Shoulder prostheses treating cuff tear arthropathy: a comparative biomechanical study

Involvement of the scapulothoracic articulation after well-functioning reverse total shoulder arthroplasty

BACKGROUND

Shoulder movements result from the combined movements of the glenohumeral (GH) joint and the scapulothoracic (ST) joint, following a rhythm known as the scapulohumeral rhythm (SHR). The SHR represents the ratio of glenohumeral movement to scapulothoracic movement during arm elevation. Numerous studies have demonstrated that scapulothoracic kinematics and the SHR are significantly modified after reverse total shoulder arthroplasty (rTSA). This study aimed to analyze scapulothoracic motion post rTSA and compare it to that of an asymptomatic healthy shoulder.

MATERIALS AND METHODS

Thirteen shoulders formed the rTSA group. All patients had undergone rTSA for primary osteoarthritis (9 shoulders) or cuff tear arthropathy (5 shoulders) between 2017 and 2022. The healthy group consisted of 25 adult volunteers with no previous medical or surgical history involving the shoulder or spine. Analyses were performed bilaterally. For each patient, clinical (shoulder range of motion), functional (Constant score, American Shoulder and Elbow Surgeons Standardized Shoulder Assessment Form, and Subjective Shoulder Value), and scapular kinematic assessments were conducted using miniature inertial measurement units.

RESULTS

Scapular upward rotation was significantly increased in rTSA patients compared with healthy controls starting from 60° of elevation (P < .01). Scapular retraction was also significantly higher in rTSA patients compared with healthy controls from the beginning of elevation (P < .01). No significant difference in scapular tilt was found between rTSA and healthy patients. The SHR of healthy controls varies during abduction, from 7 at 30° to 4 at the end of the movement, indicating increased involvement of the ST joint relative to the GH joint. In contrast, the SHR in the rTSA cohort remains stable throughout abduction, from 2.4 at the beginning to 2.8 at the end, suggesting a greater contribution of the ST joint to abduction in rTSA patients than in the healthy controls, especially at the movement's initiation.

CONCLUSION

This study demonstrates that the kinematics of the ST joint during overhead motion after a well-functioning rTSA are significantly altered compared with a healthy shoulder. Overhead motion was associated with a significant increase in scapular external rotation and retraction in rTSA patients, with no difference in scapular tilt. The ST contribution to overall shoulder movement is significantly increased in patients with an rTSA compared with a healthy shoulder. Better understanding of ST motion after rTSA could help improve rehabilitation protocols and preoperative planning for rTSA.

Impact of reverse shoulder arthroplasty design and patient shoulder size on moment arms and muscle fiber lengths in shoulder abductors

BACKGROUND

Reverse shoulder arthroplasty (RSA) increases the moment arm of the deltoid; however, there is limited knowledge on the accompanying changes in muscle architecture that play a role in muscle force production. The purpose of this study was to use a geometric shoulder model to evaluate the anterior deltoid, middle deltoid, and supraspinatus regarding (1) the differences in moment arms and muscle-tendon lengths in small, medium, and large native shoulders and (2) the impact of 3 RSA designs on moment arms, muscle fiber lengths, and force-length (F-L) curves.

METHODS

A geometric model of the native glenohumeral joint was developed, validated, and adjusted to represent small, medium, and large shoulders. Moment arms, muscle-tendon lengths, and normalized muscle fiber lengths were assessed for the supraspinatus, anterior deltoid, and middle deltoid from 0° to 90° of abduction. RSA designs were modeled and virtually implanted, including a lateralized glenosphere with an inlay 135° humeral component (lateral glenoid-medial humerus [LGMH]), a medialized glenosphere with an onlay 145° humeral component (medial glenoid-lateral humerus [MGLH]), and a medialized glenosphere with an inlay 155° humeral component (medial glenoid-medial humerus [MGMH]). Descriptive statistics were used to compare moment arms and normalized muscle fiber lengths.

RESULTS

As shoulder size increased, the moment arms and muscle-tendon lengths for the anterior deltoid, middle deltoid, and supraspinatus increased. All RSA designs achieved greater moment arms for the anterior and middle deltoid, with the MGLH design achieving the largest increase. The resting normalized muscle fiber length of the anterior and middle deltoid was substantially increased in the MGLH (1.29) and MGMH (1.24) designs, shifting the operating ranges of these muscles to the descending portions of their F-L curves, whereas the LGMH design maintained a resting deltoid fiber length (1.14) and operating range similar to the native shoulder. All RSA designs demonstrated a decrease in the native supraspinatus moment arm in early abduction, with the largest decrease in the MGLH design (-59%) and minimal decrease in the LGMH design (-14%). The supraspinatus operated on the ascending limb of its F-L curve in the native shoulder and remained on this portion of the F-L curve for all RSA designs.

CONCLUSION

Although the MGLH design maximizes the abduction moment arm for the anterior and middle deltoid, overlengthening of the muscle may compromise deltoid muscle force production by forcing the muscle to operate on the descending portion of its F-L curve. In contrast, the LGMH design increases the abduction moment arm for the anterior and middle deltoid more modestly while allowing the muscle to operate near the plateau of its F-L curve and maximizing its force-producing potential.

Three-Dimensional Finite Element Analysis of a Notched Insert Design for Reverse Total Shoulder Arthroplasty to Prevent Scapular Notching

Purpose

Reverse total shoulder arthroplasty (RSA) is an effective treatment option for rotator cuff arthropathy. Scapular notching following RSA remains a major complication and has a high incidence. This finite element analysis (FEA) study provides a future reference for the optimal design of the insert component of RSA. This study aims to clarify the effect of a new design RSA with a notched insert on the range of adduction, scapular notching, and stress variation of its insert component using three-dimensional (3D) FEA.

Methods

3D nominal Grammont-type monobloc RSA implant components are modeled on the sawbones glenohumeral joint. The polyethylene insert is redesigned with notching of the inferior part. The comparison of standard and notched designs was performed by FEA for stress pressure of scapular notching and the degree of adduction. 3D mesh models are created for stress analysis to compare the results between standard and notched inserts for the adduction.

Results

The redesigned notched inserts had an additional ~ 11.2° on adduction and prevented scapular notching. The stress analysis results for the notched insert design were lower than the standard ones (4.7 vs 22.4 Kpa).

Conclusions

Notched insert design of Grammont-type RSA could provide additional adduction with lower stress on the glenoid, leading to less scapular notching. Further experimental and clinical studies on different RSA types are needed to verify this effect.

Study Design

Basic Science Study; Biomechanics and Computer Modeling.

Rehabilitation progress following reverse total shoulder replacement and internal fixation for geriatric three and four-part proximal humerus fractures - a propensity score matched comparison

BACKGROUND

Proximal humerus fracture is one of the most common fractures in the elderly population. However, in patients with complex fracture patterns, there is still no general consensus in the best treatment method. This study aims to evaluate the outcomes between those treated with reverse total shoulder arthroplasty (rTSA) and open reduction internal fixation (ORIF).

METHODS

All geriatric patients (> 60 years of age) with proximal humerus fractures undergoing surgical treatment were analysed. There were 25 patients treated with rTSA and 75 with ORIF. Propensity score matching was used to select 25 matching patients from the ORIF group according to age and gender. All patients underwent surgical intervention within 7 days (mean 3.8 days). All patients followed a protocol-driven rehabilitation programme with outcome assessment at 3, 6, 12 and 24 months. Constant score, qDASH, range of motion, rate of complications and revision surgery were recorded and compared.

RESULTS

Twenty-five rTSA were age and gender matched with 25 ORIF patients. The average age of patients in rTSA and ORIF groups were 77.0 years and 75.2 years respectively. At 3 months, mean Constant score was 37.7 (rTSA) vs 45.5 (ORIF) (p = 0.099). Mean qDASH score was 50.6 (rTSA) vs 29.4 (ORIF) (p = 0.003). Mean forward flexion range was 72.9° (rTSA) vs 94.4° (ORIF) (p = 0.007). Mean abduction range was 64.0° (rTSA) vs 88.6° (ORIF) (p = 0.001). At 2 years, mean Constant score was 72.8 (rTSA) vs 70.8 (ORIF) (p = 0.472). Mean qDASH score was 4.50 (rTSA) vs 11.0 (ORIF) (p = 0.025). Mean forward flexion range was 143° (rTSA) vs 109° (ORIF) (p < 0.001). Mean abduction range was 135° (rTSA) vs 110° (ORIF) (p = 0.025). There was a higher number of complications observed for ORIF (3) than rTSA (1) (p = 0.297) and a higher number of re-operations for ORIF (3) than rTSA (1) (p = 0.297), which was not statistically significant.

