Operative Techniques in Orthopaedics
Volume 21, Issue 1 , Pages 52-59, March 2011

Reverse Total Shoulder Arthroplasty—Biomechanics and Rationale

Department of Orthopaedics, Columbia University Medical Center, New York, NY. Center for Shoulder, Elbow and Sports Medicine, Department of Orthopaedic Surgery, New York Presbyterian Hospital/Columbia University Medical Center, New York, NY

Article Outline

Reverse total shoulder arthroplasty (RTSA) was designed to provide pain relief, improve functional results, and reduce the risk of implant failure in patients with a painful rotator cuff-deficient shoulder. Understanding the biomechanics and rationale behind the surgical technique and its relationship to implant design is essential. The design and biomechanics of RTSA is based upon principles put forth by Grammont. These are inherent prosthetic stability, convexity of the glenoid components with complementing concavity of the humeral component, glenosphere center placement at or within the glenoid neck, and a medialized and distalized center of rotation. In addition, patient selection, surgical technique, and postoperative management all factor in the functional success of RTSA. Component sizing, version, glenoid baseplate fixation and placement, humeral neck-shaft angle and distalization, and surgical approach are all choices made by the informed surgeon. Each factor plays a role in the functional outcome of an RTSA and its potential complications.

Keywords: biomechanics, cuff tear arthropathy, reverse total shoulder arthroplasty

 

Conventional total shoulder arthroplasty (TSA) is designed to improve function by the use of an intact rotator cuff and extrinsic shoulder musculature by restoring native glenohumeral anatomy. This differs from the design of the reverse total shoulder arthroplasty (RTSA), which is intended to provide a stable fulcrum for shoulder motion of rotator cuff deficient patients. The necessity of design differences between successful conventional TSA and RTSA highlights the important biomechanical implications of rotator cuff function in shoulder arthroplasty.

In the native shoulder and conventional TSA model, the rotator cuff tendons provide dynamic shoulder stability by increasing the concavity-compression mechanism for joint stability.1, 2 When a rotator cuff deficiency exists, the normal force-couple that maintains stability at the glenohumeral joint is disrupted.3, 4 In the absence of a mechanically effective rotator cuff, the unopposed contraction of the deltoid creates a force vector that progressively displaces the humeral head superiorly rather than medially when forward elevation or abduction is attempted. This action leads to an inability to actively elevate or abduct the arm, resulting in the clinical phenomenon of pseudoparalysis. Eventually, a massive, irreparable rotator cuff tear may lead to secondary glenohumeral and acromiohumeral arthritis with osteonecrosis and collapse of the humeral head, coined “rotator cuff tear arthropathy” by Neer et al.5 This proved to be a difficult clinical entity to treat.

Attempts at conventional TSA without a functional rotator cuff resulted in uncontrolled humeral translation and eccentric loading of the glenoid. The lateral offset of the glenohumeral center of rotation was an attempt to replicate natural anatomy, therefore leading to increased forces on the glenoid-bone interface. Historically, this treatment scenario led to rapid loosening and failure of the glenoid component. Treatment options were limited at the time, and therefore, hemiarthroplasty was favored to treat massive, irreparable cuff tears or cuff tear arthropathy despite unpredictable and inconsistent improvements in pain and range of motion.5, 6, 7

In the search for a more successful treatment solution, constrained and semiconstrained RTSA designs were tested experimentally, such as the Kessel prosthesis, which was published in a series demonstrating decent pain relief, but poor function and a high reoperation rate.8, 9, 10 These designs attempted to stabilize the proximal migration of the humeral head in a rotator cuff-deficient shoulder. The early designs were still based upon a principle of replicating an anatomical center of rotation location and therefore created a long lever arm on the glenoid component. The excessive forces on the glenoid prosthesis resulted in early catastrophic loosening. These dismal results led to disinterest in the RTSA concept.

In 1987, Professor Paul Grammont introduced an improved biomechanical design with principles based upon a fixed convex glenoid component and fixed concave humeral articulating component.11 These principles addressed many of the limitations of conventional TSA for the rotator cuff deficient shoulder and previous RTSA designs. Grammont's design was further supported approximately a decade later, when Boulahia et al12 demonstrated successful clinical outcomes using this prosthesis in 16 patients with cuff tear arthropathy.

