Search

Glenoid implants used in anatomic total shoulder arthroplasties typically incorporate peripheral pegs as a design feature to support eccentric loads. These peripheral pegs and the implant‐cement‐bone interface undergo substantial cyclic tensile‐compressive loads during normal activity. Therefore, these pegs are of interest in translating the micromechanics of local implant fixation failure to the biomechanics of gross anatomic failure of the glenoid implant after total shoulder arthroplasty. This study used an in vitro peg‐cement‐bone construct which incorporated bone tissue acquired from osteoarthritic patients undergoing total shoulder arthroplasty. Strain distributions in the peripheral peg‐cement‐bone interfaces were analyzed under loading conditions emulating glenoid implant edge displacements. It was found that tensile strains in the interfaces were highest near the backside‐peg junction and were greater in magnitude than compressive strains. Notably, strains near the peg's fixation channels were relatively low. These results suggest that cracks may initiate around the peg near the backside and travel downward to cause broader fixation failure. [ABSTRACT FROM AUTHOR]

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 occurred. 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. [ABSTRACT FROM AUTHOR]

Subchondral trabecular bone (STB) undergoes adaptive changes during osteoarthritic (OA) disease progression. These changes alter both the mineralization patterns and structure of bone and may contribute to variations in the mechanical properties. Similarly, when images are downsampled – as is often performed in micro finite element model (microFEM) generation – the morphological and mineralization patterns may further alter the mechanical properties due to partial volume effects. MicroFEMs accounting for material heterogeneity can account for these tissue variations, but no studies have validated these with robust full‐field testing methods. As such, this study compared homogeneous and heterogeneous microFEMs to experimentally loaded trabecular bone cores from the humeral head combined with digital volume correlation (DVC). These microFEMs were used to compare apparent mechanical properties between normal and OA STB. Morphological and mineralization patterns between groups were also compared. There were no significant differences in tissue or bone mineral density between groups. The only significant differences in morphometric parameters were in trabecular thickness between groups. There were no significant differences in linear regression parameters between normal and OA STB apparent mechanical properties estimated using heterogeneous microFEMs with an element‐wise bilinear elastic‐plastic constitutive model. Clinical significance: Validated heterogeneous microFEMs applied to STB of the humeral head have the potential to significantly improve our understanding of mechanical variations in the bone that occur during OA progression. [ABSTRACT FROM AUTHOR]

Osteoarthritis (OA) is characterized by morphological changes that alter bone structure and mechanical properties. This study compared bone morphometric parameters and apparent modulus between humeral heads excised from end‐stage OA patients undergoing total shoulder arthroplasty (n = 28) and non‐pathologic normal cadavers (n = 28). Morphometric parameters were determined in central cores, with regional variations compared in four medial to lateral regions. Linear regression compared apparent modulus, morphometric parameters, and age. Micro finite element models estimated trabecular apparent modulus and derived density–modulus relationships. Significant differences were found for bone volume fraction (p < 0.001) and trabecular thickness (p < 0.001) in the most medial regions. No significant differences occurred between morphometric parameters and apparent modulus or age, except in slope between groups for apparent modulus versus trabecular number (p = 0.021), and in intercept for trabecular thickness versus age (p = 0.040). Significant differences occurred in both slope and intercept between density–modulus regression fits for each group (p ≤ 0.001). The normal group showed high correlations in the power‐fit (r2 = 0.87), with a lower correlation (r2 = 0.61) and a more linear relationship, in the OA group. This study suggests that alterations in structure and apparent modulus persist mainly in subchondral regions of end‐stage OA bone. As such, if pathologic regions are removed during joint replacement, computational models that utilize modeling parameters from non‐pathologic normal bone may be applied to end‐stage OA bone. An improved understanding of humeral trabecular bone variations has potential to improve the surgical management of end‐stage OA patients. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:503–509, 2020 [ABSTRACT FROM AUTHOR]