Fundamentals of the Biomechanical Characteristics Related to the Loading of Reverse Total Shoulder Arthroplasty Implants and the Development of a Wear Simulation Strategy

A greater understanding of reverse total shoulder arthroplasty (RTSA) in terms of articular contact mechanics and wear is essential for the optimization of current surgical technique and future implant design. Despite the prevalence of RTSA for shoulder reconstruction, there is little information in current literature regarding the effects of changes in implant parameters on articular contact mechanics and wear. The present work describes the use of in-vitro cadaveric studies, a computational model of the articular contact surfaces, and the development and implementation of a wear simulation strategy to assess RTSA contact mechanics and wear. The articular loading characteristics of RTSA were assessed using an in-vitro shoulder joint motion simulator and a custom instrumented implant, including the effects of changes in implant configuration. Decreasing neck-shaft angle and cup depth was not found to affect joint load or muscle forces. Increasing glenosphere diameter increased adduction range of motion (ROM), but also slightly increased joint load. The contact mechanics of RTSA were then investigated for the same implant configurations as above. The location of the contact patch and peak contact stress was typically in the inferior humeral cup quadrant, coincident with the location of clinical retrieval damage. Reducing neck-shaft angle and decreasing cup depth reduced contact area and increased peak contact stress, which may negatively impact implant performance. Increasing size came at no cost in terms of contact mechanics. A wear simulation strategy was developed based on the loading and motion characteristics of the cadaveric study, and produced a mean wear rate of 201.1±86.5 mm3/Mc, which was higher than previously published data, and created damage in the cup inferior quadrant. The number of \u27cycles\u27 per year for RTSA reconstructed shoulders was estimated between 0.33-1.5 Mc/yr, suggesting a similar order of magnitude as the lower extremities. The present work advances knowledge regarding RTSA biomechanics and tribology. Specific tradeoffs in terms of ROM and contact mechanics were observed for the reduction of both neck-shaft angle and cup depth, whereby increased motion came at the cost of reduced contact area and increased peak contact stress. Increasing size improved ROM without any negative consequences on contact mechanics

The Effect of Radial Head Hemiarthroplasty Stem Fit on Radiocapitellar Contact Mechanics: Is Loose Fit better than Rigidly Fixed?

Background/Methods: Radial head hemiarthroplasty is commonly employed to manage comminuted displaced fractures. With regards to implant fixation, current designs vary with some prostheses aiming to achieve a tight \u27fixed\u27 fit, and others utilizing a smooth stem with an over reamed \u27loose\u27 fit. The purpose of the present study was to evaluate the effect of radial head hemiarthroplasty stem fit on radiocapitellar contact using a finite element model which simulated both fixed (size-for-size) and loose (1, 2 & 3mm over reamed) stem fits. Hypothesis: It was hypothesized that a loose stem fit would improve radiocapitellar contact mechanics, with increased contact area and decreased contact stress, by allowing the implant to find its \u27optimal\u27 position with respect to the capitellum. Results/Discussion: This data suggests that the loose smooth stem radial head implant may be functioning like a bipolar implant in optimizing radiocapitellar contact. The \u27loose\u27 over reamed stem provided optimal contact mechanics of the metallic axisymmetric radial head implant compared to the \u27fixed\u27 stem. The 1mm over reamed stem reduced maximum contact stress without affecting contact area. Over reaming of 2mm provided the greatest decrease in maximum contact stress, albeit with a significant reduction in contact area. Over reaming of 3mm produced a larger amount of stress concentrations on the capitellum suggesting there may be a limit to how loose a smooth stem implant should be implanted

Mixed reality visualization in shoulder arthroplasty: is it better than traditional preoperative planning software?