The principles of Grammont's unique design were (1) inherent prosthetic stability, (2) convexity of the glenoid components with complementing concavity of the humeral component, (3) glenosphere center placement at or within the glenoid neck, and (4) a medialized and distalized center of rotation. Intrinsic stability is ensured with large spherical glenoid component with no neck on the glenoid side and a small semiconstrained cup at a nonanatomic 155° inclination angle on the humeral side. This conforming but not completely constrained design allows a greater range of motion before impingement occurs while maintaining prosthetic stability. By medializing and distalizing the center of rotation, the deltoid tension and length is increased, therefore recruiting more deltoid fibers and allowing the deltoid lever arm to be maximized (Fig. 1). Placing the center of rotation in a medial and inferior position also minimizes the shear forces and torque on the glenoid component.

(Reproduced with permission from Boileau et al.10) (Figure courtesy of Center for Shoulder, Elbow and Sports Medicine.)

Long-term follow-up of current RTSA prosthesis designs based upon Grammont's principles are still pending, but many short- and mid-term studies suggest good clinical outcomes, although complications, such as scapular notching, instability, and glenoid component failure, remain a concern. Complication rates are reported up to 75% in some series, at least 3 times that of conventional TSA.10, 13, 14, 15 These concerns have stimulated recent clinical and biomechanics research to improve implant function and survivability, minimize complications, and increase clinical success.

With the availability of a proven RTSA design, patient selection is important because performing RTSA for incorrect indications may lead to poor outcomes with consequences that are difficult to salvage. RTSA has been used to treat recalcitrant cuff tear arthropathy,5 in rheumatoid arthritis, severe proximal humerus fractures, in revision of failed fracture reconstruction, or failed arthroplasty with instability. It is ideally indicated in patients who present with clinically symptomatic massive, irreparable rotator cuff tears, and resultant pseudoparalysis but also in the previously mentioned clinical presentations. Deltoid integrity is necessary for a functional RTSA because it provides the most power contributing to restoration of forward elevation and abduction. Adequate glenoid bone stock and quality is also necessary for component fixation and may be evaluated preoperatively by the use of radiographs and computed tomography. Contraindications include inadequate glenoid or humeral bone stock, infection, a neuropathic joint, or neurologic deficits. Because RTSA is a technically demanding procedure with a high complication rate, preoperative planning is essential to a successful outcome.

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Stability 

The RTSA is stable as the result of Grammont's design principles. Glenohumeral translation is limited by the constraint of a deep conforming concave humeral cup that articulates with the larger glenosphere.10 Lack of translation eliminates the chances of edge loading, which may lead to cold flow, or permanent polymer deformation at working temperatures, of the polyethylene glenoid component rim. It also decreases the risk of glenoid component loosening.

The shape of the complementing convex glenosphere and concave humeral cup are conforming, therefore creating the basis for a concentric motion arc. As opposed to a conventional TSA in which there is a radii mismatch between the glenoid and humeral head to allow both translation and rotation, the components of an RTSA have no mismatch. Current designs have a larger convex component, allowing a greater range of motion before impingement. The concave component is also sufficiently large and deep, increasing intrinsic stability.16, 17, 18

Component size is important for stability as well as range of motion. Stability of the articulating components is related to the both the diameter and depth within the humeral socket and the radius of the glenosphere. Stability is increased as the ratio between the depth and diameter of the concave humeral socket increases relative to their absolute size.19 However, a prosthesis with a high ratio between the depth and diameter of the humeral component indicating more stability is more likely to lead to scapular notching, as opposed to a lower-ratio, less-stable component.20

This nonanatomic design allows stabilization of the glenosphere within the humeral socket, maintaining the net joint reaction force exerted by the glenoid convexity within 45° of the center of the humeral articular concavity and providing a greater stable range of motion. The maximal net angle that the net joint reaction force can form with the concavity before dislocation occurs is defined by Matsen et al21 as the balance stability angle. This angle for the RTSA is greater compared with conventional TSA, which has a balance stability angle of ≤30°.

