Ilizarov wire tensioning and holding methods: a biomechanical study

Two Versus Four Ilizarov Rings in All Schanz Screw Fixation for Fractures of the Tibia: Experimental Mechanical Testing

The mechanical performance of the Ilizarov system is closely related to the biologic status of the fracture or osteotomy line. The aim of this study was to compare the mechanical performance of 2 different Ilizarov frame configurations. Two different frame models were applied to cow tibiae, 1 with 2 rings only, 1 proximal and another distal, vs the conventional 4 rings, 2 proximal and 2 distal. The 2 models were applied to both tibiae of the same animal to overcome variations between the animal's bones. The midpoint of the tibial bones was osteotomized, and the osteotomy was distracted for 2 cm. Six identical samples for each model were created, and each identical sample of each model was tested under axial compression. Mechanical testing was performed with a load cell of 20,000 N and a speed of 2 mm/min. Measurements were evaluated using load cell and goniometric grading; load deformation was calculated using load-deformation graphs. Our comparison showed that the 4-ring frame had 37.5% greater stiffness than the 2-ring frame under direct loading under larger loads (P<.01). A comparison between the 4- and 2-ring configurations demonstrated that the 4-ring frame had 7% greater stiffness than the 2-ring frame under direct loading under smaller loads (P>.05). An excellent outcome can be achieved by using a conventional 4-ring frame, but the use of the 2-ring frame remains an available option for fixation of fractures in low-weight patients. [Orthopedics. 20XX;XX(X):xx-xx.].

Influence of Different Connecting Rod Configurations on the Stability of the Ilizarov/TSF Frame: A Biomechanical Study

Aim

The Ilizarov external fixator (IEF) is frequently used in trauma and elective orthopaedics. Many of its biomechanical variables (ring size, wire diameter, wire number, half pins vs wires, etc.) and their influence on stability and stiffness have been investigated. There is, however, a paucity in the literature regarding the influence of the connecting rod numbers and configurations between the rings on IEF stability. The primary aim of this biomechanical study was to compare the stability between four- and three-rod IEF configurations. Secondarily to assess the difference in stability between symmetrical and asymmetrical spacing of the IEF rods.

Materials and methods

A custom jig was designed to facilitate mounting of a basic two-ring IEF in a hydraulic press. Controlled centre and off-centre (thus simulated bending) axial loading was then applied across the frame. The configurations were loaded up to 4,000 N. The frame deformation was plotted and the data were then analysed and interpreted.

Results

Negligible differences were observed between different four- and three-rod configurations as long as the applied force at the loading point (LP) was within the area of support (AOS) created by the rods. The different four-rod constructs were always more stable than the three-rod constructs during bending.

Conclusion

There is comparable stiffness between a four-rod and a three-rod IEF construct as long as the LP is within the AOS created by the rods. A four-rod IEF is stiffer than a three-rod IEF in bending.

Clinical significance

This study will possibly change some paradigms regarding the planning and application of IEFs by Orthopaedics Traumatologists and Reconstruction Surgeons.

How to cite this article

Thiart G, Herbert C, Sivarasu S, et al. Influence of Different Connecting Rod Configurations on the Stability of the Ilizarov/TSF Frame: A Biomechanical Study. Strategies Trauma Limb Reconstr 2020;15(1):23–27.

Biomechanical Analysis of the Behaviour at the Metaphyseal-Diaphyseal Junction of Complex Tibial Plateau Fractures Using Two Circular Fixator Configurations

Background

High-energy tibial plateau fractures are challenges in treatment with controversy over operative stabilisation, especially for fractures with metaphyseal–diaphyseal dissociation. Treatment with percutaneous or minimally invasive direct reduction techniques, usually associated with circular external fixation, has generated interest although there is no consensus regarding the type of external fixation to be used.

Aim

This study aims to compare the two hybrid circular external fixation mountings used to treat the high-energy tibial plateau fractures.

Methods

Two different groups of hybrid circular external fixation frame mountings were assembled using composite tibiae with proximal metaphyseal osteotomies simulating tibial plateau fractures with metaphyseal–diaphyseal dissociation. The standard all-wire frame mounting was assembled, and the comparison frame mounting had the distal K-wires replaced with half-pins. Both groups were tested through cyclic loading between 300 and 1000 N for 10,000 cycles. Interfragmentary linear and rotational displacements were analysed.

