Abstract
Introduction
The purpose was to compare postoperative outcomes and functional improvement between patients with preoperative aER deficits vs. preserved aER function.
Results
There were 115 patients in the <0° aER group and 314 in the ≥30° aER group. Preoperative patients in the <0° group were worse for all measures except subjective pain while post-operatively, they had significantly greater improvement for all measures of motion. Postoperatively, both groups achieved comparable scores for forward elevation, pain, SST and ASES.
Conclusion
This study demonstrates that patients with a complete aER deficit can recover substantial and comparable function after RTSA.
Keywords: Reverse total shoulder, External rotation, Outcomes, Cuff tear arthropathy, Rotator cuff tear
1. Introduction
Reverse total shoulder arthroplasty (RTSA) has proven effective in improving comfort and function in patients with irreparable rotator cuff tears, cuff tear arthropathy, advanced shoulder arthritis, inflammatory arthopathy and posttraumatic arthritis. Its indications have expanded considerably in recent years and its prevalence now surpasses anatomic shoulder arthroplasty. Because many patients who require RTSA have tear configurations that involve the infraspinatus and/or teres minor, external rotation weakness is common to the pre-operative presentation. External rotation weakness in patients presenting for RTSA may also occur due to anatomical derangement of the glenohumeral joint such as posterior, superior, and/or medial erosion, common in cuff tear arthropathy cases. Such joint deformity may alter the length-tension relationship of the posterior rotator cuff and posterior deltoid, reducing the mechanical advantage of those muscles to rotate the arm against gravity. Such biomechanical alterations may compound the rotational impairment that occurs with posterior-superior rotator cuff tears by reducing the ability of the teres minor and/or posterior deltoid to compensate for infraspinatus deficiency.
Inability to externally rotate the arm can be functionally limiting because it significantly impairs the patient's ability to position the hand away from the body. Restoration of active external rotation (aER) after RTSA in those with substantial preoperative deficits can have a beneficial impact on overall functional outcomes, and failure to restore aER may be a source of patient dissatisfaction with the procedure. While RTSA can reliably restore forward elevation function, its ability to restore active external rotation is less predictable in those with preoperative aER deficits. Tendon transfer procedures were introduced to compensate for lack of external rotation improvement that was seen with Grammont-style RTSA implants.1 While some authors have reported improvement with latissimus dorsi transfer or combined latissimus dorsi and teres major tendon transfer, these procedures do not always have predictable outcomes, may sometimes only provide a tenodesis effect against the external rotation lag, and add potential surgical morbidity.1, 2, 3, 4, 5
The posterior deltoid, though not specifically an external rotator, may provide some external rotator moment after RTSA depending on the degree to which the procedure facilitates recruitment of its fibers. Biomechanical studies have demonstrated that different RTSA systems may result in different degrees of posterior deltoid recruitment depending on whether the glenosphere has a medialized or lateralized center of rotation (COR) design and the degree of offset between the axis of the humeral shaft and the deepest part of the humeral liner (humeral lateralization).6,7 As the position of the humerus is lateralized, more posterior deltoid fibers are recruited medial to the humeral component, which may facilitate aER following RTSA. Lateralization, which may be achieved through either a larger glenosphere, a lateralized center of rotation, an onlay humeral design or a thicker humeral liner, can also lengthen any remaining posterior rotator cuff thereby potentially improving its length-tension relationship.
To date the literature on how effectively implant design and configuration affects improvement in external rotation is controversial with some studies showing improvement with larger glenospheres, a lateralized center or rotation, or increased humeral offset and other studies showing no difference.8, 9, 10, 11, 12, 13, 14, 15, 16, 17 As such it is difficult to forecast the effect that RTSA will have on aER function in any given clinical case, including patients with an intact posterior rotator cuff and preserved preoperative aER function. Patients with well-compensated cuff function before surgery may be at risk to lose rotational range of motion due to muscle moment arm changes. This is a potential risk for patient dissatisfaction despite improvements in pain and overall outcome.
The purpose of this study was to examine the effect of a medialized glenoid, lateralized humeral RTSA design on postoperative aER and overall functional improvement, comparing patients with a preoperative aER deficit of <0° to those with preserved preoperative aER function of ≥30°. We hypothesize isolated RTSA with this biomechanical design can improve aER and overall function in those with preoperative deficits without tendon transfer. Further, we hypothesize that patients with intact aER function prior to surgery experience less aER improvement after RTSA and are at risk to lose some of this function despite overall improved outcomes.
