Research On Bone Cutting And A Novel Tool Development

With the advances in electrification technology and robot technology,more and more automated surgical systems and medical surgical robots have been used clinically in recent years.Bone cutting surgery is an important operation in surgery,with the development of electrification and automation technology,the cutting methods are becoming more and more diverse,traditional mechanical processing methods such as drilling,grinding,milling and other meothers aare widely applied in the bone surgery.However,the cutting of bone material is essentially a kind of destructive processing the the bone tissue needs to be eliminated and useing its re-growth action to achieve medical purposes.The cutting force,cutting temperature and the abnormal cutting state during the cutting process can cause serious damage to the bone tissue and its surrounding tissue which would restrict the patient’s postoperative rehabilitation.In addition,as an anisotropic material,the bone structure is similar to fiber reinforced composite,and showing a brittle property,thus in the cutting process crack is easily to be gerneated and propergates to the internal layer of bone.Therefore,it is very important to study the mechanism of bone cutting to maintain the integrity of bone tissue structure during the cutting process,while also assisting in the orthopedic technique and related equipment development.In this study,starting with observing the chip formation process in bone orthogonal cutting a three modes cutting mechanism is proposed,as shear cutting,shear-crack cutting and fracture cutting.The cutting deformation was modeled by using the fracture mechanics,and the removal mechanism of different chip morphology during bone orthogonal cutting process was explained based on the model with its anisotropic property.Based on this model,the chip morphology of the bone material cutting process can be predicted,and the critical cutting thickness of the transition between different cutting modes was obtained.The cutting force and the microstructure of the cutting surface were analyzed by experiments.The cutting performance of the bone under different cutting modes and the surface damage were also studied.Based on the structure of bone,the shear strength was modeled with relating to the osteon direction.On the basis of the material model,the dynamic milling force model was established considering the initial osteon orientation,cutting deformation mode,ploughing effect.The experimental results show that the model can accurately predict the dynamic cutting force of the bone milling process,and provide the guiding for the selection of cutting parameters of bone milling process.A special cutting temperature measurement experiment of bone milling process was designed with the thermocouple position optimized in the cutting area to observe the real temperature distribution during bone milling.The distribution characteristics of bone milling temperature were analyzed,and the temperature field distribution model of the milling process of the bone was established by the cutting theory.The validity of the model was verified by experiments.The main factors influencing the thermal effect of bone material during the milling process were analyzed by the temperature field distribution and the depth of temperature penetration of the bone.Based on the cutting mechanism of bone material,a fracture-shear composite milling tool was designed to achieve the high efficiency and low temperature of bone cutting.Through the special spiral surface structure design the tool allows to work under the shear or shear-crack cutting mode at a low feed rate,and at a high feed rate the tool can work under the combined fracture-shear cutting mode,thus to achieve the purpose of reducing the surface damage.The tool prototype were manufactured by using polycrystalline diamond PCD,and the experimental results with comparison of normal tool showed that the proposed tool can improve the cutting surface quality of the bone under the same cutting parameters,reducing the cutting force and cutting temperature,which lead to improve the cutting efficiency and shorten the surgical cutting time.The effect of using acoustic emission signal on the monitoring of bone material cutting process was studied in view of the characteristics of shear-crack and fracure deformation of bone cutting.The wavelet energy ratio of the acoustic emission signal was extracted by wavelet packet decomposition to realize the monitoring of the depth of the cutting layer of bone material.A multi-scale hybriod hidden Markov model with timedomain and frequency-domain axes of wavelet coefficients was established,and the tool wear state of bone material cutting process was monitored by applied this model.The experimental results showed that the acoustic emission signal can effectively characterize the depth of the cutting layer and the tool wear conditon during the cutting process of the bone material.