Shoulder:Biomechanics - Revision history

Alexandre.laedermann at 04:14, 17 August 2021

2021-08-17T04:14:50Z

Alexandre.laedermann at 03:43, 17 August 2021

2021-08-17T03:43:42Z

Show changes

Alexandre.laedermann at 03:49, 29 July 2021

2021-07-29T03:49:45Z

Alexandre.laedermann: /* Anatomic total shoulder arthroplasty */

2021-07-29T03:30:01Z

Anatomic total shoulder arthroplasty

Alexandre.laedermann: /* Distalization of the humerus */

2021-07-29T03:25:48Z

Distalization of the humerus

Alexandre.laedermann: /* Stability */

2021-07-29T03:19:20Z

Stability

Show changes

Alexandre.laedermann: /* Baseplate design */

2021-07-29T03:06:03Z

Baseplate design

Alexandre.laedermann: /* Medialization of the joint center of rotation */

2021-07-29T03:00:38Z

Medialization of the joint center of rotation

Alexandre.laedermann: /* Modifications in muscle recruitment */

2021-07-29T02:39:29Z

Modifications in muscle recruitment

Alexandre.laedermann: /* Modifications in muscle recruitment */

2021-07-29T02:31:18Z

Modifications in muscle recruitment

← Older revision		Revision as of 04:14, 17 August 2021
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	The specificity of biomechanically relevant parameters, such as, for example, joint reaction forces, is that they cannot be measured in vivo without invasive procedures.<ref>Vidt ME, Santago AC, II, Marsh AP, Hegedus EJ, Tuohy CJ, Poehling GG, Freehill MT , Miller ME , Saul KR. Modeling a rotator cuff tear: individualized shoulder muscle forces influence glenohumeral joint contact force predictions. Clin Biomech (Bristol, Avon) 2018;60:20–29</ref> Our knowledge therefore mainly relies on experimental cadaveric studies<ref>Williamson P, Mohamadi A, Ramappa AJ, DeAngelis JP, Nazarian A. Shoulder biomechanics of RC repair and instability: a systematic review of cadaveric methodology. J Biomech 2019;82:280–290</ref> or computational modelling.<ref>Saul KR, Hu X, Goehler CM, Vidt ME, Daly M, Velisar A, Murray WM. Benchmarking of dynamic simulation predictions in two software platforms using an upper limb musculoskeletal model. Comput Methods Biomech Biomed Engin 2015;18:1445–1458</ref> These simulations have become more sophisticated in recent years, allowing the inclusion of an increasing number of variables with the ability to adjust both pathology and patient-specific characteristics.<ref>Nikooyan AA, Veeger HE, Westerhoff P, Graichen F, Bergmann G, van der Helm FC. Validation of the Delft Shoulder and Elbow Model using in-vivo glenohumeral joint contact forces. J Biomech 2010;43:3007–3014</ref> This ongoing process will without doubt call into question prior assumptions and allow further insights into shoulder biomechanics. It is crucial to understand the basic principles of shoulder biomechanics and their modifications in the most common pathologies encountered in daily practice.		The specificity of biomechanically relevant parameters, such as, for example, joint reaction forces, is that they cannot be measured in vivo without invasive procedures.<ref>Vidt ME, Santago AC, II, Marsh AP, Hegedus EJ, Tuohy CJ, Poehling GG, Freehill MT , Miller ME , Saul KR. Modeling a rotator cuff tear: individualized shoulder muscle forces influence glenohumeral joint contact force predictions. Clin Biomech (Bristol, Avon) 2018;60:20–29</ref> Our knowledge therefore mainly relies on experimental cadaveric studies<ref>Williamson P, Mohamadi A, Ramappa AJ, DeAngelis JP, Nazarian A. Shoulder biomechanics of RC repair and instability: a systematic review of cadaveric methodology. J Biomech 2019;82:280–290</ref> or computational modelling.<ref>Saul KR, Hu X, Goehler CM, Vidt ME, Daly M, Velisar A, Murray WM. Benchmarking of dynamic simulation predictions in two software platforms using an upper limb musculoskeletal model. Comput Methods Biomech Biomed Engin 2015;18:1445–1458</ref> These simulations have become more sophisticated in recent years, allowing the inclusion of an increasing number of variables with the ability to adjust both pathology and patient-specific characteristics.<ref>Nikooyan AA, Veeger HE, Westerhoff P, Graichen F, Bergmann G, van der Helm FC. Validation of the Delft Shoulder and Elbow Model using in-vivo glenohumeral joint contact forces. J Biomech 2010;43:3007–3014</ref> This ongoing process will without doubt call into question prior assumptions and allow further insights into shoulder biomechanics. It is crucial to understand the basic principles of shoulder biomechanics and their modifications in the most common pathologies encountered in daily practice.
		+
		+	==Acromioclavicular Joint==
		+	The acromioclavicular joint is stabilized both by static and dynamic stabilizers. The static stabilizers include 1) the four acromioclavicular ligaments (superior, inferior, anterior, and posterior), 2) the lateral coracoclavicular ligaments (conoid and trapezoid), 3) the medial coracoclavicular ligaments (Figure and Video) and 4) the coracoacromial ligament.<ref name=":1">Stimec BV, Lädermann A, Wohlwend A, Fasel JH. Medial coracoclavicular ligament revisited: an anatomic study and review of the literature. Arch Orthop Trauma Surg 2012;132:1071-5</ref><ref>Moya D, Poitevin LA, Postan D, Azulay GA, Valente S, Giacomelli F, Mamone LA. The medial coracoclavicular ligament: anatomy, biomechanics,and clinical relevance-a research study. JSES Open Access. 2018 Sep 22;2(4):183-189</ref> The latter, when transferred during standard Weaver-Dunn repair is only 1/4 as strong as the intact coracoclavicular ligaments; such technique of stabilization does not provide sufficient strength and is considered by many as obsolete.<ref>Weaver JK, Dunn HK. Treatment of acromioclavicular injuries, especially complete acromioclavicular separation. J Bone Joint Surg Am 1972;54:1187-94.</ref><ref>Costic RS, Labriola JE, Rodosky MW, Debski RE. Biomechanical rationale for development of anatomical reconstructions of coracoclavicular ligaments after complete acromioclavicular joint dislocations. Am J Sports Med 2004;32:1929-36.</ref><ref name=":4">Mazzocca AD, Santangelo SA, Johnson ST, Rios CG, Dumonski ML, Arciero RA. A biomechanical evaluation of an anatomical coracoclavicular ligament reconstruction. Am J Sports Med 2006;34:236-46</ref><br />
		+	{\| class="wikitable"
		+	\|+
		+	\|[[File:1562643381001-lg.jpg\|Medial coracoclavicular ligament (asterisk) in a right shoulder region. View from in front. C clavicle, CP coracoid process (horizontal portion), DM deltoid muscle (resected), PM pectoralis minor, SM subclavius muscle. Reprinted from Stimec et al.,<ref name=":1" /> with permission.\|alt=\|left\|thumb\|800x800px]]
		+	\|[[File:1562643389406-lg.mp4\|alt=\|thumb\|500x500px\|Video]]
		+	\|}
		+	<br />
		+
		+
		+	[[File:1562643398219-lg.mp4\|425x425px\|alt=\|thumb\|Video]]
		+
		+	The capsular ligaments acted as a primary restraint to posterior displacement of the clavicle (Video).<ref>Fukuda K, Craig EV, An KN, Cofield RH, Chao EY. Biomechanical study of the ligamentous system of the acromioclavicular joint. J Bone Joint Surg Am 1986;68:434-40.</ref>
		+
		+	The superior ligament is the strongest, followed by posterior. Both ligaments provide the most restraint to posterior translation of the acromioclavicular joint and must be preserved during a Mumford procedure. The coracoclavicular ligaments (trapezoid and conoid) provides vertical stability. The dynamic stabilizers include the deltoid and trapezius muscles.<ref>Abrassart S, Gagey O, Hoffmeyer P. La chape trapézo-deltoïdienne : réalité ou illusion d’optique. Revue de Chirurgie Orthopédique et Réparatrice de l'Appareil Moteur 2007;93:96-7.</ref>
		+
		+	<br>The coracoclavicular ligaments’ main contribution is to vertical stability. However, its double bundle configuration contributes also partially to horizontal stability due to their relative orientation.<ref>Lädermann A, Gueorguiev B, Stimec B, Fasel J, Rothstock S, Hoffmeyer P. Acromioclavicular joint reconstruction: a comparative biomechanical study of three techniques. J Shoulder Elbow Surg 2013;22:171-8.</ref><ref>Yoo YS, Tsai AG, Ranawat AS, et al. A biomechanical analysis of the native coracoclavicular ligaments and their influence on a new reconstruction using a coracoid tunnel and free tendon graft. Arthroscopy 2010;26:1153-61.</ref>
		+
		+	<br>After lesion of the acromioclavicular ligaments, the conoid ligament acts as the primary restraint against anterior and superior loading, while the trapezoid functioned as the primary restraint against posterior loading.<ref>Debski RE, Parsons IMt, Woo SL, Fu FH. Effect of capsular injury on acromioclavicular joint mechanics. J Bone Joint Surg Am 2001;83-A:1344-51.</ref> When a load is applied in a superior direction, the conoid ligament fails first in its midsubstance region.<ref>Costic RS, Labriola JE, Rodosky MW, Debski RE. Biomechanical rationale for development of anatomical reconstructions of coracoclavicular ligaments after complete acromioclavicular joint dislocations. Am J Sports Med 2004;32:1929-36.</ref> <ref>Mazzocca AD, Spang JT, Rodriguez RR, et al. Biomechanical and radiographic analysis of partial coracoclavicular ligament injuries. Am J Sports Med 2008;36:1397-402.</ref>
		+
		+	<br>During elevation of the arm, the clavicle with respect to the thorax generally undergoes elevation (11 to 15 degrees), retraction (15 to 29 degrees), and posterior long-axis rotation (15 to 31 degrees). Motion of the scapula (protraction-retraction) plays a major role in the motion at the acromioclavicular joint.<ref>Ludewig PM, Behrens SA, Meyer SM, Spoden SM, Wilson LA. Three-dimensional clavicular motion during arm elevation: reliability and descriptive data. The Journal of orthopaedic and sports physical therapy 2004;34:140-9.</ref>

