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Bone Remodelling following THR; Short stems are less likely to lead to bone resorption (Presented at ISTA 2011 | International Society for Technology in Arthroplasty)

In this presentation, we look at bone resorption around hip stems, in particular periprosthetic bone loss. A number of factors influence the amount of bone loss over time & the mechanical environment following total hip replacement (THR) is important; conventional long stem prostheses have been shown to transfer loads distally, resulting in bone loss of the proximal femur. More conservative, short stems have been recently introduced to attempt to better replicate the physiological load distribution in the femur. FE models of two implants, a short (Minihip, Corin, UK) & long (Metafix, Corin, UK) hip stem were used to simulate bone remodeling under a physiological load conditions (20% gait). A strain-adaptive remodelling theory was utilised to simulate remodelling in the bone after virtual implantation. Bone mass was adapted using a site-specific approach in an attempt to return the local remodelling stimulus to the equilibrium stimulus level (calculated from the un-implanted femur). The period of implantation analysed was 2 years. The analysis concluded in there being considerably more bone resorption occurring in Gruen zone 7 with the long stem, where long stem designs disrupt the mechanical environment more than short stems, & lead to a greater bone mineral reduction over time.

Continuum Blue Bone Remodelling FEA ISTA Conference 2011

Finite Element Investigation of the Effect of Spina Bifida on Loading of the Vertebral Isthmus (EFORT Conference 2012)

Spondylolysis (SL) of the lower lumbar spine is frequently associated with spina bifida occulta (SBO). In axial rotation, the intact vertebral arch allows torsional load to be shared between the facet joints, it is hypothesised that In SBO case, the load cannot be shared across the arch, theoretically increasing the mechanical demand of the vertebral isthmus during combined axial loading & rotation. In this study a finite element model of both natural & SBO (L4-S1) including ligaments were loaded axially to 1kN & combined with axial rotation of 3°. Bilateral stresses, alternating stresses & shear fatigue failure on intact & SBO L5 isthmus were assessed & compared. Under 1kN axial load, the von Mises stresses observed in SBO & in the intact cases were very similar, having a maximum at the ventral end of the isthmus that decreases monotonically to the dorsal end. However, under 1kN axial load & rotation, the maximum von Mises stresses observed in the ipsilateral L5 isthmus in the SBO case was significantly higher than the intact case indicating a lack of load sharing across the vertebral arch in SBO. When assessing the equivalent alternating shear stress amplitude, it is estimated that shear fatigue failure will occur in under 70,000 cycles, under repetitive axial load & rotation conditions in the SBO case, while in the intact case, fatigue failure will occur above 10 million cycles. In conclusion, SBO predisposes spondylolysis by generating increased stresses across the inferior isthmus of the inferior articular process, specifically in combined axial rotation & anteroposterior shear.

Continuum Blue Spine SBO FEA EFORT Conference 2012

Multiphase, Dual Polymer Injection Molding & Cooling of Open Cavity (COMSOL Conference, Boston & Milan 2009)

With the advancement of medical devices and implants, many now require more advanced nonlinear, hyper-elastic materials such as elastomers to be extensively utilized in the body. This combined with the need to allow for considerably different, varying and graduated material responses within the three-dimensional device, poses a difficult challenge to manufacturing an elastomeric implant in a single process. A method of producing a complex three-dimensional, homogeneous body with distinct and graduated material properties is assessed using a multi-polymer injection process into an open cavity mold at elevated temperatures. A COMSOL multiphysics model is used to assess the multiphase, dual polymer injection and cooling process to form the required material properties across an implantable body, which is highly dependent on the flow and the mixing of two polymer blends at elevated temperatures.

Continuum Blue COMSOL Conferences - Boston & Milan 2009