DVC is a novel full-field and contactless measurement technique for calculating displacements and strains inside bones (Grassi and Isaksson 2015) through the comparison of 3D reconstructions (CT, micro-CT, MRI, etc.) from unloaded and loaded samples. Recent in zero-strain tests to estimate the measurement precision by applying a known state of strain (Palanca, Tozzi et al. 2015) suggested that DVC is suitable to identify regions where bone tissue is yielded (i.e. subjected to high strains). Conversely to reliably measure strain in the physiological range a severe compromise with spatial resolution is necessary (Dall'Ara, Barber et al. 2014, Palanca, Tozzi et al. 2015). In order to use DVC to explore the relationship between the local physiological strain and bone microarchitecture, an error lower than 200 microstrain (an order of magnitude lower than the mean strain) and a spatial resolution of the strain measurement lower than 100 μm is required. The aim of this work is to define if, and to what extend, high-quality images obtained by synchrotron radiation micro computed tomography (SR-μCT) improve the precision of a global DVC approach.

Cylindrical specimens of cortical and trabecular bone were extracted from a fresh bovine femur and embedded in acrylic resin. Both samples were scanned twice without any repositioning (‘repeated scantest’) at beamline l13–2 of Diamond Light Source (Oxford, UK). 4000 projections of 53 ms exposure were collected via fly-scanning with a CdWO₄scintillator-coupled pco.edge 5.5 detector with 4× magnification and an effective pixel size of 1.6μm. Strains were evaluated using a global DVC approach (ShIRT-FE) in two cubic volumes of interest (VOI) of 1,000 voxels in side length, for each specimen, exploring a DVC spatial resolution from 16 to 498 μm. The precision of measurements was evaluated extracting a similar indicator to (Liu and Morgan 2007).

Precision improved with decreasing spatial resolution, confirming a trend similar to that obtained with ‘laboratory source’ μCT on similar specimens (Palanca, Tozzi et al. 2015). To obtain a precision of better than 200 microstrains the cortical and trabecular samples required spatial resolutions of 41 and 80 μm respectively. Comparing these results to those of previous studies, where similar specimens were scanned with ‘laboratory source’ μCT (effective voxel size of the order of ten μm) the errors were vastly reduced (approximately one order of magnitude). In fact, in order to obtain a precision of better than 200 microstrain, spatial resolutions of 550 (cortical) and 480 (trabecular) μm were needed (Dall'Ara, Barber et al. 2014).

This work showed that using high-quality tomograms obtained by synchrotron radiation μCT decreases the measurement uncertainties of a global DVC approach with respect to those obtained with laboratory source μCT. DVC could therefore be used with μCT data to evaluate displacement and strain in the physiological range with remarkable spatial resolution.

Summary Statement

An organ culture experiment was simulated to explore the mechanisms that can link cell death to mechanical overload in the intervertebral disc. Coupling cell nutrition and tissue deformations led to altered metabolic transport that largely explained cell viability measurements.

Purpose: We carried out a biomechanical study by finite element analysis to compare treatment with a plate and treatment with a nail in pseudoarthrosis of the humeral shaft.

Materials and methods: We used a cadaver humerus and the two fixation devices to generate the geometry with design software (CATIA® v4.2). We then modelled the shapes with finite element analysis software (MSC.Patran®) and created three experimental models: healthy humerus, humerus with shaft pseudoarthrosis stabilised with AO plate and humerus with shaft pseudoarthrosis stabilised with locking nail. Both implants were titanium. The three models were subjected to nine different load conditions and the results compared.

Results: The nail model is stiffer than the plate in compression (3002.80 vs 789.68 N/mm), traction (6576.73 vs 1559.90 N/mm) and torsion (4.67 vs 2.73 N/mm). However, the plate model is biomechanically superior to the nail under other load conditions (mediolateral flexion, anteroposterior flexion, anteroposterior shear and mediolateral shear).

Conclusions: Although we can understand and compare the stability of the plate model with the nail, joint clinical and biomechanical studies are needed to determine the minimum stiffness required so that it will not interfere with the process of union under different load conditions.

Aim: Investigate the influence of various types of allograft (from the tibia, femur, and fibula) through finite element analysis to evaluate the best clinical configuration.