2. Methods
Information was gathered from a prospectively collected, multicenter database of patients who have undergone RTSA with at least two years follow-up. Informed consent was obtained for all subjects. All cases were performed by fellowship trained shoulder arthroplasty surgeons using the same implant (Exactech Equinoxe, Exactech, Inc., Gainesville, FL). This implant has a medialized center of rotation and achieves lateralization through the humeral implant by virtue of an onlay design. Glenospheres are available in 38, 42 and 46 mm and humeral offset can be achieved by different adaptor tray thicknesses (0, 5 mm, 10 mm) and liner thicknesses (0, 2.5 mm). Augmented baseplates are also available for cases with glenoid erosion including an 8° posterior augment, a 10° superior augment and a combined posterior/superior augment. These augmented baseplates provide some glenosphere lateralization which generally compensates for medial erosion that occurs from pathologic glenoid wear.
The database collects demographic, diagnosis-related, comorbidity and implant information along with preoperative and postoperative range of motion and outcomes measures. The database was queried for patients with <0° preoperative active external rotation (<0 aER) and those with ≥30° active external rotation (≥30 aER). Patients with a diagnosis of fracture were excluded as were patients who underwent a concomitant latissimus dorsi transfer. Groups were then compared for demographic, diagnosis and comorbidity differences. In addition, implant configuration including glenosphere diameter and combined humeral offset, as defined by the sum of thicknesses between the humeral adaptor tray and the humeral liner, were compared between groups.
Groups were also compared for preoperative and postoperative forward elevation, abduction, external and internal rotation range of motion, subjective daily pain (0–10), subjective function (0–10), SST and ASES scores. Improvement from pre-to postoperative measures was also compared between groups. Comparisons were performed using a 2-tailed Student t-test with significance set at P < 0.05. Regression analysis was used to determine if there was any correlation between implant configuration and improvement in active external rotation and other outcome measure.
A subgroup analysis was also performed for patients in the >30 aER groups whose postoperative active external rotation was unchanged or worse versus those for whom it was improved postoperatively. This was performed to determine if there were specific features that would predict which patients are at risk to lose aER after RTSA. The same statistical tests were performed for this analysis.
Regression analysis was further used to determine if there was any correlation between demographic, diagnostic, comorbidity or preoperative measures and improvement in aER.
3. Results
The database contains 1154 patients who have undergone RTSA with at least 2-years follow-up, including 987 patients (86%) with complete information on implant configuration and postoperative clinical measures. Of these 314 patients (31%) had ≥ 30° aER preoperatively and 115 patients (12%) had < 0° aER. Table 1 shows demographic and diagnostic information. There were no differences in average age, gender distribution or BMI between groups. Diagnoses of osteonecrosis and post-traumatic arthritis were significantly more common in the <0° group while the diagnosis of rotator cuff tear was significantly more common in the ≥30°. Patients in the ≥30° group were significantly more likely to have undergone prior surgery (33% vs 20%, p = 0.01). There was no difference in the rate of comorbidities between groups including diabetes, smoking, coronary artery disease, congestive heart failure, chronic liver disease and chronic renal disease. Follow-up interval did not differ between groups.
Table 1.
Demographic and diagnostic information for each group.
| ≥30° ER | <0° ER | P | |
|---|---|---|---|
| N | 314 | 115 | |
| Average Follow-up (months) | 40 (25–84) | 38 (24–96) | NS |
| Average Age | 72.8 ± 6.9 | 71.2 ± 8.3 | NS |
| Age Range | (50–90) | (53–88) | |
| Male/Female % | 43%/57% | 36%/64% | NS |
| Body Mass Index | 27.8 ± 5.3 | 28.5 ± 6.3 | NS |
| Prior Surgery % | 33% | 20% | 0.01 |
| Osteoarthritis | 51% | 49% | NS |
| Osteonecrosis | 1% | 7% | 0.006 |
| Rotator Cuff Tear | 49% | 25% | 0.01 |
| Cuff Tear Arthropathy | 45% | 46% | NS |
| Rheumatoid Arthritis | 4% | 5% | NS |
| Post-traumatic Arthritis | 0.50% | 9% | <0.001 |
NS: Not significant.
Table 2 shows the frequency of use of different implants for each group. There were no differences in the rates of use different glenosphere diameters or humeral liner thicknesses, though patients in <0° group did have a significantly greater frequency combined tray and liner thickness > 0 mm and 0 mm (p = 0.02). Augmented glenoid components were used more frequently in the ≥30° group (15%) compared to the <0° group (8%) (p = 0.03). There was no difference in the rate of subscapularis repair between groups.
