Unilateral External Fixator For Tibia Fractures Biology Essay

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This study demonstrates the effectiveness of wire in unilateral external fixator for tibia fractures. It tries to combine the beneficial characteristic of unilateral and ring fixators to test the effectiveness of wire addition, on DePuy ACE-Fischer fixator (DePuy International Limited). Longitudinal force was applied to bone and two types of results were given. First, by controlling the displacement change, the amount of applied load was achieved. Second, the quantity of gap displacement was found by holding the input load roughly constant. This experiment was repeated 3 times on three different tibias.

Introduction:

Fractures are one of the most common types of orthopaedic problems and leading cause of admission to hospitals. All the bones can be broken from different parts and in various forms such as transverse (straight break), oblique (slant break), spiral (winding break) etc. However according to NHS annual statistics of 2009-2010, from 343,536 hospital admissions for fracture, 32% had transverse shaft fracture in their long bones (femur and tibia). Tibial shaft fracture is one of the most common types of long bone fractures and as Court-brown[1] found, there are 2 tibial shaft fractures per 1000 individuals per year in an average population. These fractures can be stabilised with different methods of fixation. And selection of best method depends on ''complexity of damage to bone and soft tissues, patient age, Anatomical access restriction''[2] and many other factors. However, Base on experimental results on 42 patient ''the external fixator was the best method of treatment in 87.27% of fractures'' [2, 3]. External fixators are versatile tools that have advantages of "percutaneous application and modifiable biomechanical characteristics''[4]

Generally, fracture fixation is a method to bring the fracture part of the bone all together and stabilise them until "they become strong enough for weight-bearing"[5]. It also assists the healing process to become faster. In addition, if the bone fragments place in a correct anatomical position, the result of fixation will be optimum and essential functionality of the damaged bone will return after healing.

There are two main types of fracture fixation: cast treatment and surgical treatment. Which internal and external fixation are the most important division of surgical techniques. As can be seen in figure1, each method can be divided into different systems as well; which will be discussed later.

Figure : Different types of fixators

Cast and splint treatment:

This method is suitable for simple fractures due to their limitation in control and movements of fracture segments. Since they place around the bone and soft tissues, the control of fragments should be performed by surrounding tissues which is not accurate. In contrast, they decrease the risk of infection because they are not in direct contact with skeletal system of human body.

Surgical treatment:

Internal fixator:

Internal fixation refers to some mechanical devices such as screws, plates, wires or rods implant into the body (underneath the skin) and fix around the fracture parts. So in this manner, position and movements of bone segments can be controlled.

Also, intramedullary nail or rod which is a tube-like implant, place in the middle of the bone and fixe there by some screws which are known as interlocking screws. They are usually used for long bone fractures like femoral shaft. However occasionally they may damage blood supply and form a new problem for healing procedure.

These implants can last into body forever. Whereas sometimes, high stiffness of implants could generate new fracture around the previous fracture parts, according to stress increase. Furthermore, internal fixation which implant inside the body, is an external device and the body may invade it. And it will increase the risk of infection in fracture site and make the non-union or delayed union problems.

External fixator:

External fixation can be used for fixation of fracture or to correct the limb deformity and limb lengthening. In this method, pins and wires which attached to the bone fragments should connect to external rod/ring by using some clamps. These devises can be assembled in various configurations and the final shape depends on type of fracture and the tissue injuries around the fracture site and surgeon experience. In addition, it should be mentioned that the pins and wires must insert to the bone from some predefined safe zone to avoid the risk of damage to tendons and neurovascular organs during their insertion. Therefore, some knowledge about the cross-sectional anatomy of the body limbs is necessary.

Some of the key advantages of external fixation which make them more common are:

They have good control on treating soft tissues injuries and open fractures. So when the extremity skin has trauma problem or it has poor ability to heal as in case of "rheumatoid, diabetes and Charcot disease''[6] it will be vital to avoid additional damage to already compromised limb.