	==Instability==		==Instability==

@@ Line 1: / Line 1: @@
-From <ref>Goetti P, Denard PJ, Collin P, Ibrahim M, Hoffmeyer P, Lädermann A. Shoulder biomechanics in normal and selected pathological conditions. EFORT Open Rev. 2020;5(8):508-518</ref>, with permission.
+From Goetti et al.,<ref>Goetti P, Denard PJ, Collin P, Ibrahim M, Hoffmeyer P, Lädermann A. Shoulder biomechanics in normal and selected pathological conditions. EFORT Open Rev. 2020;5(8):508-518</ref><ref name=":40" /> with permission.
 ==Bullet Points==
@@ Line 97: / Line 97: @@
 The primary goal of cuff repair is to be as anatomic as possible and to create a biomechanically favourable environment for tendon healing. Rehabilitation protocols must logically be adapted to the strength of the repair and tissue quality. Basic science research has mainly focused on the effect of mechanical loading on tendon-to-bone repair during the acute phase of healing using rat models.<ref name=":11">Hettrich CM, Gasinu S, Beamer BS, Stasiak M, Fox A, Birmingham P, Ying O, Deng XH, Rodeo SA. The effect of mechanical load on tendon-to-bone healing in a rat model. Am J Sports Med 2014;42:1233–1241</ref> While some authors reported improved tendon-to-bone healing with immobilization,<ref name=":11" /><ref>imbel JA, Van Kleunen JP, Williams GR, Thomopoulos S, Soslowsky LJ. Long durations of immobilization in the rat result in enhanced mechanical properties of the healing supraspinatus tendon insertion site. J Biomech Eng 2007;129:400–404</ref> others have found that limited early (during the first six weeks after a repair) tensile load is beneficial for viscoelastic tendon properties.<ref>Mazuquin BF, Wright AC, Russell S, Monga P, Selfe J, Richards J. Effectiveness of early compared with conservative rehabilitation for patients having rotator cuff repair surgery: an overview of systematic reviews. Br J Sports Med 2018;52:111–121</ref><ref>Tirefort J, Schwitzguebel AJ, Collin P, Nowak A, Plomb-Holmes C, Lädermann A. Postoperative mobilization after superior rotator cuff repair: sling versus no sling: a randomized prospective study. J Bone Joint Surg Am 2019;101:494–503</ref> However, uncontrolled tensile load (as seen with open chain exercises, eccentric muscle activation and motion beyond repair elasticity), leads to impaired tissue healing and can predispose to re-tear or repair tissue elongation.<ref>Killian ML, Cavinatto L, Galatz LM, Thomopoulos S. The role of mechanobiology in tendon healing. J Shoulder Elbow Surg 2012;21:228–237</ref><ref>Galatz LM, Charlton N, Das R, Kim HM, Havlioglu N, Thomopoulos S. Complete removal of load is detrimental to rotator cuff healing. J Shoulder Elbow Surg 2009;18:669–675</ref><ref>Thomopoulos S, Williams GR, Soslowsky LJ. Tendon to bone healing: differences in biomechanical, structural, and compositional properties due to a range of activity levels. J Biomech Eng 2003;125:106–113</ref> Excessive compressive loads, typically increased by postoperative scapular protraction,<ref>Kibler WB, Ludewig PM, McClure PW, Michener LA, Bak K, Sciascia AD. Clinical implications of scapular dyskinesis in shoulder injury: the 2013 consensus statement from the ‘Scapular Summit’. Br J Sports Med 2013;47:877–885</ref> do further impair tissue healing.<ref name=":11" /><ref>Carpenter JE, Thomopoulos S, Flanagan CL, DeBano CM, Soslowsky LJ. Rotator cuff defect healing: a biomechanical and histologic analysis in an animal model. J Shoulder Elbow Surg 1998;7:599–605</ref> Lastly, Sonnabend et al., in a primate model, reported that while eight weeks after cuff repair the tissue appeared macroscopically healed, mature healing with Sharpey fibres started at 12 weeks, therefore supporting a 12–15 week rehabilitation programme.<ref>Sonnabend DH, Howlett CR, Young AA. Histological evaluation of repair of the rotator cuff in a primate model. J Bone Joint Surg Br 2010;92:586–594</ref> Further studies are needed to provide guidelines for rehabilitation based on tear size and type of repair.
-Reproduced from Goetti et al., with permission.<ref>Goetti P, Denard PJ, Collin P, Ibrahim M, Mazzolari A, Lädermann A. Biomechanics of anatomic and reverse shoulder arthroplasty. EFORT Open Rev In Press</ref>
+Reproduced from Goetti et al., with permission.<ref name=":40">Goetti P, Denard PJ, Collin P, Ibrahim M, Mazzolari A, Lädermann A. Biomechanics of anatomic and reverse shoulder arthroplasty. EFORT Open Rev In Press</ref>
 ==Anatomic total shoulder arthroplasty==
 Anatomy is key to successfully reproduce patient’s physiologic joint kinematics. By virtue of its mobility, the glenohumeral joint is predisposed to instability. One factor affecting stability is the radius of curvature mismatch between the humeral head and glenoid. Further, only 20 to 30% of the humeral head is in contact with the glenoid.<ref name=":20">McPherson EJ, Friedman RJ, An YH, Chokesi R, Dooley RL. Anthropometric study of normal glenohumeral relationships. J Shoulder Elbow Surg. 1997;6(2):105-12</ref> The rotator cuff acts as an essential dynamic stabilizing force centering the humeral in the mid-portion of range of motion and is crucial for an anatomic total shoulder arthroplasty to be effective.<ref name=":4">Sharkey NA, Marder RA. The rotator cuff opposes superior translation of the humeral head. Am J Sports Med. 1995;23(3):270-5</ref> The supraspinatus helps to center the humeral head against the force of the deltoid in lower degrees of abduction, while the infraspinatus and teres minor help to clear the greater tuberosity under the coraco-acromial arch when the arm is moved in abduction and external rotation.<ref name=":4" /><ref>Lee SB, Kim KJ, O'Driscoll SW, Morrey BF, An KN. Dynamic glenohumeral stability provided by the rotator cuff muscles in the mid-range and end-range of motion. A study in cadavera. J Bone Joint Surg Am. 2000;82(6):849-57</ref> Lastly, even though the shoulder is not a weight-bearing joint, joint reaction forces as high as 2,4 times body weight have been reported during shoulder rehabilitation.<ref>Bergmann G, Graichen F, Bender A, Rohlmann A, Halder A, Beier A, et al. In vivo gleno-humeral joint loads during forward flexion and abduction. Journal of biomechanics. 2011;44(8):1543-52</ref>

