William E. Caler

CYCLIC MECHANICAL PROPERTY DEGRADATION DURING FATIGUE LOADING OF CORTICAL BONE

Fatigue damage accumulation has been demonstrated in living bone and postulated as a stimulus to the bone modeling and remodeling response. Mechanical property degradation is one manifestation of fatigue damage accumulation. This study examines changes in secant modulus and cyclic energy dissipation behavior during axial load-controlled fatigue loading of cortical bone specimens. The findings suggest that secant modulus degradation and cyclic energy dissipation are greatly increased at loading levels above critical damage strain thresholds of 2500 and 4000 mu epsilon in tensile and compressive fatigue, respectively. Tensile and compressive fatigue loading also caused different forms of modulus degradation at loading levels above these thresholds. Bone behaves as a linear viscoelastic material below these thresholds, even after prior property degradation at higher loading levels. Cyclic energy dissipation was proportional to the 2.1 power of the applied effective strain range for all loadings below 2500 mu epsilon. Above 2500 mu epsilon, tensile fatigue loading caused cyclic energy dissipation proportional to the 5.8 power of the applied effective strain range. Compressive fatigue loading dissipated cyclic energy proportional to the 4.9 power of applied effective strain range over 4000 mu epsilon. Lifetime energy dissipation over all fatigue tests to fracture at a single loading level was well fitted by the same power law in the number of cycles to failure raised to the 0.6 power. Loading levels of 2500 mu epsilon in tension and 4000 mu epsilon in compression are within the ranges observed in living animals, and thus these phenomena may play a role in initiating the remodeling response in live bone tissue.

Bone creep-fatigue damage accumulation

Creep and fatigue tests were performed on human femoral cortical bone and the results were compared to a cumulative damage model for bone fracture. Fatigue tests in tension, compression, and reversed loading with a tensile mean stress were conducted at 2 Hz and 0.02 Hz. Load frequency had a strong influence on the number of cycles to failure but did not influence the total time to failure. Bone displayed poor creep-fracture properties in both tension and compression. The fracture surfaces of the tensile creep specimens are distinctly different than those of the compressive specimens. The results suggest that tensile cyclic loading creates primarily time-dependent damage and compressive cyclic loading creates primarily cycle-dependent damage. However, data for load histories involving both tensile and compressive loading indicate lower time to failure than predicted by a simple summation of time-dependent and cycle-dependent damage.

Cycle-Dependent and Time-Dependent Bone Fracture With Repeated Loading

Fatigue tests of human cortical bone (up to 1.74 X 10(6) cycles) were conducted under tension-compression (T-C) and zero-tension (O-T) modes with a 2Hz, stress controlled, sinusoidal loading history. Tensile creep-fracture tests at constant stress levels were also performed. The relationship between the initial cyclic strain range and cycles to failure with the T-C specimens were consistent with that derived previously in low-cycle fatigue under strain control. Using a time-dependent failure model, the creep-fracture data was found to be consistent with previous studies of the influence of strain rate on the monotonic tensile strength of bone. The model also predicted quite well the time to failure for the O-T fatigue specimens, suggesting that creep damage plays an important role in O-T fatigue specimens.

Uniaxial fatigue of human cortical bone. The influence of tissue physical characteristics

Abstract Monotonic tensile tests and uniaxial fatigue tests were conducted on devitalized human cortical bone specimens. Fatigue testing was conducted with strain rantes (Δe) from 0.005 to 0.010 and mean strains of either −0.002, 0.0, or +0.002. The stress range of the first loading cycle (Δσ0) was documented for each specimen. The results showed that the number of cycles to failure (Nf) was independent of mean strain and that fatigue is more strongly controlled by strain range than stress range. Data scatter was much more pronounced in plots of Δσ0 versus Nf than in plots of Δe versus Nf. Statistical analysis showed that much of the data scatter in the Δσ0 versus Nf curve could be explained by a strong positive correlation between specimen modulus and fatigue life. There was a weak negative correlation between specimen porosity and fatigue life and a weak positive correlation between Nf and bone density but no correlation between Nf and bone ash fraction. In analyzing the Δe versus Nf plot, there was a weak positive correlation between Nf and specimen modulus and a weak negative correlation between Nf and bone porosity. No significant correlations were found between fatigue life and bone density or ash fraction. The data also show that the fatigue resistance of bone is much lower than indicated by previous bending fatigue tests. The results suggest that the fatigue strength of cortical bone at 107 cycles may be closer to 7 than 40 MPa as indicated by previous bending fatigue tests.

Resultant loads and elastic modulus calibration of long bone cross sections

Abstract A method of calibrating a long bone cross-section for the resultant axial force and bending moments is presented. At least three single gages must be bonded at circumferential locations at the section of interest. In the laboratory, strains are recorded when known anterior-posterior and lateral-medial bending moments are applied. Strains are also recorded when a known axial force (of unknown eccentricity) is applied. From these measurements one can calculate the resultant loads acting at the section whenever the three simultaneously recorded strain values are known. In addition, the section axial stiffness and effective axial elastic modulus of the section can be determined. This calibration method is especially pertinent in the analysis of in vivo bone strains which have been previously recorded.

In vivo intracortical loading histories calculated from bone strain telemetry

Strain gages have been used by many researchers to measure bone strain in animals during gait (Evans, 1953; Lanyon and Smith, 1969, 1970; Lanyon 1973; Barnes and Pinder, 1974; Cochran, 1974; Turner et al. 1975; Rybicki et al. 1977; Carter et al. 1980). Sumner-Smith et al. (1977) reported the use of a telemetry system to record information from a strain gage rosette attached to the horse metacarpal. The telemetry system allows strain measurements to be taken at various levels of exercise without introducing gait constraints. In this paper, intracortical strain distributions have been calculated for the midshaft canine radius from telemetered and hard-wired in vivo strain gage information.