Background Preoperative traditional software planning (TSP) is a method used to assist surgeons with implant selection and glenoid guide-pin insertion in shoulder arthroplasty. Mixed reality (MR) is a new technology that uses digital holograms of the preoperative plan and guide-pin trajectory projected into the operative field. The purpose of this study was to compare TSP to MR in a simulated surgical environment involving insertion of guide-pins into models of severely deformed glenoids. Methods Eight surgeons inserted guide-pins into eight randomized three-dimensional-printed severely eroded glenoid models in a simulated surgical environment using either TSP or MR. In total, 128 glenoid models were used and statistically compared. The outcomes compared between techniques included procedural time, difference in guide-pin start point, difference in version and inclination, and surgeon confidence via a confidence rating scale. Results When comparing traditional preoperative software planning to MR visualization as techniques to assist surgeons in glenoid guide pin insertion, there were no statistically significant differences in terms of mean procedure time (P=0.634), glenoid start-point (TSP=2.2±0.2 mm, MR=2.1±0.1 mm; P=0.760), guide-pin orientation (P=0.586), or confidence rating score (P=0.850). Conclusions The results demonstrate that there were no significant differences between traditional preoperative software planning and MR visualization for guide-pin placement into models of eroded glenoids. A perceived benefit of MR is the real-time intraoperative visibility of the surgical plan and the patient’s anatomy; however, this did not translate into decreased procedural time or improved guide-pin position. Level of evidence Basic science study, biomechanics

Electromagnetic Tracking of the Kinematics of Articulating Joints

© 2017 Elsevier Inc. All rights reserved. The tracking of the kinematics of articulating joints during in vitro testing is of interest in determining the motion of healthy intact joints, as well as the changes after joint realignment, joint reconstruction, and/or soft tissue degeneration. Joint reconstruction can alter the kinematics of the joint, often resulting in a reduced range of motion and/or a significant change to soft tissue and interarticular loading. In vitro cadaveric testing, coupled with motion tracking of the bones of the joint, can provide insight into the changes when the native kinematics are compared to the reconstructed joint, permit the digitization of biologic structures using tracing techniques, and allow for the measurement of the relative motion of one structure (e.g., implanted or biological) with respect to another. The ability to track the motion of the bones associated with the articulating joint is of paramount importance in the study of joint motion. Numerous techniques have been employed, including goniometers, optical tracking systems, and electromagnetic tracking (ET) systems. However, ET offers the advantage of rapid real-time data acquisition in six degrees of freedom and eliminates line-of-sight issues common to optical techniques. Therefore, this chapter explains how to perform ET testing on articulating joints, as well as how to analyze, present, and interpret results

Hemiarthroplasty implants should have very low stiffness to optimize cartilage contact stress

© 2020 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. Hemiarthroplasty is often preferred to total arthroplasty as it preserves native tissue; however, accelerated wear of the opposing cartilage is problematic. This is thought to be caused by the stiffness mismatch between the implant and cartilage-bone construct. Reducing the stiffness of the implant by changing the material has been hypothesized as a potential solution. This study employs a finite element model to study a concave-convex hemiarthroplasty articulation for various implant materials (cobalt-chrome, pyrolytic carbon, polyether ether ketone, ultra-high-molecular-weight polyethylene, Bionate-55D, Bionate-75D, and Bionate-80A). The effect of the radius of curvature and the degree of flexion-extension was also investigated to ensure any relationships found between materials were generalizable. The implant material had a significant effect (P \u3c.001) for both contact area and maximum contact pressure on the cartilage surface. All of the materials were different from the native state except for Bionate-80A at two of the different flexion angles. Bionate-80A and Bionate-75D, the materials with the lowest stiffnesses, were the closest to the native state for all flexion angles and radii of curvature. No evident difference between materials occurred unless the modulus was below that of Bionate-55D (288 MPa), suggesting that hemiarthroplasty materials need to be less stiff than this material if they are to protect the opposing cartilage. This is clinically significant as the findings suggest that the development of new hemiarthroplasty implants should use materials with stiffnesses much lower than currently available devices

The effect of stem fit on the radiocapitellar contact mechanics of a metallic axisymmetric radial head hemiarthroplasty: is loose fit better than rigidly fixed?

