骨折髋支撑关节治疗股骨颈骨折的有限元力学分析_英文_

中国组织工程研究与临床康复 第 14 卷 第 13 期 2010–03–26 出版 Journal of Clinical Rehabilitative Tissue Engineering Research March 26, 2010 Vol.14, No.13 ISSN 1673-8225 CN 21-1539/R CODEN: ZLKHAH 2462 1Military Orthopedic Center, Urumqi General Hospital of Lanzhou Military Area Command of Chinese PLA, Urumqi 830000, Xinjiang Uygur Autonomous Region, China; 2Department of Precision Instrument, Tsinghua University, Beijing 100084, China Shi Zhen-man, Chief physician, Military Orthopedic Center, Urumqi General Hospital of Lanzhou Military Area Command of Chinese PLA, Urumqi 830000, Xinjiang Uygur Autonomous Region, China gszlch@yahoo.com. cn Supported by: A Grant from Tenth Five-Year Plan of Chinese PLA, No. 04Z04* Received: 2009-10-13 Accepted: 2009-12-15 (20090412003/G) Shi ZM, Shi J, Wang X, Guo SZ, Wu Y, Peng J. Finite element mechanical analysis on fracture hip supporting joint for treatment of femoral neck fracture. Zhongguo Zuzhi Gongcheng Yanjiu yu Linchuang Kangfu. 2010;14(13): 2462-2466. [http://www.crter.cn http://en.zglckf.com] Finite element mechanical analysis on fracture hip supporting joint for treatment of femoral neck fracture* Shi Zhen-man1, Shi Jiang1, Wang Xin1, Guo Shu-zhang1, Wu Yue2, Peng Jiang1 Abstract BACKGROUND: For treatment of femoral neck fracture, all therapies with the exception of joint replacement encounter the problems including slow healing, poor prognosis, various complications, and unable to bear weight for long time. Fracture hip supporting joint (FHSJ) is a novel unlimited hip support implement that possesses the double functions of fracture fixation and joint supporting and can be used to prevent and treat the complications of femoral neck fracture in young people. OBJECTIVE: To investigate the mechanical effects of FHSJ on treatment of femoral neck fracture. METHODS: Three types of two-dimensional finite element models were constructed by AutoCAD: normal hip (group A), femoral neck fracture fixed with two screws (group B), and femoral neck fracture fixed with two screws and FHSJ (group C). The grids of two-dimensional four nodal point elements were divided by ANSYS (PLANE82). Under the identical condition, the calculations were performed respectively. RESULTS AND CONCLUSION: The stress peak value of femoral head weight-bearing zone was 1.029 and 1.63 MPa in group A and group B, respectively, and that in the group C was 0.1-0.4 MPa. The stress peak value of the screws was 37.186 and 7.474 MPa in the group B and group C, respectively. These results indicate that FHSJ installation based on fixation of multiple screws could promote the recovery of femoral head and neck, which exhibits promising prospects in treatment of femoral neck fracture in young people. INTRODUCTION Fracture of the femoral head is a severe, relatively uncommon injury; typically, it occurs following traumatic posterior dislocation of the hip joint. Diagnosis is aided by a complete history, physical examination, and imaging, including computed tomography. Treatment consists of urgent closed reduction of the dislocated hip followed by nonsurgical or surgical management of any associated fractures. Complications associated with fracture of the femoral head and subsequent treatment include osteonecrosis, posttraumatic osteoarthritis, and heterotopic. For treatment of femoral neck fracture, all therapies with the exception of joint replacement encounter the problems including slow healing, poor prognosis, various complications, and unable to bear weight for long time[1]. Femoral neck fracture poses a significant health care problem all over the world, with an annual incidence of approximately 1.7 million, and the number of femoral neck fracture is expected to triple in the next 50 years. Although the fixation methods reform over 60 years, the nonunion rate still remains at approximately 10%, and the rates of femoral head necrosis and collapse are not decreased. One-year disability rates currently range from 14%-36%[2]. Caring for these patients represents a major global economic burden. Surgical options for the management of femoral neck fractures are closely linked to individual patient factors and to the location and degree of fracture displacement. To prevent and treat the complications of femoral neck fracture in young people, we developed fracture hip supporting joint (FHSJ), a novel unlimited hip supporting implement that possesses the double functions of fracture fixation and joint supporting, and performed finite element mechanical analysis, hopefully investigating the underlying mechanisms of action and providing theoretical evidence for clinical application. MATERIALS AND METHODS Materials Self-made