Results

The standard frame mounting behaved similarly to a classic Ilizarov frame, allowing greater axial movement (mean, 3.76 ± 0.26 mm in the standard group and 3.02 ± 0.23 mm in the test group) and smaller mediolateral displacement compared with the test frame (mean, 0.17 ± 0.16 mm compared to 0.56 ± 0.12 mm). The test frame behaved more similarly to a linear external fixator and provided greater axial stability, similar anteroposterior displacement, and lower mediolateral stability. Despite these differences, in both groups the axial displacement was greater than the prejudicial nonaxial movements.

Conclusion

Increasing the number of half-pins and decreasing the number of K-wires in hybrid circular external fixation generate frames that tend to behave more similarly to the linear external fixators.

How to cite this article

Cardoso GS, Amorim R, Penha FM, et al. Biomechanical Analysis of the Behaviour at the Metaphyseal–Diaphyseal Junction of Complex Tibial Plateau Fractures Using Two Circular Fixator Configurations. Strategies Trauma Limb Reconstr 2020;15(3):138–145.

Experimental model for controlling shear using the Ilizarov frame

OBJECTIVE

This study has been designed to ascertain whether a standard configuration Ilizarov tibial frame can control shear motion at the fracture site of an oblique fracture within acceptable limits for axial micro-motion, and therefore promote bony union. If not, are there simple modifications to the frame design that can achieve this?

METHODS

Four Ilizarov frame designs were tested on a load test rig. Fracture site shear displacement was measured using a clip gauge attached to an automated data recording system.

FINDINGS

The standard Ilizarov frame allowed 4mm of fracture site displacement at 340N of load. Our fourth frame design, the locked olive frame, allowed a maximum of 0.61mm displacement at 700N.

INTERPRETATION

This is one of only two studies to look at circular frame fracture site control in oblique fractures. This is the only study to characterise Ilizarov frame stiffness in oblique fractures. We have shown that the standard Ilizarov frame design is inadequate for control of oblique fractures in this mechanical model. Our data show that with the application of simple principles, the Ilizarov frame can be modified to provide better fracture site control. These frame designs can be applied clinically to reduce fracture site shear motion in oblique fractures, resulting in improved union rates.

The influence of a weight-bearing platform on the mechanical behavior of two Ilizarov ring fixators: tensioned wires vs. half-pins

BACKGROUND

A weight-bearing platform applied at the distal end of an Ilizarov external frame allows patients with hindfoot transfixations, foot deformities or plantar skin lesions to bear weight. This leads to an indirect loading of the fracture or osteotomy site. However, the effect on the fracture/osteotomy site's motion or compressive loads is unknown. The aim of this study was to analyze the mechanical effects of a weight-bearing platform on the traditional all-wire, four-ring frame in comparison to a two-ring frame consisting of half-pins.

METHODS

Two frame configurations, with either anatomically positioned wires or half-pins, were analyzed with and without a weight-bearing platform applied underneath the distal ring. Composite tibiae with a mid-diaphyseal osteotomy of 3.5 mm were used in all the experiments. An axial load was applied with the use of a universal test machine (UTS®). Interfragmentary movements, the relative movements of bone fragments and movements between rings were recorded using displacement transducers. Compressive loads at the osteotomy site were recorded with loading cells.

RESULTS

Indirect loading with a weight-bearing platform altered the force transmission through the osteotomy. Indirect loading of the tibiae decreased the extent of the axial micro-motion by 50% under the applied weight load when compared to direct weight loading (p < 0.05). The half pin frame was 25% stiffer than the wire frame under both direct and indirect loading of the tibiae (p < 0.05). Compressive loads under indirect loading were reduced by 67% in the wire frame and by 57% in the half-pin frames compared to direct loading of the bones (p < 0.05). While axial loading in the wire frames resulted in plain axial movements at the site of the osteotomy, it was coupled with translational movements and angular displacements in the half pin mountings. This effect was more apparent in the case of indirect loading.