@@ Line 97: / Line 97: @@
 The primary goal of cuff repair is to be as anatomic as possible and to create a biomechanically favourable environment for tendon healing. Rehabilitation protocols must logically be adapted to the strength of the repair and tissue quality. Basic science research has mainly focused on the effect of mechanical loading on tendon-to-bone repair during the acute phase of healing using rat models.<ref name=":11">Hettrich CM, Gasinu S, Beamer BS, Stasiak M, Fox A, Birmingham P, Ying O, Deng XH, Rodeo SA. The effect of mechanical load on tendon-to-bone healing in a rat model. Am J Sports Med 2014;42:1233–1241</ref> While some authors reported improved tendon-to-bone healing with immobilization,<ref name=":11" /><ref>imbel JA, Van Kleunen JP, Williams GR, Thomopoulos S, Soslowsky LJ. Long durations of immobilization in the rat result in enhanced mechanical properties of the healing supraspinatus tendon insertion site. J Biomech Eng 2007;129:400–404</ref> others have found that limited early (during the first six weeks after a repair) tensile load is beneficial for viscoelastic tendon properties.<ref>Mazuquin BF, Wright AC, Russell S, Monga P, Selfe J, Richards J. Effectiveness of early compared with conservative rehabilitation for patients having rotator cuff repair surgery: an overview of systematic reviews. Br J Sports Med 2018;52:111–121</ref><ref>Tirefort J, Schwitzguebel AJ, Collin P, Nowak A, Plomb-Holmes C, Lädermann A. Postoperative mobilization after superior rotator cuff repair: sling versus no sling: a randomized prospective study. J Bone Joint Surg Am 2019;101:494–503</ref> However, uncontrolled tensile load (as seen with open chain exercises, eccentric muscle activation and motion beyond repair elasticity), leads to impaired tissue healing and can predispose to re-tear or repair tissue elongation.<ref>Killian ML, Cavinatto L, Galatz LM, Thomopoulos S. The role of mechanobiology in tendon healing. J Shoulder Elbow Surg 2012;21:228–237</ref><ref>Galatz LM, Charlton N, Das R, Kim HM, Havlioglu N, Thomopoulos S. Complete removal of load is detrimental to rotator cuff healing. J Shoulder Elbow Surg 2009;18:669–675</ref><ref>Thomopoulos S, Williams GR, Soslowsky LJ. Tendon to bone healing: differences in biomechanical, structural, and compositional properties due to a range of activity levels. J Biomech Eng 2003;125:106–113</ref> Excessive compressive loads, typically increased by postoperative scapular protraction,<ref>Kibler WB, Ludewig PM, McClure PW, Michener LA, Bak K, Sciascia AD. Clinical implications of scapular dyskinesis in shoulder injury: the 2013 consensus statement from the ‘Scapular Summit’. Br J Sports Med 2013;47:877–885</ref> do further impair tissue healing.<ref name=":11" /><ref>Carpenter JE, Thomopoulos S, Flanagan CL, DeBano CM, Soslowsky LJ. Rotator cuff defect healing: a biomechanical and histologic analysis in an animal model. J Shoulder Elbow Surg 1998;7:599–605</ref> Lastly, Sonnabend et al., in a primate model, reported that while eight weeks after cuff repair the tissue appeared macroscopically healed, mature healing with Sharpey fibres started at 12 weeks, therefore supporting a 12–15 week rehabilitation programme.<ref>Sonnabend DH, Howlett CR, Young AA. Histological evaluation of repair of the rotator cuff in a primate model. J Bone Joint Surg Br 2010;92:586–594</ref> Further studies are needed to provide guidelines for rehabilitation based on tear size and type of repair.
 ==Anatomic total shoulder arthroplasty==
 Anatomy is key to successfully reproduce patient’s physiologic joint kinematics. By virtue of its mobility, the glenohumeral joint is predisposed to instability. One factor affecting stability is the radius of curvature mismatch between the humeral head and glenoid. Further, only 20 to 30% of the humeral head is in contact with the glenoid.<ref name=":20">McPherson EJ, Friedman RJ, An YH, Chokesi R, Dooley RL. Anthropometric study of normal glenohumeral relationships. J Shoulder Elbow Surg. 1997;6(2):105-12</ref> The rotator cuff acts as an essential dynamic stabilizing force centering the humeral in the mid-portion of range of motion and is crucial for an anatomic total shoulder arthroplasty to be effective.<ref name=":4">Sharkey NA, Marder RA. The rotator cuff opposes superior translation of the humeral head. Am J Sports Med. 1995;23(3):270-5</ref> The supraspinatus helps to center the humeral head against the force of the deltoid in lower degrees of abduction, while the infraspinatus and teres minor help to clear the greater tuberosity under the coraco-acromial arch when the arm is moved in abduction and external rotation.<ref name=":4" /><ref>Lee SB, Kim KJ, O'Driscoll SW, Morrey BF, An KN. Dynamic glenohumeral stability provided by the rotator cuff muscles in the mid-range and end-range of motion. A study in cadavera. J Bone Joint Surg Am. 2000;82(6):849-57</ref> Lastly, even though the shoulder is not a weight-bearing joint, joint reaction forces as high as 2,4 times body weight have been reported during shoulder rehabilitation.<ref>Bergmann G, Graichen F, Bender A, Rohlmann A, Halder A, Beier A, et al. In vivo gleno-humeral joint loads during forward flexion and abduction. Journal of biomechanics. 2011;44(8):1543-52</ref>

