The effect of glenosphere lateralization and inferiorization on deltoid force in reverse total shoulder arthroplasty

Robert H. Cofield, MD, Award for Best Oral Presentation 2023: Up to 8 mm of glenoid-sided lateralization does not increase the risk of acromial or scapular spine stress fracture following reverse shoulder arthroplasty with a 135° inlay humeral component

BACKGROUND

Glenoid-sided lateralization in reverse shoulder arthroplasty (RSA) decreases bony impingement and improves rotational range of motion, but has been theorized to increase the risk of acromial or scapular spine fractures (ASFs). The purpose of this study was to assess if glenoid-sided lateralization even up to 8 mm increases the risk for stress fracture following RSA with a 135° inlay humeral component.

METHODS

A retrospective review was performed from a multicenter prospectively collected database on patients who underwent primary RSA from 2015 to 2021. All RSAs were performed with a 135° inlay humeral component. Varying amounts of glenoid lateralization were used from 0 to 8 mm. Preoperative radiographs were reviewed for the presence of acromial thinning, acromiohumeral distance (AHD), and inclination. Postoperative implant position (distalization, lateralization, and inclination) as well as the presence of ASF was evaluated on minimum 1-year postoperative radiographs. Regression analyses were performed on component and clinical variables to assess for factors predictive of ASF.

RESULTS

Acromial or scapular spine fractures were identified in 26 of 470 shoulders (5.5%). Glenoid-sided lateralization was not associated with ASF risk (P = .890). Furthermore, the incidence of fracture did not vary based on glenoid-sided lateralization (0-2 mm, 7.4%; 4 mm, 5.6%; 6 mm, 4.4%; 8 mm, 6.0%; P > .05 for all comparisons). RSA on the dominant extremity was predictive of fracture (odds ratio [OR] 2.21, 95% confidence interval [CI] 1.20-5.75; P = .037), but there was no relationship between patient age, sex, preoperative acromial thinning, or diagnosis and risk of fracture. Although there was no difference in mean postoperative AHD between groups (P = .443), the pre- to postoperative delta AHD was higher in the stress fracture group (2.0 ± 0.7 cm vs. 1.7 ± 0.7 cm; P = .015). For every centimeter increase in delta AHD, there was a 121% increased risk for fracture (OR 2.21, 95% CI 1.33-3.68; P = .012). Additionally, for every 1-mm increase in inferior glenosphere overhang, there was a 19% increase in fracture risk (P = .025).

CONCLUSION

Up to 8 mm of glenoid-sided metallic lateralization does not appear to increase the risk of ASF when combined with a 135° inlay humeral implant. Humeral distalization increases the risk of ASF, particularly when there is a larger change between pre- and postoperative AHD or higher inferior glenosphere overhang. In cases of pronounced preoperative superior humeral migration, it may be a consideration to avoid excessive postoperative distalization, but minimizing bony impingement via glenoid-sided lateralization appears to be safe.

Biomechanical Characteristics of Glenosphere Orientation Based on Tilting Angle and Overhang Changes in Reverse Shoulder Arthroplasty

Background

Glenoid position and inclination are important factors in protecting against scapular notching, which is the most common complication that directly affects the longevity of reverse shoulder arthroplasty (RSA). This study aimed to investigate the biomechanical characteristics of glenosphere orientation, comparing neutral tilt, inferior overhang with an eccentric glenosphere at the same placement of baseplate, and inferior tilt after 10° inferior reaming in the lower part of the glenoid in RSA.

Methods

Nine cadaveric shoulders were tested with 5 combinations of customized glenoid components: a centric glenosphere was combined with a standard baseplate (group A); an eccentric glenosphere to provide 4-mm inferior overhang than the centric glenosphere was combined with a standard baseplate (group B); a centric glenosphere was combined with a wedge-shaped baseplate tilted inferiorly by 10° with the same center of rotation (group C); an eccentric glenosphere was attached to a wedge-shaped baseplate (group D); and 10° inferior reaming was performed on the lower part of the glenoid to apply 10° inferior tilt, with a centric glenosphere secured to the standard baseplate for simulation of clinical tilt (group E). Impingement-free angles for adduction, abduction, forward flexion, external rotation, and internal rotation were measured. The capability of the deltoid moment arm for abduction and forward flexion, deltoid length, and geometric analysis for adduction engagement were evaluated.

