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.     

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").     

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.     

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.     

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.     

Mechanical induction of critically delayed bone healing in sheep: radiological and biomechanical results

This study aimed to mechanically produce a standardized ovine model for a critically delayed bone union. A tibial osteotomy was stabilized with either a rigid (group I) or mechanically critical (group II) external fixator in sheep. Interfragmentary movements and ground reaction forces were monitored throughout the healing period of 9 weeks. After sacrifice at 6 weeks, 9 weeks and 6 months, radiographs were taken and the tibiae were examined mechanically. Interfragmentary movements were considerably larger in group II throughout the healing period. Unlike group I, the operated limb in group II did not return to full weight bearing during the treatment period. Radiographic and mechanical observations showed significantly inferior bone healing in group II at 6 and 9 weeks compared to group I. After 6 months, five sheep treated with the critical fixator showed radiological bridging of the osteotomy, but the biomechanical strength of the repair was still inferior to group I at 9 weeks. The remaining three animals had even developed a hypertrophic non-union. In this study, mechanical instability was employed to induce a critically delayed healing model in sheep. In some cases, this approach even led to the development of a hypertrophic non-union. The mechanical induction of critical bone healing using an external fixation device is a reasonable attempt to investigate the patho-physiological healing cascade without suffering from any biological intervention. Therefore, the presented ovine model provides the basis for a comparative evaluation of mechanisms controlling delayed and standard bone healing.     

Biomechanical consequences of callus development in Hoffmann, Wagner, Orthofix and Ilizarov external fixators.

A theoretical analysis by a finite elements model (FEM) of some external fixators (Hoffmann, Wagner, Orthofix and Ilizarov) was carried out. This study considered a logarithmic progress of callus elastic characteristics. A standard configuration of each fixator was defined where design and application characteristics were modified. A comparison among standard configurations and influence of every variation was made with regard to displacement and load transmission at the fracture site. An experimental evaluation of standard configurations was performed with a testing machine. After experimental validation of the theoretical model was achieved, an application of physiological loads which act on a fractured limb during normal gait was analysed. A minimal contribution from an external fixator to the total rigidity of the bone-callus-fixator system was assessed when a callus showing minimum elastic characteristics had just been established. Insufficient rigidity from the fixation devices to assure an adequate immobilization during the early stages of fracture healing was verified. However, regardless of the external fixator, callus development was the overriding element for the rigidity of the fixator-bone system.     

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.     

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.     

The critical size bony defect in a small animal for bone healing studies (II): implant evolution and surgical technique on a rat's femur.

In the preclinical field of orthopaedic and trauma surgery critical size bony defects (CDS) were used to evaluate the biocompatibility and allow to investigate the osteoinductivity and -conductivity of bone substitutes. Concerning the anatomical size the laboratory rat indicates a lower limit in small animals which are appropriate for experiments on bone. The aim of this study was to define a CSD, to develop a suitable fixation system to stabilize bony fragments in CSD and to point out the specialities of the surgical technique. These informations should help for to design and practice studies concerning bone healing on rat's femur. Based on previously acquired anatomical data of rat's femur, the technical challenges and anatomical specialities of different osteosynthesis techniques in rat's femur surgery are demonstrated. Our experiences with different fixation systems and techniques lead to the development of an external fixator, which guarantees for a stable bone fragment fixation, prevents severe soft tissue damage, allows of a roentgenologic evaluation of the defect zone and prevents from undesired direct biomaterial-implant interactions. Neither the proximal nor the distal femoral nailing technique is appropriate for a stable fixation in CSD of rat's femur. To evaluate the reliability of an own developed external fixator 42 nude rats with a 4.0 mm CSD were investigated clinically and roentgenologically over 10 weeks. The external fixator showed only a small implant failure rate. A solid fusion of the bone fragments was not observed within the 10 weeks follow-up period.     

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.     

Tensioning technique and fixator stability of a circular external fixator with threaded fine wires

Hybrid external fixators have become more popular over recent years, and although biomechanical tests show varying results from paper to paper, a consistent finding is shear motion occurring at the fracture/osteotomy site during loading. This can be reduced to some extent by frame configuration, and with the use of olive stop wires, but both of these have limitations. We have investigated the use of threaded fine wires in a circular hybrid fixator. A reliable tensioning method is described which minimises movement at the wire-bone interface during tensioning, and threaded wires are seen to offer a significant improvement in fracture site shear stiffness when compared to smooth fine wires.     