@@ Line 161: / Line 161: @@
 There is, however, a major drawback of center of rotation medialization, in the form of impingement between the scapular neck and humeral prosthetic component defined as scapular notching.<ref>Lädermann A, Gueorguiev B, Charbonnier C, Stimec BV, Fasel JH, Zderic I, et al. Scapular Notching on Kinematic Simulated Range of Motion After Reverse Shoulder Arthroplasty Is Not the Result of Impingement in Adduction. Medicine (Baltimore). 2015;94(38):e1615</ref><ref>Simovitch RW, Zumstein MA, Lohri E, Helmy N, Gerber C. Predictors of scapular notching in patients managed with the Delta III reverse total shoulder replacement. J Bone Joint Surg Am. 2007;89(3):588-600</ref> Several technical factors improve impingement free range of motion. One option is placing the glenosphere (not the baseplate) below the inferior glenoid rim or using an inferior eccentric glenosphere.<ref name=":29">Nyffeler RW, Werner CM, Gerber C. Biomechanical relevance of glenoid component positioning in the reverse Delta III total shoulder prosthesis. J Shoulder Elbow Surg. 2005;14(5):524-8</ref> De Wilde et al. et reported that a 5 mm overhang could improve impingement free adduction by 39 degrees.<ref name=":30">de Wilde LF, Poncet D, Middernacht B, Ekelund A. Prosthetic overhang is the most effective way to prevent scapular conflict in a reverse total shoulder prosthesis. Acta Orthop. 2010;81(6):719-26</ref> Abduction is also positively correlated with acromiohumeral distance (r = 0.93; p < 0.001) which is increased with an eccentric glenosphere.<ref>Lädermann A, Denard PJ, Collin P, Zbinden O, Chiu JC, Boileau P, et al. Effect of humeral stem and glenosphere designs on range of motion and muscle length in reverse shoulder arthroplasty. Int Orthop. 2020;44(3):519-30</ref> The ideal amount of overhang relative to the glenoid appears to be about 2.5 mm based on clinical evidence.<ref>Haidamous G, Lädermann A, Hartzler RU, Parsons B, Lederman E, Tokish J, et al. Radiographic parameters associated with excellent versus poor range of motion outcomes following reverse shoulder arthroplasty. Shoulder & Elbow. 2020;9:1758573220936234</ref> Alternatively, glenosphere diameter can be increased, therefore upsizing the diameter from 38 to 46 mm was reported to not only increase range of motion by 39% but also stability by a 36% increase in jump distance.<ref>Roche C, Flurin PH, Wright T, Crosby LA, Mauldin M, Zuckerman JD. An evaluation of the relationships between reverse shoulder design parameters and range of motion, impingement, and stability. J Shoulder Elbow Surg. 2009;18(5):734-41</ref> Accenter of rotationding to a computer simulation of impingement free range of motion, the single most effective modification in prosthetic design is the change of humeral neck-shaft angle from the classic 155 towards a more anatomic angle.<ref name=":26">Gutierrez S, Comiskey CAt, Luo ZP, Pupello DR, Frankle MA. Range of impingement-free abduction and adduction deficit after reverse shoulder arthroplasty. Hierarchy of surgical and implant-design-related factors. J Bone Joint Surg Am. 2008;90(12):2606-15</ref><ref>Lädermann A, Denard PJ, Boileau P, Farron A, Deransart P, Terrier A, et al. Effect of humeral stem design on humeral position and range of motion in reverse shoulder arthroplasty. Int Orthop. 2015;39(11):2205-13</ref>
-While joint center of rotation needs to be medialized in regard to the native center of rotation, slight lateralization of the glenosphere from the glenoid can further enhance compressive forces, which are thought to overcome the increased shear forces at the bone-component interface.<ref name=":25" /> Basic science studies show several benefits of lateralization. In both sawbone<ref>Gutierrez S, Levy JC, Frankle MA, Cuff D, Keller TS, Pupello DR, et al. Evaluation of abduction range of motion and avoidance of inferior scapular impingement in a reverse shoulder model. J Shoulder Elbow Surg. 2008;17(4):608-15   Gutierrez S, Levy JC, Frankle MA, Cuff D, Keller TS, Pupello DR, et al. Evaluation of abduction range of motion and avoidance of inferior scapular impingement in a reverse shoulder model. J Shoulder Elbow Surg. 2008;17(4):608-15</ref> and computer models,<ref name=":26" /><ref>Kim SJ, Jang SW, Jung KH, Kim YS, Lee SJ, Yoo YS. Analysis of impingement-free range of motion of the glenohumeral joint after reverse total shoulder arthroplasty using three different implant models. J Orthop Sci. 2019;24(1):87-94</ref><ref name=":27">Lädermann A, Tay E, Collin P, Piotton S, Chiu CH, Michelet A, et al. Effect of critical shoulder angle, glenoid lateralization, and humeral inclination on range of movement in reverse shoulder arthroplasty. Bone Joint Res. 2019;8(8):378-86</ref> lateralization improves range of motion in all directions.<ref name=":27" /> There is an ongoing debate regarding the impact of lateralization on the risk for acromial stress fractures. Finite element analysis has suggested a 17,2% increased acromial stress secondary to 10 mm lateralization.<ref>Wong MT, Langohr GDG, Athwal GS, Johnson JA. Implant positioning in reverse shoulder arthroplasty has an impact on acromial stresses. J Shoulder Elbow Surg. 2016;25(11):1889-95</ref> Clinically, distalization has been implicated as more of a culprit than lateralization.<ref>Haidamous G, Lädermann A, Frankle MA, Gorman RA, 2nd, Denard PJ. The risk of postoperative scapular spine fracture following reverse shoulder arthroplasty is increased with an onlay humeral stem. J Shoulder Elbow Surg. 2020;29(12):2556-63</ref> Glenosphere lateralization has further a linear correlation with baseplate micromotion<ref name=":31">Berliner JL, Regalado-Magdos A, Ma CB, Feeley BT. Biomechanics of reverse total shoulder arthroplasty. J Shoulder Elbow Surg. 2015;24(1):150-60</ref> and therefore exposes to the risk of aseptic loosening.<ref>Lädermann A, Schwitzguebel AJ, Edwards TB, Godeneche A, Favard L, Walch G, et al. Glenoid loosening and migration in reverse shoulder arthroplasty. Bone Joint J. 2019;101-B(4):461-9</ref> Giles et al. tested the effect of glenoid and humeral lateralization on deltoid muscle load in vitro using a simulator. They reported that 10 mm of humeral lateralization was the only parameter that actually decreased deltoid force in abduction (65 ± 8%), however, warned that this benefit may not compensate for the negative effects induced by glenosphere lateralization.<ref>Giles JW, Langohr GD, Johnson JA, Athwal GS. Implant Design Variations in Reverse Total Shoulder Arthroplasty Influence the Required Deltoid Force and Resultant Joint Load. Clin Orthop Relat Res. 2015;473(11):3615-26</ref> Lastly, Boileau et al. proposed a bony increased-offset reverse shoulder arthroplasty to lateralize the glenosphere however maintaining center of rotation at the prosthesis-bone interface and thereby minimizing torque stress.<ref>Boileau P, Moineau G, Roussanne Y, O'Shea K. Bony increased-offset reversed shoulder arthroplasty: minimizing scapular impingement while maximizing glenoid fixation. Clin Orthop Relat Res. 2011;469(9):2558-67</ref>