CONCLUSIONS

A weight-bearing platform has substantial influence on the biomechanical performance of an Ilizarov external fixator. Half-pins induce greater stiffness to the Ilizarov external fixator and allow the usage of only one ring per bone segment, but shear stresses at the osteotomy under axial loading should be considered. The results allow an estimation of the size and direction of interfragmentary movements based on the extent of weight bearing.

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.

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.

Analytical modelling of Kirschner wires in Ilizarov circular external fixator as pretensioned slender beams

Transfixing thin Kirschner wires (K-wires) are the key components of the Ilizarov fixator regarding its axial stiffness, which affects the mechanobiological environment in which bone is healed. Mechanically speaking, K-wires are slender beams that are axially tensioned, then fixed and transversely loaded. The existing solutions to such a problem either do not accommodate any axial loading prior to transverse loading, or do not account for the change in the axial load (reaction) due to transverse loading. Their applicability is also limited vis-à-vis applied loads and beam dimensions. This work seeks to address those problems by providing a mathematical formulation for a pretensioned slender beam that accounts for the change in the beam tension due to lateral loading. Central loading of a pretensioned beam was studied and new polynomial equations have been derived, the roots of which yield the final tension for a (i) long, slender and heavily loaded beam and (ii) relatively thicker beam subjected to a lower load. Results were produced and discussed for the specific application of pretensioned K-wires in circular (ring) external fixators in orthopaedics (such as Ilizarov's), which were checked (validated) via two- and three-dimensional finite-element analyses.

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.

Wire tension in the Ilizarov system: accuracy of the wire-tensioning device

The Ilizarov fixator consists of tensioned wires that attach bone segments to a modular frame. The aim of this study was to establish the accuracy and precision of the wire-tensioning device supplied with the Ilizarov external fixation system. The device was used to tension a wire in direct opposition to a calibrated load cell. Five subjects tested three devices, at each of their four tension settings, in two separate sessions. Subjects could not see the true tension during the test. There were significant differences between the results for different subjects (p < 0.01) and instruments (p < 0.01) but not for different tension settings or between the two sessions. Overall mean measured tensions were 4.9 per cent (standard deviation, 4.4 per cent) below intended values. Tensions obtained at the maximum edge (completely occluded) on the scale markings were significantly (p < 0.001) closer to the nominal values (mean discrepancy, 3.6 per cent) than those at the minimum edge (mean discrepancy, 17.6 per cent). Several factors influence wire tension. Tensioning devices are not identical and the results obtained with them depend on the user. If the scale markings are completely occluded, the discrepancy between intended and actual tensions of around 5 per cent is likely to be adequate for clinical practice since surgeons do not select the most suitable tension following quantitative data assessment, but rather it is a judgement based on surgical experience and consideration for the patient weight and expected level of activity.

The accuracy of fine wire tensioners: a comparison of five tensioners used in hybrid and ring external fixation

OBJECTIVES

To compare the accuracy of 5 commonly available fine wire tensioners used in hybrid and ring external fixation.

DESIGN

A laboratory investigation.

SETTING

The testing of 5 commonly available tensioners was performed with a servohydraulic test frame (MTS Bionix 858, Minneapolis, MN).

MAIN OUTCOME MEASUREMENT

The real wire tension data of each tensioner provided by the MTS were compared with corresponding nominal values. The percent error for each tensioner was calculated. Clinical ease of usage of the wire tensioners was also evaluated.

RESULTS

The EBI tensioner was the most accurate (-0.17% to 0.09% error). The Smith and Nephew tensioner had a -13.97% to -8.61% error, the How medica tensioner a -12.48% to -10.86% error, and the Synthes tensioner a -0.2% to 24.28% error. The DePuyACE tensioner was the least accurate, with errors ranging from -36.76% to -30.92%. The Howmedica tensioner was the easiest to use, followed by the Smith and Nephew tensioner, the DePuyACE tensioner, the Synthes tensioner, and the EBI tensioner.

CONCLUSIONS

Most commonly available tensioners tend to undertension. Future efforts should focus on the development of wire tensioners that combine accuracy and ease of usage.