The hybrid ring tubular external fixator: a biomechanical study

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Clinical Biomechanics Vol. 12. NJ. 4, pp. 259-266, 1997 c) 1997 Elsevicr Science Limited. All rights reserved

Printed in Great Britain 02hX-lm3/97 $17.00+0.00

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The hybrid ring tubular external fixator: a biomechanical study

H Stein’, R Mosheiff*, F Baumgart3, R Frigg3, S M Perren3, J Cordey3

‘Department of Orthoptic Surgery Rambam Medical Center, Haifa, Israel; 2Department of Orthoptic Surgery Hadassah University Hospital, Jerusalem, Israel; 3A/0 Research Institute, Davos Platz, Switzerland

Abstract Objective. To measure and compare the mechanical properties in bending of the four-ring, and three-ring/one-tube hybrid external fixation frames. Design. In vitro measurements of the mechanical behaviour of ring and ring-tubular external fixation frames. In the latter, one ring of the full circular frame was replaced by one tube and Schanz screws. Background. The mechanical properties of the classical llizarov four-ring external fixation frames has been compared to those of other external fixation frames by various authors. However, in clinical practice the hybrid fixation frame is being used with increasing frequency. Therefore the mechanical properties of the latter are of immediate interest and clinical value. M&hods. On explanted sheep tibiae with single and double osteotomies, frame stiffness in the four-point bending mode was measured at different K-wire tensions, comparing the values obtained from four-ring frames, to those of three-ring-tubular hybrid frames. These measurements were made under conditions of (a) bone distraction (BD), and (b) segment transport (ST), both at the initial and final stages of this procedure. Results. In circular frames, frame stiffness in bending for increasing K-wire tension showed a Gaussian distribution both in distraction and post-ST with an optimum at 1000 N. In ring tubular hybrid frames, however, frame stiffness showed a more linear relationship to K-wire tension. Conclusions. In the four-ring llizarov external fixation frame, the exchange of one ring with one tube and one Schanz screw both reduced frame stiffness in bending and converted to linear its relationship to K-wire tension.

Relevance Under clinical conditions, the use of a similar ring tubular hybrid external fixator allows the adjustment of frame stiffness in a simple and practical way. This is not the case with the original ring fixation frame. 0 1997 Elsevier Science Ltd.

Key words: llizarov external fixator, frame stiffness, K-wire tension, ring/tubular hybrid frame

Clin. Biomech. Vol 12, No. 4, 259-266, 1997

Introduction nutrient vessels. They glide against the tissues while

The Ilizarov ring external skeletal fixator is based upon tensioned Kirschner wires transfixing bone as the sole means of osseous lixation’,2. These tensioned wires supported by rings provide a three-dimensional system of external fixation. Its proposed advantages are the smooth wires, which cause little if any damage to the osteogenic elements of bone marrow or the

Received: 22 December 1995; Accepted 17 January 1997 Correspondence and reprint requests to: Haim Stein MD,

D.Phil(Oxon) Department of Orthoptic Surgery, Rambam Medical Center, Haifa, Israel

the ring apparatus resists torsion, shear, and bending. Yet it retains an axial excursion due to its low stiffness. This stiffness depends upon ring diameter and material properties as well as upon K-wire tension. Therefore the Ilizarov system was regarded as being axially dynamic’T2.

The surgical intervention for its application is minimally invasiveim4. Since only thin Kirschner wires are employed, two or more wires must cross at each level of fixation to stabilize the osseous fragments. It is extremely versatile and has a wide range of clinical indications for use, i.e. fractures, pseudarthroses,

260 Clin. Biomech. Vol. 12, No. 4, 1997

large bone defects, limb shortening, malalignment, and deformities.

Often, beaded or kinked wires were utilized to prevent bone fragments from sliding along the wires’,*. They were later replaced by ‘olive’ wires. Its mechanical behaviour is non-linear. This fixation frame promotes osteogenesis by traction and reduces tissue strair+.

Based upon this concept, Monticelli and Spinelli in Italy’, J.M. Hardy and his group in Frances, Wasser- stein in Germany’ and others developed similar fixators. The latter system are versatile yet difficult to apply and time consuming to manage postoperatively.

