Multiscale characterization and representation of composite materials during processing.

Given the importance of residual stresses and dimensional changes in composites manufacturing, process simulation has been the focus of many studies in recent years. Consequently, various constitutive models and simulation approaches have been developed and implemented for composites process simulation. In this paper, various constitutive models, ranging from elastic to nonlinear viscoelastic; and simulation approaches ranging from separated flow/solid phases to multiscale integrated phases are presented and their applicability for process simulation is discussed. Attention has been paid to practical aspects of the problem where the complexity of the model coupled with the complexity and size scaling of the structure increases the characterization and simulation costs. Two specific approaches and their application are presented in detail: the pseudo-viscoelastic cure hardening instantaneously linear elastic (CHILE) and linear viscoelastic (VE). It is shown that CHILE can predict the residual stress formation in simple cure cycles such as the one-hold cycle for HEXCEL AS4/8552 where the material does not devitrify during processing. It is also shown that using this simple approach, the cure cycle can be modified to lower the residual stress level and therefore increase the mechanical performance of the composite laminate. For a more complex cure cycle where the material is devitrified during a post-cure, it is shown that a more complex model such as VE is required. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.

In vivo evaluation of calcium polyphosphate for bone regeneration.

Current problems associated with bone allografts include risk of disease transmission, limited availability, and cost. Synthetic scaffolds have been proposed as substitute graft materials to address these issues. Calcium polyphosphate is a novel synthetic scaffold material that has shown good mechanical properties and biocompatibility. Here, we evaluated calcium polyphosphate in terms of its ability to support cell proliferation and differentiation in vivo. Calcium polyphosphate, morsellized cancellous bone, and hydroxyapatite/tricalcium phosphate particles were seeded with marrow stromal cells and implanted subcutaneously in the back of NOD/Scid mice. At 7, 14, and 28 days the samples were harvested and the proliferation characteristics and gene expression were analyzed. All tested graft materials had similar proliferation characteristics and gene expression. The subcutaneous environment had a stronger impact on the proliferation and differentiation of the cells than the scaffold material itself. However, it was shown that calcium polyphosphate is superior to hydroxyapatite/tricalcium phosphate and bone in its ability to support cell survival in vivo. The study confirmed that calcium polyphosphate has potential for replacing morsellized cancellous bone as a graft material for bone regeneration.

Mechanisms of stem subsidence in femoral impaction allografting.

Failure of the femoral component of total hip arthroplasty is often accompanied by bone loss that can pose a significant challenge to the orthopaedic surgeon. Femoral impaction allografting has attractive potential for restoring bone stock in deficient femurs. However, there have been reports of problematic postoperative stem subsidence with this procedure. Subsidence is highly variable among patients, and there is disagreement over the mechanisms that cause it. This article reviews the various mechanisms that can contribute to subsidence in femoral impaction allografting. Variables such as graft density, cement penetration profile, use of synthetic graft substitutes, or other graft additives are discussed, as well as their potential impact on subsidence. Finally, recommendations are made for future studies aiming to reduce the risk of excessive subsidence in femoral impaction allografting.

The effect of abductor muscle and anterior-posterior hip contact load simulation on the in-vitro primary stability of a cementless hip stem.

BACKGROUND: In-vitro mechanical tests are commonly performed to assess pre-clinically the effect of implant design on the stability of hip endoprostheses. There is no standard protocol for these tests, and the forces applied vary between studies. This study examines the effect of the abductor force with and without application of the anterior-posterior hip contact force in the in-vitro assessment of cementless hip implant stability. METHODS: Cementless stems (VerSys Fiber Metal) were implanted in twelve composite femurs which were divided into two groups: group 1 (N = 6) was loaded with the hip contact force only, whereas group 2 (N = 6) was additionally subjected to an abductor force. Both groups were subjected to the same cranial-caudal hip contact force component, 2.3 times body weight (BW) and each specimen was subjected to three levels of anterior-posterior hip contact load: 0, -0.1 to 0.3 BW (walking), and -0.1 to 0.6 BW (stair climbing). The implant migration and micromotion relative to the femur was measured using a custom-built system comprised of 6 LVDT sensors. RESULTS: Substantially higher implant motion was observed when the anterior-posterior force was 0.6BW compared to the lower anterior-posterior load levels, particularly distally and in retroversion. The abductor load had little effect on implant motion when simulating walking, but resulted in significantly less motion than the hip contact force alone when simulating stair climbing. CONCLUSIONS: The anterior-posterior component of the hip contact load has a significant effect on the axial motion of the stem relative to the bone. Inclusion of the abductor force had a stabilizing effect on the implant motion when simulating stair climbing.

Effect of bone graft substitute on marrow stromal cell proliferation and differentiation.

