rotator cuff is antagonist following rtsa · rotator cuff is antagonist following rtsa 1 1...

Rotator Cuff is Antagonist Following RTSA

1

ABSTRACT: 1

Introduction: There is disagreement regarding whether, when possible, the rotator cuff should be 2

repaired in conjunction with reverse total shoulder arthroplasty (RTSA). Therefore, we 3

investigated the effects of rotator cuff repair in RTSA models with varying magnitudes of humeral 4

and glenosphere lateralization (HLat & GLat). 5

Methods: Six fresh-frozen cadaveric shoulders were tested on a validated in-vitro muscle driven 6

motion simulator. Each specimen was implanted with a custom adjustable, load-sensing RTSA 7

after creation of a simulated rotator cuff tear. The effects of 4 RTSA configurations (0&10mm x 8

HLat & GLat) on deltoid force and joint load during abduction with/without rotator cuff repair 9

were assessed. 10

Results: Increasing HLat and GLat significantly affected deltoid force (-2.5±1.7%BodyWeight 11

(%BW), p=0.016 & +7.7±5.6%BW, p=0.016). Rotator cuff repair interacted with HLat & GLat 12

(p=0.005) such that with no HLat, GLat increased deltoid force without cuff repair (8.1±5.1%BW, 13

p=0.012) and this effect was increased with cuff repair (12.8±7.8%BW, p=0.010), but addition of 14

HLat mitigated this effect. Rotator cuff repair increased joint load (+11.9±5.1%BW, p=0.002), as 15

did GLat (+13.3±3.7%BW, p<0.001), and these interacted such that increasing GLat markedly 16

increases cuff repair's negative effects (9.4±3.2%BW, p=0.001 vs 14.4±7.4%BW, p=0.005). 17

Conclusion: Rotator cuff repair, especially in conjunction with GLat, produces an antagonistic 18

effect that increases deltoid and joint loading. The long-term effects of this remain unknown; 19

however, combining these factors may prove undesirable. HLat improves joint compression 20

through deltoid wrapping and increases the deltoid’s mechanical advantage. Therefore, it could be 21

used in place of rotator cuff repair, thus avoiding its complications. 22

Level of Evidence: Basic Science Study 23


2

Keywords: Reverse Total Shoulder Arthroplasty, RTSA, RSA, shoulder, rotator cuff, simulator, 24

cuff tear arthropathy 25

26


3

INTRODUCTION: 27

Reverse Total Shoulder Arthroplasty (RTSA) is primarily indicated for the treatment of rotator 28

cuff tear arthropathy or massive rotator cuff tears that are deemed irrepairable9,15,17,20. Despite 29

these indications, it is often possible to repair portions of the subscapularis and infraspinatus/teres 30

minor; however, there is disagreement regarding whether these tissues should be repaired as their 31

effects on RTSA biomechanics and outcomes remain unclear6,7,10,16. Additionally, the indications 32

for RTSA have expanded to include surgical conditions with an intact rotator cuff, such as the 33

management of A2, B2, or C glenoid erosions8,22. As such, the surgeon has the option to preserve 34

or release the rotator cuff in these scenarios. 35

Some have advocated repair of these tissues on the basis that they increase RTSA stability and 36

decrease the incidence of dislocation6,10,21,23 but clinical series by Clark et al. and Wall et al. have 37

disputed this effect7,24. However, a review by Wall et al. did find that repair of the subscapularis 38

may still be warranted because it significantly improves post-operative internal rotation24. In 39

contrast, Boulahia et al.5 suggested that subscapularis repair may detrimentally affect external 40

rotation through antagonistic loading against the already weakened posterior cuff; however, this 41

has not been confirmed in other reports, which specifically investigated subscapularis repair7,24. 42

Although to date the discussion of whether to repair the rotator cuff has primarily focused on 43

post-operative joint stability, the potential effect on muscle and joint loading must also be 44

considered. As is the case in the native glenohumeral joint, the concentric loads applied by a 45

repaired rotator cuff can be expected to counter the deltoid’s eccentric joint loads. However, in an 46

in-vitro study of one RTSA implant configuration, Ackland et al. demonstrated that the function 47

of the subscapularis is markedly shifted towards adduction – especially early in motion – compared 48

to its native role1. This finding suggests that repair of the subscapularis may resist abduction and 49


