John W. Melvin

Mechanical properties of cranial bone

Samples of human and Macaca mularta cranial bone have been tested quasistatically in tension, compression, simple shear, and torsion. The results of these experiments have been analyzed, taking into account observed anisotrophies and varying structures. Statistical correla- tions of properties have been made with density and a model proposed that summarizes these results. The cranial bones appear to be transversely isotropic and they are generally much stronger and stiffer in the transverse or tangent to the skull direction in comparison to the radial direction. The structure of the dip& region was found to be highly variable and this strongly influenced many of the mechanical responses. The model, however. explains much of the ob- served variation.

Failure properties of passive human aortic tissue. II—Biaxial tension tests

Descending mid-thoracic aortas were obtained from 16 autopsies and biaxial inflation tests performed on the tissue at dynamic (approximately 20 s-1) and quasi-static (approximately 0.01 s-1) strain rates. A bubble inflation technique was developed for this purpose. Extension histories of the specimens were recorded photographically and values of ultimate stresses and extension ratios in biaxial stretch have been calculated. Under conditions of uniform biaxial stretch the tissue consistently failed in a direction perpendicular to the long axis of the aorta and pressure values at failure were greater by a factor of two in the dynamic tests than those in the quasi-static tests.

Impact Injury Mechanisms in Abdominal Organs

Blunt abdominal trauma is a major cause of death in the United States. However, little experimental work has been done to clarify the mechanism of blunt abdominal injury and to quantify tolerance parameters for the abdominal organs. This paper describes a joint study by the Highway Safety Research Institute and the Section of General Surgery of The University of Michigan in which direct impacts were applied to livers and kidneys. The tests were performed in a high-speed testing machine at a controlled ram velocity and stroke limit. The organ was surgically mobilized in anesthesized Rhesus monkeys and then placed on a load cell while still being perfused in the living animal. Tests were performed at ram speeds of 120, 6000, and 12000 in/min (5, 250, and 500 cm/s). The resulting load-deflection data were normalized and average stress-strain curves plotted for each test. In addition, the resulting injury severity was estimated immediately after impact using an injury scale of 1 to 5. A discussion of the injury mechanisms observed in the tests is given, and correlation between injury severity and the mechanical parameters of stress, strain, and strain energy produced in the tissue of the organ is presented.

Dynamic Characteristics of the Tissues of the Head

The mechanical causes of head injury have been the subject of much research and controversy. While there is a large amount of literature concerning the overall physiological and pathological effects of head injury, there is considerably less information available on the mechanical characteristics of the tissues of the head. Yet it is these characteristics that determine the mechanism and extent of injury resulting from a blow to the head. As so well put by Ommaya (1968), ‘an understanding of the effects of trauma and the development of an exact, rational prophylaxis and therapy for head injury cannot be satisfactorily achieved without a quantitative description of the mechanical properties of the tissues involved’.

RESPONSE OF THE CERVICAL SPINE TO SUPERIOR-INFERIOR HEAD IMPACT

A test series using 12 unembalmed cadavers was conducted to investigate factors affecting the creation of cervical spine damage due to impact to the top of the head. The test subjects were instrumented to measure head, T8 thoracic spine, and sternum acceleration responses. Photographic targets on the head and torso allowed analysis of impact motions from high-speed movies. The stationary test subject was struck by a guided, moving impactor mass of 56 Kg at 4.6-5.6 m/s. The impactor striking surface consisted of a biaxial load cell with padding to vary the contact force-time characteristics of the head/impactor. The orientation of the head, cervical spine, and torso was adjusted relative to the impactor axis to investigate the effect of spinal configurations on the damage patterns. Load and acceleration data are presented as functions of time and as functions of frequency in the form of mechanical impedance. Damage to the cervical spine was produced in all but one test, including fractures of the spinous processes, laminae, transverse processes, and the bodies of the vertebrae as well as ruptured discs and torn ligaments. Both anterior and posterior damage was produced and the sites of the damage ranged from C2 to T4. The peak forces produced during the impacts ranged from 1.8 kN to 11.1 kN. The limited response data of this pilot study do not allow any specific conclusions with regard to cervical spine tolerance levels. However, it does attest to the influence of spinal configuration and impact conditions on both response and damage of the spine due to crown impact.

HEAD AND NECK RESPONSE TO AXIAL IMPACTS

Two series of impacts to the head in the superior-inferior direction using 19 unembalmed cadavers are reported. The first series of five tests was aimed at generating kinematic and dynamic response to sub-injurious impacts for the purpose of defining the mechanical characteristics of the undamaged head-neck-spine system in the S-I direction. The second series of fourteen tests was intended to define injury tolerance levels for a selected subject configuration. A 10-kg impactor was used to deliver the impact to the crown at a nominal velocity of 8 m/s for the first series, and between 7 and 11 m/s for the second series. Measurements made include the impact velocity, force, and energy, the head three-dimensional kinematics, forces and moments at the occipital condyles, and accelerations of the T1, T6, and T12 vertebrae.

