Cj Burgoyne

Relation between age, femoral neck cortical stability, and hip fracture risk

BACKGROUND Hip fracture risk rises 100 to 1000-fold over 60 years of ageing. Loss of resistance to bending is not a major feature of normal ageing of the femoral neck. Another cause of fragility is local buckling or elastic instability. Bones adapt to their local experience of mechanical loading. The suggestion that bipedalism allows thinning of the underloaded superolateral femoral neck cortex arises from the failure of walking to transmit much mechanical load to this region. We aimed to measure whether elastic instability increases greatly with age since it might trigger hip fracture in a sideways fall. METHODS We measured with computed tomography the distribution of bone in the mid-femoral neck of 77 proximal femurs from people who died suddenly aged 20-95 years. We then calculated the critical stress, from the geometric properties and density of the cortical zone most highly loaded in a sideways fall, as a threshold for elastic instability. FINDINGS With normal ageing, this thin cortical zone in the upper femoral neck became substantially thinner. Relative to mean values at age 60 years, female cortical thickness declined by 6.4% (SD 1.1) per decade (p<0.0001), and critical stress by 13.2% (4.3) per decade (p=0.004) in the superoposterior octant compressed most in a sideways fall. Similar, but significantly smaller, effects were evident in men (p=0.004). This thinning compromised the capacity of the femur to absorb energy independently of osteoporosis. Patients with hip fracture had further reduced stability. INTERPRETATION As women age, hip fragility increases because underloading of the superolateral cortex leads to atrophic thinning. Because walking does not sufficiently load the upper femoral neck, the fragile zones in healthy bones may need strengthening, for example with more well targeted exercise.

Femoral neck trabecular bone: loss with aging and role in preventing fracture.

Hip fracture risk rises 100‐ to 1000‐fold over six decades of age, but only a minor part of this increase is explained by declining BMD. A potentially independent cause of fragility is cortical thinning predisposing to local crushing, in which bone tissues material disintegrates at the microscopic level when compressed beyond its capacity to maintain integrity. Elastic instability or buckling of a much thinned cortex might alternatively occur under compression. In a buckle, the cortex moves approximately at right angles to the direction of load, thereby distorting its microstructure, eventually to the point of disintegration. By resisting buckling movement, trabecular buttressing would protect the femoral neck cortex against this type of failure but not against crushing. We quantified the effect of aging on trabecular BMD in the femoral neck and assessed its contribution to cortical elastic stability, which determines resistance to buckling. Using CT, we measured ex vivo the distribution of bone in the midfemoral necks of 35 female and 33 male proximal femurs from cases of sudden death in those 20–95 yr of age. We calculated the critical stress σcr, at which the cortex was predicted to buckle locally, from the geometric properties and density of the cortical zone most highly loaded in a sideways fall. Using long‐established engineering principles, we estimated the amount by which stability or buckling resistance was increased by the trabecular bone supporting the most stressed cortical sector in each femoral neck. We repeated these measurements and calculations in an age‐ and sex‐matched series of femoral necks donated by women who had suffered intracapsular hip fracture and controls, using histological measurements of cortical thickness to improve accuracy. With normal aging, trabecular BMD declined asymmetrically, fastest in the supero‐lateral one‐half (in antero‐posterior projection) of the trabecular compartment. When viewed axially with respect to the femoral neck, the most rapid loss of trabecular bone occurred in the posterior part of this region (supero‐posterior [S‐P]), amounting to a 42% reduction in women (34% in men) over five decades of adult age. Because local cortical bone thickness declined comparably, age had no significant effect on the relative contributions of cortical and trabecular bone to elastic stability, and trabecular bone was calculated to contribute 40% (in men) and 43% (in women) to the S‐P cortex of its overall elastic stability. Hip fracture cases had reduced elastic stability compared with age‐matched controls, with a median reduction of 49% or 37%, depending on whether thickness was measured histologically or by CT (pQCT; p < 0.002 for both). This effect was because of reduced cortical thickness and density. Trabecular BMD was similar in hip fracture cases and controls. The capacity of the femur to resist fracture in a sideways fall becomes compromised with normal aging because cortical thickness and trabecular BMD in the most compressed part of the femoral neck both decline substantially. This decline is relatively more rapid than that of femoral neck areal BMD. If elastic instability rather than cortical crushing initiates the fracture event, interventions that increase trabecular bone in the proximal femur have great potential to reduce fracture risk because the gradient defining the increase in elastic stability with increasing trabecular BMD is steep, and most hip fracture cases have sufficient trabecular bone for anabolic therapies to build on.

