Parametric analyses of pin-bone stresses in external fracture fixation devices

Problems occurring in the use of external fixators for bone fractures include pin-bone interface necrosis, infection, and loosening. These may be initiated and enhanced by pin-bone interface stress levels. Based on stress data from finite element method (FEM) models, an analytical "closed-form" model of the local pin-bone configuration in long-bone fracture fixation is developed. This model is relatively simple and useful for routine applications in combination with clinical studies and animal experiments. Although approximate, the most significant stress predictions in the pin-bone structure compare well with more sophisticated FEM analysis results. The analytical model is used for extensive parametric analyses, investigating the effects on the pin-bone interface stress distribution of frame configuration parameters, pin diameter and modulus, bone dimensions, and elastic characteristics. The results indicate that these stresses may reach very high levels under unfavorable circumstances but can be drastically reduced by increasing the bending rigidity of the pin, reducing the side-bar separation and applying full-pin configuration in favor of half-pins.     

The effect of pin location on the rigidity of the halo pin-bone interface

Optimal insertion of halo fixation pins to maximize the rigidity of the interface between the halo pins and the outer table of the skull is important in reducing the incidence of pin loosening. An in vitro biomechanical study was performed using cadaver skulls to investigate the effects of pin location on the rigidity of this pin-bone interface. Halo pins were inserted at nine positions within a recommended "safe zone" for pin insertion. It was found that the rigidity of the pin-bone interface progressively decreased as pins were inserted more superiorly on the calvaria. The rigidity of the interface did not change significantly when the location of the pins was varied in the horizontal axis. This reduction in interface rigidity associated with inserting pins more superiorly on the skull may be related to an increase in the angle of insertion of the pins with respect to the surface of the calvaria. Based on this study, a change in the technique of halo pin insertion is recommended. Pins should be placed as inferiorly as possible, close to the supraorbital ridge, to achieve the most perpendicular angle of insertion and thus the most rigid fixation. The improved rigidity obtained with perpendicular pin insertion may minimize the rate of pin loosening and other complications associated with use of the halo orthosis.     

A comparison of various angles of halo pin insertion in an immature skull model.

STUDY DESIGN: A basic science biomechanical study involving an animal model. OBJECTIVES: To evaluate the effect of varying angles of halo pin insertion on the force generated at the pin-bone interface, and thereby the stability of the halo pin-bone interaction during insertion. BACKGROUND DATA: Because of variations in the shape and size of the pediatric skull, halo pins often are inserted at various angles rather than perpendicular to the skull. Concern exists that the high complication rate associated with pediatric halo use may result in part from less than ideal structural properties at the halo pin-bone interface. METHODS: The authors used a fetal calf skull model to simulate the thickness and structural properties of the pediatric skull. Halo pins were inserted at angles of 0 degree (perpendicular), 10 degrees, 15 degrees, and 30 degrees into skull segments via a halo ring. Load generated at the pin-bone interface was measured using a modified mechanical testing device. Twenty trials were conducted per angle, with the endpoint being specimen failure, pin penetration, or maximum load. RESULTS: Mean maximum loads per unit thickness were 82.15 +/- 7.54 N/mm at 0 degree, 68.80 +/- 4.79 N/mm at 10 degrees, 51.49 +/- 5.08 N/mm at 15 degrees, and 42.38 +/- 3.51 N/mm at 30 degrees, There was a significant difference between perpendicular insertion (0 degree) and 15 degrees angles of insertion. There was also a significant difference between the 10 degrees and 30 degrees angles of insertion. CONCLUSIONS: Perpendicular halo pin insertion in an immature skull model was shown to result in increased load at the pin-bone interface. This improved structural behavior may help to reduce the incidence of complications of halo application in children.     

