Full-field comparisons between strains predicted by QCT-derived finite element models of the scapula and experimental strains measured by digital volume correlation

Time-elapsed microstructural imaging of failure of the reverse shoulder implant

BACKGROUND

Reverse Shoulder Arthroplasties (RSA) have become a primary choice for improving shoulder function and pain. However, the biomechanical failure mechanism of the humeral component is still unclear. The present study reports a novel protocol for microstructural imaging of the entire humerus implant under load before and after fracture.

METHODS

A humerus specimen was obtained from a 75-year-old male donor. An expert surgeon implanted the specimen with a commonly used RSA implant (Aequalis reversed II, Stryker Orthopaedics, USA) and surgical procedure. The physiological glenohumeral contact force that maximized the distal implant migration was selected from a public repository ( orthoload.com ). Imaging and concomitant mechanical testing were performed using a large-volume micro-CT scanner (Nikon XT H 225 ST) and a custom-made compressive stage. Both when intact and once implanted, the specimen was tested under a pre-load and by imposing a constant deformation causing a physiological reaction load (650 N, 10 degrees adducted). The deformation of the implanted specimen was then increased up to fracture, which was identified by a sudden drop of the reaction force, and the specimen was then re-scanned.

RESULTS

The specimen's stiffness decreased from 874 N/mm to 464 N/mm after implantation, producing movements of the bone-implant interface consistent with the implant's long-term stability reported in the literature. The micro-CT images displayed fracture of the tuberosity, caused by a combined compression and circumferential tension, induced by the distal migration of the implant.

CONCLUSION

The developed protocol offers detailed information on implant mechanics under load relative to intact conditions and fracture, providing insights into the failure mechanics of RSA implants. This protocol can be used to inform future implant design and surgical technique improvements.

Full-field experimental analysis of the influence of microstructural parameters on the mechanical properties of humeral head trabecular bone

Understanding the mechanical properties of trabecular bone within the metaphysis of the proximal humerus is becoming increasingly important for the design of humeral head joint replacement components that prioritize bone preservation. The aim of this study was to perform full-field mechanical testing methods on isolated trabecular bone cores from the humeral head to experimentally measure the local magnitude of strain before macroscopic failure and to characterize the ultimate strength of each core. Isolated cubic trabecular bone cores were extracted from the center of humeral head osteotomies retrieved from (1) patients with end-stage osteoarthritis (OA) undergoing total shoulder arthroplasty (TSA) and (2) normal nonpathologic cadaveric humeral heads. A custom computed tomography (CT)-compatible loading device was used to perform compressive mechanical testing. For 10 of the OA specimens, stepwise loading was performed directly within a microCT scanner and digital volume correlation (DVC) was used to measure full-field strains throughout the trabecular structure. A higher variability in ultimate strength was measured for the trabecular cores retrieved from OA humeral heads (range: 2.8-7.6 MPa) compared to the normal cadaveric humeral heads (range: 2.2-5.4 MPa), but no statistically significant difference between the groups was found (p = 0.06). Ultimate strength was strongly correlated with bone volume fraction (OA r²  = 0.72; normal r²  = 0.76) and bone mineral content (OA r²  = 0.79; normal r²  = 0.77). At the trabecular level, 95th percentile of third principal strains, measured at a subvolume size of 152 µm, exceeded 19,000 µε for each of the 10 specimens (range: -19,551 to -36,535 µε) before macroscopic failure of the cores occured. No strong linear correlations (r²  ≥ 0.50) were found between the median or 95th percentile of DVC third principal strain and the corresponding morphometric parameters of each individual bone core. The results of this study indicate that bone volume fraction and bone mineral content heavily influence the apparent ultimate strength of trabecular bone cores collected from OA patients undergoing TSA. Clinical significance: The strong correlations observed within this study further emphasize the importance of considering bone mineral content or bone volume fraction measurements in assessing the localized risk of trabecular bone fracture for orthopedic applications.

