RESUMEN
BACKGROUND: Total shoulder arthroplasty is commonly performed to treat glenohumeral osteoarthritis (OA); however, little is understood of the mechanics of the reconstructed OA shoulder. We sought to establish the effects of OA-induced changes in bone density and retroversion angle on load transfer and stress distribution in the bone-implant system of the scapula. METHODS: We developed finite element models of reconstructed healthy and OA scapulas with a virtually implanted glenoid prosthesis design. For the OA scapula, models with uncorrected and corrected retroversion were created. Loads were applied at the center or posteriorly on the glenoid surface. RESULTS: Our results suggest that with reconstruction of the corrected glenoid with a contemporary implant, cement stresses increase and the load transfer pattern changes with eccentric loads. The load transfer and local stresses in the bone-implant system in the retroverted glenoid are less sensitive to changes in loading location. Furthermore, the load transfer in the OA glenoid is less sensitive to the effect of peg proximity to the cortical shell than in the healthy glenoid. CONCLUSION: We provided evidence of how load sharing is altered among healthy, corrected OA, and retroverted OA glenoids. We demonstrated that correction of retroversion in OA glenoids may actually increase the risk for stress shielding and cement failure compared with retroverted glenoids, and OA patients can accommodate shorter pegs because of the higher glenoid bone stiffness in the OA glenoid.
Asunto(s)
Artroplastia de Reemplazo , Osteoartritis/fisiopatología , Articulación del Hombro/fisiopatología , Anciano de 80 o más Años , Fenómenos Biomecánicos , Densidad Ósea , Retroversión Ósea , Cementación , Femenino , Análisis de Elementos Finitos , Humanos , Persona de Mediana Edad , Osteoartritis/cirugía , Diseño de Prótesis , Escápula/fisiopatología , Escápula/cirugía , Articulación del Hombro/cirugía , Estrés Mecánico , Estrés Fisiológico , Soporte de PesoRESUMEN
BACKGROUND: The major complication in nonconforming total shoulder replacement (TSR) is glenoid loosening and is attributed to posteriorly directed humeral head translations. Whether the posterior translations observed clinically are induced by radial mismatch is unclear. The objective of our study was to explain the posterior glenohumeral translations observed clinically after TSR by determining the glenohumeral translation and contact force as a function of radial mismatch. We hypothesized that the posterior direction of glenohumeral translation during scaption would be related to the radial mismatch and that the joint contact force would increase as the radial mismatch increased. METHODS: A 6-degrees-of-freedom computational model of the glenohumeral joint was developed. We determined the muscle forces, joint contact force, and glenohumeral translation for radial mismatches from 1 mm to 20 mm with the shoulder positioned from 20° to 60° of scaption. RESULTS: As the radial mismatch increased, the contact location of the humeral head moved posteriorly and inferiorly. The middle deltoid force decreased by 3%, while the supraspinatus and infraspinatus muscle forces increased by 9% and 11%, respectively. The joint contact force remained relatively constant. CONCLUSIONS: Increased posterior glenohumeral translations were observed with increased radial mismatch. Clinical observations of posterior translation may be attributed to the balancing forces of the middle deltoid, infraspinatus, and supraspinatus muscles. High radial mismatches may lead to eccentric posterior loading on the glenoid component, which could lead to implant loosening and failure.
Asunto(s)
Artroplastia de Reemplazo , Escápula/fisiopatología , Articulación del Hombro/cirugía , Artroplastia de Reemplazo/efectos adversos , Fenómenos Biomecánicos , Simulación por Computador , Análisis de Falla de Equipo , Humanos , Cabeza Humeral/fisiopatología , Músculo Esquelético/fisiopatología , Escápula/cirugía , Articulación del Hombro/fisiopatologíaRESUMEN
BACKGROUND: Glenoid loosening is the primary reason for failure after a total shoulder arthroplasty (TSA), but the failure mechanism is not yet known. This study determined how the load transfer and stress distribution are affected by the introduction of a glenoid implant. METHODS: We developed a finite-element model of a scapula with and without a virtually implanted modern glenoid prosthesis design. Two load magnitudes were considered: normal and high. Loading locations were simulated at the center and at 4 eccentric positions on the glenoid. A metal-backed implant was also simulated to understand the effect of fixation stiffness. RESULTS: In the intact glenoid, for both center and eccentric loading, the majority of stress was distributed in the cancellous bone, whereas after a reconstruction, stresses in that region were lower. Metal-backed implants further decreased the joint load carried by the bone. Stresses in the cement layer increased during eccentric and high-magnitude loading. CONCLUSION: This study provided a basic understanding of the load-sharing phenomenon after a TSA that could explain glenoid loosening failure. Our results suggest that with reconstruction of the glenoid with a contemporary implant, the load transfer pattern is significantly altered, with eccentric and high-magnitude loads increasing stresses in the cement indicating potential for failure. The use of a metal-backed implant reduces the load carried by the bone, which may be detrimental to long-term TSA survival.
Asunto(s)
Artroplastia de Reemplazo , Escápula/fisiopatología , Articulación del Hombro/fisiopatología , Anciano de 80 o más Años , Cementos para Huesos , Cementación , Simulación por Computador , Análisis de Falla de Equipo , Femenino , Análisis de Elementos Finitos , Humanos , Prótesis Articulares , Diseño de Prótesis , Radiografía , Escápula/diagnóstico por imagen , Escápula/cirugía , Articulación del Hombro/diagnóstico por imagen , Articulación del Hombro/cirugía , Estrés MecánicoRESUMEN
We recently reported that masticatory ('superfast') myosin is expressed in jaw-closing muscles of some rodent species. Most mammalian limb muscle fibers express tropomyosin-ß (Tm-ß), along with fast-type or slow-type tropomyosin-ß (Tm-ß), but jaw-closing muscle fibers in members of Carnivora express a unique isoform of Tm [Tm-masticatory (Tm-M)] and little or no Tm-ß. The goal of this study was to determine patterns of Tm and troponin-T (TnT) isoform expression in the jaw-closing muscles of rodents and other vertebrate species that express masticatory myosin, and compare the results to those from members of Carnivora. Comparisons of electrophoretic mobility, immunoblotting and mass spectrometry were used to probe the Tm and fast-type TnT isoform composition of jaw-closing and limb muscles of six species of Carnivora, eight species of Rodentia, five species of Marsupialia, big brown bat, long-tailed macaque and six species of Reptilia. Extensive heterogeneity exists in Tm and TnT isoform expression in jaw-closing muscles between phylogenetic groups, but there are fairly consistent patterns within each group. We propose that the differences in Tm and TnT isoform expression patterns between phylogenetic groups, which share the expression of masticatory myosin, may impart fundamental differences in thin-filament-mediated muscle activation to accommodate markedly different feeding styles that may require high force generation in some species (e.g. many members of Carnivora) and high speed in others (e.g. Rodentia).