+While joint center of rotation needs to be medialized in regard to the native center of rotation, slight lateralization of the glenosphere from the glenoid can further enhance compressive forces, which are thought to overcome the increased shear forces at the bone-component interface.<ref name=":25" /> Basic science studies show several benefits of lateralization. In both sawbone<ref>Gutierrez S, Levy JC, Frankle MA, Cuff D, Keller TS, Pupello DR, et al. Evaluation of abduction range of motion and avoidance of inferior scapular impingement in a reverse shoulder model. J Shoulder Elbow Surg. 2008;17(4):608-15   Gutierrez S, Levy JC, Frankle MA, Cuff D, Keller TS, Pupello DR, et al. Evaluation of abduction range of motion and avoidance of inferior scapular impingement in a reverse shoulder model. J Shoulder Elbow Surg. 2008;17(4):608-15</ref> and computer models,<ref name=":26" /><ref>Kim SJ, Jang SW, Jung KH, Kim YS, Lee SJ, Yoo YS. Analysis of impingement-free range of motion of the glenohumeral joint after reverse total shoulder arthroplasty using three different implant models. J Orthop Sci. 2019;24(1):87-94</ref><ref name=":27">Lädermann A, Tay E, Collin P, Piotton S, Chiu CH, Michelet A, et al. Effect of critical shoulder angle, glenoid lateralization, and humeral inclination on range of movement in reverse shoulder arthroplasty. Bone Joint Res. 2019;8(8):378-86</ref> lateralization improves range of motion in all directions.<ref name=":27" /> There is an ongoing debate regarding the impact of lateralization on the risk for acromial stress fractures. Finite element analysis has suggested a 17,2% increased acromial stress secondary to 10 mm lateralization.<ref>Wong MT, Langohr GDG, Athwal GS, Johnson JA. Implant positioning in reverse shoulder arthroplasty has an impact on acromial stresses. J Shoulder Elbow Surg. 2016;25(11):1889-95</ref> Clinically, distalization has been implicated as more of a culprit than lateralization.<ref name=":37">Haidamous G, Lädermann A, Frankle MA, Gorman RA, 2nd, Denard PJ. The risk of postoperative scapular spine fracture following reverse shoulder arthroplasty is increased with an onlay humeral stem. J Shoulder Elbow Surg. 2020;29(12):2556-63</ref> Glenosphere lateralization has further a linear correlation with baseplate micromotion<ref name=":31">Berliner JL, Regalado-Magdos A, Ma CB, Feeley BT. Biomechanics of reverse total shoulder arthroplasty. J Shoulder Elbow Surg. 2015;24(1):150-60</ref> and therefore exposes to the risk of aseptic loosening.<ref>Lädermann A, Schwitzguebel AJ, Edwards TB, Godeneche A, Favard L, Walch G, et al. Glenoid loosening and migration in reverse shoulder arthroplasty. Bone Joint J. 2019;101-B(4):461-9</ref> Giles et al. tested the effect of glenoid and humeral lateralization on deltoid muscle load in vitro using a simulator. They reported that 10 mm of humeral lateralization was the only parameter that actually decreased deltoid force in abduction (65 ± 8%), however, warned that this benefit may not compensate for the negative effects induced by glenosphere lateralization.<ref name=":38">Giles JW, Langohr GD, Johnson JA, Athwal GS. Implant Design Variations in Reverse Total Shoulder Arthroplasty Influence the Required Deltoid Force and Resultant Joint Load. Clin Orthop Relat Res. 2015;473(11):3615-26</ref> Lastly, Boileau et al. proposed a bony increased-offset reverse shoulder arthroplasty to lateralize the glenosphere however maintaining center of rotation at the prosthesis-bone interface and thereby minimizing torque stress.<ref>Boileau P, Moineau G, Roussanne Y, O'Shea K. Bony increased-offset reversed shoulder arthroplasty: minimizing scapular impingement while maximizing glenoid fixation. Clin Orthop Relat Res. 2011;469(9):2558-67</ref>
 ===Baseplate design===
@@ Line 172: / Line 172: @@
 ===Distalization of the humerus===
-While distalization of the humerus is a central point in reverse shoulder arthroplasty with the primary goal of increasing the lever arm of the deltoid and improving functional outcomes, there are consequences to lengthening. Optimal lengthening is thought to be around 2 cm but is still debated (153). While insufficient lengthening (particularly in the revision setting) has been shown to be a critical factor regarding joint instability (153, 154), downsides of excessive lengthening include increasing the risk of a neurological lesion (neurapraxia) and over-tensioning resulting in a decreased range of motion as well as increased joint reaction forces (155, 156). Furthermore, lengthening via an onlay humeral component has been associated with an increased risk of acromial stress fracture compared to inlay components (129). While there is no current consensus regarding the optimal way to increase soft-tissue tension while avoiding complications (144, 157), recent biomechanical data suggests that humeral lateralization could potentially be a solution to improve joint and muscle loading (132, 158, 159). However, one must keep in mind that humeral lateralization also leads to distalization. In addition to the aforementioned consequences, distalization also changes the force vectors of the remaining rotator cuff. The latter may be particularly important in the use of reverse shoulder arthroplasty for diagnoses other than rotator cuff arthropathy in which much of the rotator cuff is still functional such as primary glenohumeral arthritis with posterior subluxation and a biconcave glenoid. Thus, there are not only trade-offs to distalization, but the ideal amount may also vary by diagnosis.
+While distalization of the humerus is a central point in reverse shoulder arthroplasty with the primary goal of increasing the lever arm of the deltoid and improving functional outcomes, there are consequences to lengthening. Optimal lengthening is thought to be around 2 cm but is still debated.<ref name=":39">Lädermann A, Williams MD, Melis B, Hoffmeyer P, Walch G. Objective evaluation of lengthening in reverse shoulder arthroplasty. J Shoulder Elbow Surg. 2009;18(4):588-95</ref> While insufficient lengthening (particularly in the revision setting) has been shown to be a critical factor regarding joint instability,<ref name=":39" /><ref>Lädermann A, Walch G, Lubbeke A, Drake GN, Melis B, Bacle G, et al. Influence of arm lengthening in reverse shoulder arthroplasty. J Shoulder Elbow Surg. 2012;21(3):336-41</ref> downsides of excessive lengthening include increasing the risk of a neurological lesion (neurapraxia) and over-tensioning resulting in a decreased range of motion as well as increased joint reaction forces.<ref>Athwal GS, MacDermid JC, Reddy KM, Marsh JP, Faber KJ, Drosdowech D. Does bony increased-offset reverse shoulder arthroplasty decrease scapular notching? J Shoulder Elbow Surg. 2015;24(3):468-73</ref><ref>Tashjian RZ, Burks RT, Zhang Y, Henninger HB. Reverse total shoulder arthroplasty: a biomechanical evaluation of humeral and glenosphere hardware configuration. J Shoulder Elbow Surg. 2015;24(3):e68-77</ref> Furthermore, lengthening via an onlay humeral component has been associated with an increased risk of acromial stress fracture compared to inlay components.<ref name=":37" /> While there is no current consensus regarding the optimal way to increase soft-tissue tension while avoiding complications,<ref name=":34" /><ref>Pegreffi F, Pellegrini A, Paladini P, Merolla G, Belli G, Velarde PU, Porcellini G. Deltoid muscle activity in patients with reverse shoulder prosthesis at 2-year follow-up. Musculoskelet Surg. 2017;101(Suppl 2):129-35</ref> recent biomechanical data suggests that humeral lateralization could potentially be a solution to improve joint and muscle loading.<ref name=":38" /><ref>Liou W, Yang Y, Petersen-Fitts GR, Lombardo DJ, Stine S, Sabesan VJ. Effect of lateralized design on muscle and joint reaction forces for reverse shoulder arthroplasty. J Shoulder Elbow Surg. 2017;26(4):564-72</ref><ref>Hamilton MA, Diep P, Roche C, Flurin PH, Wright TW, Zuckerman JD, et al. Effect of reverse shoulder design philosophy on muscle moment arms. J Orthop Res. 2015;33(4):605-13</ref> However, one must keep in mind that humeral lateralization also leads to distalization. In addition to the aforementioned consequences, distalization also changes the force vectors of the remaining rotator cuff. The latter may be particularly important in the use of reverse shoulder arthroplasty for diagnoses other than rotator cuff arthropathy in which much of the rotator cuff is still functional such as primary glenohumeral arthritis with posterior subluxation and a biconcave glenoid. Thus, there are not only trade-offs to distalization, but the ideal amount may also vary by diagnosis.
 ==Conclusions==

