Zhibin Fang

Evaluation of the mechanical behavior of a direct compression molded poroustantalum-UHMWPE construct: a microstructural model

UNLABELLED Monoblock constructs, for example, porous tantalum and ultra high molecular weight polyethylene (UHMWPE), offer an opportunity to combine the fixation properties of metal-backing and the wear properties of UHMWPE for total joint replacement in one component. AIM The objective of this research was to develop a two-dimensional microstructural finite element (FE) model to represent a monoblock-constructs structure and to investigate the influence of various design parameters on the constructs structural response. METHOD A parametric study to evaluate the mechanical behavior of a direct compression molded porous tantalum - UHMWPE construct was performed through a microstructural FE modeling approach. RESULTS Using a factorial analysis of the overall stiffness, it was found that the most significant design parameters were the Trabecular MetalTM porosity and UHMWPE thickness. It was shown that under normal implant operating conditions (linearly elastic stress and small strain epsilon <0.002), increasing the porosity level and polyethylene layer thickness decreased the structures stiffness. CONCLUSIONS Based on different values for apparent elastic modulus from the literature and the parametric analysis, a preliminary UHMWPE thickness could be determined for a proposed design. This approach could help to develop designs in which implant stiffness is sought to be similar to the original stiffness of the biological structure and to better understand the interaction between the main parameters.

A Mutlti-Scale FE Modeling Approach to Investigate the Contact Mechanics and Interfacial Stress of a Low Stiffness Porous Metal-Backed Acetabular Cup

In modern Total Hip Arthroplasty (THA), modular metal-backed acetabular cups consisting of a metal shell backing with porous coatings for fixation and a modular polyethylene liner for articulation are currently the most widely used cementless acetabular cups. Modular acetabular cups give surgeons the flexibility to change femoral head size, liner offset, and liner-lip buildup during hip arthroplasty as well as the ability to change the liner without removing a bone-ingrowth metal shell during revision surgery. However, concerns have been noted with modular metal backed acetabular cups. Poor locking mechanisms have been blamed for backside wear and polyethylene liner dislodgement as well as debris which may lead to osteolysis [1]. In addition, the study of the load transfer around acetabular cups has shown that a stiff metal backing generates high stress peaks around the acetabular rim while it reduces the stresses transferred at the central part of acetabulum potentially causing stress shielding at the dome of acetabulum [2].Copyright

Evaluation of Fixation Stability for MIS Friendly Cemented Tibial Trays Using Finite Element Method: Cement Bonding Condition, Stem Geometry and Tray Stiffness

Due to smaller incision and soft tissue sparing techniques, minimally invasive surgery (MIS) in total knee arthroplasty (TKA) has benefited many patients in terms of shortened hospital stays and quicker recoveries when compared to traditional TKA surgery [1]. A traditional TKA surgery generally requires an 8″ incision while MIS can be performed with a 3–5″ incision. To accommodate the smaller incision used in MIS TKA, MIS friendly tibial tray designs have shorter stems or use modular stem extensions as compared to traditional stemmed tibial tray designs. One of the design considerations is that the shorter stem may compromise the tibial tray’s fixation stability especially for Posterior Stabilized (PS) loading scenarios in which the tibial tray may be subjected to large anterior/posterior (A/P) load through the engagement of the tibial insert and the femoral component and cause tipping of the tray.Copyright

Using Triple-Density Bone Analog to Evaluate the Initial Fixation of a Cementless Tibial Implant

To achieve stable initial fixation and longevity of a cementless tibial implant, low relative motion between implant and bone is vital. Although many studies have been conducted to evaluate the micromotion in-vivo and in-vitro, the methods have been either too complicated to repeat consistently or too simple to truly represent proximal tibial behavior. In this investigation, an experimental micromotion test method was developed to test a commercially available cementless tibial implant using a 3-layer bone analog simulating subchondral, trabecular, and cortical bone and the results compared with the micromotion measurement from a single density polyurethane foam. The results indicate that there is a significant difference in micromotion between the single density foam and the 3-layer bone analog when an applied tipping moment increases.Copyright