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.     

DLS 5.0 - The Biomechanical Effects of Dynamic Locking Screws

Introduction

Indirect reduction of dia-/metaphyseal fractures with minimally invasive implant application bridges the fracture zone in order to protect the soft-tissue and blood supply. The goal of this fixation strategy is to allow stable motion at the fracture site to achieve indirect bone healing with callus formation. However, concerns have arisen that the high axial stiffness and eccentric position of locked plating constructs may suppress interfragmentary motion and callus formation, particularly under the plate. The reason for this is an asymmetric fracture movement. The biological need for sufficient callus formation and secondary bone healing is three-dimensional micro movement in the fracture zone. The DLS was designed to allow for increased fracture site motion. The purpose of the current study was to determine the biomechanical effect of the DLS_5.0.

Methods

Twelve surrogate bone models were used for analyzing the characteristics of the DLS_5.0. The axial stiffness and the interfragmentary motion of locked plating constructs with DLS were compared to conventional constructs with Locking Head Screws (LS_5.0). A quasi-static axial load of 0 to 2.5 kN was applied. Relative motion was measured.

Results

The dynamic system showed a biphasic axial stiffness distribution and provided a significant reduction of the initial axial stiffness of 74.4%. Additionally, the interfragmentary motion at the near cortex increased significantly from 0.033 mm to 0.210 mm (at 200N).

Conclusions

The DLS may ultimately be an improvement over the angular stable plate osteosynthesis. The advantages of the angular stability are not only preserved but even supplemented by a dynamic element which leads to homogenous fracture movement and to a potentially uniform callus distribution.     

Loads acting in an intramedullary nail during fracture healing in the human femur.

The form and function of the musculo-skeletal system is closely related to the forces acting in its components. Significant forces are present in the long bones, but their magnitudes have so far only been estimated from mathematical models. Fracture fixation by means of metal implants provides an opportunity to measure the implant-born forces and to estimate the long bone forces before healing occurs. The load changes during fracture healing may provide additional information. Therefore, a telemetrized, interlocking femoral nail for wireless transmission of forces and moments acting across the fracture site was developed. The design was based on the geometry and material of a 16 mm AO nail with a circular, closed cross-section allowing full protection of the electronic circuits from the body fluids. After careful testing, it was implanted in a 33-year-old patient who had sustained a multifragmentary fracture of the left femur. Measurements at a rate of approx. 0.4 Hz were performed in different patient postures between the 2nd and 26th postoperative week. Significant axial forces and bending moments were measured during several activities such as sitting, unsupported leg elevation and partial weight bearing in a standing position. Forces orthogonal to the nail axis remained small. The reductions of the implant loads due to fracture consolidation were in the order of 50%. Dynamization of the nail did not change the forces. Even though the telemetry system did not allow for dynamic measurements and the results presented here provide data from one subject only, the new information will be useful with respect to implant design, biomechanics of fracture fixation and evaluation of healing progression.     

Interaction of Age and Mechanical Stability on Bone Defect Healing: An Early Transcriptional Analysis of Fracture Hematoma in Rat

Among other stressors, age and mechanical constraints significantly influence regeneration cascades in bone healing. Here, our aim was to identify genes and, through their functional annotation, related biological processes that are influenced by an interaction between the effects of mechanical fixation stability and age. Therefore, at day three post-osteotomy, chip-based whole-genome gene expression analyses of fracture hematoma tissue were performed for four groups of Sprague-Dawley rats with a 1.5-mm osteotomy gap in the femora with varying age (12 vs. 52 weeks - biologically challenging) and external fixator stiffness (mechanically challenging). From 31099 analysed genes, 1103 genes were differentially expressed between the six possible combinations of the four groups and from those 144 genes were identified as statistically significantly influenced by the interaction between age and fixation stability. Functional annotation of these differentially expressed genes revealed an association with extracellular space, cell migration or vasculature development. The chip-based whole-genome gene expression data was validated by q-RT-PCR at days three and seven post-osteotomy for MMP-9 and MMP-13, members of the mechanosensitive matrix metalloproteinase family and key players in cell migration and angiogenesis. Furthermore, we observed an interaction of age and mechanical stimuli in vitro on cell migration of mesenchymal stromal cells. These cells are a subpopulation of the fracture hematoma and are known to be key players in bone regeneration. In summary, these data correspond to and might explain our previously described biomechanical healing outcome after six weeks in response to fixation stiffness variation. In conclusion, our data highlight the importance of analysing the influence of risk factors of fracture healing (e.g. advanced age, suboptimal fixator stability) in combination rather than alone.     