In a cadaver study, Favre et al22 discuss intrinsic stability by testing range of motion and component positioning. Intrinsic stability was most influenced by the spatial positions of the concave humerus component, followed closely by the degree of positional abduction. They demonstrated a 60% greater stability ratio in 90° abduction, theorized to be the result of possible decoaptation of the components attributable to impingement at the scapular neck with the arm in resting position. These authors also demonstrated that humeral version highly affected stability and recommended placement of the humeral component in neutral to slight anteversion. With the arm resting at the side, a change of 10° in humeral version affected the stability ratio 27%, whereas the same degree change in glenoid version resulted in a 15% loss of stability ratio. The recommendation for glenoid version, which is a secondary component of intrinsic stability, is to avoid retroversion >10°. Mole and Favard's series23 also agrees with this recommendation; in their study, they demonstrate better clinical and radiographic outcomes with minimal retroversion or slight anterversion.

In addition to medial impingement of the humeral component on the scapula in adduction, Gerber et al20 also noted that with such a large excursion of the humeral component, there may be greater tuberosity impingement at the scapular spine, causing levering of the humerus out of the socket, especially with lateralized center of rotation designs. Range-of-motion studies demonstrate impingement of the humerus at the acromion at the end ranges of abduction. Biomechanical research by Gutierrez et al24 determined that neck-shaft angle, center of rotation, and implant position were important in predicting adduction deficit postoperatively. Prosthetic instability may also be attributed to medial impingement or excessive medialization of the humerus. Therefore, it is important to check impingement with range of motion both in adduction and abduction when testing trial prosthetic stability.

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Center of Rotation 

The location of the center of rotation in successful RTSA designs has traditionally been medial to the anatomic center of rotation in accordance with Grammont's principles. He believed that to recruit more deltoid fibers and to diminish the shear forces across the glenoid component, the center of rotation should be at implant-bone interface of the scapula. Several newer designs use a relatively more lateral center of rotation that still remains medial to the anatomic center of rotation. Location of the center of rotation affects the potential range of motion to impingement, deltoid tension, the lever arm of the deltoid, and the torque at the baseplate-bone interface.

Recent studies in which the authors used computer simulations and biomechanical studies report that total range of motion is increased and adduction deficit is improved with a lateralized design (Greiwe et al, unpublished data, 2010).24, 25, 26 However, the clinical benefits of a lateralized center of rotation have not been established.

Deltoid tension in the traditional designs has been provided through distalization of the humerus.10 Proponents of the lateralized design theorized that tensioning can occur via lengthening in a distal and lateral direction. Deltoid recruitment for the initiation of abduction occurs based on the location of the center of rotation. In a medialized center of rotation design, more anterior and posterior fibers are recruited for abduction. Conversely, a lateralized center of rotation design decreases the recruitment of fibers but has the benefit of improved deltoid tensioning so that the force required to abduct the arm is decreased.

Torque at the baseplate-bone interface is increased due to the net force is acting at the center of rotation from the point of fixation of the glenoid. Concerns were raised when lateralized designs demonstrated a significant (12%) rate of glenoid failure.27 The authors hypothesized that the torque from a lateralized design resulted in micromotion at levels that precluded osseous integration. Improved fixation may mitigate concerns over glenoid loosening from excessive torque as newer studies have demonstrated improved glenoid loosening rates (<1%) with a newer locking screw design.28 Therefore, there is still debate as to the most ideal location for the center of rotation to maximize implant function while preventing component failure.

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Baseplate Fixation 

Failure strength is also related to implant fixation. In initial designs, glenoid component fixation with cement or large pegs was inadequate in the long term. Newer designs have used compression with screws or a combination of screws and a press fit peg with a porous surface to enhance bone ingrowth or on growth. Frankle et al27 reported a 12% failure rate at the baseplate-glenoid junction and attributed these failures to excessive early micromotion despite a porous coating. The design was modified with larger diameter locking screws to improve initial fixation. Biomechanical research demonstrated maximum failure load increased 2.3 times with a central screw design compared with a central peg.29

Micromotion is a critical factor in initial implant stability and influenced by the component center of rotation. Virani et al30 used finite element models to demonstrate increasing micromotion as designs moved the center of rotation laterally. A study by Harman et al31 also demonstrated that micromotion was increased by lateral offset in an in vitro design where arm abduction was simulated. Therefore, to lessen the likelihood of implant loosening, micromotion must be minimized to enhance bone formation and avoid soft tissue interposition. Studies by Engh et al,32 Jasty et al,33 and Pilliar et al34 demonstrated that implants with micromotion >150 μm are likely to have fibrous ingrowth, rather than bone ingrowth.33, 35, 36 The authors of recent studies suggest that the threshold for bone ingrowth may be even smaller than 150 μm.33, 35 The potential bone ingrowth quality may be affected by certain factors, including postoperative activity, bone quality, center of rotation, and initial strength and durability of fixation with cyclic loading.