CONCLUSION

rTSA appears to yield a slower recovery at 3 months but a better outcome at 2 years. It is a promising treatment for geriatrics with three- and four-part proximal humerus fractures aiming for a better long-term functional outcome.

A numerical study of the contact geometry and pressure distribution along the glenoid track

The glenoid track geometry and the contact forces acting on the glenohumeral joint at static positions of 30°, 60°, 90° and 120° of abduction with 90° of external rotation were evaluated using a finite element model of the shoulder that, differently from most usual approximations, accounts the humeral head translations and the deformable-to-deformable non-spherical joint contact. The model was based on data acquired from clinical exams of a single subject, including the proximal humerus, scapula, their respective cartilages concerning the glenohumeral joint, and the rotator cuff and deltoid muscles. The forces acting on the glenohumeral joint were estimated using a simulation framework consisting of an optimization procedure allied with finite element analysis that seeks the minimum muscle forces that stabilize the joint. The joint reaction force magnitude increases up to 680.25 N at 90° of abduction and decreases at further positions. From 60° onward the articular contact remains at the anterior region of the glenoid cartilage and follows an inferior to superior path at the posterior region of the humeral head cartilage. The maximum contact pressure of 3.104 MPa occurs at 90° abduction. Although translating inferiorly throughout the movement, the projection of the humeral head center at the glenoid plane remains at the central region of the glenoid surface. The model results qualitatively matched the trends observed in the literature and supports the consideration of the translational degrees of freedom to evaluate the joint contact mechanics.

The 50 Highest Cited Papers on Shoulder Arthroplasty

The purpose of this study was to determine the 50 most cited articles on shoulder arthroplasty (SA) and their characteristics. The Thomson ISIWeb of Science was searched with the following search terms: "shoulder arthroplasty", "shoulder replacement", "shoulder prosthesis" and "shoulder implant". All papers dealing with SA, including its perioperative and postoperative management, were included in this study. Citations ranged from 797 to 52 for the 50 highest cited papers on SA. According to absolute numbers, the top 10 papers were cited at least 118 times. Overall, 78% (n = 43) were clinical and the remaining articles were basic science research (one anatomic, six biomechanical). The most prevalent level of evidence was IV (72%). The Journal of Shoulder and Elbow Surgery published 40% of the studies. The majority of studies were conducted in the United States and eight other countries. The publication years of the most-cited articles ranged from 1991 to 2020, with the 2000s accounting for the most articles (96%) and the period from 2006 to 2010 with the absolute largest number of articles (17). This article provides a building block in the SA surgery.

Reverse Shoulder Arthroplasty Biomechanics

The reverse total shoulder arthroplasty (rTSA) prosthesis has been demonstrated to be a viable treatment option for a variety of end-stage degenerative conditions of the shoulder. The clinical success of this prosthesis is at least partially due to its unique biomechanical advantages. As taught by Paul Grammont, the medialized center of rotation fixed-fulcrum prosthesis increases the deltoid abductor moment arm lengths and improves deltoid efficiency relative to the native shoulder. All modern reverse shoulder prostheses utilize this medialized center of rotation (CoR) design concept; however, some differences in outcomes and complications have been observed between rTSA prostheses. Such differences in outcomes can at least partially be explained by the impact of glenoid and humeral prosthesis design parameters, surgical technique, implant positioning, patient-specific bone morphology, and usage in humeral and glenoid bone loss situations on reverse shoulder biomechanics. Ultimately, a better understanding of the reverse shoulder biomechanical principles will guide future innovations and further improve clinical outcomes.

Effect of Rotator Cuff Deficiencies on Muscle Forces and Glenohumeral Contact Force After Anatomic Total Shoulder Arthroplasty Using Musculoskeletal Multibody Dynamics Simulation

Anatomic total shoulder arthroplasty (ATSA) is widely used to treat the diseases of the glenohumeral (GH) joint. However, the incidence of rotator cuff tears after ATSA increases during follow-up. The effects of rotator cuff deficiencies after ATSA on the biomechanics of the GH joint are to be investigated. In this study, a musculoskeletal multibody dynamics model of ATSA was established using a force-dependent kinematics (FDK) method. The biomechanical effects were predicted during arm abduction under different rotator cuff deficiencies. The deltoid forces were increased under the rotator cuff deficiencies, the maximum deltoid forces were increased by 36% under the subscapularis deficiency and by 53% under the supraspinatus, infraspinatus, subscapularis, and teres minor deficiencies. The maximum GH contact forces were decreased by 11.3% under supraspinatus and infraspinatus deficiencies but increased by 24.8% under subscapularis deficiency. The maximum subscapularis force was decreased by 17% under only infraspinatus tear during arm abduction. The results suggested that the changes in the biomechanics of the GH joint induced by rotator cuff deficiencies after ATSA increase the deltoid muscle energy expenditure and joint instability, which result in postoperative less satisfactory clinical outcomes. The changes in rotator cuff muscle forces deserve more attention for understanding the evolution of rotator cuff tear after ATSA.

Is limited shoulder abduction associated with poor scapulothoracic mobility after reverse shoulder arthroplasty?

INTRODUCTION

The contribution of the glenohumeral joint to shoulder abduction is acknowledged as an important factor for reverse total shoulder arthroplasty (RTSA) patients. In contrast, the degree of scapulothoracic joint contribution and its relation to RTSA patients with poor to excellent shoulder abduction are unclear.

MATERIALS AND METHODS

Twenty-three selectively recruited patients (74 ± 7 years, 11 males) with shoulder abduction ranging from poor to excellent at least 6 months after primary, unilateral RTSA participated in this study. Individual scapulothoracic and glenohumeral contributions at maximum shoulder abduction in the scapular plane were measured using 3D motion capture and correlations between scapulothoracic and glenohumeral contributions to shoulder abduction were assessed. Multiple regression analysis was used to determine the influence of age, body mass index, follow-up period, abduction strength and passive glenohumeral mobility on scapulothoracic and glenohumeral function.

RESULTS

Maximum shoulder abduction (range 48°-140°) was not significantly correlated with the scapulothoracic contribution (range 39°-75°, r = 0.40, p = 0.06), but there was a strong and significant correlation with the glenohumeral contribution (range - 9°-83°, r = 0.91, p < 0.001). Abduction strength was strongly associated with glenohumeral (p = 0.006) but not scapulothoracic (p = 0.34) joint contributions.

CONCLUSIONS

Limited shoulder abduction is not associated with insufficient scapulothoracic mobility, which rather provides a basic level of function for RTSA patients. Good to excellent shoulder abduction could only be achieved by increasing the glenohumeral contribution that was associated with postoperative abduction strength.

Stress distribution in the humerus during elevation of the arm and external abduction

Objective

The purpose of this study is to estimate stress distribution occurring in the humerus during elevation and external rotation of the arm.Methods: contact forces and moments were estimated using telemeterized shoulder implants. An accurate three-dimensional (3D) finite element (FE) model of the natural scapula was developed, and loaded by data obtained by instrumented prosthesis.

Results

Stresses of about 40 MPa were found on the homerus during the elevation phase acting at 30° and 80°, while a peak of 60 MPa was found during the external rotation phase at 20°. The stress aging on scapula was of about 45 MPa, while the acromion was subjected at about 30 MPa. Stresses aging on ligaments were of about 15 MPa.

Conclusion

These results indicated that the transfer of major muscle and joint reaction take place predominantly through the thick bony ridges, and stresses induced can be dangerous especially for patients with shoulder problems or during the first post-operative weeks after shoulder fractures or joint replacements.

Isokinetic Evaluation of Shoulder Strength and Endurance after Reverse Shoulder Arthroplasty: A Comparative Study

OBJECTIVE

The present study aimed to compare the isometric strength and endurance of shoulder abduction and internal and external rotation between operated shoulders and nonoperated, contralateral shoulders of patients who underwent reverse shoulder replacement due to unilateral rotator cuff tear arthropathy.

