Mechanical properties of fused sagittal sutures in scaphocephaly.

BACKGROUND: Craniosynostosis in newborns is caused by the premature closure of the cranial sutures leading to cranial vault deformity. It results in aesthetic imbalance and developmental disabilities and surgery is frequent during the first months of growth. Our study focused on scaphocephaly defined as the premature closure of the sagittal suture. We hypothesised that the effective mechanical properties of sutures were altered as compared to those of the parietal adjacent tissue considered as control. METHODS: The population consisted of seven males and four females (mean age 4.9 months). Sixteen suture samples and thirty-four parietal tissue samples were harvested during corrective surgery and investigated by using three-point bending tests to obtain the structure-stiffness of specimens. An energy model was used to derive the effective Young's modulus. A histological study complemented the experimental protocol. FINDINGS: Fused sutures were thicker than adjacent bone and the natural curvature of sutures did not influence the static mechanical response. The stiffness of stenotic sutures was significantly higher than that of the parietal bone. The effective Young's modulus of stenotic sutures was significantly lower than that of the parietal adjacent tissue. The parietal tissue showed a parallel bone architecture whereas the central stenotic tissue was disorganised with more vascularisation. INTERPRETATION: The stenotic suture differed in structural and mechanical terms from the adjacent bone during calvarial growth in the first year of life. Our study emphasised the alteration of effective tissue properties in craniosynostosis.

Identification of effective elastic modulus using modal analysis; application to canine cancellous bone.

Mechanical properties of cancellous bone is of increasing interest due to its involvement in aging pathologies and oncology. Characterization of fragile bone tissue is challenging and available methodologies include quasi-static compressive tests of small size specimens, ultrasound and indentation techniques. We hypothesized that modal analysis of flexure beams could be a complementary methodology to obtain Young modulus. The sampling methodology was adapted such that the uniqueness of the linear dynamic response was available to determine the elastic modulus from natural frequencies and mode shapes. In a first step, the methodology was validated using a synthetic bone model as control. Then, water-jet cutting allowed collecting fourteen small beam-like specimens in canine distal femurs. X-ray microtomography confirmed the microarchitecture preservation, the homogeneity and the isotropy at the specimen scale to derive effective properties. The first natural frequency in clamped-free boundary conditions was used to obtain mean values of Young modulus, which ranged from 210 MPa to 280 MPa depending on the specimen collection site. Experimental tests were rapid and reproducible and our preliminary results were in good agreement with literature data. In conclusion, beam modal analysis could be considered for exploring mechanical properties of fragile and scarce biological tissues.

Biomechanical response of the CNS is associated with frailty in NPH-suspected patients.

Frailty is known to predict dementia. However, its link with neurodegenerative alterations of the central nervous system (CNS) is not well understood at present. We investigated the association between the biomechanical response of the CNS and frailty in older adults suspected of normal pressure hydrocephalus (NPH) presenting with markers of multiple co-existing pathologies. The biomechanical response of the CNS was characterized by the CNS elastance coefficient inferred from phase contrast magnetic resonance imaging and intracranial pressure monitoring during a lumbar infusion test. Frailty was assessed with an index of health deficit accumulation. We found a significant association between the CNS elastance coefficient and frailty, with an effect size comparable to that between frailty and age, the latter being the strongest known risk factor for frailty. Results were independent of CSF dynamics, showing that they are not specific to the NPH neuropathological condition. The CNS biomechanical characterization may help to understand how frailty is related to neurodegeneration and detect the shift from normal to pathological brain ageing.

Quantitative 3D comparison of biofilm imaged by X-ray micro-tomography and two-photon laser scanning microscopy.