Bone quality also has a significant influence on initial implant stability. The failure rate of implants fixed in glenoids with bone defects has not been reported, although aseptic loosening was noted in 3 patients in a revision setting in which bone loss was more likely.17 Codsi and Iannotti37 performed a biomechanical study looking at micromotion in the presence of a significant bone defect, simulating the revision setting. In each case, a large bone defect resulted in greater micromotion than standard bone. Mroczkowski38 demonstrated the effect of simulated bone density on implant micromotion. Micromotion in poor quality bone may result in significant micromotion causing absent, delayed or poor bone ingrowth.

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Baseplate Position and Inclination 

Recommendations for optimal baseplate position have been evolving. Initial recommendations were to place the implant at the center of the glenoid for maximum bone contact. Recent studies now advocate an inferior placement of the baseplate with inferior inclination to reduce the incidence of scapular notching and altered baseplate fixation. Scapular notching is considered one of the most common complications of RTSA. Determining the natural history of scapular notching and its effect on clinical outcomes remains controversial. It is theorized to occur from impingement of the medial humeral neck component on the inferior and posterior glenoid pole during adduction. Retrieval studies confirmed marked polyethylene wear at the medial border of the prosthesis and radiographs demonstrated a variable size radiolucency involving the medial scapular border.39 Authors hypothesize that mechanical impingement and polyethylene wear debris are the factors that most contribute to scapular notching. The incidence of scapular notching is reported to be 50%-96% in clinical series10, 17, 40 and usually appears from 3 to 6 months postoperatively.41 Sirveaux et al15 described a radiographic classification of inferior scapular notching in 4 grades. Grade 1, with a defect at the pillar alone, grade 2, notching with contact with the lower baseplate screw, grade 3, extension over the lower screw, and grade 4, extension under the baseplate (Fig. 2). The correlation of radiographic scapular notching to clinical outcomes remains unclear. Levigne et al42 demonstrated that clinical outcomes, such as range of motion, Constant score, and pain were not affected by notching, whereas Sirveaux et al15 demonstrated a negative clinical association of grades 3 and 4 notching with lower Constant scores. Levigne also showed a greater rate of radiologic notching in association with certain etiologies prompting RTSA, with cuff tear arthropathy the greatest at 76%. Goutallier stage 3 and 4 fatty infiltration of the infraspinatus, superior glenoid erosion (Favard type E243), and a narrow acromiohumeral distance were also associated with notching.

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  • Figure 2. 

    Sirveaux classification of radiographic notching. Grade 1: defect at pillar, Grade 2: notching with contact at inferior screw, Grade 3: extension over lower screw, and Grade 4: extension under baseplate.

(Figure courtesy of Center for Shoulder, Elbow and Sports Medicine.)

To reduce scapular notching, it has been advocated to place the inferior part of the glenosphere at the inferior portion of the glenoid with inferior tilt. Placing the baseplate and glenosphere in 15°-20 ° of inferior tilt improved survival outcomes and decreased the incidence of inferior scapular notching (Fig. 3).39 A biomechanical study by Kelly et al44 used a computed tomography templating protocol that demonstrated reduced scapular notching rates after 12-mm inferior placement of the baseplate with respect to the center of the native glenoid. Nyffeler et al39 also demonstrated that placing the glenoid lower was more important than adding neck length or increasing inferior tilt alone. Levigne et al42 recommended placing the baseplate flush with the inferior osseous glenoid rim so that the glenosphere extended 4 mm beyond the rim inferiorly (Fig. 4).

(Figure courtesy of Center for Shoulder, Elbow and Sports Medicine.)

(Figure courtesy of Center for Shoulder, Elbow and Sports Medicine.)