PATIENTS AND METHODS

With a diagnosis of cuff tear arthropathy, 41 consecutive patients (mean age of 70.8 years; age range, 57 to 84; 36 females, 5 males) who underwent unilateral reverse shoulder arthroplasty were reviewed based on functional and radiological data. In all cases, cuff tear arthropathy was unilateral and contralateral shoulder was asymptomatic, with normal shoulder function. The average length of follow-up was 34 months (range of 12-67). To assess patients' functional level, the Constant score and the Disabilities of the Arm, Shoulder, and Hand (the Quick-DASH) outcome measure were used preoperatively and at the final examination. The primary outcomes of the present study were measurement of isokinetic strength and endurance of shoulder abduction and internal and external rotation using an isokinetic evaluator.

RESULTS

Patients exhibited marked improvement in functional level as reflected by a significant increase in the mean Constant score from 38 preoperatively to 65 at the final follow-up (p = 0.03). The functional improvement was supported by a decrease in the mean Quick-DASH from 64 preoperatively to 26 at the final follow-up (p = 0.018). In the comparison of the isokinetic strength and endurance of shoulder abduction, no statistical difference was observed between operated shoulders and contralateral shoulders (p > 0.05). However; the strength and endurance of internal and external rotation were lower in operated shoulders than in contralateral shoulders (p < 0.05). Similarly, there was no statistically significant difference in comparisons of the durability of abduction (p > 0.05); however, the durability of internal and external rotation were significantly lower in operated shoulders (p < 0.05).

CONCLUSION

In terms of durability and strength of abduction, similar results with the unaffected shoulder may be accomplished; nonetheless, the surgeon should be aware that durability and strength of rotation would be weak.

LEVEL OF EVIDENCE

Level III, Therapeutic Study.

Radiologic Comparison of Humeral Position according to the Implant Designs Following Reverse Shoulder Arthroplasty: Analysis between Medial Glenoid/Medial Humerus, Lateral Glenoid/Medial Humerus, and Medial Glenoid/Lateral Humerus Designs

Background

The currently available reverse shoulder arthroplasty (RSA) designs can be classified into medial glenoid/medial humerus (MGMH), lateral glenoid/medial humerus (LGMH), and medial glenoid/lateral humerus (MGLH) prosthesis designs. The purpose of this study was to radiologically analyze the effect of different RSA designs on humeral position following RSA.

Methods

A total of 50 patients who underwent primary RSA were retrospectively analyzed. Among 50 patients, 33 patients (group A: MGMH) underwent RSA with Aequalis system (Wright, Inc, Bloomington, MN, USA), 6 (group B: LGMH) with Aequalis system using bony increased offset, and 11 (group C: MGLH) with Aequalis Ascend Flex system. The acromiohumeral distance, acromioepiphyseal distance (AED), lateral humeral offset (LHO), LHO from the center of rotation (LHO^COR), and deltoid length were radiologically measured to quantify the distalization and lateralization of the humerus.

Results

The increment in postoperative AED was 19.92 ± 3.93 mm in group A, 24.52 ± 5.25 mm in group B, and 25.97 ± 5.29 mm in group C, respectively (p=0.001). The increment in postoperative LHO was 0.13 ± 6.30 mm, 8.00 ± 12.14 mm, and 7.42 ± 6.88 mm, respectively (p=0.005). The increment in postoperative LHO^COR was 20.76 ± 6.06 mm, 22.04 ± 5.15 mm, and 28.11 ± 4.14 mm, respectively (p=0.002).

Conclusions

The radiologic analysis of the effect of different RSA designs on humeral position following RSA showed significant differences in the increment in postoperative AED, LHO, and LHO^COR between the 3 groups. Therefore, MGLH design seems to be more effective for humeral distalization and lateralization compared to original Grammont design.

Biomechanical evaluation of cable and suture cerclages for tuberosity reattachment in a 4-part proximal humeral fracture model treated with reverse shoulder arthroplasty

BACKGROUND

Sufficient tuberosity fixation in proximal humeral fractures treated with shoulder arthroplasty is essential to gain a good clinical outcome. This biomechanical study evaluated the strength of the reattached tuberosities in reverse total shoulder arthroplasty fixed with cables or with sutures in a cerclage-like technique. Considering the mechanical advantages of flexible titanium alloy cables compared with conventional sutures for cerclage-like fixations, we hypothesized that titanium alloy cables would achieve higher fixation strengths of the tuberosities compared with heavy nonabsorbable sutures.

METHODS

A 4-part fracture was created on 8-paired proximal human humeri. The tuberosities were reduced anatomically and fixed by 2 heavy nonabsorbable sutures (suture group) or by two 1-mm titanium alloy cables (cable group) in a cerclage-like technique around the neck of the prosthesis. The humeri were placed in a custom-made test setup enabling internal and external rotation. Cyclic loading with a stepwise increasing load magnitude was applied with a material testing machine, starting with 1 Nm and increasing the load by 0.25 Nm after each 100th cycle until failure of the fixation occurred (>15° rotation of the tuberosities). Any motion of the tuberosities was measured with a 3-dimensional ultrasound motion analysis system.

RESULTS

Overall, the cable group reached 1414 ± 372 cycles, and the suture group reached 1257 ± 230 cycles until the fixations failed (P = .313). The suture group showed a significantly higher rotation of the lesser tuberosity relative to the humerus shaft axis after 200, 400, and 600 cycles compared with the cable group (P = .018-.043).

CONCLUSIONS

Tuberosities reattached with cable cerclages showed higher fixation strength and therefore less rotation compared with suture cerclages in a 4-part proximal humeral fracture model treated with reverse total shoulder arthroplasty. Whether this higher fixation strength results in higher bony ingrowth rates of the tuberosities and thus leads to a better clinical outcome needs to be investigated in further clinical studies.

Deltoid muscle activity in patients with reverse shoulder prosthesis at 2-year follow-up

PURPOSE

The reverse shoulder prosthesis (RSP) was developed to relieve pain and improve functional outcomes in patients with glenohumeral arthritis and deficiency of the rotator cuff. Even if clinical and functional outcomes regarding the use of the RSP were reported by literature, data concerning progressive deltoid adaptation to this non-anatomic implant are still missing. The purpose of our study was to correlate clinical and functional outcomes with deltoid fibers activity and muscle fatigability in patients with reverse shoulder prosthesis at 2 years follow-up.

METHODS

Twenty patients with reverse shoulder prosthesis due to symptomatic deficient or nonfunctional rotator cuff associated with osteoarthritis were referred by Cervesi Hospital Shoulder and Elbow Surgery Unit. Exclusion criteria were: axillary nerve palsy, a nonfunctioning deltoid muscle, diabetes, previous trauma, malignancy. Furthermore patients who received the RSP for revision arthroplasty, proximal humerus fractures were excluded. All the patients underwent clinical and functional evaluation with the support of electromyography measurement focused on deltoid activity.

RESULTS

RSP surgical treatment in shoulder osteoarthritis confirms his good outcome in terms of pain relief. At 2 years anterior and lateral deltoid electromyographic activity was significantly lower compared with contralateral side (p < 0.001). Posterior deltoid activity was no detectable. Range of motion at 2 years of follow-up decreased in terms of forward flexion (p = 0.045), abduction (p = 0.03) and external rotation (p < 0.001).

CONCLUSIONS

Our study demonstrates that even if the patients remain pain-free, progressive deterioration of the deltoid activity is unavoidable and may lead to poor functional outcomes overtime.

Use of cuff tear arthroplasty head prosthesis for rotator cuff arthropathy treatment in elderly patients with comorbidities

Objective

To evaluate the clinical and functional behavior of patients undergoing cuff tear arthroplasty at different stages of the disease.

Methods

Cuff tear arthroplasty hemiarthroplasties were performed in 34 patients with rotator cuff arthropathy and associated comorbidities, classified according to Seebauer. The mean age was 76.3 years, and the sample comprised 23 females (67.6%) and 11 males (32.4%). The mean follow-up period was 21.7 months, and evaluations were performed using the Visual Analog Scale for pain and the Constant scale.

Results

There were no statistically significant differences in the mean reduction in the Visual Analog Scale or in the Constant scale increase between the female and male groups. The variation between the pre- and postoperative Visual Analog Scale and Constant scale evaluations was significant. There was also no statistically significant difference between the Seebauer classification groups regarding the mean Visual Analog Scale reduction, or the mean Constant scale increase.

