Methods for dynamic material testing of human cortical bone specimens (2004)

A number of researchers have investigated the material properties of human cortical bone and the effects of different parameters such as: tension, compression, age, bone density, direction dependence, regional variation, and rate dependence. Although all of the aforementioned parameters have been investigated, previous studies have focused on the long bones of the body and none have addressed more than two or three of these parameters within a single controlled study. In addition, material property testing of human cortical bone is also desirable to obtain the material properties of a specific body in order to model specific tests conducted with that body. Therefore, there is still a necessity for continued material property testing of the human skeletal system. The purpose of this study was to develop a method for preparing and testing human cortical bone coupons at dynamic rates in both tension and compression to obtain accurate, comprehensive material property data. The methods presented in this paper were developed through 195 practice tests on human tibia cortical bone. A detailed specimen preparation technique was developed to obtain cortical bone test specimens while maintaining proper specimen alignment and hydration. Methods to minimize the effects of bending due to misalignment of the test setup and test specimens were addressed. The use of a slack adapter provided constant strain rates. A strain gage, potentiometer, extensometer, laser vibrometer, and the MTS internal LVDT were evaluated for displacement measurement accuracy under static and dynamic loading. The laser vibrometer, which has nanometer scale accuracy and a high frequency response of 200 kHz, was found to be the best displacement measurement device when testing at dynamic rates. The results of the coupon tests conducted on human tibia cortical bone proved that the specimen preparation and test methods presented in this study are both accurate and precise for determining cortical bone material properties. INTRODUCTION here have been numerous studies that have invested the material properties of human cortical bone and the effects of different parameters such as: tension, compression, age, bone mineral density, direction dependence, regional variation, and rate dependence. Dempster (1952) presented one of the first studies using human femur and tibia bones. This study conducted tension and compression tests in both the axial and lateral directions, but only quasi-static loading rates were tested. Therefore, viscoelastic effects needed for application to the automobile safety field were not investigated. Evans (1956) performed tests on human T Injury Biomechanics Research 32 femur, tibia, and fibula cortical bone specimens in tension and looked at the effects of regional variation. However, like Dempster (1952), these tests were conducted at quasi-static rates and did not examine viscoelastic effects. McElhaney (1966) presented tests on human femur bones in compression, but did not look at any tests in tension. Reilly (1975) performed tests on human femur specimens in tension and compression and in three loading directions. However, this study was limited also to only quasi-static loading rates and did not perform high-rate loading tests. Finally, Saha (1974) conducted dynamic tests but only in the tension and axial directions. Although all of the effects of the aforementioned parameters have been investigated, previous studies have focused on the long bones of the body and none have addressed more than two or three of these parameters within a single controlled study. In addition, material property testing of human cortical bone is also desirable to obtain the properties of a specific body in order to model specific tests conducted with that body. Therefore, there is still a necessity for continued material property testing of the human skeletal system. The purpose of this study was to develop a method for preparing and testing human cortical bone coupons at dynamic rates in both tension and compression to obtain accurate, comprehensive material property data. METHODS The methods for dynamic material testing of human cortical bone presented in this paper were developed through 195 practice tests on human tibia cortical bone, and are presented in four sections: specimen preparation, detailing proper alignment and hydration issues; alignment of test setup, detailing specimen and grip centerline conformance; slack adapter, detailing advantages and operation; and evaluation of displacement measurement devices, detailing advantages and disadvantages of each. Specimen Preparation In order to conduct material property testing on human cortical bone, the bone coupon must first be machined into a testable geometry. This was done through numerous steps of detailed preparation. First, an oscillating bone saw was used to make two cuts to separate the tibia from the body (Figure 1).