The effect of rigidity on fracture healing in external fixation

Knowledge of the basic biomechanics of external fixation is necessary to obtain the full benefits of the technique for bone fracture treatment. The rigidity of external fixation, including pin-bone interface stresses, is discussed and bone healing and remodeling under different fixation stiffnesses and fracture gap conditions are described. The rigidity of fixation ultimately depends on the biomechanical characteristics of the fracture, the accuracy of reduction, and the amount of physiologic loading. Comparative experiments using a canine tibial fracture model have suggested that fixation rigidity is important in early bone healing and in the prevention of pin loosening. Bone union can be achieved under external fixation through different pathways, ranging from callus-free gap healing under a rigid neutralization configuration to direct-contact healing with periosteal new bone formation under axially dynamized stable fixation. Cortical reconstruction by secondary osteons seems to be important for the ultimate strength of the bone union.     

Thermal response and torque resistance of five cortical half-pins under simulated insertion technique

A model was developed that can quantitate heat generation during placement of half-pins in cortical bone. Five half-pins were tested to assess differences in insertion torque, heat generation, and microdamage at the pin-bone interface. Thin thermocouple probes were placed 0.5 mm from the track of the pin and within the pin to measure its temperature during insertion. Scanning electron microscopy was used to view the pin-bone interface to assess the microdamage during placement. The design of the tip of the pin influenced insertion torque and heat generation. Higher heat generation was measured when a thermocouple was placed within the pin itself and less was measured when thermocouple probes were placed within bone samples 0.5 mm from the impending pin track. Furthermore, insertion torque and thermal responses were related, but there were no significant differences in microdamage to bone when different pins and drilling/tapping techniques were used. Due to the significant heat generation at the pin-bone interface, proper cooling with saline irrigation should be applied during pin insertion regardless of the design of the pin. The microdamage observed at the surface of the pin track may have significant implications with regard to loosening of pins, but such effects must be studied with in vivo models.     

The effect of angled insertion on halo pin fixation

This study evaluated the effect of angled insertion of halo pins on the biomechanical characteristics of the halo pin-calvarium complex. Halo pins were inserted into isolated calvarium sections at 90 degrees, 75 degrees, and 60 degrees to the surface of the bone at an insertional torque of 0.68 N-m (6 inch-pounds). Initial rigidity, load at failure, and deformation at failure of the pin-bone complex were assessed during transverse shear loading. The structural properties of the pin-bone complex were maximized at loads approaching failure when pins were inserted perpendicular (90 degrees) to the bony surface and significantly decreased at more acute angles of insertion. Perpendicular insertion of halo pins maximizes the structural properties of the complex formed by the halo pin and the calvarium. This improved structural behavior may minimize the incidence of pin loosening clinically, and may reduce the frequency of other complications currently associated with the use of the halo orthosis.     

可吸收锥形针固定手部骨折的生物力学研究

目的 研究可吸收锥形针的生物力学性能,探讨其在手部骨折中应用的可行性.方法 取新鲜冰冻尸体掌、指骨共32根,左右配对,斜形截骨后分别用2根1.0 mm克氏针和2根1.3 mm 可吸收锥形针固定,通过折弯试验和疲劳试验对两种材料进行生物力学测试.结果 可吸收锥形针的抗弯刚度和最大载荷均显著小于克氏针;疲劳试验后两种材料的抗弯刚度差异无统计学意义,克氏针的最大载荷显著大于可吸收锥形针,而可吸收锥形针经疲劳试验后形变显著小于克氏针;疲劳试验后可吸收锥形针与克氏针的最大载荷比大于折弯试验中两者的最大载荷比,差异有统计学意义,而抗弯刚度比差异无统计学意义.结论 可吸收锥形针强度小于克氏针,但足以对抗术后功能锻炼时的应力,由于具有良好的弹性,在术后功能锻炼方面比克氏针更优越.     