A computer simulation model performed by the author's institution demonstrated a linear relationship between position of the glenosphere with respect to the center of the glenoid and range of motion improvement. In 2 separate implant designs, inferior positioning improved range of motion reflecting an improvement in adduction (Greiwe et al, unpublished data, 2010) (Fig. 5). When moving the baseplate inferiorly, surgeons should be cautious because of the limited amount of bone stock available for screw purchase. Inferior screw placement below the scapular pillar or superior screw placement below the base of the coracoid could be detrimental to future stability. Also, this study determined that an inferior glenosphere inclination of 20° improved adduction deficit and total arc of motion over other tilt angles.

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  • Figure 5. 

    Illustration of range of motion to impingement in adduction, based upon glenoid component positioning: (A) glenoid baseplate at neutral height and 0° tilt; (B) glenoid baseplate at inferior edge height and 0° tilt; and (C) glenoid baseplate at inferior edge height and 15° inferior tilt.

(Figure courtesy of Center for Shoulder, Elbow and Sports Medicine.)

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Distalization of Humerus 

As previously described, distalization of the humerus to lengthen the deltoid fibers and maximize the deltoid lever arm for abduction and elevation is a core concept in the functional outcome of the RTSA. Distalization, as well as medialization, of the humerus increases arm length an average of 1.5-3 cm (Fig. 6).10, 23 Lowering and translating the humerus medially retensions the deltoid, improving abduction and stability; however, it also realigns the vectors of the remaining rotator cuff muscles, decreasing their ability to provide external rotation. Less posterior deltoid can be used to compensate for the lack of external rotation with medialization of the humerus and center of rotation. Therefore, external rotation is often limited with RTSA, especially with absent or fatty infilitration of the teres minor. The ability to actively externally rotate is one of the proposed advantages of the lateralized center of rotation design possibly due to the more anatomic center of rotation and humeral position, allowing the deltoid to retension the necessary posterior deltoid and remaining rotator cuff.31

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  • Figure 6. 

    Distalization and medialization of the center of rotation and humerus with RTSA: (D) deltoid lengthening, allowing for retensioning, (L) increased lever arm for abduction of the humerus as the center of rotation in medialized and distalized, and (F) increase in deltoid lever arm torque.

(Figure courtesy of Center for Shoulder, Elbow and Sports Medicine.)

Ackland et al45 recently tested the moment arms of the shoulder girdle musculature in cadavers with implanted reverse total shoulder prostheses. He demonstrated that with the RTSA, there is an increase in the moment arms of the anterior and middle subregions of the deltoid, and that the posterior deltoid is also recruited as an abductor. The superior pectoralis (clavicular fibers) and anterior deltoid were the most effective flexors, whereas the subscapularis functioned as an extensor, abductor, and adductor, possibly resulting in a compressive joint force that could contribute to increasing prosthetic stability. The maximum torque peaked at approximately 90° abduction as the maximum deltoid tension was attained.

The tension of the conjoint tendon may also be used to help determine appropriate humeral lengthening, as it should be taut with the RTSA reduced, arm at side, and elbow extended, but relaxed with the elbow flexed.10 When reduced, there should be minimal translation, minimal “shuck” of about 2 mm, stability with abduction and internal rotation, and a snug fit of the reduction to ensure appropriate component sizing and placement.21

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Neck-Shaft Angle 

The neck-shaft angle, or the implant-neck angle of the humeral components influences range of motion, impingement, adduction deficit, and scapular notching (Greiwe et al, unpublished data, 2010).25, 41 These 2 angles are used to describe the same feature in implant design. The neck-shaft angle in RTSA is determined by the angle subtended by a perpendicular line to the shaft of the implant and a line along the humeral cup (including the polyethylene; Fig. 7). On average, the anatomical neck-shaft angle is approximately 135°, whereas different RTSA angles vary between each manufacturer's prostheses, ranging from 125° to 155°. The implant-neck angle is formed by a line down the implant and a line along the humeral polyethylene cup (Fig. 8). These range from 35° to 65°.

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  • Figure 7. 

    Neck-shaft angle measurement: the angle measured between the axial shaft of the humerus and angle of the humeral component neck, 150° for the implant as illustrated on this anteroposterior radiograph.

(Figure courtesy of Center for Shoulder, Elbow and Sports Medicine.)

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  • Figure 8. 