Conclusion

Cuff tear arthroplasty shoulder hemiarthroplasty is a good option for rotator cuff arthropathy in patients with comorbidities.

Is radiographic measurement of acromiohumeral distance on anteroposterior view after reverse shoulder arthroplasty reliable?

BACKGROUND

Reverse shoulder arthroplasty leads to arm lengthening. Different techniques have been described to determine postoperative lengthening. The purpose of this study was to evaluate the reliability of the acromiohumeral distance (AHD) in determining arm lengthening after reverse shoulder arthroplasty.

METHODS

At 2 centers, 44 patients who had received an onlay design reverse shoulder arthroplasty were observed for a minimum of 6 months. Examination followed a standardized protocol including preoperative and postoperative radiographs on anteroposterior view in neutral rotation. Two orthopedic surgeons independently performed the measurements in random order.

RESULTS

Mean arm lengthening averaged 2.5 cm (range, 0.3-3.9 cm) according to AHD measurement. Significant differences in interobserver and intraobserver variability for postoperative AHD measurements were found (P < .01). The mean intrapatient difference was 0.5 cm (range, 0.02-1.5 cm).

CONCLUSION

According to our study, the AHD is not a reliable measurement technique to determine arm lengthening after reverse shoulder arthroplasty.

The effect of deltoid lengthening on functional outcome for reverse shoulder arthroplasty

PURPOSE

Reverse shoulder arthroplasty (RSA) is based on the biomechanical advantages of lengthening the deltoid while medializing its center of rotation (COR). Little evidence exists describing the clinical benefits of these biomechanical advantages. The purpose of this study is to assess the relationship between deltoid lengthening and medialization of the COR with functional outcomes for RSA patients.

METHODS

We retrospectively reviewed patients treated with primary RSA. Radiographic measurements of deltoid length and COR, assessment of forward elevation (FE) and external rotation (ER), and functional outcome scores were obtained pre- and postoperatively. Linear regression analyses were performed to assess the relationship between these radiographic measurements and changes in shoulder functionality.

RESULTS

On average, patients improved significantly in function and functional outcome scores. Postoperative COR correlated weakly with postoperative FE. There were weakly negative correlations between increase in acromion to greater tuberosity distance and postoperative FE, and between deltoid lengthening and FE improvement.

CONCLUSIONS

Our results suggest that deltoid lengthening does not correlate with improvements in active FE or ER. These findings could indicate that change in deltoid length is less important than previously thought. Furthermore, the negative correlations seen could indicate that there is over-tensioning of the deltoid in specific cases. Further studies are needed to better assess the role of deltoid length and other factors that may impact RSA outcomes.

How sensitive is the deltoid moment arm to humeral offset changes with reverse total shoulder arthroplasty?

BACKGROUND

Reverse total shoulder arthroplasty commonly treats cuff-deficient or osteoarthritic shoulders not amenable to rotator cuff repair. This study investigates deltoid moment arm sensitivity to variations in the joint center and humeral offset of 3 representative reverse total shoulder arthroplasty subjects. We hypothesized that a superior joint implant placement may exist, indicated by muscle moment arms, compared with the current actual surgical implant configuration.

METHODS

Moment arms for the anterior, lateral, and posterior aspects of the deltoid muscle were determined for 1521 perturbations of the humeral offset location away from the surgical placement in a subject-specific musculoskeletal model with motion defined by subject-specific in vivo abduction kinematics. The humeral offset was varied from its surgical position ±4 mm in the anterior/posterior direction, ±12 mm in the medial/lateral direction, and -10 to 14 mm in the superior/inferior direction.

RESULTS

The anterior deltoid moment arm varied in humeral offset and center of rotation up to 20 mm, primarily in the medial/lateral and superior/inferior directions. The lateral deltoid moment arm varied in humeral offset up to 20 mm, primarily in the medial/lateral and anterior/posterior directions. The posterior deltoid moment arm varied up to 15 mm, primarily in early abduction, and was most sensitive to humeral offset changes in the superior/inferior direction.

DISCUSSION

High variations in muscle moment arms were found for all 3 deltoid components, presenting an opportunity to dramatically change the deltoid moment arms through surgical placement of the reverse shoulder components and by varying the overall offset of the humerus.

LEVEL OF EVIDENCE

Basic Science Study; Computer Modeling.

How do deltoid muscle moment arms change after reverse total shoulder arthroplasty?

BACKGROUND

Although many advantages of reverse total shoulder arthroplasty (RTSA) have been demonstrated, a variety of complications indicate there is much to learn about how RTSA modifies normal shoulder function. This study used a subject-specific computational model driven by in vivo kinematic data to assess how RTSA affects deltoid muscle moment arms after surgery.

METHODS

A subject-specific 12 degree-of-freedom musculoskeletal model was used to analyze the shoulders of 26 individuals (14 RTSA and 12 normal). The model was modified from the work of Holzbaur to directly input 6 degree-of-freedom humeral and scapular kinematics obtained using fluoroscopy.

RESULTS

The moment arms of the anterior, lateral, and posterior aspects of the deltoid were significantly different when RTSA and normal cohorts were compared at different abduction angles. Anterior and lateral deltoid moment arms were significantly larger in the RTSA group at the initial elevation of the arm. The posterior deltoid was significantly larger at maximum elevation. There was large intersubject variability within the RTSA group.

CONCLUSIONS

Placement of implant components during RTSA can directly affect the geometric relationship between the humerus and scapula and the muscle moment arms in the RTSA shoulder. RTSA shoulders maintain the same anterior and posterior deltoid muscle moment-arm patterns as healthy shoulders but show much greater intersubject variation and larger moment-arm magnitudes. These observations provide a basis for determining optimal implant configuration and surgical placement to maximize RTSA function in a patient-specific manner.

Scapular Spine Stress Fracture as a Complication of Reverse Shoulder Arthroplasty

We report a case of a stress fracture of the scapular spine which developed as a late complication of a reverse shoulder arthroplasty. After initially doing well after surgery, our patient developed pain and decreased shoulder function. A nondisplaced scapular spine fracture was noted on radiographs. Because reverse shoulder arthroplasty is a relatively new procedure in this country, radiologists may be unfamiliar with its potential complications. Stress or insufficiency fractures of the scapular spine and acromion are a potential complication of reverse shoulder arthroplasty, due to increased functional demands of the deltoid muscle, which is often combined with deficiency of acromial bone due to rotator cuff arthropathy and osteopenia.

Biomechanical evaluation of inferior scapula notching of reverse shoulder arthroplasty depending on implant configuration and scapula neck anatomy

Purpose:

The presence of inferior scapula notching is significantly affected by the anatomy the scapula and can be influenced by the glenosphere design and position and the onlay type.

Materials and Methods:

A biomechanical study was undertaken with 13 human shoulder specimens in a robot-assisted shoulder simulator. Inferior scapula contact during adduction of the humerus was detected using a contact pressure film. Computed tomography scans with three-dimensional reconstructions of each specimen were performed.

Results:

The greatest improvement of the scapula notching angle (SNA) was achieved by simultaneous implantation of a shallow humeral onlay and an eccentric glenosphere design: 16.3-19.0° (P < 0.005). The SNA was significantly decreased by 5.8° when shifting from a 38 mm centric glenosphere to a 42 mm centric glenosphere (P < 0.005) and by 8.9° comparing the 38 mm centric glenosphere with 38 mm eccentric glenosphere (P < 0.005). The solitary implantation of a shallow onlay significantly decreased the SNA depending on the glenosphere size between 7.4° and 8.0° (P = 0.001). A more inferior position of the metaglene as well as a long scapula neck (P = 0.029) and a large lateral scapula pillar angle (P = 0.033) were correlated with a lower SNA.

Conclusion:

This study demonstrates the importance of inferior glenosphere placement and the benefit of eccentric glenosphere and shallow humeral cup design to reduce the adduction deficit of the reverse shoulder. The presence of a short neck of the scapula can have a negative prognostic effect on inferior impingement during adduction of the arm.

Level of Evidence:

Basic Science Study

Does lateralisation of the centre of rotation in reverse shoulder arthroplasty avoid scapular notching? Clinical and radiological review of one hundred and forty cases with forty five months of follow-up

PURPOSE

Our purpose was to evaluate the influence of metallic lateralisation of the centre of rotation (COR) in reverse shoulder arthroplasty (RSA) on the incidence of scapular notching and its eventual clinical and radiological consequences.