Supra-acetabular placement of external fixator pins: a safe and expedient method of providing the injured pelvis with stability

Applying a stable anterior pelvic external fixator frame is a skill that should be mastered by all orthopedic surgeons who treat acutely injured patients. Splinting of an unstable pelvis during resuscitation can help to reduce the volume of the true pelvis, pending definitive surgical stabilization of the pelvic ring. Supra-acetabular pin placement, less familiar to most surgeons than iliac wing pin placement is, can provide a more reliable pin-bone interface and thus allow improved reduction ability with fewer soft-tissue complications. Because of their location, supra-acetabular pins also seem to be better tolerated than iliac crest pins when used for definitive management of the pelvic ring disruption. A young man who sustained a type II anteroposterior compression injury in a motor vehicle accident presented with symphyseal disruption (7 cm wide) and left anterior sacroiliac joint disruption. During resuscitation, the pelvis was anatomically reduced and stabilized with a supra-acetabular pin-based external fixator. Pin locations, chosen using palpable and cutaneous landmarks, were inserted without additional imaging guidance. The fracture was reduced anatomically, and the frame was used for definitive management of the pelvic ring injury.     

The effect of radial preload on the implant-bone interface: a cadaveric study

The most common complication of external fixation is pin loosening. Preloading the implant-bone interface is believed to retard this process. Radial preload, in particular, may be useful, as it allows loading in more than one direction. To investigate the effect of varying degrees of radial preload on the pin-bone interface, 30 freshly thawed human cadaveric tibiae were sectioned into 4-cm segments. Uniform drill holes were produced in the anterior tibial ridge of all segments and custom experimental bolts, oversized in diameter by as much as 1 mm, were pressed into each specimen. Macroscopic surface fractures were noted at the time of bolt insertion for misfits greater than 0.2 mm. Following histologic preparation, the implant-bone interface was evaluated microscopically based on the appearance of osteonal compression, lamellar distortion, and microfractures. Insertion of external fixator pins with misfits of greater than 0.4 mm resulted in significant microscopic structural damage to the bone surrounding the pin. High degrees of radial preload, exceeding the elastic limit of cortical bone, may be produced around pin holes by a small misfit. The use of oversized pins or screws must therefore be questioned.     

Mechanical performance of pin clamps in external fixators

Insufficient holding strength on a pin within a clamp may result in the diminution of the overall fixation rigidity as well as pin movement at the pin-bone interface. In this study the holding strength of pin fixation clamps in two representative external fixators (Hoffmann and Orthofix) was evaluated by determination of the torque resistance of pins within a clamp. In the standard Hoffman clamp, the pin-clamping effect was satisfactory in the symmetric two-, three-, and four-pin configurations, whereas the standard Orthofix clamp provided higher holding strength in the symmetric two- and three-pin configurations. All other pin configurations in both clamps resulted in a high variation of pin torsional resistance, and sometimes one of the pins registered low or had no resistance to torsion. The results indicated that the holding power of the clamps was adequate only if certain guidelines were followed at the time of external fixator application. An instrumented torque wrench may be helpful to assess the pin-fastening strength within the clamp. This wrench should also be used to introduce appropriate and uniform tightening torques to the pin clamp screws. However, these results do not apply to those fixators in which each pin will be tightened individually.     

The effects of additional hollow cylinder coated to external fixator screws for treating pilon fracture: A biomechanical perspective

An external fixator is a promising medical device that could provide optimum stability and reduce the rate of complications in treating bone fracture during intervention period. It is noted that the biomechanics behaviour of device can be altered by introducing more features such as material suitability and additional components. Therefore, this study was conducted via finite element method to investigate the effects of additional hollow cylinder coated with external fixator screws in treating Type III pilon fracture. Finite element models which have been validated with experimental data were used to simulate stresses at the pin-bone interface and relative micromovement at interfragmentary fractures during swing (70 N load) and stance phases (350 N load). All bones and external fixators were assigned with isotropic material properties while the cartilages were simulated with hyper-elastic. For the hollow cylinder, polyethylene was assigned due to its properties which are equivalent to the bone. From the results, it is found that stresses at the pin-bone interface for the coated screws were reduced to 54% as compared to the conventional fixator. For the micromovement, there was no difference between both models, whereby the value was 0.03 mm. The results supported previously published literature, in which high stresses are unavoidable at the interface, fortunately, those stresses did not exceed the ultimate strength of bone, which is safe for treating patients. In conclusion, if patients are allowed to bear weight bearing, the external fixator with coated screws is a more favourable option to be fixed into the bone to avoid complications at the interface.     

