Quantifying the competing relationship between adduction range of motion and baseplate micromotion with lateralization of reverse total shoulder arthroplasty

Reverse Total Shoulder Arthroplasty Using Lateralized Glenoid Baseplates Has Superior Patient-determined Outcome Scores at Short-term Follow-up

INTRODUCTION

There are a variety of baseplate options when performing reverse total shoulder arthroplasty (RTSA). Currently, there is no consensus on the optimal glenoid baseplate. The hypothesis of this study was that the use of lateralized baseplates would improve patient-determined outcomes and postoperative range of motion after RTSA compared with standard baseplates without increasing the risk of complications.

METHODS

Patients undergoing RTSA were stratified into a standard baseplate group (SBG) and a lateralized baseplate group (LBG). The LBG included 3 mm lateralization, 6 mm lateralization, and full-wedge augmentation (8 mm lateralization). The Western Ontario Osteoarthritis Score, American Shoulder and Elbow Surgeons score, Single Assessment Numeric Evaluation (SANE), and Simple Shoulder Test (SST) were recorded at baseline, 1 year, and 2 years. Range of motion was recorded at baseline and 1 year. Differences in complications between groups were recorded.

RESULTS

The LBG included 187 patients, and the SBG included 51 patients. No difference was observed in any patient-determined outcome score at 1-year follow-up. At 2 years, there were greater Western Ontario Osteoarthritis Score (84 ± 16 versus 74 ± 19; P = 0.01), American Shoulder and Elbow Surgeons score (81 ± 15 versus 70 ± 20; P = 0.001), SST (8.0 ± 2.4 versus 6.6 ± 2.6; P = 0.007), and SANE (82 ± 17 versus 68 ± 25; P = 0.0005). The improvement in SST (5.0 ± 2.7 versus 3.3 ± 3.6; P = 0.02) and SANE (54 ± 26 versus 37 ± 30; P = 0.004) at 2 years compared with baseline was greater in the LBG compared with the SBG. No difference was observed in any range-of-motion metric between groups. Total complications were similar between groups (P = 0.91). Scapular notching was more prevalent in the SBG (7.8% versus 1.6%; P = 0.01).

CONCLUSION

The LBG had better patient-determined outcome scores compared with the SBG at 2-year follow-up with a similar rate of overall complications but a lower rate of scapular notching. Range of motion was not improved by the use of a lateralized baseplate compared with a standard baseplate.

Ten technical aspects of baseplate fixation in reverse total shoulder arthroplasty for patients without glenoid bone loss: a systematic review

The aim of this systematic review was to collect evidence on the following 10 technical aspects of glenoid baseplate fixation in reverse total shoulder arthroplasty (rTSA): screw insertion angles; screw orientation; screw quantity; screw length; screw type; baseplate tilt; baseplate position; baseplate version and rotation; baseplate design; and anatomical safe zones. Five literature libraries were searched for eligible clinical, cadaver, biomechanical, virtual planning, and finite element analysis studies. Studies including patients >16 years old in which at least one of the ten abovementioned technical aspects was assessed were suitable for analysis. We excluded studies of patients with: glenoid bone loss; bony increased offset-reversed shoulder arthroplasty; rTSA with bone grafts; and augmented baseplates. Quality assessment was performed for each included study. Sixty-two studies were included, of which 41 were experimental studies (13 cadaver, 10 virtual planning, 11 biomechanical, and 7 finite element studies) and 21 were clinical studies (12 retrospective cohorts and 9 case-control studies). Overall, the quality of included studies was moderate or high. The majority of studies agreed upon the use of a divergent screw fixation pattern, fixation with four screws (to reduce micromotions), and inferior positioning in neutral or anteversion. A general consensus was not reached on the other technical aspects. Most surgical aspects of baseplate fixation can be decided without affecting fixation strength. There is not a single strategy that provides the best outcome. Therefore, guidelines should cover multiple surgical options that can achieve adequate baseplate fixation.

How to avoid baseplate failure: the effect of compression and reverse shoulder arthroplasty baseplate design on implant stability

BACKGROUND

Failure to achieve fixation of the glenoid baseplate will lead to clinical failure. The fixation of the baseplate to the scapula must be able to withstand sufficient shear forces to allow bony ingrowth. The importance of compression to neutralize the forces at the baseplate-bone interface has been assumed to be critical in limiting excessive micromotion. The purpose of this study is to determine the effect of compression on implant stability with different baseplate designs.

