Reverse total shoulder arthroplasty component center of rotation affects muscle function

doi:10.1016/j.jse.2013.11.025

Journal of Shoulder and Elbow Surgery

Volume 23, Issue 8, August 2014, Pages 1128-1135

https://doi.org/10.1016/j.jse.2013.11.025 Get rights and content

Background

Medialization of the glenohumeral center of rotation alters the moment arm of the deltoid, can affect muscle function, and increases the risk for scapular notching due to impingement. The objective of this study was to determine the effect of position of the glenosphere on deltoid efficiency and the range of glenohumeral adduction.

Methods

Scapulohumeral bone models were reconstructed from computed tomography scans and virtually implanted with primary or reverse total shoulder arthroplasty implants. The placement of the glenosphere was varied to simulate differing degrees of “medialization” and inferior placement relative to the glenoid. Muscle and joint forces were computed during shoulder abduction in OpenSim musculoskeletal modeling software.

Results

The average glenohumeral joint reaction forces for the primary total shoulder arthroplasty were within 5% of those previously reported in vivo. Superior placement or full lateralization of the glenosphere increased glenohumeral joint reaction forces by 10% and 18%, respectively, relative to the recommended reverse total shoulder arthroplasty position. The moment arm of the deltoid muscle was the highest at the recommended baseline surgical position. The baseline glenosphere position resulted in a glenohumeral adduction deficit averaging more than 10° that increased to more than 25° when the glenosphere was placed superiorly. Only with full lateralization was glenohumeral adduction unaffected by superoinferior placement.

Discussion/Conclusion

Selecting optimum placement of the glenosphere involves tradeoffs in bending moment at the implant-bone interface, risk for impingement, and deltoid efficiency. A viable option is partially medializing the glenosphere, which retains most of the benefits of deltoid efficiency and reduces the risk for scapular notching.

Section snippets

Construction of bone geometry

In a previous study, computed tomography (CT) scans were obtained from cadaveric shoulder specimens (N = 40) and segmented by the commercially available software Mimics (Materialise, Leuven, Belgium).¹² The 3-dimensional CT reconstructions were validated by physical measurements and surgical reconstruction. Differences between physical measurements made on cadaveric specimens and virtual measurement on 3-dimensional CT reconstructions ranged from 0.7 to 2.7 mm. In addition, cadaveric

Results

The glenohumeral joint reaction force for the primary total shoulder arthroplasty condition at 90° of abduction was 66% body weight and was within 5% of the force reported in vivo with use of an instrumented shoulder prosthesis (Fig. 3, A).¹ This result supported the validity of our rigid-body dynamics approach for predicting shoulder forces. Simulating the RTSA condition, using recommended placement to medialize the center of rotation, and placing the glenosphere at the inferior margin of the

Discussion

The rationale for the RTSA design is to convert the glenohumeral joint into a more stable ball-and-socket joint. This design facilitates shoulder abduction without requiring a functioning rotator cuff. However, surgical recommendations for glenosphere placement are conflicting. Medializing the center of rotation is recommended to reduce the bending moment at the implant-bone interface. On the other hand, lateralizing the center of rotation is proposed to reduce laxity of the external rotators,

Conclusion

Our study quantifies the tradeoffs in selecting optimum placement of the glenosphere in RTSA. Medialization does reduce the bending moment at the implant-bone interface but requires inferior placement to reduce the risk for impingement and scapular notching. Lateralization with a bone graft can reduce the risk for impingement but carries the risk of higher bending moment at the host-graft interface (at least until healing). One viable option is partially medializing the glenosphere in RTSA,

Disclaimer

The authors, their immediate families, and any research foundation with which they are affiliated did not receive any financial payments or other benefits from any commercial entity related to the subject of this article.

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Cited by (51)