Structural behavior of the halo orthosis pin-bone interface: biomechanical evaluation of standard and newly designed stainless steel halo fixation pins

The structural response of the halo orthosis pin-bone interface to transverse loading was evaluated on an Instron testing machine using fresh cadaver calvarium sections. Commercially available stainless steel (control) pins and newly designed stainless steel experimental pins were evaluated. Cyclic loading tests and load-to-failure tests were performed. Of the many designs tested, one pin demonstrated an improvement in structural properties at the pin-bone interface compared with the control pin. Furthermore, the new pin design was more resistant to insertional torque reduction when subjected to cyclic loading after insertions at 4 and 6 in-lb. Both the control and experimental pins exhibited improved structural behavior at 8 in-lb of insertional torque compared to the currently recommended 6 in-lb.     

Effect of callus development on the deformation of external fixation frames

Purpose

We designed a sensor that measures the bending moments at the articulations and the torque of the rod of a Hoffmann II® external fixation. We considered the effect of the callus formation in the stabilisation of a “fracture-fixation system.”

Methods

Four Hoffmann II® frame configurations were mechanically tested. Two carbon fibre tubes represent the bone fragments (length 180 mm, outer diameter 25 mm, inner diameter 19 mm). The callus is represented by the interposition of springs of different rigidity (10–405 N/mm) in the fracture gap between the tubes.

Results

The deformation of the frame is in inverse proportion to the stiffness of the callus; the slope of the curve drops rapidly during early development of the callus, to reach a plateau after some 50 % of recovery of the normal mechanical characteristics of the bone. This simulation supports the theoretical approach, i.e. the external frame resists larger stresses at the start of the fracture healing. Over a callus stiffness of some 200 N/mm the pattern of the curves remains similar, regardless of the frame configuration.

Conclusion

An optimisation of the frame is possible, adapted to the actual mechanical situation of the callus. A monitoring system is deemed reliable after making sure that the elementary components behave the same way in the clinical condition as in the laboratory. In an experimental set up we confirmed its reliability in a clinical-like situation.     

External fixator pin insertion techniques: biomechanical analysis and clinical relevance.

A series of identically matched pairs of fresh-frozen canine femora (approximating human radii in size and dimension) were used to mechanically compare pull-out strength between 4 mm predrilled, self-tapping, half-pins and 4 mm self-drilling, self-tapping half-pins with drill bit-like cutting flutes. A second biomechanical and videotape analysis was done comparing the differences of pin insertion by power versus hand drilling. Results indicated a mean 22% reduction in bone purchase of self-drilling, self-tapping pins compared with that of predrilled pins and a marked increase in depth of insertion required of the self-drilling pins for comparable pin purchase (10 mm). It was also observed that a visible "wobble factor" exists, which tends to weaken the pin-bone interface when hand drilling is performed.     

Experimental analysis of Hoffmann external fixation in various mountings

The mechanical properties of four different configurations of the Hoffmann external fixation system were tested in an experimental setup using 4-mm stainless steel pins. Furthermore, stainless-steel pins and titanium pins 5 mm in diameter were applied in one of the fixators. The evaluation of the results was based on a paired t-test, with P less than 0.05 as the significance level. The quadrilateral transfixing Hoffmann-Vidal frame (HVF) proved to be the most rigid fixator under all loading conditions. The unilateral frame type supplied with two connecting bars and four ball joints was significantly more rigid than the unilateral single-bar device. By increasing the pin diameter from 4 to 5 mm a further increase of stiffness could be achieved, thus reaching about two-thirds of the rigidity of the HVF. The difference between titanium and stainless steel pins was not significant. Common for all the frame mountings was a fivefold greater rigidity in the lateral direction (in the plane of the pins) than in the anteroposterior direction (perpendicular to the pins).     