FHSJ of stainless steel; cancellous bone screw with a diameter of 4.62 mm; anterioposterior X-ray image of the hip. Model establishment[3] Three types of two-dimensional finite element model were established according to anterioposterior X-ray image of the hip by AutoCAD. Group A: the normal hip; group B: femoral neck fracture fixed with two screws; group C: femoral neck fracture fixed with two screws and FHSJ. The models included pelvis and proximal femur, which have identical sizes. The diameter of femoral head bony part was 45 mm, the thickness of articular cartilage surface was 2.0 mm, and the thickness of sub-cartilaginous bone plate was 0.6 mm. The stress distribution on coronal plane of the femoral head spherical center was simulated. The pelvic anteversion angle was neglected, which was vertical to the horizontal plane. The Pauwel’s angle of femoral neck fracture was 70° and the two screws were upper-below paralleled through alignment, which had 30°included angle with horizontal plane. The assisting acetabulum was fixed in the iliac bone of acetabular superior border to broaden the acetabulum. The assisting femoral head was fixed in the femur with plate and screws, thus the non-weight-bearing surface of femoral head became weight-bearing surface. At the same time, the assisting acetabulum and assisting femoral head constituted the assisting hip joint (Figure 1). Shi ZM, et al. Finite element mechanical analysis on fracture hip supporting joint for treatment of femoral neck fracture ISSN 1673-8225 CN 21-1539/R CODEN: ZLKHAH 2463 www.CRTER.org Unit type and material characteristics The AutoCAD models established were loaded into ANSYS10.0 software (ANSYS, USA). The grids of two-dimensional four nodal point elements (Figure 2) were divided by ANSYS (PLANE 82). Total 14 regions were given and each region was assumed to be homogeneous, isotropic and linearly elastic material. Elastic modulus and Poisson’s ration are given in Table 1. Hypothesis and calculation Before calculation, the hypothesis of reduced fracture and being contacted each other, which was permitted by the finite element calculation and did not impact the final conclusion. There were two contact pairs being defined: between cortical bones and between cancellous bones. The influence of screw type was still neglected. If the condition that FHSJ, screw, and bone were firmly fixed, without moving each other but with sufficient screw strength to avoid fragmentation was provided, then walking, one foot standing, and suffering force in static state, were simulated. The element of link10 was selected to simulate the muscle tensile force which suffered from tensile force but no pressure force. The abductor tensile force (2 000 N) and adductor tensile force (100 N) as element incipient variable value were loaded. X and Y degree of freedom catenated with PLANE82 element corresponding nodal point degree of freedom, Z degree of freedom was designated as zero. The boundary condition was according to the method of Wei HW, body weight 600 N was loaded onto two keypoints at the right superior angle; the red downward arrow respectively indicated 300 N. In the model, the red upward arrow represented the self-weight of model. In the ANSYS, it was force of inertia (Figure 3). Under the identical condition, the calculations were performed respectively for each fixed model. a: Normal hip b: Fixed with two screws c: Fixed with two screws and fractured hip supporting joint Figure 1 Three types of two-dimensional finite element models a: Normal hip b: Fixed with two screws c: Fixated with two screws and fractured hip supporting joint Figure 2 Three types of the two-dimensional finite element model grids division Table 1 Material characteristics Region Elastic modulus (MPa) Poisson’s ratio Cortical bone Subchondral plate Cancellous bone Articular cartilage Stainless less Pelvis Neck of femur Femur Acetabulum Femoral head Pelvis Femoral head Neck of femur Femur Acetabulum Femoral head Screw Supporting joint of fractured hip 17 000 2 000 17 000 700 1 100 600 600 1 000 600 15 15 210 000 210 000 0.3 0.3 0.28 0.3 0.3 0.3 0.3 0.3 0.3 0.45 0.45 0.3 0.3 Figure 3 Simulation of the weight loading with single leg Shi ZM, et al. Finite element mechanical analysis on fracture hip supporting joint for treatment of femoral neck fracture P.O. Box 1200, Shenyang 110004 cn.zglckf.com 2464 www.CRTER.org RESULTS AND ANALYSIS Results The Von mises stress distribution peak value (MPa) of three finite element models is displayed in Table 2. Stress distribution on hip joint Group A: the stress concentrated on femoral cortical bone, with a peak value of 45.86 MPa. Group B: the stress concentrated on