External system can reduce body reaction against foreign implanted material, as most part of this fixator place outside of the skin. So they can also reduce the probability of infection which will be an essential factor to consider.[6]

External fixators can be divided into a few major categories:

Unilateral fixator:

In fact, unilateral fixation is one of the most popular types of frame, especially in the tibial shaft fracture. It is made of half pins which are fixed to the bone in linear configuration and act as cantilever. Also, they could be shaped as biplane configuration; which are known as bilateral and as a time being, they are rarely used. Beneficial features of this fixation method refer to their ''ease of application, access to soft tissues, and their ability to modify''[7]the frame configuration over time and its application is too simple and quick as well. The weakest point of this technique refers to lack of axial micro-motion due to high rigidity. [7, 8]

Taylor Spatial Frame (TSF):

This fixator made of ''two ring and 6 telescopic struts''[9, 10]. By adjusting the length of struts, the ring can repositioned with respect to other ring in every dimension so it give six degree of freedom to the system and make it capable of correcting "multi-planar deformities''[9] It has high accuracy in limb lengthening, correction of deformities and fractures. And the use of this fixator is easy as well.[9-12]

Ring fixator:

This fixation system consists of two major components, Rings and connecting rods. Rings can present as full ring, semi ring (partial ring) or arches. Ilizarove is the most common type of ring fixators which was initiated by Professor Ilizarov in 1950s.

In this technique, when the wires insert into the bone, they should be tensioned first. Then connect to the ring by bolts to ''suspend the bone securely within ring''[13]. So rings are used to maintain the wire tension and transfer the force from wires to the longitudinal elements[13]

Generally, full rings provide higher stability than arches. In contrast, the partial rings and arches can position better and can make the normal function of extremities much easier, especially for lower limbs applications.

In addition, there is a direct link between the ring properties and the frame stability. It means that the rings with smaller diameter are more stable. And according to results of Gasser et al s experiment[14], decreasing the ring diameter by 2 cm can increase axial frame stiffness by 70%. But the ring should not be too small to attach the limb. And at least minimum distance of "3.5 cm or two finger breath''[13] should be considered.[13, 15]Furthermore, the "distance between the rings and type of ring"[6] are other main factors to achieve a stable frame.

In spite of advantages of this method, it has lots of disadvantages as well. For instance, since in this method pins and wires enter to the limb from different directions, they might be dangerous for tissue, blood supply, and Nerves system. Moreover, application of this frame is very time consuming and it approximately takes" 6 hours ''[15]to be assembled.

As a matter of fact, ring fixation and unilateral are more common among large group of external fixators. To have a thorough understanding over these two techniques their mechanical properties should be compared, For instance stability, stiffness, rigidity etc.

First, the ideal external fixator should provide good stability for bone fragments in their alignment and they should be stiff enough to tolerate against the loads that people may apply during their daily activities without any deformation and crash. But on the other hand, they should allow axial micro-motion of the fragments by their elasticity. So the fixator should have stability and elasticity simultaneously. The ring fixator can achieve these two characteristics by wires. Wires with diameter of 1.6 mm and 1.8 mm are mostly used in this method and they are strong enough to tolerate the expected amount of load but they have low stiffness to allow axial micro-motion of bone pieces too. Also, the wire stiffness increase according to tension rise. So by applying more axial load, the wires become tenser and their stiffness will increase too, that is an important factor to avoid large movement of fragments. And a rigidity of tensed wire is equal to half pins but it still has its elasticity and low axial stiffness. Quite the opposite, the unilateral fixator are rigid and do not allow axial micro-motion of bone fragments and their dynamization is not as effective as ring fixator.[6, 8, 11, 13, 16]

In the next step, the acting force on the surface of the bone in unilateral fixation is just supported from one direction because the pins are like cantilever. Base of mechanics of deflection, the range of deflection in cantilever is higher than fixed- fixed beam (wire) and large loading on cantilever can cause an uncontrollable moment at fracture site.(fig 2)

Afterwards, since the wires are thinner, the dimension of the wire holes in the bone is much smaller than pin holes.

But the most vulnerable stage of ring fixator refers to time of assembly. Since assembling and stabilisation of configuration on patient is very time consuming. So it could be dangerous for elder patient or for patient with such cardiac disease to remain anesthetized for a long period.

Another critical parameter for ring fixator is its appearance which is too bulky and heavy as well. So it could make lots of problem for patient during his/her daily activities. This importance will come into sight especially for lower limb applications such as foot.

And at last but not least, the cost and appearance of ring fixators are other important parameters that made the patient to reject them.