4

thus increase muscle and joint loading, but it is unclear if this change in function holds across the 50

full range of RTSA configurations used clinically. 51

With the conflicting clinical information and the relative paucity of biomechanical evidence in 52

mind, we sought to investigate the effects of rotator cuff repair, or preservation in glenoid erosion 53

cases, on functional shoulder outcomes and joint kinetics. As well, we investigated how these 54

effects are influenced by changes in two geometrical implant parameters that have previously been 55

shown to have a strong influence on shoulder biomechanics. Specifically, we wanted to clarify 56

how rotator cuff repair affects active internal/external rotation range of motion and external 57

rotation strength, while also determining if it had a detrimental impact on deltoid and joint loading. 58

We hypothesized that rotator cuff repair would resist abduction thus increasing deltoid muscle 59

force requirements and the resulting joint load. As well, we hypothesized that glenosphere 60

lateralization would have no effect on internal/external rotation but would exacerbate the negative 61

effects of rotator cuff loading while humeral lateralization would improve internal/external 62

rotation and mitigate the effects of rotator cuff repair. 63


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MATERIALS & METHODS: 64

Instrumented RTSA Implant 65

In this in-vitro biomechanical study, it was possible to measure joint loads and investigate the 66

effects of systematic adjustments to implant geometry using a previously described custom 67

modular implant system with a built-in load sensor (Figure 1)14,19. Four combinations of humeral 68

and glenosphere lateralization were investigated (respectively: 0&0; 0&10; 10&0; 10&10mm) 69

where, when both variables are at 0mm, the configuration is considered to be neutral, 70

corresponding to a traditional Grammont-style implant. Specifically, neutral was defined as the 71

glenoid baseplate level with the inferior glenoid rim, the glenosphere center of rotation coincident 72

with the glenoid surface, neutral humeral version with a 155° head-neck angle, and a 12.5-mm 73

lateral offset between the humeral stem and deepest point of the cup. The 10mm offset 74

configurations were achieved by making mechanical adjustments to the custom implant without 75

altering the surgical fixation. To ensure accurate mechanical properties, commercially available 76

polyethylene humeral cups (38mm, Delta Xtend; DePuy, Warsaw, IN, USA) were used. The 77

glenosphere was custom fabricated to accommodate a 6 degree of freedom load cell (Nano25, 78

ATI-IA; Apex, NC, USA) that attached medially to a glenoid fixation baseplate, which was 79

recessed into the glenoid vault to allow neutral glenosphere positioning. 80

81

Active Motion Simulator & Specimen Preparation 82

Six fresh frozen cadaveric shoulders (60±21 years) without signs of cuff deficiency or prior surgery 83

were prepared and the humerus was transected distal to the deltoid tuberosity. To enable repeated 84

access to the glenohumeral joint throughout the testing protocol, the subscapularis muscle was 85

elevated from the scapula and reflected laterally without disrupting its insertion on the lesser 86


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tuberosity. A full thickness superior rotator cuff tear was simulated by releasing the entire 87

supraspinatus and upper portion of the infraspinatus from the greater tuberosity. The specimens 88

were then implanted with the above described custom adjustable RTSA implant positioned in the 89

neutral configuration including 0° retroversion relative to the trans-epicondylar axis and with 90

humeral distalization dictated by aligning the superior humeral cup with the superior aspect of the 91

greater tuberosity for all specimens. Following implantation, the three deltoid heads were sutured 92

at their insertion, and the subscapularis and inferior infraspinatus/teres minor musculotendinous 93

junctions were sutured across their width using a running locking stitch. Specimen preparation was 94

then completed as described by Giles et al,12 including fixation of optical trackers to the scapula 95

(Optotrak Certus, NDI, Waterloo, ON, Canada) and insertion of an instrumented intramedullary 96

humeral rod that could provide optical motion tracking and data regarding the loads applied to the 97

rod by the experimenter. These data were recorded using a six degree of freedom load cell (Mini45, 98