Table 2.
Implant and surgery-related information for each group.
| ≥30° ER | <0° ER | P | |
|---|---|---|---|
| Subscapularis Repair | 50% | 51% | NS |
| Glenosphere Diameter 38/42/46 | 57%/40%/3% | 48%/49%/3% | NS |
| Humeral Linter 0/2.5 | 80%/20% | 75%/25% | NS |
| Combined Humeral Offset 0/2.5/5/7.5 | 77%/19%/3%/1% | 69%/21%/5.7%/4.3% | 0.02 |
| Augmented Baseplate | 15% | 8% | 0.03 |
NS: Not Significant.
Table 3 demonstrates preoperative range of motion, pain, function and outcome scores. Patients in the <0° group had significantly lower measures for all parameters except for pain which did not differ between groups. Notably average preoperative aER was −18 ± 8° (range −50 to −5°) for the <0° group and 41 ± 13° (range 30–85°) for the ≥30° group.
Table 3.
Preoperative range of motion and outcome data for each group.
| ≥30° ER | <0° ER | P | |
|---|---|---|---|
| Preop Abduction | 86 ± 34 | 56 ± 27 | <0.001 |
| Preop Forward Elevation | 101 ± 41 | 72 ± 33 | <0.001 |
| Preop IR Score | 4.0 ± 1.7 | 2.4 ± 1.5 | <0.001 |
| Preop External Rotation | 41 ± 13 | −18 + 8 | <0.001 |
| Preop Daily Pain | 5.8 ± 2.2 | 5.6 ± 2.0 | NS |
| Preop Function Score | 4.2 ± 2.1 | 2.7 ± 1.5 | <0.001 |
| Preop SST | 4.8 ± 3.0 | 2.4 ± 1.8 | <0.001 |
| Preop ASES | 40 ± 16 | 34 ± 14 | 0.004 |
NS: Not Significant.
Table 4 demonstrates postoperative measures for each group indicating nearly equal outcomes for all measures except for average abduction and aER. Average forward elevation, internal rotation, daily pain and function scores, SST and ASES scores were not significantly different. While average postoperative aER was better for the ≥30° group (41 ± 18°) compared to the <0° group (25 ± 18°), 183/314 (58%) patients in the ≥30° group were unchanged or worse in terms of postoperative aER. Within this subgroup, there were no differences in age, gender, BMI, diagnosis, comorbidities or implant configuration. Of note, patients in this group of 183 whose aER was unchanged or worse had an average preoperative aER of 45 ± 9° vs 36 ± 7° (p = 0.001). This suggests that patients with well-preserved preoperative aER are at substantial risk for losing some of this range after RTSA. There was no significant correlation between preoperative aER and improvement in aER for either group (R = 0.11 for <0°group; R = 0.24 for ≥ 30° group).
Table 4.
Postoperative range of motion and outcome data for each group.
| ≥30° ER | <0° ER | P | |
|---|---|---|---|
| Postop Abduction | 120 ± 29 | 106 ± 23 | <0.001 |
| Postop Forward Elevation | 139 ± 27 | 137 ± 24 | NS |
| Postop IR Score | 4.7 ± 1.6 | 4.6 ± 1.5 | NS |
| Postp External Rotation | 41 ± 18 | 25 ± 18 | <0.001 |
| Postop Daily Pain | 1.3 ± 2.1 | 1.2 ± 1.9 | NS |
| Postop Function Score | 8.0 ± 1.9 | 7.8 ± 1.8 | NS |
| Postop Much Better/Better | 69%/23% | 64%/22% | NS |
| Postop Unchanged/Worse | 4.3%/3.7% | 11%/3% | NS |
| Postop SST | 9.9 ± 2.6 | 9.6 ± 2.7 | NS |
| Postop ASES | 82 ± 20 | 81 ± 17 | NS |
NS: Not Significant.
Table 5 compares improvement in range of motion, pain and outcome measures from preoperative to postoperative. Patients in the <0° has significantly greater improvement for all measures except for daily pain and ASES score. Of note, average improvement in aER was −2 ± 21° for the ≥30° group vs 44 ± 20° for the <0° group (p < 0.001). Regression analysis demonstrated that lateral humeral offset had no correlation with aER improvement for the <0°group (R = 0.02). For the ≥30° group, regression analysis demonstrated a stronger but still weak correlation indicating less aER improvement in those with 0 mm of lateralization (R = 0.45).