Experimental Study of Influence of Bond on Flexural Behavior of Concrete Beams Pretensioned with Aramid Fiber Reinforced Plastics

An experimental program was formulated to investigate the flexural behavior of concrete prestressed with aramid fiber reinforced plastic (AFRP) tendons. The particular focus was the influence of the bond between an AFRP tendon and concrete on the flexural response of a beam. In the main test series, pretensioned concrete beams were cast using either one of two types of AFRP tendons or steel tendons. The influence of bond was studied by testing beams with fully bonded tendons, unbonded tendons, or partially bonded tendons. It was found that, although the fully bonded beams had a high ultimate load capacity, only limited rotation occurred prior to failure. In contrast, large rotations were noted in the unbonded beams, but the strengths of these members were significantly (25%) lower than those of the fully bonded beams. The only beams that achieved both a high rotation capacity and a high ultimate load capacity were the beams with partially bonded tendons. It is suggested that the use of partially bonded tendons could provide the basis of a new design method for concrete beams prestressed with fiber reinforced plastic tendons.

Advanced Composites in Civil Engineering in Europe

Composite materials have been considered for use in structures in Europe for many years. The materials used for structures are all characterised by low creep, as would be expected when the structur...

Statistical variability in the strength and failure strain of aramid and polyester yarns

The scatter in the failure strain, load and stress of high-tenacity polyester and aramid yarns is studied experimentally. From the data, the failure strains of polyester and aramid yarns can be fitted to a two-parameter Weibull distribution. However, the log-log dependence of the strain on the gauge length is best represented by the Watson-Smith modification. Whereas the strengths of polyester yarns are best described by the two-parameter Weibull distribution, those of aramid yarns are best represented by the Gumbel distribution. The effect of strain rate on the strength distribution of aramid yarns is also examined. The strength of aramid yarns decreases slightly with an increase in the strain rate. This is contrary to theoretical predictions but in line with other test data.

Creep behaviour of a parallel-lay aramid rope

A series of dead load tests on a parallel-lay aramid rope has been conducted with the purpose of studying its creep behaviour. The main variable considered in the tests was the applied stress which varied from 24.5%–81.6% ultimate tensile strength. It was found that creep and recovery are adequately described by a logarithmic time law and that the creep coefficient for the material can be considered stress independent. An empirical expression for prediction of long-term creep at ambient temperature is presented.

Modeling the Structural Effects of Rust in Concrete Cover

Vast governmental budgets are spent annually to face corrosion problems of steel reinforcement in concrete bridges attributable to the extensive use of deicing salts. Corrosion controls the lifetime of a bridge, which has two distinct periods. During the first period, chlorides diffuse through the cover. When sufficient chlorides are formed at the rebars, corrosion initiates. This marks the start of the second period, during which rust with higher volume to bare steel is produced. The rust puts pressure on the cover, which finally leads to cover spalling. In this paper, a model is developed to determine the time span of the second period. The model includes a volume compatibility condition that allows for the proper introduction of compaction of all materials that contribute to cover spalling, including the rust. A new condition for marking failure of the cover is also established, based on fracture mechanics and strain energies. Finally, a new formula is proposed for the rate of rust production, which allows for the constant rust production at early and nonlinear diffusion dependant rates at latter stages of corrosion. DOI: 10.1061/(ASCE)EM.1943-7889.0000215.

Expansive cement couplers - a means of pre-tensioning fibre reinforced plastic tendons

Abstract Fibre reinforced plastics describes a group of materials composed of inorganic or organic fibres embedded in a resin matrix. frps are strong, non-magnetic, light-weight and for the most part, non-corrodable. There is great scope for the use of frps as concrete reinforcement and the high strength of the materials is conducive to prestressed applications. However, finding a suitable method of anchoring the tendons without inducing stress concentrations in the fibres has been identified as a problem. The current paper investigates the potential for the use of expansive cement couplers as a means of pretensioning frp tendons. An experimental study was carried out on couplers to join steel reinforcing bars and then extended to include the coupling of frp materials to steel prestress wire.

ANALYSIS OF CONCRETE BEAMS WITH PARTIALLY BONDED COMPOSITE REINFORCEMENT

Beams prestressed with partially bonded fiber-reinforced plastic (FRP) tendons have high strength and rotation capacity but cannot be modeled by conventional techniques. This paper assumes that all deformation takes place at cracks between rigid bodies. By setting up appropriate compatibility and equilibrium equations, the behavior at a single crack can be modeled, which then allows predictions to be made as to which of four possible events will occur next. These lead either to beam failure or to changes in the geometry that can be analyzed using the same techniques. Comparisons are made with test results, and reasonable agreement is shown.

Parafil ropes for prestressing applications

Parafil ropes, which contain a core of parallel filaments of aramid yarns within a polymeric sheath, have been used for prestressing concrete and for a variety of other structural applications. The construction, properties and methods of using the ropes are described, as are the results of tests of beams prestressed with the ropes. A number of real applications of the ropes are also given.