METHODS

Various baseplate designs (1-piece monolithic central screw [1P], 2-piece locking central screw [2PL], and 2-piece nonlocking center screw [2PNL]) were investigated at 3 different compressive forces (high [810 N], medium [640 N], and low [530 N]). Synthetic bone cylinders were instrumented, and peripheral screws were used in all models. The combination of 1 locking and 3 nonlocking peripheral screw fixation was selected as worst-case scenario. Dynamic testing protocol followed the ASTM F2028-17 standard. The baseplate micromotion at high compression was compared to low compression. Additionally, the baseplate micromotion for each design was compared at baseline (first 50 cycles) and at 10,000 cycles for the 3 different compressive forces where motion above 150 μm was defined as failure.

RESULTS

Baseplate micromotion was found to negatively correlate with compression (rpb = -0.83, P < .0001). At baseline, all baseplate designs were considered stable, regardless of compression. With high compression, average micromotion at the glenoid baseplate-bone interface remained below the 150-μm threshold for all baseplate designs at 10,000 cycles (1P: 50 ± 10 μm; 2PL: 78 ± 32 μm; 2PNL: 79 ± 8 μm; P = .060). With medium compression, average micromotion at 10,000 cycles for all 3 designs remained below the 150-μm threshold (1P: 88 ± 22 μm; 2PL: 132 ± 26 μm; 2PNL: 107 ± 39 μm). The 2PL design had the highest amount of micromotion (P = .013). With low compression, both 2-piece designs had an average micromotion above the 150-μm threshold whereas the 1-piece design did not (1P: 133 ± 35 μm; 2PL: 183 ± 21 μm; 2PNL: 166 ± 39 μm). The 2PL design had significantly higher micromotion when compared to 1P design (P = .041).

DISCUSSION

The stability of a central screw baseplate correlates with the amount of compression obtained and is affected by implant design. For the same amount of compression, more micromotion is observed in a 2-piece design than a 1-piece design.

How does computed tomography inform our understanding of shoulder kinematics? A structured review

The objective of this structured review was to review how computed tomography (CT) scanning has been used to measure the kinematics of the shoulder. A literature search was conducted using Evidence-based Medicine Reviews (Embase) and PubMed. In total, 29 articles were included in the data extraction process. Forty percent of the studies evaluated healthy participants' shoulder kinematics. The glenohumeral joint was the most studied, followed by the scapulothoracic, acromioclavicular, and sternoclavicular joints. Three-dimensional computed tomography (3DCT) and 3DCT with biplane fluoroscopy are the two primary imaging techniques that have been used to measure shoulder joints' motion under different conditions. Finally, many discrepancies in the reporting of the examined motions were found. Different authors used different perspectives and planes to report similar motions, which results in confusion and misunderstanding of the actual examined motion. The use of 3DCT has been widely used in the examination of shoulder kinematics in a variety of populations with varying methods employed. Future work is needed to extend these methodologies to include more diverse populations, to examine the shoulder complex as a whole, and to standardize their reporting of motion examined to make study to study comparisons possible.

Central fixation element type and length affect glenoid baseplate micromotion in reverse shoulder arthroplasty

BACKGROUND

Reverse shoulder arthroplasty (RSA) is commonly used to treat patients with rotator cuff tear arthropathy. Loosening of the glenoid component remains one of the principal modes of failure and represents a significant complication that requires revision surgery. This study assessed the effects of various factors on glenoid baseplate micromotion for primary fixation of RSA.

MATERIALS AND METHODS

A half-fractional factorial design of experiment was used to assess 4 factors: central element type (central peg or screw), central cortical engagement according to length (13.5 or 23.5 mm), anterior-posterior peripheral screw type (nonlocking or locking), and cancellous bone surrogate density (160 or 400 kg/m³, 10 or 25 PCF). Glenoid baseplates were implanted into high- or low-density Sawbones rigid polyurethane foam blocks and cyclically loaded at 60° for 1000 cycles (500-N compressive force range) using a custom-designed loading apparatus. Micromotion at the 4 peripheral screw positions was recorded using linear variable differential transformers.