Figure

After all the comparison between ring and unilateral fixation methods, it has found out that one of the most influential parameters which improve the stiffness of ring fixators and determine the forces which the bone is subjected to is wire. So this study tries to combine the beneficial characteristic of unilateral and ring fixators and test the effectiveness of wire addition, on DePuy ACE-Fischer fixator, since it is assumed that wire can make the fixator stiffer more stable. ''This fixator use both tensioned wire and half-pins, so that complex fractures can be supported with improved stiffness characteristics while still allowing easy wound access''[13]

Methods:

Experiment procedure:

Generally, this experiment consists of two main stages: first, preparation of bone, then test the fixator. The main process steps of each section are:

Preparation of tibia:

Design a configuration for external fixator

Drill the pin and wire holes on tibia

Frame assembly

Tension the wire

Make the fracture

Test the fixator:

Displacement control test: keep the gap displacement fixed to find the load variation

Load control test: investigate the displacement of the fracture gap during constant application of load

Tibia preparation:

Since tibial shaft fracture is one of the frequent types of long bone fractures it was decided to set the unilateral construct for a mid-shaft fracture. Thus the standardized transverse diaphyseal fractures of the tibia in Sawbones (#3401; Medium Left Fourth Generation Composite Tibia, Sawbones Europe AB Inc) was simulated and it was stabilized with two semi-rings (2/3 ring), two wires, three pins with diameter of 5mm and three connecting rods. One ring per each limb segment was applied on either side of proposed fracture with the upper rings positioned 86.5 mm and the lower rings 57.5 mm from the fracture site. The superior ring was mounted to the tibia with two 1.6 mm wires positioned 40/140 to each other and two half-pins.and the third pin was inserted into the bone through the lower ring (fig. 3a-b). Pins and wire were inserted perpendicular to the bone and attached directly to the ring using appropriate pin-holding clamps and cannulated bolts respectively.

both rings were connected by three connecting rods equidistant to each other. And all wires were inserted and tensioned to 600 N using an Instron tensioning machine.

The reasons for making the assembled constructions like this, was maximising the mechanical characteristics of fixation as possible. For instance, usually two wires are applied on each ring to achieve an adequate ''rigidity against rotational and translational bending''[6, 8, 9, 11, 13, 16]. Even though, sometimes additional wires are essential to reach the desire rigidity but it should be mentioned that it increase the risk of complication as well[17]. Also, crossing angle of the wire could effectively change the bending rigidity of fixator and It is better that crossing angle of wires be as close to 90°/90° as possible; because the results of previous experiments show 90° crossing angle produce the more stable configuration[13]. But because for this experiment 90° was not possible according to position of other devices the crossing angle of 40°/140° was selected. Moreover, the connecting rods which are used to connect the rings are other influential factors for stable assembly. Most of the fixators are made of two or three longitudinal elements. But when the tensioned wires are used in the configuration, at least three rods should be used.

By defining general configuration of the fixator, the pin and wire holes should be drilled.

(b)

(a)

Figure : the final configuration of the fixator: (a) Position and angle of wires with respect to rings. (b) Distance between pins wires and rings

The important point in drilling procedure is about the heat generation during the drilling that is more important in real bone drilling because it can make bone or soft tissue necrosis and may cause loosening of wire, so frequent stops during drilling can decrease the heat creation.

As it mentioned before, Since the position of the pins and wires were defined previously, by drilling the first hole for upper pin as reference hole other holes were drilled more easily; by dimensioning them according to the first one.

Figure : (a) drilling temple to fix tibia during drilling with drilling guide .(b)orientation plate which adjust the tibia in correct angle and also act as an adaptor to make a flat surface at both ends of tibia and align the holes in exact position

After drilling stage, the frame was assembled on prepared tibia. Initially, all three pins were inserted to the bone with pin inserter until they fixed firmly inside the bone and stay perpendicular to longitudinal axis of tibia. Then pin holders were inserted and secured on the pins. After that, the upper ring was attached to the pin holder. Followed by, attaching the lower ring to the lower pin holder in a way to position inline and parallel with upper ring and each segment of tibia. Due to 145 mm distance between 2 rings, all of the connecting rods were measures and fixed at length of 145 mm(they should be completely fastened that their length do not change). Then they were attached to the rings and fixed there. At last, all of the pin holders (their connection part to the ring and pins) and connecting rods were fastened to make sure that whole structure was stable enough. And the wire fixation clamps were placed in their predefined position according to the design of structure. (fig. 3a) these process steps should be done according to the above order to attain stable fixation.

In the next stage, wire was inserted into the bone and it was passed through the wire fixation device (clamp) on both sides of the ring. But one end of wires was fastened securely to guarantee that during the tensioning process, wire would never slip from that fixed end. Then the fastened end of wire was clamped and attached to the base part of tensioning machine and the loose end of wire was attached to the steel fixture. This steel fixture was attached to the load cell of Instron testing machine. The point in this machine is that the axis of wire should be parallel to the axis of testing machine to achieve an exact tension in wire.