METHODS

We analysed 140 RSAs with a lateralised design (Arrow, FH Orthopedics, Mulhouse, France) implanted for massive rotator cuff tear with/without arthritis. Mean follow-up was 45 months (range 24-120, standard deviation 20). Patients were evaluated clinically using the Constant and Murley (Clin Orthop Relat Res 214:160-164, 1987) score and active range of motion (ROM) and radiologically using standard anteroposterior and axillary view. Scapular notching was assessed according to Sirveaux classification Simovitch et al. (J Bone Joint Surg Am, 89:588-600, 2007), and patients were separated into two groups (scapular notch/no scapular notch) and compared.

RESULTS

Forty-one notches (29 %) were found: 20 grade 1, 18 grade 2, and three grade 3. The latter three patients had a follow-up of 44, 70 and 84 months, respectively, and the scapular notch did not evolve in the final two years. Better pre-operative function was significantly associated with scapular notching (p < 0,05 for flexion and abduction), but no final clinical differences in ROM and Constant score were found between groups. A body mass index (BMI) <30 increased the risk of scapular notching, which was observed in 43 % of patients with a BMI < 30 and 30 % of patients with a BMI > 30 p = 0,048). Pre-operative narrowing of the subacromial space increased the risk of scapular notching, but age or gender showed no influence. However, there was a significantly greater number of scapular notches in patients operated on the dominant side (p = 0,04). No significant difference in lateral offset was found between groups (p = 0,99). Glenoid implantation in an excessively high position (p = 0,033) and absence of inferior tilt (p = 0,0029) were significantly associated with scapular notching.

CONCLUSIONS

In this series, metallic lateralisation of the COR in RSA did not impair clinical results, with patients achieving good flexion recovery ROM increase in rotations. Metallic lateralisation of the COR in RSA leads to a lower incidence of scapular notching (29 %) compared with previously reported results using other arthroplastic systems with a more medialised COR. However, although scapular notching was not totally eliminated, those that were found did not evolve over time. Several factors increased the incidence of scapular notching: BMI <30, better preoperative ROM, an excessively high glenoid implant and absence of inferior tilt.

Scapulohumeral rhythm in shoulders with reverse shoulder arthroplasty

BACKGROUND

Little is known about kinematic function of reverse total shoulder arthroplasty (RTSA). Scapulohumeral rhythm (SHR) is a common metric for assessing muscle function and shoulder joint motion. The purpose of this study was to compare SHR in shoulders with RTSA to normal shoulders.

METHODS

Twenty-eight subjects, more than 12 months after unilateral RTSA, were recruited for an Institutional Review Board-approved study. Subjects performed arm abduction in the coronal plane with and without a 1.4-kg hand-held weight. Three-dimensional model-image registration techniques were used to measure orientation and position for the humerus and scapula from fluoroscopic images. Analysis of variance and Tukey tests were used to assess groupwise and pairwise differences.

RESULTS

SHR in RTSA shoulders (1.3:1) was significantly lower than in normal shoulders (3:1). Below 30° abduction, RTSA and normal shoulders show a wide range of SHR (1.3:1 to 17:1). Above 30° abduction, SHR in RTSA shoulders was 1.3:1 for unweighted abduction and 1.3:1 for weighted abduction. Maximum RTSA shoulder abduction in weighted trials was lower than in unweighted trials. SHR variability in RTSA shoulders decreased with increasing arm elevation.

CONCLUSION

RTSA shoulders show kinematics that are significantly different from normal shoulders. SHR in RTSA shoulders was significantly lower than in normal shoulders, indicating that RTSA shoulders use more scapulothoracic motion and less glenohumeral motion to elevate the arm. With these observations, it may be possible to improve rehabilitation protocols, with particular attention to the periscapular muscles, and implant design or placement to optimize functional outcomes in shoulders with RTSA.

Effect of reverse shoulder design philosophy on muscle moment arms

This study analyzes the muscle moment arms of three different reverse shoulder design philosophies using a previously published method. Digital bone models of the shoulder were imported into a 3D modeling software and markers placed for the origin and insertion of relevant muscles. The anatomic model was used as a baseline for moment arm calculations. Subsequently, three different reverse shoulder designs were virtually implanted and moment arms were analyzed in abduction and external rotation. The results indicate that the lateral offset between the joint center and the axis of the humerus specific to one reverse shoulder design increased the external rotation moment arms of the posterior deltoid relative to the other reverse shoulder designs. The other muscles analyzed demonstrated differences in the moment arms, but none of the differences reached statistical significance. This study demonstrated how the combination of variables making up different reverse shoulder designs can affect the moment arms of the muscles in different and statistically significant ways. The role of humeral offset in reverse shoulder design has not been previously reported and could have an impact on external rotation and stability achieved post-operatively.

Conversion of hemi into reverse shoulder arthroplasty: implant design limitations

PURPOSE

Cranial migration of shoulder hemiarthroplasties due to rotator cuff insufficiency typically requires conversion into a reverse total shoulder arthroplasty. This study was conducted to analyze differences between the height and offset of six implants designed to enable conversion of a hemiarthroplasty into a reverse system.

METHODS

Anteroposterior radiographs of 40 shoulders were taken. An image analyzing software was used to simulate the implantation of the hemiprostheses. Then the implant was dissembled, leaving on the stem within the humeral shaft. Finally, the implantation of a reverse system was simulated using the stem in the same position. Values are reported as ∆-height and ∆-offset ± standard deviation. Significance was assumed for P < 0.05.

RESULTS

The least decrease in height was determined for Implantcast with 11.6 ± 3.3 mm, followed by DePuy (16 ± 5.7 mm) and the greatest for Tornier with 33 ± 5.3 mm. No significant differences were found among Exactech, Mathys and Zimmer. The largest offset-deviation was calculated for DePuy (-21.7 ± 3.7 mm) and the smallest for Implantcast (-3.3 ± 2.8 mm) and Tornier (1.5 ± 5.7 mm).

CONCLUSIONS

Due to the modular stem, the system of Implantcast can be converted in a reverse system with the least changes in height and offset. For the other manufacturers it does not seem possible to convert a hemiprosthesis to a reversed prosthesis without accepting additional tension of the deltoid muscle. Further experimental studies have to analyze the changes in deltoid abduction moments after conversion of a hemi- into a reversed prosthesis.

Results of 30 reverse shoulder prostheses for revision of failed hemi- or total shoulder arthroplasty

PURPOSE

Revision surgery for shoulder prosthesis remains a difficult task in shoulder surgery. The purpose of this retrospective study was to evaluate the clinical and radiological outcomes of a series of 30 reverse shoulder prostheses performed as revision of failed hemi- or total shoulder arthroplasty. The most relevant technical points in surgery are described, as are other surgical options; a rational strategy for the treatment of these patients is proposed.

MATERIALS AND METHODS

Thirty patients (average age 69.5) were included. Mean follow-up was 36.4 months (range 24-100 months). There were 14 patients in group 1 (Delta III) and 16 in group 2 (Reverse Arrow).

RESULTS

A total of 83% were satisfied (16 cases) or very satisfied (9 cases), and 17% were disappointed (5 patients). The mean Constant score increased from a mean of 25-52. The mean score for pain improved from 5 (range 0-15) to 13 (range 5-15) (p < 0.001). The mean score of strength improved from 1 (range 0-6) to 5 (range 0-10) (p < 0.001). The forward elevation changed from a mean of 55° (range 0-120) to 108° (range 40-160) (p < 0.001). There was no significant improvement of external rotation at 0° abduction (range 14°-18°) or internal rotation (range 5-4.63). There were 4 scapular notching. We could not find the influence of scapular notching on Constant Score. Complication rate was 26.6%.

CONCLUSION

Reverse total shoulder arthroplasty prosthesis represents an available option in difficult cases of failed hemiarthroplasty or total shoulder arthroplasty when the rotator cuff is irreparable and the glenoid bone stock is sufficient.

LEVEL OF EVIDENCE

Level 4.