© 2019 Journal of Shoulder and Elbow Surgery Board of Trustees Background: Radial head hemiarthroplasty is commonly used to manage comminuted displaced fractures. Regarding implant fixation, current designs vary, with some prostheses aiming to achieve a tight “fixed” fit and others using a smooth stem with an over-reamed “loose” fit. The purpose of this study was to evaluate the effect of radial head hemiarthroplasty stem fit on radiocapitellar contact using a finite element model that simulated both fixed (size-for-size) and loose (1-, 2-, and 3-mm over-reamed) stem fits. It was hypothesized that a loose stem fit would improve radiocapitellar contact mechanics, with an increased contact area and decreased contact stress, by allowing the implant to find its “optimal” position with respect to the capitellum. Methods: Finite element models of the elbow were produced to compare the effects of stem fit on radiocapitellar contact of a metallic axisymmetric radial head implant. Radiocapitellar contact mechanics (contact area and maximum contact stress) were computed for 0°, 45°, 90°, and 135° of elbow flexion with the forearm in neutral rotation, pronation, and supination. Results: The data suggest that the loose smooth stem radial head implant may be functioning like a bipolar implant in optimizing radiocapitellar contact. Over-reaming of 3 mm produced a larger amount of stress concentration on the capitellum, suggesting there may be a limit to how loose a smooth stem implant should be implanted. Conclusions: The loose 1 to 2 mm over-reamed stem provided optimal contact mechanics of the metallic axisymmetric radial head implant compared with the fixed stem

The effect of inhomogeneous trabecular stiffness relationship selection on finite element outcomes for shoulder arthroplasty

Copyright © 2019 by ASME. An important feature of humeral orthopedic finite element (FE) models is the trabecular stiffness relationship. These relationships depend on the anatomic site from which they are derived; but have not been developed for the humerus. As a consequence, humeral FE modeling relies on relationships for other anatomic sites. The variation in humeral FE outcomes due to the trabecular stiffness relationship is assessed. Stemless arthroplasty FE models were constructed from CT scans of eight humeri. Models were loaded corresponding to 45 deg and 75 deg abduction. Each bone was modeled five times with the only variable being the trabecular stiffness relationship: four derived from different anatomic-sites and one pooled across sites. The FE outcome measures assessed were implant-bone contact percentage, von Mises of the change in stress, and bone response potential. The variance attributed to the selection of the trabecular stiffness relationship was quantified as the standard deviation existing between models of different trabecular stiffness. Overall, variability due to changing the trabecular stiffness relationship was low for all humeral FE outcome measures assessed. The variability was highest within the stress and bone formation potential outcome measures of the trabecular region. Variability only exceeded 10% in the trabecular stress change within two of the eight slices evaluated. In conclusion, the low variations attributable to the selection of a trabecular stiffness relationship based on anatomic-site suggest that FE models constructed for shoulder arthroplasty can utilize an inhomogeneous site-pooled trabecular relationship without inducing marked variability in the assessed outcome measures

Comparing daily shoulder motion and frequency after anatomic and reverse shoulder arthroplasty

