Biomechanics of skull fracture.

This study was conducted to determine the biomechanics of the human head under quasistatic and dynamic loads. Twelve unembalmed intact human cadaver heads were tested to failure using an electrohydraulic testing device. Quasistatic loading was done at a rate of 2.5 mm/s. Impact loading tests were conducted at a rate of 7.1 to 8.0 m/s. Vertex, parietal, temporal, frontal, and occipital regions were selected as the loading sites. Pathological alterations were determined by pretest and posttest radiography, close-up computed tomography (CT) images, macroscopic evaluation, and defleshing techniques. Biomechanical force-deflection response, stiffness, and energy-absorbing characteristics were obtained. Results indicated the skull to have nonlinear structural response. The failure loads, deflections, stiffness, and energies ranged from 4.5 to 14.1 kN, 3.4 to 16.6 mm, 467 to 5867 N/mm, and 14.1 to 68.5 J, respectively. The overall mean values of these parameters for quasistatic and dynamic loads were 6.4 kN (+/- 1.1), 12.0 mm (+/- 1.6), 812 N/mm (+/- 139), 33.5 J (+/- 8.5), and 11.9 kN (+/-0.9), 5.8 mm (+/- 1.0), 4023 N/mm (+/- 541), 28.0 J (+/- 5.1), respectively. It should be emphasized that these values do not account for the individual variations in the anatomical locations on the cranium of the specimens. While the X-rays and CT scans identified the fracture, the precise direction and location of the impact on the skull were not apparent in these images. Fracture widths were consistently wider at sites remote from the loading region. Consequently, based on retrospective images, it may not be appropriate to extrapolate the anatomical region that sustained the impact forces. The quantified biomechanical response parameters will assist in the development and validation of finite element models of head injury.

Evoked potential studies of the effects of impact acceleration on the motor nervous system.

The initial results of a continuing investigation into the effects of various levels of impact acceleration on the functional integrity of the motor nervous system are summarized. The results are based on the measurement of alterations in neural transmission along the motor pathway of the Rhesus monkey as revealed by latency and amplitude changes in the motor pathway evoked potential (EP) following the delivery of various levels of impact acceleration to a test vehicle. The EPs were produced by electrical stimulation of and recording from the motor pathway of experimental animals subjected to -Y (lateral impact) acceleration and animals subjected to -X (frontal impact) acceleration. High resolution latency and amplitude measures of the EP recorded from these animals before and after impact were tracked so that the time course of recovery of nerve propagation following impact could be accurately assessed. Analysis of these EP measures revealed that the time course of recovery to preimpact values is directly related to the intensity of the acceleration impulse delivered to the test vehicle.

Early somatosensory evoked potentials.

The early somatosensory evoked potential secondary to median nerve stimulation in the human had an onset latency of 9--12 msec when recorded from scalp electrodes at vertex-to-mastoid, vertex-to-inion or at the base of the skull. Similar latencies were observed from responses recorded over the cervical dorsal columns during neurologic surgery. A latency difference of 1.5 msec was observed between the early response and the responses recorded from the junction of medial lemniscus and nucleus ventralis posterior lateralis of the thalamus during human stereotaxic surgery. Cervical cord transections and transection at the midpontine levels of the monkey showed that the evoked potential was due to generators between these levels. Depth recording of the monkey indicate that the early evoked potential originates in the region of dorsal column nuclei, while the later components are secondary to generators in cerebral cortex.