Methods: A non-linear 3D finite element model of a lumbar spine L3–L5 was used as a physiologic model (Noailly, 2003). The model was modified with the insertion of a transpedicular instrumentation (Surgival SA, Spain) and the removal of the L4 body and two adjacent discs. CT scans of a femur, tibia and fibula from the same patient were performed. Fragments of each bone were reconstructed and inserted within the model. Four configurations of allografts were investigated: one femur fragment, one tibial fragment, three fragments of fibula, six fragments of fibula. Four types of loadings were applied: compression (1000N), flexion, extension, and rotation (15Nm). Strain and stresses were calculated in large displacement (MARC, MSC Software).

Results: Von Mises stresses within the internal fixator are well below the Yield stress and the fatigue limit and therefore no fracture of the fixator is foreseen. The use of a fixator to create fusion of the two vertebras makes the lumbar spine much stiffer. The geometry and configuration of the allografts have a large influence on the strain and stresses within the adjacent vertebrae with a reduction of strains and stresses. The use of fragments of fibula gives the most stable configuration. However, this is also the configuration that changes most the maximal principal strains within the vertebrae. Results obtained with the femur or the tibia are very similar between each other. However, due to its ellipsoidal geometry, the allograft in tibia gives more asymmetric deformations than the femur.

Conclusion: Allografts harvested from the femur seems to be more reliable and change least the strain and stress distributions within the lumbar spine compared to allografts from the tibia or fibula.

Aim: Investigate the influence of end-plate preparation in a model of corporectomy to evaluate the best biomechanical configuration.

Methods: A non-linear 3D finite element model of a lumbar spine L3–L5 was used as a physiologic model (Noailly, 2003). The model was modified with the insertion of a transpedicular instrumentation (Surgival SA, Spain) and the removal of the L4 vertebral body and two adjacent discs. A femur allograft was inserted anteriorly. Four configurations were investigated: with allograft supported on the entire end-plate, with allograft supported on the half of cartilage endplate thickness, with allograft supported on the subcondral cortical shell and, finally, with allograft supported on the trabecular bone. Four types of loadings were applied: compression (1000N), flexion, extension, and rotation (15Nm). Strain and stresses were calculated in large displacement (MARC, MSC Software).

Results: Results indicate that the preparation of the end-plates has a minor influence on the strain and stresses within the adjacent vertebrae when rigid transpedicular instrumentation was placed. The use of a fixator to create fusion of the two vertebras makes the lumbar spine much stiffer. The resection of the cartilage and support the allograft in the cortical shell changes most the maximal principal strains in the remaining end-plate, and creates a peak stress in the contact area. On the other hand, complete resection of cartilage and subcondral cortical end-plate is the configuration that changes least the maximal principal strains within the adjacent vertebrae.

Conclusion: Preservation of the cortical end-plate may not offer a significant biomechanical advantage in reconstructing the anterior column when rigid transpedicular instrumentation was used.

Purpose: Finite element analysis can be used to assess the behaviour of loaded structures. We used this method to evaluate the influence of glenoid implant design on the behaviour of an osteoarthritic scapula.

Material and methods: A 76-year-old female patient scheduled for a shoulder prosthesis underwent preoperative computed tomography of the osteoarthritic shoulder. Two polyethylene implants were evaluated: one with a triangular stem and the same prosthesis with three studs. 3D reconstruction of the glenoid cavity with the implants was then obtained and processed with the finite elements method. Three loadings were applied to the model: centred loading to reproduce the case of an ideally stable prosthesis with a normal tendinomuscular environment and excentred loading to simulate a deficient rotator cuff or prosthesis instability.

Results: With centred loading, stress remained low, to the order of 7 MPa, at the stem-glenoid cavity interface. Excentered loading produced peak stress on the borders of the glenoid implants, directly under the loading zone and at the tip of the stem, at the bone-cement interface, reaching 20 MPa. The implant tended to bend in the anteroposterior direction producing strong shear forces on the posterior part of the glenoid cavity. These forces caused micromovement at the cement-bone interface. There was no significant difference between the stem and stud implants.

Discussion: Eccentric loading of the glenoid implant appears to have a negative effect on long-term survival, the stress reaching levels greater than the values of cement fatigue fracture. Peak stress was situated on the posterior border of the cement layer due to the small space available between the implant the cortical bone in the posterior part of the osteoarthritic scapula. In this situation, the tip of the stem or the studs tend to come into contact with the posterior cortical of the scapula. When inserting a total shoulder prosthesis, it appears to be more important to keep in mind the geometry and the mechanical properties of the scapula than the implant design.