Results

Compared with neutral tilt, inferior tilt at the same position showed no significant difference in impingement-free angle, moment arm capability, and deltoid length. However, group D resulted in better biomechanical properties than a central position, regardless of inferior tilt. Group E demonstrated a greater range of adduction, internal and external rotation, and higher abduction and forward flexion capability with distalization, compared to corresponding parameters for inferior tilt with a customized wedge-shaped baseplate.

Conclusions

A 10° inferior tilt of the glenosphere, without changing the position of the baseplate, had no benefit in terms of the impingement-free angle and deltoid moment arm. However, an eccentric glenosphere had a significant advantage, regardless of inferior tilt. Inferior tilt through 10° inferior reaming showed better biomechanical results than neutral tilt due to the distalization effect.

The Long Head of the Biceps Has a Stabilizing Effect on the Glenohumeral Joint in Simulated Infraspinatus or Subscapularis but Not Supraspinatus Rotator Cuff Deficiency: A Biomechanical Study

PURPOSE

To investigate the stabilizing role of the long head of the biceps (LHB) for different simulated rotator cuff (RC) tears.

METHODS

Human cadaveric specimens (n = 8) were fixed in a robotic-based experimental setup with a static loading of the RC, deltoid, and the LHB. RC tears were simulated by unloading of the corresponding muscles. A throwing motion and an anterior load-and-shift test were simulated under different RC conditions by unloading the supraspinatus (SS), subscapularis (SSc), infraspinatus (IS), and combinations (SS + SSc, SS + IS, SS + SSc + IS). The LHB was tested in 3 conditions: unloaded, loaded, and tenotomy. Translation of the humeral head and anterior forces depending on loading of the RC and the LHB was captured.

RESULTS

Loading of LHB produced no significant changes in anterior force or glenohumeral translation for the intact RC or a simulated SS tear. However, if SSc or IS were unloaded, LHB loading resulted in a significant increase of anterior force ranging from 3.9 N (P = .013, SSc unloaded) to 5.2 N (P = .001, simulated massive tear) and glenohumeral translation ranging from 2.4 mm (P = .0078, SSc unloaded) to 7.4 mm (P = .0078, simulated massive tear) compared to the unloaded LHB. Tenotomy of the LHB led to a significant increase in glenohumeral translation compared to the unloaded LHB in case of combined SS + SSc (2.6 mm, P = .0391) and simulated massive tears of all SS + SSc + IS (4.6 mm, P = .0078). Highest translation was observed in simulated massive tears between loaded LHB and tenotomy (8.1 mm, P = .0078).

CONCLUSIONS

Once SSc or IS is simulated to be torn, the LHB has a stabilizing effect for the glenohumeral joint and counteracts humeral translation. With a fully loaded RC, LHB loading has no influence.

CLINICAL RELEVANCE

With an intact RC, the condition of the LHB showed no biomechanical effect on the joint stability. Therefore, from a biomechanical point of view, the LHB could be removed from the joint when the RC is intact or reconstructable. However, since there was a positive effect even of the unloaded LHB in this study when SSc or IS is deficient, techniques with preservation of the supraglenoid LHB origin may be of benefit in such cases.

The Influence of Component Design and Positioning on Soft-Tissue Tensioning and Complications in Reverse Total Shoulder Arthroplasty: A Review

» Reverse total shoulder arthroplasty was designed to function in the rotator cuff deficient shoulder by adjusting the glenohumeral center of rotation (COR) to maximize deltoid function.» Adjustments in the COR ultimately lead to changes in resting tension of the deltoid and remaining rotator cuff, which can affect implant stability and risk of stress fracture.» Soft-tissue balance and complication profiles can be affected by humeral component (version, neck shaft angle, and inlay vs. onlay) and glenoid component (sagittal placement, version, inclination, and lateralization) design and application.» A good understanding of the effects on soft-tissue balance and complication profile is critical for surgeons to best provide optimal patient outcomes.

