Introduction. Reverse Shoulder Arthroplasty (RSA) is recognized to be an effective solution for rotator cuff deficient arthritic shoulders, but there are still concerns about impingement and range of motion (ROM). Several RSA biomechanical studies have shown that humeral lateralization can increase ROM in planar motions (e.g. abduction). However, there is still a debate whether humeral lateralization should be achieved with a larger sphere diameter or by lateralizing the center of rotation (COR). The latter has shown to decrease the deltoid moment arm and increase shear forces, where the former may pose challenges in implanting the device in small patients. The aim of this study was to evaluate how humeral lateralization achieved by varying COR lateral offset and glenosphere diameter in a reverse implant can affect impingement during activities of daily living (ADLs). Methods. Nine shoulder CT scans were obtained from healthy subjects. A reverse SMR implant (LimaCorporate, IT) was virtually implanted on the glenoid and humerus (neck-shaft angle 150°) as per surgical technique using Mimics software (Materialise NV). Implant positioning was assessed and approved by a senior surgeon. The 3D models were imported into a validated shoulder computational model (Newcastle Shoulder Model) to study the effects of humeral lateralization. The main design parameters considered were glenosphere diameter (concentric Ø36mm, Ø40mm, Ø44mm) and COR offset (standard, +2mm, +5mm), for a total of 9 combinations for each subject; −10°, 0° and 10° humeral components versions were analyzed. The model calculated the percentage of impingement (intra-articular, contact of cup with scapula neck and glenoid border; extra-articular, contact of humerus with acromion and coracoid) during 5 ADLs (hand to opposite shoulder, hand to back of head, hand to mouth, drink from mug and place object to head height). Results. On average, the Ø40mm and Ø44mm glenosphere resulted in significantly less impingement across ADLs compared to Ø36mm (−31% and −35% respectively). Humeral version and lateralization had no significant effect on impingement for the Ø44mm glenosphere. However, lateralization of +5 mm substantially reduced impingement on the Ø36mm glenosphere but the effect was significant only for the neutral 0° version (−42%) and 10° (−50%) anteversion. Discussion and Conclusions. The results of this study suggest that, for the SMR Reverse prosthesis, humeral lateralization through the increase of glenosphere diameter was the most efficient way to reduce impingement during ADLs compared to the lateralization of the COR. Humeral version can also affect the impact of lateralization on impingement during ADLs; in this study, the impingement for the Ø36mm glenosphere with 10° retroversion was not decreased through lateralization; this may be related to the combined effect of version and scapular morphology. Considering that using larger glenosphere diameter without offsetting the COR theoretically does not reduce overall deltoid lever arm nor increase the shear forces on the glenoid component, this should be the preferable option whenever possible. However, concerns over soft tissue over-tensioning may necessitate the use of a smaller diameter glenosphere in some patients

There are a variety of sizes currently available for reverse total shoulder arthroplasty (RTSA) implant systems. Common sizing options include a smaller 36 to 38 mm or a larger 40 to 42 mm glenosphere, and are typically selected based on surgeon preference or patient size. Previous studies have only evaluated the abduction and adduction range of motion within a single plane of elevation, providing a limited view of the joint's possible range of motion. The purpose of this study was to use computer modeling to evaluate the abduction and adduction range of motion across multiple planes of elevation for a range of glenosphere sizes. Computed tomography images of four cadaveric specimens (age: 54 ± 24 years) were used to obtain the osseous anatomy to be utilised in the model. Solid-body motion studies of the RTSA models were constructed with varying glenosphere diameters of 33, 36, 39, 42, and 45 mm in Solidworks (Dassault Systems, US). The implant components were scaled, while maintaining a consistent centre of rotation. Simulations encompassing the full range of abduction and adduction were conducted for the planes of elevation between −15˚ and 135˚ at 15˚ intervals, with the motion of the humerus being constrained in neutral internal-external rotation throughout all planes. Angles of elevation were obtained utilising the humeral long axis and the RTSA centre of rotation. Statistical analysis was performed using repeated measures ANOVA. Glenosphere diameter was found to significantly affect the adduction range of motion (p=0.043), in which the largest size provided approximately 17˚ more adduction range of motion than the smallest. However, abduction range of motion was not found to be significantly affected through the alteration of glenosphere size (p=0.449). The plane of elevation was not found to significantly affect abduction or abduction (p=0.585 & p=0.225, respectively). Increasing glenosphere diameter resulted in an increased adduction range of motion when averaged across the tested planes of elevation; however the observed influence on abduction was not significant. These are similar to the trends observed in the previous single plane of elevation studies. These findings illustrate the importance of implant sizing related to range of motion. Further studies are required to determine the influence of glenosphere size on internal and external range of motion

