A series of forty experiments has been performed on Rhesus monkeys in which the heads were subjected to a controlled single approximately sinusoidal pulse of angular acceleration about a fixed axis perpendicular to the sagittal plane. The head was constrained to undergo planar motion with a total angular displacement of 60 degrees in each case. Angular acceleration values ranged up to 1.2 x 10 5 rad/sec 2 , and peak values of tangential acceleration at the center of the mass of the brain reached 1300 g's. Physiological and neurological data including EKG, EEG, systemic arterial pressure, intracranial pressure, respiration, corneal reflex, were recorded.
The post insult state was evaluated for each subject in accordance with a scale of Experimental Trauma Severity (ETS) based on the observed changes in the physiological and neurological variables. The ETS scale ranges from 0 (absence of any changes) to 6 (instantaneous death with gross brain fragmentation). The severity of the injury correlated well with the mechanical input in terms of acceleration (angular or tangential) and in particular, with the maximum value of the resultant tangential force (ranging up to 910 Newtons) applied to the brain. The occurrence of subdural hematomas originating in the parasagittal bridging veins in 16 of the experiments is well correlated with peak values of tangential acceleration with onset occurring at values of 700 g's. Results are also given in terms of the incidence of cerebral concussion (occurring in 27 out of the 40 studies) as a function of mechanical variables including angular acceleration, brain mass, and pulse duration.
The results display a statistically significant gradation of reversible trauma effects with increasing mechanical input. Subdural hematoma formation appears as a threshhold phenomenon, the consequences of which dominate the clinical outcome.