Measurement of lateralization and distalization in Grammont and onlay reverse total shoulder arthroplasty
2023, Seminars in Arthroplasty JSES
Reverse total shoulder arthroplasty (RTSA) implants allow variation in lateral and distal offset (LO and DO). The aim of this study was to measure, compare, and classify LO and DO and to assess the effects of modular changes using 2 humeral RTSA implants (one Grammont and one onlay-type) paired with a common modular glenoid component.
Three thousand eight hundred sixty virtual RTSA models were created. Measurements of LO and DO between reference points (B—center of the ‘baseplate;’ C—‘glenosphere’ center of rotation; P—pivot point of the ‘insert;’ M—midpoint of the distal surface of the ‘tray’ in the onlay humeral implant or midpoint of the proximal surface of the ‘spacer’ in the Grammont implant; J—junction of humeral stem and ‘tray,’ ‘liner’ or ‘spacer;’ H—distal end of stem) were taken. ‘Glenoid offset’ was distance BP (comprising BC and CP); ‘humeral offset’ was distance PH (comprising PJ, JM, and MH); their sum was the ‘global offset.’
Global LO and DO measurements were 14.9-30.9 mm and 13.1-36.3 mm, for the Grammont implant, and 23.0-52.6 mm and 18.0-42.2 mm for the onlay implant. Glenoid LO and DO measurements were 8.2-17.3 mm and 16.3-25.0 mm for the Grammont implant and 11.6-23.1 mm and 12.8-23.3 mm for the onlay implant. LO and DO did not vary proportionally in either implant type. Baseplate modularity affects distance BC; ‘glenosphere’ modularity affects distance BC, (‘glenosphere’ eccentricity and lateralization) and/or distance CP (‘glenosphere’ diameter). Humeral modularity affects distance PH (stem, ‘insert,’ ‘tray,’ and ‘spacer’ geometry, overall neck shaft angle [NSA]) and distance CP (overall NSA). Overall NSA also dictated the relative amount of humeral LO and DO resulting from other humeral modular changes.
Both Grammont and onlay humeral RTSA implants paired with a common modular glenoid component can be configured to create a large range of LO and DO, but values are typically greater when using the onlay humeral component. This is despite the higher NSA of the Grammont implant which would be expected to result in more DO. The described classification of DO together with LO may be beneficial. Previous classifications of RTSA based on their ‘glenoid’ (distance BP) and ‘humeral’ (distance PH) offsets are misleading. Distance BC should be considered as the measure of ‘glenoid’ and distance CH as the measure of ‘humeral’ offset. The ‘glenosphere’ is best thought of as a ‘nonglenoid, nonhumeral spacer’ that can affect both BC and CH.
Effect of humeral tray offset on clinical outcomes and computed tomography parameter in reverse total shoulder arthroplasty
2023, Seminars in Arthroplasty JSES
Despite the evolution of reverse total shoulder arthroplasty (RTSA), limitations in range of motion (ROM) and strength leading to limited increases in functional scores have been reported. This study was undertaken to determine the effects of humeral tray lateralization.
A retrospective clinical study was performed on 35 patients who underwent RTSA with a single implant (Aequalis Ascend Flex short stem; Tornier SAS–Wright Medical Inc., Bloomington, MN, USA) and a humeral tray offset of 1.5 mm from July 2016 to November 2020 by a single surgeon. Patients were classified into two groups: an inferiorly positioned humeral tray group resulting in humeral lateralization (the inferior group [n = 22]) and a superiorly positioned humeral tray group resulting in distalization (the superior group [n = 13]). Constant-Murley score, University of California-Los Angeles shoulder scale, Simple Shoulder Test, Activity scale, American Shoulder and Elbow Surgeons shoulder score, and pain visual analog score were evaluated. ROM for forward flexion and abduction was measured. Strengths of forward flexion, internal rotation at 90° abduction, and external rotation at side (ER) were measured. Computed tomography was performed postoperatively to measure the amount of humeral lateralization.
Postoperative ER power was greater in the inferior group (32.39 N ± 12.08 N) than in the superior group (23.77 N ± 10.96 N; P = .043). Postoperative abduction ROM was also greater in the inferior group (158.18° ± 18.36° vs. 128.08° ± 34.97°; P = .011), and humeral tray center to greater tubercle distance was larger in this group (54.68 mm ± 5.31 mm vs. 50.76 mm ± 3.65 mm; P = .025).
Lateralization by inferior humeral tray positioning provided better results than distalization by superior humeral tray positioning in RTSA in terms of ER strength and abduction ROM. RTSA with humeral lateralization appears to result in better functional outcomes in certain conditions.
Biomechanical consequences of glenoid and humeral lateralization in reverse total shoulder arthroplasty
2023, Journal of Shoulder and Elbow Surgery
The objective of our study was to quantify the biomechanical effectiveness of lateralization in RTSA with respect to glenoid and humeral component configurations.