Table 5.
Improvement in range of motion and outcomes from pre-to postoperative for each group.
| ≥30° ER | <0° ER | P | |
|---|---|---|---|
| Δ Abduction | 34 ± 40 | 50 ± 31 | <0.001 |
| Δ Forward Elevation | 37 ± 42 | 66 ± 38 | <0.001 |
| Δ IR Score | 0.7 ± 1.8 | 2.2 ± 2.0 | <0.001 |
| Δ External Rotation | −2.0 + 21 | 44 ± 20 | <0.001 |
| Δ Daily Pain | 4.2 ± 2.3 | 4.6 ± 2.9 | NS |
| Δ Function Score | 4.0 ± 2.6 | 5.1 ± 1.9 | <0.001 |
| Δ SST | 5.2 ± 3.3 | 7.2 ± 2.9 | <0.001 |
| Δ ASES | 42 ± 23 | 45 ± 19 | NS |
NS: Not Significant.
Regression analysis demonstrated that for both groups there was no significant correlation between any demographic, diagnostic, comorbidity or implant-related parameter and improvement in external rotation or any other outcome measure. This suggests that, for this clinical series, there are no apparent factors that can be used to predict who will experience the most versus the least improvement after RTSA.
4. Discussion
This study demonstrates that use of an RTSA system with a medialized COR and lateralized humeral design can result in significant improvement in aER in patients with <0° aER and comparable outcomes to those with preserved aER before surgery. Patients with preserved aER preoperatively are at substantial risk to lose some external rotation range of motion after RTSA despite large gains in forward elevation, abduction and overall outcome. In fact, the average improvement in aER in patients with ≥30° preoperatively was essentially flat at −2°. While the final postoperative average aER remained better for those with preserved preoperative aER function, the degree of improvement was much greater for those with preoperative deficits. This was true for all other outcome measures suggesting that margin for improvement is an important consideration when discussing expected outcomes of RTSA with patients.
Historically, the ability of RTSA to restore aER has been considered unpredictable. Early series with the Grammont-design demonstrated reliable improvement in elevation and abduction but limited improvement in aER.18 This was particularly found to be true when patients had fatty atrophy of the teres minor on preoperative imaging.19,20 Combined RTSA with latisimuss dorsi transfer was introduced to address this problem and has been proven effective in restoring ER in patients with preoperative deficiency, particularly those with combined loss of elevation and external rotation or so-called psueodoparalysis.1,5 It remains somewhat unclear to what degree tendon transfer patients effectively recruit this muscle as an external rotator or whether it works by a passive tenodesis effect. Most series reporting on this technique also do not have a comparative control group to determine the degree to which aER improvement is the result of the tendon transfer.
Subsequent generations of reverse implants have introduced designs with biomechanical differences relative to the Grammont design. These include implants with a lateralized center of rotation and inlay humerus (lateral glenoid/medial humerus [LG/MH]) and those with a medialized center of rotation and onlay humerus (medial glenoid/lateral humerus [MG/LH]).21 By lateralizing the greater tuberosity either through the glenoid or humeral components, each of these designs can increase the passive tension in any remaining posterior rotator cuff theoretically improving the length/tension relationship. Hamilton et al. demonstrated that while all reverse designs shorten the external rotators, the MG/LH design shortened them the least and that the moment arm of the external rotators is sensitive to lateral humeral offset.6,7 These studies have also shown the MG/LH design can recruit more posterior deltoid fibers compared to the LG/MH design by medializing the center of rotation. Deltoid fibers lateral to the center of rotation may have a weak external rotation moment suggesting that the design used in the current study could improve external rotation through both of these means. The restoration of aER in those with preoperative deficits in this study was comparable that achieved by prior studies combining RTSA with latissimus dorsi transfer.1, 2, 3, 4 Berglund et al. found similar results in restoring aER without tendon transfer for their series using a LG/MH design.8 Collectively, these studies may suggest that implant selection can play a role in the degree of improvement achieved after RTSA. Unfortunately, comparative clinical studies between implants with different designs that were normalized to account for all variable that could potentially affect outcomes would be nearly impossible to perform with sufficient power to prove this concept conclusively.
