Med. & Biol. Eng. ~&Comput., 1977, 15, 207-208
Technical note Instrumentation for measuring the tensile properties of femoral cortical bone Keywords--Bone, Instrumentation, Tensile strength Introduction THE raECHANICALproperties of h u m a n cortical bone have received considerable attention (REILLY and BURSTEIN, 1974), but, to date, there is little published information on these properties when bone is subjected to loads at a fast rate to simulate trauma (MC]ELHANEY, 1966; CROWNINSHIELD and POPE, 1974).
E
E
oli radii 0-50
/
X "X
12
20.50
30'00 air dimensions mm
Fig. 1 Testpiece dimensions
oscillating at + 1 mm amplitude. The maximum ram speed is 50 mm/s. A strain-gauged extensometer was attached across the gauge length of the specimen, see Fig. 2. A small preload was applied to the arms of the extensometer before testing. Using an extensometer such as this is inherently the most accurate method of measuring the increase in length of a tensile test specimen under load. This method measures the change in length directly, and is not influenced by any embedding of the specimen in the grips. On calibration, the extensometer had a linear range of _+3 m m and it has a natural frequency of. 2 kHz. The calibration curve was repeatable to within 1%. The load exerted on the specimen was measured by a load cell incorporated in the crosshead of the testing machine. Recent work has brought attention to the importance of energy absorption to fracture as an important mechanical property of bone (PIEKARSKI, 1970; POPE and MvRenv, 1974). An electronic integrating device was built to record the transient load-displacement characteristics, and from these to compute the area under the load-displacement curve; this is the energy absorbed by the specimen. Fig. 3 is a block diagram illustrating its operation. The device has two modes of operation: 'read' and 'write'. In the 'write" mode, the analogue information from the load and displacement transducers is converted
In order to apply and monitor the load and displacement on the test specimen, an instrumentation system, based on a servo-hydraulic testing machine, has been developed. The system described (vide infra) is being used to determine the effects of immobility on the mechanical properties of bone.
Material R o u n d dumb-bell-shaped specimens were machined from the middle third of fresh, human, femoral, cortical bone, obtained at post mortem. The dimensions of the test piece are shown in Fig. 1. The specimens were kept wet, with physiological saline, while being machined and tested. Considerable care was taken in the selection of specimens. Clinical histories were obtained, and for normal subjects bone was not used if there was any possible skeletal defect. The criterion used for the immobilised bone was six weeks immobilisation. Instrumentation system The tensile specimens were inserted into two carefully constructed grips mounted on the servo-hydraulic testing machine. The hydraulically driven, servocontrolled ram has a fiat response from 0 to 4 Hz when First received 22nd April and in final form 7th June 1976
Medical & Biological Engineering & Computing
Fig. 2 Extensometer in position on test piece
March 1977
207
to a digital signal and stored in a 256 bit memory. A warning light indicates if the store capacity has been exceeded. In the 'read' mode the stored information is reprocessed to produce the analogue information from the transducers, the period of scan is set by a 5-position rotary switch and allows the stored information to be read out at a fast or slow rate for display on an Oscilloscope or an xy-plotter. Start and stop pushbuttons are provided to allow full control of the read process. The computation of energy absorption is automatic and a continuous summation is displayed on a 5-digit display.
I
input.j~
~
mpul
~
~
: ="
] ~
--output
A tensile test was carried out at a strain rate of I" 3/s; the time to fradture was about 30 ms. The load displacement characteristic was recorded on a n xy-plotter and the energy absorption recorded as described above. Results Fig. 4 shows a typical load-displacement curve for normal bone. The line AB represents the elastic region and BC the so-called plastic region. The area under ABC is the energy absorbed to fracture. Results for 50 normal specimens show a highly significant correlation between ultimate tensile strength and energy absorbed (P < 0' 001). Preliminary results for immobilised bone show a lower ultimate tensile strength and energy absorption, the most probable reason being the greatly increased porosity in immobilised bone. Conclusion The system described allows an accurate recording of transient load-displacement information to be made. It simultaneously computes energy absorbed to fracture, saving much manual effort. The results to date suggest, for human femoral cortical bone, that the tensile strength and energy absorbed to fracture decrease with immobility. Clinically, this result is important when considering patients after long periods of bed rest.
Fig. 3 Operation of the integrating device
500
~o0
Acknowledgment--The authors would like to express their thanks to R. Jayes, for his invaluable technical help. R. P. DICKENSON W. C. HUTTON
Medical Engineering Group Division of Engineering Polytechnic of Central London 115 New CavendJsh Street London WI M 8JS, England
~ 100 A 0
I
0.10 020 displacement, mm
I
030
Fig. 4 Load displacen,~ i "urve for normal bone The device will resolve +0".5 bit or +0"29/00 in load and displacement. The control over the read process allows energy absorption to be computed over any portion of the load-displacement graph. This is particularly useful when it is required to compute separately the elastic and plastic energy absorption. This system represents a considerable improvement over the other possible way of monitoring transient load-displacement information, namely by photographing an oscilloscope trace.
208
References CROWNINSHIELD, R. D. and POPE, M. H. (1974) The response of compact bone in tension at various strain rates. Ann. Biomed. Eng. 2, 2t7-225. MCELHANEY, J. H. (1966) Dynamic response of bone and muscle tissue. J. AppL Physiol. 21, 1231-1236. PIEKARSK/, K. (1970) Fracture of bone. J, AppL Phys. 41, 215-223. POPE, M. H. and MURPHY, M. C. (1974) Fracture energy of bone in a shear mode. Med. &Biol. Eng. 12, 763-767. REILLY, D. T. and BURSTEIN, A. H. (1974) The mechanical properties of cortical bone. J. Bone & Joint Surgery 56A, 1001-1021.