Eight cadaveric shoulders were tested in a custom shoulder testing system. Three parameters, including the glenosphere thickness, humeral tray offset, and insert thickness, were assessed by implanting 8 configurations on each specimen. Humeral position, maximum internal rotation, and maximum external rotation (ER) before impingement were quantified at 0° and 30° glenohumeral abduction. The adduction angle at which the humeral component contacted the inferior scapular neck and the abduction angle where acromial notching occurred were also measured. The simulated active range of motion, including ER and abduction capability, was tested by increasing the load applied to the remaining posterior cuff and middle deltoid, respectively. Stability was evaluated by the forces that induced anterior dislocation at 30° abduction.
The thicker glenosphere affected only lateralization, whereas the centric humeral tray and thicker insert significantly affected humeral lateralization and distalization simultaneously. Greater adduction and ER angles were found in more lateralized humerus. A significant positive correlation between humeral lateralization and ER capability was observed; however, lateralization did not significantly improve implant stability in this cadaveric testing system.
Lateralization is achievable at both the glenoid and humeral sides but has different effects; therefore, lateralized implant options should be selected according to patients’ needs. Lateralization is an effective strategy for reducing adduction notching while increasing ER capability. Thicker glenospheres only affected humeral lateralization. The centric humeral tray would be selected for less distalization to avoid overlengthening, whereas an eccentric humeral tray is the most effective for distalization and medialization in reducing abduction notching to the acromion and for patients with pseudoparalysis.
Does isolated glenosphere lateralization affect outcomes in reverse shoulder arthroplasty?
2023, Orthopaedics and Traumatology: Surgery and Research
While lateralization of the glenohumeral center of rotation during reverse shoulder arthroplasty (RSA) has benefits of maintaining tension on the remaining rotator cuff and decreasing implant impingement on the glenoid, few clinical studies have evaluated the isolated effect of glenoid lateralization in RSA. The purpose of this study was to evaluate if clinical outcomes are affected by isolated glenosphere lateralization using a single implant design.
A retrospective review from a multicenter shoulder arthroplasty research database was performed between 2011 and 2018 using a single implant system to perform this case-controlled study. Inclusion criteria included primary RSAs with adequate preoperative and postoperative active and passive range of motion (ROM) measurements, outcome scores, and a minimum two-year follow-up. Revision shoulder arthroplasties and RSA for fractures were excluded from analysis. 102 RSAs (61 females, 41 males) using a +4 mm lateralized glenosphere were compared to 102 sex, age, and glenosphere diameter matched control shoulders with standard glenospheres (whose center of rotation (CoR) is 2 mm lateral to the glenoid fossa). The mean age at surgery was 70.4 years. Mean follow up was 43.6 + 18.9 months. All RSAs were performed with the same implant system (Equinoxe, Exactech, Gainesville, FL). Clinical outcome measures included ROM, ASES, Constant, UCLA, SST, SPADI scores, and VAS pain scores. We used the chi-squared test and Fisher exact test for bivariate analysis and the student t-test for continuous variables.
Both groups were of similar average age and follow-up. They also had comparable rates of prior surgery and comorbidities. The lateralized glenosphere group had a slightly higher BMI (31.2 vs. 29.2, p = 0.04). Both groups demonstrated significant improvements in all outcome scores that exceeded the MCID and the SCB. The groups demonstrated similar preoperative, postoperative and improvements in ROM as well as outcome scores. The overall complication rate was similar between groups (4% in lateralized and 5% in controls, p = 0.73). Scapular or acromial fractures differences were not statistically significant between groups (1% in lateralized group vs. 3% in standard group, p = 0.31). Scapular notching was more frequent in the standard group compared to the lateralized group (9% vs. 2%, p = 0.03).
In a medialized glenosphere/lateralized humerus design, a +4 mm lateralized glenosphere provided no significant advantage in postoperative pain, ROM, or outcome scores. However, lateralized glenospheres did demonstrate significantly lower scapular notching rates.
III; retrospective cohort comparison; treatment study.
Reverse total shoulder glenoid component inclination affects glenohumeral kinetics during abduction: a cadaveric study
2022, Journal of Shoulder and Elbow Surgery
Optimal implant placement in reverse total shoulder arthroplasty (rTSA) remains controversial. Specifically, the optimal glenoid inclination is unknown. Therefore, a cadaveric shoulder simulator with 3-dimentional human motion specific to rTSA was used to study joint contact and muscle forces as a function of glenoid component inclination.