Impact Response and Tolerance of the Lower Extremities

This paper presents the results of direct impact tests and driving point impedance tests on the legs of seated unembalmed human cadavers. Variables studied in the program included impactor energy and impact direction (axial and oblique). Multiple strain gage rosettes were applied to the bone to determine the strain distribution in the bone. The test results indicate that the unembalmed skeletal system of the lower extremities is capable of carrying significantly greater loads than those determined in tests with embalmed subjects (the only similar data reported in the present literature). The strain analysis indicated that significant bending moments are generated in the femur with axial knee impact. The results of the impedance tests are used to characterize the load transmission behavior of the knee-femur-pelvis complex, and the impact test results are combined with this information to produce suggested response characteristics for dummy simulation of knee impact response.

Material identification of soft tissue using membrane inflation

The constitutive equation for large elastic deformations is often used to model the mechanical response of soft tissue. This paper is concerned with the applications of the method of material identification to the determination of the strain energy density functions ( W) in such a mode, under the assumption that the tissue is incompressible and isotropic. It is shown that an identification experiment based on inflation by lateral pressure ofan initially flat circular membraneous specimen has a number of advantages. These are: the method of clamping the specimen, the ease of labelling material particles and measuring current coordinates, the easily determined domain of identification of I% and a means of systematically determining Wover a large deformation range. An example in the form of a hypothetical experiment is presented. 1. lNTRODUCDON The constitutive equation for large elastic defor- mations is often used to model the mechanical re- sponse of soft biological tissues as well as rubbery materials. Its application depends on the knowledge of the strain energy density function (W) for the material under consideration. Forms for W for incompressible rubbery materials are reasonably well established. On the other hand, a significant amount of activity is concerned with the determination of strain energy density functions for soft tissues. The procedures which have been used to establish forms for W for rubber appear to be the guide for some current approaches for the determination of W for soft tissue. It will be useful, for present purposes, to briefly review the determination of W for rubber. The initial forms for W were developed from an experimental program based on subjecting specimens to unequal biaxial homogeneous deformattons. In order to assess the Mooney form of W in predicting non- homogeneous deformations, Adkins and Rivlin (1952) used it in the calculation of the deformed profiles of a clamped, initially flat, circular rubber membrane which had been pressurized on one side. These were then compared with actual profiles which had been previously measured by Treloar (1944). Agreement between the measured and calculated profiles was satisfactory at low inflation levels but less so at higher levels. Later, Klingbeil and Shield (1964), and then Hart-Smith and Crisp (1967) introduced improved forms for W for which the match between measured and calculated profiles was very good. Recently, a method has been suggested which combines these stages of construction from homo- geneous deformations, verification against non- homogeneous deformations and subsequent improve-

Bolster impacts to the knee and tibia of human cadavers and an anthropomorphic dummy

Knee bolsters on the lower instrument panel have been designed to control occupant kinematics during sudden deceleration. However, a wide variability in car occupant anthropometry and choice of seating posture indicates that lower-extremity contacts with the impingement bolster could predominantly load the flexed leg through the knee (acting through the femur) or through the tibia (acting through the knee joint). Potential injuries associated with these types of primary loading may vary significantly and an understanding of potential trauma mechanisms is important for proper occupant restraint. Impacts of the bolster panel against the knee or lower leg were simulated in 10 human cadaver and anthropomorphic dummy tests and the following aspects were assessed: (1) biomechanical response for lower-extremity impacts, (2) potential mechanisms of skeletal and ligamentous trauma, (3) differences between human cadavers and an anthropomorphic test dummy response, and (4) knee-joint ligament failure characteristics in isolated knee-joint tests. For the covering abstract see IRRD 810752. (TRRL)

Impact trauma of the human temporal bone.

A cooperative study between the Department of Otorhinolaryngology and the Highway Safety Research institute of the University of Michigan was designed to study temporal bone fracture produced in cadavers subjected to realistic automotive impact situations. Utilizing sled and piston impact configurations frontal and parietal impacts were noted to produce ipsilateral and contralateral fractures of nine temporal bones in seven cadavers. The impact velocities varied between 18.1 and 25.0 mph. Using standard otologic microsurgical techniques, the temporal bones were dissected and numerous gross and microscopic injuries to middle and inner ear structures were found. The authors conclude that extensive comminuted fracture of the human temporal bone is seen with realistic crash situations of low velocity, and that lateral impact which produces a longitudinal fracture with a posterior fossa comminution is associated with disruption of the cochlea and facial nerve, as well as of middle ear structures. The classical transverse fracture of extensive skull trauma lies medial to these structures and does not involve the otologic contents of the human temporal bone. Associated brain and skull injuries are also described.