A different external fixation concept was developed by Heinz Wagner. It consisted of a rigid unilateral (cantilever pin) frame used for relatively rapid distraction ‘” This device has been used by . other authors for distraction osteogenesis with good bone formation”. In 1977 DeBastiani et al. further developed this concept by adding a telescoping component that could be released for dynamic bone loading . ‘* Kenwright added cyclic loading to a similar framei3. Half-pins have also been described by Fleming et ali4 and Green”,i5 as causing minimal damage to the surrounding soft tissue and allowing a high degree of freedom for their insertion into anatomically safe areas. As a rule, cantilever fixators, although slow in promoting the formation of bone regeneration, are simple, readily accepted by patients, and easy to care for. Further advantages of half-pins include less pin site infection, preservation of the range of adjacent joint movement, reduced require- ments for pain medication and higher ambulatory capacity i1,‘*,i6. However, these fixators do not permit immediate weight-bearing and leave large holes in bone when the pins are removed.

In order to preserve Ilizarov’s concept of distraction osteogenesis while using simple unilateral cantilever pin hxator constructions, Alonso and Regazzoni modified the popular AO/ASIF tubular fixator (Synthes Co. Ltd, Switzerland) for the treatment of segmental defectsi6.

In Lecco, Cattaneo and Catangi have replaced part of the rings in the Ilizarov frame with half or one-third rings, fixed to bone with Schanz screws. Thus the Ring Hybrid frame came into clinical use. However, the mechanical properties of such a device need to be defined so that its clinical applications will be based on measured data.

In order to retain the versatility, stability, and biomechanical advantages of the ring external fixation frame and to combine it with the construction simpli- city and implant tolerance of the monoplane cantilever external tixators, the authors designed a hybrid model which is based on two very popular systems: the AO/ASIF tubular external f?xator and the Ilizarov ring apparatus. Special rings, bolts and nuts were designed in order to achieve a comfortable connec-

tion between the two systems. The hybrid ring tubular frame was constructed by exchanging one ring of the Ilizarov four-ring frame with one tube and one Schanz screw. Its mechanical properties were studied and compared to the analogous ring fixator. The two devices were examined in vitro while the K-wire tension was gradually elevated. Both the distraction and segment-transport configurations were analysed. The results of these measurements are the subject of the following report.

Methods

Tibiae from six Swiss mountain sheep were harvested from the lower limb between the knee and the ankle. The covering soft tissues were removed. These explanted bones were deep frozen and then thawed for analysis at room temperature.

To create the hybrid system, one A0 external fixator rod with its 4.5 mm Schanz screws was connected to specially designed stainless steel rings and connecting devices machined at the A0 Research Institute (ARI, Davos, Switzerland) for this experiment. Frame components were ring diameter = 110 mm, standard 1.6 mm K-wires, threaded external fixation rods 8 mm in diameter, nuts and washers (Synthes Co., Ltd. Bettlach, Switzerland). This modified modular system was assembled into modular frames which simulated clinical treatment modalities (Figure 1). Bone with a 2 cm osteotomy gap was investigated under conditions of:

1. bone distraction; 2. segment transport, with the segment in its original

position; 3. segment transport, after 2.0 cm of distal transport.

For modes 2 and 3 a second osteotomy was added, 2.0 cm proximal to the original resection osteotomy.

All models were examined in pairs, ring opposite the comparable hybrid frame (Figure 1). Mechanical testing was performed in a four-point bending mode in the anterior-posterior plane. These tests were performed on a Microtron testing machine (Rumul, Russenberg and Multer, Schafhausen, Switzerland), with a four-point testing devicei7. This device has been designed by one of the authors (JC) and has been in continuous use since 1980 (Figure 2). The machine is load cell equipped for measuring the applied load; maximal load admitted by it is 1 kN with an accuracy better than 2%.

The bones were treated in bending for the following reasons:

1. The geometrical bone axis, the centroid of its cross-section, is the neutral axis.

2. Axial load is the most commonly applied load vector to long bones. Since it cannot follow the neutral axis, it results in bending strains.

Stein et al: Hybrid ring tubular external fixator 261

Ring Fixator

Model 1: Bone Distraction

Model 2: M;del3: Segment transport Segment transport before transport. after transport.