@@ Line 164: / Line 164: @@
 ===Baseplate design===
-To allow bone ingrowth, baseplate micromotion must be inferior to 150 um (134). As baseplates are screwed down to the glenoid, research focused on the optimal configuration to enhance initial stability on polyurethane foam models. While increased screw length (>17 mm inside the glenoid) or screw diameter (3.5 vs. 5.0 mm) was shown to additionally reduce micromotion by up to 30%, inclining screws by 30 degrees (compared to 0 degree) was the most effective as it led to a 50% reduction of micromotion (111, 135). With a central post design, the most important screw in the baseplate is thought to be the inferior one, as tensile forces are the highest at the inferior border secondary to humeral loading. A locking screw should therefore be favored in this particular location as a 7% enhanced load to failure was reported compared to standard cortical screws (136). Regarding the total number of screws, a cadaveric study comparing a two peripheral screws flat-backed baseplate construct (superior and inferior one) with a four screws construct found no statistical difference regarding motion during cyclic loading (137). Regarding baseplate design, the central screw does not seem superior to the post regarding load to failure compared to the central post (138). Lastly, Gutierrez et al. investigated optimal baseplate position using a computer model. According to their work that focused on uniform force distribution, a 15 degrees inferior tilt is best suited for concentric or lateral eccentric glenosphere, for inferior eccentric glenosphere a neutral inclination (0 degree) is the preferred orientation (139, 140). Superior tilt should always be avoided as stress at the bone interface increases. Boileau et al. suggested that superior tilt is commonly underestimated during reverse shoulder arthroplasty planification (141). As the baseplate is implanted in the inferior part of the glenoid, they introduced the reverse shoulder arthroplasty angle, defined as the angle between the inferior part of the glenoid fossa and the perpendicular to the floor of the supraspinatus. Compared to the anatomic total shoulder arthroplasty angle (β angle or global glenoid inclination angle), the reverse shoulder arthroplasty angle is 8 ± 4 degrees larger.
+To allow bone ingrowth, baseplate micromotion must be inferior to 150 um.<ref>Jasty M, Bragdon C, Burke D, O'Connor D, Lowenstein J, Harris WH. In vivo skeletal responses to porous-surfaced implants subjected to small induced motions. J Bone Joint Surg Am. 1997;79(5):707-14</ref> As baseplates are screwed down to the glenoid, research focused on the optimal configuration to enhance initial stability on polyurethane foam models. While increased screw length (>17 mm inside the glenoid) or screw diameter (3.5 vs. 5.0 mm) was shown to additionally reduce micromotion by up to 30%, inclining screws by 30 degrees (compared to 0 degree) was the most effective as it led to a 50% reduction of micromotion.<ref name=":25" /><ref>Hopkins AR, Hansen UN, Bull AM, Emery R, Amis AA. Fixation of the reversed shoulder prosthesis. J Shoulder Elbow Surg. 2008;17(6):974-80</ref> With a central post design, the most important screw in the baseplate is thought to be the inferior one, as tensile forces are the highest at the inferior border secondary to humeral loading. A locking screw should therefore be favored in this particular location as a 7% enhanced load to failure was reported compared to standard cortical screws.<ref>Chebli C, Huber P, Watling J, Bertelsen A, Bicknell RT, Matsen F, 3rd. Factors affecting fixation of the glenoid component of a reverse total shoulder prothesis. J Shoulder Elbow Surg. 2008;17(2):323-7</ref> Regarding the total number of screws, a cadaveric study comparing a two peripheral screws flat-backed baseplate construct (superior and inferior one) with a four screws construct found no statistical difference regarding motion during cyclic loading.<ref>James J, Allison MA, Werner FW, McBride DE, Basu NN, Sutton LG, et al. Reverse shoulder arthroplasty glenoid fixation: is there a benefit in using four instead of two screws? J Shoulder Elbow Surg. 2013;22(8):1030-6</ref> Regarding baseplate design, the central screw does not seem superior to the post regarding load to failure compared to the central post.<ref>Bonnevialle N, Geais L, Muller JH, Shoulder Friends I, Berhouet J. Effect of RSA glenoid baseplate central fixation on micromotion and bone stress. JSES Int. 2020;4(4):979-86</ref> Lastly, Gutierrez et al. investigated optimal baseplate position using a computer model. According to their work that focused on uniform force distribution, a 15 degrees inferior tilt is best suited for concentric or lateral eccentric glenosphere, for inferior eccentric glenosphere a neutral inclination (0 degree) is the preferred orientation.<ref>Gutierrez S, Keller TS, Levy JC, Lee WE, 3rd, Luo ZP. Hierarchy of stability factors in reverse shoulder arthroplasty. Clin Orthop Relat Res. 2008;466(3):670-6</ref><ref>Gutierrez S, Walker M, Willis M, Pupello DR, Frankle MA. Effects of tilt and glenosphere eccentricity on baseplate/bone interface forces in a computational model, validated by a mechanical model, of reverse shoulder arthroplasty. J Shoulder Elbow Surg. 2011;20(5):732-9</ref> Superior tilt should always be avoided as stress at the bone interface increases. Boileau et al. suggested that superior tilt is commonly underestimated during reverse shoulder arthroplasty planification.<ref>Boileau P, Gauci MO, Wagner ER, Clowez G, Chaoui J, Chelli M, et al. The reverse shoulder arthroplasty angle: a new measurement of glenoid inclination for reverse shoulder arthroplasty. J Shoulder Elbow Surg. 2019;28(7):1281-90</ref> As the baseplate is implanted in the inferior part of the glenoid, they introduced the reverse shoulder arthroplasty angle, defined as the angle between the inferior part of the glenoid fossa and the perpendicular to the floor of the supraspinatus. Compared to the anatomic total shoulder arthroplasty angle (β angle or global glenoid inclination angle), the reverse shoulder arthroplasty angle is 8 ± 4 degrees larger.
 ===Stability===

