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In vivo measurement of bending stiffness in fracture healing.

Hente R, Cordey J, Perren SM - Biomed Eng Online (2003)

Bottom Line: Measurement of stiffness data over time revealed a significant logarithmic increase between the third and seventh weeks, whereby the logarithmic rate of change among sheep was similar, but started from different levels.Comparative measurements showed that early individual changes between the third and fourth weeks can be used as a predictor of bending stiffness at seven weeks (r = 0.928) and at ten weeks (r = 0.710).Bending stiffness can be measured precisely, with less error in the case of pin loosening.

View Article: PubMed Central - HTML - PubMed

Affiliation: University of Regensburg, Trauma Centre Regensburg, Franz-Josef-Strauss-Allee 11, D-93042 Regensburg, Germany. reiner.hente@klinik.uni-regensburg.de

ABSTRACT

Background: Measurement of the bending stiffness a healing fracture represents a valid variable in the assessment of fracture healing. However, currently available methods typically have high measurement errors, even for mild pin loosening. Furthermore, these methods cannot provide actual values of bending stiffness, which precludes comparisons among individual fractures. Thus, even today, little information is available with regards to the fracture healing pattern with respect to actual values of bending stiffness. Our goals were, therefore: to develop a measurement device that would allow accurate and sensitive measurement of bending stiffness, even in the presence of mild pin loosening; to describe the course of healing in individual fractures; and help to evaluate whether the individual pattern of bending stiffness can be predicted at an early stage of healing.

Methods: A new measurement device has been developed to precisely measure the bending stiffness of the healing fracture by simulating four-point-bending. The system was calibrated on aluminum models and intact tibiae. The influence of pin loosening on measurement error was evaluated. The system was tested at weekly intervals in an animal experiment to determine the actual bending stiffness of the fracture. Transverse fractures were created in the right tibia of twelve sheep, and then stabilized with an external fixator. At ten weeks, bending stiffness of the tibiae were determined in a four-point-bending test device to validate the in-vivo-measurement data.

Results: In-vivo bending stiffness can be measured accurately and sensitive, even in the early phase of callus healing. Up to a bending stiffness of 10 Nm/degree, measurement error was below 3.4% for one pin loose, and below 29.3% for four pins loose, respectively. Measurement of stiffness data over time revealed a significant logarithmic increase between the third and seventh weeks, whereby the logarithmic rate of change among sheep was similar, but started from different levels. Comparative measurements showed that early individual changes between the third and fourth weeks can be used as a predictor of bending stiffness at seven weeks (r = 0.928) and at ten weeks (r = 0.710).

Conclusion: Bending stiffness can be measured precisely, with less error in the case of pin loosening. Prediction of the future healing course of the individual fracture can be assessed by changes from the third to the fourth week, with differences in stiffness levels. Therefore, the initial status of the fracture seems to have a high impact on the individual healing course.

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Principle drawing of the bending stiffness measurement device. The measurement device is adjusted to the external fixator, aligning the longitudinal rod of the device with the neutral axis of the bone, and the strain gauges (rotational center) positioned at the fracture site. The longitudinal rod of the external fixator is then removed. When applying compressive or distractive forces between the clamps on each side, the measurement device bends through the center of the fracture, producing a four-point-bending moment between the inner pins. The angular deflection is measured by means of strain gauges.
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Figure 2: Principle drawing of the bending stiffness measurement device. The measurement device is adjusted to the external fixator, aligning the longitudinal rod of the device with the neutral axis of the bone, and the strain gauges (rotational center) positioned at the fracture site. The longitudinal rod of the external fixator is then removed. When applying compressive or distractive forces between the clamps on each side, the measurement device bends through the center of the fracture, producing a four-point-bending moment between the inner pins. The angular deflection is measured by means of strain gauges.

Mentions: In a former experimental study [28], we used a prototype of the measurement device in an animal model. The custom-made instrument for measuring the bending stiffness consisted of a U-frame that could be attached to the external fixator pins (Fig. 1). On both sides of the fracture, a rigid reversed U-profile was mounted on the two pins of each fragment. The clamps of the instrument were fastened to the pins, similar to a unilateral fixator. At the lower part of the U-frame, the junction bars of the instrument were connected to each other by two flexible blades, instrumented by strain gauges and functioning as angular displacement transducers (Fig. 1). The height of the instrument was positioned so that the central axis of the bone was in one plane with both goniometric blades (Fig. 2). To achieve this position, a postoperative radiograph was taken with the center beam exactly at the center of the gap, perpendicular to the plane of the external fixator. The distance from the outer surface of the proximal inner fixation clamp of the external fixator to the central axis of the bone was subtracted from the height of the measurement device. For each animal, a spacer of this distance was fabricated and slipped over the inner proximal pin to assure reproducible positioning of the measurement device.


In vivo measurement of bending stiffness in fracture healing.

