James H. McElhaney

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.

A viscoelastic study of scalp, brain, and dura

Abstract A series of creep and relaxation experiments on scalp, brain, and dura from both human and monkey is described. An analysis of a practical creep loading with a rise time is given. An empirical expression for the observed creep curves is developed and deviations from classical viscoelasticity theory noted. A four parameter Maxwell-Kelvin fluid model is proposed and fitted to the creep data. In addition, a free vibration test using the same equipment as the creep and relaxation experiments is presented. Analysis of this experiment yields values of the complex modulus in the frequency range 10–40 Hz.

Driving point impedance characteristics of the head

The mechanical impedance of the human and monkeytMoc~crc.o ~nulurtu I head u a\ determined over the frequency range 30-1X)00 Hz Miniature accelerometers and pressure transducers were placed in the brain to measure its response to vibration at constant g-levels and variable frequency. The maximum acceleration studied was 20 gs, /rl rirro experiments on a fresh human cadaver and irk uico and it? ci?ro experiment> on monkeys were performed. The effect of varying blood pressure was investigated as well as the contribution to the mechanical impedance of the scalp. skull and brain. A linear two-degree-of-freedom model that summarizes the result, with acceptable accuracy is presented. Certain non-linear responses were observed for various input accelerations. No significant effect on impedance due to time after death was found for times up to five hours. The implantable accelerometer and pressure transducer experiments indicated that the brain is very nearly critically damped. Raising the blood pressure was shown to stiffen the hram. causing the resonance frequency of the head to increase.

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’.

The Charge Distribution on the Human Femur Due to Load

A method is presented for measuring accurately the piezoelectric modulus of small areas on the surface of whole dried bone and of specimens cut from dried bone. The results indicate that the ordered matrix necessary for the piezoelectric effect varies from place to place giving rise to regions of positive and negative charge on the surface of the bone. A map of the charge distribution on the surface of a typical femur under load is presented, and a model of the collagen fiber orientation consistent with the known organization of bone is proposed that explains to some extent the variability of the surface charge-load observations.

Electric fields and bone loss of disuse

Abstract The object of this research was to investigate the effect of electric fields on bone loss due to disuse. Plaster casts embedded with electric field generating plates were applied to the right legs of forty-eight male rats. Fields of various types were applied over a period of 28 days. A paired analysis of the properties of the right and left femurs was compared to a sham group which received no field treatments. Parameters measured included bone weight, specific gravity, cortical area, ultimate strength, modulus of elasticity, hardness, osteon count, and chemical analysis. It was found that the bone weight loss and cortical area reduction caused by immobilization were reduced by electric field treatments. Eight bone tumors were observed on eighteen femurs treated with the electric fields. No tumors were observed on the sham group.

Biomechanical Aspects of Head Injury

With the advent of high speed air and land transportation, engineers have become increasingly aware of the mechanical frangibility of the human body. Thus, we have seen the evolution of various isolating and load distributing devices ranging from seat belts and padded sun visors, to ejection seats, crash helmets, and acceleration couches. While there is a large amount of information available regarding the response of inanimate systems to vibration and impact, there is a comparable dearth of knowledge pertaining to the mechanical responses of biological systems. Therefore, the design of much supporting and protective equipment is often based on intuition because of the lack of information available about the mechanical behavior of the human body. In addition, such knowledge would be helpful in the treatment of injury by serving to identify the mechanism of trauma. Thus, both a rational design procedure for impact protection and a rational therapy for treatment of trauma cannot be developed until a quantitive description of the mechanical responses of the human body is obtained.

Human Torso Response to Blunt Trauma

The most frequent causes of blunt abdominal injury are the steering wheel and the lap belt. The organs most often injured are the liver, pancreas, spleen and intestines. Based on this information, a series of animal abdominal impacts were designed to study the relationship between shape and type of impactor, velocity and direction of impact, body region impacted and injury level. The results of this study are given in the form of an experimental scaling factor which relates the sensitivity to impacts of the various body regions. This scaling factor was found to be dependent of body weight, making it applicable for evaluating human tolerances to abdominal impacts.

Side impact tolerance to blunt trauma

The object of this research program has been to extend the scope of earlier work to include long-duration head impacts and to develop new scaling relationships to allow extrapolation of impact data from infrahuman primates to living humans. A series of living primate side impacts to the head and torso was conducted in parallel with a series of impacts to human cadavers. Dimensional analysis techniques were employed to estimate in vivo human tolerance to side injury. The threshold of closed brain injury to humans was found to be 76 g for a pulse duration of 20 ms and an impact velocity of 43 ft/s (13.2 m/s). The maximum tolerable penetration to the chest was found to be 2.65 in (6.72 cm) for both the left and right sides. Scaling of abdominal injuries to humans was accomplished by employing a factor that relates impact contact area, animal mass, impact force, and pulse duration to injury severity. The maximum tolerable contact pressure to the upper abdomen of a human was found to be 32 lbf/inu2 (220 kPa).

DOOR CRASHWORTHINESS CRITERIA

The object of the program was to make long duration head impacts and to develop scaling relationships to allow extrapolation of impact data for infra-human primates to living humans. A series of primate side impacts, to the head and body was conducted in parallel with a series of impacts to human cadavers. Dimensional analysis techniques were employed to estimate in vivo human tolerance to side impacts. The threshold of closed brain injury to humans was found to be 76Gs for a pulse duration of 20 msec and an impact velocity of 29.5 mph. The maximum tolerable penetration to the chest was found to be 2.65 inches for both the left and right sides. Scaling of abdominal injuries to humans was accomplished by employing a factor which relates impact contact area, animal mass, impact force, and pulse duration, to injury severity. The maximum tolerable contact pressure to the upper abdomen of a human was found to be 32 psi.