater ADLEIR, ASES, and Single Assessment Numeric Evaluation (SANE) scores in patients who underwent bilateral TSA. Postoperatively, TSA patients also had significantly greater forward elevation, external rotation, and internal rotation when compared to RSA patients [20]. However, when assessing pre- to postoperative improvements in ROM, only external rotation remained significant [20]. Kurowicki et al. [21] also noted a greater SF-12 physical component subscore and ASES and SANE scores at a mean follow-up of 51 months among bilateral TSA patients; however, the pre- to postoperative improvement of these clinical outcomes scores did not differ significantly between the two groups. Only the change in pre- to postoperative internal rotation was significantly greater in TSA patients [21].

In a meta-analysis comparing outcomes of patients who underwent bilateral TSA versus bilateral RSA, the mean ASES and SANE scores were 8.96 and 13.06 points higher, respectively, in TSA patients compared to RSA patients [5]. While statistically significant, these values do not reach clinical significance, as the minimal clinically important difference (MCID) is 20.9 points for ASES and 14.9 points for SANE [23,24]. The authors also documented significantly greater mean active forward elevation and external rotation in their TSA cohort as well [5]. However, improvements in ROM from the preoperative state were not assessed.

An additional study by Triplet et al. [22] evaluated functional internal rotation tasks between 47 bilateral TSA patients, 17 bilateral RSA patients, and 9 TSA/RSA patients with a mean follow-up of 72 years. While baseline internal rotation was similar in the three cohorts, the bilateral TSA group had the greatest improvement, demonstrating significantly greater postoperative internal rotation compared to the bilateral TSA and TSA/RSA cohorts [22]. However, while a greater proportion of bilateral RSA patients lost the ability to wash their back (52.9% vs. 30.4% TSA/TSA and 33.3% TSA/RSA), tuck a shirt-tail into the back of their pants (29.4% vs. 10.6% TSA/TSA and 11.1% TSA/RSA), and manage toileting (11.8% vs. 0% in both TSA/TSA and TSA/RSA) compared to the other two cohorts, these differences were not significant [22].

Moreover, Turnbull et al. compared outcomes of TSA and RSA after a prior contralateral TSA in patients with bilateral primary osteoarthritis and an intact rotator cuff [9]. While the postoperative outcomes of the first TSA in both cohorts were similar, patients who underwent a subsequent RSA contralaterally demonstrated significantly superior postoperative ASES, SST, Constant, Shoulder Pain and Disability Index, and University of California—Los Angeles scores in addition to active forward elevation and abduction compared to patients who underwent a subsequent TSA procedure [9]. No significant differences in active external and internal rotation or complications were found either [9]. The pre- to postoperative improvements in all outcome scores and ROM were not statistically significant, which may suggest a similar change overall; however, the authors noted that a significantly greater proportion of patients undergoing RSA exceeded the MCID for the Constant and ASES scores as well as the substantial clinical benefit for forward elevation [9].

Patients with bilateral RSA demonstrate favorable outcomes and can perform most internal rotation tasks similarly to patients who undergo bilateral TSA as well as TSA/RSA. While postoperative outcomes may differ between various shoulder arthroplasty combinations, the effectiveness of treatment—represented by the change in pre- to postoperative function—is comparable.

Several studies to date have investigated the role of glenoid component positioning on ROM after RSA. A series of computed tomography (CT)-based modeling studies have attempted to provide guidelines on the maximum-possible impingement-free ROM after RSA. Li et al. [10] demonstrated that, in a CT-based computer model, 10-mm lateralization with 6 mm of inferior translation of the glenosphere resulted in maximal impingement-free internal and external rotation. Inferior tilts of the glenosphere 15° and 30° also improved internal and external impingement-free rotation [10]. Arenas-Miquelez et al. [11] showed in a similar CT-based computer simulation study that moderate lateralization (≤4 mm) was the most-effective method for increasing total global ROM and that further extreme lateralization (≤12 mm) did not provide significant additional benefit. However, these findings were limited, as computer models do not account for soft tissue tension. Another CT-based computer modelling study by Huish et al. [12] found that a larger, lateralized, and inferiorized glenosphere had the greatest improvements in impingement-free motion. Of these factors, the greatest improvement in internal rotation was inferior positioning on the glenosphere [12]. These and other studies [27,28] suggest that an inferiorly placed, lateralized, larger, and inferiorly tilted glenosphere may minimize bony impingement between the scapular neck and the humeral implant. This in turn may result in increased internal rotation. However, routine placement of the glenosphere in this position does carry risks, such as overlengthening and increased shear forces across the baseplate and glenoid. Additionally, glenoid retroversion in virtual ROM studies have been found to increase internal rotation. Budge et al. [13] found that the greatest achievable internal rotation was at a native retroversion of 10° to −10°. Keener et al. [27] found that 20° of retroversion had a significant effect.