Hybrid Fixator Figure 1. Graphic computer design of the ring (top) and ring hybrid (bottom) frames used in this study.

3.

7.

Bending strains are linearly proportional in magnitude to the distance from the applied axial strain to the neutral axis. The larger amount of this distance is outside the bone. From the biomechanical point of view, bone is prone to fail where the bending strain is largest. Therefore, in vitro testing of bone in bending is more physiological than testing under axial strain. Stiffness is also defined as the ratio between load and deformation. In fractures or osteotomies, the application of a constant bending moment throughout the intact bone ends will induce angular displacement at the fracture site. In four- point bending, the bending moment remains constant along the whole bone axis. Measuring the angle induced by four-point bending is very exact since it is measured over a constant length of bone (Figure 2a). The bone ends were fixed in PMMA cylinders, which were inserted into the loading machine (Figure 2b, 2~). It could be turned around its longitudinal axis, tension being applied at the upper and compression at the lower end (Figure 2~). When force F is applied to the loading bars, F/2 is applied to each loading block, and L is the distance between the axes of the bars which pull up and those which

8.

pull down. The resulting bending moment M therefore is:

M = F.L/2 Angular displacement was measured using a self- designed strain-gauged goniometer”. It consisted of a flexible blade deformed as a cantilever fixed to one diaphyseal fragment by four, and to often diaphysed fragment by one K-wire (Figure 2d). The blade was calibrated in degrees” with the osteotomy gap opposite the middle portion. The flexible length of the blade was 50 mm. The angular displacement was measured for a bending moment of 10 Nm. The stiffness of the fixation frame and bone was

defined as the bending stiffness measured. The latter was the ratio between applied load-bending moment and the resultant angular displacement (Nm/degree).

The axial displacement in the osteotomy gap does not influence the angle measured by the flexible blade. It was measured with a goniometric blade instrumented with strain gauges (Figure 2d). Precision was very high, accuracy being -t 0.25”.

Stiffness relates inversely to the angle of deformation. In segment transport bending the two main segments may result in the free segment being shifted in an unpredictable direction in an in vitro situation;


the latter may even project out of the bending curve. Such a situation cannot develop in vivo, where the surrounding soft tissues create ligamentotaxis.

During testing, the bones were kept moist. A removable tensioner was used to achieve predeter- mined values of K-wire tension. The investigated tension values were 500, 1000 and 1300 N. A four- point bending force was applied continuously between zero and 400 N to each of the bones with the pretensioned frame in place. The available six tibiae (animals) were tested in pairs. Each frame fixation was independently tested three times.

The degree of angulation indicated the deformation of the apparatus at a given value of K-wire tension. It was recorded on an x/y recorder (Graphtec Co Ltd.). Using these data, the relationship of deformation/K-wire tension was measured for all models, so that the angulation bending force curve was displayed (Figure 3). The applied bending moment

a

b

C

d

Figure 2. Graphic representation of the four-point bending test. (a) The principle. (b) Fixation of bone and frame of both ends with PMMA cylinders. (c) Design of the testing mode. (d) Measurement of the bending angle.

0 12 3 (NW Figure 3. Typical example. Compliance curves during a four- point bending test of the ring frame at 500 N K-wire tension segment transport model.

was within the physiological range (the bending moment for failure of a human tibia is 50 Nm).

Statistics were not used in this study. The question was: how does the mechanical change in the configuration of the four-ring external fixation frame, induced by the replacement of one ring with one tube, influence frame stiffness, in vitro? With 4-10 measurements for each studied point and an accuracy of measurements of bending in the range of +0.25”, statistical evaluation is both superfluous and meaningless.

The force applied in the measuring 400 N, therefore the resulting bending was:

M = 400N x 0.05 m/2 = 10 Nm

machine was moment (M)

Model 1: bone distraction, after 2.0 cm of elongation (Figure 1)

A ring fixator system consisting of four rings was applied to the sheep tibia. Two K-wires crossed at 90” in each ring. They were used to mount the external fixator to bone. A 2 cm standardized bone segment was resected with the proximal cut 12 cm above the tip of the media] malleolus (10% of the diaphyseal length). To create a parallel hybrid system, the upper (proximal) ring was exchanged for a 4.5 mm Schanz screw and an 8 mm transport rod; Figure 1 shows the ring frames in the top row, and the corresponding hybrid models in the bottom row.

