Three-dimensional load measurements in an external fixator

On the basis of a six-degree-of-freedom adjustable fracture reduction hexapod external fixator, a system which can be used for measuring axial and shear forces as well as torsion and bending moments in the fixator in vivo was developed. In a pilot study on 9 patients (7 fresh fractures and 2 osteotomies of the tibia), the load in the fixator during the healing process was measured after 2, 4, 8 and 12 weeks and at fixator removal. The measured values enabled both the type of fracture to be determined as well as the monitoring of the healing process. In well-reduced type A3 fractures small axial (direction of the bone axis) forces were found in the fixator. A2, B2 and C3 fractures showed distinct axial forces, which decreased during the healing process, according to an increasing load transfer over the bone. Bending moments in the fixator showed good correspondence with the clinical healing process, except in the case of a C3 fracture. A combination of bending moment and axial force proved to be particularly suitable to assess fracture healing. In transverse fractures, the well-known resorption phenomenon of bone in the fracture gap at approximately 4 weeks was detected by the system. Compared with other external fixator load measurements in vivo, the hexapod offers the advantage of being able to measure all forces and moments in the fixator separately and with a relatively simple mechanical arrangement. In our opinion, it will be possible to control fracture healing using this system, thereby minimizing radiation exposure from radiographs. Furthermore, the measurement system is a step towards the development of external fixator systems that enable automatic adjustments of the callus mechanical situation ("automatic dynamization") and inform the patients about the optimal weight bearing of their extremity ("intelligent fixator").     

Kinematic simulation of fracture reduction and bone deformity correction under unilateral external fixation

Combined kinematic analysis and graphic models of two unilateral external fixators are presented to simulate and visualize the correction of bone fracture deformities through systematic adjustments of the fixator joints. The models were developed as rigid linkage systems, and the analysis utilized the 4x4 transformation matrices and the kinematic chain theory to obtain the necessary rotations and translations at each joint of the fixator to correct bone deformities at the fracture site. Three-dimensional malalignments with fracture gaps were simulated to correct the deformities. Due to the redundant pair variables in the fixator joints and other problems in obtaining unique solutions, an optimization technique was used to solve the governing linkage loop equations. For each adjustment solution, the bone correction paths were infinite but a unique and optimal reduction path was obtained by applying corrections to all joints simultaneously and in small increments. When the deformity exceeded a certain range, no admissible solution could be obtained, partially due to the limitation of the unilateral fixator configuration and partially due to the restricted joint rotation and translation in the fixator design. The present models and analysis technique can be used to investigate a fixator's adjustability to correct a 3-D bone deformity at a fracture or lengthening site facilitating patient care planning and medical personnel training.     

Adjustable Stiffness,External Fixator for the Rat Femur Osteotomy and Segmental Bone Defect Models

The mechanical environment around the healing of broken bone is very important as it determines the way the fracture will heal. Over the past decade there has been great clinical interest in improving bone healing by altering the mechanical environment through the fixation stability around the lesion. One constraint of preclinical animal research in this area is the lack of experimental control over the local mechanical environment within a large segmental defect as well as osteotomies as they heal. In this paper we report on the design and use of an external fixator to study the healing of large segmental bone defects or osteotomies. This device not only allows for controlled axial stiffness on the bone lesion as it heals, but it also enables the change of stiffness during the healing process in vivo. The conducted experiments have shown that the fixators were able to maintain a 5 mm femoral defect gap in rats in vivo during unrestricted cage activity for at least 8 weeks. Likewise, we observed no distortion or infections, including pin infections during the entire healing period. These results demonstrate that our newly developed external fixator was able to achieve reproducible and standardized stabilization, and the alteration of the mechanical environment of in vivo rat large bone defects and various size osteotomies. This confirms that the external fixation device is well suited for preclinical research investigations using a rat model in the field of bone regeneration and repair.     