Reverse total shoulder arthroplasty component center of rotation affects muscle function

BACKGROUND

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

METHODS

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

RESULTS

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

DISCUSSION/CONCLUSION

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

[Cuff tear arthropathy – long-term results of reverse total shoulder arthroplasty]

The results after reverse total shoulder arthroplasty for cuff tear arthropathy are superior and the complications fewer than for other etiologies, such as rheumatoid arthritis, fracture, fracture sequelae or even revision. The improvements in function and pain are excellent whereas rotation may be unaffected. Revisions and complications can be encountered in the first 3 years and the survival curve of the prosthesis is still good after 10 years with 90%. Progressive functional and radiological deterioration is observed after 9 years in approximately 30% of the patients without apparent problems with the prosthesis. Notching is progressive with incidence and size over time, without a proven correlation to the functional results but remains a major concern. Modification of the implants, the operative techniques and experience could significantly improve the results and reduce the rate of complications. Lateralization of the center of rotation and smaller inclination angles have a positive effect on the rate of notching and the range of motion, especially for the rotation. New prosthetic designs and operative techniques attempt to implement a combination of the biomechanical improvements. Reverse shoulder arthroplasty remains a challenging operation with a high rate of complications. The results depend on the etiology and the function of the remaining muscles and therefore on the experience and the skill of the surgeon to implement the appropriate biomechanical factors. Because of the concerns regarding the longevity, reverse shoulder arthroplasty should be reserved for the elderly over 70 years of age.

Electromyographic analysis of reverse total shoulder arthroplasties

BACKGROUND

Understanding how reverse total shoulder arthroplasty (RTSA) affects muscle activation may help refine it. This study evaluated deltoid and upper trapezius activity during shoulder abduction, flexion, and external rotation in RTSA recipients.

METHODS

Fifty individuals were recruited for this study: 33 were ≥6 months postunilateral RSTA, and 17 comprised our control group. Control individuals easily performed all functional tasks and had no history of shoulder pathology or pain. RTSA participants were divided into 3 groups according to implant design. Participants performed weighted and unweighted abduction in the coronal plane, forward flexion in the sagittal plane, and unweighted external rotation. Electromyography activation of the anterior, lateral, and posterior aspects of the deltoid and the upper trapezius muscles was recorded bilaterally. Motion capture using passive reflective markers quantified 3-dimensional motions of both shoulders.

RESULTS

During abduction and flexion, deltoid and upper trapezius activity was significantly increased in RTSA shoulders. Posterior deltoid activation was highest in shoulders with the medial glenosphere/lateral humerus implant. Medial glenosphere/medial humerus shoulders were most similar to the control group's anterior, lateral, and posterior deltoid muscle activation during weighted flexion.

CONCLUSIONS

RTSA increases muscle activation compared with normal shoulders. RTSA often restores stability and motion but not normal deltoid or upper trapezius activation. Increased muscle activation in shoulders with RTSA suggests less efficiency. RTSAs with lateral or medial glenosphere centers of rotation had mostly similar muscle activation. Average posterior deltoid activation did not exceed 20% of maximal voluntary isometric contraction for any group during unweighted external rotation, and differences between groups were <5% maximal voluntary isometric contraction.

Arm lengthening after reverse shoulder arthroplasty: a review

PURPOSE

The purpose of this review is to provide a better understanding of biomechanical changes induced by reverse shoulder arthroplasty (RSA), discuss the different techniques of radiographic assessment of upper limb lengthening after RSA and determine the ideal soft tissue tension that provides the best functional outcome without increasing the risk of complications.

METHODS

Inclusion criteria were articles in which the primary interest was the technique of measuring upper-extremity lengthening after complications related to lengthening and its role in postoperative function; those written in English, French or German; and those that provided evidence levels I-IV relevant to search terms.

RESULTS

Seven articles met our inclusion criteria. Postoperatively, changes in humeral length varied from minus five to five millimetres, and changes in upper-extremity length varied from 15 mm to 27 mm. The acromiohumeral distance averaged 23 mm. Humeral and arm shortening increased the risk of dislocation and led to poor anterior active elevation. The type of surgical approach did not play a role in postoperative function. Subclinical neurological lesions were frequent.

CONCLUSIONS

Studies in this systematic review indicate that deltoid tensioning by restoring humeral length and increasing the acromiohumeral distance is critical for adequate postoperative function and to prevent dislocation. Excessive arm lengthening should be avoided, with zero to two centimetres of lengthening being a reasonable goal to avoid postoperative neurological impairment.

Uncommon indications for reverse total shoulder arthroplasty

Total shoulder arthroplasty and shoulder hemiarthroplasty have been the traditional method for treating a variety of shoulder conditions, including arthritis, cuff tear arthropathy, and some fracture types. However, these procedures did not provide consistently good results for patients with torn rotator cuffs. The development of the reverse prosthesis by Grammont in the late 20th century revolutionized the treatment of the rotator-cuff-deficient shoulder with arthritis. The main indication for the reverse prosthesis remains the patient with cuff tear arthropathy who has pain and loss of motion. Because the reverse total shoulder arthroplasty produced such good results in these patients, the indications for the reverse prosthesis have expanded to include other shoulder conditions that have previously been difficult to treat successfully and predictably. This review discusses and critically reviews these newer indications for the reverse total shoulder arthroplasty.

Influence of prosthetic humeral head size and medial offset on the mechanics of the shoulder with cuff tear arthropathy: a numerical study

This numerical study assesses the influence of an oversized humeral hemiprosthesis with a larger medial offset on the mechanics of the shoulder with cuff tear arthropathy (CTA). Shoulder elevation in the scapular plane is performed, and a Seebauer Type IIa CTA is simulated: a massive rotator cuff tear, a proximal and static migration of the humeral head, and two contacts with friction (glenohumeral and acromiohumeral). The CTA model without a prosthesis (friction coefficient 0.3) is evaluated first as a reference model. Then, three humeral head prosthetic geometries (friction coefficient 0.15) are evaluated: anatomical head, oversized head, and oversized head with a large medial offset. The function of the middle deltoid (i.e. moment arm, applied force, and strength), the contact forces, and the range of motion are studied. The anatomical head, which reduces friction by half, decreases the middle deltoid force (25%) and the contact forces (glenoid 7%; acromion 25%), and increases the range of motion from 41 to 54°. The oversized head increases the moment arm (15%) and the middle deltoid strength (13%), which further decreases the deltoid force (7%) and the contact forces (glenoid 7%; acromion 17%), and increases the range of motion from 54° to 69°. The oversized head with a large medial offset enhances these effects: the moment arm increases by another 3.1%, the deltoid force decreases by another 5% and the acromiohumeral contact force by another 12%, and the range of motion increases from 69° to 84°. These results suggest that increasing the medial offset and oversizing the hemiprosthetic head improve the function of the deltoid, reduce acromial solicitation, and restore elevation to almost 90°.

Reverse total shoulder arthroplasty for cuff tear arthropathy: the clinical effect of deltoid lengthening and center of rotation medialization

BACKGROUND

Reverse total shoulder arthroplasty (RSA) for cuff tear arthropathy improves shoulder function and reduces pain. Implant position and soft tissue balancing are important factors to optimize outcome. Tensioning the deltoid and increasing the deltoid moment arm by medializing the center of rotation are biomechanically advantageous. The purpose of this study was to correlate RSA functional outcomes with deltoid lengthening and center of rotation medialization.

MATERIALS AND METHODS

This prospective cohort study enrolled 49 consecutive patients who underwent RSA for cuff tear arthropathy. Preoperative and serial postoperative physical examinations, radiographs, and American Shoulder and Elbow Surgeons and Simple Shoulder Test scores were evaluated. Deltoid lengthening and medialization of the center of rotation were measured radiographically and correlated with functional outcome scores, range of motion, and complications.

RESULTS

At final follow-up (average, 16 ± 10 months), 37 of 49 patients (76%) were available for analysis. Deltoid lengthening (average, 21 ± 10 mm) correlated significantly (P = .002) with superior active forward elevation (average, 144° ± 19°). Medialization of the center of rotation (average, 18 ± 8 mm) did not correlate with active forward elevation or subjective outcomes. Deltoid lengthening that achieved an acromion-greater tuberosity distance exceeding 38 mm had a 90% positive predictive value of obtaining 135° of active forward elevation. Two patients (4%) required revision surgery, and 68% of patients developed scapular notching (average grade, 1.3 ± 1.2) at final follow-up.

CONCLUSION

Deltoid lengthening improves active forward elevation after RSA for cuff tear arthropathy.

Mechanical analysis of cuff tear arthropathy during multiplanar elevation with the AnyBody shoulder model

BACKGROUND

This numerical study analysed the mechanics of cuff tear arthropathy with the AnyBody shoulder model.

