Study On 4-axis CNC Milling Of A Patient’s Hip Joint Prosthesis With Irregular Freeform Surface By CT Scanning

The aging population leads to the increase of patients with hip arthroplasty,but the low matching between the clinical standard prosthesis stem and irregular curved surface of human femur inhibits the growth of bone tissue around the prosthesis and results in prosthesis loosening.Although cement is used clinically,its service life is short.Therefore,the hip joint of patients was scanned by computed tomography(CT),stripping the tissue and extracting the geometric data of femoral lumen,to reconstruct the digital model of the irregular free-form surface of hip prosthesis.Based on the point cloud of the reconstructed prosthesis free-form surface,the tool contact of the prosthesis surface is interpolated,the milling tool path is designed,and a four-axis milling method with adaptive and controllable tool attitude is proposed to avoid the superposition of mechanical errors caused by five-axis linkage.Moreover,the microgroove array structure with biological characteristics is processed on the prosthesis surface,and the manufacturing theory and method of personalized hip prosthesis stem with high-precision biological anastomosis are explored.Firstly,in order to solve the trajectory design issue of four-axis machining of irregular free-form surfaces,The matching and reconstruction method of CT scan data of hip joint prosthesis is proposed.In the study,the CT scanning graphics of hip joint were denoised and smoothed,the matching and polar coordinate reconstruction model of an irregular spatial point cloud of the prosthesis was established,the wedge-shaped prosthesis with a small reaming range in operation was designed,and the characteristic point cloud interpolation model of four-axis tool trajectory was constructed.The results show that the area with a distance of less than 1 mm between the prosthesis reconstruction model with a design length of 75 mm and the femoral lumen accounted for 89%,and the average design error was 430μm。Then aiming at the instability of speed and vector in the machining of irregular surfaces,a tool spatial attitude control method for four-axis milling of the spatial irregular surface is proposed.Therefore,two algorithms of tool attitude angle with fixed offset angle and fixed attitude angle are deduced.The theoretical analysis shows that the constant attitude angle control can stabilize the cutting speed of the tool location more than the constant offset angle,but there is a linear error.Therefore,the design of densified tool positions can reduce the shape error by about 26%.The experimental verification of milling at a fixed attitude angle of25° shows that the average shape error of the hip prosthesis can reach 106 μm.It can form a smooth curved surface,and the Ra can reach 0.28-0.42 μm.Next,in order to explore the biocompatibility of the surface microstructure morphology of the titanium alloy prosthesis,the V-shaped tip of the diamond grinding wheel with CNC trimming was used to grind the spherical surface of the millimeter-scale cemented carbide tool with a micro-edged array.The microscopic morphology showed that the micro-cutting edge is integrated and sharp,and the tip radius can reach 2-5 μm.Based on the micro-blade array tool,the micro-groove array milling and parameterized model of the prosthesis surface are proposed,and the micro-blade array structure of the tool can be copied to the surface of the prosthesis in CNC milling with low rotational speed.The results show that although the over-cutting leads to the microgroove angle error,the processing morphology of the micro-groove array structure on the surface of the prosthesis is complete,and its tip radius can reach about 5 μm.Finally,the experimental study on the adhesion and proliferation of osteoblasts on the rough surface of the titanium alloy biomaterial and the surface of the microgroove structure of the prosthetic titanium alloy was carried out.The results show that the arithmetic means height and fractal dimension of roughness did not correlate with cell adhesion.Although the mechanisms of initial adhesion and cell proliferation before and after seven days are different,the microgroove structure and its depth-width ratio can reflect the biocompatibility.The surface of the microgroove array structure is more conducive to the initial adhesion of cells than the rough surface.It can cross the groove to realize the interweaving of cells into a network,especially after seven days,it has obvious proliferation characteristics.