© 2017 Journal of Shoulder and Elbow Surgery Board of Trustees Background Both anatomic (TSA) and reverse total shoulder arthroplasty (RTSA) are common interventions for glenohumeral arthrosis, with the goal of relieving pain and restoring mobility. Understanding shoulder arthroplasty motion and frequency is of interest in evaluating effectiveness and in predicting bearing wear for implant development and optimization. The purpose of this study was to measure and compare the total daily shoulder motion of patients after TSA and RTSA. Methods Thirty-six human subjects who had undergone shoulder arthroplasty wore a custom instrumented garment that tracked upper extremity motion for the waking hours of 1 day. The 3-dimensional orientation of each humeral sensor was transformed with respect to the torso to calculate total joint motion and frequency, with comparison of TSA to RTSA. In addition, the yearly motion of the shoulder was extrapolated. Results The majority of shoulder motion occurred below 80° of elevation (P \u3c.001), totaling on average 821 ± 45 and 783 ± 27 motions per hour for TSA and RTSA, respectively. Conversely, elevations \u3e80° were significantly less frequent, totaling only 52 ± 44 (P \u3c.001) and 38 ± 27 (P \u3c.001) motions per hour for TSA and RTSA, respectively. No significant differences were detected between TSA and RTSA shoulders (P =.22) or their respective contralateral asymptomatic sides (P =.64, P =.62). When extrapolated, it was estimated that each TSA and RTSA shoulder elevated above 60° approximately 1 million and 0.75 million cycles per year, respectively. Discussion Mean shoulder motions after TSA or RTSA were not significantly different from the contralateral asymptomatic side. In addition, no significant differences were detected in shoulder motion or frequency between TSA and RTSA

Implant Design Variations in Reverse Total Shoulder Arthroplasty Influence the Required Deltoid Force and Resultant Joint Load

© 2015, The Association of Bone and Joint Surgeons®. Background: Reverse total shoulder arthroplasty (RTSA) is widely used; however, the effects of RTSA geometric parameters on joint and muscle loading, which strongly influence implant survivorship and long-term function, are not well understood. By investigating these parameters, it should be possible to objectively optimize RTSA design and implantation technique. Questions/purposes: The purposes of this study were to evaluate the effect of RTSA implant design parameters on (1) the deltoid muscle forces required to produce abduction, and (2) the magnitude of joint load and (3) the loading angle throughout this motion. We also sought to determine how these parameters interacted. Methods: Seven cadaveric shoulders were tested using a muscle load-driven in vitro simulator to achieve repeatable motions. The effects of three implant parameters—humeral lateralization (0, 5, 10 mm), polyethylene thickness (3, 6, 9 mm), and glenosphere lateralization (0, 5, 10 mm)—were assessed for the three outcomes: deltoid muscle force required to produce abduction, magnitude of joint load, and joint loading angle throughout abduction. Results: Increasing humeral lateralization decreased deltoid forces required for active abduction (0 mm: 68% ± 8% [95% CI, 60%–76% body weight (BW)]; 10 mm: 65% ± 8% [95% CI, 58%–72 % BW]; p = 0.022). Increasing glenosphere lateralization increased deltoid force (0 mm: 61% ± 8% [95% CI, 55%–68% BW]; 10 mm: 70% ± 11% [95% CI, 60%–81% BW]; p = 0.007) and joint loads (0 mm: 53% ± 8% [95% CI, 46%–61% BW]; 10 mm: 70% ± 10% [95% CI, 61%–79% BW]; p \u3c 0.001). Increasing polyethylene cup thickness increased deltoid force (3 mm: 65% ± 8% [95% CI, 56%–73% BW]; 9 mm: 68% ± 8% [95% CI, 61%–75% BW]; p = 0.03) and joint load (3 mm: 60% ± 8% [95% CI, 53%–67% BW]; 9 mm: 64% ± 10% [95% CI, 56%–72% BW]; p = 0.034). Conclusions: Humeral lateralization was the only parameter that improved joint and muscle loading, whereas glenosphere lateralization resulted in increased loads. Humeral lateralization may be a useful implant parameter in countering some of the negative effects of glenosphere lateralization, but this should not be considered the sole solution for the negative effects of glenosphere lateralization. Overstuffing the articulation with progressively thicker humeral polyethylene inserts produced some adverse effects on deltoid muscle and joint loading. Clinical Relevance: This systematic evaluation has determined that glenosphere lateralization produces marked negative effects on loading outcomes; however, the importance of avoiding scapular notching may outweigh these effects. Humeral lateralization’s ability to decrease the effects of glenosphere lateralization was promising but further investigations are required to determine the effects of combined lateralization on functional outcomes including range of motion