A biomechanical analysis of the Ilizarov external fixator

Five configurations of the Ilizarov fixator were analyzed in vitro. The overall stiffness, shear stiffness, and axial motion of the fracture site were determined. The data were compared with the results of eight conventional one-half frame fixators previously tested in the same manner. The Ilizarov fixator allowed significantly more axial motion at the fracture site during axial compression than the other fixators tested. The overall stiffness and shear rigidity of the Ilizarov external fixator were similar to those of the one-half pin fixators in bending and torsion. The stability of the Ilizarov fixator was a function of bone position within the fixator rings and fixation wire tension. The use of olive stop wires increased the shear resistance of the Ilizarov system.     

Rigidity of half-pins for the Ilizarov external fixator.

The success of the Ilizarov technique is due to the combination of the biomechanics of its external fixator and the biology of distraction osteogenesis. The stability and stiffness of the conventional Ilizarov fixator is attributed to its use of a K-wire cross structure. The disadvantages of this structure include pain, and possible neurologic and vascular injuries when the wires are introduced into crucial neurovascular areas, as well as increased frame complexity and construction. Reducing the number of wires decreases these problems, but also decreases the stiffness of the system. Hybrid (wire and half-pins) Ilizarov fixators and half-pin fixators are also being used in an attempt to alleviate these problems. Half-pins have been described by Fleming et al. and Green as causing minimal transfixation of the surrounding soft tissues and capable of and being inserted into anatomically safe areas. The stiffnesses of the hybrid systems have not been measured. This study reports on the stiffness of different wire and half-pin systems that were biomechanically tested in axial loading, anterior-posterior bending, medial-lateral bending, and torsional loading. The results demonstrated that the conventional Ilizarov fixator with wires possesses a high axial stiffness, whereas the fixator with half-pins possesses higher stiffness under bending and torsional loads. To obtain adequate stability, the use of hybrid Ilizarov frames with one wire and two or more half-pins (5 or 6 mm in diameter), or larger half-pin frames (5 or 6 mm) with three pins is recommended.     

Control of interfragmentary micromotion in Ilizarov distractional osteogenesis.

Growth of the regenerate is affected by various factors during the course of Ilizarov distractional osteogenesis. One of the chief biomechanical factors that influences the quality and rate of bone formation is fixator stiffness. A four-ring Ilizarov apparatus was configured around a synthetic tibia. In a series of trials involving a uniform axial load, different transfixing wire tensions, and the separation of paired proximal and distal rings, fragment displacement was measured. Preliminary results suggest that the effect produced by the distraction of ring pairs on interfragmentary micromotion is as significant as pretensioning of the wires.     

The effect of wire configuration on the stability of the Ilizarov external fixator.

The stability of the basic unit for fixation of the Ilizarov external fixation system was tested in several loading modes. The effects of varying the number of wires and the orientation of wire placement were studied. The fixation units were mounted on a plastic, simulated, long bone and tested by loading in several directions. The Ilizarov fixation ring was found to be relatively stiff in axial compression and torsion. Its stiffness in this mode was directly proportional to the number of wires in the system and independent of the configuration of wire placement. Loading in bending and in shear provided much lower levels of stiffness, and this was dependent on the angles formed between the wires. The addition of a wire at a minimum distance of 4 cm from the primary ring significantly improved bending stiffness. The use of opposed olive wires also improved shear stiffness.     

Mechanisches Verhalten von Ilizarov-Ringfixateuren Einfluss der Gestaltparameter auf die Steifigkeit und Konsequenzen für den klinischen Einsatz

Using a mechanical testing procedure, various fixator constructs were tested in vitro. In addition, the influence of the passive soft tissue structures on the fixation stiffness was determined. An increased number of Schanz' screws or Kirschner wires led to a comparable increase in stiffness than that observed with an increasing screw or wire diameter. In consequence, larger diameters should be preferred over an additional screw or wire where clinically applicable. With diaphyseal telescoping rods only the axial stiffness decreased. As expected, large ring diameters as well as titanium wires reduced stiffness components. Bracing the outer rings caused a reduction of the overall stiffness. Asymmetric pre-tensioning of the K-wires resulted in a significant reduction of tension in the neighboring wire. Removal of the soft tissues reduced stiffness to a similar extend as experienced in a fibula defect situation. The study demonstrates the correlation between design parameters, passive soft tissues and fixation stiffness and presents guidelines for an optimized fixator design.     

