A Study On The Behaviours Of Radial Fretting Damages Of Human Cortical Bone

Endosteal implants in human body are most usual in fixation for clinical implanting. Forces transfer to the interface between bone and implant and then to the body by implanters when human in the activities. Joint replacement, fracture fixation and teeth implant endure the forces are cyclic and oscillatory loading, under which the compact interface will occur fretting. Cortical bone is going to be damaged, which exhibit wear and loss even microcracky on the surface as it is relative weak in fretting. It results that the interface is broken and the implanter is loosening or failure. The cortical bone provides the major fixation to implanter and the mode of action between bone and implanter is multiforce but most radial based on the analysis of mechanics. So the experiments build the radial fretting mode on the bone and implanter in order to study the behaviours of radial fretting damage of the interface.The study used fresh human femur cortical bone against 10mm and 40mm diameter titanium (TA2) balls which were most popular in the current clinical treatment. The tests were on the tension-compression hydraulic machine, which was designed for a ball-on-flat contact configuration. Maximum normal load (F_n) were set as 100N,200N,300N and minimum normal load was 10N to keep the two parts contacting persistently. Study of microcracks used 10mm diameter titanuim ball under the load of 400N. With a constant speed of 12mm/min, we took 1 to 10~5 cycles. The fretting curve printed at the same time represented the kinetics characters of cortical bone in the wear under different loads. The scars morphous were examined by optics microscope (OM), laser Confocal scanning microscope (LCSM) and scanning electron microscope (SEM), different extent damages and deformations were observed; cracks of scar were observed by SEM and projection microscope (PM). Take the experiments of X-ray Diffraction (XRD) and Energy Dispersive Spectrum Analysis (EDS) on the scars to detect whether the cortical bone had the crystal lattice changed and the transferation of titanium from the ball. It concluded that the femur cortical bone was damage in radial fretting and the surface displayed wears and cracks. The damages were in an aggravating manner with the increase of the normal loads and wears caused byΦ10mm TA2 were more severe thanΦ40mm TA2. The curve of normal load against displacement demostrated under a lower normal load (F_n≤100N), contact surface mainly occured elastic deformation. With a mean load (F_n≤200N) the surface of bone occured elastic and plastic deformation. With a high load (F_n≤300N), the plastic deformation in the earlier time on the cortical bone surface transits into an elastic and plastic deformation cooperation manner afterward. The curves of phase cyclic test and dissipated energy displayed that with lower load the dissipated energy was gradually increasing. With the mean load the dissipated energy in a decreasing manner during the whole circling process under the coordination of elastic and plastic deformation. With the high load there was a notable increase with the dissipated energy and displacement, it may caused by formation of cracks. In the morphous graph of scars, the group of big titanium ball had slight damage with three normal loads. The group of small titanium ball displayed evident annular slip region at the mean and high load. The slip region enlarged to central zone with the increasing of load and cyclic numbers. The damage in the center of scars was in a material detachment and adhesion. Micro cracks formed at the edge of contact areas only in the high load condition, it had the consistency with the undulation of dissipated energy. So it concluded that the load is the threshold of crack formation. The observation of cracks on the surface or in the section of bone scars found that cracks emerged with four patterns of propagation on the interface of bone among which the cement line propagating way was most popular. The major area that cracks extended to the deep layer was at the edge of contact zone. The Havers' canals were not only the passage of releasing energy, but also the weak places of producting cracks. It could change the direction of cracks reached the canal into the axial direction of Havers' system. In the XRD and EDS experiments, it found that the damage of cortical bone in the radial fretting could be regarded as the physical changes resulted from broken of crystal lattice, and it had not detected the titanium transferred from the counterbody.Based on these findings, damages on the cortical bone of radial fretting are the important manner to form wears and micro cracks which have the grave influences to the stability of implanter. Loads in the experiments and critical load for crack production were deduced to be conducted as the physical fundament of reducing the stress of the interface. Furthermore, the methods that coated with the same materials on the surface of implanter to avoid the damages caused by radial fretting on the bone-implant interfaces were also discussed.