screws, with a peak value of 46.175 MPa. Group C: the stress concentrated on FHSJ, with a peak value of 158.654 MPa. The stress peak value of group C was approximately 5 times that of group A and group B(Figure 4). Stress distribution on proximate femur Group A: the stress was distributed from femoral head ecto-anodic weight-bearing area to trabecular bone and concentrated on calcar femorale via neck of femur. The stress peak value was 6.823 MPa. Group B and group C: the stress band was divided by bone fracture and screw, and a twist moment formed between distal end of fracture (calcar femorale) and screw tip. A shear force pair formed between proximate and distal ends, and the stress concentrated on screws. The stress peak value of screw in the turning curve area was 37.186 MPa in the group B, 7.474 MPa in the group C. The stress peak value of group B was approximately 5 times that of group B (Figure 5). Stress distribution on femoral head Group A: the stress of femoral head ecto-anodic weight-bearing area was 1.029 MPa. The stress was distributed from trabecular bone and concentrated in the junctional zone of femoral head and neck of femur, with a peak value of 1.282 MPa. Group B and group C: the stress band was divided by bone fracture and screw. The stress of group B was concentrated at the femoral head weight-bearing area which was above the upper screw tip, with a peak value of 1.63 MPa. The stress of group C was concentrated at the medial-anodic femoral head, with a peak value of 0.759 MPa. The stress peak value of weight-bearing area was only 0.1-0.4 MPa. The stress peak value of femoral head weight-bearing area of group V was 1.6 times of group A and 5 times of group C (Figure 6). Results demonstrated that when the fractured femoral neck was fixed by two screws, the stress was concentrated on the screws and stress peak value of the femoral head weight-bearing area was higher than the normal femoral head; when the fractured femoral neck was fixed by two screws and FHSJ, the stress was concentrated on the FHSJ, which not only decreased the stress peak value of screw and femoral head weight-bearing area but also made the stress peak value of femoral head moving toward medial side, and the stress direction was in concord with the screw way. Table 2 Vos mises stress distribution peak value among three groups (MPa) Group Calcar femorale A B C Hip joint Proximate femur Proximate end of fracture Screw 45.860 46.175 148.654 6.823 7.596 5.164 0.844 0.574 37.186 7.474 Group Interior femo- ral head A B C Hip joint Femoral head Weight-bearing area 45.860 46.175 148.654 1.282 1.630 0.759 1.029 1.630 0.1-0.4 Group A: normal hip joint; group B: fixed with two screws; group C: fixed with two screws and supporting joint of fractured hip a: Normal hip b: Fixed with two screws c: Fixated with two screws and fractured hip supporting joint Figure 4 Von mises stress distribution on hip joint after loading a: Normal hip b: Fixed with two screws c: Fixated with two screws and fractured hip supporting joint Figure 5 Von mises equivalent stress distribution on proximate femur among three groups Shi ZM, et al. Finite element mechanical analysis on fracture hip supporting joint for treatment of femoral neck fracture ISSN 1673-8225 CN 21-1539/R CODEN: ZLKHAH 2465 www.CRTER.org DISCUSSION Pathological characteristics of femoral neck fracture Special blood supply: There are the internal and external capsule vascular rings in the femoral neck that form the two class branches supplying blood to the femoral head. At the same time, there is no periosteum in the femoral neck, and the vascular rings and branches were tight on the bone surface. The histological findings demonstrate that once the displacement of fractured femoral neck damages the blood vessel and vascular rings, the ischemic incidence rate of femoral head would be up to 85%. Following fracture reduction, fracture union depends on the new vessels of neck getting into the femoral head, which contributes to femoral head surviving and new bone formation. However, the speed of neigenesis was slowly, and fracture union usually needs 7-8 months[4]. Necrosis of the femoral head represents a special form of aseptic bone necrosis because it develops in the high load-bearing region of the hip joint. The mechanisms underlying femoral head necrosis is not precisely known. Characteristic macroscopic findings include articular cartilage detachment in the load-bearing zone immediately below the subchondral osseous lamella, the disperse necrosis of the marrow space and bone cells, and the gradual loss of normal morphology of the femoral head[5-12]. The necrosis rate of the femoral head is 11.9%-37.4% in