@@ Line 154: / Line 154: @@
 ===Modifications in muscle recruitment===
 [[File:Capture d’écran 2021-07-29 à 04.32.18.png|alt=(A) Native shoulder. The center of rotation is in the humeral head, and the level of arm of deltoid does not allow consequent deltoid recruitment. (B) RSA with a medial glenoid/lateral humerus design in case of massive and irreparable rotator cuff lesion. Medialization of the center of rotation and humeral lateralization allows important deltoid recruitment. (C) Lateral glenoid/medial humerus RSA. As in native shoulders, the bony lateralization of the center of rotation decreases recruitment of the deltoid for rotation but allows for a retensioning of the rotator cuff.|thumb|(A) Native shoulder. The center of rotation is in the humeral head, and the level of arm of deltoid does not allow consequent deltoid recruitment. (B) RSA with a medial glenoid/lateral humerus design in case of massive and irreparable rotator cuff lesion. Medialization of the center of rotation and humeral lateralization allows important deltoid recruitment. (C) Lateral glenoid/medial humerus RSA. As in native shoulders, the]]
-The aforementioned modifications to physiologic shoulder anatomy lead to a 42% increased deltoid lever arm, as well as an increased recruitment of anterior deltoid muscle fibers to perform abduction.<ref>Kontaxis A, Johnson GR. The biomechanics of reverse anatomy shoulder replacement--a modelling study. Clin Biomech (Bristol, Avon). 2009;24(3):254-60</ref> The original design with a 155 degrees non-anatomic stem further enhanced the deltoid lever arm by distalization of the humerus.<ref>Boileau P, Watkinson DJ, Hatzidakis AM, Balg F. Grammont reverse prosthesis: design, rationale, and biomechanics. J Shoulder Elbow Surg. 2005;14(1 Suppl S):147S-61S</ref> The anterior deltoid becomes consecutively an important contributor to flexion and abduction moment arms.<ref>Schwartz DG, Kang SH, Lynch TS, Edwards S, Nuber G, Zhang LQ, et al. The anterior deltoid's importance in reverse shoulder arthroplasty: a cadaveric biomechanical study. J Shoulder Elbow Surg. 2013;22(3):357-64</ref> In case of a deficient anterior deltoid (i.e., revision surgery with detached or paretic anterior deltoid)<ref>Lädermann A, Walch G, Denard PJ, Collin P, Sirveaux F, Favard L, et al. Reverse shoulder arthroplasty in patients with pre-operative impairment of the deltoid muscle. Bone Joint J. 2013;95-B(8):1106-13</ref> compensation for abduction relies on significantly enhanced force of the subscapularis (195%) and middle portion of the deltoid (26%).<ref>Gulotta LV, Choi D, Marinello P, Wright T, Cordasco FA, Craig EV, et al. Anterior deltoid deficiency in reverse total shoulder replacement: a biomechanical study with cadavers. J Bone Joint Surg Br. 2012;94(12):1666-9</ref> There are, however, drawbacks to these anatomic modifications of physiologic moment arms. While the anterior and posterior deltoid as well as pectoralis major are recruited as additional flexors and abductors, the latissimus dorsi, teres major, and lower part of the pectoralis major have increased adductor and extensor moment arms, therefore directly limiting their participation in active internal and external rotation.<ref>Ackland DC, Richardson M, Pandy MG. Axial rotation moment arms of the shoulder musculature after reverse total shoulder arthroplasty. J Bone Joint Surg Am. 2012;94(20):1886-95</ref><ref>Ackland DC, Roshan-Zamir S, Richardson M, Pandy MG. Moment arms of the shoulder musculature after reverse total shoulder arthroplasty. J Bone Joint Surg Am. 2010;92(5):1221-30</ref> As lever arms of the anterior and posterior cuff are already decreased secondary to humeral medialization, this adds to a further weakening of active internal and external rotation.<ref>Herrmann S, Konig C, Heller M, Perka C, Greiner S. Reverse shoulder arthroplasty leads to significant biomechanical changes in the remaining rotator cuff. J Orthop Surg Res. 2011;6:4</ref><ref>Simovitch RW, Helmy N, Zumstein MA, Gerber C. Impact of fatty infiltration of the teres minor muscle on the outcome of reverse total shoulder arthroplasty. J Bone Joint Surg Am. 2007;89(5):934-9</ref> This issue can either be addressed by the addition of a tendon transfer or by modifying the classic reverse shoulder arthroplasty design to a “lateralized” one.<ref>Shi LL, Cahill KE, Ek ET, Tompson JD, Higgins LD, Warner JJ. Latissimus Dorsi and Teres Major Transfer With Reverse Shoulder Arthroplasty Restores Active Motion and Reduces Pain for Posterosuperior Cuff Dysfunction. Clin Orthop Relat Res. 2015;473(10):3212-7</ref> This modification will preserve rotational moment arms of the subscapularis and teres minor and therefore enhance active range of motion in the axial plane (Figure).<ref>Greiner S, Schmidt C, Konig C, Perka C, Herrmann S. Lateralized reverse shoulder arthroplasty maintains rotational function of the remaining rotator cuff. Clin Orthop Relat Res. 2013;471(3):940-6</ref> Finally, while the postoperative range of motion takes place inside the prosthetic joint, scapulothoracic participation is significantly increased after reverse shoulder arthroplasty.<ref>Kwon YW, Pinto VJ, Yoon J, Frankle MA, Dunning PE, Sheikhzadeh A. Kinematic analysis of dynamic shoulder motion in patients with reverse total shoulder arthroplasty. J Shoulder Elbow Surg. 2012;21(9):1184-90</ref>
+The aforementioned modifications to physiologic shoulder anatomy lead to a 42% increased deltoid lever arm, as well as an increased recruitment of anterior deltoid muscle fibers to perform abduction.<ref name=":24">Kontaxis A, Johnson GR. The biomechanics of reverse anatomy shoulder replacement--a modelling study. Clin Biomech (Bristol, Avon). 2009;24(3):254-60</ref> The original design with a 155 degrees non-anatomic stem further enhanced the deltoid lever arm by distalization of the humerus.<ref>Boileau P, Watkinson DJ, Hatzidakis AM, Balg F. Grammont reverse prosthesis: design, rationale, and biomechanics. J Shoulder Elbow Surg. 2005;14(1 Suppl S):147S-61S</ref> The anterior deltoid becomes consecutively an important contributor to flexion and abduction moment arms.<ref>Schwartz DG, Kang SH, Lynch TS, Edwards S, Nuber G, Zhang LQ, et al. The anterior deltoid's importance in reverse shoulder arthroplasty: a cadaveric biomechanical study. J Shoulder Elbow Surg. 2013;22(3):357-64</ref> In case of a deficient anterior deltoid (i.e., revision surgery with detached or paretic anterior deltoid)<ref>Lädermann A, Walch G, Denard PJ, Collin P, Sirveaux F, Favard L, et al. Reverse shoulder arthroplasty in patients with pre-operative impairment of the deltoid muscle. Bone Joint J. 2013;95-B(8):1106-13</ref> compensation for abduction relies on significantly enhanced force of the subscapularis (195%) and middle portion of the deltoid (26%).<ref>Gulotta LV, Choi D, Marinello P, Wright T, Cordasco FA, Craig EV, et al. Anterior deltoid deficiency in reverse total shoulder replacement: a biomechanical study with cadavers. J Bone Joint Surg Br. 2012;94(12):1666-9</ref> There are, however, drawbacks to these anatomic modifications of physiologic moment arms. While the anterior and posterior deltoid as well as pectoralis major are recruited as additional flexors and abductors, the latissimus dorsi, teres major, and lower part of the pectoralis major have increased adductor and extensor moment arms, therefore directly limiting their participation in active internal and external rotation.<ref>Ackland DC, Richardson M, Pandy MG. Axial rotation moment arms of the shoulder musculature after reverse total shoulder arthroplasty. J Bone Joint Surg Am. 2012;94(20):1886-95</ref><ref>Ackland DC, Roshan-Zamir S, Richardson M, Pandy MG. Moment arms of the shoulder musculature after reverse total shoulder arthroplasty. J Bone Joint Surg Am. 2010;92(5):1221-30</ref> As lever arms of the anterior and posterior cuff are already decreased secondary to humeral medialization, this adds to a further weakening of active internal and external rotation.<ref>Herrmann S, Konig C, Heller M, Perka C, Greiner S. Reverse shoulder arthroplasty leads to significant biomechanical changes in the remaining rotator cuff. J Orthop Surg Res. 2011;6:4</ref><ref>Simovitch RW, Helmy N, Zumstein MA, Gerber C. Impact of fatty infiltration of the teres minor muscle on the outcome of reverse total shoulder arthroplasty. J Bone Joint Surg Am. 2007;89(5):934-9</ref> This issue can either be addressed by the addition of a tendon transfer or by modifying the classic reverse shoulder arthroplasty design to a “lateralized” one.<ref>Shi LL, Cahill KE, Ek ET, Tompson JD, Higgins LD, Warner JJ. Latissimus Dorsi and Teres Major Transfer With Reverse Shoulder Arthroplasty Restores Active Motion and Reduces Pain for Posterosuperior Cuff Dysfunction. Clin Orthop Relat Res. 2015;473(10):3212-7</ref> This modification will preserve rotational moment arms of the subscapularis and teres minor and therefore enhance active range of motion in the axial plane (Figure).<ref>Greiner S, Schmidt C, Konig C, Perka C, Herrmann S. Lateralized reverse shoulder arthroplasty maintains rotational function of the remaining rotator cuff. Clin Orthop Relat Res. 2013;471(3):940-6</ref> Finally, while the postoperative range of motion takes place inside the prosthetic joint, scapulothoracic participation is significantly increased after reverse shoulder arthroplasty.<ref>Kwon YW, Pinto VJ, Yoon J, Frankle MA, Dunning PE, Sheikhzadeh A. Kinematic analysis of dynamic shoulder motion in patients with reverse total shoulder arthroplasty. J Shoulder Elbow Surg. 2012;21(9):1184-90</ref>
 ===Medialization of the joint center of rotation===
-The biomechanical benefit of a medialized joint center of rotation is to convert torque forces into compressive forces across the bone-glenosphere interface and therefore provide stability and enhanced component integration (111). As the rotator cuff no longer provides its compressive forces, the fixed center of rotation allows the deltoid to compensate and provide the needed compression to stabilize the joint (99). While in anatomic total shoulder arthroplasty joint reaction forces can reach up to 90% of body weight at 90 degrees of abduction, reverse shoulder arthroplasty design reduces both compressive and shear stress and therefore joint reaction forces by up to 42%. This further allows active abduction with a 20% decreased deltoid activity in a cuff deficient shoulder (112-114).