The literature addressing the impact of implant configuration on outcomes of reverse arthroplasty is controversial. Werner et al. found that larger glenospheres correlated with improved external rotation range of motion.17 Muller et al. also found that larger glenospheres demonstrated better external rotation at the side and greater abduction strength.13 Mollon et al. found similar results with improved rotation and elevation range of motion with larger glenospheres.12 Valenti et al. determined that RTSA designs with less medialization of the center of rotation demonstrated gains in aER.16 Tashjian found that increasing humeral liner thickness decreased passive rotation likely due to a tenodesis effect of increasing capsular tension.15
Sabesan on the other hand found no correlation between glenosphere size and degree of improvement in range of motion or overall outcomes after RTSA.14 Langhor and colleagues similarly found that glenosphere size did not affect postoperative range of motion.22 Berglund found that in LG/MH design, glenosphere size and total prosthesis lateralization did not correlate without improvement in external rotation or outcome.8 Our results support these latter studies finding no correlation between glenosphere size or combined humeral offset with any parameter of postoperative function or outcome improvement. This suggests that while certain implant configurations may impart theoretical biomechanical advantages, improvement in motion, strength and function is complex and multifactorial.
While both groups in this analysis experienced comparable postoperative outcomes, those with preoperative ER deficits had statistically greater functional improvement. These patients were also comparatively worse in terms of all other preoperative measures except for daily pain score. Interestingly, this group was less likely to have a diagnosis of rotator cuff tear than for patients with preserved external rotation function. This implies that shoulder pathology that severely compromises external rotation also has substantial impact on elevation, abduction and internal rotation. Furthermore, it suggests that loss of function may stem from causes other than cuff pathology such as the effect of bone erosion on shoulder biomechanics and the role of stiffness in motion loss. Unfortunately, we were not able to collect data on the pattern and degree of cuff pathology in these patients, nor other measures of cuff function such as atrophy and fatty infiltration to determine a correlation between these and outcome improvement. Berglund et al. found by regression analysis in their study that Goutallier score did not correlate with improvement in external rotation nor were there any other clear-cut predictors.8
The greater degree of improvement in patients with preoperative aER deficits demonstrates that margin for improvement is an important consideration in determining the relative benefit patients can expect from RTSA. The fact that 58% of patients with preserved preoperative aER function were unchanged or worse in terms of their postoperative aER is important information in terms of counseling patients on the potential for selective loss of motion despite gains in other areas such as elevation. Depending on a patient's specific expectations for surgery, this could be a possible source of dissatisfaction although the percentage of patient who rated themselves as unchanged or worse in this series was not statistically different for this group versus for those who aER was improved.
5. Conclusion
This study demonstrates that using a MG/LH design prosthesis, patients with <0° preoperative aER can achieve postoperative outcomes that are comparable to those with preserved aER and can expect a degree of improvement that is actually significantly higher for range of motion, subjective function and SST. Pain improvement and ASES improvement were comparable between groups. We were not able to demonstrate a correlation between improvement and implant configuration to make specific recommendations. Future studies should aim to further our understanding of how preoperative cuff pathology and joint derangement impact function improvement and whether specific implant designs and configurations can be intelligently chosen to address biomechanical deficiencies in a manner that will maximize functional gains.
Author contributions
Moby Parsons - Conceptualization; Investigation; Methodology; Project administration; Resources; Supervision; Validation; Visualization; Roles/Writing - original draft; Writing - review & editing.
Howard D. Routman - Conceptualization; Investigation; Methodology; Resources; Validation; Visualization; Roles/Writing - original draft; Writing - review & editing.
Christopher P. Roche - Data curation; Formal analysis; Investigation; Methodology; Resources; Software; Supervision; Validation; Visualization; Roles/Writing - original draft; Writing - review & editing.
Richard J. Friedman - Conceptualization; Data curation; Funding acquisition; Investigation; Methodology; Project administration; Resources; Supervision; Validation; Visualization; Roles/Writing - original draft; Writing - review & editing.
Disclosures
Dr. Parsons is a consultant for Exactech Inc. for product design and medical education. Dr. Routman is a consultant for Exactech, Inc. and receives research support and royalties. Mr. Roche is a paid employee of Exactech, Inc. and is a shareholder. Dr. Friedman is a consultant for Exactech, Inc.
Institutional review board (IRB)
The Institutional Review Board of the Medical University of South Carolina approved this study (Pro 00030914) on September 11, 2018. The approval letter and most recent continuing review approval is included within the submission.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of competing interest
Richard Friedman: This author received consultant payments from Exactech, which is directly related to the subject of this work.
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