Hente R, Cordey J, Perren SM - Biomed Eng Online (2003)

Principle drawing of the bending stiffness measurement device. The measurement device is adjusted to the external fixator, aligning the longitudinal rod of the device with the neutral axis of the bone, and the strain gauges (rotational center) positioned at the fracture site. The longitudinal rod of the external fixator is then removed. When applying compressive or distractive forces between the clamps on each side, the measurement device bends through the center of the fracture, producing a four-point-bending moment between the inner pins. The angular deflection is measured by means of strain gauges.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC153498&req=5

Figure 2: Principle drawing of the bending stiffness measurement device. The measurement device is adjusted to the external fixator, aligning the longitudinal rod of the device with the neutral axis of the bone, and the strain gauges (rotational center) positioned at the fracture site. The longitudinal rod of the external fixator is then removed. When applying compressive or distractive forces between the clamps on each side, the measurement device bends through the center of the fracture, producing a four-point-bending moment between the inner pins. The angular deflection is measured by means of strain gauges.
Mentions: In a former experimental study [28], we used a prototype of the measurement device in an animal model. The custom-made instrument for measuring the bending stiffness consisted of a U-frame that could be attached to the external fixator pins (Fig. 1). On both sides of the fracture, a rigid reversed U-profile was mounted on the two pins of each fragment. The clamps of the instrument were fastened to the pins, similar to a unilateral fixator. At the lower part of the U-frame, the junction bars of the instrument were connected to each other by two flexible blades, instrumented by strain gauges and functioning as angular displacement transducers (Fig. 1). The height of the instrument was positioned so that the central axis of the bone was in one plane with both goniometric blades (Fig. 2). To achieve this position, a postoperative radiograph was taken with the center beam exactly at the center of the gap, perpendicular to the plane of the external fixator. The distance from the outer surface of the proximal inner fixation clamp of the external fixator to the central axis of the bone was subtracted from the height of the measurement device. For each animal, a spacer of this distance was fabricated and slipped over the inner proximal pin to assure reproducible positioning of the measurement device.

Bottom Line: Measurement of stiffness data over time revealed a significant logarithmic increase between the third and seventh weeks, whereby the logarithmic rate of change among sheep was similar, but started from different levels.Comparative measurements showed that early individual changes between the third and fourth weeks can be used as a predictor of bending stiffness at seven weeks (r = 0.928) and at ten weeks (r = 0.710).Bending stiffness can be measured precisely, with less error in the case of pin loosening.

View Article: PubMed Central - HTML - PubMed

Affiliation: University of Regensburg, Trauma Centre Regensburg, Franz-Josef-Strauss-Allee 11, D-93042 Regensburg, Germany. reiner.hente@klinik.uni-regensburg.de

ABSTRACT

Background: Measurement of the bending stiffness a healing fracture represents a valid variable in the assessment of fracture healing. However, currently available methods typically have high measurement errors, even for mild pin loosening. Furthermore, these methods cannot provide actual values of bending stiffness, which precludes comparisons among individual fractures. Thus, even today, little information is available with regards to the fracture healing pattern with respect to actual values of bending stiffness. Our goals were, therefore: to develop a measurement device that would allow accurate and sensitive measurement of bending stiffness, even in the presence of mild pin loosening; to describe the course of healing in individual fractures; and help to evaluate whether the individual pattern of bending stiffness can be predicted at an early stage of healing.

Methods: A new measurement device has been developed to precisely measure the bending stiffness of the healing fracture by simulating four-point-bending. The system was calibrated on aluminum models and intact tibiae. The influence of pin loosening on measurement error was evaluated. The system was tested at weekly intervals in an animal experiment to determine the actual bending stiffness of the fracture. Transverse fractures were created in the right tibia of twelve sheep, and then stabilized with an external fixator. At ten weeks, bending stiffness of the tibiae were determined in a four-point-bending test device to validate the in-vivo-measurement data.

Results: In-vivo bending stiffness can be measured accurately and sensitive, even in the early phase of callus healing. Up to a bending stiffness of 10 Nm/degree, measurement error was below 3.4% for one pin loose, and below 29.3% for four pins loose, respectively. Measurement of stiffness data over time revealed a significant logarithmic increase between the third and seventh weeks, whereby the logarithmic rate of change among sheep was similar, but started from different levels. Comparative measurements showed that early individual changes between the third and fourth weeks can be used as a predictor of bending stiffness at seven weeks (r = 0.928) and at ten weeks (r = 0.710).

Conclusion: Bending stiffness can be measured precisely, with less error in the case of pin loosening. Prediction of the future healing course of the individual fracture can be assessed by changes from the third to the fourth week, with differences in stiffness levels. Therefore, the initial status of the fracture seems to have a high impact on the individual healing course.

Show MeSH
Related in: MedlinePlus