Optical imaging techniques for biofilm observation, like laser scanning microscopy, are not applicable when investigating biofilm formation in opaque porous media. X-ray micro-tomography (X-ray CMT) might be an alternative but it finds limitations in similarity of X-ray absorption coefficients for the biofilm and aqueous phases. To overcome this difficulty, barium sulphate was used in Davit et al. (2011) to enable high-resolution 3D imaging of biofilm via X-ray CMT. However, this approach lacks comparison with well-established imaging methods, which are known to capture the fine structures of biofilms, as well as uncertainty quantification. Here, we compare two-photon laser scanning microscopy (TPLSM) images of Pseudomonas Aeruginosa biofilm grown in glass capillaries against X-ray CMT using an improved protocol where barium sulphate is combined with low-gelling temperature agarose to avoid sedimentation. Calibrated phantoms consisting of mono-dispersed fluorescent and X-ray absorbent beads were used to evaluate the uncertainty associated with our protocol along with three different segmentation techniques, namely hysteresis, watershed and region growing, to determine the bias relative to image binarization. Metrics such as volume, 3D surface area and thickness were measured and comparison of both imaging modalities shows that X-ray CMT of biofilm using our protocol yields an accuracy that is comparable and even better in certain respects than TPLSM, even in a nonporous system that is largely favourable to TPLSM.

Prediction of the intramembranous tissue formation during perisprosthetic healing with uncertainties. Part 1. Effect of the variability of each biochemical factor.

A stochastic model is proposed to predict the intramembranous process in periprosthetic healing in the early post-operative period. The methodology was validated by a canine experimental model. In this first part, the effects of each individual uncertain biochemical factor on the bone-implant healing are examined, including the coefficient of osteoid synthesis, the coefficients of haptotactic and chemotactic migration of osteoblastic population and the radius of the drill hole. A multi-phase reactive model solved by an explicit finite difference scheme is combined with the polynomial chaos expansion to solve the stochastic system. In the second part, combined biochemical factors are considered to study a real configuration of clinical acts.

Prediction of the intramembranous tissue formation during perisprosthetic healing with uncertainties. Part 2. Global clinical healing due to combination of random sources.

This work proposes to examine the variability of the bone tissue healing process in the early period after the implantation surgery. The first part took into account the effect of variability of individual biochemical factors on the solid phase fraction, which is an indicator of the quality of the primary fixation and condition of its long-term behaviour. The next issue, addressed in this second part, is the effect of cumulative sources of uncertainties on the same problem of a canine implant. This paper is concerned with the ability to increase the number of random parameters to assess the coupled influence of those variabilities on the tissue healing. To avoid an excessive increase in the complexity of the numerical modelling and further, to maintain efficiency in computational cost, a collocation-based polynomial chaos expansion approach is implemented. A progressive set of simulations with an increasing number of sources of uncertainty is performed. This information is helpful for future implant design and decision process for the implantation surgical act.

Biomechanical comparison of two locking plate constructs under cyclic torsional loading in a fracture gap model. Two screws versus three screws per fragment.

OBJECTIVES: The number of locking screws required per fragment during bridging osteosynthesis in the dog has not been determined. The purpose of this study was to assess the survival of two constructs, with either two or three screws per fragment, under cyclic torsion. METHODS: Ten-hole 3.5 mm stainless steel locking compression plates (LCP) were fixed 1 mm away from bone surrogates with a fracture gap of 47 mm using two bicortical locking screws (10 constructs) or three bicortical locking screws (10 constructs) per fragment, placed at the extremities of each LCP. Constructs were tested in cyclic torsion (range: 0 to +0.218 rad) until failure. RESULTS: The 3-screws constructs (29.65 ± 1.89 N.m/rad) were stiffer than the 2-screws constructs (23.73 ± 0.87 N.m/rad), and therefore, were subjected to a greater torque during cycling (6.05 ± 1.33 N.m and 4.88 ± 1.14 N.m respectively). The 3-screws constructs sustained a significantly greater number of cycles (20,700 ± 5,735 cycles) than the 2-screws constructs (15,600 ± 5,272 cycles). In most constructs, failure was due to screw damage at the junction of the shaft and head. The remaining constructs failed because of screw head unlocking, sometimes due to incomplete seating of the screw head prior to testing. CLINICAL SIGNIFICANCE: Omitting the third innermost locking screw during bridging osteosynthesis led to a reduction in fatigue life of 25% and construct stiffness by 20%. Fracture of the screws is believed to occur sequentially, starting with the innermost screw that initially shields the other screws.