    Implant-neck angle measurement: the angle measured between the axial shaft of the humeral component and the face of the humerosocket, 65° for the implant as illustrated on this anteroposterier radiograph.

(Figure courtesy of Center for Shoulder, Elbow and Sports Medicine.)

In a study at our institution, 2 different implant designs were compared with the use of computer-simulated motion analysis. Each implant had a different center of rotation and implant-neck angle, but similar socket radius of curvatures. In this study, we found that a smaller implant-neck angle (comparing a 60° and 65° implant-neck angles) and a larger center of rotation minimized the amount of medial scapular impingement, therefore minimizing the adduction deficit (Greiwe et al, unpublished data, 2010). We found that implant design plays a large role in scapular impingement and overall glenohumeral range of motion, but does not play a role in maximum abduction achieved. Implant design features that contribute to these findings are increased center of rotation and decreased implant-neck angle. A glenosphere position of 10 mm below the center of the glenoid demonstrated improved adduction deficit, abduction angle, and total glenohumeral arc compared with other baseplate positions. Inferior glenosphere inclination of 20° improved adduction deficit and total arc of motion over other tilt angles (Greiwe et al, unpublished data, 2010).

Gutierrez et al19, 25 used both computer simulation and saw bone models to analyze multiple factors that may affect impingement and motion. They found that the adduction deficit can be improved upon by choosing a prosthesis with a varus (smaller) neck-shaft angle. Their study demonstrated that the neck-shaft angle was the primary factor affecting the adduction deficit. The use of a 130° versus 150° or 170° neck-shaft angle implant provided the smallest adduction deficit, of 5.8° with a center of rotation at 10 mm. Each prosthetic system varies, but generally investigators agree that a more varus humeral neck shaft angle helps to decrease the risk of scapular notching. Humeral cup sizing is customized intraoperatively, including height and tilt until good stability with functional range of motion is achieved. Therefore, appreciating a combination of component design factors with surgical technique is essential to RTSA.

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Surgical Approach 

Surgical approach can influence the stability of the joint after RTSA. Most often, a superolateral (anterolateral) or deltopectoral approach is chosen. Transacromial approaches are also described, but are contraindicated in patients with poor bone quality or a thin acromion, as demonstrated by the failures in acromion osteosynthesis post operatively by Rittmeister and Kerschbaumer.40 Mole et al23 compared a superolateral versus deltopectoral approach; they found the superolateral approach demonstrated better postoperative stability (0 vs 5.1%), better prevention of acromial and scapular fracture, and better preserved the subscapularis. Prosthetic dislocation has decreased with the anterosuperior trans-deltoid approach, but scapular notching was found radiographically more often (86% vs 56%), likely attributable to an inability to place the glenoid inferiorly from this approach.42

In contrast, the deltopectoral approach improves glenoid exposure, improving implant positioning and tilt. This has resulted in improved outcomes with less component loosening and inferior scapular notching. In addition, better retention of active external rotation, as well as preserving the deltoid origin, the main power of motion behind the RTSA, were reported in Mole et al's series.23 The deltopectoral approach allows identification and protection of the axillary nerve as well as access to the humerus in cases of revision, but does require a subscapularis tenotomy and repair. The large French series also demonstrated no statistical difference on outcome of subscapularis repair versus tenotomy without repair in his series. However, in a recent study Edwards et al46 followed a prospective series of 138 consecutive RTSAs and had outcomes complicated by 7 postoperative dislocations only in patients with irreparable subscapularis tendons. The influence of subscapularis repair on postoperative instability remains controversial.

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Conclusions 

Surgical indications, operative management, the resultant outcomes, and recognition of complications of RTSA are increasingly complex and rely on understanding the core principles of biomechanics providing the function and survivorship of the prosthesis. Although these core biomechanical principles originally described by Grammont provide the basis for current reverse total shoulder prostheses available, surgical approaches and techniques, including component positioning, fixation, sizing, and deltoid tensioning are technically difficult and require the surgeon to be familiar with the procedure and the prosthesis itself.

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References 

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PII: S1048-6666(10)00062-5

doi:10.1053/j.oto.2010.10.006

Operative Techniques in Orthopaedics
Volume 21, Issue 1 , Pages 52-59, March 2011

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