ATI-IA; Apex, NC, USA) interposed between the proximal rod, which was inserted into the 99

humeral canal, and the distal rod, which was mated to the testing simulator during passive testing 100

and fitted with brass weights – to simulate the mass of the transected distal arm – during active 101

motion testing. The scapula was cemented to the simulator, and all muscles were connected to 102

computer controlled pneumatic actuators through physiologically accurate lines of action. This 103

simulator produces accurate and repeatable motions along predefined glenohumeral rotation 104

profiles by using a previously validated multi-PID (Proportional-Integral-Differential) control 105

system, which employs real time feedback of the humerus and scapula orientation, to produce 106

independent loads for each muscle group 13. The simulator also continuously rotated the scapula 107

to maintain a physiologically accurate glenohumeral-to-scapulothoracic rhythm. 108

109


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Testing Protocol 110

Four RTSA configurations with 0 or 10mm of glenosphere lateralization and 0 or 10mm of 111

humeral lateralization were tested in random order. For each configuration, fully unconstrained 112

and simulator-guided muscle driven motions and strength tests were conducted. Fully 113

unconstrained active abduction was simulated from an adducted position (lowest level of 114

abduction without humeral cup impingement) to 90° of humerothoracic abduction at a rate of 1°/s. 115

To ensure proper joint loading was achieved, a physiologic glenohumeral-to-scapulothoracic 116

rhythm of 2:1 was maintained by rotating the scapula using feedback of the instantaneous level of 117

glenohumeral abduction. Additionally, to assess the effect of repairing the rotator cuff, active 118

motion trials were performed with and without (order of testing cuff repair was randomized) 119

subscapularis and infraspinatus/teres minor loading. 120

Simulator-guided muscle driven active internal and external rotational range of motion (IR/ER 121

ROM) was assessed in full adduction by attaching the distal humeral rod (with weights removed) 122

to a previously described guiding arc12 that constrained ad-abduction and flexion-extension while 123

leaving axial rotation free to vary. To realistically replicate the pattern of muscle forces present 124

during IR/ER motions, muscle loading ratios were calculated using previously reported pCSA 125

(physiological Cross-Sectional Area) data2 and EMG signals recorded during IR and ER motions11. 126

The resulting ratios were, for internal rotation: 1, 0.16, 0.16, 0, 0, and for external rotation: 0.32, 127

1, 0, 0.22, 0.3, for the subscapularis, infraspinatus/teres minor, anterior, middle, and posterior 128

deltoid, respectively. Maximum rotational ROM for each motion was recorded after the load of all 129

muscles was increased until the prime mover – the subscapularis for internal rotation and 130

infraspinatus/teres-minor for external rotation – was loaded to 50N. External rotation strength was 131

also assessed by applying the same ramp loading protocol while holding the humerus in neutral 132


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rotation using the simulator’s guiding arc and recording the maximum axial torque measured by 133

the humeral rod’s load cell. 134

135

Outcome Variables and Statistical Analyses 136

For active abduction, the effects of rotator cuff repair status (yes/no), and humeral and glenosphere 137

lateralization on resultant joint load, total deltoid muscle force, and joint load angle were assessed. 138

The force components recorded by the glenosphere load cell were transformed into a coordinate 139

system aligned with a standard glenoid coordinated system, which was coincident with the 140

glenosphere center. The resultant joint load was then calculated from these data and the joint 141

loading angle was found using the superiorly and laterally directed forces such that a 0° angle is 142

purely compressive, and positive angles are directed superiorly. The total deltoid force outcome 143

was calculated through the summation of the force on the three deltoid heads at each point in time. 144

Resultant joint load and total deltoid force data were subsequently expressed as a percentage of 145

the specimen’s total body weight (%BW). All outcomes for active abduction were evaluated every 146

7.5° of humerothoracic abduction (5° glenohumeral and 2.5° scapulothoracic) from 15° to 82.5° 147

as this was the range achieved by all conditions without implant impingement. Each active motion 148

outcome variable was statistically tested using a 4-way (cuff status, humeral lateralization, 149

glenosphere lateralization, abduction level) repeated-measures analysis of variance (RM-150

ANOVA) with follow-up analyses of interactions where appropriate and Bonferroni-corrected 151

pairwise comparisons with significance set at p<0.05. A power analysis for each outcome variable 152

indicated that testing of six shoulder specimen would enable a clinically meaningful change of 153

