| 1. Background and Objective:The optimal fitness of the femoral stem into the proximal femoral canal is the vital factor to determine the primary stability and long term survival of the femoral stem in total hip arthroplasty. The fitness of the stem into the femoral canal demands the contour conformity of the two parts, especially in the crucial area which contributes a lot to the axial and rotational stability of the prosthesis. Moreover it needs precise pre-operative plan and intro-operation manipulation because the more conformity it has the more difficulty it will encounter during inserting the stem. Many studies on the morphology of the proximal medullary canal and the calcar femorale are only the two-dimensional or three-dimensional delineation, and the mechanical philosophy was not included in them. So these studies do not help the optimization and operation of the femoral stem of THA so much.This study try to incorporate mechanical philosophy into the morphology study of the femoral canal, and do the femoral stem morphology and operation optimization. The aim is to find the proper femoral stem with the parameters that fit the femoral canal of Chinese people. 2. Materials and Methods2.1 Preparation and Measurement of the femurs50 normal femurs were harvested form the Anatomy department of Jilin University. First, anteroposterior and lateral radiography were taken to all the specimens in order to rule out the ill ones and to make comparative study to the three-dimensional study. Coordinate system was made of wood panel. Then each femur was put in is and the X-ray taken. The X-ray conditions were as follows: 50KV, 0.2mA, and 0.2S. The distance between the panel and the tube is 1.2 meter.The following parameters were measured directly on each specimen: total length of femur, length of femur neck, collodiaphyseal angle, and femur neck anteversion. And the following parameters were measured on the X-rays: diameter of femur head, length of femur neck, collodiaphyseal angle, diameter of isthmus, canal flare index and metaphyseal canal flare index.Then all the specimens were numbered and put in the refrigerator in the temperature of -24℃. Before CT scanning the specimens were thawed in the room temperature for 20 minutes.2.2 Three-dimensional CT scan and measurementWe performed the CT scan with GE Lightspeed 16. The scan began 1cm proximal to the upper edge of femur head and ended 1cm distal to the lower edge of femur condyles. The thickness of scan is 1.25mm, and the conditions were 120Kv, 100mA. The femur specimen was put in the CT machine with its long axis parallel to the direction of scan.MB-ruler v3.4 was used to do the measurements. The following parameters were measured on the selected transverse plane: the direction of the long axis of transverse plane, the direction of the long axis of calcar femorale and its angular relation with the tangent direction of the anterior cortical wall of metaphyseal medullary canal. The following parameters were measured on the selected coronal plane: the inclination angle of the medial wall of metaphyseal medullary canal(IAMMC) and the inclination angle of the lateral wall of metaphyseal medullary canal(IALMC). The following parameters were measured on the selected sagittal plane: the inclination angle of the anterior wall of metaphyseal medullary canal (IAAMC), the intersection angle of the long axe of metaphyseal medullary canal and proximal diaphyseal medullary canal(AMDC), and the intersection angle of the long axe of calcar femorale and proximal diaphyseal medullary canal(ACDC). Also the posterior inclination angle of the longitudinal tangent line of the proximal anterior cortex and the anterior inclination angle of the longitudinal long axis of the calcar femorale were measured on the sagittal plane that passing the middle line of the calcar femorale.On the basis of these data the following parameters were calculated: canal flare index(CFI), metaphyseal canal flare index(MCFI), three-dimensional canal flare index(TCFI), three-dimensional metaphyseal canal flare index(TMCFI), canal torsional angle(CTA).2.3 Three-dimensional reconstruction of proximal medullary canal and the optimization ideas of the femoral stemThree-dimensional reconstruction of proximal medullary canal: First the CT scan data were copied in the DICOM format, and then these were input into the software of Mimics to perform three-dimensional reconstruction. Basing on the 3-D model, the former measured data of the medullary canal, and the rules of design of cementless femoral stem the optimization was going to be done in the following aspects:①The optimization of the proximal part of femoral stem: The shape was modified according to IAMMC, IALMC, IAAMC and ACDC.②The optimization of the distal part of femoral stem: Modify the torsional angle of the long axis of transverse plane of the distal part of femoral stem to enhance the rotational stability of the femoral prosthesis.