Creep behavior of hand-mixed Simplex P bone cement under cyclic tensile loading

Acrylic cement, used for the fixation of total hip replacements and other orthopedic implants, is a subject of renewed scientific interest as a result of recent hypotheses about dynamic, long-term mechanical failure mechanisms suspected to play a role in prosthetic loosening. Little is known, however, about the long-term mechanical behavior of cement. In this study, the dynamic creep deformation of hand mixed acrylic cement was examined in laboratory tests. Strain patterns found represented the familiar creep process consisting of a primary, a secondary, and a tertiary creep phase. Specimens dynamically loaded with a maximum stress of 3 MPa from 0 were subject to creep of about 50% of the elastic strain after 250 000 loading cycles. A linear relationship between the logarithmic values of the creep-strain and the number of loading cycles was found. Specimens exposed to higher loads showed significantly higher creep-strains. No relationship could be established between the strain levels and the porosity of the specimens. Specimens dynamically loaded with a maximal stress of 7 or 11 MPa from 0 failed during the tests. The number of loading cycles to failure was similar to fatigue strength data reported in earlier literature. [Journal Article; In English; United States

Mechanical failure of cemented femoral total hip replacement

A review, with 17 refs. Factors which affect mech. failure of cemented femoral total hip replacement, i.e., external load, materials, geometry, and interfaces, are discussed

The effect of vancomycin and tobramycin on the tensile properties of cured low viscosity bone cements

Samples of plain and vancomycin-tobramycin-loaded low viscosity cements were evaluated for their tensile mech. properties. Vancomycin is effective against nearly all resistant pathogens now involved in prosthesis infection. The mech. properties of vancomycin-impregnated cement have never been studied. Tobramycin extends the spectrum to Gram neg. bacteria and has been well studied as used in bone cement. This antibiotic mixt. covers most of the pathogens resulting from arthroplasty thus providing an active local prophylaxis against infection. Specimens of 4 low-viscosity bone cements were machined, radiographed and tested. The addn. of 2 g vancomycin in 40 g cement powder did not significantly affect the tensile properties of the four cements. Simultaneous addn. of vancomycin (2 g) and tobramycin (1 g) significantly decreased the tensile strength and fracture strain of one cement, but the abs. values remained equal to the others or higher and well above the levels reported with std. viscosity cements. Vancomycin-tobramycin fulfill the criteria required for diffusion and antimicrobial activity after admixing in bone cement. The effects of such a combination on the tensile properties of 4 low viscosity bone cements are acceptable. [on SciFinder (R)

Finite element and experimental models of cemented hip joint reconstructions can produce similar bone and cement strains in pre-clinical tests

Finite element (FE) models could be used for pre-clinical testing of cemented hip replacement implants against the damage accumulation failure scenario. To accurately predict mechanical failure, the models should accurately predict stresses and strains. This should be the case for various implants. In the current study, two FE models of composite hip reconstructions with two different implants were validated relative to experimental bone and cement strains. The objective was an overall agreement within 10% between experimental and FE strains. Two stem types with different clinical results were analyzed: the Lubinus SPII and the Mueller Curved with loosening rates of 4% and 16% after 10 yr, respectively (Prognosis of total hip replacement. 63rd Annual Meeting of the American Academy of orthopaedic surgeons, Atlanta, USA). For both implant types, six stems were implanted in composite femurs. All specimens were subjected to bending. The Mueller Curved specimens were additionally subjected to torsion. Bone strains were recorded at 10 locations on the cortex and cement strains at three locations within the cement mantle. An FE model was built for both stem types and the experiments were simulated. Bone and cement strains were calculated at the experimental gauge locations. Most FE bone strains corresponded to the mean experimental strains within two standard deviations; most FE cement strains within one standard deviation. Linear regression between the FE and mean experimental strains produced slopes between 0.82 and 1.03, and R/sup 2/ values above 0.98. Particularly for the Mueller Curved, agreement improved considerably when FE strains were compared to the strains from the experimental specimen used to build the FE model. The objective of overall agreement within 10% was achieved, indicating that both FE models were successfully validated. This prerequisite for accurately predicting long-term failure has been satisfied

Functional biomechanical performance of a novel anatomically shaped polycarbonate urethane total meniscus replacement