Medical & Biological Engineering & Computing
March 1977
Technical note Instrumentation for measuring the tensile properties of femoral cortical bone Keywords--Bone, Instrumentation, Tensile strength Introduction THE raECHANICALproperties of h u m a n cortical bone have received considerable attention (REILLY and BURSTEIN, 1974), but, to date, there is little published information on these properties when bone is subjected to loads at a fast rate to simulate trauma (MC]ELHANEY, 1966; CROWNINSHIELD and POPE, 1974).
E
E
oli radii 0-50
/
X "X
12
20.50
30'00 air dimensions mm
Fig. 1 Testpiece dimensions
oscillating at + 1 mm amplitude. The maximum ram speed is 50 mm/s. A strain-gauged extensometer was attached across the gauge length of the specimen, see Fig. 2. A small preload was applied to the arms of the extensometer before testing. Using an extensometer such as this is inherently the most accurate method of measuring the increase in length of a tensile test specimen under load. This method measures the change in length directly, and is not influenced by any embedding of the specimen in the grips. On calibration, the extensometer had a linear range of _+3 m m and it has a natural frequency of. 2 kHz. The calibration curve was repeatable to within 1%. The load exerted on the specimen was measured by a load cell incorporated in the crosshead of the testing machine. Recent work has brought attention to the importance of energy absorption to fracture as an important mechanical property of bone (PIEKARSKI, 1970; POPE and MvRenv, 1974). An electronic integrating device was built to record the transient load-displacement characteristics, and from these to compute the area under the load-displacement curve; this is the energy absorbed by the specimen. Fig. 3 is a block diagram illustrating its operation. The device has two modes of operation: 'read' and 'write'. In the 'write" mode, the analogue information from the load and displacement transducers is converted
In order to apply and monitor the load and displacement on the test specimen, an instrumentation system, based on a servo-hydraulic testing machine, has been developed. The system described (vide infra) is being used to determine the effects of immobility on the mechanical properties of bone.
Material R o u n d dumb-bell-shaped specimens were machined from the middle third of fresh, human, femoral, cortical bone, obtained at post mortem. The dimensions of the test piece are shown in Fig. 1. The specimens were kept wet, with physiological saline, while being machined and tested. Considerable care was taken in the selection of specimens. Clinical histories were obtained, and for normal subjects bone was not used if there was any possible skeletal defect. The criterion used for the immobilised bone was six weeks immobilisation. Instrumentation system The tensile specimens were inserted into two carefully constructed grips mounted on the servo-hydraulic testing machine. The hydraulically driven, servocontrolled ram has a fiat response from 0 to 4 Hz when First received 22nd April and in final form 7th June 1976
Medical & Biological Engineering & Computing
Fig. 2 Extensometer in position on test piece
March 1977
207
to a digital signal and stored in a 256 bit memory. A warning light indicates if the store capacity has been exceeded. In the 'read' mode the stored information is reprocessed to produce the analogue information from the transducers, the period of scan is set by a 5-position rotary switch and allows the stored information to be read out at a fast or slow rate for display on an Oscilloscope or an xy-plotter. Start and stop pushbuttons are provided to allow full control of the read process. The computation of energy absorption is automatic and a continuous summation is displayed on a 5-digit display.
I
input.j~
~
mpul
~
~
: ="
] ~
--output
A tensile test was carried out at a strain rate of I" 3/s; the time to fradture was about 30 ms. The load displacement characteristic was recorded on a n xy-plotter and the energy absorption recorded as described above. Results Fig. 4 shows a typical load-displacement curve for normal bone. The line AB represents the elastic region and BC the so-called plastic region. The area under ABC is the energy absorbed to fracture. Results for 50 normal specimens show a highly significant correlation between ultimate tensile strength and energy absorbed (P < 0' 001). Preliminary results for immobilised bone show a lower ultimate tensile strength and energy absorption, the most probable reason being the greatly increased porosity in immobilised bone. Conclusion The system described allows an accurate recording of transient load-displacement information to be made. It simultaneously computes energy absorbed to fracture, saving much manual effort. The results to date suggest, for human femoral cortical bone, that the tensile strength and energy absorbed to fracture decrease with immobility. Clinically, this result is important when considering patients after long periods of bed rest.
Fig. 3 Operation of the integrating device
500
~o0
Acknowledgment--The authors would like to express their thanks to R. Jayes, for his invaluable technical help. R. P. DICKENSON W. C. HUTTON
Medical Engineering Group Division of Engineering Polytechnic of Central London 115 New CavendJsh Street London WI M 8JS, England
~ 100 A 0
I
0.10 020 displacement, mm
I
030
Fig. 4 Load displacen,~ i "urve for normal bone The device will resolve +0".5 bit or +0"29/00 in load and displacement. The control over the read process allows energy absorption to be computed over any portion of the load-displacement graph. This is particularly useful when it is required to compute separately the elastic and plastic energy absorption. This system represents a considerable improvement over the other possible way of monitoring transient load-displacement information, namely by photographing an oscilloscope trace.
208
References CROWNINSHIELD, R. D. and POPE, M. H. (1974) The response of compact bone in tension at various strain rates. Ann. Biomed. Eng. 2, 2t7-225. MCELHANEY, J. H. (1966) Dynamic response of bone and muscle tissue. J. AppL Physiol. 21, 1231-1236. PIEKARSK/, K. (1970) Fracture of bone. J, AppL Phys. 41, 215-223. POPE, M. H. and MURPHY, M. C. (1974) Fracture energy of bone in a shear mode. Med. &Biol. Eng. 12, 763-767. REILLY, D. T. and BURSTEIN, A. H. (1974) The mechanical properties of cortical bone. J. Bone & Joint Surgery 56A, 1001-1021.
Medical & Biological Engineering & Computing
March 1977