Ilisarov ring fixation and its technical application]

The Ilisarov ring fixator is an extremely versatile external fixation device. There is a wide range of indications for its use, e.g. fractures, pseudarthroses and non-unions, large defects, deformities and limb shortening. The device consists of rings or half-rings, which are connected to each other by threaded rods or plates and to the bone by wires under tension. This design allows neutral fixation, compression or distraction of the bone fragments. Because of its low axial stiffness at low loads and increased stiffness at higher loads, this fixator promotes osteogenesis and reduces strain on the tissues in functional treatment. Surgical intervention for its application is minimally invasive. This is a useful addition to the orthopaedic surgeon's armamentarium, providing proper attention is paid to preoperative planning, the principle rules of frame design and wire insertion technique, and postoperative management.     

Comparison of mechanical performance among different frame configurations of the Ilizarov external fixator: experimental study

The circular frame of the Ilizarov fixator can be modified to reflect anatomic variations and treatment aims. However, these modifications in the frame system cannot always achieve the mechanical performance of the standard frame system. A standard system has two rings in each bone fragment connected by four longitudinal bars on each side of the ring. In this study, the mechanical performances of one standard and eight modified frame systems were compared. Each system was loaded on a material testing machine, with calculation of axial compression, four-point bending, and torsion. As a result, systems that were modified with drop wires and Schanz screws 45 degrees oblique to the wires on the proximal ring provided a mechanical performance closer to the standard system than systems with other modifications.     

Determination of ankle external fixation stiffness by expedited interactive finite element analysis

Interactive finite element analysis holds the potential to quickly and accurately determine the mechanical stiffness of alternative external fixator frame configurations. Using as an example Ilizarov distraction of the ankle, a finite element model and graphical user interface were developed that provided rapid, construct-specific information on fixation rigidity. After input of specific construct variables, the finite element software determined the resulting tibial displacement for a given configuration in typically 15s. The formulation was employed to investigate constructs used to treat end-stage arthritis, both in a parametric series and for five specific clinical distraction cases. Parametric testing of 15 individual variables revealed that tibial half-pins were much more effective than transfixion wires in limiting axial tibial displacement. Factors most strongly contributing to stiffening the construct included placing the tibia closer to the fixator rings, and mounting the pins to the rings at the nearest circumferential location to the bone. Benchtop mechanical validation results differed inappreciably from the finite element computations.     

Direct and indirect loading of the Ilizarov external fixator: the effect on the interfragmentary movements and compressive loads

The amount of weight bearing and the force transmission to the frame have an important influence on the results of treatment with an Ilizarov external fixator. The frame provides beneficial interfragmentary movements and compressive loads at the fracture site through elastic wires. Mobilisation can be achieved by applying a weight-bearing platform at the distal end of the fixator. The effect on the interfragmentary movements and the compressive loads in indirect and direct loading were analysed in this study using a composite tibia bone model. Displacement transducers were attached to measure the interfragmentary movements and to detect relative movements of the bone fragments and movements between the rings. The compressive loads in the osteotomy were measured with loading cells in the defect zone. The weight-bearing platform had a substantial effect on the biomechanical behaviour of the frame. It led to an indirect force transmission through the fixator with respect to the osteotomy, resulting in lower compressive loads, lower interfragmentary movements and higher mechanical stress on the frame.     

Relationship between locking-bolt torque and load pre-tension in the Ilizarov frame

The wire-bolt interface in an Ilizarov frame has been mechanically tested. The optimal torque to be applied to the frame locking-bolts during physiological loading has been defined. The set-up configuration was as is used clinically except a copper tube was used to simulate bone. The force-displacement curves of the Ilizarov wires are not altered by locking-bolt torque. The force in the bone model at which pre-tension is lost increases as the locking-bolts are tightened to 14 Nm torque, but decreases if torque exceeds 14 Nm. Thus, 14 Nm is the optimal locking-bolt torque in frame. The relationship between pre-tension versus load for different locking-bolt torques arises because at low and high clamping torques poor wire holding and plastic deformation respectively occur. Wire damage was seen under light and electron microscopy. Clinically, over or under-tightening locking-bolts will cause loss of pre-tension, reduction in frame stiffness and excessive movement at the fracture site, which may be associated with delayed union.     