METHODS

The model simulated three frequent characteristics of cuff tear arthropathy: A supero-posterior massive rotator cuff tear, a proximal and static migration of the humeral head, and a contact between the humeral head and the scapula (glenoid & acromion) with friction. The mechanics of the cuff tear arthropathy with and without friction were studied by analysing: the mechanics of the deltoid (i.e. length & strength), the gleno-humeral and acromio-humeral contact forces, the friction moment, and the maximum elevation angle. Elevations in the frontal, scapular and sagittal planes were simulated.

FINDINGS

Compared to an intact condition, the cuff tear arthropathy model without friction estimated a deltoid strength of -18% (frontal=-13%, scapular=-17%, sagittal=-25%), a gleno-humeral contact force of -34% (frontal=-60%, scapular=-46%, sagittal=+5%), estimated an acromio-humeral contact force of 240 N (frontal=213 N, scapular=184 N, sagittal=324 N) and a maximum elevation angle of 77° (frontal=80°, scapular=87°, sagittal=65°). Contact friction enhanced this behaviour, decreasing even more the gleno-humeral contact force and the maximum elevation angle, while increasing the acromio-humeral contact force.

INTERPRETATION

This novel cuff tear arthropathy model suggests that friction and plane of elevation greatly influence the mechanics of the shoulder with cuff tear arthropathy. It also shows that the AnyBody simulation tool may be useful to study musculoskeletal pathologies and not only normal conditions.

Functional and clinical outcome of total shoulder arthroplasty with oversized glenoid

BACKGROUND

The Epoca-Reconstruction-(Reco)®-Glenoid has been developed to treat patients with cuff-tear-arthropathy. The glenoid component of this system has a hemispheric shape that canopies the humeral head. This design is believed to provide a stable fulcrum and restore normal deltoid function. The purpose of this study was to analyse strengths and disadvantages of the Epoca-Reco®-Glenoid in cuff-tear-arthropathy patients. Changes in functional outcome using Constant-Murley-Scoring(CMS), CMS sub-scoring parameters and radiological outcome were analysed. For this purpose, a classification for radiologic lucency was proposed. Diverging results, influencing factors and alternative treatment options have been discussed to analyse weaknesses and enhance future development of this arthroplastic model.

MATERIALS AND METHODS

23 patients aged 68 ± 8.4 years with irreparable cuff-tear-arthropathy refractory to physiotherapeutic treatment were treated with Reco-Glenoid Total-Shoulder-Arthroplasty (TSA). Pre-operative standardized evaluations included history, physical examination, radiographs, computer tomography and clinical scorings. The post-operative controls included physical examination with CMS, video documentation and radiological evaluation.

RESULTS

After a median follow-up time of 38 ± 18 months, the CMS had been significantly improved (p < 0.001) from (17.4 ± 5.8) to (43.2 ± 19.2) points. Significant improvement in pain, activities of daily life, range of motion (p < 0.001) and power (p = 0.006) was achieved. Significantly, inferior results in functional outcome and higher lucency rates were observed in female patients and in patients treated with small glenoid components. The follow-up rate was 100 %.

CONCLUSION

The semi-constraint reconstruction glenoid prosthesis model in cuff-tear-arthropathy patients significantly improves shoulder function, however, it yields controversial results, with satisfactory results in male and poor results in female patients. Revision rate of the female cohort and loosening of the glenoid component in this short-term follow-up is of concern. Further investigations taking BMD, osteopenic conditions and influence of surface area in smaller individuals into consideration are recommended, to determine whether this is the underlying cause of the inferior results in females.

An experimental glenoid rim strain analysis for an improved reverse anatomy shoulder implant fixation

Loosening of glenoid components in TSA is a main cause of failure. In reverse anatomy TSA designs used for unstable joints, fixation is particularly demanding. Strains developed around the glenoid rim of biomechanical sawbone scapulae implanted with (a) the original fixed-fulcrum Bayley-Walker glenoid prosthesis in current clinical use, and (b) a revised version with conical cross-section, were compared. The conical shape of the revised design was hypothesized to produce greater strains in the glenoid rim than the original tapered screw design. The 2D strain field at three accessible locations around the rim of each scapula was measured with three-element rosette strain gauges for two types of simulated cancellous bone fill under applied physiologically relevant loads. The average strain energy densities around the rim for the conical design were greater than for the original design by a factor of 1.55-2.25 for all loading conditions. Results indicate that a significantly greater proportion of load was directed toward cortical bone in the conical design, thus promoting cortical bone loading.

Polyethylene wear in retrieved reverse total shoulder components

BACKGROUND

Reverse total shoulder arthroplasty has been used to treat rotator cuff tear arthropathy and proximal humerus fractures, as well as for failed conventional total shoulder prostheses. It has been suggested that polyethylene wear is potentially higher in reverse shoulder replacements than in conventional shoulder replacements. The modes and degree of polyethylene wear have not been completely elucidated. The purpose of this study was to evaluate polyethylene wear patterns in 7 specimens retrieved at revision arthroplasty and identify factors that may be associated with increased wear.

METHODS

Reverse total shoulder components were retrieved from 7 patients during revision arthroplasty for loosening and/or pain. Preoperative glenoid tilt and placement and scapular notching were evaluated by use of preoperative radiographs. Polyethylene wear was evaluated via micro-computed tomography and optical microscopy.

RESULTS

Wear on the rim of the polyethylene humeral cup was identified on all retrieved components. The extent of rim wear varied from a penetration depth of 0.1 to 4.7 mm. We could not show a correlation between scapular notching and rim wear. However, rim wear was more extensive when the inferior screw had made contact with the liner. Metal-on-metal wear between the humeral component and the inferior screw of 1 component was also observed. Wear of the intended bearing surface was minimal.

DISCUSSION

Rim damage was the predominant cause of polyethylene wear in our retrieved specimens. Direct contact between the humeral component and inferior metaglene screws is concerning because this could lead to accelerated ultra-highmolecular weight polyethylene wear and also induce mechanical loosening of the glenoid component.

Soft tissue structures resisting anterior instability in a computational glenohumeral joint model

The glenohumeral joint is the most dislocated joint in the body due to the lack of bony constraints and the dependence on soft tissue for stability. The roles that various structures provide to joint function are important for understanding injury treatment and orthopaedic device design purposes. The goal of this study was to develop a computational model of the glenohumeral joint whereby joint behaviour was dictated by articular contact, ligamentous constraints, muscle loading and external perturbations. The bone structure of the computational model consisted of assembled computer tomographic images of the scapula, humerus and clavicle. The soft tissue elements were composed of forces and tension-only springs that represented muscles and ligaments. Validation of this model was achieved by comparing computational predictions to the results of a cadaveric experiment in which the relative contribution of muscles and ligaments to anterior joint stability was examined. The computational model predicted an anterior subluxation force that was similar to the cadaveric experimental results in humeral external rotation. The individual structure results showed the subscapularis to be critical to stabilisation in both neutral and external rotations, the biceps stabilised the joint in neutral but not in external rotation, and the inferior glenohumeral ligament resisted anterior displacement only in external rotation. The model's predictions were similar to the conclusions of the cadaveric experiment and the literature. Knowledge gained from this type of model could assist in further understanding the contribution of soft tissue stabilisers to joint function, pre-operative planning or the design of orthopaedic implants.

Do less medialized reverse shoulder prostheses increase motion and reduce notching?

BACKGROUND

Cuff tear arthropathy is the primary indication for total reverse shoulder arthroplasty. In patients with pseudoparalytic shoulders secondary to irreparable rotator cuff tear, reverse shoulder arthroplasty allows restoration of active anterior elevation and painless shoulder. High rates of glenoid notching have also been reported. We designed a new reverse shoulder arthroplasty with a center of rotation more lateral than the Delta prosthesis to address this problem.

QUESTIONS/PURPOSES

Does reduced medialization of reverse shoulder arthroplasty improve shoulder motion, decrease glenoid notching, or increase the risk of glenoid loosening?

PATIENTS AND METHODS

We retrospectively reviewed 76 patients with 76 less medialized reverse shoulder prostheses implanted for pseudoparalytic shoulder with rotator cuff deficiency between October 2003 and May 2006. Shoulder motion, Constant-Murley score, and plain radiographs were analyzed. Minimum followup was 24 months (mean, 44 months; range, 24-60 months).