+The biomechanical benefit of a medialized joint center of rotation is to convert torque forces into compressive forces across the bone-glenosphere interface and therefore provide stability and enhanced component integration.<ref name=":25">Harman M, Frankle M, Vasey M, Banks S. Initial glenoid component fixation in "reverse" total shoulder arthroplasty: a biomechanical evaluation. J Shoulder Elbow Surg. 2005;14(1 Suppl S):162S-7S</ref> As the rotator cuff no longer provides its compressive forces, the fixed center of rotation allows the deltoid to compensate and provide the needed compression to stabilize the joint.<ref name=":24" /> While in anatomic total shoulder arthroplasty joint reaction forces can reach up to 90% of body weight at 90 degrees of abduction, reverse shoulder arthroplasty design reduces both compressive and shear stress and therefore joint reaction forces by up to 42%. This further allows active abduction with a 20% decreased deltoid activity in a cuff deficient shoulder.<ref>Terrier A, Reist A, Merlini F, Farron A. Simulated joint and muscle forces in reversed and anatomic shoulder prostheses. J Bone Joint Surg Br. 2008;90(6):751-6</ref><ref>Ackland DC, Roshan-Zamir S, Richardson M, Pandy MG. Muscle and joint-contact loading at the glenohumeral joint after reverse total shoulder arthroplasty. J Orthop Res. 2011;29(12):1850-8</ref><ref>Rugg CM, Coughlan MJ, Lansdown DA. Reverse Total Shoulder Arthroplasty: Biomechanics and Indications. Curr Rev Musculoskelet Med. 2019;12(4):542-53</ref>
-There is, however, a major drawback of center of rotation medialization, in the form of impingement between the scapular neck and humeral prosthetic component defined as scapular notching (115, 116). Several technical factors improve impingement free range of motion. One option is placing the glenosphere (not the baseplate) below the inferior glenoid rim or using an inferior eccentric glenosphere. De Wilde et al. reported that a 5 mm overhang could improve impingement free adduction by 39 degrees (117-119). Abduction is also positively correlated with acromiohumeral distance (r = 0.93; p < 0.001) which is increased with an eccentric glenosphere (120). The ideal amount of overhang relative to the glenoid appears to be about 2.5 mm based on clinical evidence (121). Alternatively, glenosphere diameter can be increased, therefore upsizing the diameter from 38 to 46 mm was reported to not only increase range of motion by 39% but also stability by a 36% increase in jump distance (122). Accenter of rotationding to a computer simulation of impingement free range of motion, the single most effective modification in prosthetic design is the change of humeral neck-shaft angle from the classic 155 towards a more anatomic angle (123, 124).
+There is, however, a major drawback of center of rotation medialization, in the form of impingement between the scapular neck and humeral prosthetic component defined as scapular notching.<ref>Lädermann A, Gueorguiev B, Charbonnier C, Stimec BV, Fasel JH, Zderic I, et al. Scapular Notching on Kinematic Simulated Range of Motion After Reverse Shoulder Arthroplasty Is Not the Result of Impingement in Adduction. Medicine (Baltimore). 2015;94(38):e1615</ref><ref>Simovitch RW, Zumstein MA, Lohri E, Helmy N, Gerber C. Predictors of scapular notching in patients managed with the Delta III reverse total shoulder replacement. J Bone Joint Surg Am. 2007;89(3):588-600</ref> Several technical factors improve impingement free range of motion. One option is placing the glenosphere (not the baseplate) below the inferior glenoid rim or using an inferior eccentric glenosphere.<ref>Nyffeler RW, Werner CM, Gerber C. Biomechanical relevance of glenoid component positioning in the reverse Delta III total shoulder prosthesis. J Shoulder Elbow Surg. 2005;14(5):524-8</ref> De Wilde et al. et reported that a 5 mm overhang could improve impingement free adduction by 39 degrees.<ref>de Wilde LF, Poncet D, Middernacht B, Ekelund A. Prosthetic overhang is the most effective way to prevent scapular conflict in a reverse total shoulder prosthesis. Acta Orthop. 2010;81(6):719-26</ref> Abduction is also positively correlated with acromiohumeral distance (r = 0.93; p < 0.001) which is increased with an eccentric glenosphere.<ref>Lädermann A, Denard PJ, Collin P, Zbinden O, Chiu JC, Boileau P, et al. Effect of humeral stem and glenosphere designs on range of motion and muscle length in reverse shoulder arthroplasty. Int Orthop. 2020;44(3):519-30</ref> The ideal amount of overhang relative to the glenoid appears to be about 2.5 mm based on clinical evidence.<ref>Haidamous G, Lädermann A, Hartzler RU, Parsons B, Lederman E, Tokish J, et al. Radiographic parameters associated with excellent versus poor range of motion outcomes following reverse shoulder arthroplasty. Shoulder & Elbow. 2020;9:1758573220936234</ref> Alternatively, glenosphere diameter can be increased, therefore upsizing the diameter from 38 to 46 mm was reported to not only increase range of motion by 39% but also stability by a 36% increase in jump distance.<ref>Roche C, Flurin PH, Wright T, Crosby LA, Mauldin M, Zuckerman JD. An evaluation of the relationships between reverse shoulder design parameters and range of motion, impingement, and stability. J Shoulder Elbow Surg. 2009;18(5):734-41</ref> Accenter of rotationding to a computer simulation of impingement free range of motion, the single most effective modification in prosthetic design is the change of humeral neck-shaft angle from the classic 155 towards a more anatomic angle.<ref name=":26">Gutierrez S, Comiskey CAt, Luo ZP, Pupello DR, Frankle MA. Range of impingement-free abduction and adduction deficit after reverse shoulder arthroplasty. Hierarchy of surgical and implant-design-related factors. J Bone Joint Surg Am. 2008;90(12):2606-15</ref><ref>Lädermann A, Denard PJ, Boileau P, Farron A, Deransart P, Terrier A, et al. Effect of humeral stem design on humeral position and range of motion in reverse shoulder arthroplasty. Int Orthop. 2015;39(11):2205-13</ref>
-While joint center of rotation needs to be medialized in regard to the native center of rotation, slight lateralization of the glenosphere from the glenoid can further enhance compressive forces, which are thought to overcome the increased shear forces at the bone-component interface (111). Basic science studies show several benefits of lateralization. In both sawbone (125) and computer models,(123, 126, 127) lateralization improves range of motion in all directions.(127). There is an ongoing debate regarding the impact of lateralization on the risk for acromial stress fractures. Finite element analysis has suggested a 17,2% increased acromial stress secondary to 10 mm lateralization (128). Clinically, distalization has been implicated as more of a culprit than lateralization (129). Glenosphere lateralization has further a linear correlation with baseplate micromotion (130) and therefore exposes to the risk of aseptic loosening (131). Giles et al. tested the effect of glenoid and humeral lateralization on deltoid muscle load in vitro using a simulator. They reported that 10 mm of humeral lateralization was the only parameter that actually decreased deltoid force in abduction (65 ± 8%), however, warned that this benefit may not compensate for the negative effects induced by glenosphere lateralization (132). Lastly, Boileau et al. proposed a bony increased-offset reverse shoulder arthroplasty to lateralize the glenosphere however maintaining center of rotation at the prosthesis-bone interface and thereby minimizing torque stress (133).
+While joint center of rotation needs to be medialized in regard to the native center of rotation, slight lateralization of the glenosphere from the glenoid can further enhance compressive forces, which are thought to overcome the increased shear forces at the bone-component interface.<ref name=":25" /> Basic science studies show several benefits of lateralization. In both sawbone<ref>Gutierrez S, Levy JC, Frankle MA, Cuff D, Keller TS, Pupello DR, et al. Evaluation of abduction range of motion and avoidance of inferior scapular impingement in a reverse shoulder model. J Shoulder Elbow Surg. 2008;17(4):608-15   Gutierrez S, Levy JC, Frankle MA, Cuff D, Keller TS, Pupello DR, et al. Evaluation of abduction range of motion and avoidance of inferior scapular impingement in a reverse shoulder model. J Shoulder Elbow Surg. 2008;17(4):608-15</ref> and computer models,<ref name=":26" /><ref>Kim SJ, Jang SW, Jung KH, Kim YS, Lee SJ, Yoo YS. Analysis of impingement-free range of motion of the glenohumeral joint after reverse total shoulder arthroplasty using three different implant models. J Orthop Sci. 2019;24(1):87-94</ref><ref name=":27">Lädermann A, Tay E, Collin P, Piotton S, Chiu CH, Michelet A, et al. Effect of critical shoulder angle, glenoid lateralization, and humeral inclination on range of movement in reverse shoulder arthroplasty. Bone Joint Res. 2019;8(8):378-86</ref> lateralization improves range of motion in all directions.<ref name=":27" /> There is an ongoing debate regarding the impact of lateralization on the risk for acromial stress fractures. Finite element analysis has suggested a 17,2% increased acromial stress secondary to 10 mm lateralization.<ref>Wong MT, Langohr GDG, Athwal GS, Johnson JA. Implant positioning in reverse shoulder arthroplasty has an impact on acromial stresses. J Shoulder Elbow Surg. 2016;25(11):1889-95</ref> Clinically, distalization has been implicated as more of a culprit than lateralization.<ref>Haidamous G, Lädermann A, Frankle MA, Gorman RA, 2nd, Denard PJ. The risk of postoperative scapular spine fracture following reverse shoulder arthroplasty is increased with an onlay humeral stem. J Shoulder Elbow Surg. 2020;29(12):2556-63</ref> Glenosphere lateralization has further a linear correlation with baseplate micromotion<ref>Berliner JL, Regalado-Magdos A, Ma CB, Feeley BT. Biomechanics of reverse total shoulder arthroplasty. J Shoulder Elbow Surg. 2015;24(1):150-60</ref> and therefore exposes to the risk of aseptic loosening.<ref>Lädermann A, Schwitzguebel AJ, Edwards TB, Godeneche A, Favard L, Walch G, et al. Glenoid loosening and migration in reverse shoulder arthroplasty. Bone Joint J. 2019;101-B(4):461-9</ref> Giles et al. tested the effect of glenoid and humeral lateralization on deltoid muscle load in vitro using a simulator. They reported that 10 mm of humeral lateralization was the only parameter that actually decreased deltoid force in abduction (65 ± 8%), however, warned that this benefit may not compensate for the negative effects induced by glenosphere lateralization.<ref>Giles JW, Langohr GD, Johnson JA, Athwal GS. Implant Design Variations in Reverse Total Shoulder Arthroplasty Influence the Required Deltoid Force and Resultant Joint Load. Clin Orthop Relat Res. 2015;473(11):3615-26</ref> Lastly, Boileau et al. proposed a bony increased-offset reverse shoulder arthroplasty to lateralize the glenosphere however maintaining center of rotation at the prosthesis-bone interface and thereby minimizing torque stress.<ref>Boileau P, Moineau G, Roussanne Y, O'Shea K. Bony increased-offset reversed shoulder arthroplasty: minimizing scapular impingement while maximizing glenoid fixation. Clin Orthop Relat Res. 2011;469(9):2558-67</ref>
 ===Baseplate design===