Die Verwendung eines Nervenstimulators zur sicheren Platzierung von Ilisarov-Drähten

OBJECTIVE: Avoidance of potential iatrogenic nerve injury during insertion of Ilizarov fine wires into areas of high anatomic risk by using a modified nerve stimulation technique. INDICATIONS: Application of the Ilizarov ring fixator to areas of high anatomic hazard, in situations where anatomic topography may be distorted by previous surgery, trauma, or congenital anomalies. CONTRAINDICATIONS: Use of systemic muscle relaxants. Caution in patient with cardiac pacemaker. SURGICAL TECHNIQUE: Preliminary experiments showed that a standard nerve-stimulating device can deliver a negatively charged, monophasic square pulse of current through Ilizarov wires. During the application of an Ilizarov frame to potentially hazardous anatomic regions, providing no systemic muscle relaxants are used, a voltage field sufficient to cause nerves in close proximity to the Ilizarov wire to depolarize is produced. Identification of a distal muscle twitch provoked by the stimulation may indicate a potential for iatrogenic nerve injury. RESULTS: Results show that with the nerve stimulator set at 2.5 mA (pulsed at a frequency of 2 Hz), peripheral nerves are stimulated if they lie within 5 mm of the wires. Should a distal muscle twitch occur, wires should be repositioned so that equivalent stimulation produces no twitch. The technique was used during Ilizarov frame application in ten patients, with only a single occurrence of distal muscle twitches in a lower-leg frame. Following repositioning of the Ilizarov wire in this case, no further twitches were observed, indicating that no Ilizarov wire was inserted close to peripheral nerves. No neurologic impairment was present postoperatively.     

Mechanical stress on tensioned wires at direct and indirect loading: a biomechanical study on the Ilizarov external fixator

Background

The biomechanical effect of indirect weight loading with the Ilizarov ring fixator using a weight-bearing platform has not yet been investigated. The problem of wire loosening and breakage occurs more frequently when patients are mobilised with a weight-bearing platform. Therefore, the aim of this research was to compare the influence of direct and indirect weight loading on the tensioned wires.

Method

A universal testing machine (UTS, Germany) was used in this study. A composite tibia model with a standard four-ring Ilizarov fixator and 1.8-mm wires in anatomical position was used to simulate a clinical situation. Wire strain was measured with two strain gauges positioned at the ring–wire interface of each wire. After a standardised 2-mm mid-diaphyseal osteotomy, an axial load of up to 1000 N was applied to the bone; the different methods of weight loading were evaluated in two experimental set-ups.

Results

A higher axial load was necessary to achieve an osteotomy gap closure at indirect loading. Mechanical stress on the tensioned wires was 400% higher on the proximal wires and 250% higher on the distal wires at a maximum axial loading of 1000 N. Mechanical stress remained on the wires in indirect loading, even after bone end contact, and led to excessive stress under higher weight-bearing amounts.

Conclusion

There is a substantial change in the biomechanical characteristics of the Ilizarov ring fixator when mobilising a patient with a weight-bearing platform. The considerable higher mechanical stress on the wires needs to be considered when patients are mobilised with a weight-bearing platform.     

Management of unstable open and closed tibial fractures using the Ilizarov method.