Scapular notching is a common problem following reverse shoulder arthroplasty (RSA). This is due to impingement between the humeral polyethylene cup and scapular neck in adduction and external rotation. Various glenoid component strategies have been described to combat scapular notching and enhance impingement-free range of motion (ROM). There is limited data available detailing optimal glenosphere position in RSA with an onlay configuration. The purpose of this study was to determine which glenosphere configurations would maximise impingement free ROM using an onlay RSA prosthesis. A three-dimensional (3D) computed tomography (CT) scan of a shoulder with Walch A1, Favard E0 glenoid morphology was segmented using validated software. An onlay RSA prosthesis was implanted and a computer model simulated external rotation and adduction motion of the virtual RSA prosthesis. Four glenosphere parameters were tested; diameter (36mm, 41mm), lateralization (0mm, 3mm, 6mm), inferior tilt (neutral, 5 degrees, 10 degrees), and inferior eccentric positioning (0.5mm, 1.5mm. 2.5mm, 3.5mm, 4.5mm). Eighty-four combinations were simulated. For each simulation, the humeral neck-shaft angle was 147 degrees and retroversion was 30 degrees. The largest increase in impingement-free range of motion resulted from increasing inferior eccentric positioning, gaining 15.0 degrees for external rotation and 18.8 degrees for adduction. Glenosphere lateralization increased external rotation motion by 13. 6 degrees and adduction by 4.3 degrees. Implanting larger diameter glenospheres increased external rotation and adduction by 9.4 and 10.1 degrees respectively. Glenosphere tilt had a negligible effect on impingement-free ROM. Maximizing inferior glenosphere eccentricity, lateralizing the glenosphere, and implanting larger glenosphere diameters improves impingement-free range of motion, in particular external rotation, of an onlay RSA prosthesis. Surgeons’ awareness of these trends can help optimize glenoid component position to maximise impingement-free ROM for RSA. Further studies are required to validate these findings in the context of scapulothoracic motion and soft tissue constraints

Is there an optimal way to place a glenoid in reverse total shoulder arthroplasty (RTSA)? Four of the six parameters that a surgeon can control in a RTSA involve the glenoid. The parameters are: inferior tilt, increased lateral or inferior offset and increased glenosphere diameter. The theoretical challenges are further complicated by the normal variations that exist in the bony anatomy of the scapula and pathological abnormalities prevalent in as many as 40% of patients undergoing RTSA. Over the last 5 years there has been a growing body of data and study on the biomechanics, clinical outcomes and complications of this prosthesis. What have we learned? How does a surgeon incorporate this into their practice? The goal of this talk is to briefly review the current status of biomechanics on the impact of glenosphere positioning and offset on the outcome of reverse arthroplasty

Introduction. Acromial and scapular fractures are a rare but difficult complication with reverse total shoulder arthroplasty (rTSA), with an incidence rate reported from 1–10%. The risk factors associated with these fractures types is largely unknown. The goal of this study is to analyze the clinical outcomes, demographic and comorbidity data, and implant sizing and surgical technique information from 4125 patients who received a primary rTSA with one specific prosthesis (Equinoxe, Exactech, Inc) and were sorted based on the radiographic documentation of an acromial and/or scapula fracture (ASF) to identify factors associated with this complication. Methods. 4125 patients (2652F/1441M/32 unspecified; mean age: 72.5yrs) were treated with primary rTSA by 23 orthopaedic surgeons. Revision and fracture reverse arthroplasty cases were excluded. The radiographic presence of each fracture was documented and classified using the Levy classification method. 61 patients were identified as having ASF, 10 patients had fractures of the Type 1, 32 patients had Type 2, and 18 patients had Type 3 fractures according to Levy's classification. One fracture was not classifiable. Pre-op and post-op outcome scoring, ROM as well as demographic, comorbidity, implant, and surgical technique information were evaluated for these 61 patients and compared to the larger cohort of patients to identify any associations. A two-tailed, unpaired t-test identified differences (p<0.05). Results. The overall rate of ASF was 1.48% with the average time after surgery occurring at 12.9 ± 17.9 months (range 1 day to 78 months). Men had an ASF rate of 0.69% (10 of 1441); whereas women had a rate of 1.92% (51 of 2652). Patients with ASF were observed to be significantly shorter than patients without ASF (65.1 in vs 63.3 in, p=0.0004). ASF were more common in females (p=0.0019), have Rheumatoid Arthritis (p=0.0051), Cuff Tear Arthropathy (p=0.0093), or previous shoulder surgery (p=0.0189). Patient's weight did not correlate, nor did BMI. No difference was observed in humeral stem size, glenosphere diameter, or the humeral tray offset, humeral liner offset, or combined humeral tray+liner offset. The average number of screws used in the fracture group was significantly more than in the non-fracture group (p=0.0327), and 93% of patients in the fracture group had a screw in the superior hole of the baseplate. Pre-operatively, patients who developed ASF had significantly worse ASES (p=0.0104) and SPADI (p=0.0136) scores and also had significantly worse forward elevation (p=0.0237) and internal rotation (p=0.0054) than those who did not develop ASF. At latest follow-up, patients with ASF had significantly worse SST, UCLA, ASES, Constant, and SPADI scores (all p<0.0001); significantly worse abduction, forward elevation, internal rotation, strength (all p<0.0001); and significantly less preop-to-postop improvement in all measured outcomes, except for external rotation (all p<0.0001). Finally, 24% of fractures were identified as being caused by a traumatic event, 28% of patients with fractures had a previous acromioplasty, and 53% of fractures were Levy type 2. Discussion. Acromial and scapular fractures after rTSA are a rare complication, with an incidence of 1.48% in this analysis of 4125 patients with a single rTSA prosthesis. These fractures were observed to occur at an average of 12.9 months after surgery, but were observed as early as 1 day and as late as 6.5 years. Female patients, Rheumatoid Arthritis, Cuff Tear Arthropathy, previous shoulder surgery, relatively worse pre-operative ASES or SPADI scores, relatively decreased pre-operative forward elevation and internal rotation as well as a larger number of screws placed in the baseplate all were significantly associated with the occurrence of ASF. Although 93% of patients with ASF had a screw placed in the superior hole of the baseplate, we cannot conclude that this is a driving factor at this time, as the superior screw number for the non-fracture group was not recorded. Future work should evaluate if usage of a superior glenoid baseplate screw and previous acromioplasty are also risk factors for these fracture types after rTSA. This study is the largest ever performed analysis of this rare complication and provides news insight into the predisposing risk factors to consider when evaluating patients for rTSA

