Measurement of fracture gap motion in external fixation

Fracture site motion with Ilizarov and "hybrid" external fixation

OBJECTIVES

To evaluate differences in fracture site motion by using different external fixators.

DESIGN

A wooden dowel was used to simulate a long bone with a transverse diaphyseal fracture. Ilizarov, "hybrid," and strutaugmented "hybrid" external fixation was used to stabilize the "fracture." The wooden dowel was subjected to separate axial, four-point bending, and torsional loads. Fracture site motion in the axial plane, off-axis motion (shear and bending), and rotation were measured.

SETTING

All mechanical testing was performed with a sevohydraulic test frame (MTS Systems, Minneapolis, MN, U.S.A.). Fracture site motion was measured with an interfragment motion device developed in this laboratory.

INTERVENTION

Comparison was made between a traditional fourring Ilizarov fixator, a "hybrid" fixator using rings and threaded pins attached by a unilateral aluminum bar, and a "hybrid" fixator augmented with a V-shaped strut.

MAIN OUTCOME MEASUREMENT

Load-deformation behavior in axial displacement, shear displacement, and bending displacement were compared between the different configurations under identical conditions of axial loading, torsional loading, and four-point bending. In torsional loading, rotational displacement was also measured.

RESULTS

The Ilizarov configuration allowed significantly less off-axis fracture site motion in all loading modes than either "hybrid" configuration while still allowing axial compression of the fracture ends.

CONCLUSIONS

In a completely unstable fracture with poor bone apposition, the mechanical behavior of a four-ring Ilizarov external fixator is superior to the mechanical behavior of a unilateral "hybrid" frame.

Stabilization of an inserted tricalcium phosphate spacer enhances the healing of a segmental tibial defect in sheep

The effect of inserting a tricalcium phosphate (TCP) spacer stabilized by a rigid or non-rigid fixation technique on the healing of segmental tibial defects of critical size was established. The osteotomized tibiae, 11 with and 8 without TCP spacers, were fixed by an external circular device in 11 mature sheep and by plates in 8 mature sheep, respectively. Healing was evaluated roentgenographically 16 weeks after the operation. Compared with the defects without TCP spacers, enhanced stability and healing were observed in the defects with TCP spacers under an identical external fixation. Furthermore, a significantly higher incidence of healing was obtained with plate fixation than with external device fixation in the TCP-implanted defects (P < 0.04). An abundant bridging callus was roentgenograpically demonstrated in most of the healed defects, but none in the unhealed defects. The TCP spacer with its mechanical integrity enhances the stability of external fixation, and the stable immobilization provided by rigid fixation is essential for osteoconduction of an inserted TCP spacer in the healing of segmental diaphyseal defects in sheep.

Effect of loading and fracture motions on diaphyseal tibial fractures

A computerized motion sensor was used to record the three-dimensional components of interfragmentary motion during healing in three patients with closed, low-energy fractures of the tibial diaphysis treated with functional braces. At the first measurement session 2 weeks after fracture, the patients applied approximately 15 kg to the injured limb. Although this produced 1-4 mm of translation of the fragments, this was recovered when the load was removed. The maximum rotational and angulatory displacements often occurred as the patients rose from the chair with no weight applied to the limb and frequently were reduced as the 15 kg of load was applied. Under load, the maximum axial rotation was 3 degrees and the maximum angular displacement was 1 degree. As with the translations, the initial rotational and angulatory positions of the fragments were recovered when the load was removed and the patient returned to the seated position. At 8 weeks, the patient applied full body weight, producing a maximum interfragmentary translation of 0.5 mm and maximum axial rotation or angulation of 0.5 degrees. Abundant peripheral callus formed in all three fractures, and they healed by 15 weeks through typical gradual consolidation and mineralization of the callus, accompanied by a corresponding reduction in interfragmentary motions.

Three-dimensional movement at externally fixated tibial fractures and osteotomies during normal patient function

Three-dimensional movements at four externally fixated tibial fractures and four leg-lengthening osteotomies were measured using the Oxford micromovement transducer. By measuring movements in 6 degrees of freedom during normal patient activity, both shear and angulatory displacements have been found to be of the same order as axial displacements, and are often more dominant. The magnitude of such movements may be as high as 2 mm and 2 degrees in early healing to a small fraction of this in later stages, regardless of whether the fractures are initially reduced and whether they are oblique or transverse. Also, the main bars of certain fixators have torsional 'backlash' looseness, causing significant torsional movement at the fracture and shear perpendicular to the plane of the bone screws. Further interesting findings concern the movement of fractures during walking, 'dynamization' (telescopic movement of the fixator bar) and muscle activity. Axial, angular, and shear movements reverse direction in phase with the 'heel-strike' and 'toe-off' sequence of walking. During dynamization (where the fixator bar is 'loose'), fracture movements at a given load are often less than when not dynamizing (where the fixator bar is 'locked'). Active muscle response can cause movements of 5-6 times greater than movements arising from the transfer of force through the tibia during weight bearing. Also, the tensile stiffness during the above activities has been shown to be more than an order of magnitude lower than the apparent compressive stiffness.