新型可透X线骨外固定器的研制

目的治疗近关节骨折、复杂骨折、骨缺损及骨不连等的一种新型可透x线骨外固定器。方法由固定部件、连接杆、万向头、连接杆锁紧螺帽、骨折固定螺钉、固定螺钉锁紧螺帽构成,通过x线能够全方位地观察骨折情况,调整骨外固定器使骨折对位对线。结果可避免因传统金属骨外固定器对术后骨折愈合情况观察,固定安全可靠,特别适用于战时、紧急情况下对骨折的固定,固定不超关节,故可以术后即刻关节功能锻炼。结论固定器设计合理,构思新颖,值得推广应用。     

A rigorous method for evaluation of the 6D compliance of external fixators

External fixators are standard devices to stabilize bone fractures and their compliance aims at producing an interfragmentary motion that promotes rapid and successful healing. While evaluation of their axial compliance is a routine test, the quantification and interpretation of their full 6 × 6 compliance matrix is an extensive and delicate task. In this context, the objective of this study was to develop, validate and demonstrate the potential of a rigorous method to quantify their 6 × 6 compliance matrix. An experimental system was developed to apply six independent static forces and moments to an external fixator in the field of view of two infrared cameras quantifying the induced motion. The system was then tested with a calibration structure which compliance could be calculated analytically and numerically. Finally, the system was applied to compare three configurations of a commercial external wrist fixator. The results of the method proved to be reproducible and highly consistent with the linear elasticity theory in the physiological range of small deformations. A rigorous method for evaluation of the 6D compliance becomes therefore available for research in mechanobiology of fracture healing by external fixation.     

Complications of vascularized fibula graft for reconstruction of long bones

The clinical results and complications of the vascularized fibular graft for the reconstruction of various long bone defects were reviewed in 60 cases. Bony reconstruction was achieved in 57 of the 60 cases; however, various postoperative complications occurred in 54 percent of the cases. One case of arterial thrombosis of an anastomosed vessel and nine cases of venous congestion of the monitoring flap occurred in the early postoperative periods. The authors managed the nine cases of venous congestion of the flap conservatively, and all flaps survived. Partial necrosis of the flap was noted in eight of these nine cases, but additional surgical intervention was required in only four cases. Treatment included a gastrocnemius musculocutaneous flap in one case and a full-thickness skin graft in three cases. The vascularized fibula survived and bony fusion was achieved in all of these cases. The one case of arterial thrombosis resulted in graft failure due to a delay in the decision to perform a thrombectomy. Graft fracture occurred in 13 cases as the mechanical stress to the graft increased. In two cases of femoral reconstruction, graft fracture occurred during dynamization of the graft, despite the use of an Ilizarov external fixator. Correct alignment between the recipient bone and the external fixator is a prerequisite to preventing graft fracture. Vascularized fibular grafting offers the patient a great deal of benefit; however, this graft has a concomitant high risk of complications. Great attention to detail must be paid to prevent postoperative complications.     

Development and validation of a new approach for computer-aided long bone fracture reduction using unilateral external fixator

An innovative computer-aided method to plan and execute long bone fracture reduction using Dynafix unilateral external fixator (EF) is presented and validated. A matrix equation, which represents a sequential transformation from proximal to distal ends, was derived and solved for the amount of rotation and translation required at each EF joint to correct for a displaced fracture using a non-linear least square optimization method. Six polyurethane-foam models of displaced fracture tibiae were used to validate the method. The reduction accuracy was quantified by calculating the residual translations (xr, yr, zr), the residual displacement (dr), and the residual angulations (alphar, betar, gammar) based on the X-Y-Z Euler angle convention. The experiment showed that the mean+/-S.D. of alphar, betar, gammar, xr, yr, zr and dr were 1.57+/-1.14 degrees, 1.33+/-0.90 degrees, 0.71+/-0.70 degrees, 0.98+/-1.85, 0.80+/-0.67, 0.30+/-0.27, and 0.50+/-0.77 mm, respectively, which demonstrated the accuracy and reliability of the method. Instead of adjusting the fixator joints in-situ, our method allows for off-site adjustment of the fixator joints and employs the adjusted EF as a template to guide the surgeons to manipulate the fracture fragments to complete the reduction process. Success of this method would allow surgeons to perform fracture reduction more objectively, efficiently and accurately yet reduce the radiation exposure to both the involved clinicians and patients and lessen the extent of periosteum and soft tissue disruption around the fracture site.     