Digital volume correlation for the characterization of musculoskeletal tissues: Current challenges and future developments

Biological tissues are complex hierarchical materials, difficult to characterise due to the challenges associated to the separation of scale and heterogeneity of the mechanical properties at different dimensional levels. The Digital Volume Correlation approach is the only image-based experimental approach that can accurately measure internal strain field within biological tissues under complex loading scenarios. In this minireview examples of DVC applications to study the deformation of musculoskeletal tissues at different dimensional scales are reported, highlighting the potential and challenges of this relatively new technique. The manuscript aims at reporting the wide breath of DVC applications in the past 2 decades and discuss future perspective for this unique technique, including fast analysis, applications on soft tissues, high precision approaches, and clinical applications.

3D strain analysis of trabecular bone within the osteoarthritic humeral head subjected to stepwise compressive loads

Understanding the local mechanical properties of trabecular bone at the humeral head-neck junction is essential for the safe design of stemless humeral head implants. Recent advancements in mechanical testing coupled with volumetric imaging have allowed for the ability to quantify full-field strain distributions throughout trabecular bone. Within this study, digital volume correlation (DVC) was applied to micro-computed tomography images to investigate the local load carrying characteristics of trabecular bone within osteoarthritic (OA) humeral heads subjected to stepwise loading. A multi-pegged indenter was used to transfer loads from a custom-fabricated loading apparatus to trabecular bone on the resection surface of OA humeral head osteotomies retrieved from patients undergoing total shoulder arthroplasty (TSA). In regions of trabecular bone that eventually fractured, third principal strains were significantly higher (95^th percentile third principal strain = -12,558 μstrain, p < 0.001) compared to regions that did not fracture (95^th percentile third principal strain = -7,806 μstrain). As well, bone volume fraction (p = 0.012), trabecular separation (p = 0.014), and trabecular number (p = 0.007) were found to influence the likelihood of trabecular bone fracture. Collectively, this work has led to a deeper understanding of the local load carrying characteristics of trabecular bone specific to patients receiving TSA for osteoarthritis.

Finite element modelling and experimental validation of a total implanted shoulder joint

Background Replicating a total shoulder arthroplasty in laboratory is a difficult task due to complex geometry of the structures and degrees of freedom of the joint. Implanted joint shoulders have been investigated using numerical tools, but models developed lack of experimental validation. The objective of this study was to develop a finite element model that replicated correctly an experimental simulator of an implanted joint shoulder based on the comparison of measured and calculated strains. The methods used include a non-cemented Anatomical Comprehensive© Total Shoulder System that was implanted in 4^th generation composite bones. The finite element model designed replicates adequately the experimental model. Both models included the most important muscles of shoulder abduction and the same boundary conditions (loads, fixation, and interface conditions). Strain gauge rosettes were used to measure strain responses on the shoulder in 90° abduction. The results of linear regression analysis between numerical and experimental results present a high correlation coefficient of 0.945 and a root-mean-square-error of 35 µε, suggesting adequate agreement between the experimental and the numerical models. Small strains were obtained and changes in load distribution from posterior to anterior region were observed. As conclusion we can say that the experiments allowed good replication of the finite element model, and the use of strain gauges is suitable for numerical-experimental validation of bone joints.

In Vivo Deformation and Strain Measurements in Human Bone Using Digital Volume Correlation (DVC) and 3T Clinical MRI

Strains within bone play an important role in the remodelling process and the mechanisms of fracture. The ability to assess these strains in vivo can provide clinically relevant information regarding bone health, injury risk, and can also be used to optimise treatments. In vivo bone strains have been investigated using multiple experimental techniques, but none have quantified 3D strains using non-invasive techniques. Digital volume correlation based on clinical MRI (DVC-MRI) is a non-invasive technique that has the potential to achieve this. However, before it can be implemented, uncertainties associated with the measurements must be quantified. Here, DVC-MRI was evaluated to assess its potential to measure in vivo strains in the talus. A zero-strain test (two repeated unloaded scans) was conducted using three MRI sequences, and three DVC approaches to quantify errors and to establish optimal settings. With optimal settings, strains could be measured with a precision of 200 με and accuracy of 480 με for a spatial resolution of 7.5 mm, and a precision of 133 με and accuracy of 251 με for a spatial resolution of 10 mm. These results demonstrate that this technique has the potential to measure relevant levels of in vivo bone strain and to be used for a range of clinical applications.