Marrow stromal cells (MSCs) are ideally suited for tissue engineered bone grafts since they have the potential to regenerate bone, but may also maintain the homeostasis of the repaired tissue through their ability for self-renewal. An ideal bone graft substitute should support MSC self-renewal as well as differentiation to ensure complete bone defect regeneration and maintenance. The purpose of this investigation was to determine the effect of different substrate materials on MSC expansion and differentiation. Calcium polyphosphate (CPP), bone and hydroxyapatite/tricalcium phosphate (HA/TCP) were seeded with rat MSCs and maintained in culture conditions that promote cell expansion. At 0, 3, 7, 14, and 21 days cell numbers were determined by measuring their metabolic activity using a MTT assay and the frequency of cycling cells by 24 hr BrdU incorporation. Osteogenic, chondrogenic, and adipogenic marker expression in these cultures was measured by qRT-PCR. An initial drop in cell numbers was observed on all substrates. CPP and bone, but not HA/TCP supported an increase in proliferating cells at day 14 and 21. In addition, no upregulation of mature bone markers was observed in cells cultured on CPP and bone, which suggests that these substrates support the expansion of undifferentiated MSCs. In contrast, cell numbers on HA/TCP decreased with time and only rare BrdU positive cells were observed. This decrease in proliferation correlated with the down regulation of osteogenic progenitor markers and the substantial increase in mature osteocyte markers, indicating that HA/TCP favors MSC differentiation and maturation along the osteogenic lineage.

Influence of cement penetration and graft density on stem stability in impaction allografting: a finite element study.

BACKGROUND: Excessive stem migration is often problematic after impaction allografting. The mechanisms responsible for migration are not known, but achieving a dense graft bed has traditionally been believed to be essential for stem stability. When the stem is cemented into the allograft bed, however, the graft becomes infiltrated with bone cement. Extensive cement penetration into the graft has been observed in previous studies, resulting in regions of cement-endosteum contact. METHODS: This study explored the effects of graft density and cement penetration on stem motion using a finite element model that was validated against experimental data. FINDINGS: Cement penetration has a considerable stabilizing effect on stem motion, whereas graft density is important only when there is no cement-endosteum contact. Stem migration can be attributed primarily to slippage at the endosteum and stem-cement interfaces rather than to shear failure within the graft. INTERPRETATION: Partial cement penetration to the endosteum increases the likelihood of meeting clinical requirements of early implant stability, particularly when a dense graft bed cannot be achieved.

Impaction allografting--the effect of impaction force and alternative compaction methods on the mechanical characteristics of the graft.

Revision total hip replacement with impaction allografting has an attractive potential for restoring bone stock, however, fractures and implant migration remain problematic. Postoperative graft deformation is believed to contribute to migration. Under compressive loading, the fluid in the graft takes up the load initially through fluid pressure that dissipates over time. Given the short duration of an impaction, we proposed two novel graft compaction techniques that allow more time for fluid flow: holding a compression force constant for 90 s (creep technique), and cycles of compressing the graft to a given force and then holding the displacement constant (cyclic relaxation technique). This study examined the effect of the impaction force on the density and mechanical characteristics of the graft, and explored the potential benefit of the proposed alternative compaction techniques. Increasing the impaction force from "low" to "high" increased graft density by 41%, and this translated to stiffness and shear strength increases of 93 and 164%, respectively. The creep technique improved the stiffness and shear strength by 14 and 16%, respectively, when compared with impaction, while the cyclic relaxation technique did not improve the mechanical properties. Although the creep technique could potentially provide a lower risk of intraoperative fracture over the use of larger impaction forces, any clinical benefit of the creep technique warrants further investigation. Our results point to the importance of maximizing impaction forces in impaction allografting surgery.

Structural characteristics of impaction allografting for revision total hip arthroplasty.

BACKGROUND: The impaction allografting procedure for treatment of failed hip reconstructions has shown promising but variable results. The objective of this study was to compare the structural characteristics of revision total hip arthroplasty constructs with impaction allografting (cement+morsellized bone) with all-cement and all-morsellized bone constructs. METHODS: Uniaxial cyclic compression was applied to a simplified uniaxial, parallel, aluminum tube model to simulate normal gait. Applied force and axial stem displacement were recorded to determine stem subsidence and construct stiffness. FINDINGS: Introduction of a small amount of cement into the bone graft, as suggested in an impaction allografting procedure previously reported, makes the construct behave structurally more similar to an all-cemented construct than to an all-bone graft construct. INTERPRETATION: The results suggest that the structural properties achieved in an impaction allografting construct are sensitive to the amount of cement in the graft and that care should be taken clinically to achieve consistent constructs.