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Comparison between different methods for biomechanical assessment of ex vivo fracture callus stiffness in small animal bone healing studies.

Steiner M, Volkheimer D, Meyers N, Wehner T, Wilke HJ, Claes L, Ignatius A - PLoS ONE (2015)

Bottom Line: Differences in the experimental results between the bending tests originate in their respective location of maximal bending moment induction.Compared to four-point bending, three-point bending is easier to apply on small rat and mouse bones under realistic testing conditions and yields robust measurements, provided low variation of the callus shape among the tested specimens.Axial compressive testing was highly sensitive to embedding variations, and therefore cannot be recommended.

View Article: PubMed Central - PubMed

Affiliation: Institute of Orthopedic Research and Biomechanics, Center of Musculoskeletal Research Ulm, University Hospital Ulm, Ulm, Germany.

ABSTRACT
For ex vivo measurements of fracture callus stiffness in small animals, different test methods, such as torsion or bending tests, are established. Each method provides advantages and disadvantages, and it is still debated which of those is most sensitive to experimental conditions (i.e. specimen alignment, directional dependency, asymmetric behavior). The aim of this study was to experimentally compare six different testing methods regarding their robustness against experimental errors. Therefore, standardized specimens were created by selective laser sintering (SLS), mimicking size, directional behavior, and embedding variations of respective rat long bone specimens. For the latter, five different geometries were created which show shifted or tilted specimen alignments. The mechanical tests included three-point bending, four-point bending, cantilever bending, axial compression, constrained torsion, and unconstrained torsion. All three different bending tests showed the same principal behavior. They were highly dependent on the rotational direction of the maximum fracture callus expansion relative to the loading direction (creating experimental errors of more than 60%), however small angular deviations (<15°) were negligible. Differences in the experimental results between the bending tests originate in their respective location of maximal bending moment induction. Compared to four-point bending, three-point bending is easier to apply on small rat and mouse bones under realistic testing conditions and yields robust measurements, provided low variation of the callus shape among the tested specimens. Axial compressive testing was highly sensitive to embedding variations, and therefore cannot be recommended. Although it is experimentally difficult to realize, unconstrained torsion testing was found to be the most robust method, since it was independent of both rotational alignment and embedding uncertainties. Constrained torsional testing showed small errors (up to 16.8%, compared to corresponding alignment under unconstrained torsion) due to a parallel offset between the specimens' axis of gravity and the torsional axis of rotation.

No MeSH data available.


Related in: MedlinePlus

Results of the torsional testing.A) Torsional stiffness GI for the five different model designs S0, S1, S2, T7, and T14, under unconstrained (UT-left) and constrained (CT-right) axial torsion. B) The respective errors of the means in relation to the reference mean value (i.e. stiffness of S0 under unconstrained torsion). C) Reaction forces F at the gimbal in axial direction (z) and the secondary directions (x and y). D) Reaction moments M at the gimbal in axial direction (z) and the secondary directions (x and y). *p<0.05, ***p<0.001, ****p<0.0001.
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pone.0119603.g006: Results of the torsional testing.A) Torsional stiffness GI for the five different model designs S0, S1, S2, T7, and T14, under unconstrained (UT-left) and constrained (CT-right) axial torsion. B) The respective errors of the means in relation to the reference mean value (i.e. stiffness of S0 under unconstrained torsion). C) Reaction forces F at the gimbal in axial direction (z) and the secondary directions (x and y). D) Reaction moments M at the gimbal in axial direction (z) and the secondary directions (x and y). *p<0.05, ***p<0.001, ****p<0.0001.

Mentions: The torsional stiffnesses GICT measured in constrained torsional testing were 394±21 Nmm2/°, 418±22 Nmm2/°, 463±22 Nmm2/°, 396±21 Nmm2/°, and 406±9 Nmm2/° (mean±SD), for S0, S1, S2, T7, and T14 specimens, respectively (cf. Fig. 6). The measured torsional stiffness was significantly different solely between S2 and all other specimen designs. Furthermore, large constraining forces and moments (i.e. the same order of magnitude as in torsional direction) were measured in the secondary directions (Fig. 6C, D). Example graphs are shown in S5 Fig.