adults and 21%-87.5% in children. If the necrosis occurs in 1 year after fracture, it is usually detected at 9-12 months. The necrosis rate is 56.8%, 96.08%, and 3.92% at 2-3 years, 5 years, and over 5 years after fracture, respectively. The collapse rate of the femoral head is 24.09%. Time period from initial necrosis to collapse averages 11.1 months. When necrosis and collapse occur, the necrosis zone usually resides in weight-bearing area of the femoral head, and when necrosis stops, the necrosis zone is located outside of the femoral head weight bearing area[13-14]. Special anatomy: Femoral neck fractures in the geriatric patient continue to represent a therapeutic challenge. Despite advances in surgical techniques and medical care, the risk of nonunion and osteonecrosis after fixation have not changed appreciably in the last 50 years[15]. The included angle between the femoral neck and the femoral shaft is 130°. The curved angle and the twist moment make femoral neck receive complicated force. Even though the internal fixation is firm, loosing, breakage, and bone nonunion easily occur. A union rate of 80%-90% is described by most authors. Limb length, rotational and angular deformities can be corrected at the same time. A total hip replacement is probably the best option for patients with 65-80 years old[16], but it is not suitable for young patients[17-18]. To prevent and treat nonunion and necrosis, effective measures must be explored, which would be able to decrease the compressive stress of the femoral head, reduce the complex forces of the femoral head, promote the regeneration of new bone, and protect the long course of treatment. Multiple screws fixation for femoral neck fracture Screws bearing high stress: After the fractured femoral neck is fixed with multiple screws, the stress zone of proximate femur is divided, the femoral head endures the downward pressure with the upward tensile force of gluteus and the supporting force of femoral shaft, and these forces form couple[19-20]. That is to say, the maximum shearing force and the twist moment form at the fracture site, and the stress is concentrated at the screws used for fracture fixation. The stress peak value of the screws is 5 times of the proximate femur. The high stress of the screws would lead to loosing, breakage, and bone nonunion. The nonunion of fracture would result in a problem that newly formed vessels at the neck hardly pass through the fracture interspace and grow into the femoral head, which influence the transformation and union of the femoral head. Femoral head bearing high stress: after the fractured femoral neck is fixed with multiple screws or total hip replacement[19-20], the stress zone of the femoral head is divided, the stress is concentrated on the femoral head weight-bearing area, which is above the upper screw tip, the distance between stress surface and stress concentrated zone of the femoral head is shortened. However, the stress peak value is high up to 1.6 times of the normal femoral head. If the head suffers from the weight loading, collapsed fracture easily occurs. The femoral neck fracture was fixed with multiple screws, which depends on the high stress screw to retain the bone connection, but has few effects on diminishing the complex stress of fracture zone, promoting the regeneration of new bone, and protecting the long course of treatment. Meanwhile, it also induces an increase in the stress peak value of femoral head weight-bearing area higher than the normal femoral head, which damages the femoral head. Multiple screws and FHSJ fixation for femoral neck fracture Stress bypass: the fractured femoral neck was fixed using multiple screws and FHSJ, during which, a stress bypass (it starts from the acetabulum, via the femoral head and neck, to the femur) is established to substitute the intrinsical pathway of stress delivery. The FHSJ suffers from high stress in articulus, diminishes the compressive stress of the femoral head, and a: Normal hip b: Fixed with two screws c: Fixated with two screws and fractured hip supporting joint Figure 6 Von mises equivalent stress distribution on femoral head among three groups Shi ZM, et al. Finite element mechanical analysis on fracture hip supporting joint for treatment of femoral neck fracture P.O. Box 1200, Shenyang 110004 cn.zglckf.com 2466 www.CRTER.org decreases the shearing force and twist moment of the fracture. The fixation using multiple screws and FHSJ makes the stress peak value of screw being 1/5 of the fixation using simple multiple screws, avoiding screw loosing, breakage, and nonunion of fracture. The FHSJ protects union of fracture, i.e., allowing the new vessels of neck