For wire tensioning, there is not any ideal and specific amount of tension, however wires usually are tensioned in range of ''500N-1300N to obtain the greatest stiffness without introducing excessive risk of wire failure'' [17-19]. Although this amount of pre-tension always decrease during the treatment, due to slippage of wire inside the cannulated bolt and reduction of axial stress. Since these bolts apply large amount of force on wires to secure and keep them in place. As a result, the clamp ''squeeze the volume of wire axially outwards''[18] in the direction of applied force and due to this reduction of wire diameter the Poisson's ratio cause reduction in axial tensile strain which cause the reduction in axial tensile stress as well.

In this experiment it was supposed to tensioned the wire for 500N but according to the tension reduction which mentioned above and also because of the fixator configuration; because when the fixator was set into the Instron machine the axis of wire was not parallel to the machine axis; a little bit of tension was lost. And the resultant tension was less than 500 N so the wires were pre-tensioned for 600N to approximately achieve 500 N tensed wires. After that, the second bolt was fixed as well, to keep the wire tensed. Then the frame was removed from tensioning machine. . And the same procedure was done for the second wire.

Then, the osteotomy was completed using a reciprocating saw to cut the bone from identified point (it was in the middle of the shaft). Then the lengths of connecting rods were increased for about 2 millimetres to standardize a 2 mm fracture gap.

At last, to make the mechanical loading parallel to the mechanical axis of the bone and place the tibia perfectly straight in the testing machine, two steel pot shape instruments were mounted at both ends of the bone. Then the molded bone cement was poured in the pots to solidify there and keep the bone straight (fig.5a). The pots were parallel to each other and perpendicular to the mechanical axis of the tibia.

Test the fixator:

The fixator was tested using Instron 5848 materials testing machine (2 kN load cell in accuracy of 0.5±1N (mean ± standard deviation, SD); displacement range of 114mm and accuracy of 0.005mm).

The structure was fixed to the Instron machine with grippers in the way that bone positioned in vertical direction without any angulations. Four separate frame configurations were tested: (1) a fixator frame with 2 wires and 3 pins (2) 1 wire and 3 pins frame (3) 0 wire and 3pins frame and (4) structure without wire and with only 2 pins

Also, two types of test were done on each test. In one of them, 30 cycles of sinusoidal load with frequency of 1 HZ and gap displacement of 1mm was applied to the fixator to achieve the load variation which is applied on structure due to displacement change. And in the second mode, axial loads were applied and the fixator was cycled with 100 N at a frequency of 1 Hz for 30 cycles. Thus Under cyclic loading conditions, axial displacement of fracture gap could be measured.

The selected value of frequency refers back to the results of some experiment like Mireille et al[20], which shows during the walking the usual range of frequency change in both legs vary between the range of 0.75-1 Hz so the load of 1 Hz was applied into the bone. And the test was repeated three times, on three different tibias.

Figure : (a) tibia with assembled fixator that place in Instron machine. (b) Final configuration of tibia and fixator. (C) Instron tensioning machine which is used for tensioning the wires.

Results:

Figure 6 shows the graphs of displacement plotted against load for first tibia. And illustrate the essential force to displace fractured bone for 1mm.The load was applied parallel to longitudinal axis of the bone. As can be seen, the test with construction of 2wires and 3pins required more force to make the movement of 1 mm (about 80 N), and this load decreased to 76 N and 72 N for 1wire and without wire tests respectively; but the number of pins are still the same. And the most noticeable load change was found by removing one of the pins as well as wires which decreased the load to 60 N. Therefore, by gradual wire removal, the amount of applied force was decreased and displacement became easier. But for same level of displacement, the lowest amount of load was applied when one of the pins was taking away in addition to both wires.

On the other hand, the results of load control tests confirm the same theory, which application of equal amount of load in different modes of testing, cause higher displacement of fractured bone when the number of wires is less. As part (c) and(d) of figure 7 demonstrate, when the last wire was removed the displacement raise around 0.04 mm and pin removal increase this displacement further to 0.86 mm. But according to experimental results, the pins play more effective role than wires to stabilise the fixator and make it stiffer, because the effect of removing one pin is more noticeable than wire removal. For instance, comparison between part (c) and (d) of figure 6, prove that removing the second wire increase the load for about 4N. While taking out one of the pins make this load change equal to 12 N.

Alternatively, When investigating the stiffness characteristics of this fixator,(Stiffness values were defined as the slope of the load-displacement curve) it was found that it reduced by removing the wire and it reached its minimum value, when frame was assembled with only two pins. As can be seen form table 1, the results were the same in all three tibias. Therefore it can be says that existence of wire can increase the fixator stiffness.