5%BW to be detected with ≥80% power. 154


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For active range of motion trials, the reported ROM value was taken as the amount of rotation 155

from the neutral position achieved during maximal muscle loading. Active external rotation 156

strength was recorded as the torque applied to the humeral rod in Newton-Meters (Nm) during 157

maximal muscle loading. For consistency, the ROM and strength outcomes were recorded after 158

holding the maximum muscle loads for 5 seconds. For each of these three outcomes, a 2-way 159

(humeral lateralization, glenosphere lateralization) RM-ANOVA was performed with analyses of 160

interactions where appropriate. 161

162

163


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RESULTS: 164

165

Active Abduction: 166

Deltoid Force 167

Results for total deltoid force indicated that rotator cuff loading (i.e. clinical rotator cuff repair or 168

intact anterior and posterior cuff) did not have a significant main effect (p=0.3) when assessed 169

across all implant configurations and abduction angles; however, in addition to the expected main 170

effect of changing abduction angle (p=0.001), both humeral and glenosphere lateralization 171

significantly affected deltoid force (p<0.021). Specifically, a 10mm increase in humeral 172

lateralization significantly decreased deltoid force (average±SD: 3±2%BW, p=0.016) – although 173

this decrease cannot be considered clinically meaningful – while a 10mm increase in glenosphere 174

lateralization significantly increased deltoid force (8±5%BW, p=0.016). Additionally, there was a 175

significant three-way interaction between cuff status, humeral lateralization, and glenosphere 176

lateralization (p=0.005, Figure 2) such that when there was no humeral lateralization, increasing 177

glenosphere lateralization significantly increased deltoid force without rotator cuff loading 178

(8±5%BW, p=0.012) and this effect was exacerbated when cuff loads were applied (13±8%BW, 179

p=0.010). However, this negative effect on deltoid force was mitigated by humeral lateralization 180

to the extent that deltoid force increases due to glenosphere lateralization were no longer 181

significant (p>0.05). 182

183

Resultant Joint Load 184

In contrast to the effects on total deltoid force, resultant joint load data does indicate that rotator 185

cuff loading has a significant main effect (p=0.002) as does abduction (p<0.001) and glenosphere 186


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lateralization (p<0.001), but not humeral lateralization (p=0.195). Repairing the rotator cuff 187

resulted in a 12±5%BW (p=0.002) increase in resultant joint load across all implant configurations, 188

while increasing glenosphere lateralization increased joint load by 13±4%BW (p<0.001). 189

Additionally, cuff status and glenosphere lateralization interacted (Figure 3) such that increasing 190

glenosphere lateralization significantly increases the negative effect of loading the rotator cuff 191

(Unloaded vs Loaded: 0mm=9±3%BW (p=0.001), 10mm=14±7%BW (p=0.005)). 192

193

Joint Load Angle 194

Analysis of the joint load angle data found that abduction (p<0.001), rotator cuff loading 195

(Unloaded vs Loaded: 40±14° vs 34±13°, p<0.001), and humeral lateralization (0 vs 10mm: 196

40±13° vs 33±14°, p<0.001; Figure 4) all have significant main effects such that load angle 197

becomes more compressive as these factors increase but glenosphere lateralization had no effect 198

(0 vs 10mm: 37±14° vs 37±13°, p=0.83). Both cuff status and glenosphere lateralization 199

significantly interact with abduction angle (p<0.001) such that their effects on joint load angle are 200

reduced as abduction progresses. Conversely, the effect of humeral lateralization was almost 201

completely constant (variation <0.7°) across abduction. 202

203

Active Internal-External Rotation: 204

Range of Motion & Strength 205

A two-way RM-ANOVA for active internal rotation found that neither implant variable had a 206

significant main effect (p>0.05) but humeral and glenosphere lateralization did produce a 207

significant cross-over interaction (p=0.002; Figure 5). Specifically, with 0mm of glenosphere 208

lateralization, increasing humeral lateralization by 10mm significantly increased range of motion 209