Introduction:. Given that factors like center of rotation (COR), neck shaft angle, glenosphere diameter and component tilt alter the biomechanics of reverse total shoulder arthroplasty (rTSA), the performance of the total rTSA system is of interest. This study compared the composite performance of two rTSA systems that were designed around a medialized or lateralized glenohumeral COR. The objective was to quantify the following outcome measures: 1) COR & humeral position; 2) range of glenohumeral abduction; 3) force to abduct; and 4) range of internal (IR)/external (ER) rotation. Methods:. Seven pairs of shoulders were tested with a biomechanical shoulder simulator. Beads were implanted in the scapula and humerus to quantify bone positions with a fluoroscope. Spectra lines simulated the deltoid and the rotator cuff. Linear actuators simulated muscle excursion while load cells recorded applied force. Diode arrays were used to quantify arm position and calculate the humeral center of rotation. Native specimens were tested where a motion path was recorded from resting to peak glenohumeral abduction in the scapular plane. The trajectory was replayed and deltoid force vs. arm position was recorded. With the elbow flexed, the arm was articulated to maximal internal and external rotation to determine ROM limits due to impingement or soft tissue constraint. Specimens were implanted with a Tornier Aequalis Reversed Shoulder prosthesis (“A,” 36 mm glenosphere, 10° humeral retroversion, 9 mm poly insert – “medial”) or a DJO Surgical Reverse Shoulder Prosthesis (“R,” 32 mm, 30° retroversion, neutral insert/shell – “lateral”). Implants were randomized between shoulders in a pair. After implantation the test protocol was repeated. Paired-t tests (p ≤ 0.050) were adjusted with Holm's step-down correction for multiple comparisons. Results:. Joint COR shifted inferiorly (A = 7 ± 3 mm, R = 4 ± 2 mm) and medially (A = 19 ± 4 mm, R = 12 ± 3 mm) for both systems with respect to native (p≤0.007, between systems p≤0.037). All humeri shifted inferiorly with respect to native (Fig. 1, p = 0.000, between systems p = 0.718). The RSP maintained a nearly anatomic medial/lateral humerus position, whereas the Aequalis medialized the humerus (p = 0.007). Both rTSA systems showed adduction deficit versus native arms (Fig. 2, p ≤ 0.046). Peak passive abduction, IR and ER were not significantly different between systems (p ≥ 0.113) or with respect to native (p ≥ 0.085). Deltoid force required to elevate the arm decreased ∼25% after rTSA (p ≤ 0.049), but did not differ between systems (p ≥ 0.117). Discussion:. Understanding the implications of implant configuration is imperative to improving implant design and optimizing patient outcomes. As tested, the configurations represent over 70% of respective clinical cases. The systems varied in COR offset, humeral component version/tilt, glenosphere placement, and insert thickness, yet few kinematic differences arose. The RSP COR was more lateral than the Aequalis, yet both were medial to native. Accordingly, both systems provided a similar mechanical advantage by reducing the abduction forces. The RSP had the least adduction deficit, which could indicate increased inferior clearance around the more lateral COR. Inferior and medial humerus shift could negatively impact external rotation capability by moving the posterior cuff line of action below the COR and reducing muscle tension (Fig. 3)

Aims

The liner design is a key determinant of the constraint of a reverse total shoulder arthroplasty (rTSA). The aim of this study was to compare the degree of constraint of rTSA liners between different implant systems.