Comparison between different methods for biomechanical assessment of ex vivo fracture callus stiffness in small animal bone healing studies.

Steiner M, Volkheimer D, Meyers N, Wehner T, Wilke HJ, Claes L, Ignatius A - PLoS ONE (2015)

Results of the torsional testing.A) Torsional stiffness GI for the five different model designs S0, S1, S2, T7, and T14, under unconstrained (UT-left) and constrained (CT-right) axial torsion. B) The respective errors of the means in relation to the reference mean value (i.e. stiffness of S0 under unconstrained torsion). C) Reaction forces F at the gimbal in axial direction (z) and the secondary directions (x and y). D) Reaction moments M at the gimbal in axial direction (z) and the secondary directions (x and y). *p<0.05, ***p<0.001, ****p<0.0001.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4363594&req=5

pone.0119603.g006: Results of the torsional testing.A) Torsional stiffness GI for the five different model designs S0, S1, S2, T7, and T14, under unconstrained (UT-left) and constrained (CT-right) axial torsion. B) The respective errors of the means in relation to the reference mean value (i.e. stiffness of S0 under unconstrained torsion). C) Reaction forces F at the gimbal in axial direction (z) and the secondary directions (x and y). D) Reaction moments M at the gimbal in axial direction (z) and the secondary directions (x and y). *p<0.05, ***p<0.001, ****p<0.0001.
Mentions: The torsional stiffnesses GICT measured in constrained torsional testing were 394±21 Nmm2/°, 418±22 Nmm2/°, 463±22 Nmm2/°, 396±21 Nmm2/°, and 406±9 Nmm2/° (mean±SD), for S0, S1, S2, T7, and T14 specimens, respectively (cf. Fig. 6). The measured torsional stiffness was significantly different solely between S2 and all other specimen designs. Furthermore, large constraining forces and moments (i.e. the same order of magnitude as in torsional direction) were measured in the secondary directions (Fig. 6C, D). Example graphs are shown in S5 Fig.

Bottom Line: Differences in the experimental results between the bending tests originate in their respective location of maximal bending moment induction.Compared to four-point bending, three-point bending is easier to apply on small rat and mouse bones under realistic testing conditions and yields robust measurements, provided low variation of the callus shape among the tested specimens.Axial compressive testing was highly sensitive to embedding variations, and therefore cannot be recommended.

View Article: PubMed Central - PubMed

Affiliation: Institute of Orthopedic Research and Biomechanics, Center of Musculoskeletal Research Ulm, University Hospital Ulm, Ulm, Germany.

ABSTRACT
For ex vivo measurements of fracture callus stiffness in small animals, different test methods, such as torsion or bending tests, are established. Each method provides advantages and disadvantages, and it is still debated which of those is most sensitive to experimental conditions (i.e. specimen alignment, directional dependency, asymmetric behavior). The aim of this study was to experimentally compare six different testing methods regarding their robustness against experimental errors. Therefore, standardized specimens were created by selective laser sintering (SLS), mimicking size, directional behavior, and embedding variations of respective rat long bone specimens. For the latter, five different geometries were created which show shifted or tilted specimen alignments. The mechanical tests included three-point bending, four-point bending, cantilever bending, axial compression, constrained torsion, and unconstrained torsion. All three different bending tests showed the same principal behavior. They were highly dependent on the rotational direction of the maximum fracture callus expansion relative to the loading direction (creating experimental errors of more than 60%), however small angular deviations (<15°) were negligible. Differences in the experimental results between the bending tests originate in their respective location of maximal bending moment induction. Compared to four-point bending, three-point bending is easier to apply on small rat and mouse bones under realistic testing conditions and yields robust measurements, provided low variation of the callus shape among the tested specimens. Axial compressive testing was highly sensitive to embedding variations, and therefore cannot be recommended. Although it is experimentally difficult to realize, unconstrained torsion testing was found to be the most robust method, since it was independent of both rotational alignment and embedding uncertainties. Constrained torsional testing showed small errors (up to 16.8%, compared to corresponding alignment under unconstrained torsion) due to a parallel offset between the specimens' axis of gravity and the torsional axis of rotation.

No MeSH data available.


Related in: MedlinePlus