12

(7±5°, p=0.019). However, with 10mm of glenosphere lateralization, the lateralized humeral 210

configuration produced less internal rotation compared to the neutral (i.e. 0mm) humeral 211

configuration (5±7°), but this difference was not significant (p=0.092). 212

Similar analysis of the active external rotation data found that it was significantly increased by 213

both humeral (10±6°, p=0.008) and glenosphere lateralization (10±9°, p=0.033) (Figure 5). 214

A two-way RM-ANOVA for active external rotation strength found that neither humeral 215

lateralization (p=0.99) or glenosphere lateralization (p=0.61) produced a significant main effect. 216

217


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DISCUSSION: 218

Although RTSA is primarily indicated as a treatment for rotator cuff tear arthropathy or massive 219

cuff tears, in many cases, portions of the degenerated infraspinatus and subscapularis are amenable 220

to repair despite having decreased contractile capabilities. Previous studies, focusing on the effects 221

of cuff repair on RTSA stability/dislocation and internal-external rotation range of motion5–222

7,10,21,23,24, have disagreed on whether these tissues should be repaired when possible. This study is 223

unique as it focuses on the kinetic effects of rotator cuff repair and how this is affected by RTSA 224

configuration. We have found that rotator cuff repair increases the demands on the deltoid during 225

abduction and this in turn increases joint loading. This effect is further exacerbated by glenosphere 226

lateralization. Conversely, humeral lateralization reduced the deltoid muscle’s required force, 227

which mitigated the increases in joint loading caused by rotator cuff repair. Furthermore, humeral 228

lateralization produced a more compressive joint load, which effectively stabilizes the joint, but 229

did so without increasing joint load magnitude. 230

Effects on Active Abduction 231

As expected, repair and loading of the rotator cuff significantly (p=0.001) shifted the joint 232

loading angle towards compression (~6°). Additionally, results indicate that increasing humeral 233

lateralization from neutral to 10mm produces a similar compressive shift in joint angle of 7° 234

irrespective of cuff repair status, but if combined with cuff repair, the loading angle can be shifted 235

~13° towards compression. Shifting of the joint load towards greater compression is desirable as 236

it is less challenging to glenosphere baseplate fixation, which is especially important to the 237

promotion of osseous integration in the early post-operative phase. These data also suggest that 238

both cuff repair and humeral lateralization are effective at stabilizing an RTSA by producing a 239

more compressive overall joint load, which increases concavity compression. However, while 240


14

rotator cuff repair produces this affect through larger muscle and joint loading, humeral 241

lateralization causes the deltoid to wrap around the greater tuberosity to a greater extent and this 242

redirects its tension through the humeral head and articulation thus resulting in a more 243

compressively directed joint load without introducing additional force into the joint (Figure 6). 244

Therefore, with respect to joint load angle which affects functional stability and implant fixation, 245

repair of the rotator cuff and use of humeral lateralization are equally effective; however, these 246

factors must be viewed in light of the manner in which they produce these effects and their other 247

impacts on RTSA biomechanics. 248

As previously reported, these data demonstrate that glenosphere lateralization significantly 249

increases joint loads (14%BW) as a result of decreasing the deltoid’s mechanical advantage14,18,19. 250

Rotator cuff repair was also found to significantly increase joint load magnitude (12%BW). 251

Additionally, the interaction between these two factors meant that if the rotator cuff was repaired, 252

and the glenosphere was lateralized – for instance to avoid scapular notching – the joint load 253

increased by approximately 29%. The sensitivity of humeral polyethylene liners to increases in 254

load has not be clearly defined in the literature, an increase of this magnitude may lead to increased 255

wear3. 256

Although repairing the cuff caused a significant increase in joint load magnitude, this outcome 257

alone does not clearly elucidate the cuff’s role in abduction following RTSA, as this increase may 258

be attributable to the added rotator cuff loading, or, alternatively, could be an indicator that the 259

cuff acts as an adductor thus increasing the required deltoid force and thus the overall joint load. 260

This distinction can be made by looking at deltoid muscle force. Specifically, when assessing the 261

average effect across all RTSA configurations, these data suggest that the cuff is not an adductor 262

as there are no significant increases in deltoid force when the cuff is repaired. However, there is a 263