Model 2: segment transport, before transport (Figure 1)

A 2 cm bone segment was resected (10% of the diaphyseal length) as in model 1. A second osteotomy was performed 2.0 cm proximal to the resected area. Thus two osteotomies produced a free 2.0 cm long bone segment. The fifth ring of the apparatus carried


this fragment by two transfixing K-wires crossing at 90”. In this model the bone fragment was left in place to simulate the clinical situation before transport.

Model 3: segment transport, after transport (Figure I)

The configuration was identical to model 2. In this model, the bone segment was moved on the frame to dock with the distal tibia. This simulated the clinical situation following transport.

Results

Compliance is the inverse of stiffness and appears to be a more ‘user friendly’ term for surgeons than stiffness (the latter being better accepted by engineers). Higher compliance allows higher deformation. There- fore, angular deformation curves for a bending moment of 10 Nm display the compliance of the system.

Angulation/bending moment curves were measured for the ring fixators at various values of K-wire tension. This relationship was non-linear. Figure 3 demonstrates the compliance curve of the ring fixation frame before and after segment transport with K-wire tensioned to 500 N. Data on the compliance and stiffness of the distraction model, in the two frames, and at various K-wire tensions are described in Table 1. Elevation of K-wire tension caused an increase in frame stiffness in a Gaussian-like distribution. Hence, too high a K-wire tension ( > 1000 N) led to the risk of failure in frame stiffness (Figure 4a,c). This loss of stiffness in spite of the increase in K-wire tension was called the ‘yield point’ of the system. The pattern of biomechanical behaviour under progressive elevation of K-wire tension was similar in the ring frames both in distraction (model 1, Figure 4a) and in post-ST transport (model 3, Figure 4c) with a loss of stiffness at K-wire tensions above 1000 N.

Hybrid frames exhibited similar patterns of mechanical behaviour and stiffness at low K-wire tension. Elevating the wire tension in them from 500 to 1000 N increased frame stiffness. A ‘yield point’ was not found (Figure 4a-c) in either of these three

Table 1. Data of compliance and stiffness of the lengthening model, in either one of the two frames, at the three measured K-wire tensions

K-wire tension Compliance Stiffness

Lengthening without callus, with ring fixator 500 N 4.5 [“/IO Nm]

1000 N 2.9 [“/IO Nm] 1300 N 6.6 [“/IO Nm]

Lengthening without callus, with hybrid fixator 500 N 9.0 [“/IO Nm]

1000 N 8.7 [“/I 0 Nm] 1300 N 8.1 [“/IO Nm]

2.2 [NmP] 3.5 [NmP] 1.5 [Nm/“]

1.1 [NmP] 1.15 [NmP] 1.2 (Nm/“]

models or in the ring frame of the ST before transport model 2 (Figure 4b). This latter model, however, has no clinical application.

Discussion

The mechanical properties of external fixation frames have a direct influence on the rate of fracture healing'-16.1%27, i.e. their biomechanical action is directly influenced by these properties. This demonstrates the intimate interrelationship of the mechanical properties of fixation devices used in medical practice and their biomechanical function, an issue recently addressed by Chao2”. A study of this nature must firstly be performed in an in vitro situation, since the in vivo conditions are complex and multi- factorial. Therefore, the isolation for analysis isolated forces is impossible.

Hybrid ring fixation frames of various configurations are gaining clinical popularity. This is based both on patients’ improved acceptance of these frames and on easier insertion of Schanz screws into areas rich in neurovascular structures, thus presenting to surgeons a complex and difficult surgical anatomyih.‘“. The mechanical properties of such frames are not well defined, and their biomechanical capability to promote or retard bone healing has not been documented.

In this study the mechanical properties of the ring and ring tubular hybrid frame were compared in vitro under conditions simulating their clinical use. The K-wire tensions were those used in clinical practice.

The measured values relate to frame stiffness, i.e. the slope of the linear part of the curve. During the first load application to the system, this slope became almost constant. Therefore the values are reported as points. These very small statistical variations (i.e. the standard deviation of the measured values) could be seen as a precision, and show that it is smaller than what we would expect if we were to repeat the measurements in a series of newly constructed models.