Numerical Simulation of Callus Healing for Optimization of Fracture Fixation Stiffness

The stiffness of fracture fixation devices together with musculoskeletal loading defines the mechanical environment within a long bone fracture, and can be quantified by the interfragmentary movement. In vivo results suggested that this can have acceleratory or inhibitory influences, depending on direction and magnitude of motion, indicating that some complications in fracture treatment could be avoided by optimizing the fixation stiffness. However, general statements are difficult to make due to the limited number of experimental findings. The aim of this study was therefore to numerically investigate healing outcomes under various combinations of shear and axial fixation stiffness, and to detect the optimal configuration. A calibrated and established numerical model was used to predict fracture healing for numerous combinations of axial and shear fixation stiffness under physiological, superimposed, axial compressive and translational shear loading in sheep. Characteristic maps of healing outcome versus fixation stiffness (axial and shear) were created. The results suggest that delayed healing of 3 mm transversal fracture gaps will occur for highly flexible or very rigid axial fixation, which was corroborated by in vivo findings. The optimal fixation stiffness for ovine long bone fractures was predicted to be 1000–2500 N/mm in the axial and >300 N/mm in the shear direction. In summary, an optimized, moderate axial stiffness together with certain shear stiffness enhances fracture healing processes. The negative influence of one improper stiffness can be compensated by adjustment of the stiffness in the other direction.     

Measurement of fracture movement in patients treated with unilateral external skeletal fixation

Axial movement occurring at the fracture site has been determined in a group of healing tibial fractures treated by external skeletal fixation. Fracture movement was determined via a strain gauge transducer which was attached to the column of the external fixator and measured the deflection of the bone screw adjacent to the fracture site and the active loading or weight bearing given by the patient to the fractured limb was monitored using a force platform. The results for 27 subjects show that, with a rigid unilateral fixator, the axial movement occurring at the fracture site was initially small (mean = 0.28 mm at 5 weeks post fracture). This movement increases to reach a mean maximum value of 0.43 mm at 11 weeks post-fracture and then decreases, despite increased weight bearing, as fracture healing progresses. In the early stages of healing, the movement can be increased slightly if the fixator is fitted with a module which permits additional fracture site movement, although the resultant increase in movement is only a small proportion of the potential available with this module.     

A 3D computational simulation of fracture callus formation: influence of the stiffness of the external fixator

The stiffness of the external fixation highly influences the fracture healing pattern. In this work we study this aspect by means of a finite element model of a simple transverse mid-diaphyseal fracture of an ovine metatarsus fixed with a bilateral external fixator. In order to simulate the regenerative process, a previously developed mechanobiological model of bone fracture healing was implemented in three dimensions. This model is able to simulate tissue differentiation, bone regeneration, and callus growth. A physiological load of 500 N was applied and three different stiffnesses of the external fixator were simulated (2300, 1725, and 1150 N/mm). The interfragmentary strain and load sharing mechanism between bone and the external fixator were compared to those recorded in previous experimental works. The effects of the stiffness on the callus shape and tissue distributions in the fracture site were also analyzed. We predicted that a lower stiffness of the fixator delays fracture healing and causes a larger callus, in correspondence to well-documented clinical observations.     

锁骨骨折内固定失效的原因分析及对策

目的：探讨锁骨骨折切开复位内固定术后内固定失效的原因,并寻找补救方案。方法：选择我院2007年5月~2010年5月收治的184例锁骨骨折患者,其中男性123例,女性61例,年龄24~76岁,对手术切开复位内固定失败病例的内固定方法进行对比,分析内固定失效的原因,并选取记忆合金环抱器或天鹅型记忆接骨器作为再次手术的内固定器械,分析其临床疗效。结果：本组所有病例均获得6~24个月随访,所有患者首次手术均行钢板内固定治疗,其中应用重建钢板治疗48例,解剖钢板治疗86例,锁骨钩钢板治疗50例,3例术后发生钢板或螺钉断裂患者选用记忆合金环抱器重新手术内固定治疗,2例痊愈,1例记忆合金环抱器再次发生断裂,改用天鹅型记忆接骨器治疗获得痊愈。3例骨不连患者均选择天鹅型记忆接骨器配合植骨内固定治疗,术后恢复良好。结论：锁骨骨折切开复位内固定术后内固定失败的原因主要与所使用内固定技术不合理及患者早期不正确的功能锻炼有关。记忆合金环抱器和天鹅型记忆接骨器均可作为钢板内固定失败术后的补救方案,但天鹅型记忆接骨器较记忆合金环抱器可提供更好的纵向加压作用,治疗钢板断裂及骨不连患者更为可靠。     

Biomechanical assessment and clinical analysis of different intramedullary nailing systems for oblique fractures

The aim of this study is to evaluate the fracture union or non-union for a specific patient that presented oblique fractures in tibia and fibula, using a mechanistic-based bone healing model. Normally, this kind of fractures can be treated through an intramedullary nail using two possible configurations that depends on the mechanical stabilisation: static and dynamic. Both cases are simulated under different fracture geometries in order to understand the effect of the mechanical stabilisation on the fracture healing outcome. The results of both simulations are in good agreement with previous clinical experience. From the results, it is demonstrated that the dynamization of the fracture improves healing in comparison with a static or rigid fixation of the fracture. This work shows the versatility and potential of a mechanistic-based bone healing model to predict the final outcome (union, non-union, delayed union) of realistic 3D fractures where even more than one bone is involved.     