Eight human cadaver shoulders were tested before and after rTSA implantation. Scapular plane abduction kinematics from control subjects and those with rTSA drove a cadaveric shoulder simulator with 3-dimentional scapulothoracic and glenohumeral motion. Glenoid inclination varied from −20° to +20°. Outputs included compression, superior-inferior (S/I) shear, and anterior-posterior shear forces from a 6° of freedom load cell in the joint, and deltoid and rotator cuff muscle forces. Data were evaluated with statistical parametric mapping and t-tests.
Inferior glenoid inclination (−) reduced S/I shear by up to 125% relative to superior inclination, with similar compression to the neutral condition (0°). Superior inclinations (+) increased the S/I shear force by approximately the same magnitude, yet decreased compression by 25% in the most superior inclination (+20°). There were few differences in deltoid or rotator cuff forces due to inclination. Only the middle deltoid decreased by approximately 7% for the most inferior inclination (−20°). Compared with native shoulders, the neutral (0°) rTSA inclination showed reduced forces of 30%-75% in the anterior deltoid and a trend toward decreased forces in the middle deltoid. Force demands on the rotator cuff varied as a function of elevation, with a trend toward increased forces in rTSA at peak glenohumeral elevation.
Inferior inclination reduces superior shear forces, without influencing compression. Superior inclination increased S/I shear, while decreasing compression, which may be a source of component loosening and joint instability after rTSA. Inferior inclination of the rTSA glenoid may reduce the likelihood of glenoid loosening by reducing the magnitude of cyclic shear and compressive loading during arm elevation activities, although this may be altered by specific-subject body habitus and motion. These factors are especially important in revision rTSA or glenoid bone grafting where there is already a 3-fold increase in glenoid baseplate loosening vs. primary rTSA.
Reverse shoulder arthroplasty with metallic augments to preserve bone and restore joint line in patients with glenoid bone loss
2022, Seminars in Arthroplasty JSES
Glenoid bone loss poses significant challenges for reverse shoulder arthroplasty. In these patients, excess reaming can lead to further bone loss and medialization of the joint line. Metallic augments have been described as a technique for addressing this issue. However, there are currently no studies evaluating the effect of metallic augments on anatomic outcomes. This study evaluates the effect of metallic augments on the glenohumeral joint line and bone preservation. We hypothesize that metallic augments can preserve bone while preventing further medialization of the joint line in patients with preoperative glenoid bone loss.
A prospective single-center, 2-surgeon consecutive case series was evaluated with postoperative computed tomography scans. Three-dimensional reconstruction was used to create models of the scapula and humerus. Comparisons were made with preoperative computed tomography scans using volumetric analysis to evaluate bone loss. The base of the coracoid was used as a constant landmark to assess the joint line position and, by proxy, the center of rotation. Further evaluation of the glenoid version and inclination was measured on the reconstructed models through standardized techniques.
Fifteen patients (16 implants) underwent preoperative planning and reverse shoulder arthroplasty with metallic augments from January 1, 2018, to January 1, 2021. The mean follow-up time was 13.1 months. Analysis revealed that augments prevented further medialization of the joint line and corrected the deficit on average by 10.2 mm (range 6.9-14.3; standard deviation 2.0). The mean bone volume removed during surgery was 1292 mm³ (range 525-2256; standard deviation 527), with this value inclusive of the baseplate post volume (∼800 mm³). Inclination and version were restored to acceptable limits in all cases.
Reverse shoulder arthroplasty with metallic augments reliably preserves bone and prevents medialization of the joint line in patients with preoperative glenoid bone loss. Further research is required to assess the long-term and functional outcomes of this technique.

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Basic ScienceReverse total shoulder arthroplasty component center of rotation affects muscle function

Background

Methods

Results

Discussion/Conclusion

Section snippets

Construction of bone geometry

Results

Discussion

Conclusion

Disclaimer

J Biomech

J Orthop Res

J Shoulder Elbow Surg

J Shoulder Elbow Surg

J Shoulder Elbow Surg

Orthop Clin North Am

J Shoulder Elbow Surg

Grammont reverse prosthesis: design, rationale, and biomechanics

J Shoulder Elbow Surg

Tendons, ligaments, and capsule of the rotator cuff. Gross and microscopic anatomy

J Bone Joint Surg Am

Reverse shoulder arthroplasty for the treatment of rotator cuff deficiency

J Bone Joint Surg Am

Basic Science
Reverse total shoulder arthroplasty component center of rotation affects muscle function