Measurement of implant deployment and related forces in kyphoplasty by percutaneous approach.

BACKGROUND: The treatment of osteoporotic vertebral compression fractures using a transpedicular approach and cement injection has grown significantly over the last two decades. METHODS: The aim was to study the deployment of an implant dedicated to the vertebral augmentation by percutaneous approach (kyphoplasty). Its kinematics and the related forces have been investigated. A theoretical model of deployment has been proposed and the ancillary was instrumented with strain gauges and Hall effect sensors to measure kinematics and force in the deployment actuator (tensile rod). The methodology was first evaluated ex-vivo in a test-bench with boundary conditions monitored by a tensile machine. Then, a cadaver study was carried out in three lumbar and thoracic vertebral segments of normal and osteoporotic spines. FINDINGS: The relationships between ancillary internal forces, deployment, and cranio-caudal pushing force have been obtained. The test-bench experiment showed quasi-proportional relationship between force distribution and kinematics during the deployment. Ex-vivo cranio-caudal pushing forces were measured. Cadaver studies showed cranio-caudal pushing forces comprised between 100N and 200N. These forces were dependent upon the implant location in the vertebral body and bone stock. INTERPRETATION: The methodology was related to the analysis of load distribution and kinematics of a deployable implant for vertebral augmentation. The ancillary instrumentation contributed to the objective quantification of the surgical technique. The cadaver study in normal and osteoporotic spines exhibited the role of bone properties and implant location in implant deployment. This pilot study showed a methodology to improve the kyphoplasty surgery and patient comfort in clinical routine.

Ex vivo cyclic mechanical behaviour of 2.4 mm locking plates compared with 2.4 mm limited contact plates in a cadaveric diaphyseal gap model.

OBJECTIVES: To compare the mechanical properties of locking compression plate (LCP) and limited contact dynamic compression plate (LC-DCP) constructs in an experimental model of comminuted fracture of the canine femur during eccentric cyclic loading. METHODS: A 20 mm mid-diaphyseal gap was created in eighteen canine femora. A 10-hole, 2.4 mm stainless steel plate (LCP or LC-DCP) was applied with three bicortical screws in each bone fragment. Eccentric cyclic loadings were applied at 10 Hertz for 610,000 cycles. Quasistatic loading / unloading cycles were applied at 0 and 10,000 cycles, and then every 50,000 cycles. Structural stiffness was calculated as the slope of the linear portion of the load-displacement curves during quasistatic loading / unloading cycles. RESULTS: No bone failure or screw loosening occurred. Two of the nine LCP constructs failed by plate breakage during fatigue testing, whereas no gross failure occurred with the LC-DCP constructs. The mean first stiffness of the LCP constructs over the course of testing was 24.0% lower than that of constructs stabilized by LC-DCP. Construct stiffness increased in some specimens during testing, presumably due to changes in bone-plate contact. The first stiffness of LC-DCP constructs decreased by 19.4% and that of locked constructs by 34.3% during the cycling period. A biphasic stiffness profile was observed: the second stiffness was significantly greater than the first stiffness in both groups, which allowed progressive stabilization at elevated load levels. CLINICAL SIGNIFICANCE: Because LCP are not compressed to the bone, they may have a longer working length across a fracture, and thus be less stiff. However, this may cause them to be more susceptible to fatigue failure if healing is delayed.

Effective mechanical properties of diaphyseal cortical bone in the canine femur.

