A novel device for resistance-free biomechanical testing of the metaphysis of long bones

Abstract

Background: Biomechanical testing is an essential component of bone research. In order to test the metaphyseal region of long bones, a typical location for the nowadays increasing field of osteoporotic bone changes, three-point bending and breaking test devices are suitable and widely used. The aim of our study was to increase the effectiveness of this method by using a newly developed ball-mounted platform design. This new design eliminates the negative effects of friction, present in previous studies, caused by the lengthening of the distal tibia along its diaphyseal axis while sliding over the surface of a fixed aluminum block.

Methods: 70 tibiae of 35 twelve week old, female Sprague Dawley rats were separated into two groups for a metaphyseal bending/breaking test. Group 1 was made up of the rat's right tibiae, Group 2 of the left tibiae. Group 1 was tested on a solid metal block according to previously established testing devices whereas Group 2 was tested on the newly designed device: the resistance-free gliding, ball-mounted platform. Stiffness (N/mm), yield Load (N), and failure Load (N) were registered. In the evaluation of both testing procedures, the results of the right and left tibiae were compared according to the rat they originated from.

Results: Stiffness (S) showed highly significant differences (p = 0.002) with 202.25 ± 27.010 N/mm SD (Group 1) and 184.66 ± 35.875 N/mm SD (Group 2). Yield Load (yL) showed highly significant differences (p < 0.001) with 55.31 ± 13.074 N SD (Group1) and 37.17 ± 12.464 N SD (Group2). The mean failure Load (fL) did not differ significantly (p < 0.231) between Group 1: 81.34 ± 11.972 N SD and Group 2: 79.63 ± 10.345 N SD.

Conclusions: We therefore conclude that, used in the three-point bending/breaking test, the mobile, ball-mounted platform device is able to efficiently eliminate the influence of friction in terms of stiffness and yield load. Failure Load was not affected. We suggest that the new ball-mounted platform device, when compared to other existing techniques, generates more accurate test results when used in the three-point bending/breaking test of the metaphysis of long bones.

Figure 1

Concept of the interaction of frictional forces and strength application needed for bending/breaking the tibia metaphysis. Both constructions are designed to provide surface contact for the tibia at the same height. A) Frictional forces are created (small green arrows) when, through the axial lengthening of the tibia diaphysis, the distal tibia moves across the metal plate (star) and thus a larger force for bending/breaking is needed (large red arrow). B) Frictional forces are eliminated through the mobile, ball-mounted platform (blue double arrow) and less force is needed (orange arrow) to bend/break the tibia metaphysis.

Figure 2

The newly designed, mobile, ball-mounted platform for the three-point bending/breaking test. A) The three-point bending/breaking device with the mobile, ball-mounted platform as a contact point for the distal diaphyseal tibia. B) Range of frictionless motion of the mobile, ball-mounted platform on the three-point bending/breaking device.

Figure 3

Three-point bending/breaking test set-up with the two different platform options. A) The three-point bending/breaking device strapped into a ZWICK-testing machine type Z020/TND (ZWICK-/Roell, Ulm, Germany) with a solid aluminum block as contact point for the distal diaphyseal tibia. B) Overview of the ZWICK-testing machine and the three-point bending/breaking device with the mobile, ball-mounted platform as contact point for the distal diaphyseal tibia.

Figure 4

Schematic drawing of the stamp which was connected to the ZWICK-testing machine and lowered onto the marked location of tibia metaphysis. The radius of the tip is 2.5 mm. The dimensions in the drawing are all in millimeter (mm).

Figure 5

Example of the graphical visualization of the data (distance (mm) travelled by the stamp and force (N) exerted by the stamp) recorded by the “testXpert” software during the bending and breaking test performed on the newly designed mobile, ball-mounted platform.

Figure 6

Results for stiffness, yield load, and failure load of the three-point bending/breaking test for Group 1 (solid aluminum block) and Group 2 (mobile, ball-mounted platform). The differences for stiffness were significant (p < 0.002) and the differences for yield load were significant (p < 0.000) (stars).