Implant migration and functional outcome of Reverse Shoulder Lateralized Glenosphere Line Extension System: a study protocol for a randomized controlled trial

Background

Inferior scapular notching is a complication unique to reverse shoulder arthroplasty. The most efficient technique to avoid inferior scapular notching has been reported to be lateralization of the glenoid offset. This study aims to compare radiological and functional outcomes of the DELTA Xtend® Reverse Shoulder System Lateralized Glenosphere Line Extension (intervention group) with the Standard DELTA Xtend® Reverse Shoulder System (control group). We hypothesize that the lateralization improves the patient outcome by decreasing the risk of inferior scapular notching without increasing the risk of migration and loosening of glenoid component.

Methods

In this randomized controlled trial, all Danish citizens with rotator cuff arthropathy or degeneration of the glenohumeral joint with severe posterior wear and allocated for a reverse total shoulder arthroplasty at the department of orthopaedic surgery at Herlev and Gentofte Hospital, Copenhagen University Hospital, will be considered for participation. The exclusion criteria are as follows: below 50 years of age, cognitive or linguistic impairment, insufficient glenoid bone stock, previous fracture in the upper extremity and autoimmune-mediated inflammatory arthritis. There will be included a total of 122 patients of which 56 will participate in the radiostereometric analysis. This number of patients allows 20% to drop out. The co-primary outcomes are the pattern and magnitude of the migration of the glenoid component assessed by radiostereometric analysis and the Western Ontario Osteoarthritis of the Shoulder index. The secondary outcomes are inferior scapular notching, patient-reported and functional outcomes (Oxford shoulder score, Constant-Murley score and pain), side effects and complications, changes in bone mineral density and economy. The included patients will be examined before the surgery, within 1 week and 3, 6, 12 and 24 months after.

Discussion

No previous studies have compared the conventional reverse shoulder arthroplasty with the lateralized reverse shoulder arthroplasty in a randomized controlled trial regarding migration and functional outcome. Furthermore, radiostereometric analysis has not been used to evaluate the migration of reverse shoulder arthroplasty in a randomized controlled trial. This study intends to determine which treatment has the most optimal outcome for the benefit of future patients with an indication for reverse shoulder arthroplasty.

Trial registration

The study has been notified to Pactius and has approval number P-2021-231. Furthermore, the study will be registered on Clinicaltrials.gov before starting the inclusion.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13063-022-06482-8.

In Vitro Simulation of Shoulder Motion Driven by Three-Dimensional Scapular and Humeral Kinematics

In vitro simulation of three-dimensional (3D) shoulder motion using in vivo kinematics obtained from human subjects allows investigation of clinical conditions in the context of physiologically relevant biomechanics. Herein, we present a framework for laboratory simulation of subject-specific kinematics that combines individual 3D scapular and humeral control in cadavers. The objectives were to: (1) robotically simulate seven healthy subject-specific 3D scapulothoracic and glenohumeral kinematic trajectories in six cadavers, (2) characterize system performance using kinematic orientation accuracy and repeatability, and muscle force repeatability metrics, and (3) analyze effects of input kinematics and cadaver specimen variability. Using an industrial robot to orient the scapula range of motion (ROM), errors with repeatability of ±0.1 mm and <0.5 deg were achieved. Using a custom robot and a trajectory prediction algorithm to orient the humerus relative to the scapula, orientation accuracy for glenohumeral elevation, plane of elevation, and axial rotation of <3 deg mean absolute error (MAE) was achieved. Kinematic accuracy was not affected by varying input kinematics or cadaver specimens. Muscle forces over five repeated setups showed variability typically <33% relative to the overall simulations. Varying cadaver specimens and subject-specific human motions showed effects on muscle forces, illustrating that the system was capable of differentiating changes in forces due to input conditions. The anterior and middle deltoid, specifically, showed notable variations in patterns across the ROM that were affected by subject-specific motion. This machine provides a platform for future laboratory studies to investigate shoulder biomechanics and consider the impacts of variable input kinematics from populations of interest, as they can significantly impact study outputs and resultant conclusions.