15

significant interaction between cuff repair status, humeral and glenosphere lateralization such that 264

increases in deltoid force due to cuff repair (i.e. the slope of the lines in Figure 2) were exacerbated 265

when glenosphere lateralization was increased (~5%BW) irrespective of the level of humeral 266

lateralization. This demonstrates that glenosphere lateralization (through implant or bony means – 267

as in the BIO-RSA which uses a bone graft to achieve lateralization) causes the rotator cuff to act 268

as an adductor and antagonist to the deltoid. Therefore, the combination of these factors should be 269

employed with caution due to the ~13%BW increase in the deltoid force required to produce 270

motion. 271

Interestingly, the three-factor interaction in the deltoid force data also indicates that when the 272

glenosphere is lateralized, although humeral lateralization did not alter cuff repair’s effect of 273

increasing deltoid force, it did decrease the magnitude of deltoid load nearly equally for both cuff 274

repair states (~5%BW). This consistent decrease in deltoid force caused by adjusting humeral 275

lateralization indicates that it increases the deltoid mechanical advantage, thus decreasing the 276

deltoid’s required force, but it does not markedly change the antagonistic adduction effect of the 277

rotator cuff. Therefore, whereas the concurrent use of rotator cuff repair and glenosphere 278

lateralization should be considered cautiously, the addition of humeral lateralization in cases where 279

the glenosphere is lateralized can improve function and reduce deltoid fatigue irrespective of 280

rotator cuff repair status. 281

282

Effects on Internal-External Rotation 283

Evaluation of internal rotation range of motion using physiologically accurate muscle loading 284

ratios found that neither implant variable significantly affected motion but that they interacted with 285

one another in a cross-over pattern caused by changing combinations of impingement and soft 286


16

tissue restraint. Specifically, IR could be increased by lateralizing the glenosphere (~7°) as a result 287

of decreased contact with the anterior glenoid and coracoid, or by lateralizing the humerus (~10°) 288

due to increases in the cuff’s IR moment arms. However, IR is reduced compared to these two 289

configurations (by ~2° and ~5°, respectively), when both parameters are lateralized, due to 290

overstuffing of the joint and over tensioning of the posterior soft tissues. As such, over tensioning 291

of the posterior soft tissues in the horizontal plane results in a posterior tether to internal rotation. 292

In the case of external rotation range of motion, both humeral and glenosphere lateralization 293

produced significant and equivalent increases (10°, respectively). These increases can be explained 294

similarly to those initially seen with internal rotation; however, in this case, rotation was not 295

limited by over tensioning of the anterior soft tissues when both lateralizations were applied as 296

was the case with internal rotation. Therefore, these results help to confirm the clinical findings of 297

Boileau et al. that BIO-RSA can increase motion by decreasing impingement4, but also supports 298

the value of humeral lateralization in increasing motion by improving the rotator cuff’s mechanical 299

advantage. However, it has been shown that these improvements in internal rotation can be 300

partially negated by combined lateralization. As such, it is important to specifically assess IR range 301

of motion when using lateralized glenospheres or a BIO-RSA and to appropriately manage the soft 302

tissues. If IR is restricted with lateralization, it may be prudent to release contracted posterior 303

capsular tissues. 304

External rotation strength is critically important to performing activities of daily living and thus 305

was of interest in this protocol; however, neither implant variable assessed in this study showed 306

any effect on strength. Despite not effecting external rotation strength in the tested neutral position, 307

it is possible that humeral lateralization increases external rotation strength with the arm in 308

externally rotated orientations by causing the greater tuberosity and cuff insertion to lay more 309


17

eccentric relative to the cuff’s line of action, which should increase its mechanical advantage and 310

thus strength. However, further investigations are required to test this hypothesis. 311