Forty-one consecutive tibial diaphyseal fractures that required operative stabilization were treated using the external fixator and concepts of compression-distraction of Ilizarov. Eleven fractures had bone loss greater than 1 cm and were managed by simultaneously compressing the fracture gap and distracting through a corticotomy site to maintain extremity length. Thirty tibial fractures consisted of closed unstable and open fractures that were managed using the external fixator, emphasizing immediate weight bearing and gradual compression at the fracture site. Twenty-six fractures in 23 patients were available for follow-up evaluation six to 9.5 months after bone healing. There were six closed, two Grade I, eight Grade II, five Grade IIIA, and five Grade IIIB fractures. Serial wound debridements, wet-to-dry-dressing changes, wound- and fracture-site compressions (13 fractures), and split-thickness skin grafts (eight wounds) were used to accomplish wound closure. Chronic infections did not occur. All fractures healed from 12 to 47 weeks without bone grafting. Eight transosseous fixation wires are used, only two of which transfixed significant muscle. Approximately 10% of the 248 wire sites became inflamed and nine wire sites were treated for infection with antibiotics, skin release around the offending wire, or wire removal. Three wires fractured and one wire was replaced. One ring sequestrum occurred and responded to curettement. Angulation of 7 degrees-9 degrees occurred in five fractures (19%). The results were good or excellent in 25 fractures. One patient with 9 degrees varus in a distal fracture refused correction. Operative time was 60 to 90 minutes after developing a satisfactory protocol for frame application. This method allows immediate functional stabilization of tibial diaphyseal fractures and postoperatively allows ease of fracture gap closure and compression. The frame can be left in place for the duration of the fracture care. Application of the Ilizarov external fixator is slightly more complicated than traditional large pin fixators and requires more attention to detail intraoperatively and postoperatively, but can be a versatile tool in the management of complex tibial shaft fractures.     

Biomechanics of the ring fixator--contributions of individual structural elements]

The mechanical behaviour of the circular external fixation device using thin, pre-tensioned wires is difficult to predict, because the individual elements, such as rings, rods and wires, can be assembled in a large variety of configurations. This behaviour is of particular importance in the treatment of large segmental defects and in bone lengthening, because most of the load is transmitted through the fixator. In this study, the mathematical basis of the widely varying definitions of stiffness under axial, bending and torsional loads is presented. The principal variables influencing a particular configuration and their effects are indicated. Particular consideration must be given to the fact that the wires under tension generate a non-linear behaviour of the frame. The information needed for reporting on different fixator systems is indicated. Finally, the effect of individual structural elements, such as number, direction and tension of the wires, number of threaded rods and distance between the planes of the rings on the different stiffnesses, is presented and discussed with respect to clinical significance.     

Hybrid external fixation of distal tibial fractures: new strategy to place pins and wires without penetrating the anterior compartment

Introduction Impalement of the anterior compartment musculature remains a problem in the hybrid external fixation of distal tibial fractures. The purposes of this study were to develop a tensioned wire configuration which does not violate the anterior compartment and to analyze the biomechanical implications of new wire configuration.Materials and methods Thirty-seven adult volunteers without known pathology around either tibia were recruited. Axial computed tomography of the distal tibia was performed at 5-mm slices from the plafond to the upper margin of the syndesmosis. The wire convergence angle was measured at the 1-, 2-, and 2.5-cm levels using the following landmarks: tibialis anterior tendon (TA), tibialis posterior tendon (TP), peroneus brevis tendon (PB), anterolateral border of the lateral malleolus (LM). Two straight lines were drawn by connecting TA and PB and connecting TP and LM. The wire convergence angle was defined as an acute angle between these two lines. Then the orientation of the bisector axis of the wires was measured. As a second part of this study, a validated three-dimensional hybrid external fixator model was developed using finite elements modeling to analyze the stiffness of the frames constructed according to the measured wire convergence angle and orientation. Five simulated configurations were tested. The stiffness of each frame was analyzed under four load conditions: torsion, axial compression, side bending, and anteroposterior bending.Results The mean convergence angle was 30° irrespective of the level. The bisector axis was oriented towards the anterolateral direction about 20° from the coronal plane. The stiffness of the frame constructed with a wire convergence angle of 30° and an anterolateral wire orientation of 20° was 20–30% less than that of the frame constructed with 60° wires oriented in a coronal plane. The addition of an anteromedial half-pin increased the stiffness significantly.Conclusions Two tensioned wires may be placed without violating the anterior compartment by using the above four clinically identifiable landmarks. Addition of a half-pin on the anteromedial surface of the distal articular fragment makes the frame markedly stiffer than is possible using the standard wire configuration.     

Stabile Ringfixateurmontage beim Segmenttransport

The stability of bone fragments within the Ilizarov external frame has a substantial effect on regenerate bone healing. The biomechanical performance of the frame is affected not only by manipulation of the different fixation parameters (rings, struts, wires, half pins) but also by the mode of loading and the patient??s ability of weight bearing. Because frame stability depends on complex interactions of the different frame components small modifications may have a substantial influence on interfragmentary movements within the regenerate and at the docking site. Manipulation of the different fixation components specifically to control the frame stability requires a precise understanding of the biomechanical principles of the external ring fixator. This article focuses on the biomechanical influence of the different fixation parameters as well as the mode of mechanical loading and common mechanical problems and complications.     