\u3cp\u3ePURPOSE: To evaluate the functional biomechanical performance of a novel anatomically shaped, polycarbonate urethane total meniscus implant.\u3c/p\u3e\u3cp\u3eMETHODS: Five human cadaveric knees were flexed between 0° and 90° under compressive loads mimicking a squat movement. Anteroposterior (AP) laxity tests were performed in 30° and 90° flexion. Meniscal kinematics and knee laxity were quantified using roentgen stereophotogrammetric analysis. Tibial cartilage contact mechanics were determined in 90° flexion. Measurements were repeated for the native medial meniscus, the implant, after total medial meniscectomy and allograft transplantation.\u3c/p\u3e\u3cp\u3eRESULTS: The implant and allograft displayed increased posterior and medial displacements compared to the native meniscus, yet no differences were found between the implant and allograft. Meniscal condition did not affect rotational laxity. Compared to the native joint, AP laxity for the implant was increased in 30° flexion, but not in 90°. The implant reduced the mean contact pressure compared to meniscectomy but could not restore contact pressures to native meniscus levels. Compared to the native meniscus, the implant significantly increased the peak pressure, while the contact area was reduced. Contact mechanics of the implant and allograft were never statistically different.\u3c/p\u3e\u3cp\u3eCONCLUSIONS: Biomechanical performance was similar for the implant and allograft. However, both meniscal replacements could not restore outcomes to native meniscus levels or sufficiently improve outcomes after meniscectomy. This was presumably caused by the mobility allowed by the suture-only horn fixation. The similarity of implant and allograft performance suggests that the novel implant has the biomechanical potential to serve as an alternative to meniscal allograft transplantation.\u3c/p\u3

Improving peri-prosthetic bone adaptation around cementless hip stems:a clinical and finite element study

\u3cp\u3eThis study assessed whether the Symax™ implant, a modification of the Omnifit\u3csup\u3e®\u3c/sup\u3e stem (in terms of shape, proximal coating and distal surface treatment), would yield improved bone remodelling in a clinical DEXA study, and if these results could be predicted in a finite element (FE) simulation study.In a randomized clinical trial, 2 year DEXA measurements between the uncemented Symax™ and Omnifit\u3csup\u3e®\u3c/sup\u3e stem (both n=25) showed bone mineral density (BMD) loss in Gruen zone 7 of 14% and 20%, respectively (p<0.05). In contrast, the FE models predicted a 28% (Symax™) and 26% (Omnifit\u3csup\u3e®\u3c/sup\u3e) bone loss. When the distal treatment to the Symax™ was not modelled in the simulation, bone loss of 35% was predicted, suggesting the benefit of this surface treatment for proximal bone maintenance.The theoretical concept for enhanced proximal bone loading by the Symax™, and the predicted remodelling pattern were confirmed by DEXA-results, but there was no quantitative match between clinical and FE findings. This was due to a simulation based on incomplete assumptions concerning the yet unknown biological and mechanical effects of the new coating and surface treatment.\u3c/p\u3

Three-dimensional ultrasound strain imaging of skeletal muscles

\u3cp\u3eIn this study, a multi-dimensional strain estimation method is presented to assess local relative deformation in three orthogonal directions in 3D space of skeletal muscles during voluntary contractions. A rigid translation and compressive deformation of a block phantom, that mimics muscle contraction, is used as experimental validation of the 3D technique and to compare its performance with respect to a 2D based technique. Axial, lateral and (in case of 3D) elevational displacements are estimated using a cross-correlation based displacement estimation algorithm. After transformation of the displacements to a Cartesian coordinate system, strain is derived using a least-squares strain estimator. The performance of both methods is compared by calculating the root-mean-squared error of the estimated displacements with the calculated theoretical displacements of the phantom experiments. We observe that the 3D technique delivers more accurate displacement estimations compared to the 2D technique, especially in the translation experiment where out-of-plane motion hampers the 2D technique. In vivo application of the 3D technique in the musculus vastus intermedius shows good resemblance between measured strain and the force pattern. Similarity of the strain curves of repetitive measurements indicates the reproducibility of voluntary contractions. These results indicate that 3D ultrasound is a valuable imaging tool to quantify complex tissue motion, especially when there is motion in three directions, which results in out-of-plane errors for 2D techniques.\u3c/p\u3

Accelerated 4D self-gated MRI of tibiofemoral kinematics

\u3cp\u3eAnatomical (static) magnetic resonance imaging (MRI) is the most useful imaging technique for the evaluation and assessment of internal derangement of the knee, but does not provide dynamic information and does not allow the study of the interaction of the different tissues during motion. As knee pain is often only experienced during dynamic tasks, the ability to obtain four-dimensional (4D) images of the knee during motion could improve the diagnosis and provide a deeper understanding of the knee joint. In this work, we present a novel approach for dynamic, high-resolution, 4D imaging of the freely moving knee without the need for external triggering. The dominant knee of five healthy volunteers was scanned during a flexion/extension task. To evaluate the effects of non-uniform motion and poor coordination skills on the quality of the reconstructed images, we performed a comparison between fully free movement and movement instructed by a visual cue. The trigger signal for self-gating was extracted using principal component analysis (PCA), and the images were reconstructed using a parallel imaging and compressed sensing reconstruction pipeline. The reconstructed 4D movies were scored for image quality and used to derive bone kinematics through image registration. Using our method, we were able to obtain 4D high-resolution movies of the knee without the need for external triggering hardware. The movies obtained with and without instruction did not differ significantly in terms of image scoring and quantitative values for tibiofemoral kinematics. Our method showed to be robust for the extraction of the self-gating signal even for uninstructed motion. This can make the technique suitable for patients who, as a result of pain, may find it difficult to comply exactly with instructions. Furthermore, bone kinematics can be derived from accelerated MRI without the need for additional hardware for triggering.\u3c/p\u3