The effective elastic modulus, yield strength, yield strain, ultimate strength, ultimate strain, strain energy density at yield and strain energy density at ultimate failure of femoral diaphyseal cortical bone were investigated on canine femurs. Four femurs representative of the canine population were selected from four statistically-determined clusters based on increasing size and weight comprising the Toy poodle (5 kg), Poodle (12 kg), German shorthaired pointer (25 kg) and Doberman (50 kg). The zones of interest were the lateral, medial, cranial, and caudal quadrants of the mid-diaphysis. Effective mechanical properties were measured using quasi-static three-point bending tests on strips. The averages ± SD were 15.6 ± 2.6 GPa for effective elastic modulus, 174.3 ± 32.1 MPa for yield strength, 0.012 ± 0.003 for yield strain, 251.0 ± 49.1 MPa for ultimate strength, 0.021 ± 0.005 for ultimate strain, 10.7 ± 4.0J m(-3) × 10(5) for strain energy density at Yield and 33.0 ± 14.1 Jm(-3)× 10(5) for strain energy density at ultimate failure. Significant differences were found between dogs and the effective elastic modulus increased with breed weight and size (13.9 GPa for the Toy poodle to 17.2 GPa for the Doberman). The ultimate strength σ(u) and strain energy density at ultimate failure U(u) were significantly lower in the Toy poodle than in the Poodle and German shorthaired pointer indicating that the cortical bone material in the Toy poodle differed from that of the other dogs. Examination of the zones of interest revealed that the cranial quadrant showed the greatest stiffness, whereas strength was highest at the medial site. The caudal cortex was less stiff and strong than the cranial cortex.

Is ICP solid or fluid? In vitro biomechanical model using a fluid-saturated gel.

Intracranial pressure is mainly considered to be hydrostatic pressure, but observations demonstrated that ICP is heterogeneous within brain suggesting the presence of a solid pressure. Brain tissue is a biphasic material composed of solid and fluid phases. We hypothesized that in a saturated porous model, fluid and solid phases yielded two pressures. Our brain model was 0.5% agar gel. A quasi static compression was applied using a tensile machine. Pressures were gauged within the gel using two different microsensors. One sensor (A) has an open sensitive area measuring the total pressure, whereas the other sensor (B) has a pressure-sensitive area design that gauges mainly the fluid pressure. There was very good agreement between the pressure applied to the gel and the pressure inside the gel measured with sensor A. However, sensor B systematically underestimated the pressure in the gel. We assume that sensor A gauged the total pressure, which is the sum of the pore fluid pressure and mechanical stress, whereas sensor B probably measured only the fluid pressure. The difference between the two sensors reflects the solid part of the total pressure. ICP has to be considered to be the sum of fluid pressure and solid stress.

Influence of fluid-flow direction on effective permeability of the vertebral end plate: an analytical model.

Convective transports in the vertebral end plate (VEP) play a significant role in the homeostasis of the spine. A few studies hypothesised that the hydraulic resistance or effective permeability of the VEP could be dependant upon fluid-flow direction. Results were influenced by species, region of interest within the end plate and pathology. Some results were contradictory. We propose an analytical model based on steady-state Newtonian flows in capillary media to develop a phenomenological analysis of convective transport through the VEP. This dependence was established using a biquadratic analytical function involving porosities of subchondral bone, capillary bed and cartilage end plate. Discussion of results provided a theoretical justification for variable and/or contradictory experimental results concerning the amount of energy lost by fluid during its course through the end plate. Tissue porosities and, especially, those relative to the capillary bed could strongly influence the dependence of fluid energy loss on flow direction and could potentially modify tissue homeostasis related to the day and night cycle.

Substructuring and poroelastic modelling of the intervertebral disc.

We proposed a substructure technique to predict the time-dependant response of biological tissue within the framework of a finite element resolution. Theoretical considerations in poroelasticity preceded the calculation of the sub-structured poroelastic matrix. The transient response was obtained using an exponential fitting method. We computed the creep response of an MRI 3D reconstructed L(5)-S(1) intervertebral disc of a scoliotic spine. The FE model was reduced from 10,000 degrees of freedom for the full 3D disc to only 40 degrees of freedom for the sub-structured model defined by 10 nodes attached to junction nodes located on both lower and upper surfaces of the disc. Comparisons of displacement fields were made between the full poroelastic FE model and the sub-structured model in three different loading conditions: compression, offset compression and torsion. Discrepancies in displacement were lower than 10% for the first time steps when time-dependant events were significant. The substructuring technique provided an exact solution in quasi-static behavior after pressure relaxation. Couplings between vertical and transversal displacements predicted by the reference FE model were well stored by the sub-structured model despite the drastic reduction of degrees of freedom. Finally, we demonstrated that substructuring was very efficient to reduce the size of numerical models while respecting the time-dependant behavior of the structure. This result highlighted the potential interest of substructure techniques in large-scale models of musculoskeletal structures.