Angle-fixed plate fixation or double-plate osteosynthesis in fractures of the proximal humerus: a biomechanical study

Internal fixation of fractures of the proximal humerus needs a high stability of fixation to avoid secondary loss of fixation. This is especially important in osteoporotic bone. In an experimental study, the biomechanical properties of the angle-fixed Philos plate (internal fixator) and a double-plate osteosynthesis using two one-third tubular plates were assessed. The fracture model was an unstable three-part fracture (AO type B2). Eight pairs of human cadaveric humeri were submitted to axial load and torque. In the first part of the study, it was assessed to which degree the original stiffness of the humeri could be restored after the osteotomy by the osteosynthesis procedure. Subsequently, subsidence during 200 cycles of axial loading and torque was analysed. During axial loading, the Philos plate was significantly stiffer and showed less irreversible deformation. Two double-plate fixations, but none of the Philos plate osteosynthesis, failed. During torsion, there were no significant differences between the two implants. From the biomechanical point of view, the angle-fixed Philos plate represents the implant of choice for the surgical fixation of highly unstable three-part fractures of the proximal humerus, as the internal fixator system is characterised by superior biomechanical properties.     

Evaluation of linear finite-element analysis models' assumptions for external fixation devices.

Linear finite-element models (FEMs) have enjoyed an increased use in orthopaedic research, including the use for modeling external fixation devices. These fixator FEMs depend on a number of basic assumptions concerning the overall fixation frame stability and the components' rigidity. Among the more important ones are: (i) rigid fixation at both ends of the pin and sidebar; (ii) that the sidebar can be treated essentially as a rigid entity, with all bending occurring in the bone pins; and (iii) that the system can be treated as linearly elastic. Prior work done by the authors questions some of these assumptions. Thus, this study sought an empirical evaluation of the validity of some of these a priori assumptions. A Hoffmann single half-frame was tested in its standard form and then according to a stepwise protocol wherein the frame was welded to eliminate any possible points of instability. These tests looked at the stability and rigidity in various modes (axial compression, torsion, and medial-lateral and anterior-posterior four-point bending). The basic assumptions concerning the frame stability, frame rigidity and the frame's response to loads were found to be erroneous. Component failure was common under minimal loads and statistically significant differences (p less than 0.05) of up to 75% were noted in frame rigidity among the various frame forms tested. Thus, considerable caution must be exercised when employing the FEM technique for evaluating the fixator properties.     

Rigidity and stress analyses of external fracture fixation devices—A theoretical approach

The rigidity and stresses in external fracture fixation devices were studied by means of the finite element method. Different geometries and material parameters were simulated using a beam element model. Axial, bending and torsional loads were applied through the bone ends and the displacement obtained at the fracture sites was used to calculate the fracture fixation stiffness. The key parameters which increased fixation rigidity were identified. High pin stresses were predicted under certain application conditions. Possible clinical implications for the use of such apparatus are discussed in the light of bone fracture healing. The present results are expected to have a significant impact on future design modifications and clinical applications of this popular instrument in orthopedic surgery and traumatology.     

Design and performance of a fracture monitoring transducer

The answer to the question, ‘when is a fracture healed?’ is not simple, since the healing process is progressive and it is not possible to specify a time when the fracture can be said to have healed. In the past the assessment of fracture healing has, in the main, been subjective, relying upon the skill of the interpreter. A more objective method would be an advantage for many reasons, and since the bone is intended to be load bearing it is reasonable to assess healing by measuring the mechanical integrity of the bone. to do this a ‘clamp on’ transducer has been developed which, when fitted to the support column of an external fixator, enables the stiffness of a fracture to be determined during the healing process. Over the past 6 years this system has been used for both clinical and research work. It has enabled various forms of treatment to be evaluated in terms of ‘rate of healing’ and it also indicates the safe point at which the fixator can be removed.     