The investigated variety of the hybrid ring/tubular tixator was found to have mechanical properties which are easier to manipulate and control by increasing K-wire tension. This tension appears to decline spontaneously because of the frame’s mechanical propertieP”.

The biomechanics of ring and cantilever fixation are different in principle. Ring fixation incorporates a ‘trampoline’ effect that allows axial compression and distraction during gait’. This effect occurs both during distraction and bone healing. Bending and shear stresses are minimized by perpendicularly placed transfixing wires. These stresses may be further decreased by the use of olive wires, which prevent lateral bone slippage. Tensioned wires and multiple rings per bone segment further reduce


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Figure 4. Frame deformation in four-point bending of the ring and ring hybrid frames at different K-wire tensions. Each point was measured 4-10 times, SD smaller than the size of displayed points. (a) Bone distraction - model. (b) Segment transport (before transport) - model. (c) Segment transport (after transport) - model.


bending and shear stresses. The ring frame remains less rigid in the modes critical for promoting bone healing than cantilever fixator5,6,32. The number and diameter of K-wires in each ring determine the local contact pressure transmitted from wire to bone.

Cantilever fixation results in resistance to bending forces only in the plane of the fkator, while less rigidity is present in the plane perpendicular to the fixator frame. Hence tensile force (in the plane of each pin) at the osteotomy site is applied eccentric- ally by half-pins and concentrically by full pins. Circular frames with their transfixing K-wires load more evently across the bone defect in bending and torsion compared with uniplanar frames, i.e. there is less deformity in the former.

The measurements in this study consisted of a preliminary series of four successive measurements under continuous loading and unloading of the frame in bending. The resulting angular deformation was measured in relationship to the bending moment applied. On these curves, an initial shift of the angular deformation during loading and unloading was observed. When the curve stabilized, it become reproducible. Then three repeated measurements were performed, the results being identical. This behaviour may be explained by the initial adaptation of the frame (and particularly of the K-wires within the bone) under the first loads. Later the frame becomes stable (steady state of motion). The difference between loading and unloading displayed the K-wire slippage which occurs in these conditions3”,“‘.

In the ring frame, K-wire tension greatly contri- butes to frame stiffness, but the exact significance of the latter has not been clearly defined. Aronson and Harp6 have reported that the magnitude of wire tension is critical for the amount of new bone formation in a canine fracture model.

We found a non-linear relationship between wire tension and frame stiffness. Hence the clinical success of the ring fixator might be related to this axial non-linearity. This behaviour is not based on friction, wire slippage, or plastic deformation of the constit- uents. It is a reversible phenomenon of pure geometric non-linearity. The low stiffness at low loads may be used to promote fracture healing, whereas the increased stiffness at higher loads may protect the repair tissues from excessive strains. This is supported by the finding of Gasser et al.” and agrees with current concepts of bone healing. The enhanced rate of bone healing creates biological stability/stiffness of the healing bone gap.

The ring/hybrid frame demonstrated low axial stiffness at low loads and increased stiffness at higher loads. Thus it maintains the advantages of the Ilizarov frame, i.e. it may promote osteogenesis with immediate full weight-bearing, yet it is lighter and more user friendly for both surgeon and patient.

A consistent difference between the biomechanical performance of the ring and hybrid frames was observed both in distraction and in segment transport, with a rise in stiffness related to higher K-wire tension. This relationship approached linearity in the hybrid frame. In comparison to the ring fixator, the hybrid ring frame becomes more rigid when higher tension is applied to the transfixing wires. It can bear higher values of K-wire tension without reaching a ‘yield point’.

Thus, in the ring external fixator. replacing one ring by a 4.5 mm Schanz screw with a connecting tube created a hybrid frame less rigid than the original ring fixator, but with an improved and more linear relationship between K-wire tension and frame stiffness. This may be further altered by the insertion of more than one half-pin as proposed by Calhoun et a1.“2-“‘. Furthermore, the ring/tubular hybrid is mechanically controllable and can be adjusted by the surgeon to existing clinical conditions.

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The hybrid ring tubular external fixator: a biomechanical study

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