Implant characteristics affect in vivo shoulder kinematics during multiplanar functional motions after reverse shoulder arthroplasty

The purpose of this study was to determine how implant characteristics affect in vivo shoulder kinematics after reverse shoulder arthroplasty (RSA). Kinematics of the affected upper limb were measured in 32 participants during five motions (scapular plane abduction, hand-to-head, hand-to-back, internal/external rotation at 90° abduction, and circumduction) using optical motion capture. Shoulder abduction, plane of elevation, and internal/external rotation range of motion (ROM), peak angles, and continuous kinematics waveforms were calculated for each motion. Multiple regression was used to identify associations between kinematics and implant characteristics of lateralization, humeral retroversion, glenosphere size, glenosphere tilt, glenoid eccentricity, and implant neck-shaft angle (135° or 145°). Less humeral retroversion was associated with greater shoulder rotation ROM (p = 0.036) and greater plane of elevation ROM (p = 0.024) during circumduction, while less eccentricity was associated with more posterior plane of elevation during hand-to-back (p = 0.021). The 145° implant was associated with greater internal/external shoulder rotation ROM (p < 0.001), greater internal shoulder rotation (p = 0.002), and greater plane of elevation ROM (p = 001) during the hand-to-back. The 145° implant was also associated with more internal/external rotation ROM (p = 0.043) during shoulder rotation and more abduction ROM during circumduction (p = 0.043). During the hand-to-back motion, individuals having 135° neck-shaft angle implants were more abducted from 21 to 51% of the motion and were less internally rotated from 70 to 100% of the motion, while more lateralization was associated with less internal rotation from 90 to 100% of the motion. Retroversion and implant neck-shaft angle are the primary implant characteristics associated with in vivo shoulder kinematics during complex motions after RSA.

Factors Influencing Acromial and Scapular Spine Strain after Reverse Total Shoulder Arthroplasty: A Systematic Review of Biomechanical Studies

BACKGROUND

Acromial and scapular spine fractures after reverse total shoulder arthroplasty (RTSA) can be devastating complications leading to substantial functional impairments. The purpose of this study was to review factors associated with increased acromial and scapular spine strain after RTSA from a biomechanical standpoint.

METHODS

A systematic review of the literature was conducted based on PRISMA guidelines. PubMed, Embase, OVID Medline, and CENTRAL databases were searched and strict inclusion and exclusion criteria were applied. Each article was assessed using the modified Downs and Black checklist to appraise the quality of included studies. Study selection, extraction of data, and assessment of methodological quality were carried out independently by two of the authors. Only biomechanical studies were considered.

RESULTS

Six biomechanical studies evaluated factors associated with increased acromial and scapular spine strain and stress. Significant increases in acromial and scapular spine strain were found with increasing lateralization of the glenosphere in four of the included studies. In two studies, glenosphere inferiorization consistently reduced acromial strain. The results concerning humeral lateralization were variable between four studies. Humeral component neck-shaft angle had no significant effect on acromial strain as analysed in one study. One study showed that scapular spine strain was significantly increased with a more posteriorly oriented acromion (55° vs. 43°; p < 0.001). Another study showed that the transection of the coracoacromial ligament increased scapular spine strain in all abduction angles (p < 0.05).

CONCLUSIONS

Glenoid lateralization was consistently associated with increased acromial and scapular spine strain, whereas inferiorization of the glenosphere reduced strain in the biomechanical studies analysed in this systematic review. Humeral-sided lateralization may increase or decrease acromial or scapular spine strain. Independent of different design parameters, the transection of the coracoacromial ligament resulted in significantly increased strains and scapular spine strains were also increased when the acromion was more posteriorly oriented. The results found in this systematic review of biomechanical in-silico and in-vitro studies may help in the surgical planning of RTSA to mitigate complications associated with acromion and scapular spine fracture.