312

Limitations & Future Work 313

The results of this study should be viewed in light of its limitations. First, as with all in-vitro 314

testing, the use of cadaveric specimens may affect results; specifically, in this study specimens did 315

not exhibit existing rotator cuff tears and thus the testing of lateralized RTSA configurations with 316

simulated rotator cuff repair did not result in the large passive joint forces that could occur 317

clinically due to tendon retraction. However, from a clinical standpoint, over tensioned soft tissues 318

often relax post-operatively and thus this study’s rotator cuff repair model can be considered 319

analogous to this relaxed state. Second, in conditions where rotator cuff repair was simulated, the 320

muscle loads used during testing were guided by previously reported EMG data for healthy 321

patients as none exist post-RTSA. The use of these healthy EMG signals may influence the 322

magnitude of the observed trends but because all conditions were tested with the same EMG 323

patterns, comparisons between implant configurations should still yield meaningful results. Third, 324

the current simulator is not able to simulate all shoulder muscle groups, specifically 325

humerothoracic muscles, thus measurements of active IR-ER rotation could only form a basis for 326

comparison between implant configurations and not to define the precise range of motion. Fourth, 327

the current protocol only assessed external rotation strength in neutral rotation, which did not show 328

an affect; however, further investigations in other orientations may have more comprehensively 329

described the effects of the tested conditions. Finally, we believe that further research is required 330

to elucidate the effects on active abduction of independently repairing the subscapularis and 331


18

infraspinatus muscles, which would require simulation of humerothoracic muscles not currently 332

included in this model. 333


19

CONCLUSION 334

Rotator cuff repair in the setting of RTSA produces an antagonistic effect that increases the deltoid 335

muscle’s work during elevation and this effect is exacerbated by increased glenosphere 336

lateralization. The combination of these rotator cuff forces and the increased demands on the 337

deltoid muscle significantly increases joint loads. The long-term effects of these changes remain 338

unknown but their magnitude is likely to be clinically meaningful. Humeral lateralization, 339

however, produces increased deltoid wrapping that results in improved joint compression and 340

stability – without the added muscle and joint loading associated with rotator cuff repair – and 341

increases the deltoid’s mechanical advantage which reduces the deltoid force required for 342

elevation. Therefore, humeral lateralization could be used in conjunction with rotator cuff repair 343

to maximize joint stability, or in its place to produce a similar stabilizing effect while decreasing 344

the demands on the deltoid and avoiding complications associated with rotator cuff repair. 345

346


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FIGURE & TABLE LEGENDS 422

Figure 1 - Computer rendering of the custom instrumented RTSA composed of a glenosphere 423

fixation baseplate (i), a six degree of freedom load cell (ii), glenosphere lateralization spacer (iii), 424

custom 38mm glenosphere with hollow to accommodate ii & iii and a corresponding humeral cup 425

(Depuy) (iv), humeral head-neck angle component (v), and baseplate and fixation stem which 426

facilitates humeral lateralization (vi). 427

Figure 2 - Effects of rotator cuff repair status, HLat, & GLat on deltoid force (mean±SD). Note 428

that p-values indicate the effect of GLat with and without loading on the rotator cuff. 429

Figure 3 - Effects of rotator cuff repair status and GLat on joint load (mean±SD) across abduction. 430

Note that shaded areas represent ±1 Standard Error of the Mean and p-values are for effect of 431

rotator cuff repair status averaged across abduction 432

Figure 4 – Effects of rotator cuff repair status and HLat on joint loading angle (mean±SD). 433

Included p-values indicate the significant difference between the two levels of each factor averaged 434

across the levels of the other factor. Note that a 0° joint load corresponds to pure compression and 435

positive values are superiorly directed. 436

Figure 5 – Effect of HLat and GLat on internal and external rotation range of motion (mean±SD). 437

Note that * and † indicate the significant difference (p=0.008 & p=0.033) between the two levels 438

of the humeral and glenosphere lateralization factors averaged across the levels of the other factor 439

for external rotation range of motion. 440

Figure 6 – Illustration of the deltoid’s compressive effect on the glenohumeral joint for RTSA 441

configurations with (right) and without (left) humeral lateralization. Note that the white arrows 442

represent the deltoid muscle force direction acting at the level at which the tissue contacts the 443

greater tuberosity and the black arrows represent the force imparted on the humeral head. This 444


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imparted force is related to the component of the muscle forces perpendicular to the tangent of the 445

greater tuberosity’s surface at the point of contact. Also note the increase in deltoid moment arm 446

caused by humeral lateralization (i.e. left vs right pane). 447

rotator cuff is antagonist following rtsa · rotator cuff is antagonist following rtsa 1 1...

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