RESULTS

The absolute Constant-Murley score increased from 24 to 59, representing an increase of 35 points. The range of active anterior elevation increased by 61°, and the improvement in pain was 10 points. The gain in external rotation with elbow at the side was 15°, while external rotation with 90° abduction increased by 30°. Followup showed no glenoid notching and no glenoid loosening with these less medialized reverse prostheses.

CONCLUSIONS

Less medialization of reverse shoulder arthroplasty improves external and medial rotation, thus facilitating the activities of daily living of older patients. The absence of glenoid notching and glenoid loosening hopefully reflects longer prosthesis survival, but longer followup is necessary to confirm these preliminary observations.

LEVEL OF EVIDENCE

Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Grammont's idea: The story of Paul Grammont's functional surgery concept and the development of the reverse principle

BACKGROUND

The increased use of the reverse prosthesis over the last 10 years is due to a large series of publications using the reverse prosthesis developed by Paul Grammont. However, there is no article reporting the story of the concepts developed by Grammont.

QUESTIONS/PURPOSES

The purposes of this review are to describe the principles developed by Grammont, the chronology of development, and the biomechanical concepts and studies that led to the current design of the reverse prosthesis.

METHODS

We selectively reviewed literature and provide personal observations.

RESULTS

From phylogenetic observations, Grammont developed the principle of functional surgery applied to the rotator cuff tears. To increase the deltoid lever arm, he imagined two possibilities: the lateralization of the acromion, which facilitates the action of the rotator cuff, and the medialization of the center of rotation, which has been developed to respond to situations of rotator cuff deficiency. Grammont proposed the use of an acromiohumeral prosthesis, which was quickly abandoned due to problems of acromial loosening. Finally, Grammont used the principle of reverse prosthesis developed in the 1970s, but made a major change by medializing the center of rotation in a nonanatomic location. In 1985, Grammont validated the concept by an experimental study and the first model using a cemented sphere was implanted.

CONCLUSIONS

The development of the modern reverse prosthesis is the result of the intellectual and experimental work conducted by Grammont and his team for 20 years. Knowledge of this history is essential to envision future developments.

Effects of tilt and glenosphere eccentricity on baseplate/bone interface forces in a computational model, validated by a mechanical model, of reverse shoulder arthroplasty

HYPOTHESIS/BACKGROUND

Reverse shoulder arthroplasty is being used with greater frequency for patients with severe rotator cuff deficiency. There are several commercially available reverse shoulder devices, each with different glenosphere options. The purpose of this study was to determine: (1) forces at the baseplate-bone interface in glenospheres with centers of rotation located concentrically and eccentrically to the center of the baseplate; and (2) if baseplate-bone forces can be optimized by altering tilt of the baseplate.

METHODS

A validated computer model was used to compare concentric glenospheres with neutral offset to eccentrically offset glenospheres (6 mm inferior or 6 mm lateral) in 3 baseplate tilts: 15° inferior, neutral, or 15° superior. A baseplate, simulated bone, screws, and humeral component were modeled, and forces underneath the baseplate were calculated as the arm was abducted through 90° of glenohumeral motion.

RESULTS

For lateral and concentric glenospheres, inferior tilt provides the most even distribution of forces (mean difference in force between superior and inferior portions of baseplate: 11.3 N and 24.7 N, respectively) and superior tilt provides the most uneven distribution of forces (109.3 N and 78.7 N, respectively). For inferior eccentric glenospheres, inferior tilt provides the most uneven distribution of forces (58.7 N) and neutral tilt provides the most even distribution of forces (27.7 N).

CONCLUSION

This is the first study to investigate force distribution under the baseplate in inferior eccentric glenospheres. Although inferior tilting of the baseplate is recommended for concentric and laterally offset glenospheres, this same recommendation may be detrimental to inferiorly offset glenospheres and warrants further investigation.

Reverse anatomy shoulder replacement: comparison of two designs

The aim of this study was to use a biomechanical model of the shoulder to examine the characteristics of two concepts of reverse anatomy implant; those that preserve the natural glenohumeral centre such as the Bayley-Walker (B-W) replacement and those medializing it such as Delta. A biomechanical model of the shoulder was used to evaluate the biomechanical characteristics of these prostheses. The moment arms of the shoulder muscles were predicted for the implanted models over a wide workspace and were compared to those of natural anatomy. The deltoid muscle moment arms through almost all of the range of motion in the Delta model were greatest. As a result the shoulder joint contact force was decreased for this implant. Furthermore, although the B-W model predicted similar moment arms to natural anatomy, the contact force decreased owing to change in the joint constraints. Finally, it was concluded that the reverse anatomy can compensate for loss of rotator cuff muscles in terms of joint stability, yet the shoulder function depends on the individual's musculature.

Prosthetic overhang is the most effective way to prevent scapular conflict in a reverse total shoulder prosthesis

BACKGROUND AND PURPOSE

Despite good clinical results with the reverse total shoulder arthroplasty, inferior scapular notching remains a concern. We evaluated 6 different solutions to overcome the problem of scapular notching.

METHODS

An average and a "worst case scenario" shape in A-P view in a 2-D computer model of a scapula was created, using data from 200 "normal" scapulae, so that the position of the glenoid and humeral component could be changed as well as design features such as depth of the polyethylene insert, the size of glenosphere, the position of the center of rotation, and downward glenoid inclination. The model calculated the maximum adduction (notch angle) in the scapular plane when the cup of the humeral component was in conflict with the scapula.

RESULTS

A change in humeral neck shaft inclination from 155° to 145° gave a 10° gain in notch angle. A change in cup depth from 8 mm to 5 mm gave a gain of 12°. With no inferior prosthetic overhang, a lateralization of the center of rotation from 0 mm to 5 mm gained 16°. With an inferior overhang of only 1 mm, no effect of lateralizing the center of rotation was noted. Downward glenoid inclination of 0º to 10º gained 10°. A change in glenosphere radius from 18 mm to 21 mm gained 31° due to the inferior overhang created by the increase in glenosphere. A prosthetic overhang to the bone from 0 mm to 5 mm gained 39°.

INTERPRETATION

Of all 6 solutions tested, the prosthetic overhang created the biggest gain in notch angle and this should be considered when designing the reverse arthroplasty and defining optimal surgical technique.

Observations on retrieved humeral polyethylene components from reverse total shoulder arthroplasty

HYPOTHESIS

Analyses of polyethylene components retrieved at revision of total knee, hip, and shoulder replacements have been used to study the effect of design, patient, and surgical factors on initial implant performance, but no studies have reported similar types of findings in retrieved humeral polyethylene components in reverse total shoulder arthroplasty. Our hypothesis is that while the conforming surface of the humeral polyethylene may predispose it to surface wear modalities, as seen in total hip arthroplasty, the presence of clinical instability may also increase the occurrence of focal contact stresses leading to subsurface fatigue failure.

MATERIALS AND METHODS

Fourteen humeral polyethylene components were retrieved from revision surgery at a single institution. Each polyethylene was analyzed for 9 modes of damage in each of 4 quadrants into which the bearing surface was divided. For each implant, the most recent radiographs before removal were scored using an adapted radiolucency score, and glenosphere positioning was measured.

RESULTS

Despite the short mean length of implantation (0.46 ± 0.5 years), scratching and abrasion were seen in 14 and 13 components, respectively, followed by third-body debris and pitting. The modes of damage observed were most severe in the inferior quadrant of the humeral polyethylene. Scapular notching, glenoid, and humeral radiolucencies were prevalent on preoperative radiographs, but their long-term significance has not yet been elucidated. Increased glenosphere inclination was associated with decreased superior and total glenoid radiolucency, along with total polyethylene wear scores.

DISCUSSION

Promising early, functional results with the use of reverse total shoulder arthroplasty has led to the increased expansion of its use, but high complication and revision rates continue to raise concerns regarding implant longevity. The presence of a clinical, adduction deficit may predispose patients to inferior quadrant polyethylene wear.

CONCLUSIONS

Impingement of the humeral polyethylene at the lateral edge of the scapula leads to inferior quadrant wear and associated polyethylene failure, and implant instability may predispose the components to fatigue wear mechanisms. Analysis of retrieved humeral polyethylene components, along with patient, design, and surgical factors, provide important information on the causes of component failure.