③Modify the sagittal shape of the femoral stem according to AMDC to further enhance the rotational stability of the femoral prosthesis using the crank arm effect. Modify the proximal and distal shape of femoral stem basing on the results of three-dimensional canal flare index(TCFI) and three-dimensional metaphyseal canal flare index(TMCFI).3. Results3.1 The results of X-rays: The results of this group are parallel to those reported by other domestic scholars. But they are more than the counterparts of those published on westerners.3.2 The results of CT scan and reconstruction: On the plane of femur neck osteotomy there was strong correlation between the intersection angle of the anterior cortical tangent line and the long axis of the osteotomy plane. The long axis of the femoral canal transverse plane rotated anteriorly proximally to distally till it reached sigittal line and then went back. Due to the running direction of the calcar femorale, the long axis of the metaphyseal canal and that of the proximal diaphyseal canal are nearly in coincidence. So the proximal effective canal has no posterior bow. Thus the stem which is a true match to the proximal femur effective canal should be a straight stem. IAMMC, IALMC and IAAMC are vital parameters for the Effective Wedging of the optimized femoral stem. TCFI and TMCFI are more capable on revealing the real shape of the medullary canal than CFI and MCFI.3.3 Reconstruction of the proximal medullary canal by Mimics10.01 revealed some findings that confirmed the results got form 3-D CT scan and reconstruction views.3.4 The optimization ideas were as follows:①Effective wedging is an ideal loading pattern, and its advantage is the stable and effective loading pattern. First wedging is a stable state between and the stem and host bone, and the more the stress the more stability it achieves. Moreover effective wedging can have an optimized vast contact area between the stem and host bone, thus insure the safe and effective load transfer. This study revealed that the shape of the transverse sagittal and coronal planes are all wedging shaped, so the triple-taper shape of the femoral stem and the triple-wedging design philosophy can achieve an ideal fitness of the optimized stem and the medullary canal of Chinese people and to avoid stress shield and stress concentration.②Due to the running direction of the calcar femorale, the long axis of the metaphyseal canal and that of the proximal diaphyseal canal are nearly in coincidence. So the proximal effective canal has no posterior bow. Thus the stem which is a true match to the proximal femur effective canal should be a straight stem.③On the aspect of the integrated shape of the optimized femoral stem: The long axis of the transverse plane of the proximal femoral medullary canal rotated anteriorly from proximal plane to distal plane till it reached sagittal direction then went back. So the optimized stem was integrated torsional angle of 75orelative to the proximal part of the stem in the distal part of it to enhance the rotational stability of the stem. Moreover the larger contact area decreases the possibility of thigh pain. And the angle between the proximal and distal part of the femoral stem was optimized according to AMDC and ACDC, to further enhance the rotational stability by the crank arm effect.④On the transformation of the shape of the femoral stem proximally to distally: The average TCFI is 2.9 and the average TMCFI is 2.3, and the variability of these two parameters was significantly less than that of CFI and MCFI. The size of the femoral stem was optimized according the two parameters.3.5 The optimization ideas of stem insertion.The relationship between the following two angles is strong: the intersection angle of the long axis of the calcar femorale and the long axis of the osteotomy plane, and the angle of the tangent line of the anterior metaphyseal cortex and the long axis of the osteotomy plane. And the relationship between the following two angles is also strong: the posterior inclination angle of the longitudinal tangent line of the proximal anterior cortex and the anterior inclination angle of the longitudinal long axis of the calcar femorale. So the longitudinal and transverse direction of the proximal anterior cortex can be used as reliable guide for broaching the medullary canal and inserting the femoral stem.On the sagittal plane the long axis of the calcar femorale runs anteriorly from proximally to distally. And the intersection angle between it and the long axis of the proximal diaphyseal medullary canal is 4.8o. That is to say that the two axe run almost in the same direction. So the reaming and inserting of the stem should be according to direction of the longitudinal direction of the calcar femorale. And entry point should be on the relatively posterior aspect of the sinus piriformis.4. Conclusion4.1 The long axis of the transverse plane of the proximal femoral medullary canal rotated anteriorly from proximal plane to distal plane, and the rates varied among different parts of the proximal femoral canal.4.2 The proximal femur has posterior bow on appearance. But due to the running direction of the calcar femorale, the long axis of the metaphyseal canal and that of the proximal diaphyseal canal are nearly in coincidence. So the proximal effective canal has no posterior bow. Thus the stem which is a true match to the proximal femur effective canal should be a straight stem.4.3 IAMMC, IALMC and IAAMC are vital parameters for the Effective Wedging of the optimized femoral stem, especially the IAMMC.4.4 TCFI ans TMCFI are more capable on revealing the real shape of the medullary canal than CFI and MCFI.4.5 The longitudinal and transverse direction of the proximal anterior cortex can be used as reliable guide for broaching the medullary canal and inserting the femoral stem.4.6 Reconstruction of the proximal medullary canal by Mimics10.01 revealed some findings that confirmed the results got form 3-D CT scan and reconstruction views.4.7 Because the proximal effective canal has no posterior bow so the stem which is a true match to the proximal effective canal should be a straight stem. The posterior aspect of the stem should be straight and the inclination angle of the other three aspects should be optimized according to the former angular findings.
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