Influence of the positioning of a cementless glenoid prosthesis on its interface micromotions.

The positioning of the glenoid component in total shoulder arthroplasty is complicated by the limited view during operation. Malalignment and/or motion of the glenoid component with respect to the bone can be a cause of, or contribute to, failure of the implant. The aim of this paper is to determine the effect of the positioning of a cementless glenoid component on the micromotions between the implant and the bone during normal loading after surgery. For this study a three-dimensional finite element model of a complete scapula with a cementless glenoid component was used. In total, eight positions of the upper arm in both abduction and anteflexion were chosen to represent the patient's arm movement postoperatively. A previously published musculoskeletal model was used to determine the joint and muscle forces on the scapula with implant in each arm position. Five different alignments of the glenoid component (neutral, anterior, inferior, posterior, and superior inclinations), two different implantation depths ('optimal' and 'deeper' implantations), and two bone qualities (healthy and rheumatoid arthritis (RA) bone) were considered. Inclinations of 10 degrees with respect to a neutral alignment did not affect the overall interface micromotions in the optimal implantation depth. However, when the implantation depth was 3 mm deeper, anterior and inferior inclinations were more favourable than a neutral alignment and other inclinations. Micromotions in RA bone were always larger than in healthy bone.

Dynamic mechanical properties of 3D fiber-deposited PEOT/PBT scaffolds: an experimental and numerical analysis.

Mechanical properties of three-dimensional (3D) scaffolds can be appropriately modulated through novel fabrication techniques like 3D fiber deposition (3DF), by varying scaffold's pore size and shape. Dynamic stiffness, in particular, can be considered as an important property to optimize the scaffold structure for its ultimate in vivo application to regenerate a natural tissue. Experimental data from dynamic mechanical analysis (DMA) reveal a dependence of the dynamic stiffness of the scaffold on the intrinsic mechanical and physicochemical properties of the material used, and on the overall porosity and architecture of the construct. The aim of this study was to assess the relationship between the aforementioned parameters, through a mathematical model, which was derived from the experimental mechanical data. As an example of how mechanical properties can be tailored to match the natural tissue to be replaced, articular bovine cartilage and porcine knee meniscus cartilage dynamic stiffness were measured and related to the modeled 3DF scaffolds dynamic stiffness. The dynamic stiffness of 3DF scaffolds from poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) copolymers was measured with DMA. With increasing porosity, the dynamic stiffness was found to decrease in an exponential manner. The influence of the scaffold architecture (or pore shape) and of the molecular network properties of the copolymers was expressed as a scaffold characteristic coefficient alpha, which modulates the porosity effect. This model was validated through an FEA numerical simulation performed on the structures that were experimentally tested. The relative deviation between the experimental and the finite element model was less than 15% for all of the constructs with a dynamic stiffness higher than 1 MPa. Therefore, we conclude that the mathematical model introduced can be used to predict the dynamic stiffness of a porous PEOT/PBT scaffold, and to choose the biomechanically optimal structure for tissue engineering applications.