RESULTS

Central peg fixation generated 358% greater micromotion at all peripheral screw positions compared with central screw fixation (P < .001). Baseplates with short central elements that lacked cortical bone engagement generated 328% greater micromotion than those with long central elements (P = .001). No significant effects were observed when varying anterior-posterior peripheral screw type or bone surrogate density. There were significant interactions between central element type and length (P < .001).

DISCUSSION

A central screw and a long central element that engaged cortical bone reduced RSA baseplate micromotion. These findings serve to inform surgical decision making regarding baseplate fixation elements to minimize the risk of glenoid loosening and, thus, the need for revision surgery.

Complications of Reverse Total Shoulder Arthroplasty: A Computational Modelling Perspective

Reverse total shoulder arthroplasty (RTSA) is an established treatment for elderly patients with irreparable rotator cuff tears, complex proximal humerus fractures, and revision arthroplasty; however, with the increasing indications for RTSA over the last decade and younger implant recipients, post-operative complications have become more frequent, which has driven advances in computational modeling and simulation of reverse shoulder biomechanics. The objective of this study was to provide a review of previously published studies that employed computational modeling to investigate complications associated with RTSA. Models and applications were reviewed and categorized into four possible complications that included scapular notching, component loosening, glenohumeral joint instability, and acromial and scapular spine fracture, all of which remain a common cause of significant functional impairment and revision surgery. The computational shoulder modeling studies reviewed were primarily used to investigate the effects of implant design, intraoperative component placement, and surgical technique on postoperative shoulder biomechanics after RTSA, with the findings ultimately used to elucidate and mitigate complications. The most significant challenge associated with the development of computational models is in the encapsulation of patient-specific anatomy and surgical planning. The findings of this review provide a basis for future direction in computational modeling of the reverse shoulder.

Lateralized versus nonlateralized reverse total shoulder arthroplasty

Throughout the history of reverse total shoulder arthroplasty, the extent of lateral offset has changed considerably from "too lateral" to "too medial" and has been lately swinging back towards a point somewhere in between. Nonlateralized designs minimize shear forces on the glenoid and decrease force required by the deltoid. Glenoid lateralization decreases impingement and scapular notching and improves range of motion. Humeral lateralization achieves a more anatomic position of the tuberosities while maintaining a nonlateralized center of rotation. Several factors play a role in choosing the extent of lateral offset and method of lateralization.

In vivo reverse total shoulder arthroplasty contact mechanics

BACKGROUND

Several in vitro studies have investigated the biomechanics of reverse total shoulder arthroplasty (RTSA); however, few in vivo studies exist. The purpose of this study was to examine in vivo RTSA contact mechanics in clinically relevant arm positions. Our hypothesis was that contact would preferentially occur in the inferior region of the polyethylene liner.

METHODS

Forty patients receiving a primary RTSA were recruited for a prospective cohort study. All patients received the same implant design with a nonretentive liner. Stereo radiographs were taken at maximal active range of motion. Model-based radiostereometric analysis was used to identify implant position. Contact area between the polyethylene and glenosphere was measured as the geometric intersection of the 2 components and compared with respect to polyethylene liner size, arm position, and relative position within the liner.

RESULTS

There were no differences in the proportion of contact area in any arm position between polyethylene liner sizes, ranging from 30% ± 17% to 38% ± 23% for 36-mm liners and 32% ± 21% to 41% ± 25% for 42-mm liners. Contact was equally distributed between the superior and inferior halves of the liner at each arm position (P = .06-.79); however, greater contact area was observed in the outer radius of the liner when the arm was flexed (P = .002).

CONCLUSION

This study highlights that contact mechanics are similar between 36- and 42-mm liners. Contact area is generally equally distributed throughout the liner across the range of motion and not preferentially in the inferior region as hypothesized.

Offset in Reverse Shoulder Arthroplasty: Where, When, and How Much

Since the advent of Paul Grammont medialized reverse shoulder prosthesis in the 1980s, shoulder surgeons have had a reliable option for treating glenohumeral joint disease in the rotator cuff-deficient shoulder. However, the prosthesis is not without complications, including scapular notching, instability, and limited rotational motion. Implants have been modified from the original design in an effort to reduce the risk of these complications. Increasing the offset, or lateralization, of the glenosphere may reduce the rate of scapular notching, reduce impingement, increase stability, and improve rotational motion. However, a more lateralized glenosphere could lead to baseplate loosening, decreased deltoid efficiency, and increased risk of acromial fracture. Increasing the offset on the humeral side, rather than the glenosphere side, may be able to reduce the rate of scapular notching and improve rotational motion without an increased risk of baseplate loosening. Humeral lateralization also improves tension of the rotator cuff and maintains good deltoid efficiency. However, humeral lateralization provides little stability benefit, and acromial fracture remains a risk. Ultimately, the surgeon must be familiar with the implants he or she is using and the options for both glenosphere and humeral lateralization to ensure that risks and benefits can be weighed for each patient.