But during the test of second tibia, due to reciprocal motion of bone, one of the grippers which was fixing the fixator and Instron machine was loosed so the bone started to tilting as well as longitudinal motion. Therefore, the graphs of second tibia are not entirely linear (fig.8). But on the other hand, it makes the test more similar to the real situation. Because, it is obvious that during the normal body activities such as walking, lots of forces are applying on the bone from different direction and could affect the fracture site. And longitudinal forces are just a simple division of these various forces. So result of test on second tibia could be more similar to in-vivo applications. As a result, the figure 8 shows that wire and pin removal can cause more significant and visible changes in load application when the tilting is added to axial motion. And the stiffness decrease rapidly too from 74.432 to 51.218 N/m. But still the fixator stiffness was at maximum value in 2 wire and 3 pins test.

Figure : results of displacement control test on first tibia in four conditions; the load variation is presented by keeping the displacement change at same level. (a) Test with 2 wire and 3 pins. (b) 1 wire and 3 pins. (c) 0 wire and 3 pins. (d) 0 wire and 2 pins.

Figure : : results of load control test on third tibia in four conditions; in all of them load remains approximately at same level to measure the displacement change. (a) Test with 2 wire and 3 pins. (b) 1 wire and 3 pins. (c) 0 wire and 3 pins. (d) 0 wire and 2 pins.

Figure : results of displacement control tests on second tibia in three conditions. (a) Displacement control test of 1 wire and 3 pins. (b) Displacement control test with 0 wire and 3pins. (c) Displacement control test on 0 wire and 2 pins construction

Table : fixator stiffness in different construction in all three tibias

Discussion:

Wire is an important factor influencing the stiffness and stability of an external fixator that normally is used in ring fixators. A number of recent studies have investigated the change in stiffness of ring fixators that can occur due to wire application. These studies have all shown that the tensed wire can determine the forces which the bone is subjected to and can increase the construction stiffness. The results of this study do not disagree with the findings of previous studies. And the results of the entire tests confirm that wires can increase the stiffness of structure even if they apply on unilateral fixations. Therefore, there is no difference between theoretical and experimental methods.

From mechanical point of view, according to fig.9 the lower half of the bone was fixed to the machine base and the load was applied on the upper half. And due to reciprocal motion of the bone, it can be assumed as simple spring in mechanical model (Eqn 4.2). Since the applied force just moves the upper half of the bone, it just affects the 2 wires and 2 pins. Therefore, the reaction force of 4 devices that connected to the upper bone responded to the applied load (Eqn 4.1)

On the other hand, in this model it was assumed that the bone is a rigid body, thus the displacement of whole bone should be equal to displacement of each pin and wire. (Eqn 4.3)

Finally, Eqn 4.4 shows that the stiffness of fixator is affected by stiffness of every connected pin and wire. for instance, by removing the first wire, the stiffness of whole structure will be decreased. Or even if one of the wire or pins suddenly removed from the fixator structure, by application of constant amount of load, the displacement will increase.

(4.1)

(4.2)

(4.3)

= (4.3)

(4.4)

results of this study demonstrate that the wire application can increase the stiffness of fixation and according to experiment's results on all three tibias, during the constant application of load, the bone with its connected fixator undergo 0.76±0.04 mm (mean ± standard deviation, SD) displacement, and removing the wire can roughly increase the displacement for 0.02 mm. It means that the wire can approximately decrease the gap displacement for about of the total displacement. In addition, for the duration of tests over constant bone displacement of 1 mm,when the wire were removed ,the resultant load on the fixation raise for around 5±3 N. whilst the total load variation was in the region of 70±9 N. Therefore it can be says that wire can tolerate of the total load which was applied on the fixator structure. And their deductions cause a sudden load increase on the fracture site.

But this amount of practical force was just a sample for in-vitro testing, and during the in-vivo models, lots of internal forces exist that act on the tibia under daily activities[21]. And this load does not have a specific value because it varies base on patient but the study by Wehner et al [22] concluded that the highest internal force that the tibial bone should withstand during the normal gait, is about 4.7 bodyweight. Therefore, if the normal weight of a patient assume as 60 Kg, his tibia should withstand 282 Kg during his walking. So along with the experiment achievements, the wire can tolerate and decrease near 20 Kg of this load which is not negligible at all. And can affect the gap displacement and healing procedure.

Most patients with frames do not fully weight bear initially following application of their frames. Because this is a defect model, we chose less than full weight bearing to simulate the known clinical situation. This simulates partial weight bearing for a limited testing period

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