股骨骨折术后1年随访骨愈合模型的有限元分析

目的：对股骨骨折髓内钉术后1年骨愈合模型快速建模,通过有限元分析研究对比术前术后模型,通过术前判定内固定取出后骨折断端是否断裂。方法：运用Mimics、Geomagic Studio、Abaqus等软件采用快速个体化建模方法对股骨骨折髓内钉术后1年内固定取出术前后的多层螺旋CT数据进行快速建立模型,术前模型模拟剥除钢板后进行有限元分析,施加人体单腿站立时的静力载荷和约束,并将分析结果与术后模型进行对比,观察米塞斯应力分布情况、最大值及其所处部位。结果：按照材料属性进行区别显示米赛斯应力的最大值及最小值,在不同应力载荷下,手术前后各类型材料的米赛斯应力最大值及最小值部位相同,各类型材料中,最大值均没有位于骨折断端,不同方法的最大应力值部位相近,均在股骨中远端1/4交界处,手术前后应力分布基本相同。结论：采用个体化建模方法可以对骨折内固定取出前的骨愈合模型进行运算分析,快速预判术后是否导致骨折断端断裂。     

A new device to control mechanical environment in bone defect healing in rats

Mechanical conditions have a significant influence on the biological processes of bone healing. Small animal models that allow controlling the mechanical environment of fracture and bone defect healing are needed. The aim of this study was to develop a new animal model that allows to reliably control the mechanical environment in fracture and bone defect healing in rats using different implant materials. An external fixator was designed and mounted in vitro to rat femurs using four Kirschner-wires (titanium (T) or steel (S)) of 1.2mm diameter. The specimens were distracted to a gap of 1.5mm. Axial and torsional stiffness of the device was tested increasing the offset (distance between bone and fixator crossbar) from 5 to 15mm. In vivo performance (well-being, infection, breaking of wires and bone healing) was evaluated in four groups of 24 Sprague-Dawley rats varying in offset (7.5 and 15mm) and implant material (S/T) over 6 weeks. Torsional and axial stiffness were higher in steel compared to titanium setups. A decrease in all configurations was observed by increasing the offset. The offset 7.5mm showed a significantly higher torsional (S: p<0.01, T: p<0.001) and axial in vitro stiffness (S: p<0.001, T: p<0.001) compared to 15mm offset of the fixator. Although in vitro designed to be different in mechanical stiffness, no difference was found between the groups regarding complication rate. The overall-complication rate was 5.2%. In conclusion, we were able to establish a small animal model for bone defect healing which allows modeling the mechanical conditions at the defect site in a defined manner.     

Development and clinical application of an external fixator monitoring system

A monitoring system for measuring movement occurring in a dynamic external fixator used to treat fractures is described. The system measures shortening during fracture healing, micromovement at the fracture site on weight bearing and detects pin loosening. The method of calibration including cadaver experiments is presented. The clinical application is described and the reasons for measuring movement are discussed.     

Design and experimental evaluation of adjustable bone plates for mandibular fracture fixation

Conventional bone plates are commonly used for surgical mandibular fracture fixation. Improper alignment between bone segments, however, can result in malocclusion. Current methods of fixation require a surgeon to visually align segments of bone and affix a metal plate using bone screws, after which little can be done to adjust alignment. A method of adjusting fracture alignment after plate placement, without screw removal, presents an improvement over costly and risky revision surgery. A modified bone plate has been designed with a deformable section to give surgeons the ability to reduce misalignments at the fracture site. The mechanics of deformation for various adjustment mechanisms was explored analytically, numerically, and experimentally to ensure that the adjustable plate is comparable to conventional bone plates. A static force of 358.8 N is required to deform the adjustable bone plate, compared with predicted values of 351 N using numerical simulation and 362 N using a simple beam theory. Dynamic testing was performed to simulate in vivo loading conditions and evaluate load-capacity in both deformed and un-deformed bone plates. Results indicate that bending stiffness of a rectangular bone plate is 709 N/mm, compared with 174 N/mm for an octagonal plate and 176 N/mm for standard plates. Once deformed, the rectangular and octagonal plates had a stiffness of 323 N/mm and 228 N/mm, respectively. Un-deformed and deformed adjustable bone plates have efficacy in bone segment fixation and healing.