Effect of RSA glenoid baseplate central fixation on micromotion and bone stress

Background

In reverse shoulder arthroplasties (RSA), osseous in-growth is promoted if glenoid micromotion does not exceed 150 μm. The purpose of this study was to determine whether the configuration of central fixation for RSA glenoid baseplates reduces implant micromotion or changes scapula bone stresses.

Methods

Using finite element analysis, glenoid baseplate fixation was tested in a cohort of 3 male and 2 female patients who were to undergo RSA. Computer models were created for 3 different RSA glenoid baseplate and 84 glenosphere designs, a central threaded peg (1 variant, D-TP), a central unthreaded peg (2 variants, I- 85 P(15) and I-P(25)), and a central peg with a screw (2 variants, A-PS and I-PS). A compressive and shear force of 756 N was distributed across the glenosphere with the scapula anchored.

Results

Displacement was within 20-130 μm at the glenosphere baseplate-bone interface for all baseplates. The glenospheres with unthreaded pegs had intermediate displacement values (I-P(15): median, 89 μm; range, 32-112 μm; and I-P(25): median, 93 μm; range, 31-109 μm). The von Mises stresses were 1.8-7.0 MPa within cortical bone and 0.6-1.6 MPa within trabecular bone. Cortical bone stresses were similar with unthreaded pegs (I-P(15): median, 4.2 MPa; range, 1.8-6.0 MPa; and I-P(25): median, 4.2 MPa; range, 1.8-6.1 MPa), whereas mean trabecular stresses were similar for all configurations.

Conclusions

All configurations yielded adequate stability, with micromotions being below 150 μm. The unthreaded pegged designs provided a valid alternative to the stable threaded pegged convex baseplates in terms of micromotions and bone stresses.

Finite Element Analysis of Custom Shoulder Implants Provides Accurate Prediction of Initial Stability

Custom reverse shoulder implants represent a valuable solution for patients with large bone defects. Since each implant has unique patient-specific features, finite element (FE) analysis has the potential to guide the design process by virtually comparing the stability of multiple configurations without the need of a mechanical test. The aim of this study was to develop an automated virtual bench test to evaluate the initial stability of custom shoulder implants during the design phase, by simulating a fixation experiment as defined by ASTM F2028-14. Three-dimensional (3D) FE models were generated to simulate the stability test and the predictions were compared to experimental measurements. Good agreement was found between the baseplate displacement measured experimentally and determined from the FE analysis (Spearman’s rank test, p < 0.05, correlation coefficient ρs = 0.81). Interface micromotion analysis predicted good initial fixation (micromotion <150 µm, commonly used as bone ingrowth threshold). In conclusion, the finite element model presented in this study was able to replicate the mechanical condition of a standard test for a custom shoulder implants.

Clinical comparison of humeral-lateralization reverse total shoulder arthroplasty between patients with irreparable rotator cuff tear and patients with cuff tear arthropathy

Background

This study aimed to compare the clinical and radiologic outcomes between patients with irreparable cuff tears (ICTs) and those with cuff tear arthropathy (CTA) after reverse total shoulder arthroplasty (RTSA) with a humeral-lateralization prosthesis.

Methods

A total of 127 patients with ICTs and CTA who underwent RTSA were enrolled and matched at a 1:2 ratio by propensity score. Preoperative shoulder function was assessed for all patients. Radiologic parameters including the acromion-deltoid tuberosity (ADT) distance, lateral humeral offset, and scapular notching were evaluated.