Cells, growth factors and bioactive surface properties in a mechanobiological model of implant healing.

Interface conditions are of prime importance for implant fixation in the early post-operative period and modelling of specific biochemical interactions at implant surface is still missing. We hypothesized that updating osteoblast adhesion properties and growth factor source in an active zone located at the implant surface was relevant to model biochemical interactions of implant with its environment. We proposed an innovative set of diffusive-convective-reactive equations which relevant parameters were the cell decay factor, the cell motility and the growth factor balance. Initial comparison with histomorphometic results from a stable PMMA canine implant model provided an encouraging base to implement a numerical sensitivity analysis to evaluate the role of three types of bioactive surfaces: acid-etched titanium, coarse grit-blasted acid-etched titanium and coarse grit-blasted acid-etched titanium with RGDS peptide. We found that cell diffusion decrease (acid-etched+RGDS peptide vs. PMMA), and increase of local growth factor fraction (PMMA vs. acid-etched+RGDS peptide), significantly improved the amount of mineralized tissue on the implant surface. When the variation of structural fraction to cell motility and growth factor synthesis was investigated, an envelope pattern with an optimum was obtained but this could be exceeded for strong surface modifications and/or for high growth factor concentrations. The model also confirmed that implant bioactive properties should play a limited role to reduce heterogeneity of new-formed tissue. In conclusion, we suggested that our innovative theoretical approach was relevant to investigate implant fixation and could potentially help in reduction of implant revision.

Characterization of bone-implant fixation using modal analysis: application to a press-fit implant model.

Orthopaedic implant fixation is strongly dependant upon the effective mechanical properties of newly formed tissue. In this study, we evaluated the potential of modal analysis to derive viscoelastic properties of periprosthetic tissue. We hypothesized that Young's modulus and loss factor could be obtained by a combined theoretical, computational and experimental modal analysis approach. This procedure was applied to ex vivo specimens from a cylindrical experimental implant placed in cancellous bone in an unloaded press-fit configuration, obtained after a four week observation period. Four sections each from seven textured titanium implants were investigated. The first resonant frequency and loss factor were measured. Average experimentally determined loss factor was 2% (SD 0.4%) and average first resonant frequency was 2.1 KHz (SD: 50). A 2D axisymmetric finite element (FE) model identified effective Young's modulus of tissue using experimental resonant frequencies as input. Average value was 42 MPa (SD: 2.4) and no significant difference between specimens was observed. In this pilot study, the non-destructive method allowed accurate measure of dynamic loss factor and resonant frequency and derivation of effective Young's modulus. Prior to implementing this dynamic protocol for broader mechanical evaluation of experimental implant fixation, further work is needed to determine if this affects results from subsequent destructive shear push-out tests.

Evaluation of the glenoid implant survival using a biomechanical finite element analysis: influence of the implant design, bone properties, and loading location.

Total shoulder arthroplasty has become a successful surgical procedure through design improvements. However, lucent lines around the glenoid component are of major concern for leading to component loosening. To better understand the mechanism causing loosening, a finite element biomechanical model of an in vivo scapula was developed. The effect of eccentric loading was analyzed on a keel glenoid and a peg glenoid implant. Results indicated that eccentric loading greatly increases stresses in the cement mantle at the bone-cement interface, and no significant difference was predicted between keel and peg implants. The results suggested that eccentric loading is a likely cause for initiation of cracks in the cement layer especially on the posterior side. Moreover, these results, compared with other studies, indicate that geometric and bone properties of the scapula may be more important factors in the success of shoulder arthroplasty than implant design.