Control of motion of tibial fractures with use of a functional brace or an external fixator. A study of cadavera with use of a magnetic motion sensor

A computer-linked magnetic motion transducer was used to monitor and record the six components of motion of the bone fragments in eight cadaveric tibiae in which a simulated, oblique fracture of the middle of the shaft had been stabilized with a functional brace. The limbs were mounted in a servo-hydraulic testing frame, and a cyclic load of 150 newtons was applied along the axis of the tibia. Motion sensors, attached to each side of the fracture, measured and displayed the values of the three translations (axial, anterior-posterior, and medial-lateral), the axial rotation, and the two angulations (anterior-posterior and varus-valgus) as they occurred. Although only an axial load was applied, the off-axis motions were comparable in magnitude with the motion along the axis. The elastic (recoverable) translations of the fragments ranged from 0.5 to 1.9 millimeters, about four to ten times larger than the corresponding motions that were recorded in an earlier study of such fractures that had been stabilized with two types of external fixators. The recoverable rotation and angulations of the fragments of the limbs in the functional brace ranged from 0.7 to 1.2 degrees, about ten times those recorded when the external fixators were used.

Ex vivo biomechanics of Kirschner-Ehmer external skeletal fixation applied to canine tibiae

The purpose of this study was to determine the respective contribution of each of the following parameters to the compressive, bending, and torsional rigidity of the Kirschner-Ehmer (KE) external fixation splint as applied to canine tibiae with an osteotomy gap: bilateral versus unilateral splints; increasing the number of fixation pins; altering the diameter of fixation pins and side bars; decreasing side bar distances from the bone; increasing pin separation distances in each pin group; decreasing distances between pin groups; altering pin clamp orientation; and altering side bar conformation. Bilateral splints were 100% (mean) stiffer than unilateral splints, with stiffness enhanced to the greatest extent in mediolateral bending and torsion. Increasing pin numbers stiffened both bilateral (mean, 41%; 8 versus 4) and unilateral splints (mean, 14%; 8 versus 4). Medium KE splints were 85% (mean) stiffer than small KE splints. Decreasing side bar distances to the bone from 1.5 cm to 1.0 cm to 0.5 cm increased stiffness of both bilateral and unilateral splints by a mean of 13% to 35%. Widening pin spacing from 1.67 cm to 2.5 cm increased stiffness in craniocaudal bending only (56% increase, bilateral splints; 73% increase, unilateral splints). Decreasing the distance between pin groups from 5.84 cm to 2.5 cm increased stiffness in torsion between 23% (unilateral splints) and 45% (bilateral splints) and decreased stiffness of unilateral splints by 29% in craniocaudal bending. Altering pin clamp configuration so that the bolts of the clamp were inside the side bar rather than outside the side bar increased stiffness in axial compression only (73% increase, bilateral splints; 54% increase, unilateral splints). Conforming the lateral side bar to the tibiae increased only axial compressive stiffness by 77% but was no different than placing the clamps inside the side bars of an unconformed bilateral splint. These results quantify the relative importance of specific parameters affecting KE splint rigidity as applied to unstable fractures in the dog.

Evaluation of linear finite-element analysis models' assumptions for external fixation devices

Linear finite-element models (FEMs) have enjoyed an increased use in orthopaedic research, including the use for modeling external fixation devices. These fixator FEMs depend on a number of basic assumptions concerning the overall fixation frame stability and the components' rigidity. Among the more important ones are: (i) rigid fixation at both ends of the pin and sidebar; (ii) that the sidebar can be treated essentially as a rigid entity, with all bending occurring in the bone pins; and (iii) that the system can be treated as linearly elastic. Prior work done by the authors questions some of these assumptions. Thus, this study sought an empirical evaluation of the validity of some of these a priori assumptions. A Hoffmann single half-frame was tested in its standard form and then according to a stepwise protocol wherein the frame was welded to eliminate any possible points of instability. These tests looked at the stability and rigidity in various modes (axial compression, torsion, and medial-lateral and anterior-posterior four-point bending). The basic assumptions concerning the frame stability, frame rigidity and the frame's response to loads were found to be erroneous. Component failure was common under minimal loads and statistically significant differences (p less than 0.05) of up to 75% were noted in frame rigidity among the various frame forms tested. Thus, considerable caution must be exercised when employing the FEM technique for evaluating the fixator properties.

Measurement of fracture movement in patients treated with unilateral external skeletal fixation

Axial movement occurring at the fracture site has been determined in a group of healing tibial fractures treated by external skeletal fixation. Fracture movement was determined via a strain gauge transducer which was attached to the column of the external fixator and measured the deflection of the bone screw adjacent to the fracture site and the active loading or weight bearing given by the patient to the fractured limb was monitored using a force platform. The results for 27 subjects show that, with a rigid unilateral fixator, the axial movement occurring at the fracture site was initially small (mean = 0.28 mm at 5 weeks post fracture). This movement increases to reach a mean maximum value of 0.43 mm at 11 weeks post-fracture and then decreases, despite increased weight bearing, as fracture healing progresses. In the early stages of healing, the movement can be increased slightly if the fixator is fitted with a module which permits additional fracture site movement, although the resultant increase in movement is only a small proportion of the potential available with this module.

A strain-gauge study of the effect of external fixation on the canine tibia

Electrical resistance rosette strain gauges bonded directly to the surface of the midshaft of the normal canine tibia were utilized to study the effect of the AO external skeletal fixator on the magnitude and orientation of the peak strain values recorded during locomotion. The application of the fixator in a single lateral bar configuration resulted in a significant decrease in surface bone strain magnitude but in only minor changes in the orientation of those strains. However, in only one of three dogs tested did the addition of a second external connecting bar result in significant further decreases in bone strain.