Results

Thirty-four patients in the ICT group and 68 patients in the CTA group were matched for comparison. Preoperatively, mean active forward flexion in the ICT group (89.7° ± 29.4°) was significantly better than that in the CTA group (65.5° ± 24.0°, P < .001). In the CTA group, fatty infiltration of the supraspinatus was worse (3.7 ± 0.5) and the ADT distance was shorter (134.0 ± 12.0 mm) compared with the ICT group preoperatively (3.3 ± 0.8 [P = .008] and 140.7 ± 12.5 mm [P = .001], respectively). There was no significant difference in postoperative functional or radiologic outcomes between the 2 groups. However, gains in active forward flexion (37.9° in ICT group vs. 61.5° in CTA group, P < .01) and abduction (42.1° in ICT group vs. 60.6° in CTA group, P < .01) were significantly greater in the CTA group than in the ICT group.

Conclusions

Shoulder function was significantly improved after RTSA regardless of the preoperative diagnosis. Postoperatively, radiologic findings were not significantly different between the 2 groups. Due to the fact that preoperative range of motion and rotator cuff status were better in patients with ICTs, improvements in active forward flexion and abduction were significantly greater in patients with CTA.

Reverse Total Shoulder Arthroplasty: Biomechanics and Indications

PURPOSE OF REVIEW

Over the past decade, our understanding of the biomechanics of the reverse total shoulder arthroplasty (RTSA) has advanced, resulting in design adjustments, improved outcomes, and expanding indications. The purpose of this review is to summarize recent literature regarding the biomechanics of RTSA and the evolving indications for its use.

RECENT FINDINGS

While Grammont's principles of RTSA biomechanics remain pillars of contemporary designs, a number of modifications have been proposed and trialed in later generations to address complications such as impingement and glenoid failure. Clinical and biomechanical literature suggest that less medialized, more inferior glenospheres result in less impingement and notching. On the humerus, a more vertical neck cut is associated with less impingement. Indications for RTSA continue to expand beyond the classic indication of cuff tear arthropathy (CTA). Patients without a functional cuff but no arthritis now have a reliable option in the RTSA. RTSA has also replaced hemiarthroplasty as the implant of choice for displaced three- and four-part proximal humerus fractures in the elderly. Finally, updated design options and modular components now allow for treatment of glenoid bone loss, failed arthroplasty, and proximal humerus tumors with RTSA implants. Reverse total shoulder arthroplasty design has been modernized on both the glenoid and humerus to address biomechanical challenges of early implants. As outcomes improve with these modifications, RTSA indications are growing to address complex bony pathologies such as tumor and bone loss. Longitudinal follow-up of patients with updated designs and novel indications is essential to judicious application of RTSA technology.

Impact of Modeling Assumptions on Stability Predictions in Reverse Total Shoulder Arthroplasty

Reverse total shoulder arthroplasty (rTSA) is commonly used in the shoulder replacement surgeries for the relief of pain and to restore function, in patients with grossly deficient rotator cuff. Primary instability due to glenoid loosening is one of the critical complications of rTSA; the implants are designed and implanted such that the motion between the glenoid baseplate and underlying bone is minimized to facilitate adequate primary fixation. Finite element analysis (FEA) is commonly used to simulate the test setup per ASTM F2028-14 for comparing micromotion between designs or configurations to study the pre-clinical indications for stability. The FEA results can be influenced by the underlying modeling assumptions. It is a common practice to simplify the screw shafts by modeling them as cylinders and modeling the screw-bone interface using bonded contact, to evaluate micromotion in rTSA components. The goal of this study was to evaluate the effect of three different assumptions for modeling the screw-bone interface on micromotion predictions. The credibility of these modeling assumptions was examined by comparing the micromotion rank order predicted among three different modular configurations with similar information from the literature. Eight configurations were modeled using different number of screws, glenosphere offset, and baseplate sizes. An axial compression and shear load was applied through the glenosphere and micromotion at the baseplate-bone interface was measured. Three modeling assumptions pertaining to modeling of the screw-bone interface were used and micromotion results were compared to study the effect of number of peripheral screws, eccentricities, and baseplate diameter. The relative comparison of micromotion between configurations using two versus four peripheral screws remained unchanged irrespective of the three modeling assumptions. However, the relative comparison between two inferior offsets and baseplate sizes changed depending on the modeling assumptions used for the screw-bone interface. The finding from this study challenges the generally believed hypothesis that FEA models can be used to make relative comparison of micromotion in rTSA designs as long as the same modeling assumptions are used across all models. The comparisons with previously published work matched the finding from this study in some cases, whereas the comparison was contradicting in other cases. It is essential to validate the computer modeling approach with an experiment using similar designs and methods to increase the confidence in the predictions to make design decisions.