Werner et al. [29] performed a multicenter retrospective clinical study including 455 RSAs with a minimum 1 year of follow-up, which showed that glenoid lateralization of 6–8 mm was significantly associated with improved active internal rotation when compared to glenoid lateralization of <6 mm. While a larger glenosphere size was associated with improved ROM in computer modelling studies, increasing glenosphere diameter was shown to be significantly associated with decreased internal rotation at 90° of abduction [29]. Further studies with attempts at clinical correlation are needed to better understand the role of glenosphere positioning on internal rotation. The primary constraint on internal rotation on the glenoid side appears to be bony impingement, and preventing such impingement is critical not just for motion but also to minimize the risk of scapular notching.

Similarly, a number of studies have investigated the role of humeral component positioning on ROM after RSA. A cadaveric study by Gulotta et al. [14] found that, when using a humeral prosthesis with a 145° neck-shaft angle, increased humeral retroversion led to increased external rotation and decreased internal rotation. The authors found that placement of the humeral component in 0°–20° of retroversion allowed for maximum internal rotation with the arm at the side [14]. Conversely, in their multivariate analysis, Werner et al. [29] found that increased humeral retroversion was significantly associated with increased active internal rotation at 90° of abduction. Stephenson et al. [15] found in a 155° neck-shaft angle prosthesis that anteversion of the humeral stem increased internal rotation. Many have theorized that, while the total arc of humeral rotation does not change with changes in humeral version, the relative position of the arc of motion depends on the humeral component version. Again, these findings are limited by inadequate clinical correlation and an inability to model soft tissue tension and scapular mechanics in these computer modelling studies.

One additional factor to consider is the role of subscapularis repair. There is controversy regarding whether to repair the subscapularis during RSA since the constraint of the RSA implant compensates for rotator cuff deficiencies [30]. Even though the subscapularis is an internal rotator of the humerus, there are limited data to suggest a meaningful internal rotation benefit from subscapularis repair in RSA. One clinical study by Friedman et al. [31] compared 591 patients with subscapularis repair to those without using a 145° onlay prosthesis. Those with a repaired subscapularis did experience a significant improvement in internal rotation, albeit with only a modest effect size, i.e., L1–L3 versus L4–L5. Harries et al. [32] similarly reported a benefit of one vertebral level, which may not be clinically important, and other studies have shown no significant difference [33-35]. A meta-analysis of six articles studying subscapularis repair after RSA did not demonstrate a significant change in ROM in patients with subscapularis repair, although the data included in the meta-analysis were quite heterogeneous [36]. A notable finding emerged from a study by Erickson et al. [37], who performed ultrasound at 1 year after surgery in RSA patients to assess the integrity of subscapularis repair and its relationship with postoperative internal rotation. These authors found no difference in internal rotation between patients who experienced success of the subscapularis repair versus those who did not, suggesting that successful healing of the tendon did not change patients’ functional outcome [37].

Despite the above findings and improvements in preoperative planning, the effect of component positioning remains unpredictable in vivo. This is likely a reflection of the complex, multifactorial nature of motion after RSA. Currently, our understanding of scapulothoracic mechanics and their relative contribution to RSA stability remains incomplete. Similarly, the nature of soft tissue compliance and changes after RSA remain unclear. Proper soft tissue tensioning may allow more motion, but under-tensioning may increase the risk for instability. Future studies could provide a more complete picture of the complex interplay between component positioning, bony impingement, scapular motion, and soft tissue tension in RSA patients. Further clinical studies exploring the effect of various glenoid and humeral component positioning on stability, ROM, and functional outcomes are necessary.