Development of a variable stiffness external fixation system for stabilization of segmental defects of the tibia

The mechanical properties of a variable stiffness external fixation system were explored. Initial testing of a unilateral fixator configuration demonstrated that system rigidity could be increased by maximizing pin separation distance in the fracture component and the number of pins used while minimizing pin separation distance across the fracture site and the sidebar offset distance from bone. A triangulated system composed of half pin frames mounted anteriorly and medially on the tibial aspects and linked by crossbars was devised. Progressive disassembly of the frame was shown to result in progressive decreases in fixator rigidity in all planes.     

膨胀式椎弓根螺钉在骨质疏松绵羊体内界面的观察研究

目的研究膨胀式椎弓根螺钉(expansive pedicle screw,EPS)在骨质疏松绵羊的松质骨内短期的骨-螺钉界面组织学情况。方法3只经去势后骨质疏松的绵羊选取双侧股骨髁,植入EPS,饲养3个月。依次进行切片组织学观察。结果切片经骨粉染色和丽春红三色染色,见新生骨小梁长入膨胀后的螺钉缝隙中,并与EPS周围的骨小梁相延续、延伸至膨胀中心。EPS骨-钉界面及膨胀缝隙中的骨小梁与钉直接紧密无缝隙相接触。结论EPS所形成的骨中有钉、钉中有骨的立体交叉复合结构的诸多优点,说明应用在骨质疏松的状态下具有强大的稳定性。     

Development of a variable stiffness external fixation system for stabilization of segmental defects of the tibia

The mechanical properties of a variable stiffness external fixation system were explored. Initial testing of a unilateral fixator configuration demonstrated that system rigidity could be increased by maximizing pin separation distance in the fracture component and the number of pins used while minimizing pin separation distance across the fracture site and the sidebar offset distance from bone. A triangulated system composed of half pin frames mounted anteriorly and medially on the tibial aspects and linked by crossbars was devised. Progressive disassembly of the frame was shown to result in progressive decreases in fixator rigidity in all planes.     

External fixation of the leg using unilateral biplanar frames

A simple and reliable method is described for testing the stability of external fixators under stresses similar to those found in clinical practice. Unilateral uniplanar, unilateral biplanar, and bilateral uniplanar frames were used. It seemed important to measure deformations under bending stresses in different planes, because we have found a variable rigidity in some of the frames in the different planes. Our results show that a unilateral biplanar frame without transfixation pins can be set up with an overall rigidity as good as that of a bilateral frame. Using this on the leg, one can avoid putting pins through the anterolateral compartment.     

Biomechanical behavior of the tibiofibular frame in nonunion

In this paper an in vitro investigation of the biomechanical behavior of the tibiofibular frame in nonunion using electrical extensometry is presented. The intact and untreated nonunited frames are studied as well as three surgical treatments classically used in nonunion: the plating technique, the onlay graft technique and the external fixation according to Ilizarov. The monopodal position with extended knee is considered, and particular attention is paid to the different muscle insertions. The results for the plating and onlay grafting techniques resemble those for the intact frame, whereas the Ilizarov external fixator increases the overall rigidity of the frame. In the untreated nonunited tibiofibular frame, an inversion of the tension and compression areas at the level of the tibia was found. We hypothesize that this phenomenon could be one of the mechanical factors leading to nonunion. Indeed, cyclic stresses in the fracture callus might prevent bony fusion, for the areas healing under compression stresses must become tension areas once the tibia is healed and vice versa.