Biomechanical properties of external Ilizarov fixator with hybrid implants system]

Possible ways of improving the stability of the fragments of the femur under elongation by introducing hybrid implant systems (the Kirschner wires and the Schanz screws) into the Ilizarov fixator structure were explored. Experimental studies were conducted on physical models with the fixator mounted on a pipe section modelling the femur shaft. Ten modifications of the Ilizarov fixator were developed and tested. The designs differed in the kind and configuration of implants. The effect of the developed Ilizarov fixator designs was examined by comparing the coefficients of axial, transversal and torsional rigidity of the structures. The research results clearly show that the Ilizarov fixator's rigidity (particularly transverse rigidity) coefficients can be considerably increased by replacing the Kirschner wires with the Schanz screws or using hybrid systems of these implants. As a result, the stability of the femur fragments improves and so does the quality of the regenerated bone, which makes it possible to reduce the treatment time to a minimum.     

Reconstruction of infected nonunion of the distal humerus by Ilizarov external fixator

BackgroundInfected nonunion of the distal humerus represents a true challenge as the fragment is usually small and difficult for fixation. The aim of the present study is to assess the results of Ilizarov external fixator in management of infected nonunion of the distal third humerus.Material and methodsTwenty-three patients with infected nonunion of the distal humerus were included in this study. The ages ranged from 19 to 58 years with an average of 35 years. Seventeen cases were males and 6 were females. All patients were treated by radical debridement and application of Ilizarov external fixator in one stage surgery.ResultsBony union was achieved in all cases. Bone graft was required in 12 patients with hypotrophic nonunion. The external fixation time ranged from 4 to 9 months with an average of 5.6 months. Infection was controlled in all cases without recurrence during the period of follow up. The complications included pin tract infection in 7 cases, radial nerve injury in one case, elbow stiffness in 4 cases and refracture after frame removal in one case.ConclusionIlizarov external fixator is effective in management of infected nonunion of the distal humerus. The thin tensioned wires produce good grip in the small or osteoporotic bone fragments.     

Ilizarov bone transport for massive tibial bone defects

This article reports the treatment of massive tibial bone defects by bone transport using the Ilizarov external fixator. Fifteen patients were treated using this technique (3 females and 12 males). The defect size ranged between 7 and 22 cm (average: 10.6 cm). Etiology was infected nonunion in 9 patients, nonunion in 5 patients, and recurrent giant-cell tumor in 1 patient. The affected site was the tibial diaphysis in 10 patients, the lower tibial metaphysis in 4, and the upper tibial epiphysis in 1 patient. The external fixation time ranged from 9 months to 17 months (average: 12.27 months). External fixation index ranged from 21.8 to 42.5 day/cm (average: 35.7 day/cm). There was no recurrence of infection, no recurrence of the tumor, nor fractures after frame removal. We had to graft the docking site in 2 patients for delayed union and 2 patients developed equinus deformity and had tenoplasty for the Achilles tendon at the time of frame removal. Four patients had pin tract infection at > or =1 of the wires and this was successfully treated by antibiotic injection at the wire site. This study suggests that Ilizarov bone transport is a reliable method to fill massive bone defects.     

Segmenttransport

Segment transport is used to reconstruct bony defects after removal of the bone due to treatment for osteomyelitis. We use the Ilizarov fixator for the segment transport. The part of the bone that has to be moved through the defect is pulled by a lateral and a medial wire. To continuously determine the tension forces in the wires, a tension measuring cell with a resistance strain gauge was implanted in each wire. The readings showed the strength of the forces affecting the wires. A dependence of the length of the defect that has to be bridged and the forces that occur was found. Due to the differences between measured forces and expected forces complications, such as premature ossification of the transport stretch could be recognised early and treated. The aim is to develop an automatic motorised segment transport controlled by the forces created. Thus a continuous traction can be obtained with a constant distension stimulus for bone regeneration.