人脐带间充质干细胞对脊柱骨折大鼠愈合及神经功能的影响

摘要 目的：探讨人脐带间充质干细胞(Human umbilical cord mesenchymal stem cells,hUC-MSCs)对脊柱骨折大鼠愈合及神经功能的影响。方法：脊柱骨折Sprague-Dawley雄性大鼠模型30只随机分为hUC-MSCs组与对照组,各15只。hUC-MSCs组大鼠在骨折部位移植0.5 mL的hUC-MSCs(细胞浓度为2×10⁶/mL),对照组大鼠移植同体积的生理盐水,记录大鼠愈合及神经功能变化情况。结果：两组造模后15 min、30 min、90 min的平均动脉压都波动明显,不过组间对比差异无统计学意义(P>0.05)。与造模后2 w对比,两组造模后4 w的神经功能BBB评分均升高,且hUC-MSCs组造模后2 w、4 w的神经功能BBB评分都高于对照组(P<0.05)。hUC-MSCs组造模后8 w的骨体积分数高于对照组(P<0.05)。hUC-MSCs组骨折部位附近有少量骨痂生长,骨折线逐渐消失;骨痂已明显包裹骨折部位。hUC-MSCs组造模后8 w的脊髓细胞凋亡指数低于对照组(P<0.05)。结论：hUC-MSCs在脊柱骨折大鼠的应用能促进骨折愈合与改善神经功能,也可以抑制脊髓细胞凋亡,从而发挥很好的治疗作用。     

Continual measurement of intramedullary blood perfusion with laser Doppler flowmetry in intact and ostectomized tibiae of rabbits

Blood flow is important for the healing of bone fractures. Until now, however, there have been no publications on the daily, continual measurement of intramedullary blood perfusion using laser Doppler flowmetry (LDF) in the conscious animal. In this study, a model for the daily, continual measurement of intramedullary blood perfusion by LDF and the temperature near the cortex both in intact and ostectomized tibiae in the conscious rabbit is described. The probes for blood perfusion and temperature measurement were implanted permanently at three different localizations into the right tibia of 10 adult New Zealand White rabbits. The probes were held in place by a bilateral, single-plane external fixator. In five of these animals, a midshaft tibial ostectomy was created in order to simulate a fracture. Intramedullary blood perfusion and temperature were measured daily over 49 days. While in intact tibiae no significant (P > 0.05) differences were found in blood perfusion readings taken at various time points, for mean values or for blood perfusion over time, in ostectomized tibiae the differences were significant: various time points (P = 0.0056), mean values (P = 0.0034) and blood perfusion over time (P = 0.0337). Blood perfusion readings at the centre probe were elevated compared with those at the proximal and distal probes. Thus, a revascularization in the ostectomy gap during the fracture healing was proven by means of the LDF. No influence of the blood perfusion on the temperature in the ostectomy area could be determined during healing of the ostectomy. The described model seems suitable for the continual measurement of intramedullary blood perfusion both in intact and ostectomized tibiae in the conscious rabbit.     

弹性髓内钉对掌骨骨折的外科治疗

背景与目的：大多数的掌骨骨折可以采取保守治疗的方法。手术适合于一些特定的病例如：在骨折侧位片上成角大于30度、短缩超过5mm或者有旋转移位。本文的目的是分析应用预弯1.6mm克氏针治疗关节外掌骨骨折的临床效果。方法：先将克氏针弯曲成尖部弯曲大约在5mm左右,较长的轴部向着与尖部相反的方向弯曲的稳定S型。首先用2.5mm的钻头钻入掌骨基底部,然后用克氏针的尾部顺行插入,当克氏针经过骨折部位时复位骨折。应用克氏针三点固定的原理固定骨折,用手指夹板固定,早期进行掌指关节的活动。结果：自2009年到2010年我们探索性的固定了5例第五掌骨和2例第一掌骨。病人的平均年龄是35岁,其中6例男性、1例女性。外科治疗的平均时间是13天（4天至28天）,骨折全部愈合良好。克氏针拔出的平均时间是4.5周（3至6周）。结论：单根预弯克氏针治疗掌骨骨折,不仅简单、快捷而且可以早期行患手各关节功能练习,使手部关节功能获得满意恢复,是掌骨骨折治疗过程中的一种理想方法